*2.1.1 Physical activation of biochar*

Physical activation methods such as steam activation involve high-temperature steam forced through the pores of the biochar. Steam activation, which is carried out after pyrolysis, is a common modification method used to increase the structural porosity of the biochar and remove impurities such as products of incomplete combustion. According to [44], higher water flow rates and longer activation times at 800°C increased the sorption of Cd, Cu, and Zn on the surface of biochar from poultry manure feedstocks pyrolyzed at 700°C. In another study, comparison of Cu2+ adsorption for biochar from *Miscanthus* before (500°C pyrolysis) and after (800°C) steam activation showed no significant change [45]. It was found that steam activation of the biochar increased the surface area and aromaticity alongside a decrease in the abundance of functional groups [45]. Similarly, steam-activated biochar from pine sawdust increased the surface area but had little effect on the surface functional group as a result of which adsorption capacity of biochar for phosphate was reduced due to electrostatic repulsion by the negatively charged surface of biochar [46]. The steam-activated invasive plant (*Sicyos angulatus* L.)-derived biochar produced at 700°C showed 55% increase in sorption capacity of veterinary antibiotics (sulfamethazine) compared to that of nonactivated biochar produced at the same temperature [47]. Hence, steam activation could be a process for increasing the porosity and surface area of biochar along with aromaticity to obtain better adsorption of inorganic material in the wastewater.

### *2.1.2 Chemical activation using acidic and alkaline solutions*

The biochar activation using acidic solutions forms carboxylic groups on the biochar surface [48] and develops micropores, thus increasing the surface area [49]. The increase of oxygenated functional groups on biochar surfaces increases the potential of biochar to bind positively charged pollutants through specific adsorption chemically. The pH dependence of Cu2+ sorption capacity for HNO3-activated cactus fiber biochar indicated chemical sorption on oxygen-containing functional groups on the biochar surface [48]. Higher O/C ratio in the post-activation of rice straw with H2SO4 and HNO3 showed evidence of oxygen-containing functional group incorporated into the carbon structure [50]. Acid treatment of pine tree sawdust with diluted H3PO4 prior to pyrolysis increased the surface area, the total pore volume, and volume of micropores area along with P-O-P incorporation in the C structure [51]. This increased the Pb sorption capacity of the phosphoric-treated biochar by 20% in comparison to a nontreated sample, mainly due to phosphate precipitation and surface adsorption [51]. Similarly, almost double increase in cation exchange capacity was observed for pinewood biochar treated with 30% H2O2 because the oxygen-containing functional groups in the surface of biochar,

**225**

the increased PO4

tion with microorganisms [65].

NO3

<sup>−</sup>, PO4

*Biochar-Assisted Wastewater Treatment and Waste Valorization*

<sup>2</sup><sup>−</sup>, PO4

area and altercation of the functional group at the surface.

which were more abundant in the activated biochar, exchanged with cations in solution [52]. Treating a hydrochar, a carbon-enriched solid produced from hydrothermal carbonization of peanut hull, with a 10% H2O2 solution increased Pb sorption capacity compared to the unmodified hydrochar, which can be attributed to a greater abundance of carboxyl functional groups that can form complexes with Pb [53]. However, the introduction of acid or oxidizing agents dissolves mineral

the biochar matrix. These minerals in biochar are particularly important for the removal of metal cations from water due to precipitation [54], the affinity of which

Activation of biochar using alkali (most commonly KOH and NaOH) increases adsorption by increasing porosity, surface and oxygenated functional group at the surface. Oxygenated functional groups provide proton-donating exchange sites where cation such as Pb2+ adsorbs chemically [55]. The activation of ipomoea plant biochar with KOH, followed by pyrolysis (350–550°C) demonstrated an increase adsorption of Cd from aqueous solution [56]. Further evidence of kinetics of sorption fitting a pseudo-second-order model and thermodynamic studies indicating spontaneous endothermic process showed that Cu sorption on KOH-activated biochar was due to chemical adsorption [57]. The adsorption capacity of As(V) on municipal solid waste biochar was increased by 1.3 times after activation with 2 M KOH [58]. It can be concluded that activation by alkali greatly enhanced the surface

The biochar composites are prepared by embedding different materials into the biochar structure pre- or post-pyrolysis. Generally, biochar has a higher surface area, high pH, and a negative surface charge. This facilitates specific adsorption of metal ions via oxygenated functional groups, electrostatic attraction to aromatic groups, and precipitation on the mineral ash components of the biochar. But at the same time biochar is usually a poor adsorbent for oxy-anions contaminants like

metal oxide on biochar surfaces. It can be done by soaking biochars or the feedstocks in a solution of metal nitrate or chloride salt solution (common examples FeCl3, Fe, Fe(NO3)3, and MgCl2) and heated under atmospheric condition within a temperature range of 50–300°C. This process ensures removal of nitrite and chlorine leaving behind metals in the biochar matrix. Ca-, FeO-, and Fe3+-modified biochar from soaked rice husk and municipal biomass in CaO, iron powder, and FeCl3 respectively, increased the capability of biochar to remove As(V), but not as high for Cr(VI), from aqueous solution [59]. Taking into consideration that one of the main mechanisms for Cr(VI) removal is the electrostatic interaction to the positively charged functional groups on the surface of adsorbents, high Cr(VI) removal is observed at low pH values [60]. It is rather possible that the high pH values of the RH-Ca2þ, RH-Fe0, and SW-Fe0 solutions are related to the deprotonation of their functional groups and the repelling of the negatively charged Cr(VI) [60]. Similarly, a 20-time increase in the sorption of As(V) was observed when corncob biochar was modified with Fe(NO3)3 [61]. Despite the lower surface area, modification of biochars from garden wood waste and wood chips as well as corncob showed

<sup>3</sup><sup>−</sup> [44]. This can be improved by the homogenous spread of

<sup>3</sup><sup>−</sup> sorption by a factor of 12–50% [58]. Further research has been

carried out for preparing biochar-based composites by impregnation or coating the surface of the biochar with metal oxides of Al, Mn, and Mg [58]; clay minerals [62]; complex organic compounds, such as chitosan [63] or amino acids [64]; or inocula-

<sup>3</sup><sup>−</sup>) in the biochar structure and removes them from

*DOI: http://dx.doi.org/10.5772/intechopen.92288*

<sup>2</sup><sup>−</sup>, SiO4

could be reduced by the acid treatment.

*2.1.3 Biochar-based composites*

<sup>3</sup><sup>−</sup>, and AsO4

components (CO3

*Applications of Biochar for Environmental Safety*

lowing sections.

biofilms [43].

in the wastewater.

*2.1.2 Chemical activation using acidic and alkaline solutions*

The biochar activation using acidic solutions forms carboxylic groups on the biochar surface [48] and develops micropores, thus increasing the surface area [49]. The increase of oxygenated functional groups on biochar surfaces increases the potential of biochar to bind positively charged pollutants through specific adsorption chemically. The pH dependence of Cu2+ sorption capacity for HNO3-activated cactus fiber biochar indicated chemical sorption on oxygen-containing functional groups on the biochar surface [48]. Higher O/C ratio in the post-activation of rice straw with H2SO4 and HNO3 showed evidence of oxygen-containing functional group incorporated into the carbon structure [50]. Acid treatment of pine tree sawdust with diluted H3PO4 prior to pyrolysis increased the surface area, the total pore volume, and volume of micropores area along with P-O-P incorporation in the C structure [51]. This increased the Pb sorption capacity of the phosphoric-treated biochar by 20% in comparison to a nontreated sample, mainly due to phosphate precipitation and surface adsorption [51]. Similarly, almost double increase in cation exchange capacity was observed for pinewood biochar treated with 30% H2O2 because the oxygen-containing functional groups in the surface of biochar,

**2.1 Biochar modification**

*2.1.1 Physical activation of biochar*

peroxide, alkali or acid, and impregnation/coating with chemicals [41]. The detail about the modified biochar for wastewater treatment will be discussed in the fol-

Researchers have discussed several methods for modifying the properties of biochar [42]. These methodologies include treatments with steam, acids, bases, metal oxides, carbonaceous materials, clay minerals, organic compounds, and

Physical activation methods such as steam activation involve high-temperature

steam forced through the pores of the biochar. Steam activation, which is carried out after pyrolysis, is a common modification method used to increase the structural porosity of the biochar and remove impurities such as products of incomplete combustion. According to [44], higher water flow rates and longer activation times at 800°C increased the sorption of Cd, Cu, and Zn on the surface of biochar from poultry manure feedstocks pyrolyzed at 700°C. In another study, comparison of Cu2+ adsorption for biochar from *Miscanthus* before (500°C pyrolysis) and after (800°C) steam activation showed no significant change [45]. It was found that steam activation of the biochar increased the surface area and aromaticity alongside a decrease in the abundance of functional groups [45]. Similarly, steam-activated biochar from pine sawdust increased the surface area but had little effect on the surface functional group as a result of which adsorption capacity of biochar for phosphate was reduced due to electrostatic repulsion by the negatively charged surface of biochar [46]. The steam-activated invasive plant (*Sicyos angulatus* L.)-derived biochar produced at 700°C showed 55% increase in sorption capacity of veterinary antibiotics (sulfamethazine) compared to that of nonactivated biochar produced at the same temperature [47]. Hence, steam activation could be a process for increasing the porosity and surface area of biochar along with aromaticity to obtain better adsorption of inorganic material

**224**

which were more abundant in the activated biochar, exchanged with cations in solution [52]. Treating a hydrochar, a carbon-enriched solid produced from hydrothermal carbonization of peanut hull, with a 10% H2O2 solution increased Pb sorption capacity compared to the unmodified hydrochar, which can be attributed to a greater abundance of carboxyl functional groups that can form complexes with Pb [53]. However, the introduction of acid or oxidizing agents dissolves mineral components (CO3 <sup>2</sup><sup>−</sup>, SiO4 <sup>2</sup><sup>−</sup>, PO4 <sup>3</sup><sup>−</sup>) in the biochar structure and removes them from the biochar matrix. These minerals in biochar are particularly important for the removal of metal cations from water due to precipitation [54], the affinity of which could be reduced by the acid treatment.

Activation of biochar using alkali (most commonly KOH and NaOH) increases adsorption by increasing porosity, surface and oxygenated functional group at the surface. Oxygenated functional groups provide proton-donating exchange sites where cation such as Pb2+ adsorbs chemically [55]. The activation of ipomoea plant biochar with KOH, followed by pyrolysis (350–550°C) demonstrated an increase adsorption of Cd from aqueous solution [56]. Further evidence of kinetics of sorption fitting a pseudo-second-order model and thermodynamic studies indicating spontaneous endothermic process showed that Cu sorption on KOH-activated biochar was due to chemical adsorption [57]. The adsorption capacity of As(V) on municipal solid waste biochar was increased by 1.3 times after activation with 2 M KOH [58]. It can be concluded that activation by alkali greatly enhanced the surface area and altercation of the functional group at the surface.

## *2.1.3 Biochar-based composites*

The biochar composites are prepared by embedding different materials into the biochar structure pre- or post-pyrolysis. Generally, biochar has a higher surface area, high pH, and a negative surface charge. This facilitates specific adsorption of metal ions via oxygenated functional groups, electrostatic attraction to aromatic groups, and precipitation on the mineral ash components of the biochar. But at the same time biochar is usually a poor adsorbent for oxy-anions contaminants like NO3 <sup>−</sup>, PO4 <sup>3</sup><sup>−</sup>, and AsO4 <sup>3</sup><sup>−</sup> [44]. This can be improved by the homogenous spread of metal oxide on biochar surfaces. It can be done by soaking biochars or the feedstocks in a solution of metal nitrate or chloride salt solution (common examples FeCl3, Fe, Fe(NO3)3, and MgCl2) and heated under atmospheric condition within a temperature range of 50–300°C. This process ensures removal of nitrite and chlorine leaving behind metals in the biochar matrix. Ca-, FeO-, and Fe3+-modified biochar from soaked rice husk and municipal biomass in CaO, iron powder, and FeCl3 respectively, increased the capability of biochar to remove As(V), but not as high for Cr(VI), from aqueous solution [59]. Taking into consideration that one of the main mechanisms for Cr(VI) removal is the electrostatic interaction to the positively charged functional groups on the surface of adsorbents, high Cr(VI) removal is observed at low pH values [60]. It is rather possible that the high pH values of the RH-Ca2þ, RH-Fe0, and SW-Fe0 solutions are related to the deprotonation of their functional groups and the repelling of the negatively charged Cr(VI) [60]. Similarly, a 20-time increase in the sorption of As(V) was observed when corncob biochar was modified with Fe(NO3)3 [61]. Despite the lower surface area, modification of biochars from garden wood waste and wood chips as well as corncob showed the increased PO4 <sup>3</sup><sup>−</sup> sorption by a factor of 12–50% [58]. Further research has been carried out for preparing biochar-based composites by impregnation or coating the surface of the biochar with metal oxides of Al, Mn, and Mg [58]; clay minerals [62]; complex organic compounds, such as chitosan [63] or amino acids [64]; or inoculation with microorganisms [65].

Thus, the selection of biochar and modification methods for the application in wastewater treatment requires a considerable understanding of the biochar properties and mechanism by which it supports the treatment process at different stages of wastewater treatment.
