# Biochar for Wastewater Treatment--Conversion Technologies and Applications
## 4 Biochar as an Efficient Adsorbent for Organic and Inorganic Pollutants
### Heavy Metals  
Recent studies showed that biochar offers an excellent ability to remove inorganic pollutants such as heavy metals from wastewater (Table 3). It has been widely recognized that the adsorption capacity of biochar toward heavy metals largely depends on biochar characteristics and the nature of the target metals [102; 103; 104; 105]. Biochar can be an effective material for the sorption of heavy metals due to the abundance of functional groups on its surface such as phenolic, hydroxyl, and carboxyl groups, and due to its porous structure and its large surface area [106; 107]. [PERSON] et al. [108] compared between straws of canola, rice, soybean and peanut derived biochars for the adsorption of trivalent chromium. The adsorption capacities were consistent with the content of acidic functional groups of the biochars and vary in the following order peanut > soybean > canola > rice. In another study performed by [PERSON] et al. [107], the properties of biochars from rice straw, chicken manure, and sewage sludge and their effect on the removal efficiency of Pb\({}^{2+}\) and Zn\({}^{2+}\) were investigated. The highest carbon and hydrogen contents, the greatest number of functional groups (e.g., O-H and C=C/C=O), the highest pH, the most negative surface charge, and the highest physical stability were exhibited by the rice straw biochar, which also showed the highest adsorption capacity toward Pb\({}^{2+}\). Precipitation and ion exchange reactions have been widely considered as possible mechanisms for heavy metals adsorption. [PERSON] et al. [109] testing rice straw biochar for the removal of Cu\({}^{2+}\) and Zn\({}^{2+}\) reported that ion exchange of native cations with Cu\({}^{2+}\) and Zn\({}^{2+}\) cations is the dominant mechanism controlling the adsorption. [PERSON] et al. [110] separated biochar into organic and inorganic fractions and tested their capacity in removing Pb. The Pb adsorption capacity of the inorganic fraction was higher than 300 mg/g, while the adsorption was only 1 mg/g for the organic fraction; this was reported to be due to the dominance of the cation exchange and Pb precipitation during the adsorption process, while Pb complexation with organic functional groups was limited [110]. In another study performed by [PERSON] and [PERSON] [111], rice straw-derived biochar was tested for the adsorption of Cd\({}^{2+}\). The results showed that minerals on the surface of the biochar containing Mg and Ca exchanged preliminary their cations with Cd\({}^{2+}\), which then form a precipitate within the biochar structure. In comparison to other conventional adsorbents such as activated carbon, biochar could be a promising substitute for the adsorption of heavy metals. [PERSON] et al. [112] reported that in spite of the lower surface area of biochar produced by gasification (115.5 m\({}^{2}\)/g) in comparison to commercial activated carbon (759.8 m\({}^{2}\)/g), the biochar removed more efficiently heavy metal ions from aqueous solutions than activated carbon. This was reported to be due to the type and the high amount of oxygen-functional groups on the biochar surface. Activation of biochar has been found to significantly improve its adsorption capacity toward heavy metals. Potassium hydroxide activated-biochar allowed for more than three-fold increase in the adsorption capacity of Cu\({}^{2+}\) as compared to no-activated biochar [103]. Hydrochar from HTC of hickory and peanut hull was also physically activated by carbon dioxide, which significantly enhanced its adsorption capacity toward Pb\({}^{2+}\), Cu\({}^{2+}\), and Cd\({}^{2+}\). This was reported to be due to the improvement of the specific area of the hydrochar and to the introduction of acidic functional groups to its surface following the CO\({}_{2}\) activation which promoted the electrostatic attractions [113]. [PERSON] et al. [114] studied the adsorption of AsV and AsIII by Fe-coated biochars from empty fruit bunch and rice husk.