**5. Factors impacting phosphate adsorption**

### **5.1 Influence of temperature**

Liu et al. prepared a modified biochar using anaerobic digestion residue by chemical co-precipitation of Mg2+/Fe3+. Pyrolysis was performed at temperatures 500, 600, 700, and 800°C. **Figure 4** shows the effect of pyrolysis temperature on phosphate adsorption capacity. The maximum adsorption capacity on modified biochar is achieved at 600°C and adsorption decreases at 700 and 800°C. This could be explained by the disintegration of the carbon skeleton, the drop in functional groups, and reduction in surface area. It could also be explained by the pores' blockage due to their softening, carbonization, and melting during high pyrolysis temperatures [43].

On the other hand, there is no clear impact of the change of temperature on phosphate adsorption from wastewater. **Figure 5** shows no clear trend on phosphate adsorption with increasing hydrochar temperature for different tested feedstocks (W wood, D digested, and M *Miscanthus*).

## **5.2 Influence of pH**

The influence of pH changes on P adsorption from aqueous solutions and wastewaters varies slightly between studies. Ci et al. [39] used magnesiummodified corn biochar for phosphorus removal from swine wastewater. Their study investigated the impact of initial solution pH on phosphate adsorption. **Figure 6(a)** shows that Mg-impregnated biochar adsorption increases as pH increases from 6 to 10. The adsorption reached its maximum at pH 9 and then dropped at 10. The non-modified biochar is not significantly impacted by the change in pH as it increases from 6 to 10, aside from a slight decrease in phosphate adsorption starting at pH 9. The non-modified biochar adsorption capability relied on the physical structure including the surface area, distribution and quantity of mesoporous structures, and organic functional groups. However,

**79**

**Figure 4.**

**Figure 5.**

between Mg biochar and P. H2PO4

decreases as OH- competes with PO4

tion amount by Mg/biochar is 239 mg/g [39].

6 and below 7.21, while HPO4

*Comparative Evaluation of Hydrochars and Pyrochars for Phosphate Adsorption from Wastewater*

Mg-modified biochar adsorption relies on both chemical and physical properties. P is considered as a ternary acid with the following ionization constants 2.15, 7.20, and 12.33. At an acidic solution environment (pH < 6), there is a low interaction

*The pyrolysis temperature influence on P adsorption on pyrochar and hydrochar adsorption [44].*

*The pyrolysis temperature influence on P adsorption on biochar and Mg/biochar [9].*

and below 9. P adsorption increased with increasing chemical action, while it can decrease as adsorption sites are used. At pH between 9 and 10, P adsorption

On the other hand, Li et al. [2] found that phosphate adsorption continuously decreased when pH moved from 3.0 to 10.9 (**Figure 6(b)**). Other authors also reported that better adsorption is favored at lower pH for some adsorbents with metal oxides [11, 45]. The properties of the biochar and phosphate species distribution can explain the negative impact of the increase in pH on phosphate adsorption. At low pH levels, biochar has a more phosphate adsorption capacity.

<sup>−</sup> is the superior in solution form at a pH above

<sup>3</sup><sup>−</sup> on adsorption sites. The highest P adsorp-

<sup>2</sup><sup>−</sup> is the superior in solution form at a pH above 7.21

*DOI: http://dx.doi.org/10.5772/intechopen.92612*

*Comparative Evaluation of Hydrochars and Pyrochars for Phosphate Adsorption from Wastewater DOI: http://dx.doi.org/10.5772/intechopen.92612*

**Figure 4.** *The pyrolysis temperature influence on P adsorption on biochar and Mg/biochar [9].*

**Figure 5.** *The pyrolysis temperature influence on P adsorption on pyrochar and hydrochar adsorption [44].*

Mg-modified biochar adsorption relies on both chemical and physical properties. P is considered as a ternary acid with the following ionization constants 2.15, 7.20, and 12.33. At an acidic solution environment (pH < 6), there is a low interaction between Mg biochar and P. H2PO4 <sup>−</sup> is the superior in solution form at a pH above 6 and below 7.21, while HPO4 <sup>2</sup><sup>−</sup> is the superior in solution form at a pH above 7.21 and below 9. P adsorption increased with increasing chemical action, while it can decrease as adsorption sites are used. At pH between 9 and 10, P adsorption decreases as OH- competes with PO4 <sup>3</sup><sup>−</sup> on adsorption sites. The highest P adsorption amount by Mg/biochar is 239 mg/g [39].

On the other hand, Li et al. [2] found that phosphate adsorption continuously decreased when pH moved from 3.0 to 10.9 (**Figure 6(b)**). Other authors also reported that better adsorption is favored at lower pH for some adsorbents with metal oxides [11, 45]. The properties of the biochar and phosphate species distribution can explain the negative impact of the increase in pH on phosphate adsorption. At low pH levels, biochar has a more phosphate adsorption capacity.

*Applications of Biochar for Environmental Safety*

through a slow pyrolysis of AlCl3

polluted water.

capacity of 620 mg P/g.

temperatures [43].

**5.2 Influence of pH**

**5.1 Influence of temperature**

**5. Factors impacting phosphate adsorption**

(W wood, D digested, and M *Miscanthus*).

adsorption reaction time. Ming et al. [40] produced a biochar composite material which linked biochar with AlOOH nanoparticles. The biochar was produced

tion of the AlOOH biochar showed a uniform presence of AlOOH particles on biochar surface according to scanning electron microscope (SEM) studies. The Langmuir maximum capacity best describes phosphate adsorption isotherm data to be around 135 mg/g. This makes the biochar/AlOOH nanocomposite a very competitive and efficient adsorbent that can be used in the recovery of phosphate from

Jung et al. [41] used a dried microalga as a feedstock to prepare a Mg-Alassembled biochar. The biochar was prepared using an electro-assisted modification method by dipping the microalgae in MgCl2 solution with MgCl2 acting as an electrolyte. The solution pH was adjusted to 3 using NaOH solutions and 0.5 M H2SO4, and a current density was applied to the solid sample that was pyrolyzed at 600°C for 1 h at 5°C/min rate. This method reported the highest adsorption capacity of 887 mg/g according to the Langmuir-Freundlich model. Same authors [42] used the same electrochemical modification with changes in parameters such as using MgO nanocomposites instead of Mg-Al [41], resulting in a maximum adsorption

Liu et al. prepared a modified biochar using anaerobic digestion residue by chemical co-precipitation of Mg2+/Fe3+. Pyrolysis was performed at temperatures 500, 600, 700, and 800°C. **Figure 4** shows the effect of pyrolysis temperature on phosphate adsorption capacity. The maximum adsorption capacity on modified biochar is achieved at 600°C and adsorption decreases at 700 and 800°C. This could be explained by the disintegration of the carbon skeleton, the drop in functional groups, and reduction in surface area. It could also be explained by the pores' blockage due to their softening, carbonization, and melting during high pyrolysis

On the other hand, there is no clear impact of the change of temperature on phosphate adsorption from wastewater. **Figure 5** shows no clear trend on phosphate adsorption with increasing hydrochar temperature for different tested feedstocks

The influence of pH changes on P adsorption from aqueous solutions and wastewaters varies slightly between studies. Ci et al. [39] used magnesiummodified corn biochar for phosphorus removal from swine wastewater. Their study investigated the impact of initial solution pH on phosphate adsorption. **Figure 6(a)** shows that Mg-impregnated biochar adsorption increases as pH increases from 6 to 10. The adsorption reached its maximum at pH 9 and then dropped at 10. The non-modified biochar is not significantly impacted by the change in pH as it increases from 6 to 10, aside from a slight decrease in phosphate adsorption starting at pH 9. The non-modified biochar adsorption capability relied on the physical structure including the surface area, distribution and quantity of mesoporous structures, and organic functional groups. However,

<sup>−</sup> pretreated biomass at 600°C. The characteriza-

**78**

**Figure 6.**

*The pH influence on P adsorption on biochar [2, 9, 39]. (a) effect of solution pH on the P adsorption of Mg modified corn biochar, (b) effect of solution pH on the P adsorption of Mg modified sugar cane harvest residue biochar, (c) effect of solution pH on the P adsorption of Mg modified anaerobic digestion residue biochar.*

In a study by Li et al [2], phosphate existed in two forms, *HPO*<sup>4</sup> 2− and *H*2 *PO*<sup>4</sup> − over pH from 3 to 10.9. At lower initial pH, Mg and Fe oxides impregnated on biochar react with the solution to become FeOH<sup>+</sup> and MgOH<sup>+</sup> protons which can increase the pH of the solution. Those protons can interact with the anions *HPO*<sup>4</sup> 2− and *H*2 *PO*<sup>4</sup> − in an electrostatic interaction process resulting in better phosphate adsorption. The increasing pH would transform the surface to negatively charged which can cause an electrostatic repulsive interaction between phosphate anions and the

**81**

*Comparative Evaluation of Hydrochars and Pyrochars for Phosphate Adsorption from Wastewater*

surface [46]. A similar effect was observed by Liu et al. [9] who prepared a MgOmodified biochar and studied the pH effect on phosphate adsorption. **Figure 6(c)** shows that adsorption capacity increased from pH 1 to 3 and decreased from 3 to 11. The surface property is directly related to phosphate adsorption at different pH according to Li et al. [2]. As shown in **Figure 4(c)**, phosphate exists in different

<sup>2</sup><sup>−</sup>, and H2PO4

modified biochar in the pH ranges because it has lower free energy which resulting

Biochar has different components: fixed carbon, labile carbon, moisture, volatiles, and ash content. The chemical environment of the carbon in the biochar is changed during the heating process allowing the production of aromatic structures that could resist microbial decomposition. Consequently, there is a stability in biochar C compounds for long periods of time that could reach thousands of years. The biochar skeletal structure consists of different pore size minerals and carbon. Micropores control high adsorption capacity and surface area, while mesopores control liquid-solid adsorption processes and macropores are responsible for the movement of roots, hydrology, aeration, and bulk soil structure. The biochar feedstock and pyrolysis temperature are directly responsible for the pattern and pore size. SEM is used to determine the biochar pore size distribution and morphology. Biochar porous structure is composed of aromatic compounds in addition to functional groups coming from lignin biomass production. This porous structure serves as channels for the flow of nutrients in solutions such as

During pyrolysis, O and H are lost to water followed by the formation of tarrich vapors and hydrocarbons and gases (H2, CO, and CO2) [48]. During pyrolysis, some inorganic compounds volatilize while the major part does not as it takes part of the biochar surface. At low temperatures, N present in biomass, Cl, and k vaporize. At high temperatures, Mg, Si, and Ca are released while Mn, S, P, and Fe are retained in biochar. At pyrolysis temperature higher than 300°C, the biochar cross section appears as graphene sheets. The graphene is described as a polyaromatic, monolayer carbon atom structure produced at temperature 250–550°C, with high breakage resistance, stability index, and electrical conductivity [49]. Aromatic C-containing groups are dominant in biochars produced at temperatures 350°C and above; these are efficient adsorbents for hazardous molecules and heavy metals. P, S, H, N, and O related to the aromatic rings control the biochar electronegativity, which has a big influence on cation exchange capacity. The biochar surface charge contributes to the biochar interaction with its environment (soil,

At pyrolysis above 900°C, biochar surface is deformed as walls separating adjacent pores are destructed causing a widening in the micropores. Moreover, high pyrolysis temperature decreases the amount of volatile matter in the biochar and also its particle size. This results in a higher amount of graphene layers, which leads to an increase in the solid density. Overall, biochar properties depend on parameters such as heating rate, pyrolysis temperature, furnace residence time, and type of pyrolytic reactor of feedstock. Biochar derived from animal manure has more N than plant-derived biochar. On the other hand, plant-derived biochar has a more organized pore structure and was tested as a good-quality fertilizer and good heavy metal adsorbent [50]. The biochar efficiency is impacted when fungi, bacteria, or others enter the pores. The pores get clogged and the biochar adsorption capacity

<sup>−</sup> attached better to a MgO-

<sup>−</sup>, HPO4

*DOI: http://dx.doi.org/10.5772/intechopen.92612*

in higher adsorption in the pH range 2.15–7.21.

**5.3 Characteristics of adsorbed surfaces**

forms including H3PO4, H2PO4

soil solutions [47].

water, organic matter) [47].

decreases leading to the deactivation of biochar [47].

*Comparative Evaluation of Hydrochars and Pyrochars for Phosphate Adsorption from Wastewater DOI: http://dx.doi.org/10.5772/intechopen.92612*

surface [46]. A similar effect was observed by Liu et al. [9] who prepared a MgOmodified biochar and studied the pH effect on phosphate adsorption. **Figure 6(c)** shows that adsorption capacity increased from pH 1 to 3 and decreased from 3 to 11. The surface property is directly related to phosphate adsorption at different pH according to Li et al. [2]. As shown in **Figure 4(c)**, phosphate exists in different forms including H3PO4, H2PO4 <sup>−</sup>, HPO4 <sup>2</sup><sup>−</sup>, and H2PO4 <sup>−</sup> attached better to a MgOmodified biochar in the pH ranges because it has lower free energy which resulting in higher adsorption in the pH range 2.15–7.21.

### **5.3 Characteristics of adsorbed surfaces**

*Applications of Biochar for Environmental Safety*

**80**

*PO*<sup>4</sup> −

**Figure 6.**

In a study by Li et al [2], phosphate existed in two forms, *HPO*<sup>4</sup>

react with the solution to become FeOH<sup>+</sup>

pH from 3 to 10.9. At lower initial pH, Mg and Fe oxides impregnated on biochar

*The pH influence on P adsorption on biochar [2, 9, 39]. (a) effect of solution pH on the P adsorption of Mg modified corn biochar, (b) effect of solution pH on the P adsorption of Mg modified sugar cane harvest residue biochar, (c) effect of solution pH on the P adsorption of Mg modified anaerobic digestion residue biochar.*

 in an electrostatic interaction process resulting in better phosphate adsorption. The increasing pH would transform the surface to negatively charged which can cause an electrostatic repulsive interaction between phosphate anions and the

the pH of the solution. Those protons can interact with the anions *HPO*<sup>4</sup>

and MgOH<sup>+</sup>

2− and *H*2 *PO*<sup>4</sup>

protons which can increase

− over

2− and *H*2

Biochar has different components: fixed carbon, labile carbon, moisture, volatiles, and ash content. The chemical environment of the carbon in the biochar is changed during the heating process allowing the production of aromatic structures that could resist microbial decomposition. Consequently, there is a stability in biochar C compounds for long periods of time that could reach thousands of years. The biochar skeletal structure consists of different pore size minerals and carbon. Micropores control high adsorption capacity and surface area, while mesopores control liquid-solid adsorption processes and macropores are responsible for the movement of roots, hydrology, aeration, and bulk soil structure. The biochar feedstock and pyrolysis temperature are directly responsible for the pattern and pore size. SEM is used to determine the biochar pore size distribution and morphology. Biochar porous structure is composed of aromatic compounds in addition to functional groups coming from lignin biomass production. This porous structure serves as channels for the flow of nutrients in solutions such as soil solutions [47].

During pyrolysis, O and H are lost to water followed by the formation of tarrich vapors and hydrocarbons and gases (H2, CO, and CO2) [48]. During pyrolysis, some inorganic compounds volatilize while the major part does not as it takes part of the biochar surface. At low temperatures, N present in biomass, Cl, and k vaporize. At high temperatures, Mg, Si, and Ca are released while Mn, S, P, and Fe are retained in biochar. At pyrolysis temperature higher than 300°C, the biochar cross section appears as graphene sheets. The graphene is described as a polyaromatic, monolayer carbon atom structure produced at temperature 250–550°C, with high breakage resistance, stability index, and electrical conductivity [49]. Aromatic C-containing groups are dominant in biochars produced at temperatures 350°C and above; these are efficient adsorbents for hazardous molecules and heavy metals. P, S, H, N, and O related to the aromatic rings control the biochar electronegativity, which has a big influence on cation exchange capacity. The biochar surface charge contributes to the biochar interaction with its environment (soil, water, organic matter) [47].

At pyrolysis above 900°C, biochar surface is deformed as walls separating adjacent pores are destructed causing a widening in the micropores. Moreover, high pyrolysis temperature decreases the amount of volatile matter in the biochar and also its particle size. This results in a higher amount of graphene layers, which leads to an increase in the solid density. Overall, biochar properties depend on parameters such as heating rate, pyrolysis temperature, furnace residence time, and type of pyrolytic reactor of feedstock. Biochar derived from animal manure has more N than plant-derived biochar. On the other hand, plant-derived biochar has a more organized pore structure and was tested as a good-quality fertilizer and good heavy metal adsorbent [50]. The biochar efficiency is impacted when fungi, bacteria, or others enter the pores. The pores get clogged and the biochar adsorption capacity decreases leading to the deactivation of biochar [47].

### **6. Adsorption kinetic, isotherm, and thermodynamics**

### **6.1 Adsorption kinetics**

Researchers use adsorption kinetics to study phosphate adsorption over time, in terms of solute uptake rate, considered an important characteristic defining adsorption efficiency. Solute uptake by biochar can be calculated by the difference between the initial and final quantities of the solute (phosphate) concentration in the solution (mg/L) using

$$\mathbf{Q} = \mathbf{V} \left( \mathbf{C}\_0 - \mathbf{C}\_f \right) / \mathbf{M} \tag{4}$$

where M is the mass of the biosorbent (biochar) in g, V is the solution volume (mg/L), and C0-Cf represents the difference between the initial and equilibrium solute concentrations (mg/L) [40, 46, 51–53].

The behavior of biochar can be examined by studying phosphate adsorption kinetics. For that, experimental kinetics are calculated using mathematical models which are listed below; all these models were tested by [54]

$$k\frac{dq\_t}{dt} = k\_1(q\_\varepsilon - q\_t), \text{first - order} \tag{5}$$

$$k\frac{dq\_t}{dt} = k\_2 \left(q\_\epsilon - q\_t\right)^2, \text{second - order} \tag{6}$$

$$\frac{dq\_t}{dt} = k\_\text{n\text{ }} \left(q\_\text{e} - q\_t\right)^\text{N\text{ }}, \text{N\text{ }}\text{ }\text{ }\text{ }order\text{ }\tag{7}$$

$$\frac{dq\_t}{dt} = \alpha \exp\left(-\beta q\_t\right), \text{Elecich} \tag{8}$$

where k1 represents the first-order, k2 is the second-order, and kn is Nth-order apparent adsorption rate constants in (h<sup>−</sup><sup>1</sup> , kg/mg h, and kgNmg−Nh−<sup>1</sup> ). For the Elevich model, α represents the initial adsorption rate (mg/kg), and β denotes the desorption constant (mg/kg). qe characterizes the amount of phosphate adsorbed at equilibrium, and qt is the phosphate adsorbed at time t, in (mg/kg). First-order, second-order, and Nth-order characterize the solid solution kinetics system based on mononuclear, binuclear, and N nuclear adsorption, respectively. The Elevich model is used if the researchers would like to consider desorption in their calculations.

Krishnan et al. [11] used pseudo-second order to study phosphate adsorption on modified coir pith at different initial phosphate solution concentrations over time. **Figure 7** shows that the initial phosphate adsorption rate increases with an increase in phosphate concentration. This can be explained by the increase in covalent interactions of the adsorbent with phosphate H2PO4 <sup>−</sup>. Similar conclusions are also drawn by [55]. Zhang et al. performed kinetic studies using all the described models above and found the best fit to be the first-order model as shown in **Figure 8** [40].

### **6.2 Adsorption isotherms**

The following isotherms are used to simulate biochar phosphate adsorption [1, 40, 46, 52, 53]: *qe* = \_

$$q\_{\varepsilon} = \frac{KQC\_{\varepsilon}}{1 + KC\_{\varepsilon}}, \text{Langmuri} \tag{9}$$

**83**

**Figure 8.**

**Figure 7.**

*initial concentrations [11].*

*Comparative Evaluation of Hydrochars and Pyrochars for Phosphate Adsorption from Wastewater*

where K represents the Langmuir bonding term for energy interactions (L/mg)

enous surface and monolayer adsorption on its surface without molecule interactions, while Freundlich and Langmuir-Freundlich models are empirical equations

*Pseudo-second-order kinetic plots for phosphate adsorption on coir pith iron-modified biochar at different* 

*Adsorption kinetic for phosphate on biochar/AlOOH nanocomposite [40].*

*<sup>n</sup>* Freundlich (10)

kg<sup>−</sup><sup>1</sup>

. Ce symbol-

). Q represents the

*<sup>n</sup>* Langmuir − Freundlich (11)

). The Langmuir model assumes homog-

*qe* = *Kf Ce*

*n* 1 + *KCe*

and Kf represents the Freundlich affinity coefficient in mg(1−n)Ln

izes the equilibrium solution concentration of sorbate (mg L<sup>−</sup><sup>1</sup>

which describe the adsorption on heterogeneous equations.

*qe* = *KQ Ce* \_

*DOI: http://dx.doi.org/10.5772/intechopen.92612*

Langmuir maximum capacity (mg kg<sup>−</sup><sup>1</sup>

*Comparative Evaluation of Hydrochars and Pyrochars for Phosphate Adsorption from Wastewater DOI: http://dx.doi.org/10.5772/intechopen.92612*

$$q\_{\epsilon} = K\_f C\_{\epsilon}^n \text{ Freundlich} \tag{10}$$

$$
\boldsymbol{q}\_{\epsilon} = \boldsymbol{K}\_{\boldsymbol{f}} \mathbf{C}\_{\epsilon}^{n} \text{ Freundlich} \tag{10}
$$

$$
\boldsymbol{q}\_{\epsilon} = \frac{\boldsymbol{K} \boldsymbol{Q} \boldsymbol{C}\_{\epsilon}^{n}}{\mathbf{1} + \boldsymbol{K} \boldsymbol{C}\_{\epsilon}^{n}} \text{ Langgmuir - Freundlich} \tag{11}
$$

where K represents the Langmuir bonding term for energy interactions (L/mg) and Kf represents the Freundlich affinity coefficient in mg(1−n)Ln kg<sup>−</sup><sup>1</sup> . Ce symbolizes the equilibrium solution concentration of sorbate (mg L<sup>−</sup><sup>1</sup> ). Q represents the Langmuir maximum capacity (mg kg<sup>−</sup><sup>1</sup> ). The Langmuir model assumes homogenous surface and monolayer adsorption on its surface without molecule interactions, while Freundlich and Langmuir-Freundlich models are empirical equations which describe the adsorption on heterogeneous equations.

### **Figure 7.**

*Applications of Biochar for Environmental Safety*

solute concentrations (mg/L) [40, 46, 51–53].

which are listed below; all these models were tested by [54] \_ *dqt*

> \_ *dqt dt* = *k*2

\_ *dqt dt* = *k*n

apparent adsorption rate constants in (h<sup>−</sup><sup>1</sup>

actions of the adsorbent with phosphate H2PO4

**6.2 Adsorption isotherms**

[1, 40, 46, 52, 53]:

\_ *dqt*

**6.1 Adsorption kinetics**

the solution (mg/L) using

**6. Adsorption kinetic, isotherm, and thermodynamics**

Researchers use adsorption kinetics to study phosphate adsorption over time, in terms of solute uptake rate, considered an important characteristic defining adsorption efficiency. Solute uptake by biochar can be calculated by the difference between the initial and final quantities of the solute (phosphate) concentration in

where M is the mass of the biosorbent (biochar) in g, V is the solution volume (mg/L), and C0-Cf represents the difference between the initial and equilibrium

The behavior of biochar can be examined by studying phosphate adsorption kinetics. For that, experimental kinetics are calculated using mathematical models

> (*qe* − *qt*) 2

(*qe* − *qt*)

where k1 represents the first-order, k2 is the second-order, and kn is Nth-order

Krishnan et al. [11] used pseudo-second order to study phosphate adsorption on modified coir pith at different initial phosphate solution concentrations over time. **Figure 7** shows that the initial phosphate adsorption rate increases with an increase in phosphate concentration. This can be explained by the increase in covalent inter-

by [55]. Zhang et al. performed kinetic studies using all the described models above

The following isotherms are used to simulate biochar phosphate adsorption

and found the best fit to be the first-order model as shown in **Figure 8** [40].

*qe* = \_ *KQ Ce* 1 + *K Ce*

Elevich model, α represents the initial adsorption rate (mg/kg), and β denotes the desorption constant (mg/kg). qe characterizes the amount of phosphate adsorbed at equilibrium, and qt is the phosphate adsorbed at time t, in (mg/kg). First-order, second-order, and Nth-order characterize the solid solution kinetics system based on mononuclear, binuclear, and N nuclear adsorption, respectively. The Elevich model is used if the researchers would like to consider desorption in their calculations.

*dt* = <sup>α</sup> exp

Q = V (C0 − Cf)/M (4)

*dt* = *k*1(*qe* <sup>−</sup>*qt*),first <sup>−</sup> order (5)

, second − order (6)

N,N\_th − order (7)

(− *qt*),Elevich (8)

<sup>−</sup>. Similar conclusions are also drawn

,Langmuir (9)

). For the

, kg/mg h, and kgNmg−Nh−<sup>1</sup>

**82**

*Pseudo-second-order kinetic plots for phosphate adsorption on coir pith iron-modified biochar at different initial concentrations [11].*

**Figure 8.** *Adsorption kinetic for phosphate on biochar/AlOOH nanocomposite [40].*

Yao et al. [1] used these models to draw adsorption isotherm for phosphate on anaerobically digested sugar beet tailings. **Figure 9** shows that all models reproduced isotherm data correctly with correlation coefficients of 0.95. The highest adsorption capacity is presented by the Langmuir model at 133,085 mg/kg, while Freundlich and Langmuir-Freundlich models gave a better fit to the experimental data. It indicates that phosphate adsorption onto the biochar was determined by heterogeneous processes.

Zhang et al. [40] ran isotherm models of phosphate adsorption on biochar and found that both Freundlich model and Langmuir model described the isotherm data well, while the Freundlich model had a better fit for the data as shown in **Figure 10**. The maximum adsorption capacity was 135,000 mg/kg according to the Langmuir model.

**Figure 9.** *Adsorption isotherm for phosphate on biochar [1].*

**85**

**Author details**

North Carolina, USA

Aicha Slassi Sennou\*, Shuangning Xiu and Abolghasem Shahbazi Biological Engineering Program, Department of Natural Resource and Environmental Design, North Carolina A&T State University, Greensboro,

\*Address all correspondence to: aslassisennou@aggies.ncat.edu

provided the original work is properly cited.

© 2020 The Author(s). Licensee IntechOpen. 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,

*Comparative Evaluation of Hydrochars and Pyrochars for Phosphate Adsorption from Wastewater*

Biomass conversion into pyrochar and hydrochar has seen a growing interest in the last years because of its use in different applications including phosphate adsorption from wastewater. Biochar has economic and sustainability benefits. In this chapter, an overview of hydrochar and pyrochar production techniques in addition to the application of biochar for phosphate adsorption from wastewater is discussed. Biochar needs to have adequate properties to be applied for phosphate adsorption from wastewater. Several factors influence the biochar properties including feedstock, pyrolysis temperature, solution pH, modification techniques, and treatment conditions. Studies have suggested that magnetic biochar has better adsorption properties than non-magnetic biochar. The biochar adsorption mechanisms are explained including ion exchange, electrostatic attraction, and chemical precipitation. Overall, biochar was proven to offer good phosphate adsorption rate along with environmental advantages such as low carbon emissions and renewability. However, further life cycle assessment studies of biochar with an evaluation of its economic benefits and environmental impacts are necessary for long-term

The authors thank the USDA-CSREE-EVANS-ALLEN Project (NCX-272-5-13-

*DOI: http://dx.doi.org/10.5772/intechopen.92612*

**7. Conclusion**

applications.

**Acknowledgements**

130-1) for the financial support.

**Figure 10.** *Adsorption isotherm for phosphate adsorption onto biochar AlOOH nanocomposite [40].*

*Comparative Evaluation of Hydrochars and Pyrochars for Phosphate Adsorption from Wastewater DOI: http://dx.doi.org/10.5772/intechopen.92612*
