# Biochar for Wastewater Treatment--Conversion Technologies and Applications
## 3 Modification of Biochar  
Owing to the little functional groups and the small surface area and pore volume exhibited by the biochar and the fact that in case of dry pyrolysis, the pores formed during carbonization are plugged with tarred material, a subsequent chemical or physical treatment is required to improve the properties of specific surface area, pore volume, and pore structure of the biochar for its subsequent use in different environmental applications. During physical modification, biochar is subjected to a controlled gasification at high temperature and under activation atmosphere. Significant changes in textural characteristics including changes in surface area, pores volume and pores distribution and in surface chemical properties including surface functional groups, hydrophobicity, and polarity were observed after physical activation [80,81]. The widely oxidizing agent used for the biochar modification using the physical method is water steam. Chemical reactions involved during steam activation can be expressed as follows [82] (Equations (1) and (2)):  
\[\text{C}+\text{H}_{2}\text{O}==>\text{CO}+\text{H}_{2}\,,\text{H}=117\text{ KJ}/\text{mol} \tag{1}\]  
\[\text{CO}+\text{H}_{2}\text{O}==>\text{CO}_{2}+\text{H}_{2}\,,\text{H}=41\text{ KJ}/\text{mol} \tag{2}\]  
The steam activation process can selectively eliminate most reactive carbon atoms from the original biochar and then generate porosity and surface area. The reaction between steam and carbon induces an elimination of volatile materials and a decomposition of tar, which leads to the development of new micropores and the further widening of existing pores [83]. This tendency is more pronounced as the temperature increases [83]. However, at high temperatures, a significant decrease in mass yield was also observed, which does not allows the process to be economically favorable. Prolonged residence time leads to the improvement of surface area and porous structure until a limit where further increase may induce a widening of existing pores with little new pore formation, which causes the reduction in surface area and pore volume. Steam flow rate, the nature of the precursor, and its composition could also be considered as factors effecting steam activation, but there still is a luck in information to figure out their in-depth influence mechanisms. The activation with steam generally favors carbon microporosity and leads to a final activated biochar with a well-developed microporosity with a very small contribution of mesoporous [82]. Similar to steam modification, gas purging modification can also be applied to improve the structure of biochar and increase its surface area, thus enhancing its adsorption capacity. It consists of a first pyrolysis step of feedstock followed by a purging step of the biochar by carbon dioxide or ammonia gas. Carbon dioxide modification can promote the microporous structure of biochar, while ammonia gas modification introduces nitrogen-containing groups on the biochar [84]. The modification of cotton-stalk-derived biochar by a mixture of carbon dioxide and ammonia gas was reported by [PERSON] et al. [85] as more efficient to increase the biochar surface area compared with single modification using carbon dioxide or ammonia gas. During chemical activation, biochar is impregnated with chemical agents such as acids (hydrochloric acid, nitric acid, sulfuric acid, and phosphoric acid), bases (potassium hydroxide, sodium hydroxide, and potassium carbonate), and oxidants (hydrogen peroxide and potassium permanganate). The acid modification aimed mainly to introduce acid functional groups on the surface of biochar. [PERSON] et al. [86] modified reed-derived biochar by 1 M hydrochloric acid and reported an enrichment of hydrophobic adsorption sites for the adsorption of pentachlorophenol. Equally, [PERSON] et al. [87] reported that the treatment of kenaf fiber biochar by hydrochloric acid resulted in the increase of surface area of 289.5-346.6 m\({}^{2}\)/g.