# Biochar for Wastewater Treatment--Conversion Technologies and Applications
## 4 Biochar as an Efficient Adsorbent for Organic and Inorganic Pollutants
### Biochar Adsorption Mechanism  
The biochar surface is characterized by its heterogeneity allowing for different sorption mechanisms to be occurred. The adsorption mechanism is depending on the nature of the contaminants and the chemical properties of the adsorbent surface [117]. According to [PERSON] [118], the major routes of adsorption could be roughly divided into physical route where the adsorbate settles on the surface of the adsorbent, the precipitation route where the adsorbent form layers on the adsorbent surface and the porefilling route characterized by the condensation of the adsorbate into the pores of the adsorbent. For organic contaminants, the adsorption process is facilitated by electrostatic attraction, pore-filling, \(\pi\)-\(\pi\) electron-donor acceptor interaction, hydrogen-bonding, complexes adsorption, and hydrophobic interactions (Figure 2). For instance, the sorption of organic pollutants onto biochar by pore-filling is depending on the total micropore and mesopore volumes; the penetration of the pollutant on the biochar structure is more promoted as its ionic radius is low, which leads to an increase in the adsorption capacity of biochar [117; 119]. Soluble pollutants can be attached to hydrophobic biochar when they have hydrophobic functional group or precipitated on alkaline biochar surfaces. The surface of biochar is usually negatively charged, due to the dissociation of oxygen-containing functional groups, which causes electrostatic attraction between biochar and positively charged molecules [119; 120]. For biochar prepared at high temperatures the loss of oxygen-containing and hydrogen-containing functional groups make them less polar and more aromatic and then less appropriate for polar organic contaminants removal. However, the adsorption could occur by hydrogen-bonding promoted as a results of the electrostatic repulsion between negatively charged anionic organic compounds and biochar. The absence of hydrogen-bonding between water and oxygen-functional groups makes more pronounced the penetration of non-polar contaminant to hydrophobic sites [119]. For the removal of inorganic pollutants such as heavy metals, multitude mechanisms could interfere including surface precipitation under alkaline conditions, ion exchange and complexation, cationic and anionic electrostatic attraction (Figure 2). [PERSON] et al. [121] studied the relative contribution of various mechanisms to Pb adsorption on sludge-derived biochar and proposed the following mechanisms: (i) electrostatic complexation due to metal exchange with cations (potassium and sodium) available in the biochar, (ii) co-precipitation and complexation with organic matter and mineral oxides of the biochar, (iii) surface complexation with free carboxyl and hydroxyl functional groups of the biochar, and (iv) surface precipitation as lead-phosphate-silicate (5 PbO.P\({}_{2}\)O\({}_{5}\).SiO\({}_{2}\)).  
Figure 2: The proposed mechanisms for the removal of organic and inorganic contaminants by biochar.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c c c} \hline \hline
**Feedback** & **Propation Conditions** & **Yield** & **SAA** & **VV** & **Ash** & **CEC** & **pH** & \multicolumn{3}{c}{**Elemental Analysis [F3]**} & \multicolumn{3}{c}{**Ogngni Fultants**} & \multicolumn{3}{c}{**PI\({}^{\bullet}\)**} & \multicolumn{3}{c}{**T**} & \multicolumn{1}{c}{**Q\({}_{\text{mag}}\)**} & \multicolumn{1}{c}{**Ref.**} \\  & [T/a] & [m/g] & [m/g] & [m/g] & [T/a] & [m/g] & & & & & & & & & & & & &  
\begin{tabular}{c c c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Feedback**} & \multirow{2}{*}{**Propagation Conditions**} & \multirow{2}{*}{**Type