# Effects and Mechanisms of Microbial Remediation of Heavy Metals in Soil: A Critical Review
## 4 Microbial Remediation of Heavy Metal-Contaminated Soil
### Comparison of Microbial Removal Ability
#### 4.2.4 Methods for Microbial Remediation  
A clear measurement methodology for microbial remediation of heavy metals in soil has not yet been proposed. Generally, the microbial culture is established by injecting a specific microorganism into heavily polluted soil. [PERSON] et al. adapted stains that were grown in metal free and metal contaminated soil leachate [34]. After a period of time, the residual heavy metal concentration is measured by a spectrophotometer and compared to pre-injection conditions to evaluate the microbial restoration ability [22]. [PERSON] used an atomic absorption spectrophotometer to identify the concentration of unadsorbed metal ions [14]. Today, Qmax is generally used to describe the microbial adsorption capacity for heavy metal ions. Generally, this capacity varies significantly among species. Thus, there is not a preferred microorganism. However, in meta-analyses of similar subjects, Qmax = 0, indicating that metal ions cannot be adsorbed; thus, this is viewed as a baseline. Thus, the higher the Qmax, the better the adsorption ability [67]. Qmax is the maximum amount of metal ions adsorbed by bacteria per unit of dry weight and is determined using the Langmuir model discussed above.  
\[Q\text{max}=(Q\text{eq}+bC\text{eq2})/(bC\text{eq})\]  
We found that those organisms that have spores or genes that degrade heavy metals on the plasmid such as _Bacillus_ and _Pseudomonas_ often perform better than others. However, for any given organism, both the value of Qmax and the removal rate vary for different heavy metals (Table 5, [34]). [PERSON] et al. [10] showed that the enrichment ability of _Bacillus subtilis_ for Cu\({}^{2+}\) is much greater than  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Microbial** & **Metal** & **Best-Fitted Model** & **Source** \\ \hline _Bacillus Subtilis_ & Hg\({}^{2+}\) & Second-order & [31] \\ _Nauclea diderichii_ & Cd\({}^{2+}\) Hg\({}^{2+}\) & Second-order & [65] \\ _Rice husk_ & Pb\({}^{2+}\) & Second-order & [66] \\ _Kappaphycus_ sp. & Pb\({}^{2+}\) Cu\({}^{2+}\) Fe\({}^{2+}\) Zn\({}^{2+}\) & Second-order & [44] \\ _Helix pomentia_ & Fe\({}^{2+}\) Cr\({}^{3+}\) & First-order & [59] \\ _Helix pomentia_ & Cd\({}^{2+}\) Pb\({}^{2+}\) & Second-order & [59] \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparison of mechanisms of sorption.  
that of _Escherichia coli_, _Bacillus cereus_, and _Pseudomonas aeruginosa_. The amount of metal adsorbed by _Bacillus subtilis_ in \(\upmu\)mol/g at an equilibrium concentration of 1 g was 4.150 \(\upmu\)mol, whereas the absorption capacities of the other three bacteria were only 2.188, 2.576 and 2.560 \(\upmu\)mol/g, respectively. However, the enrichment capacity of _Bacillus subtilis_ for Cd is weak (only 0.147 \(\upmu\)mol/g), while that of _Escherichia coli_ reached 1.067 \(\upmu\)mol/g [57]. [PERSON] et al. [68] found that _Bacillus licheniformis_ and _Bacillus heterozygous_ had a much better effect on the adsorption of Cd and Cr than other bacteria; thus, the bacterial species showing the highest enrichment varies with heavy metal species.  
Current studies have shown that soil fungi, which form mycorrhizal plants through symbiotic relationships, are effective at removing heavy metals from soil. The heavy metal removal ability of the same fungi varies for different heavy metal elements, and for different fungi on the same element. For example, inoculation of _Gomus intraradices_ increased the accumulation of Cu in the subsurface part of plants but had no significant effect on the accumulation of Zn [41]. Inoculation of ectomycorrhizal fungi could reduce the contents of heavy metals in soils under Cu\({}^{2+}\) and Cd\({}^{2+}\) conditions by 2.64-11.79 and 1.49-7.56 times, respectively [69]. The effectiveness of _Actinomycetes_ and other microorganisms on the removal of heavy metals from soil is less studied.