# Effects and Mechanisms of Microbial Remediation of Heavy Metals in Soil: A Critical Review
## 4 Microbial Remediation of Heavy Metal-Contaminated Soil
### Remediation Mechanisms
#### 4.1.1 Biosorption
\begin{table}
\begin{tabular}{c c c c} \hline
**Type** & **Method** & **Application** & **Advantages (+) and Disadvantages (+)** \\ \hline \hline \multirow{8}{*}{Physical chemistry repair} & \multirow{8}{*}{Electro-thermal methods} & \multirow{8}{*}{Volatile heavy metals (e.g., Hg)} & (*) Economically feasible [28] \\  & & & (*) Does not stir the soil [2] \\  & & & (*) Shortens repair time [34] \\  & & & (*) Poor conductivity of high permeability sandy soil reduces effectiveness [31] \\ \cline{1-1}  & & & (*) Efficient [50] \\ \cline{1-1}  & & & (*) Can fundamentally eliminate soil heavy metal pollution [31; 34] \\ \cline{1-1}  & & & (*) Easy to destroy organic matter and water in soil [31] \\ \cline{1-1}  & & & (*) Consumes a large amount of energy [31] \\ \cline{1-1} \cline{2-3} Soil leaching & \multirow{2}{*}{Small area of severely polluted soil} & (*) Can cause some leaching and precipitation of some nutrients [31] \\ \cline{1-1} \cline{2-3}  & & & \\ \end{tabular}
\end{table} 
Table 1: _Cont._ \begin{table}
\begin{tabular}{c c c c} \hline
**Type** & **Condition** & **Mechanism** & **Example** \\ \hline Surface precipitation & Edge charges on adsorbent & & 1. & Pb precipitates in phosphoogypsum and red gypsum minerals to form stable lead sulfate minerals [35]. \\  & Edge charges on adsorbent & & stability in soil solid phase components, reduces migration and bioavailability & 2. & Phosphate formed by phosphate precipitated on the surface of soil plant roots passivated Pb [36]. \\ \hline Ion exchange & Other metal ions existing & The metal ions bound by the cell material are combined by other metal ions with stronger binding ability. & 1. & Inactive _Agrobacteriumrium rhizems_ adsorbs & Pb, Cu and releases Ca, Mg [37]. \\  & & & ions with stronger binding ability. & 2. \\ \hline Ligand exchange & There are organic functional groups, such as R-COOH, R-SH & Metal ions and ligands are bonded to the surface of the adsorbent by covalent or ionic bonding & 1. & Cr is reduced by organic matter or iron reducing substances [37]. \\  & & & 2. & _Aruscular mycorrhizal_ production of polysaccharide substance ligands chelated with heavy metals to form stable complexes [38]. \\ \hline Diffusion and chemical & Manganese oxide & Reduction of toxicity of heavy metal ions & 1. & Cr is reduced by organic matter or iron reducing substances [35]. \\ modification of adsorbent surfaces & Iron oxide & & by chemical transfer between heavy metal ions and chemical modification \\  & Zoolite & & metal ions and chemical modification \\ \hline \end{tabular}
\end{table} 
Table 2: Comparison of overall mechanisms of sorption. Heavy metal ions bind to the surface of the cell not only by electrostatic interaction and complexation but also by ion exchange to the cell surface; for example, the non-living brown algae (_Ascophyllum nodosum_) exchanges the original cell wall adsorption of K\({}^{+}\), Ca\({}^{2+}\) and Mg\({}^{2+}\) to adsorb Co\({}^{2+}\) (Figure 1, [40]). [PERSON] and [PERSON] [22] showed that yeast releases approximately 70% of K\({}^{+}\) rapidly and 60% of Mg\({}^{2+}\) slowly in the process of adsorbing Cu\({}^{2+}\). Furthermore, studies have shown that ion exchange can occur with complexation [41; 42]. However, other studies have shown that ion exchange is not the main mechanism of microbe remediation, because the amounts of released cations (Ca\({}^{2+}\) and Mg\({}^{2+}\)) are always less than those of heavy metal ions [22].