**4.1 Carbonaceous nanostructures**

The removal of heavy metals with carbon nanotubes (CNT) varies because the adsorption mechanisms depend on the affinity order of metals ions and the surface functionalization of CNT. Some examples of adsorption have been summarized by Ihsanullah et al. [47]. The mechanism is based on the release of protons (H<sup>+</sup> ) from the surface when oxidation of CNT was achieved. After that, the attachment of divalent metals occurs on the surface. This process depends on the concentration of the metal ion. A higher number of ions increase the pH value due to the increment of H<sup>+</sup> in the solution. It was found that the adsorption of divalent ions, such as Cd2+ and Zn2+, is dominated by a combination of physisorption (Van deer Waals forces) and sorption-precipitation when the pH is increasing in the solution. It means that the acid treatment and surface modification have a significant effect on the adsorption capacity of CNT. Some examples of carbon materials used for metal removal under different metal concentrations in the ppm range are listed in **Table 2** [49–52]. Those experiments were performed without temperature or pH

#### **Figure 3.**

*Fundamental mechanisms for the adsorption of heavy metal ions on porous nanostructured materials.*

**173**

less Cr3+ [55].

where the pH was adjusted.

*Low Dimensional Nanostructures: Measurement and Remediation Technologies Applied to Trace…*

**Material Copper Lead Other metals Reference**

50 ppm – 74%

709 ppm – 100% pH 4

50 ppm – 84%

10 ppm – 91% 20 ppm – 90% 40 ppm – 93%

10 ppm – 91% 20 ppm – 81% 40 ppm – 98%

10 ppm – 100% 20 ppm – 89% 40 ppm - 99%

Red volcanic rock 5 ppm – 15% 5 ppm – 14% — Mabel et al. [54]

**Chrome** 30 ppm – 50.6% 50 ppm – 48.2% **Nickel** 30 ppm – 90% 50 ppm – 87.8% **Cadmium** 30 ppm – 86% 50 ppm – 84%

**Nickel** 996 ppm – 92% pH 4

**Chrome** 30 ppm – 63% **Nickel** 30 ppm – 89.9%

0.5 ppm – 66% **Nickel** 15 ppm −75.4% **Cadmium** 6 ppm - 87%

Karnib et al. [49]

Kadirvelu et al. [50]

Sheet et al. [51]

Mubarak et al. [52]

Hodges [53]

Hodges [53]

Hodges [53]

— Gonzalez

— Gonzalez

— Gonzalez

variation, except for the analysis that uses activated carbon from coconut waste,

**Table 2** presents higher percentages of copper and lead removal when using nitrogen-doped carbon nanotubes grown on volcanic rock [53] than other carbon materials or red volcanic rock [54] without any treatment. The increased efficiency of these carbon nanotubes grown on volcanic rock can be correlated with the presence of nitrogen on their graphitic lattice. Doping carbon nanostructures with heteroatoms, such as nitrogen, increases the adsorption capacity without any acid treatment [4]. Nitrogen has a similar atom diameter but an extra electron as carbon, favoring the atom replacement, and the electron-donating properties [55]. N-doping occurs in different forms, such as pyridine, pyrrole-like, and quaternary-N salts, but may include nitrogen oxide groups and amines [56]. In particular, quaternary nitrogen atoms are considered to increase the adsorption capacity by electrostatic interaction, as the surface carbon nanostructures doped in this way are more negatively charged than the undoped ones [55, 57]. Besides, nitrogenated carbon nanostructures act as reducing agents for toxic Cr6+ converting it into harm-

*DOI: http://dx.doi.org/10.5772/intechopen.93263*

Activated carbon (coconut waste)

N-doped carbon nanotubes growth on red volcanic rock

N-doped carbon nanotubes growth on black volcanic rock

N-doped carbon nanotubes growth on yellow volcanic rock

**Table 2.**

Activated carbon — 30 ppm – 83%

126 ppm 73% pH 5

10 ppm – 82% 20 ppm – 84% 40 ppm – 51%

10 ppm – 90% 20 ppm – 95% 40 ppm - 71%

10 ppm – 92% 20 ppm – 83% 40 ppm – 63%

*Heavy metal removal percentages using different carbon-based materials.*

Carbon nanotubes 100 ppm – 41% 40 ppm – 83% **Chrome**

Graphite oxide — 30 ppm – 85%


*Low Dimensional Nanostructures: Measurement and Remediation Technologies Applied to Trace… DOI: http://dx.doi.org/10.5772/intechopen.93263*

#### **Table 2.**

*Trace Metals in the Environment - New Approaches and Recent Advances*

magnesium (Mg2+), potassium (K+

**4.1 Carbonaceous nanostructures**

adsorption of Pb ions [48].

increment of H<sup>+</sup>

Water is by far, one of the most contaminated resources in the planet; that is why the remediation and removal of contaminants are an urgent need together with easy and fast monitoring tools. The available treatments used for removal of heavy metals from water are classified as follows: chemical precipitation, membrane filtration, ion exchange, reverse osmosis, and adsorption [44]. The adsorption using nanomaterials has been of great interest since several nanostructured adsorbents have demonstrated a high performance [44–46]. Adsorption on nanostructured materials is complicated, but some authors have proposed possible mechanisms that depend mainly on the nature of the surface area. The fundamental mechanisms are based on physical adsorption (physisorption), chemical adsorption (chemisorption), electrostatic attraction, and sorption-precipitation [9, 47, 48]. **Figure 3** shows a schematic representation for the adsorption mechanisms of heavy metal on porous nanomaterials. Lu et al. reported that biochar, a 3D network, is a material rich in cations and surface interaction sites for lead adsorption. Electrostatic cation exchange or metal exchange reactions mechanisms may occur when calcium (Ca2+),

), and sodium (Na+

The removal of heavy metals with carbon nanotubes (CNT) varies because the adsorption mechanisms depend on the affinity order of metals ions and the surface functionalization of CNT. Some examples of adsorption have been summarized by Ihsanullah et al. [47]. The mechanism is based on the release of protons (H<sup>+</sup>

from the surface when oxidation of CNT was achieved. After that, the attachment of divalent metals occurs on the surface. This process depends on the concentration of the metal ion. A higher number of ions increase the pH value due to the

such as Cd2+ and Zn2+, is dominated by a combination of physisorption (Van deer Waals forces) and sorption-precipitation when the pH is increasing in the solution. It means that the acid treatment and surface modification have a significant effect on the adsorption capacity of CNT. Some examples of carbon materials used for metal removal under different metal concentrations in the ppm range are listed in **Table 2** [49–52]. Those experiments were performed without temperature or pH

*Fundamental mechanisms for the adsorption of heavy metal ions on porous nanostructured materials.*

in the solution. It was found that the adsorption of divalent ions,

in the adsorption of Pb ions, but the electrostatic interaction and surface complexation with pi-cationic and functional groups interaction, may also happen in the

) ions released from biochar

)

**172**

**Figure 3.**

*Heavy metal removal percentages using different carbon-based materials.*

variation, except for the analysis that uses activated carbon from coconut waste, where the pH was adjusted.

**Table 2** presents higher percentages of copper and lead removal when using nitrogen-doped carbon nanotubes grown on volcanic rock [53] than other carbon materials or red volcanic rock [54] without any treatment. The increased efficiency of these carbon nanotubes grown on volcanic rock can be correlated with the presence of nitrogen on their graphitic lattice. Doping carbon nanostructures with heteroatoms, such as nitrogen, increases the adsorption capacity without any acid treatment [4]. Nitrogen has a similar atom diameter but an extra electron as carbon, favoring the atom replacement, and the electron-donating properties [55]. N-doping occurs in different forms, such as pyridine, pyrrole-like, and quaternary-N salts, but may include nitrogen oxide groups and amines [56]. In particular, quaternary nitrogen atoms are considered to increase the adsorption capacity by electrostatic interaction, as the surface carbon nanostructures doped in this way are more negatively charged than the undoped ones [55, 57]. Besides, nitrogenated carbon nanostructures act as reducing agents for toxic Cr6+ converting it into harmless Cr3+ [55].

#### **4.2 Silica-based nanostructures**

The high surface area found on porous materials, such as the silica-based ones, is attributed to the 3D network that displays a hierarchical organization that can be found at different scales (nano, micro, and milli). The nanostructured organization is mainly attributed to the form and size of the pore, which allows the accommodation of other nanostructures, as well as the functionalization of the surface. The methodology to produce nanostructured porous silica is based on the liquid-crystal template mechanism. This allows getting pores around 2–10 nm in size [58]. A surfactant at high concentration is set at a specific pH and temperature values to form a liquid-crystalline phase. Then, the crystalline form may be organized by spherical or rod-shaped micelles arranged on a periodic 3D structure, which serves as a template to be filled with tetraethoxysilane (TEOS) or tetramethylorthosilica (TMOS). Finally, the surfactant is removed to form a mesoporous material [59].

Surface functionalization is a useful tool for the realization of a well-defined set of functions which improves the adsorption properties of silica. It can be carried out by chemical bonds or physical interactions. Silica surface functionalization is of interest due to the multiple options that it offers for heavy metals removal. For example, the thiol-functionalization of mesoporous silica has been proved to exhibit a high affinity for thiophilic heavy-metal ions like Cu2+, Zn2+, Cr3+, and Ni2+ but focused on highly toxic metals such Hg2+. At the same time, the amino functionalization increases the efficiency of the adsorption of Hg2+ [59, 60]. For some specific types of silica materials such as SBA-15 (Santa Barbara Amorphous-15), the imidazole functionalization improves the selective affinity adsorption of Pd2+ and Pt2+, despite the high concentration of Ni2+, Cu2+, and Cd2+ in the mixture [61]. Specific morphologies, such as PMO-like (periodic mesoporous organosilica) or spherical mesostructured, increased the surface area and the particular site of functionalization, consequently, affinity, and selectivity are improved [62, 63].

#### **4.3 Zeolites**

Zeolites are known as microporous materials with a crystalline structure composed by tetrahedral building blocks of alumina (AlO4) and silica (SiO4) that give rise to a three-dimensional network linked by oxygen atoms [64]. Zeolites are an effective, economical, and eco-friendly option to remove heavy metals and organic contaminants from wastewater [64–66]. There are more than 40 naturally occurring zeolites each with different physicochemical properties. Among their most interesting characteristics are their high chemical stability, molecular sieve, adsorption, and ion exchange capacity [67].

The Si/Al ratio in the structure of a zeolite determines its adsorption and ion exchange capacities, which are directly related to the amount of aluminum contained in the natural zeolite [66], so the isomorphic replacement of Si4+ by Al3+ (aluminum-rich zeolites) will have a more negative charge on the framework, providing a higher affinity for polar molecules. The negative charge is balanced with interchangeable cations (generally sodium, potassium, or calcium), therefore these cations are used for cation exchange processes in solutions containing lead, chromium, or mercury, among others ions [68]. The adsorption capacity of zeolites also depends on the charge density and the diameter of the hydrated ion, which is why zeolites have a great potential to remove heavy metal ions from wastewater [64].

Natural zeolites such as clinoptilolite, mordenite, and chabazite were investigated for the removal of heavy metals [64, 65]. The natural zeolite clinoptilolite has attracted attention because of its abundance. The selectivity of the sodium form of clinoptilolite, extracted from natural deposits, was found to be

**175**

*Low Dimensional Nanostructures: Measurement and Remediation Technologies Applied to Trace…*

firmed that natural zeolites have a great ability to selectively remove metallic ions from wastewater. In particular, sodium loaded zeolites resulted the most effective because sodium acts as an exchangeable ion with heavy metals [69]. Further, clinoptilolite exhibited a higher capacity than carbonaceous materials such as carbon nanotubes (CNTs) for lead removal (up to 15.5 times higher) [70, 71]. Nano zeolite [72] and modified zeolites [73–75] demonstrated a higher removal

capacity of lead than that reported by natural zeolites, even much higher than functionalized reduced graphene oxide (RGO) [71]. The silica nano-zeolite X [72] presented an adsorption capacity of 909.09 mg/g of Pb2+ being 5.5 times higher compared to that reported for natural clinoptilolite (166 mg/g) [69]. The main characteristic of nano zeolites and modified zeolites is a larger surface area and pore size, which make them more efficient and facilitate the adsorption of heavy metals and organic molecules, compared to natural zeolites [76]. The efficiency in the removal of heavy metals and other cations will strongly depend on the high surface area of the zeolites, and in their efficiency in removing the metal ions present in wastewater. The surface area in porous materials is determined by the specific surface area (BET) by standard multipoint techniques of nitrogen adsorption. There are several studies where the largest surface area is for nano zeolites (692 m<sup>2</sup>

Iron oxide nanostructures are gaining attention for metal removal from water due to their high surface area, excellent adsorption capacities, innocuousness with the environment, and easiness of separation as one can make good use of their magnetic properties. Despite their advantageous characteristics, their use in real scenarios has not been proved. It is essential to explore their removal mechanisms,

Zerovalent iron nanoparticles (nZVI) are considered a strong reducing agent that is bringing degradation to less harmful substances from a wide range of organic and inorganic pollutants. Sorption of co-precipitation of heavy metals on the surface forms an iron oxide or hydroxide shell when nZVI is exposed to air [79]. The most cited example is the transformation of Cr6+, which precipitates on nZVI with corrosion products. However, this property usually depends on the surface functionalization of the nZVI. Functionalization is important since iron nanoparticles form aggregates very easily and suffer oxidation under acidic and oxygenated environments. The best approach is to coat the nanoparticle surface with Fe3O4 or some polymer to reduce the contact with the environmental oxygen, but maintain-

Previous studies on the adsorption of Cr6+ on the surface of Fe3O4 show the formation of a different crystalline structure by chemical adsorption. For chromite (FeCr2O4), the Cr6+ is reduced to Cr3+, followed by the precipitation of Cr3+ onto the F3O4 nanoparticles. Low desorption is indicative of adsorption because desorption is due to physical adsorption, mainly by electrostatic interactions. This adsorption process predominates on γ-Fe2O3, as previously demonstrated by X Ray Photoelectron Spectroscopy (XPS). Another evidence of the physical adsorption is the non-modification of the crystallographic structure, an indication that the removal process was not due to a chemical interaction [80, 81]. The positively charged surface on the iron oxide nanoparticles determines the attraction for negatively charged pollutants; the positive charge of iron oxides depends on the

> Cu2+ > Co2+ > Cr3+ > Zn2+ > Ni2+ > Hg2+. This behavior con-

/g) [76],

/g) [72]. A smaller surface area

/g) [78].

/g) [77], and the smallest area

*DOI: http://dx.doi.org/10.5772/intechopen.93263*

followed by modified silica natural zeolites (702 m<sup>2</sup>

is found for the sodium form of clinoptilolite (70.4 m<sup>2</sup>

is for the simple natural zeolite clinoptilolite (15.36702 m<sup>2</sup>

not only for iron oxides but also for zerovalent iron nanomaterials.

Pb2+ > Cd2+ > Cs+

**4.4 Iron oxides**

ing the reactivity [79].

*Low Dimensional Nanostructures: Measurement and Remediation Technologies Applied to Trace… DOI: http://dx.doi.org/10.5772/intechopen.93263*

Pb2+ > Cd2+ > Cs+ > Cu2+ > Co2+ > Cr3+ > Zn2+ > Ni2+ > Hg2+. This behavior confirmed that natural zeolites have a great ability to selectively remove metallic ions from wastewater. In particular, sodium loaded zeolites resulted the most effective because sodium acts as an exchangeable ion with heavy metals [69]. Further, clinoptilolite exhibited a higher capacity than carbonaceous materials such as carbon nanotubes (CNTs) for lead removal (up to 15.5 times higher) [70, 71].

Nano zeolite [72] and modified zeolites [73–75] demonstrated a higher removal capacity of lead than that reported by natural zeolites, even much higher than functionalized reduced graphene oxide (RGO) [71]. The silica nano-zeolite X [72] presented an adsorption capacity of 909.09 mg/g of Pb2+ being 5.5 times higher compared to that reported for natural clinoptilolite (166 mg/g) [69]. The main characteristic of nano zeolites and modified zeolites is a larger surface area and pore size, which make them more efficient and facilitate the adsorption of heavy metals and organic molecules, compared to natural zeolites [76]. The efficiency in the removal of heavy metals and other cations will strongly depend on the high surface area of the zeolites, and in their efficiency in removing the metal ions present in wastewater. The surface area in porous materials is determined by the specific surface area (BET) by standard multipoint techniques of nitrogen adsorption. There are several studies where the largest surface area is for nano zeolites (692 m<sup>2</sup> /g) [76], followed by modified silica natural zeolites (702 m<sup>2</sup> /g) [72]. A smaller surface area is found for the sodium form of clinoptilolite (70.4 m<sup>2</sup> /g) [77], and the smallest area is for the simple natural zeolite clinoptilolite (15.36702 m<sup>2</sup> /g) [78].

#### **4.4 Iron oxides**

*Trace Metals in the Environment - New Approaches and Recent Advances*

The high surface area found on porous materials, such as the silica-based ones, is attributed to the 3D network that displays a hierarchical organization that can be found at different scales (nano, micro, and milli). The nanostructured organization is mainly attributed to the form and size of the pore, which allows the accommodation of other nanostructures, as well as the functionalization of the surface. The methodology to produce nanostructured porous silica is based on the liquid-crystal template mechanism. This allows getting pores around 2–10 nm in size [58]. A surfactant at high concentration is set at a specific pH and temperature values to form a liquid-crystalline phase. Then, the crystalline form may be organized by spherical or rod-shaped micelles arranged on a periodic 3D structure, which serves as a template to be filled with tetraethoxysilane (TEOS) or tetramethylorthosilica (TMOS). Finally, the surfactant is removed to form a mesoporous material [59]. Surface functionalization is a useful tool for the realization of a well-defined set of functions which improves the adsorption properties of silica. It can be carried out by chemical bonds or physical interactions. Silica surface functionalization is of interest due to the multiple options that it offers for heavy metals removal. For example, the thiol-functionalization of mesoporous silica has been proved to exhibit a high affinity for thiophilic heavy-metal ions like Cu2+, Zn2+, Cr3+, and Ni2+ but focused on highly toxic metals such Hg2+. At the same time, the amino functionalization increases the efficiency of the adsorption of Hg2+ [59, 60]. For some specific types of silica materials such as SBA-15 (Santa Barbara Amorphous-15), the imidazole functionalization improves the selective affinity adsorption of Pd2+ and Pt2+, despite the high concentration of Ni2+, Cu2+, and Cd2+ in the mixture [61]. Specific morphologies, such as PMO-like (periodic mesoporous organosilica) or spherical mesostructured, increased the surface area and the particular site of functionalization, consequently, affinity, and selectivity are improved [62, 63].

Zeolites are known as microporous materials with a crystalline structure composed by tetrahedral building blocks of alumina (AlO4) and silica (SiO4) that give rise to a three-dimensional network linked by oxygen atoms [64]. Zeolites are an effective, economical, and eco-friendly option to remove heavy metals and organic contaminants from wastewater [64–66]. There are more than 40 naturally occurring zeolites each with different physicochemical properties. Among their most interesting characteristics are their high chemical stability, molecular sieve, adsorp-

The Si/Al ratio in the structure of a zeolite determines its adsorption and ion exchange capacities, which are directly related to the amount of aluminum contained in the natural zeolite [66], so the isomorphic replacement of Si4+ by Al3+ (aluminum-rich zeolites) will have a more negative charge on the framework, providing a higher affinity for polar molecules. The negative charge is balanced with interchangeable cations (generally sodium, potassium, or calcium), therefore these cations are used for cation exchange processes in solutions containing lead, chromium, or mercury, among others ions [68]. The adsorption capacity of zeolites also depends on the charge density and the diameter of the hydrated ion, which is why zeolites have a great potential to remove heavy metal ions from wastewater [64]. Natural zeolites such as clinoptilolite, mordenite, and chabazite were investigated for the removal of heavy metals [64, 65]. The natural zeolite clinoptilolite has attracted attention because of its abundance. The selectivity of the sodium form of clinoptilolite, extracted from natural deposits, was found to be

**4.2 Silica-based nanostructures**

**174**

**4.3 Zeolites**

tion, and ion exchange capacity [67].

Iron oxide nanostructures are gaining attention for metal removal from water due to their high surface area, excellent adsorption capacities, innocuousness with the environment, and easiness of separation as one can make good use of their magnetic properties. Despite their advantageous characteristics, their use in real scenarios has not been proved. It is essential to explore their removal mechanisms, not only for iron oxides but also for zerovalent iron nanomaterials.

Zerovalent iron nanoparticles (nZVI) are considered a strong reducing agent that is bringing degradation to less harmful substances from a wide range of organic and inorganic pollutants. Sorption of co-precipitation of heavy metals on the surface forms an iron oxide or hydroxide shell when nZVI is exposed to air [79]. The most cited example is the transformation of Cr6+, which precipitates on nZVI with corrosion products. However, this property usually depends on the surface functionalization of the nZVI. Functionalization is important since iron nanoparticles form aggregates very easily and suffer oxidation under acidic and oxygenated environments. The best approach is to coat the nanoparticle surface with Fe3O4 or some polymer to reduce the contact with the environmental oxygen, but maintaining the reactivity [79].

Previous studies on the adsorption of Cr6+ on the surface of Fe3O4 show the formation of a different crystalline structure by chemical adsorption. For chromite (FeCr2O4), the Cr6+ is reduced to Cr3+, followed by the precipitation of Cr3+ onto the F3O4 nanoparticles. Low desorption is indicative of adsorption because desorption is due to physical adsorption, mainly by electrostatic interactions. This adsorption process predominates on γ-Fe2O3, as previously demonstrated by X Ray Photoelectron Spectroscopy (XPS). Another evidence of the physical adsorption is the non-modification of the crystallographic structure, an indication that the removal process was not due to a chemical interaction [80, 81]. The positively charged surface on the iron oxide nanoparticles determines the attraction for negatively charged pollutants; the positive charge of iron oxides depends on the

**Figure 4.**

*The different steps on removal of contaminants from water using magnetic nanoparticles.*

polarization of oxygen atoms on the surface, which can be modified by pH. The successful removal of pollutants such as Cr6+ and As5+ is highly pH-dependent [82].

#### *4.4.1 Recovery and reuse of nanostructures using iron oxides*

Some of the problems faced when iron nanostructures are used in water remediation are related to their capture, recovery, and reuse. Sometimes complicated steps are required. Many authors have proposed to take advantage of the magnetic nanoparticles as an alternative to capture contaminants and nanostructures that could remain in the environment. Goon et al. [45] studied the capture and quantification of cupric ions at trace level using a composite formed by polyethyleneimine (PEI-) coated with Fe3O4 nanoparticles. They captured trace levels (∼2 ppb) of Cu2+. The PEI is amine-rich, so it captures the Cu ions easily, while the magnetite nanoparticles allow the magnetic separation of the material from water. Hu and coworkers [44] used graphene oxide coped Fe3O4 nanoparticles for highly efficient removal of Pb+2.

The particularity of the composite with graphene oxide is that adsorption capacity improved at pH 7, the natural pH value found in a faucet. They also observed that the system graphene oxide/Fe3O4 could be recyclable because it maintains an 80% adsorption capacity after 10 adsorption-desorption cycles. This process can be generalized to the removal, or capture, of any contaminant by the interaction with a specific functionalized nanoparticle followed by its recovery from water (see the scheme in **Figure 4**). Tang and Lo [79] consider that magnetic separation could be a low-cost and a convenient method over the use of a membrane-separation filtration method because the separation of tiny magnetic nanoparticles with the adsorbed heavy metal is easier. The magnetic separation usually occurs with the help of a magnetic field or with a hand-held magnet [79].

### **5. Remediation based on nanobiotechnology**

Natural resources, such as biological systems as microorganisms and plants can also be mixed with nanostructured nanoparticles, usually by cross-linking bonding using a bifunctional reagent; nanoparticles should be inert and biocompatible materials. Even though this methodology is fast, simple, and exhibits an electron transfer, the main disadvantage is the formation of covalent bonds between the functional groups at the outer membranes of the biological living system [18]. Biological systems can be used to directly originate nanomaterials for heavy metal

**177**

*Low Dimensional Nanostructures: Measurement and Remediation Technologies Applied to Trace…*

remediation, but they can also be the active coating on nanostructured materials for similar purposes. Even more, the utilization of microbes for intracellular/extracellular synthesis of nanoparticles with different chemical composition, size/shapes, and controlled monodispersity can be a novel, economically viable, and eco-friendly strategy that can reduce toxic chemicals in the conventional protocol [83]. Studies were conducted on some bacteria to produce an iron sulfide compound, which acts as an adsorbent for several toxic metal ions [84]. Nanoparticles obtained from the plant *Noaea mucronata* were used for the accumulation of heavy metals, such as Pb, Cu, Cd, Zn, Fe, and Ni in groundwater, streams, and rivers [85]. The study conducted on plant species such as *Centaurea virgata*, *Scariola orientalis*, *Noaea mucronata*, *Chenopodium album*, *Cydonia oblonga*, *Reseda lutea*, and *Salix excelsa* revealed that these plants are very good heavy metal accumulators. Specifically, *Noaea mucronata* is a suitable accumulator for Pb to a level higher than 1000 ppm [86]. Biological substrates like bacteria, fungi, algae, yeast, and plant derivatives can be immobilized on nanomaterials or nanoparticles and play an essential role in the retention of metal ions. They offer several advantages, namely biodegradability, natural abundance, low cost, simple production, high surface to volume ratio, and various active sites such as carboxyl, hydroxy, amino, sulfate, or phosphate groups [8]. Besides, immobilized biological substrates have been employed as living and non-living cells. The use of dead bacteria offers the possibility to develop continuous flow systems on different solid supports [87]. The main advantage is that dead microorganisms avoid the risk of contamination of water with bacteria. Some examples were summarized by Escudero et al. [87] in an extensive literature review of biological substrates, which includes composites with biological materials and nanostructures as a green alternative in trace elemental preconcentration and speciation analysis. Such nano-based technology has been proved successful on the laboratory scale, but only a few have been used for small-scale testing or commercialization [88]. Specific studies are summarized in the following lines to exemplify the advantages of the biological substrate-based methods in the removal of heavy metals from water. According to the information reported by Escudero and collaborators, the most employed biological substrates are plant-derivatives, bacteria, and

*DOI: http://dx.doi.org/10.5772/intechopen.93263*

fungi, and they are mainly used for water treatments [87].

Heat inactivated *Fusarium verticillioides* filamentous fungi has been immobilized on nano-silica particles for biosorption of calcium (Ca2+) and magnesium (Mg2+) cations, in helping the preconcentration technique of solid-phase extraction to reduce the hardness of aqueous solutions. Maximum capacities were found to be 1000.0 μmol/g for magnesium and 1333.3 μmol/g for calcium [89]. Lead (Pb) has also been removed using a chromatographic column filled with biomass of this same fungal species immobilized on TiO2 nanoparticles and using hydrochloric acid (HCl) as eluent [90]. Dead coliform bacteria have been immobilized on nanoparticles of titanium oxide (TiO2) for Pb preconcentration which was then analyzed using a flow injection analysis system coupled to a flame atomic absorption spectrometer [91]. The biosorption of aluminum (Al3+) and cadmium (Cd2+) ions over an exopolysaccharide obtained from the bacterium *Lactobacillus rhamnosus* was possible due to the presence of hidroxyl (▬OH) and carboxyl (▬COOH) groups that facilitated a complex formation with the target analytes [92]. The well recognized bacteria *Escherichia coli* was immobilized on multiwalled carbon nanotubes to help in the determination of trace elements such as Cd, Co, Cu, and Ni by flame atomic absorption spectrometry [93]; while the metallothionein of the cyanobacterium genera Synechococcus decorated graphene oxide nanosheets for the selective adsorption of Cd from different waters [94]. Different mechanisms are involved in the extraction of metal ions by biological substrates. They include ion exchange, microprecipitation, complexation,

#### *Low Dimensional Nanostructures: Measurement and Remediation Technologies Applied to Trace… DOI: http://dx.doi.org/10.5772/intechopen.93263*

remediation, but they can also be the active coating on nanostructured materials for similar purposes. Even more, the utilization of microbes for intracellular/extracellular synthesis of nanoparticles with different chemical composition, size/shapes, and controlled monodispersity can be a novel, economically viable, and eco-friendly strategy that can reduce toxic chemicals in the conventional protocol [83]. Studies were conducted on some bacteria to produce an iron sulfide compound, which acts as an adsorbent for several toxic metal ions [84]. Nanoparticles obtained from the plant *Noaea mucronata* were used for the accumulation of heavy metals, such as Pb, Cu, Cd, Zn, Fe, and Ni in groundwater, streams, and rivers [85]. The study conducted on plant species such as *Centaurea virgata*, *Scariola orientalis*, *Noaea mucronata*, *Chenopodium album*, *Cydonia oblonga*, *Reseda lutea*, and *Salix excelsa* revealed that these plants are very good heavy metal accumulators. Specifically, *Noaea mucronata* is a suitable accumulator for Pb to a level higher than 1000 ppm [86].

Biological substrates like bacteria, fungi, algae, yeast, and plant derivatives can be immobilized on nanomaterials or nanoparticles and play an essential role in the retention of metal ions. They offer several advantages, namely biodegradability, natural abundance, low cost, simple production, high surface to volume ratio, and various active sites such as carboxyl, hydroxy, amino, sulfate, or phosphate groups [8]. Besides, immobilized biological substrates have been employed as living and non-living cells. The use of dead bacteria offers the possibility to develop continuous flow systems on different solid supports [87]. The main advantage is that dead microorganisms avoid the risk of contamination of water with bacteria. Some examples were summarized by Escudero et al. [87] in an extensive literature review of biological substrates, which includes composites with biological materials and nanostructures as a green alternative in trace elemental preconcentration and speciation analysis. Such nano-based technology has been proved successful on the laboratory scale, but only a few have been used for small-scale testing or commercialization [88]. Specific studies are summarized in the following lines to exemplify the advantages of the biological substrate-based methods in the removal of heavy metals from water. According to the information reported by Escudero and collaborators, the most employed biological substrates are plant-derivatives, bacteria, and fungi, and they are mainly used for water treatments [87].

Heat inactivated *Fusarium verticillioides* filamentous fungi has been immobilized on nano-silica particles for biosorption of calcium (Ca2+) and magnesium (Mg2+) cations, in helping the preconcentration technique of solid-phase extraction to reduce the hardness of aqueous solutions. Maximum capacities were found to be 1000.0 μmol/g for magnesium and 1333.3 μmol/g for calcium [89]. Lead (Pb) has also been removed using a chromatographic column filled with biomass of this same fungal species immobilized on TiO2 nanoparticles and using hydrochloric acid (HCl) as eluent [90]. Dead coliform bacteria have been immobilized on nanoparticles of titanium oxide (TiO2) for Pb preconcentration which was then analyzed using a flow injection analysis system coupled to a flame atomic absorption spectrometer [91]. The biosorption of aluminum (Al3+) and cadmium (Cd2+) ions over an exopolysaccharide obtained from the bacterium *Lactobacillus rhamnosus* was possible due to the presence of hidroxyl (▬OH) and carboxyl (▬COOH) groups that facilitated a complex formation with the target analytes [92]. The well recognized bacteria *Escherichia coli* was immobilized on multiwalled carbon nanotubes to help in the determination of trace elements such as Cd, Co, Cu, and Ni by flame atomic absorption spectrometry [93]; while the metallothionein of the cyanobacterium genera Synechococcus decorated graphene oxide nanosheets for the selective adsorption of Cd from different waters [94]. Different mechanisms are involved in the extraction of metal ions by biological substrates. They include ion exchange, microprecipitation, complexation,

*Trace Metals in the Environment - New Approaches and Recent Advances*

polarization of oxygen atoms on the surface, which can be modified by pH. The successful removal of pollutants such as Cr6+ and As5+ is highly pH-dependent [82].

*The different steps on removal of contaminants from water using magnetic nanoparticles.*

Some of the problems faced when iron nanostructures are used in water remediation are related to their capture, recovery, and reuse. Sometimes complicated steps are required. Many authors have proposed to take advantage of the magnetic nanoparticles as an alternative to capture contaminants and nanostructures that could remain in the environment. Goon et al. [45] studied the capture and quantification of cupric ions at trace level using a composite formed by polyethyleneimine (PEI-) coated with Fe3O4 nanoparticles. They captured trace levels (∼2 ppb) of Cu2+. The PEI is amine-rich, so it captures the Cu ions easily, while the magnetite nanoparticles allow the magnetic separation of the material from water. Hu and coworkers [44] used graphene oxide coped Fe3O4 nanoparticles for highly efficient

The particularity of the composite with graphene oxide is that adsorption capacity improved at pH 7, the natural pH value found in a faucet. They also observed that the system graphene oxide/Fe3O4 could be recyclable because it maintains an 80% adsorption capacity after 10 adsorption-desorption cycles. This process can be generalized to the removal, or capture, of any contaminant by the interaction with a specific functionalized nanoparticle followed by its recovery from water (see the scheme in **Figure 4**). Tang and Lo [79] consider that magnetic separation could be a low-cost and a convenient method over the use of a membrane-separation filtration method because the separation of tiny magnetic nanoparticles with the adsorbed heavy metal is easier. The magnetic separation usually occurs with the help of a

Natural resources, such as biological systems as microorganisms and plants can also be mixed with nanostructured nanoparticles, usually by cross-linking bonding using a bifunctional reagent; nanoparticles should be inert and biocompatible materials. Even though this methodology is fast, simple, and exhibits an electron transfer, the main disadvantage is the formation of covalent bonds between the functional groups at the outer membranes of the biological living system [18]. Biological systems can be used to directly originate nanomaterials for heavy metal

*4.4.1 Recovery and reuse of nanostructures using iron oxides*

magnetic field or with a hand-held magnet [79].

**5. Remediation based on nanobiotechnology**

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removal of Pb+2.

**Figure 4.**

and oxide-reduction processes whose adequate election depends basically of the target contaminant and the type of sample to be treated.
