# Effects and Mechanisms of Microbial Remediation of Heavy Metals in Soil: A Critical Review
## 5 Microbial Remediation of Heavy Metal Pollution in Soil and Water
### Microbial Living Environments
#### 5.1.3 Substrate Species  
There are three relevant factors to consider in understanding substrate species: soil type, heavy metal ions, and soil additives. The adsorption characteristics of heavy metals on different soils often vary significantly. Hu [51] showed that the soil adsorption capacity of beach tidal soil (Freundlich adsorption constant K = 93.79) is higher than that of black soil (K = 16.41), which in turn is higher than that of yellow mud (K = 1.17), and that the mean desorption rate of soil in ascending order is Lithic Ochri-Aquic Cambosos (0.67%) \(<\) Endgleyic Fe-accumilic Stagnic Anthrosols (3.62%) \(<\) Fe-accumilic Gleyic Stagnic Anthrosols (35.85%). Obviously, the adsorption rate of soil and its retention of heavy metal ions (i.e., its low desorption rate) result in low heavy metal ion mobility and render the removal of these ions by microbial adsorption difficult [74].  
Heavy metal ion species affect the removal of heavy metals by influencing the generation time of microorganisms. _Thibocillus ferrooxidans_ with sulfur as the substrate has a generation time of approximately 10-25 h, far greater than the substrate generation time on Fe, which is approximately 6.5-15 h. The solubility of different heavy metals is often different; elements such as Zn, Ni, and Cu are easily dissolved, whereas Pb\({}^{2+}\) and Cr are less soluble. Furthermore, the presence of metal ions in the soil or the presence of several metal ions also affect the enrichment of microorganisms [57]. [PERSON] et al. [77] showed that the individual bioavailability of Cd\({}^{2+}\), Pb\({}^{2+}\) and Zn\({}^{2+}\) in the soil is often greater than that of multiple metal ions. For Cd\({}^{2+}\) alone, the adsorption is 11.2 mg/g. However, in the presence of Zn\({}^{2+}\) and Pb\({}^{2+}\), its adsorption is reduced to 3.15 mg/g, with similar results observed for Zn\({}^{2+}\) and Pb\({}^{2+}\), which is reduced from 19.5 and 2.25 mg/g to 8.08 and 0.915 mg/g, respectively. Obviously, this effect is greatest for Cd and least for Zn [77].  
Soil additives can significantly increase the removal of heavy metals by microorganisms, and the concentration of additives can have varying effects on the leaching rate of heavy metal ions.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Bacteria**} & \multirow{2}{*}{**Temperature**} & \multicolumn{3}{c}{**Qmax**} \\ \cline{3-5}  & & **Cd** & **Zn** & **Cr** \\ \hline \multirow{3}{*}{_Bacillus jeotgali_ U3} & 25 \({}^{\circ}\)C & 37.3 & 105.22 & \multirow{3}{*}{} \\  & 30 \({}^{\circ}\)C & 47.5 & 222.2 & \\  & 35 \({}^{\circ}\)C & 57.9 & 128.2 & \\ \hline \multirow{3}{*}{_Bacillus licheniformis_} & 25 \({}^{\circ}\)C & 142.73 & \multirow{3}{*}{} & 62.02 \\  & 37 \({}^{\circ}\)C & 140.41 & & 63.98 \\ \hline \hline \end{tabular} The contents of the table are derived from [49; 68].  
\end{table}
Table 6: Comparison of \(Q\)max for different bacteria at different temperatures.  
[PERSON] et al. [78] showed that the addition of 20 g/L FeSO\({}_{4}\cdot 7\)H\({}_{2}\)O increased the leaching rate of Zn and Cu by factors of 2 and 1.9, respectively, but the leaching rate did not increase when the concentration was greater than 20 g/L. With this same additive, Zn and Cu removal rates were increased to 85% and 93%, respectively, 6 and 3.2 times greater than the control. Research indicates that the use of more than one additive, such as a combination of FeSO\({}_{4}\cdot 7\)H\({}_{2}\)O, results in a higher removal rate than that achieved when these additives are used individually [5].