# Biochar for Wastewater Treatment--Conversion Technologies and Applications
## 3 Modification of Biochar
giving favor to highly porous activated biochar showing a honeycomb with various sizes. Acid modification can also change the surface area and the pore structure of the biochar depending on the type and concentration of the acid [88]. [PERSON] et al. [86] found that the use of 1 M of hydrochloric acid increased the surface area of biochar from 58.8 m\({}^{2}\)/g to 88.4 m\({}^{2}\)/g. While the modification of biochar by a combination of 30% sulfuric acid and oxalic acid induces a significant increase in the surface area from 2.3 m\({}^{2}\)/g to 571 m\({}^{2}\)/g [89]. Alkaline modification increases the surface area and the oxygen-containing functional groups on the surface of biochar. In the study carried out by [PERSON] et al. [28], authors activated biochar prepared from a mixture of olive solid wastes and olive mill wastewater using potassium hydroxide as an activating agent. The activated biochar showed high surface area (1375 m\({}^{2}\)/g) and micropore volume (0.52 cm\({}^{3}\)/g) and exhibited high adsorption capacity toward indigo carmine (599 mg/g). In another study, [PERSON] et al. [90] modified municipal solid wastes-derived biochar by potassium hydroxide and showed an increase in the surface area from 14.4 m\({}^{2}\)/g to 49.1 m\({}^{2}\)/g and in the oxygen-containing functional groups on the biochar surface, which resulted in an enhancement of the As(V) removal efficiency. Contrary, a decrease in the surface area from 4.4 m\({}^{2}\)/g to 0.69 m\({}^{2}\)/g was reported by [PERSON] and [PERSON] [91] by using potassium hydroxide for the modification of wheat-straw-derived hydrochar. As compared to potassium hydroxide, sodium hydroxide modification of coconut-derived biochar significantly increased its surface area up to 2885 m\({}^{2}\)/g, which was higher than that reported for coconut shell derived biochar modified by potassium hydroxide (1940 m\({}^{2}\)/g) [92]. The characteristics of the biochar modified by alkaline agents is strongly dependent on the types of feedstock and the preparation method. The ratio between base and biochar was also reported as significant factor affecting the properties of biochar [91]. In comparison between acid and basic treatments, [PERSON] et al. [93] studied the effect of different agents (phosphoric acid, sulfuric acid, sodium hydroxide and potassium carbonate) on the properties of pork bone char. Results showed that sulfuric acid treatment produced biochar with high microporosity and the treatment with sodium hydroxide and potassium carbonate increased more the amounts of micropores and mesopores within biochar structure. While the effect of phosphoric acid on biochar was extremely aggressive and leads to a destruction of biochar pore structure. Other than acid and basic modification, the modification using oxidizing agents such as hydrogen peroxide and potassium permanganate was also reported to increase the content of oxygen-containing functional groups on the biochar. [PERSON] et al. [94] treated a hydrothermally produced hydrochar from peanut hull with hydrogen peroxide. The results showed an increase in the oxygen-containing functional groups on the hydrochar surfaces, while no enhancement in the surface area was observed, as also reported by [PERSON] et al. [95]. Contrary, the modification of hickory-wood-derived biochar by potassium permanganate enhanced its surface area from 101 m\({}^{2}\)/g to 205 m\({}^{2}\)/g and then enhanced its adsorption capacity toward Pb, Cu, and Cd [96]. The modification of biochar by organic solvents was also used. [PERSON] et al. [90] modified a municipal solid waste-derived biochar by methanol and reported an esterification between the carbonyl groups and the biochar, which resulted in a significant enhancement of adsorption capacity toward tetracycline [90]. However, the practical application of organic solvent could be limited by its high cost and its volatile characteristic.