# Biochar for Wastewater Treatment--Conversion Technologies and Applications
## 4 Biochar as an Efficient Adsorbent for Organic and Inorganic Pollutants
### Parameters Affecting the Adsorption of Pollutants on Biochar  
#### 4.4.1 Biochar Characteristics  
The sorption of an adsorbate by an adsorbent is controlled by its accessible volume of micropores [146; 147]. Adsorbent materials contain pores of various sizes that are grouped into micropores, mesopores, and macropores based on the width of opening [148]. The size and distribution of pores is largely dependent on the experimental conditions used during biochar production with temperature having the greatest impact [149]. Micropores are reported as the most abundant within the biochar structure and the responsible for its surface area and its high adsorptive capacity. [PERSON] et al. [147] reported that biochar produced by pyrolysis at high temperatures contains a high micropores volume that range between 50%-78% of the total pores. The size of the adsorbate is also an important parameter controlling the sorption rate of the biochar. While larger adsorbate size can cause exclusion or blockage of sorption sites, smaller particle sizes reduce the mass transfer limitation and increase the van der Waal force of penetration of the adsorbate into the adsorbent [150]. The adsorption rate of biochar also depends on types and levels of surface functional groups [120]. The distribution of surface functional groups is determined by the chemical composition of the feedstock, the carbonization method and the temperature of carbonization [31]. [PERSON] et al. [151] compared the properties of biochar and hydrochar obtained from pyrolysis and HTC of pig manure. Results showed that the broad peak at 3400 cm\({}^{-1}\), assigned to -OH stretching vibration in carboxyl and hydroxyl groups, becomes less obvious in biochars compared to the feedstock following the pyrolysis temperature increase. HTC-hydrochars also showed a broad band at 3400 cm\({}^{-1}\) with lower intensity than for the feedstock due to dehydration and decarboxylation reactions occurred during the HTC process. Authors concluded that pyrolysis at high temperature (600 \({}^{\circ}\)C) produced biochars with high aromatic structure, while HTC process at 200-240 \({}^{\circ}\)C during 2 h give favor to biochar with increased amount of aliphatic structures. [PERSON] et al. [120] stated that due to pyrolytic conditions, the O-H, -CH\({}_{2}\), CO=, CC=, and -CH\({}_{3}\) functional groups were altered in biochar, which promotes the biochar hydrophobic interactions. The abundance of oxygen- and nitrogen-containing functional groups determines the hydrophobic nature of biochars, the lower the oxygen- and nitrogen-containing functional groups in the biochar, the more hydrophobic the biochar [152]. Biochars with hydrophobic nature are believed to contribute in the adsorption of insoluble adsorbates, while hydrophylic biochar are reported as less effective due to the sorption of water; the presence of oxygen-containing functional groups on the hydrophilic biochar surface allows the penetration of water via hydrogen-bonding, which leads to a competitiveness between water and adsorbate for the available sites on the biochar surface [153]. In aqueous solutions, the insoluble or less soluble adsorbates are more likely to be adsorbed in the pores of the biochar [153].