# Preparation and Modification of Biochar Materials and their Application in Soil Remediation
## 3 Removal Mechanism of Major Pollutants by Biochar
### Complexation  
Complexation refers to the interaction between oxygen-containing functional groups on the surface of biochar and heavy metals to form complexes, which could be fixed. [PERSON] et al. [93] studied the aluminum phytotoxicity of cow manure biochar to wheat and concluded that the adsorption ofaluminum by biochar was mainly through the complexation of carboxyl group with [Al(OH)]\({}^{2+}\) and its monomer surface, rather than through the electrostatic attraction of Al\({}^{3+}\) with negative charge sites. [PERSON] et al. [94] believed that the adsorption of oxytetracycline by biochar was mainly mediated by \(\pi\)-\(\pi\) interaction and metal bridge, with surface complexation as the main factor, and cationic exchange might exist. In the process of adsorption, it is often not a single mechanism, but a combination of multiple adsorption mechanisms. Table 3 summarizes the adsorption mechanism of biochar for pollution restoration. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Raw Material** & **Pollutat** & **Mechanism** & **Reference** \\ \hline Municipal sewage sludge & Cd & Surface precipitation under alkaline conditions and exchange of exchangeable cations with Cd. & [95] \\ Fertillizer & Cu, Zn and Cd & Precipitate from CO\({}_{3}\)\({}^{-2}\), VO\({}_{3}\)\({}^{-3}\) on the surface of the biochar, partially by surface & [92] \\  & & complexing with Off group or delocalized \(\epsilon\) electron. & \\ Rice husk loaded with & Oxide spherical complexes and biochar surface oxygen complexes; the \(\pi\)-band electron & density of graphene-based carbon in the \(\pi\)-electron cloud system reduces vacancies on the surface of biocaffe, thereby adsorbing Pb\({}^{2+}\). & [96] \\ Wheat straw, pine needles & Zn & The components of -OH, CO\({}_{2}\)\({}^{-2}\), and Si in biochar can form precipitates with Zn\({}^{2+}\). & [97] \\  & Electron-donor-acceptor (EDA) interaction with pH \(<\) 2.0, also forms charge-assisted hydrogen bonds (CAHB) and hydrogen bonds at pH 4.0-4.5, and interaction with CAHB and EDA at pH \(>\) 7.0 & [98] \\  & & After Na-S modification, sulfur impregnated onto the biochar exactly with Hg(II) to form Hg\({}_{5}\), which greatly facilitated the sorption of HgII. Formation of surface complexes between Hg(II) and the functional groups of sorbent, such as phenolic hydroxy, carbonyling groups, these oxygen-containing functional groups exchanged ion with Hg(II). The electrostatic and EDA interaction also participated in Hg(II) sorption. & [51] \\  & & Because of the easy hydrolysis of Pb at low pH, biochar has a higher affinity for Pb than Cd. Besides, precipitation as carbonate minerals (2 PPCOs), Pb(OH)\({}_{2}\) and CodecO\({}_{3}\)) and composition with functional groups such as carboxyl and hydroxyl, were also important for adsorption of Pb and Cd by biochar. & [79] \\ Rice straw, swine manure & Tetracycline (TC) & The H-bonding, electrostatic attraction and EDA interaction might be the primary mechanism during adsorption process. & [48] \\  & & First, SSF bipolar reduced Cr(V) to Cr(III) by electrostatic adsorption. Second, with the participation of hydrogen ions and the electron donors from SBT biochar, Cr(VI) was reduced to Cr(III). Then, the function groups on the SBT biochar complexed with CF(III). & [99] \\ Empty fruit bunch, rice husk & As(III), As(V) & Surface complexes were formed between As(III) and As(V) and the functional groups (hydroxy, carboxyl), and C-O'e of alcohols) of the two biocathers. & [100,101] \\  & & The sorption of neutral sulfonamide species occurred mainly due to H-bonds followed by EDA, and by Lewis acid-base interaction. EDA was the main mechanism for the sorption of positive sulfonamides species. The sorption of negative species was mainly due to proton exchange with water forming negative CAHB, followed by the neutralization of -OH groups by H\({}^{+}\) released from functionalized biochar surface, and \(\pi\)-\(\pi\)electron-acceptor (EAA) interaction. & [102] \\ \hline \hline \end{tabular}
\end{table}