**Graphene-Based Composites and Hybrids for Water Purification Applications**

Rahul Sharma and Parveen Saini

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/63666

#### **Abstract**

The increasing water demand and the worldwide shortage of clean water call for new technologies for wastewater treatment, of which sorption is recognized as simplest and efficient method for all major water pollutants, including heavy metal ions, organic dyes and organic solvents. In this context, graphene, an atomically thin two-dimensional hexagonal arrays of carbon atoms, and its analogs have been visualized as ultimate materials for the separation of pollutants from water due to properties such as superhy‐ drophobicity, superoleophilicity, chemical-environmental-thermal stability, chemical manipulability, controllable morphology, texture, porosity and accessible surface area, facile processing, low density, biocompatibility and low-cost synthesis from a variety of abundant carbonaceous precursors. This chapter begins with an introduction of water purification, related processes and fundamentals of adsorption and absorption phenomenon. The next section emphasizes the importance and advantages of gra‐ phene as sorbent, brief about its synthesis, types of graphene-based composites/hybrid systems and their synthesis strategies. The last section throws light on the performance of graphene-based foams, aerogels and composites/hybrids for the separation of metal ions, organic dyes and various oils/organic solvents, supported by comprehensive literature account. Finally, the chapter concludes with the comments on the current status, major challenges and future scope in the direction.

**Keywords:** sorption, adsorption, absorption, graphene oxide (GO), reduced graphene oxide (RGO), superhydrophobicity, superoleophilicity

#### **1. Introduction**

In the current scenario of global socioeconomic development, environmental conservation is a major concern and numerous initiatives have been undertaken to handle air, water, soil, noise

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

andelectromagneticpollutions [1–13].Inparticular,waterpollutionis recognizedas abigthreat for society, and enormous efforts are underway to prevent contamination of freshwater and to purify contaminated water. The water pollution is considered as an offshoot of scientific and technological growth, industrial revolutionization and ignorance/unavailability of efficient waste management strategies. The discharge of domestic and industrial effluents/wastewater into water bodies not only makes water sources unfit for drinking purpose but also adversely affects its utility for pharmaceutical, agricultural, chemical and industrial applications. In particular, dumping of untreated wastewater at domestic and industrial levels (e.g., by paper, textile, food, leather and chemical industries) introduces hazardous substances, for example, dyes, chromium compounds, heavy metals such as copper, arsenic, lead, cadmium, mercury, nickel, cobalt and certain toxic organic chemicals, into rivers, lakes and other freshwater bodies [10,14–18].These contaminantsaffectthephotosynthesis inaquatic systemsanddisturbthe selfpurification mechanism [19] of water with extinction of some aquatic species as one of its worst consequence.Theiracuteexposureposesserioushealthhazardsforhuman,forexample,organic dyes and heavy metal ions are known to exert harmful effects such as an increased heart rate, vomiting, shock, cyanosis, jaundice, tissue necrosis, bone defects, renal damage, paralysis and even death [20–22] in some cases.

The leakage and spilling of industrial chemicals and oils into water bodies also disturb the water quality and adversely affect the aquatic life. The large-scale oil spill accidents (occurring during onshore/offshore handling of oils) are identified as catastrophic and devastating situation for marine life. It also adversely affects the infrastructure (e.g., beaches), underwater ecosystem (fauna and flora) and badly affects the economies in terms of losses in tourism, fishing, seafood sectors and expenditures for the restoration of the same [23–31]. Therefore, in the past half-century, an extensive research was carried out across the globe to develop a number of efficient and smart materials/technologies for the effective treatment of wastewater containing various pollutant, especially metal ions, dyes, oils and solvents [32–42,17,43–45]. Among currently available mechanical, chemical and biological water purification processes such as coagulation, flocculation, ozone treatment, chemical oxidation, membrane filtration, ion exchange, photocatalysis and sorption [46–56], sorption (adsorption-absorption) practices emerged as most preferred commercially viable solution, owing to specific advantages in terms of simplicity of process, cost-effectiveness, environmental friendliness, applicability to diversified pollutants and low-energy consumption. In particular, adsorption practices are the sole most promising alternates in the isolation of metal ions that are otherwise very difficult to remove by means of photocatalysis and membrane filtration [57,58]. However, adsorption has also been demonstrated as viable solution for the removal of hazardous organic dyes from the wastewater streams of various industries. In contrast, absorption practices are prevalent in selective removal of nonpolar (hydrophobic) pollutants such as mineral or vegetable oils, crude oils and common organic solvents [28,59,60] from their aqueous emulsion or floating layers over water.

In the past, a number of adsorbing-absorbing materials have been exploited for wastewater treatment, including zeolite [61], silica gel [62], activated alumina [63], fly ash [64], polymeric and carbon-based materials [65–68]. Among them, carbon-based systems with environmental compatibility, large-scale availability, high porosity, large specific surface area and low cost are the most promising. Activated carbon derivatives with high degree of porosity and large specific surface area (600–2000) m2 g−1 are used in adsorption-based water purification for many years [65,69–72]. However, essentiality of their surface activation to enhance adsorption capacity and release of secondary pollutants in the form of fine particles in water limited their utility. The new members of the carbon family fullerene, carbon nanotube (CNT) and graphene have truly revolutionalized the field with graphene, the latest member of the family, seems to be most promising alternate owing to its extraordinary mechanical, thermal & chemical properties coupled with very high specific surface area, inherent hydrophobicity, tunable chemical structure, cost-effective and scalable synthesis [10,73–78]. In this context, the quest for the development of highly efficient adsorbents and absorbents and advancements in graphene science and technology fuelled numerous efforts for designing/selecting highly efficient graphene-based sorbents, understanding their pollutant-capturing mechanism and assessing their actual purification performance [16,47,57,79–83,43].

It is worth mentioning that though sorption is a broad term, the underlying adsorption and absorption subprocesses are surface- and bulk-related phenomenon, respectively, and their governing factors are entirely different. For example, adsorption process depends on param‐ eters such as nature of adsorbent-adsorbate pair, accessible surface area of adsorbent, concen‐ tration of pollutants, temperature, pH, etc [15,45,48,84]. In contrast, absorption is regulated by pore size/volume of the sorbent, its texture and wettability characteristics along with physical attributes of absorbate species and absorbent-absorbate interfacial properties [75,85,86]. Therefore, detailed understanding of adsorption and absorption phenomenon, related terminology, important parameters and governing empirical equations is necessary before identifying and designing a suitable graphene-based adsorbent or absorbent.

#### **2. Adsorption**

andelectromagneticpollutions [1–13].Inparticular,waterpollutionis recognizedas abigthreat for society, and enormous efforts are underway to prevent contamination of freshwater and to purify contaminated water. The water pollution is considered as an offshoot of scientific and technological growth, industrial revolutionization and ignorance/unavailability of efficient waste management strategies. The discharge of domestic and industrial effluents/wastewater into water bodies not only makes water sources unfit for drinking purpose but also adversely affects its utility for pharmaceutical, agricultural, chemical and industrial applications. In particular, dumping of untreated wastewater at domestic and industrial levels (e.g., by paper, textile, food, leather and chemical industries) introduces hazardous substances, for example, dyes, chromium compounds, heavy metals such as copper, arsenic, lead, cadmium, mercury, nickel, cobalt and certain toxic organic chemicals, into rivers, lakes and other freshwater bodies [10,14–18].These contaminantsaffectthephotosynthesis inaquatic systemsanddisturbthe selfpurification mechanism [19] of water with extinction of some aquatic species as one of its worst consequence.Theiracuteexposureposesserioushealthhazardsforhuman,forexample,organic dyes and heavy metal ions are known to exert harmful effects such as an increased heart rate, vomiting, shock, cyanosis, jaundice, tissue necrosis, bone defects, renal damage, paralysis and

The leakage and spilling of industrial chemicals and oils into water bodies also disturb the water quality and adversely affect the aquatic life. The large-scale oil spill accidents (occurring during onshore/offshore handling of oils) are identified as catastrophic and devastating situation for marine life. It also adversely affects the infrastructure (e.g., beaches), underwater ecosystem (fauna and flora) and badly affects the economies in terms of losses in tourism, fishing, seafood sectors and expenditures for the restoration of the same [23–31]. Therefore, in the past half-century, an extensive research was carried out across the globe to develop a number of efficient and smart materials/technologies for the effective treatment of wastewater containing various pollutant, especially metal ions, dyes, oils and solvents [32–42,17,43–45]. Among currently available mechanical, chemical and biological water purification processes such as coagulation, flocculation, ozone treatment, chemical oxidation, membrane filtration, ion exchange, photocatalysis and sorption [46–56], sorption (adsorption-absorption) practices emerged as most preferred commercially viable solution, owing to specific advantages in terms of simplicity of process, cost-effectiveness, environmental friendliness, applicability to diversified pollutants and low-energy consumption. In particular, adsorption practices are the sole most promising alternates in the isolation of metal ions that are otherwise very difficult to remove by means of photocatalysis and membrane filtration [57,58]. However, adsorption has also been demonstrated as viable solution for the removal of hazardous organic dyes from the wastewater streams of various industries. In contrast, absorption practices are prevalent in selective removal of nonpolar (hydrophobic) pollutants such as mineral or vegetable oils, crude oils and common organic solvents [28,59,60] from their aqueous emulsion or floating

In the past, a number of adsorbing-absorbing materials have been exploited for wastewater treatment, including zeolite [61], silica gel [62], activated alumina [63], fly ash [64], polymeric and carbon-based materials [65–68]. Among them, carbon-based systems with environmental

even death [20–22] in some cases.

22 Diamond and Carbon Composites and Nanocomposites

layers over water.

The term adsorption implies the presence of an excess concentration of molecular species (gas or liquid), known as adsorbate (**Figure 1**), at the surface of porous solid (i.e., at solid-fluid interface), known as adsorbent, compared to its concentration in bulk of the adsorbent.

Adsorption arises due to the fact that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk. In solids, unsatisfactory valence forces of surface

**Figure 1.** Pictorial representation of adsorption process.

atoms, activate the surface. At these activated surfaces, other substances stick feasibly with a decrease in the entropy (*S*) of the system. Since every spontaneous process is driven by a decrease in Gibbs free energy (*G*) that can be expressed as follows:

$$
\Delta \mathbf{G} = \Delta \mathbf{H} \text{-T} \Delta \mathbf{S} \tag{1}
$$

Therefore, adsorption is essentially an exothermic process and the magnitude of change in enthalpy (Δ*H*) must be greater than the magnitude of term '*T*Δ*S*'. As the adsorption proceeds, Δ*H* becomes less negative and ultimately becomes equal to *TΔS*, so that Δ*G* becomes zero and equilibrium is attained.

The term adsorption involves the capture of adsorbate exclusively over the surface of adsorb‐ ent. Therefore, on the basis of type interaction between adsorbent and adsorbate (i.e., physical or chemical), adsorption can be classified into two main categories, namely physisorption that involves van der Waals binding forces and chemisorption [87] wherein a strong chemical bond formation takes place between adsorbent and adsorbate. Adsorption is a complex phenom‐ enon and requires systematic studies to understand completely its mechanism, kinetics and the thermodynamic parameters that influence adsorption capacity and rate. In particular, homogeneous and heterogeneous adsorption types have been identified with the involvement of single or multilayer adsorption of adsorbates [87]. To understand such processes and to account for the equilibrium adsorbate concentration over adsorbent and bulk of solution, several adsorption isotherms and related governing equations have been proposed, which are described in the following section.

#### **2.1. Freundlich adsorption isotherm**

Freundlich, in 1909, gave an empirical relationship between the quantity of a substance adsorbed by unit mass of solid adsorbent and the equilibrium concentration of adsorbate solution at a particular temperature for heterogeneous surfaces, which can be expressed by the following Eq. (2) [70]:

$$\propto \prime m = q\_e = kC\_e^{1/\mu} \tag{2}$$

where '*x*' is amount (in mol) of adsorbate adsorbed by '*m*' units amount (in g) of adsorbent. The constants *K* and *n* are the characteristics of given adsorbent-adsorbate pair. *C*e is the equilibrium concentration (i.e., when adsorption is complete) of adsorbate solution. The use of logarithmic function converts Eq. (2) into the following linear form:

$$\log q\_{\varepsilon} = \log \left( \mathbf{x} \,/ \, m \right) = \log K + \frac{1}{n} \log C\_{\varepsilon} \tag{3}$$

Plot of log(*x*/*m*) versus log *C*<sup>e</sup> gives a straight line and values of '*n*' and '*K*' can be calculated from its slope and intercept, respectively. The validity of Freundlich adsorption isotherm depends upon the completion of adsorption process. Equal volumes of standard solutions are added to equal amounts (*m*) of adsorbent (say charcoal) in different flasks. The final concen‐ tration (*C*e) is determined in each flask after adsorption. The difference in the initial and final concentrations gives the value of '*x*'. For each set of '*x*' and '*C*e' values, the validity of Freundlich isotherm can be established using Eq. (3).

#### **2.2. Langmuir adsorption isotherm**

atoms, activate the surface. At these activated surfaces, other substances stick feasibly with a decrease in the entropy (*S*) of the system. Since every spontaneous process is driven by a

Therefore, adsorption is essentially an exothermic process and the magnitude of change in enthalpy (Δ*H*) must be greater than the magnitude of term '*T*Δ*S*'. As the adsorption proceeds, Δ*H* becomes less negative and ultimately becomes equal to *TΔS*, so that Δ*G* becomes zero and

The term adsorption involves the capture of adsorbate exclusively over the surface of adsorb‐ ent. Therefore, on the basis of type interaction between adsorbent and adsorbate (i.e., physical or chemical), adsorption can be classified into two main categories, namely physisorption that involves van der Waals binding forces and chemisorption [87] wherein a strong chemical bond formation takes place between adsorbent and adsorbate. Adsorption is a complex phenom‐ enon and requires systematic studies to understand completely its mechanism, kinetics and the thermodynamic parameters that influence adsorption capacity and rate. In particular, homogeneous and heterogeneous adsorption types have been identified with the involvement of single or multilayer adsorption of adsorbates [87]. To understand such processes and to account for the equilibrium adsorbate concentration over adsorbent and bulk of solution, several adsorption isotherms and related governing equations have been proposed, which are

Freundlich, in 1909, gave an empirical relationship between the quantity of a substance adsorbed by unit mass of solid adsorbent and the equilibrium concentration of adsorbate solution at a particular temperature for heterogeneous surfaces, which can be expressed by

1/ / *<sup>n</sup>*

of logarithmic function converts Eq. (2) into the following linear form:

where '*x*' is amount (in mol) of adsorbate adsorbed by '*m*' units amount (in g) of adsorbent. The constants *K* and *n* are the characteristics of given adsorbent-adsorbate pair. *C*e is the equilibrium concentration (i.e., when adsorption is complete) of adsorbate solution. The use

ΔG = ΔH-TΔS (1)

*e e x m q kC* = = (2)

( ) <sup>1</sup> log log / log log *e e q xm K C <sup>n</sup>* = =+ (3)

decrease in Gibbs free energy (*G*) that can be expressed as follows:

equilibrium is attained.

24 Diamond and Carbon Composites and Nanocomposites

described in the following section.

**2.1. Freundlich adsorption isotherm**

the following Eq. (2) [70]:

Langmuir derived a quantitative expression for variation in the extent of adsorption with concentration, by assuming the equilibrium nature of adsorption and fixed number of equivalent energy adsorbing sites on adsorbent surface [70] .This model is applicable to homogeneous surfaces and assumes that only monolayer of adsorbate molecules formed on adsorbent surface and there is no interaction between the adsorbate species after adsorption. This isotherm represents the equilibrium distribution of a material in solid and liquid or gas phases and can be mathematically expressed as follows [70]:

$$\text{m} \times m = q\_{\text{e}} = \frac{qKC\_{\text{e}}}{1 + KC\_{\text{e}}} \tag{4}$$

where *q*e and *C*e are the equilibrium adsorption capacity (mol/g) and equilibrium concentration of adsorbate solution, respectively. The constants *K* and q are the Langmuir's constant and maximum monolayer adsorption capacity. The linear form of Langmuir Eq. (4) is given as follows:

$$\frac{1}{q\_s} = \frac{1}{q} + \frac{1}{qKC\_s} \tag{5}$$

By plotting 1/*q*e against 1/*C*e value of *K* and *q* can be calculated from slope and intercept, respectively, of fitted straight line. In order to judge the feasibility of adsorption, a dimen‐ sionless constant parameter *R*L called separation factor is defined, which is related to Langmuir constant and initial concentration *C*<sup>i</sup> of adsorbate solution as follows [70]:

$$R\_i = \frac{1}{1 + KC\_i} \tag{6}$$

This constant explains the characteristic features of Langmuir's adsorption isotherm, as it indicates the shape of isotherm/energy of the adsorption. The magnitude of *R*L signifies the feasibility of adsorption as *R*L>1, *R*L=1, 1<*R*L>0, *R*L=0 indicates unfavorability, linearity, favorability and irreversibility of adsorption, respectively [70].

#### **2.3. Temkin adsorption isotherm**

According to Temkin adsorption isotherm, energy of the adsorption is a linear function of adsorbent surface coverage by assuming the indirect interaction of adsorbent and adsorbate [18]. The heat of adsorption of adsorbate molecules decreases with increase in the surface coverage of adsorbent [18]. This model can be represented by the following mathematical equations:

$$q\_s = \frac{RT}{b} \ln kC\_s \tag{7}$$

$$q\_{\epsilon} = B \ln K + B \ln C\_{\epsilon} \tag{8}$$

where *q*e is the equilibrium adsorption capacity (mol g−1) and *C*e is the equilibrium concentra‐ tion of adsorbate solution. Plots of *q*e versus *C*<sup>e</sup> give a straight line whose slope provides the values of *B*, which is associated with energy of adsorption, and Temkin's constant *K* can be calculated by intercept of the line.

A number of other adsorption isotherm models, namely BET isotherms [88], Dubinin-Radushkevich isotherm [89], Toth adsorption isotherms [90] are proposed to describe the adsorption phenomenon. Since precise mechanism of adsorption in solution phases is still not known to a greater extent; therefore, a single adsorption model is not applicable universally. The validity of these models generally changes from one adsorbent-adsorbate system to another and also with pH of solution, nature of solvent, ionic strength, etc. [15,91,18]. Freund‐ lich and Langmuir adsorption isotherms are the two very common approaches generally used in the experimental practices. Since the use of adsorption isotherm equations essentially involves the 'curve-fitting' method, the model with higher value of regression coefficient is considered to be more valid than the one with lower regression coefficient value.

#### **2.4. Thermodynamic treatment of adsorption isotherm**

Since every spontaneous process proceeds by characteristic changes in thermodynamic parameters, thermodynamic parameters, such as Gibbs free energy change (Δ*G*ads), change in enthalpy (Δ*H*ads) and change in entropy (Δ*S*ads) associated with the adsorption phenomenon, provide thorough knowledge about energy changes involved in adsorption process and the other thermodynamic variables on which adsorption depends on parameters such as temper‐ ature, pressure, etc. By designing systematic experiments, we can find out the distribution coefficient *K*d. More specifically, it is equilibrium constant for the distribution of adsorbate molecules in solution phase and on adsorbent surface and given by Eq. (9):

$$K\_D = \frac{C\_{\alpha\alpha}}{C\_{\epsilon}} \tag{9}$$

where *C*ea and *C*e are the equilibrium concentrations of adsorbate molecules on adsorbent surface and solution respectively. Other thermodynamic properties can be deduced using the distribution coefficient *K*d.

$$
\Delta G\_{\text{ads}} = -RT\ln K\_d \tag{10}
$$

$$
\ln K\_d = \frac{\Delta S\_{\text{ads}}}{R} - \frac{\Delta H\_{\text{ads}}}{RT} \tag{11}
$$

The values of the Δ*H*ads and Δ*S*ads will be easily determined using the van't Hoff curves by plotting *K*d against *T*−1 which gives a straight line. The slope and intercept of line gives Δ*S*ads and Δ*H*ads, respectively. The decrease in the Gibbs free energy, that is, a negative value of Δ*G*, is the necessity of a spontaneous process.

#### **2.5. Kinetics of adsorption**

**2.3. Temkin adsorption isotherm**

26 Diamond and Carbon Composites and Nanocomposites

calculated by intercept of the line.

equations:

According to Temkin adsorption isotherm, energy of the adsorption is a linear function of adsorbent surface coverage by assuming the indirect interaction of adsorbent and adsorbate [18]. The heat of adsorption of adsorbate molecules decreases with increase in the surface coverage of adsorbent [18]. This model can be represented by the following mathematical

ln *e e*

where *q*e is the equilibrium adsorption capacity (mol g−1) and *C*e is the equilibrium concentra‐ tion of adsorbate solution. Plots of *q*e versus *C*<sup>e</sup> give a straight line whose slope provides the values of *B*, which is associated with energy of adsorption, and Temkin's constant *K* can be

A number of other adsorption isotherm models, namely BET isotherms [88], Dubinin-Radushkevich isotherm [89], Toth adsorption isotherms [90] are proposed to describe the adsorption phenomenon. Since precise mechanism of adsorption in solution phases is still not known to a greater extent; therefore, a single adsorption model is not applicable universally. The validity of these models generally changes from one adsorbent-adsorbate system to another and also with pH of solution, nature of solvent, ionic strength, etc. [15,91,18]. Freund‐ lich and Langmuir adsorption isotherms are the two very common approaches generally used in the experimental practices. Since the use of adsorption isotherm equations essentially involves the 'curve-fitting' method, the model with higher value of regression coefficient is

Since every spontaneous process proceeds by characteristic changes in thermodynamic parameters, thermodynamic parameters, such as Gibbs free energy change (Δ*G*ads), change in enthalpy (Δ*H*ads) and change in entropy (Δ*S*ads) associated with the adsorption phenomenon, provide thorough knowledge about energy changes involved in adsorption process and the other thermodynamic variables on which adsorption depends on parameters such as temper‐ ature, pressure, etc. By designing systematic experiments, we can find out the distribution coefficient *K*d. More specifically, it is equilibrium constant for the distribution of adsorbate

*ea*

*<sup>C</sup>* <sup>=</sup> (9)

*e*

considered to be more valid than the one with lower regression coefficient value.

molecules in solution phase and on adsorbent surface and given by Eq. (9):

*D*

*<sup>C</sup> <sup>K</sup>*

**2.4. Thermodynamic treatment of adsorption isotherm**

*RT q kC <sup>b</sup>* <sup>=</sup> (7)

ln ln *e e q BKBC* = + (8)

Thermodynamics deals with spontaneity of the processes, but it does not tell about rapidity (e.g., adsorption rates) of the adsorption process and its mechanistic details. Therefore, adsorption kinetic studies are employed for better understanding of adsorption mechanism and its rate. Since adsorption in solution phase involves more than two components viz. adsorbate, solvent and adsorbent, depending upon the adsorption system, several kinetic models have been proposed.

#### *2.5.1. Pseudofirst-order kinetic model*

In 1898, Lagergren proposed the pseudofirst-order kinetic model [15,92] for the adsorption of adsorbate particles from solution by a solid adsorbent and proposed the following governing mathematical Eq. (12):

$$\log(q\_\epsilon - q\_\iota) = \log q\_\iota - \frac{K\_\iota t}{2.303} \tag{12}$$

where *K*<sup>1</sup> is the pseudofirst-order rate constant, *q*e and *q*<sup>t</sup> are equilibrium adsorption capacity (mol/maximum adsorption capacity for a given amount of adsorbent beyond which no further adsorption can take place) and adsorption capacity (mol/g) at a given time (*t*), respectively. By plotting log(*q*<sup>e</sup> − *q*<sup>t</sup> ) versus '*t*' and linear data fitting values of *K*<sup>1</sup> and *q*e from slope intercept, respectively. It has been realized that pseudofirst-order kinetic model is generally applicable to the initial stage of the adsorption as it only account for the involvement of only two components in adsorption process that occurs in solution phase [93,94].

#### *2.5.2. Pseudosecond-order kinetic model*

A more appropriate kinetic model for solution phase adsorption process is proposed by Ho and McKay [95] who showed that the rate of adsorption depends upon the adsorption capacity of adsorbate on solid adsorbent rather than its concentration in the solution phase. Eq. (13) was proposed to investigate the kinetics [96]:

$$\frac{1}{q\_i} = \frac{1}{q\_e K\_2} + \frac{1}{q\_e} \tag{13}$$

where *K*2 and *q*e represent pseudo-second-order rate constant and equilibrium adsorption capacity, respectively. Again the linear fitting of '*t*/*q*<sup>t</sup> ' versus '*t*' plots allowed the determination of *K*2 and *q*e from the calculated intercept and slope values.

For a better look into the mechanistic pathways, one should accounts the three basic steps by which adsorption proceeds viz. (i) the movement of adsorbate particles through bulk solution to the external surface of adsorbent, (ii) diffusion of adsorbate particles in the pores of adsorbent (interparticle diffusion) and (iii) adsorption of the adsorbate particles on the active sites of adsorbent. Any of the above steps can be rate-determining step.

For the identification of various steps involved in the adsorption process, interparticle diffusion model is used, which is expressed as follows [97]:

$$q\_r = K\_i T^{1/2} + C \tag{14}$$

where *K*<sup>i</sup> is the interparticle diffusion rate constant and *C* is a numeric constant. If interparticle diffusion is the rate-determining step of the adsorption process, then the above equation gives straight line for *q*<sup>t</sup> versus *t* 1/2 plots.

#### **3. Absorption**

Absorption is a phenomenon in which chemical substances, such as atoms, molecules and ions, enter into the bulk phase of absorbent and distributed throughout the bulk of absorbent. The process of absorption can be either physical [98] or chemical [98] depending upon the inter‐ action between the absorbent and the absorbing species. Physical absorption is sometimes referred as a nonreactive process, for example, the absorption of air oxygen in water at airwater interface [99]. Such absorption phenomenon is highly temperature and pressure dependent. In practice, the absorbing capacity of the absorbent depends upon the interfacial surface area, contact time and the wettability characteristics of the both substances. For example, superhydrophilic materials preferentially absorb water molecules from a mixture of several substances, whereas superhydrophobic surfaces (e.g., lotus leaf) tend to exclude water. A reactive or chemical absorption process, which takes place by means of some chemical reactions, for example, the absorption of carbon dioxide by sodium hydroxide, occurs via the formation of sodium carbonate [99]. Since the absorption of adsorbate occurs through the surface of absorbent, surface properties of the solid absorbent are most critical in deciding their absorption capacity, absorption rate, etc. These properties include surface energy [100], surface wettability, [100] topological roughness [101] and contact angles [101]. For absolutely flat surfaces, surface wettability can be determined using Young's equation that correlates the interfacial surface energies with contact angle value by the following equation:

*2.5.2. Pseudosecond-order kinetic model*

28 Diamond and Carbon Composites and Nanocomposites

was proposed to investigate the kinetics [96]:

capacity, respectively. Again the linear fitting of '*t*/*q*<sup>t</sup>

of *K*2 and *q*e from the calculated intercept and slope values.

diffusion model is used, which is expressed as follows [97]:

1/2 plots.

versus *t*

where *K*<sup>i</sup>

straight line for *q*<sup>t</sup>

**3. Absorption**

sites of adsorbent. Any of the above steps can be rate-determining step.

A more appropriate kinetic model for solution phase adsorption process is proposed by Ho and McKay [95] who showed that the rate of adsorption depends upon the adsorption capacity of adsorbate on solid adsorbent rather than its concentration in the solution phase. Eq. (13)

> 2 111 *te e q qK q*

where *K*2 and *q*e represent pseudo-second-order rate constant and equilibrium adsorption

For a better look into the mechanistic pathways, one should accounts the three basic steps by which adsorption proceeds viz. (i) the movement of adsorbate particles through bulk solution to the external surface of adsorbent, (ii) diffusion of adsorbate particles in the pores of adsorbent (interparticle diffusion) and (iii) adsorption of the adsorbate particles on the active

For the identification of various steps involved in the adsorption process, interparticle

1/2

diffusion is the rate-determining step of the adsorption process, then the above equation gives

Absorption is a phenomenon in which chemical substances, such as atoms, molecules and ions, enter into the bulk phase of absorbent and distributed throughout the bulk of absorbent. The process of absorption can be either physical [98] or chemical [98] depending upon the inter‐ action between the absorbent and the absorbing species. Physical absorption is sometimes referred as a nonreactive process, for example, the absorption of air oxygen in water at airwater interface [99]. Such absorption phenomenon is highly temperature and pressure dependent. In practice, the absorbing capacity of the absorbent depends upon the interfacial surface area, contact time and the wettability characteristics of the both substances. For example, superhydrophilic materials preferentially absorb water molecules from a mixture of several substances, whereas superhydrophobic surfaces (e.g., lotus leaf) tend to exclude water. A reactive or chemical absorption process, which takes place by means of some chemical

is the interparticle diffusion rate constant and *C* is a numeric constant. If interparticle

= + (13)

*t i q KT C* = + (14)

' versus '*t*' plots allowed the determination

$$\cos \theta\_r = \frac{\mathcal{Y}\_{sv} - \mathcal{Y}\_{sl}}{\mathcal{Y}\_{lv}} \tag{15}$$

where *γsv*, *γsl*, *γsl* are surface-free energy of involved solid-gas, solid-liquid and liquid-gas interfaces, respectively, and θr is the water contact angle with solid surface (**Figure 2a**). Therefore, for a solid absorbent, low surface energy is required for higher water contact angles (WCAs). Since lower contact angles are the direct reflection of surface wettability, for achieving highly wettable or superhydrophilic surfaces, we need to design the surfaces having minimum surface energies and an opposite situation is required for superhydrophobic surfaces having excellent wettability for nonpolar liquids such as oils and organic solvents. In addition to this, surface roughness and topology are another crucial factors for high/low WCAs, which are explained on the basis of Wenzel [102] (**Figure 2b**) and Cassie-Baxter [103] (**Figure 2c**) models.

**Figure 2.** Schematic representation of wetting behavior of water droplet on solid absorbent (a) Young's model, (b) Wenzel model, (c) Cassie model. Reproduced from [75] with permission from RSC.

According to Wenzel's theory, no air bubbles are present between the liquid and solid surface. Hence, the liquid floods the roughness introduced channels (**Figure 2b**) leading to perfect wetting of the underlying surface, so that the contact angle of smooth (nontextured) and structured (rough) surfaces satisfy the following equation:

$$\cos \theta\_o = \mathbf{r} \cos \theta \tag{16}$$

where *θω* and *θ* are the contact angles of structured (rough surface) and smooth (nontextured) surfaces respectively, '*r*' is the roughness ratio (i.e., ratio of the true surface area of the structured surface to its projection area). The equation indicates that the surface roughness can alter the actual wettability of the surface. If *r*>1, then the microstructured surface can increase the hydrophobicity (if θ>90° ) and hydrophilicity (if θ<90° ) of the actual smooth surface.

In Cassie's model, due to the presence of air molecules present between the surfaces of two phases (i.e., within roughness introduced channels), solid surface is not perfectly wetted by liquid (**Figure 2c**). Hence, the contact angles of structured (rough) surface (*θc*) and smooth surface (*θ*) are related by following equation:

$$\cos \theta\_{\varepsilon} = f\_1 \cos \theta - f\_2 \tag{17}$$

where *f*1 and *f*2 are the ratios of solid-liquid interface and solid-gas interface areas to their combined (*f*1+*f*2) projection area, respectively. According to the equation, the presence of air pockets between the solid and liquid surfaces leads to increase in contact angle value. In addition to absorbent properties, the absorbate's physiochemical characteristics are also crucial in determining the absorption effectiveness of the solid absorbent. Since absorption is a mass transfer phenomenon, it heavily depends upon the density and viscosity of the absorbates. Generally, less viscous liquids (low cohesive energy) tend to absorb faster compare to highly viscous liquids [104]. As the pore volume of a given absorbent is fixed, and hence, it can only absorb a fixed volume of a liquid, the absorption capacity an absorbent can be linearly related with the densities of the absorbing liquids [105].

#### **4. Synthesis of graphene-based hybrids and composites**

A number of strategies have been used for the synthesis of variety of adsorbents and absorbents constituted by graphene-based hybrids and composites.

#### **4.1. Dip-coating process**

**Figure 3.** Schematic representation of formation of graphene-coated absorbent sponges by dip-coating process.

The dip coating is actually a two-step process (**Figure 3**) involving coating of graphene oxide (GO) over substrate (foam/particulate-solid) followed by chemical reduction of coating to reduced GO (RGO). In the typical coating process, sponge/foam substrate is immersed inside aqueous dispersion of GO for specified time and dried to form GO-coated foam. The concen‐ tration of GO solution and number of dip-dry cycles may be varied to change the coating thickness and uniformity and ultimately the GO loading. Once the desired coating/loading is achieved, the foam may be chemically reduced by exposure to chemical reducing agents such as hydrazine, ascorbic acid or other systems, resulting in the formation of RGO-coated foam. The formation of GO and RGO coatings are evident from the change in the shade of foam to brownish and blackish, respectively. The dip-coating process involved simple physical interactions between GO/RGO coating and foam and possess disadvantages in terms of coating uniformity and its poor adherence. In it modified version involving chemical grafting, suitable functionalized derivative of GO is chemically grafted over substrate (foam/particulate-solid) carrying complimentary functional groups. Such foams display better coating uniformity and adherence compared to dip-coating process.

#### **4.2. Surface coating by** *in situ* **generation of graphene via thermal treatment**

**Figure 4.** Schematic representation for the synthesis of graphene-coated sand composites. Reproduced from [106] with permission from ACS.

In such process, substrate material (e.g., sand) is treated with suitable (carbonizable) graphene precursor (e.g., sugar solution) leading to surface coating (**Figure 4**) with precursor (sugar with sand) upon drying. Further, high-temperature treatment leads to the carbonization of coated precursor and formation of graphene-/sand-type hybrids [106] (GSCs).

#### **4.3.** *In situ* **incorporation process**

alter the actual wettability of the surface. If *r*>1, then the microstructured surface can increase

In Cassie's model, due to the presence of air molecules present between the surfaces of two phases (i.e., within roughness introduced channels), solid surface is not perfectly wetted by liquid (**Figure 2c**). Hence, the contact angles of structured (rough) surface (*θc*) and smooth

> q

where *f*1 and *f*2 are the ratios of solid-liquid interface and solid-gas interface areas to their combined (*f*1+*f*2) projection area, respectively. According to the equation, the presence of air pockets between the solid and liquid surfaces leads to increase in contact angle value. In addition to absorbent properties, the absorbate's physiochemical characteristics are also crucial in determining the absorption effectiveness of the solid absorbent. Since absorption is a mass transfer phenomenon, it heavily depends upon the density and viscosity of the absorbates. Generally, less viscous liquids (low cohesive energy) tend to absorb faster compare to highly viscous liquids [104]. As the pore volume of a given absorbent is fixed, and hence, it can only absorb a fixed volume of a liquid, the absorption capacity an absorbent can be linearly related

A number of strategies have been used for the synthesis of variety of adsorbents and absorbents

**Figure 3.** Schematic representation of formation of graphene-coated absorbent sponges by dip-coating process.

The dip coating is actually a two-step process (**Figure 3**) involving coating of graphene oxide (GO) over substrate (foam/particulate-solid) followed by chemical reduction of coating to reduced GO (RGO). In the typical coating process, sponge/foam substrate is immersed inside aqueous dispersion of GO for specified time and dried to form GO-coated foam. The concen‐ tration of GO solution and number of dip-dry cycles may be varied to change the coating

) of the actual smooth surface.

= - *f f* (17)

) and hydrophilicity (if θ<90°

1 2 cos cos *<sup>c</sup>*

q

**4. Synthesis of graphene-based hybrids and composites**

the hydrophobicity (if θ>90°

surface (*θ*) are related by following equation:

30 Diamond and Carbon Composites and Nanocomposites

with the densities of the absorbing liquids [105].

constituted by graphene-based hybrids and composites.

**4.1. Dip-coating process**

In this process, the aqueous dispersion of GO (colloidal solution) is mixed with suitable secondary phase (CNTs, inorganic nanoparticles such as Fe23/Fe3O4) and subjected to further processing to form 3D self-assembled structures and cause reduction of GO phase to RGO.

**Figure 5.** Schematic representation for the synthesis of a graphene composite by *in situ* incorporation of CNTs in GO solution.

For example, GO/CNTs mixed solution is subjected to freeze drying followed by chemical reduction to form RGO/CNTs hybrid-based aerogels (**Figure 5**). The presence of CNTs prevents the stacking of RGO phase and provides improved porosity and mechanical strength to the aerogel.

**Figure 6.** Schematic representation of *in situ* incorporation process involving the mixing of GO with suitable filler fol‐ lowed by secondary processing. Reproduced from [53] with permission from RSC.

Similarly, GO/Fe3O4/glucose system is hydrothermally treated at high temperature to form the RGO/Fe3O4-based hybrids in a single step (**Figure 6**). Here, the Fe3O4 phase checks the RGO agglomeration, improves porosity and allows magnetic separation of the hybrids from the water (after the completion of uptake of pollutant).

#### **4.4. Hydrothermal process in the presence of gelation-causing agent**

This involves the hydrothermal treatment of GO dispersion in the presence of suitable gelation agent, for example, chitosan leading to the formation of hydrogel [107], which may lyophilized or supercritical dried to form the corresponding hybrid aerogel.

**Figure 7.** Photograph of GO/chitosan hydrogel and its SEM image. Reproduced from [107] with the permission from RSC.

For example, mixed aqueous dispersion of GO and chitosan may be hydrothermally converted into 3D self-assembled macroscopic hydrogel (**Figure 7**), which consists of highly porous structure as evident from the scanning electron microscope (SEM) investigations of its lyophilized version.

## **5. The water purification performance of graphene-based hybrids and composites**

As already discussed in previous section, due to its high specific surface area, tunable mor‐ phological features, controllable porosity, inherent hydrophobicity and oleophilicity, chemical manipulability, tunable surface functionality, facile processing via chemical route and good thermal/environmental/chemical stability, graphene has been widely used to design variety of sorbents to selectively remove the pollutants via adsorption (e.g., for metal ions or organic dyes) or absorption (e.g., for oils or organic solvents) processes [15,17,18,43,53,79,81,91,97,105,108–111]. The contaminant capture ability, including the affinity, toward particular type of contaminant, capturing mechanism and actual purification performance of synthesized graphene-based platforms depends upon the presence of specific set of above properties. This section provides comprehensive account of the purification performance of the graphene-based hybrids and composites for three major classes of pollu‐ tants viz. metal ions, organic dyes and oils/organic solvents, with emphasis on the role of above-mentioned specific properties and their correlation to observed performance [47,52,106,112].

#### **5.1. Adsorption of metal ions**

For example, GO/CNTs mixed solution is subjected to freeze drying followed by chemical reduction to form RGO/CNTs hybrid-based aerogels (**Figure 5**). The presence of CNTs prevents the stacking of RGO phase and provides improved porosity and mechanical strength

**Figure 6.** Schematic representation of *in situ* incorporation process involving the mixing of GO with suitable filler fol‐

Similarly, GO/Fe3O4/glucose system is hydrothermally treated at high temperature to form the RGO/Fe3O4-based hybrids in a single step (**Figure 6**). Here, the Fe3O4 phase checks the RGO agglomeration, improves porosity and allows magnetic separation of the hybrids from the

This involves the hydrothermal treatment of GO dispersion in the presence of suitable gelation agent, for example, chitosan leading to the formation of hydrogel [107], which may lyophilized

**Figure 7.** Photograph of GO/chitosan hydrogel and its SEM image. Reproduced from [107] with the permission from

lowed by secondary processing. Reproduced from [53] with permission from RSC.

**4.4. Hydrothermal process in the presence of gelation-causing agent**

or supercritical dried to form the corresponding hybrid aerogel.

water (after the completion of uptake of pollutant).

to the aerogel.

32 Diamond and Carbon Composites and Nanocomposites

RSC.

It has been well established that the presence of excessive amounts of heavy metals such as Hg (mercury), Pb (lead), As (Arsenic), Cr (chromium), Cu (copper), Ni (nickel), Cd (Cadmium), Se (selenium), etc. in water has a catastrophic effects on living systems and as they degrade the quality of water for other practices like in industries and agricultural sector. The main sources of these metal ions in water systems include effluents of various processes such as galvanization, batteries' waste disposal, mining operations and paper/textile industries, tanneries, etc. [74,91]. Therefore, the removal of above metal ions from the wastewater streams is very important and several efforts in the direction are made using graphene-based adsorb‐ ents.

Chandra et al. synthesized magnetite (Fe3O4)/reduced graphene oxide (M-RGO) composites [17] and demonstrated their good binding affinity toward As(III) and As(V) ions. The presence of Fe3O4 phase enable easy removable of spent adsorbent from water after completion of adsorption of As ions. The composites displayed arsenic removal efficiency (>99.9%) with maximum adsorption capacity (**Figure 8b**) of 13.10 mg/g and 5.83 mg/g for As(V) and As(III), respectively.

**Figure 8.** (a) Adsorption isotherms of As(III) and As(V) on the Fe3O4-RGO composite (temperature 20°C, pH 7). (b) Maximum adsorption capacity of M-RGO composites for arsenic removal from water. Kinetic absorption data plots of arsenic ions by M-RGO: (c) arsenic removal rate *q*<sup>t</sup> versus time *t* and (d) the transformed rate plot *t*/*q*<sup>t</sup> versus *t*. (e) Effect of temperature on arsenic adsorption: pH, 7; adsorption time, 2 h; adsorbent dose, 0.2 g/L; and arsenic concentration 5 ppm, (f) Effect of pH on arsenic adsorption: temperature, 20°C; adsorption time, 2 h; adsorbent dose, 0.2 g/L; and ar‐ senic concentration, 5 ppm. Adopted from [17] with permission from ACS.

The authors plotted the adsorption isotherm (**Figure 8a**) and fitted kinetic data (**Figure 8c** and **8d**) into pseudosecond-order kinetic model and showed the applicability of the model on adsorption process. They also outlined the optimum temperature (**Figure 8e**) for maximum adsorption capacity. The difference in adsorption mechanism of As(V) and As(III) on M-RGO composites was observed by studying the effect of solution pH (**Figure 8f**) on adsorption capacity. An increase in the adsorption of As(III) (present as H3AsO4 arsenous acid) in basic conditions ruled out the electrostatic interactions between positively charged adsorbent and negatively charged arsenious acid. Rather, surface complexation of As(III) ions is responsible for very high adsorption capacity in alkaline conditions. On the other hand, As(V) ions present as [H2AsO3] − species) are electrostatically adsorbed by the adsorbent surfaces at low pH values. Wang et al. [74] exploited the surface area and charge density of graphene oxide sheets in the removal of cadmium and cobalt ions. They have studied the sorbing properties of graphene oxide nanosheets for the removal of Cd(II) and Co(II) ions from aqueous solutions at different pH values and showed that the adsorption of the Co(II) ions increases with increase in the pH of solution reaching maximum at pH>9. In case of Cd(II) ions, 98% adsorption of ions occurred at pH>9 mainly due to increase in negative surface charge density of graphene nanosheets (with an increase in the pH of solution) [74], which electrostatically adsorb the positively charged metal ions. Expanding the work in this area, Fan et al. [113] fused the acid resistant, magnetic cyclodextrin with GO nanosheets (MCGN) for enhancing the adsorption capacity of the composite for Cr(VI) ions (**Figures 9** and **10**).

**Figure 9.** A schematic representation of the preparation of cyclodextrin-graphene composite and removal of Cr(VI). Reproduced from [113] with permission from RSC.

**Figure 8.** (a) Adsorption isotherms of As(III) and As(V) on the Fe3O4-RGO composite (temperature 20°C, pH 7). (b) Maximum adsorption capacity of M-RGO composites for arsenic removal from water. Kinetic absorption data plots of

of temperature on arsenic adsorption: pH, 7; adsorption time, 2 h; adsorbent dose, 0.2 g/L; and arsenic concentration 5 ppm, (f) Effect of pH on arsenic adsorption: temperature, 20°C; adsorption time, 2 h; adsorbent dose, 0.2 g/L; and ar‐

The authors plotted the adsorption isotherm (**Figure 8a**) and fitted kinetic data (**Figure 8c** and **8d**) into pseudosecond-order kinetic model and showed the applicability of the model on adsorption process. They also outlined the optimum temperature (**Figure 8e**) for maximum adsorption capacity. The difference in adsorption mechanism of As(V) and As(III) on M-RGO composites was observed by studying the effect of solution pH (**Figure 8f**) on adsorption capacity. An increase in the adsorption of As(III) (present as H3AsO4 arsenous acid) in basic conditions ruled out the electrostatic interactions between positively charged adsorbent and negatively charged arsenious acid. Rather, surface complexation of As(III) ions is responsible for very high adsorption capacity in alkaline conditions. On the other hand, As(V) ions present

Wang et al. [74] exploited the surface area and charge density of graphene oxide sheets in the removal of cadmium and cobalt ions. They have studied the sorbing properties of graphene oxide nanosheets for the removal of Cd(II) and Co(II) ions from aqueous solutions at different pH values and showed that the adsorption of the Co(II) ions increases with increase in the pH of solution reaching maximum at pH>9. In case of Cd(II) ions, 98% adsorption of ions occurred at pH>9 mainly due to increase in negative surface charge density of graphene nanosheets

species) are electrostatically adsorbed by the adsorbent surfaces at low pH values.

versus time *t* and (d) the transformed rate plot *t*/*q*<sup>t</sup>

versus *t*. (e) Effect

arsenic ions by M-RGO: (c) arsenic removal rate *q*<sup>t</sup>

34 Diamond and Carbon Composites and Nanocomposites

as [H2AsO3]

−

senic concentration, 5 ppm. Adopted from [17] with permission from ACS.

**Figure 10.** (a) MCGN in the absence (A) and presence (B) of external magnetic field (b) time profile and (c) Kinetics of Cr(VI) ions removal with 1 g/L MCGN concentration and initial Cr(VI) concentrations are 100 mg/L and 150 mg/L. Re‐ produced from [113] with the permission from RSC.

The adsorbents work better under slightly acidic conditions with good recyclability, making it a good choice for heavy metal ions removal as shown in **Figure 11**.

**Figure 11.** (a) Effect of pH on adsorption capacity of cyclodextrin-graphene nanocomposite. ([CGN] = 1 gL−1, [Cr(VI)] = 100 mgL−1, contact time: 60 min). (b) Recyclability of the cyclodextrin/graphene. Reproduced from [113] with the per‐ mission from RSC.

The selectivity of adsorbent for a specific adsorbate is highly desirable. Hao et al. [114] synthesized a SiO2-graphene composite and specifically adsorb Pb(II) ions from solution. The authors observed that adsorption capacity is highly dependent on the ionic strength of solution and decrease with ionic strength due to completion between various ions for adsorbing sites. Luo et al. [82] highlighted the performance of heavy metal salts such as ZrO2 hybridized with graphene oxide for the removal of As(III) and As(V) ions. This composite showed ionic strength independent adsorption behavior with high adsorption capacities of 95.15 mg/g and 84.89 mg/ g for As(III) and As(v) ions, respectively, with good recyclability. Ren et al. [81] outlined the role of temperature of solution in regulating adsorption. The graphene-MnO2 composite prepared by them for the removal of Ni(II) ions increase its adsorption capacity by a factor of 0.7 upon changing temperature from 298 K to 318 K. The kinetic parameters obtained from **Table 1** suggested that the pseudosecond-order kinetic model is more appropriate than pseudofirst-order kinetic model. Langmuir adsorption isotherms parameters are given in **Table 2** at various temperatures, which shows the increase in adsorption capacity at higher temperatures.


**Table 1.** The kinetics constants of the Ni (II) adsorption onto GNS/MnO2 composite.


**Table 2.** The Langmuir isotherm parameters of the Ni (II) adsorption onto GNS-MnO2 composite at various temperature (*K*).

The adsorbents work better under slightly acidic conditions with good recyclability, making

**Figure 11.** (a) Effect of pH on adsorption capacity of cyclodextrin-graphene nanocomposite. ([CGN] = 1 gL−1, [Cr(VI)] = 100 mgL−1, contact time: 60 min). (b) Recyclability of the cyclodextrin/graphene. Reproduced from [113] with the per‐

The selectivity of adsorbent for a specific adsorbate is highly desirable. Hao et al. [114] synthesized a SiO2-graphene composite and specifically adsorb Pb(II) ions from solution. The authors observed that adsorption capacity is highly dependent on the ionic strength of solution and decrease with ionic strength due to completion between various ions for adsorbing sites. Luo et al. [82] highlighted the performance of heavy metal salts such as ZrO2 hybridized with graphene oxide for the removal of As(III) and As(V) ions. This composite showed ionic strength independent adsorption behavior with high adsorption capacities of 95.15 mg/g and 84.89 mg/ g for As(III) and As(v) ions, respectively, with good recyclability. Ren et al. [81] outlined the role of temperature of solution in regulating adsorption. The graphene-MnO2 composite prepared by them for the removal of Ni(II) ions increase its adsorption capacity by a factor of 0.7 upon changing temperature from 298 K to 318 K. The kinetic parameters obtained from **Table 1** suggested that the pseudosecond-order kinetic model is more appropriate than pseudofirst-order kinetic model. Langmuir adsorption isotherms parameters are given in **Table 2** at various temperatures, which shows the increase in adsorption capacity at higher

**Adsorbents (***q***e)exp (mg g−1) Pseudofirst-order equation Pseudosecond-order equation**

**Table 1.** The kinetics constants of the Ni (II) adsorption onto GNS/MnO2 composite.

GNS/MnO2 42.855 28.372 0.00072 0.8672 46.447 0.00149 0.9993

(*q*e)cal (mg g−1) *K*1 (min−1) *R*<sup>2</sup> (*q*e)cal (mg g−1) *K*2 (g mg−1min−1) *R*<sup>2</sup>

it a good choice for heavy metal ions removal as shown in **Figure 11**.

36 Diamond and Carbon Composites and Nanocomposites

mission from RSC.

temperatures.

A systematic study was done by Liu et al. [45] for the adsorption of Co(II) ions on magnetitegraphene composite (**Figure 12**).

**Figure 12.** Effect of ionic strength (A), foreign cations (B), foreign anions (C), and sorbent content (D) on Co(II) sorption onto the M-GO composite at pH=6.8, *T*=303.15 K, *I*=0.01 M NaCl, concentration of cobalt = 10.0 mg/L from [45] with the permission from ACS.

The effect of ionic strength on adsorption capacity is negligible as shown in **Figure 12a**. But the presence of foreign ions significantly affected the adsorption behavior of the adsorbent. The pH of solution higher than 8.5 was favorable of maximum adsorption capacity. Zhou et al. [79] also reported the dependence of adsorption capacity of graphene composites on pH of solution. The material showed decreased affinity for Cr(IV) ions at higher pH as hydroxyl ion compete with the CrO4 2− ions for adsorbing sites [5]. Such studies are also reported by other groups to show how metal ion compete for adsorbing sites in the environment of excessive hydroxyl ions and hydronium ions [115]. It can be concluded from the discussion earlier that graphene-based materials are highly promising for the removal of heavy metal ions from wastewater systems.

#### **5.2 Adsorption of dyes**

Synthetic organic dyes that represent important color imparting ingredient of leather, paper, textile and rubber industries are also present in significant amount in their wastewater streams [22]. These dye stuffs have serious health hazards for both humans and aquatic systems [21,116]. In recent times, graphene analogs, including GO- or RGO-based sponges, composites and hybrids, have been successfully employed for the removal of toxic dyes from wastewater.

Liu et al. [47] used a centrifugal evaporator (**Figure 13a**) to synthesize GO-based sponge (**Figure 13b**) consisting of 3D interconnected network of graphene layers and demonstrated their potential for the removal of cationic dyes such as methylene blue (MB) and methyl violet (MV) with removal efficiency of 99.1 and 98.8%, respectively, with equilibrium reaching within 2 min. It has been highlighted that the microporous structure of GO sponge (**Figure 13c**), high negative charge density (due to rich oxygen functionalities) mediated cation-anion-type interactions, π-π stacking interactions between dye backbone and adsorbent and high specific surface area (|48.4 m2 /g), collectively contribute toward the excellent dye adsorption capacity (**Figure 13d**), that is, 397 mg/g and 467 mg/g for MB and MV, respectively (**Figure 13**).

**Figure 13.** (a) Synthetic scheme of a 3D GO sponge, (b) optical image of GO sponge, (c) SEM image of GO sponge, (d) digital images of the original MB dye solution (left), the pale color solution with precipitated MB-adsorbed GO spong‐ es (middle), and the colorless water after filtering the MB adsorbed GO sponge (right). Effect of (e) stirring rate, (f) temperature and (g) pH on the adsorption rate and capacity for MV dye. Reprinted from [47] with permission from ACS.

The effect of stirring (**Figure 13e**), temperature (**Figure 13f**) and pH (**Figure 13g**) on the adsorption rate and capacity have also been investigated, with the observation of increase in these parameters favoring adsorption rate and capacity. Once the adsorption is complete, spent adsorbent has to be removed from purified water by filtration process, which is not only bit cumbersome but also involve risks of recontamination. Gupta et al. [106] prepared sugarderived graphene sand composite (GSC) consisting of graphene-coated sand particles (**Figure 14a** and **b**) via thermal processing. This cost-effective active adsorbent material-packed column (**Figure 14c**) showed good R6G dye removal efficiency with adsorption capacity of 50– 55 mg/g. The time-dependent adsorption studies were also carried out to understand the system kinetics (**Figure 14**).

**5.2 Adsorption of dyes**

38 Diamond and Carbon Composites and Nanocomposites

surface area (|48.4 m2

ACS.

Synthetic organic dyes that represent important color imparting ingredient of leather, paper, textile and rubber industries are also present in significant amount in their wastewater streams [22]. These dye stuffs have serious health hazards for both humans and aquatic systems [21,116]. In recent times, graphene analogs, including GO- or RGO-based sponges, composites and hybrids, have been successfully employed for the removal of toxic dyes from wastewater. Liu et al. [47] used a centrifugal evaporator (**Figure 13a**) to synthesize GO-based sponge (**Figure 13b**) consisting of 3D interconnected network of graphene layers and demonstrated their potential for the removal of cationic dyes such as methylene blue (MB) and methyl violet (MV) with removal efficiency of 99.1 and 98.8%, respectively, with equilibrium reaching within 2 min. It has been highlighted that the microporous structure of GO sponge (**Figure 13c**), high negative charge density (due to rich oxygen functionalities) mediated cation-anion-type interactions, π-π stacking interactions between dye backbone and adsorbent and high specific

(**Figure 13d**), that is, 397 mg/g and 467 mg/g for MB and MV, respectively (**Figure 13**).

**Figure 13.** (a) Synthetic scheme of a 3D GO sponge, (b) optical image of GO sponge, (c) SEM image of GO sponge, (d) digital images of the original MB dye solution (left), the pale color solution with precipitated MB-adsorbed GO spong‐ es (middle), and the colorless water after filtering the MB adsorbed GO sponge (right). Effect of (e) stirring rate, (f) temperature and (g) pH on the adsorption rate and capacity for MV dye. Reprinted from [47] with permission from

The effect of stirring (**Figure 13e**), temperature (**Figure 13f**) and pH (**Figure 13g**) on the adsorption rate and capacity have also been investigated, with the observation of increase in

/g), collectively contribute toward the excellent dye adsorption capacity

**Figure 14.** (a) Digital image and (b) SEM image of GSC, (c) photograph of GSC750 packed adsorption column for the separation of R6G dye. Variation of adsorption capacity as a function of (d) GSC heating temperature and (e) GSC con‐ tent. (f) Kinetic study of dye adsorption with 30-min time interval. Inset shows the pseudofirst- and pseudosecondorder model fits. Reproduced from [106] with permission from ACS.

The authors highlighted the dependence of adsorption characteristics on GSC-heating temperature (**Figure 14d**) and adsorbent content (**Figure 14e**). The kinetic data were examined in terms of Lagergren pseudofirst-order (Section 2.5.1) and Ho's pseudosecond-order (Section 2.5.2) kinetic models [95] and the fitted data (inset of **Figure 14f**) showed that a pseudosecondorder equation is more appropriate in describing the experimental data. Li et al. [84] synthe‐ sized a Mg(OH)2-RGO composite(GMC) to improve the adsorption capacity by increasing the porosity of the adsorbent. They pointed that surface charge density and π conjugation play an important role for high removal efficiency of GMC for cationic dyes such as MB. The presence of Mg(OH)2 ensured the formation of mesoporous structure (**Figure 15** calculation of BET surface area and pore size distribution) and inhibit restacking of graphene sheets, thereby generating high accessible surface area (|104 m2 /g) for adsorption (**Figure 15**).

**Figure 15.** (a) N2 adsorption-desorption isotherm at 77 K and (b) pore width distribution of MGC. Adopted from [84] with permission from RSC.

These GMC readily desorbs the MB molecules in ethanol solution reflecting promising recyclability. It has been observed that despite their good adsorption capacity, the separation of these graphene-based powdered adsorbents is a difficult and cumbersome task. Therefore, they pose inherent disadvantages in terms of requirement of energy-intensive separation and regeneration techniques along with chances of recontamination of water with any leftover adsorbent particles. In order to overcome poor separation ability of powdered absorbents, Fan et al. [53] synthesized graphene-Fe3O4 with carbon (GFC)-type hybrid materials with coupled porosity and magnetic properties for MB removal (**Figure 16**).

**Figure 16.** (a) Magnetic hysteresis loops for pure Fe3O4, graphene-Fe3O4 and GFC hybrids, (b) adsorption curves for pure Fe3O4, graphene-Fe3O4, Fe3O4@carbon hybrid, GFC hybrid, (c) schematic representation of recyclability of GFC hy‐ brid, and (d) percentage removal of MB by GFC hybrid in water and 1 M HCl solution in five cycles. Reproduced from [53] with permission from RSC.

This material exhibits good porosity and magnetic character (**Figure 16a**) resulting in good dye adsorption response (**Figure 16b**) with good capacity (73.26 mg/g) and fast adsorption rates (i.e. removal of 80% dye within 20 min). The superiority of such materials lies in the fact that they can be easily separated after adsorption of dye from water using an external magnetic field and can be regenerated by desorbing the dye in ethanol (**Figure 16c**). The good recycla‐ bility is evident from very low decrease in adsorption capacity after five cycles of regeneration and reuse. The adsorption mechanism of MB on GFCs was examined by applying Langmuir and Freundlich isotherm models and their parameters are reported in **Table 3**.


**Table 3.** Langmuir and Freundlich parameters for the adsorption of MB on GFC hybrid.

**Figure 15.** (a) N2 adsorption-desorption isotherm at 77 K and (b) pore width distribution of MGC. Adopted from [84]

These GMC readily desorbs the MB molecules in ethanol solution reflecting promising recyclability. It has been observed that despite their good adsorption capacity, the separation of these graphene-based powdered adsorbents is a difficult and cumbersome task. Therefore, they pose inherent disadvantages in terms of requirement of energy-intensive separation and regeneration techniques along with chances of recontamination of water with any leftover adsorbent particles. In order to overcome poor separation ability of powdered absorbents, Fan et al. [53] synthesized graphene-Fe3O4 with carbon (GFC)-type hybrid materials with coupled

**Figure 16.** (a) Magnetic hysteresis loops for pure Fe3O4, graphene-Fe3O4 and GFC hybrids, (b) adsorption curves for pure Fe3O4, graphene-Fe3O4, Fe3O4@carbon hybrid, GFC hybrid, (c) schematic representation of recyclability of GFC hy‐ brid, and (d) percentage removal of MB by GFC hybrid in water and 1 M HCl solution in five cycles. Reproduced from

porosity and magnetic properties for MB removal (**Figure 16**).

with permission from RSC.

40 Diamond and Carbon Composites and Nanocomposites

[53] with permission from RSC.

On the basis of regression coefficient (*R*<sup>2</sup> ) values, Freundlich adsorption is ruled out and result better fits into the Langmuir adsorption model. The good value of obtained maximum adsorption capacity (i.e., 73.26 mg/g) has been ascribed to high surface area of GFCs, which enhances the accessibility of diffusion pathways for dye molecules in macroporous domains. Additionally, strong electrostatic interactions and π-π interactions between the aromatic cationic dye and negatively charged GFC hybrid are mainly responsible for rapid adsorption rates. Wang et al. [108] also synthesized a Fe3O4 -graphene-based magnetic composite for the removal of fuchsine dye and showed an excellent adsorption capacity of 89.4 mg/g in very shorter time period. The kinetics of adsorption was determined by fitting the results to (**Figure 17a**) pseudosecond-order kinetic model (Eq. (13)). The regression coefficient value of 0.9999 shows the applicability of the model and confirmed the chemisorption of dye molecules on adsorbent surface (**Figure 17**).

**Figure 17.** (a) Effect of adsorbent dose on removal ratio of dye in water and (b) effect of pH on removal of dye. Adopt‐ ed from [108] with permission from Elsevier.

The authors also studied the role of dye solution pH (**Figure 17b**) on the adsorption mechanism of the composite. The adsorption capacity was found to increase with pH (from 3 to 5.5) mainly due to the fact that at low pH condition protonation of basic dye (amido groups) fuchsine occurs, which leads to increase solubility of dye in water and hence its relatively poor adsorp‐ tion under slightly acidic conditions. The extensive research on development of such handy magnetic graphene materials is now of great interest as these materials have high dye removal efficiency, quick dye adsorption and facile removal of spent adsorbent from purified water after completion of pollutant adsorption. In this series, a very promising magnetic adsorbent Ni-RGO (NGC) composite is prepared by Li group [117], which almost completely removes rhodamine (RhB) and MB dyes from water within 4 h as evident from corresponding timedependent UV-visible spectra (**Figure 18**) (**Table 4**).

**Figure 18.** UV-visible spectra of original and NGC treated 50 mg organic dye solutions (a) RhB 1 × 10−5 M, 20 mL (b) 2 × 10−5M, 20 mL for various time intervals. The insets are photographs of corresponding solutions at various time inter‐ vals and magnetic separation. Reproduced from [117] with the permission from RSC.


**Table 4.** Freundlich isotherm constants and their correlation coefficients.

The authors show that the Freundlich adsorption mode is applicable to NGC with regression coefficient value (*R*<sup>2</sup> ) of 0.99 for both RhB and MB dyes, which indicates the heterogeneity of adsorbent surface. The Freundlich exponent (*n*) for RhB (1.048) is greater compared to MB (0.894) that shows the better interaction of the RhB dye with adsorbent compared to MB. This is mainly due to molecular structural differences between the two dyes. In particular, MB has a soft S-based cationic system and RhB has a hard N-based cationic system, resulting in high affinity of latter for a hard O-based anionic center. Jiang et al. [110] synthesized a magnetic chitosan-graphene oxide (MCGO) nanocomposite for the adsorption of anionic dye methyl orange (MO). The magnetic adsorbent showed extraordinary MO adsorption capacity (398.08 mg/g) and easily separated by magnetic assistance after completion of dye adsorption **Figure 19a**.

**Figure 19.** (a) Digital photograph of MO adsorption on MCGO nanocomposite and its magnetic separation. Variation in adsorption capacity with (b) adsorbent mass and (c) solution pH. Effect of (d) pH on zeta potential of the MCGO nanocomposite, (e) Experimental data and curve fits for Langmuir and Freundlich isotherms (f) contact time on MO adsorption by MCGO nanocomposite. Reproduced from [110] with permission from International Journal of Biological Macromolecules.

The authors showed that in case of MO, adsorption capacity decreases with pH of solution as the negatively charged MO molecules compete with hydroxide ions in alkaline conditions shown in **Figure 19c**. Also, zeta potential measurement **Figure 19d** showed that the MCGO nanocomposite attained isoelectric point at pH 10 and hence has poor affinity for anionic dye under high alkaline conditions. Because of high concentration gradient between adsorbate and adsorbent species, rapid dye adsorption takes place initially followed by a gradual decrease in adsorption rate (**Figure 19f**) as at this stage most of the adsorption sites are occupied by the dye molecules. The model fits of Langmuir and Freundlich isotherms for MO are shown in **Figure 19e**. It was found that the Langmuir isotherm better described the adsorption of MO with the higher regression coefficient *R*<sup>2</sup> (0.9897) than Freundlich isotherm (*R*<sup>2</sup> =0.9112). The kinetic study concluded the applicability of the pseudosecond-order kinetic model for dye adsorption over pseudofirst-order kinetic equation and curve fitting of the experimental data for the two models is shown in **Figure 19f**.

#### **5.3. Absorption of oils/organic solvents**

The authors also studied the role of dye solution pH (**Figure 17b**) on the adsorption mechanism of the composite. The adsorption capacity was found to increase with pH (from 3 to 5.5) mainly due to the fact that at low pH condition protonation of basic dye (amido groups) fuchsine occurs, which leads to increase solubility of dye in water and hence its relatively poor adsorp‐ tion under slightly acidic conditions. The extensive research on development of such handy magnetic graphene materials is now of great interest as these materials have high dye removal efficiency, quick dye adsorption and facile removal of spent adsorbent from purified water after completion of pollutant adsorption. In this series, a very promising magnetic adsorbent Ni-RGO (NGC) composite is prepared by Li group [117], which almost completely removes rhodamine (RhB) and MB dyes from water within 4 h as evident from corresponding time-

**Figure 18.** UV-visible spectra of original and NGC treated 50 mg organic dye solutions (a) RhB 1 × 10−5 M, 20 mL (b) 2 × 10−5M, 20 mL for various time intervals. The insets are photographs of corresponding solutions at various time inter‐

The authors show that the Freundlich adsorption mode is applicable to NGC with regression

adsorbent surface. The Freundlich exponent (*n*) for RhB (1.048) is greater compared to MB (0.894) that shows the better interaction of the RhB dye with adsorbent compared to MB. This is mainly due to molecular structural differences between the two dyes. In particular, MB has a soft S-based cationic system and RhB has a hard N-based cationic system, resulting in high affinity of latter for a hard O-based anionic center. Jiang et al. [110] synthesized a magnetic

) of 0.99 for both RhB and MB dyes, which indicates the heterogeneity of

vals and magnetic separation. Reproduced from [117] with the permission from RSC.

**Table 4.** Freundlich isotherm constants and their correlation coefficients.

coefficient value (*R*<sup>2</sup>

**Constant RhB MB** *K* 0.376 0.302 *N* 1.048 0.894 *R*<sup>2</sup> 0.99 0.99

dependent UV-visible spectra (**Figure 18**) (**Table 4**).

42 Diamond and Carbon Composites and Nanocomposites

Addressing the severe water pollution arising from oil spillage and chemical leakage is a challenging task. In principle, materials with both superhydrophobic and superoleophilic properties are highly desired for solving the environmental issues because of their capacity of selective absorption of oils and organic solvents while repelling water completely. As a remediation of accidents, the cleanup of leaked chemicals or oil is a global environment issue. Generally, porous materials that have high porosity and high surface areas show great potential in oil/chemicals pickup; however, these materials usually show poor selectivity toward target oil, thus easily saturated with water. Recently, graphene-based 3D porous structures, possessing superhydrophobic and superoleophilic surfaces, attracted enormous attention owing to their high affinity for various oils and organic solvents and ability to selectively absorb them from their dispersion, emulsions or floating layered system with water. In particular, some work has been done in the direction of graphene-coated polymeric foams [105] type hybrids and graphene/composites based aerogels [118]. A number of polymeric foam substrates have been exploited for coating/decoration with graphene, including polyur‐ ethane (PU), polydimethoxysiloxane (PDMS) and melamine foams.

Liu et al. [105] fabricated reduced graphene oxide-coated polyurethane sponges (rGPU) via a two-step process involving dip coating with GO followed by its chemical reduction to RGO. In this system, porous structure of PU sponge-based substrate (SEM image **Figure 20a**) is coated with RGO layers (SEM image **Figure 20b**) without disturbance of macroporous cellular architecture of the sponge. However, due to its inherent hydrophobic nature, the uniformly coated of RGO impart high degree of hydrophobicity and oleophilicity to PU sponges, as confirmed from the contact angle measurements for water (**Figure 20c**) and oil (**Figure 20d**).

**Figure 20.** (a, b) SEM images of RGPU and (c, d) optical image of a water drop and lubricating oil drop on RGPU sur‐ face. Digital images of oil (e–g) and chloroform (h–k) absorption. Absorption capacity of RGPU sponge for various oils. Reproduced from [105] with the permission from ACS.

Such a combination of macroporosity, hydrophobicity and oleophilicity of these materials leads to efficient absorption of lubricating oil (**Figure 20e–g**), chloroform (**Figure 20h–k**) and variety of other oils and organic solvents with good specific absorption capacity.

properties are highly desired for solving the environmental issues because of their capacity of selective absorption of oils and organic solvents while repelling water completely. As a remediation of accidents, the cleanup of leaked chemicals or oil is a global environment issue. Generally, porous materials that have high porosity and high surface areas show great potential in oil/chemicals pickup; however, these materials usually show poor selectivity toward target oil, thus easily saturated with water. Recently, graphene-based 3D porous structures, possessing superhydrophobic and superoleophilic surfaces, attracted enormous attention owing to their high affinity for various oils and organic solvents and ability to selectively absorb them from their dispersion, emulsions or floating layered system with water. In particular, some work has been done in the direction of graphene-coated polymeric foams [105] type hybrids and graphene/composites based aerogels [118]. A number of polymeric foam substrates have been exploited for coating/decoration with graphene, including polyur‐

Liu et al. [105] fabricated reduced graphene oxide-coated polyurethane sponges (rGPU) via a two-step process involving dip coating with GO followed by its chemical reduction to RGO. In this system, porous structure of PU sponge-based substrate (SEM image **Figure 20a**) is coated with RGO layers (SEM image **Figure 20b**) without disturbance of macroporous cellular architecture of the sponge. However, due to its inherent hydrophobic nature, the uniformly coated of RGO impart high degree of hydrophobicity and oleophilicity to PU sponges, as confirmed from the contact angle measurements for water (**Figure 20c**) and oil (**Figure 20d**).

**Figure 20.** (a, b) SEM images of RGPU and (c, d) optical image of a water drop and lubricating oil drop on RGPU sur‐ face. Digital images of oil (e–g) and chloroform (h–k) absorption. Absorption capacity of RGPU sponge for various oils.

Such a combination of macroporosity, hydrophobicity and oleophilicity of these materials leads to efficient absorption of lubricating oil (**Figure 20e–g**), chloroform (**Figure 20h–k**) and

variety of other oils and organic solvents with good specific absorption capacity.

Reproduced from [105] with the permission from ACS.

ethane (PU), polydimethoxysiloxane (PDMS) and melamine foams.

44 Diamond and Carbon Composites and Nanocomposites

**Figure 21.** (a) Absorption capacities of RGPU sponge with contact time with various oils and (b) recyclability of RGPU sponge. Reproduced from [105] with the permission from ACS.

In addition, these hybrid sponges also display high absorption rate (**Figure 21a**) with saturation attaining in 2 h, The authors also exploited the compressible nature of PU sponge for the reusability of these materials and shown that they can be reused number of times (**Fig‐ ure 21b**) without much loss in absorption capacity.

**Figure 22.** Effect of graphene loading on the water CA of the pure sponge (a). Relationship between pH values and CAs of the graphene-based sponges after 12 h floating on aqueous solutions with different pH values (b). Photograph of pure (white color) and graphene-based (black color) sponges after being placed on water, the inset is a photograph of the graphene-based sponge partially immersed in water by a force (c). Water droplets as quasi-spheres and motor oil trace on the surface of the graphene-based sponge (d), the inset in (d) is an optical image of a water droplet at a CA of 162. Reproduced from [86] with permission from RSC.

Nguyen group prepared graphene-coated melamine sponges [86] and highlighted the role of graphene loading and solution pH on the water contact angle value of synthesized hybrid foams. The presence of graphene nanolayers on the surface of melamine sponge drastically changed its surface properties attributed by the fact that the water contact angle changed from 0° (for pristine melamine sponge) to 162° (for graphene-coated melamine sponge). Further, the stability of superhydrophibicity to aggressive environment is demonstrated by the fact that the WCA remained greater than 150 degree even after 12 h of floating over aqueous solutions of different pH values. The pure sink to the bottom (**Figure 22c**), whereas the graphene-coated sponge floats over water, retard forced immersion with no water uptake, which further demonstrates its superhydrophobic nature. The silver mirror-like appearance of the immersed part of the graphene-based sponge (the inset **Figure 22c**) is attributed to a uniform air layer trapped between the water and superhydrophobic surfaces, which are referred to nonwetting Cassie-Baxter surfaces (Section 3). Interestingly, the sponge also displays superoleophilicity such that water droplets (pointed arrows) display WCA of 162° (inset **Figure 22d**) and remained unabsorbed (**Figure 22d**), whereas motor oil droplet (encircled) was immediately absorbed. The coexistence of superhydrophobic and superoleophilic surfaces is attributed to the combination of the microporous structure (with trapped air pockets) of the melamine sponge substrate and the hydrophobicity of the graphene nanosheets, and the micro-/nanotextured structure of graphene nanosheets on sponge skeletons.

These hybrid sponges display good absorption capacities (ranging from 60 to 165 times of its own weight) for motor oil (**Figure 23a**), chloroform (**Figure 23b**) and several other solvents (**Figure 23c**), good recyclability, low density and chemical inertness.

**Figure 23.** Snapshots of the removal process of motor oil (dyed with solvent red 24 for clear observation) (a) and chloroform (b) from water using the graphene-based sponges. Oil and organic solvent absorption capacities of the gra‐ phene-based sponges (c). Absorption recyclability of the graphene-based sponges for oils (d) and organic solvents (e). Reprinted from [86] with permission from RSC.

The recyclability and reusability of the graphene-based sponge also plays an important role in oil/chemical clean up applications. In general, sponge regeneration and absorbed pollutant separation can be realized either by squeezing the sponges for high-boiling materials like oils or by distillation/squeezing for low-boiling oils/solvents. It is observed that the generalized recyclable absorption response is similar for all tested oils (**Figure 23d**) with large decrease in absorption capacity after first cycle due to the residual oils inside the sponges, which cannot be removed by manual squeezing. After the second cycle, the saturated absorption capacity decreases only slightly indicating good recyclability. However, in case of organic solvents, desorbed sponges (after distillation) display no obvious change in saturated absorption capacity (**Figure 23e**) reflecting complete pollutant (solvent) recovery and good sponge recyclability, which are requisite for practical utility. Despite their great promise for oil/ chemical clean up, the poor adhesion between coated graphene and foam substrate, remained a problematic issue. An attempt has been made by Tajendra et al. to improve the adhesive forces between PU sponges and RGO via covalent bonding strategy [119] involving use of silane- or titanate-based coupling agents. The sponges exhibited absorption capacity of 30–50 g/g depending upon the nature of oil/solvent. Further, the authors reported that the covalent bonding between RGO and PU prevents the leaching problem of surface layers and hence increase the recyclability of the hybrid sponges.

Nguyen group prepared graphene-coated melamine sponges [86] and highlighted the role of graphene loading and solution pH on the water contact angle value of synthesized hybrid foams. The presence of graphene nanolayers on the surface of melamine sponge drastically changed its surface properties attributed by the fact that the water contact angle changed from

stability of superhydrophibicity to aggressive environment is demonstrated by the fact that the WCA remained greater than 150 degree even after 12 h of floating over aqueous solutions of different pH values. The pure sink to the bottom (**Figure 22c**), whereas the graphene-coated sponge floats over water, retard forced immersion with no water uptake, which further demonstrates its superhydrophobic nature. The silver mirror-like appearance of the immersed part of the graphene-based sponge (the inset **Figure 22c**) is attributed to a uniform air layer trapped between the water and superhydrophobic surfaces, which are referred to nonwetting Cassie-Baxter surfaces (Section 3). Interestingly, the sponge also displays superoleophilicity

unabsorbed (**Figure 22d**), whereas motor oil droplet (encircled) was immediately absorbed. The coexistence of superhydrophobic and superoleophilic surfaces is attributed to the combination of the microporous structure (with trapped air pockets) of the melamine sponge substrate and the hydrophobicity of the graphene nanosheets, and the micro-/nanotextured

These hybrid sponges display good absorption capacities (ranging from 60 to 165 times of its own weight) for motor oil (**Figure 23a**), chloroform (**Figure 23b**) and several other solvents

**Figure 23.** Snapshots of the removal process of motor oil (dyed with solvent red 24 for clear observation) (a) and chloroform (b) from water using the graphene-based sponges. Oil and organic solvent absorption capacities of the gra‐ phene-based sponges (c). Absorption recyclability of the graphene-based sponges for oils (d) and organic solvents (e).

(for graphene-coated melamine sponge). Further, the

(inset **Figure 22d**) and remained

0°

(for pristine melamine sponge) to 162°

46 Diamond and Carbon Composites and Nanocomposites

such that water droplets (pointed arrows) display WCA of 162°

(**Figure 23c**), good recyclability, low density and chemical inertness.

structure of graphene nanosheets on sponge skeletons.

Reprinted from [86] with permission from RSC.

**Table 5** compares the oil/organic-solvent removal performance of various graphene-coated foams based hybrids. It can be seen that the achieved absorption capacities are still lower than theoretically possible values and there is large deviation between results of different works. Further, besides hydrophobicity of the hybrid sponges, the parameters like viscosity, densities, polarities, surface tensions/energies of spilled chemicals and pore size, porosity and surface texturing of the foamed substrates, plays an important role in governing absorption charac‐ teristics (absorption capacity, absorption rate, selectivity) and need to be looked into thor‐ oughly to further understanding and improvements. The graphene coating's adherence issues are sorted out to improve the reusability and to reduce the chances of cross contamination. In order to drastically improve the gravimetric absorption capacities, eliminate the adherence issues and improve the recyclability, all graphene-based 3D self-assembled networks such as graphene aerogel seems to be promising solution [16,75].



**Table 5.** Absorption capacities (g/g) of graphene-based materials for various oils/organic solvents.

Nevertheless, huge efforts in the direction are needed for the realization of efficient, stable, cost-effective and commercially viable absorbents for large-scale oil/chemical clean up to be used at actual spill site especially under marine environment.

#### **7. Conclusion**

In summary, graphene-based composites/hybrids have a tremendous potential for water purification applications. The inherent properties of graphene in conjunction with other materials induce additional functionalities in the composites/hybrids reflected in terms of enhanced, surface areas, porosity, flexibility, induced magnetism, improved adsorption/ absorption rates and capacities. As discussed in this chapter, these materials show extraordi‐ nary chemical stabilities with good recyclability and durability, which are some of the most crucial attributes of sorbing materials. Importantly, the generation of magnetism in graphene composites/hybrids with magnetic nanomaterials prevents to ease out the sorbent operation‐ ality and its recovery process. But issues, such as desorption of pollutants for the regeneration of the active materials, are still a major challenge and need to be thoroughly investigated.

#### **Author details**

**S. No. Absorbate Absorption capacities (g/g) of various materials Graphenecoated melamine sponge**

48 Diamond and Carbon Composites and Nanocomposites

6 Used pump oil

9 Soya bean oil

13 Lubricate oil

14 Gasoline oil

**7. Conclusion**

**GO-coated PU sponges**

7 Methanol 80 – – – 8 Pump oil 95 90 100 28

10 Motor oil 90 – – – 11 Diesel oil – 75 90 – 12 Olive oil – 85 100 –

Ref. [103] [63] [63] [104]

**Table 5.** Absorption capacities (g/g) of graphene-based materials for various oils/organic solvents.

Nevertheless, huge efforts in the direction are needed for the realization of efficient, stable, cost-effective and commercially viable absorbents for large-scale oil/chemical clean up to be

In summary, graphene-based composites/hybrids have a tremendous potential for water purification applications. The inherent properties of graphene in conjunction with other materials induce additional functionalities in the composites/hybrids reflected in terms of enhanced, surface areas, porosity, flexibility, induced magnetism, improved adsorption/ absorption rates and capacities. As discussed in this chapter, these materials show extraordi‐ nary chemical stabilities with good recyclability and durability, which are some of the most crucial attributes of sorbing materials. Importantly, the generation of magnetism in graphene composites/hybrids with magnetic nanomaterials prevents to ease out the sorbent operation‐ ality and its recovery process. But issues, such as desorption of pollutants for the regeneration of the active materials, are still a major challenge and need to be thoroughly investigated.

PU—polyurethane sponges, TTEAI— titanium (IV)(triethanolaminato) isopropoxide.

used at actual spill site especially under marine environment.

90 – – –

100 92 110 –

– 100 112 –

**RGO-coated PU sponges**

**TTEAI-RGO sponge**

45

Rahul Sharma and Parveen Saini\*

\*Address all correspondence to: pksaini@nplindia.org; parveensaini580@gmail.com

Conjugated Polymer and Graphene Technology Lab, Polymeric and Soft Materials Section, Materials Physics and Engineering Division, CSIR-National Physical Laboratory, New Delhi, India

#### **References**


[26] Dalton, T. & Jin, D. Extent and frequency of vessel oil spills in US marine protected areas. *Mar. Pollut. Bull*. 60, 1939–1945 (2010).

[11] Saini,P.(2015) *Fundamentals of Conjugated Polymer Blends, Copolymers and Composites: Synthesis, Properties and Applications*. (John Wiley & Sons, Inc., 2015). Hoboken, NJ, USA.

doi: 10.1002/9781119137160.ch1 <http://doi.wiley.com/10.1002/9781119137160>

*Nanoparticle Res*. 15, (2013), 15: 1415

50 Diamond and Carbon Composites and Nanocomposites

*Manage*. 92, 407–418 (2011).

Germany. doi: 10.1002/9783527659647.ch10

separation. *J Mater Chem* 2, 2445–2460 (2014).

search/a?dbs+hsdb:@term+@DOCNO+1405>

*Soil Biol*. 1, 15–19 (2009).

for Arsenic Removal. *Acs Nano* 4, 3979–3986 (2010).

dynamic studies. *J. Hazard. Mater*. 154, 337–346 (2008).

[12] Saini, P. & Choudhary, V. Enhanced electromagnetic interference shielding effective‐ ness of polyaniline functionalized carbon nanotubes filled polystyrene composites. *J.*

[13] Saini, P. (2013) Electrical Properties and Electromagnetic Interference Shielding Response of Electrically Conducting Thermosetting Nanocomposites, in Thermoset Nanocomposites (ed V. Mittal), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim,

[14] Fu, F. & Wang, Q. Removal of heavy metal ions from wastewaters: A review. *J. Environ.*

[15] Yilmaz, M., Tay, T., Kivanc, M. & Turk, H. Removal of corper(II) Ions from aqueous

[16] Xue, Z., Cao, Y., Liu, N., Feng, L. & Jiang, L. Special wettable materials for oil/water

[17] Chandra, V. *et al*. Water-Dispersible Magnetite-Reduced Graphene Oxide Composites

[18] Tan, I. A. W., Ahmad, A. L. & Hameed, B. H. Adsorption of basic dye on high-surfacearea activated carbon prepared from coconut husk: Equilibrium, kinetic and thermo‐

[19] Whitehead, P. G. & Lack, T. Dispersion and self-purification of pollutants in surface water systems. *United Nations Educ. Sci. Cult. Organ. 7 Place Fontenoy 75700 Paris*

[20] El Qada, E. N., Allen, S. J. & Walker, G. M. Adsorption of basic dyes from aqueous

[21] TOXNET (Toxicology Database Network. at <http://toxnet.nlm.nih.gov/cgi-bin/sis/

[22] Sulak, M. T. & Yatmaz, H. C. Removal of textile dyes from aqueous solutions with eco-

[23] Kayode, J., Olowoyo, O. & Oyedeji, A. The Effects of Used Engine Oil Pollution on the Growth and Early Seedling Performance of Vigna uniguiculata and Zea mays. *Res. J.*

[25] Ordinioha, B. & Brisibe, S. The human health implications of crude oil spills in the Niger delta, Nigeria: An interpretation of published studies. *Niger. Med. J*. 54, 10 (2013).

solution onto activated carbons. *Chem. Eng. J*. 135, 174–184 (2008).

friendly biosorbent. *Desalination Water Treat*. 37, 169–177 (2012).

[24] at <https://en.wikipedia.org/wiki/Deepwater\_Horizon\_oil\_spill>

solution by a lactic acid bacterium. *Braz. J. Chem. Eng*. 27, 309–314 (2010).


[55] Jiao, T. *et al*. Reduced Graphene Oxide-Based Silver Nanoparticle-Containing Compo‐ site Hydrogel as Highly Efficient Dye Catalysts for Wastewater Treatment. *Sci. Reports* 5, 11873 (2015).

[40] Broje, V. & Keller, A. A. Improved Mechanical Oil Spill Recovery Using an Optimized Geometry for the Skimmer Surface. *Environ. Sci. Technol*. 40, 7914–7918 (2006).

[41] Goodbody-Gringley, G. *et al*. Toxicity of Deepwater Horizon Source Oil and the Chemical Dispersant, Corexit® 9500, to Coral Larvae. *Plos One* 8, e45574 (2013).

[42] Brooijmans, R. J. W., Pastink, M. I. & Siezen, R. J. Hydrocarbon-degrading bacteria: the

[43] Zhao, J., Ren, W. & Cheng, H.-M. Graphene sponge for efficient and repeatable adsorption and desorption of water contaminations. *J. Mater. Chem*. 22, 20197 (2012).

[44] Mukherjee, R., Sharma, R., Saini, P. & De, S. Nanostructured polyaniline incorporated ultrafiltration membrane for desalination of brackish water. *Env. Sci Water Res Tech‐*

[45] Liu, M., Chen, C., Hu, J., Wu, X. & Wang, X. Synthesis of Magnetite/Graphene Oxide Composite and Application for Cobalt(II) Removal. *J. Phys. Chem. C* 115, 25234–25240

[46] Zhu, L. *et al*. Mixed matrix membranes containing MIL-53(Al) for potential application

[47] Liu, F., Chung, S., Oh, G. & Seo, T. S. Three-Dimensional Graphene Oxide Nanostruc‐ ture for Fast and Efficient Water-Soluble Dye Removal. *Acs Appl. Mater. Interfaces* 4,

[48] Li, Q., Zhai, J., Zhang, W., Wang, M. & Zhou, J. Kinetic studies of adsorption of Pb(II), Cr(III) and Cu(II) from aqueous solution by sawdust and modified peanut husk. *J.*

[49] Li, B. & Cao, H. ZnO@graphene composite with enhanced performance for the removal

[50] Kornaros, M. & Lyberatos, G. Biological treatment of wastewaters from a dye manu‐ facturing company using a trickling filter. *J. Hazard. Mater*. 136, 95–102 (2006).

[51] Kalantzopoulos, G. N. *et al*. Resistance to the transport of H2 through the external surface of as-made and modified silicalite-1 (MFI). *Microporous Mesoporous Mater*. 220,

[52] Gao, W. *et al*. Engineered Graphite Oxide Materials for Application in Water Purifica‐

[53] Fan, W. *et al*. Hybridization of graphene sheets and carbon-coated Fe3O4 nanoparticles as a synergistic adsorbent of organic dyes. *J. Mater. Chem*. 22, 25108 (2012).

[54] Ackacha, M. A. Removal of Pb(II) from Aqueous Solution by <i>Por‐ tulaca oleracea</i> Leaves: Kinetic, Equilibrium and Thermodynamic

in organic solvent nanofiltration. *Rsc Adv* 5, 73068–73076 (2015).

oil-spill clean-up crew. *Microb. Biotechnol*. 2, 587–594 (2009).

*nol* 1, 893–904 (2015).

52 Diamond and Carbon Composites and Nanocomposites

(2011).

922–927 (2012).

290–297 (2016).

*Hazard. Mater*. 141, 163–167 (2007).

of dye from water. *J Mater Chem* 21, 3346–3349 (2011).

tion. *Acs Appl. Mater. Interfaces* 3, 1821–1826 (2011).

Studies. *Am. J. Anal. Chem*. 04, 27–32 (2013).


[84] Li, B., Cao, H. & Yin, G. Mg(OH)2@reduced graphene oxide composite for removal of dyes from water. *J. Mater. Chem*. 21, 13765 (2011).

[69] Namasivayam, C. & Kavitha, D. Removal of Congo Red from water by adsorption onto activated carbon prepared from coir pith, an agricultural solid waste. *Dyes Pigments* 54,

[70] Ai, L., Li, M. & Li, L. Adsorption of Methylene Blue from Aqueous Solution with Activated Carbon/Cobalt Ferrite/Alginate Composite Beads: Kinetics, Isotherms, and

[71] Foo, K. Y. & Hameed, B. H. Detoxification of pesticide waste via activated carbon

[72] Sánchez-Polo, M. & Rivera-Utrilla, J. Adsorbent-Adsorbate Interactions in the Adsorp‐ tion of Cd(II) and Hg(II) on Ozonized Activated Carbons. *Environ. Sci. Technol*. 36, 3850–

[73] Geim, A.K. & Novoselov, K.S.The rise and rise of graphene. *Nat. Nanotechnol*. 5, 755–

[74] Zhao, G., Li, J., Ren, X., Chen, C. & Wang, X. Few-Layered Graphene Oxide Nanosheets As Superior Sorbents for Heavy Metal Ion Pollution Management. *Environ. Sci.*

[75] Wang, J.-N. *et al*. Recent developments in superhydrophobic graphene and graphenerelated materials: from preparation to potential applications. *Nanoscale* 7, 7101–7114

[76] Sun, Y., Wu, Q. & Shi, G. Graphene based new energy materials. *Energy Environ. Sci*. 4,

[77] Schedin, F. *et al*. Detection of individual gas molecules adsorbed on graphene. *Nat.*

[78] Li, X. *et al*. Graphene Films with Large Domain Size by a Two-Step Chemical Vapor

[79] Zhu, J. *et al*. One-Pot Synthesis of Magnetic Graphene Nanocomposites Decorated with Core@Double-shell Nanoparticles for Fast Chromium Removal. *Environ. Sci. Technol*.

[80] Xu, J., Wang, L. & Zhu, Y. Decontamination of Bisphenol A from Aqueous Solution by

[81] Ren, Y. *et al*. Graphene/d-MnO2 composite as adsorbent for the removal of nickel ions

[82] Luo, X. *et al*. Nanocomposites of graphene oxide-hydrated zirconium oxide for simultaneous removal of As(III) and As(V) from water. *Chem. Eng. J*. 220, 98–106 (2013).

[83] Chabot, V. *et al*. A review of graphene and graphene oxide sponge: material synthesis and applications to energy and the environment. *Energy Environ. Sci*. 7, 1564 (2014).

Thermodynamics. *J. Chem. Eng. Data* 56, 3475–3483 (2011).

adsorption process. *J. Hazard. Mater*. 175, 1–11 (2010).

Deposition Process. *Nano Lett*. 10, 4328–4334 (2010).

Graphene Adsorption. *Langmuir* 28, 8418–8425 (2012).

from wastewater. *Chem. Eng. J*. 175, 1–7 (2011).

47–58 (2002).

54 Diamond and Carbon Composites and Nanocomposites

3854 (2002).

755 (2010).

(2015).

1113 (2011).

*Mater*. 6, 652–655 (2007).

46, 977–985 (2012).

*Technol*. 45, 10454–10462 (2011).


[112] Sharma, P., Hussain, N., Borah, D. J. & Das, M. R. Kinetics and Adsorption Behavior of the Methyl Blue at the Graphene Oxide/Reduced Graphene Oxide Nanosheet–Water Interface: A Comparative Study. *J. Chem. Eng. Data* 58, 3477–3488 (2013).

[98] Wang, M., Lawal, A., Stephenson, P., Sidders, J. & Ramshaw, C. Post-combustion CO2 capture with chemical absorption: A state-of-the-art review. *Chem. Eng. Res. Des*. 89,

[99] Vácha, R., Slavíc?ek, P., Mucha, M., Finlayson-Pitts, B. J. & Jungwirth, P. Adsorption of Atmospherically Relevant Gases at the Air/Water Interface: Free Energy Profiles of Aqueous Solvation of N 2, O 2, O 3, OH, H 2 O, HO 2, and H 2 O 2. *J. Phys. Chem. A* 108,

[100] Wang, S., Li, M. & Lu, Q. Filter Paper with Selective Absorption and Separation of Liquids that Differ in Surface Tension. *Acs Appl. Mater. Interfaces* 2, 677–683 (2010).

[101] Baldacchini, T., Carey, J. E., Zhou, M. & Mazur, E. Superhydrophobic Surfaces Prepared by Microstructuring of Silicon Using a Femtosecond Laser. *Langmuir* 22, 4917–4919

[102] Wenzel, R. N. RESISTANCE OF SOLID SURFACES TO WETTING BY WATER. *Ind.*

[103] Cassie, A. B. D. & Baxter, S. Wettability of porous surfaces. *Trans. Faraday Soc*. 40, 546

[104] Rizzuti, L. & Brucato, A. Liquid viscosity and flow rate effects on interfacial area in

[105] Liu, Y. *et al*. Cost-Effective Reduced Graphene Oxide-Coated Polyurethane Sponge As a Highly Efficient and Reusable Oil-Absorbent. *Acs Appl. Mater. Interfaces* 5, 10018–

[106] Gupta, S. S., Sreeprasad, T. S., Maliyekkal, S. M., Das, S. K. & Pradeep, T. Graphene from Sugar and its Application in Water Purification. *Acs Appl. Mater. Interfaces* 4, 4156–

[107] Chen, Y., Chen, L., Bai, H. & Li, L. Graphene oxide–chitosan composite hydrogels as broad-spectrum adsorbents for water purification. *J Mater Chem* 1, 1992–2001 (2013).

[108] Wang, C. *et al*. Preparation of a graphene-based magnetic nanocomposite for the removal of an organic dye from aqueous solution. *Chem. Eng. J*. 173, 92–97 (2011).

[109] Lü, W., Wu, Y., Chen, J. & Yang, Y. Facile preparation of graphene–Fe 3 O 4 nanocom‐ posites for extraction of dye from aqueous solution. *CrystEngComm* 16, 609–615 (2014).

[110] Jiang, Y. *et al*. Magnetic chitosan–graphene oxide composite for anti-microbial and dye

[111] Samad, Y. A., Li, Y., Schiffer, A., Alhassan, S. M. & Liao, K. Graphene Foam Developed with a Novel Two-Step Technique for Low and High Strains and Pressure-Sensing

removal applications. *Int. J. Biol. Macromol*. 82, 702–710 (2016).

Applications. *Small* 11, 2380–2385 (2015).

1609–1624 (2011).

56 Diamond and Carbon Composites and Nanocomposites

11573–11579 (2004).

*Eng. Chem*. 28, 988–994 (1936).

packed columns. *Chem. Eng. J*. 41, 49–52 (1989).

(2006).

(1944).

10026 (2013).

4163 (2012).

