# 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  
Microbes can accumulate heavy metals by either adsorption or absorption, which are two main ways to increase metal ions in soil [18]. The process of adsorption differs from absorption, in that a fluid (the absorbate) is dissolved by or permeates a liquid or solid (the absorbent) [19]. Thus, adsorption is a surface phenomenon, while absorption involves the entire volume of material. Table 2 describes the overall sorption mechanisms, including precipitation, chemical adsorption and ion exchange, surface precipitation, the formation of stable complexes with organic ligands, and redox reaction. However, due to the limitations of current analytical techniques and complexity of soil matrix, the exact immobilization mechanisms have not been clarified [20]. Adsorption involves complexation of heavy metals on the cell surface, from which they can be absorbed into the cell [21]. Because of the cell surface structure--principally the cell wall and mucus layer--heavy metals can be adsorbed and absorbed relatively easily. Many ions in the cell surface functional groups, such as nitrogen, oxygen, sulfur, and phosphorus (Figure 1, [22]), can be complexed with metal ions as coordination atoms. In addition, phosphoric acid anions and carboxyl anionic groups on the surface of the microbial cell wall are negatively charged, and most heavy metal surfaces carry a cationic group that interacts with the cell wall and allows the metal ions to bind or pass through the cell membrane (Figure 1, [23]). [PERSON] et al. [24] showed that the primary mechanism by which microbes accumulate heavy metal ions is adsorption, which normally does not depend on energy metabolism, rather than absorption, which depends on energy metabolism and almost exclusively occurs in living cells. Generally, microbes adsorb large amounts of heavy metal ions rapidly. It has been found that at pH 7.2, _Bacillus_ can adsorb 60% of its Cu\({}^{2+}\) capacity within the first minute and reach adsorption equilibrium within 10 min [25]. In contrast, absorption is time-consuming and relatively inefficient; although, the removal rates can be improved by 26.3 to 31.5% through the addition of agents such as lemon oil or ethylenediaminetetraacetic acid (EDTA), respectively [26]. However, the intake of essential nutrients can reduce the ability of a cell to absorb heavy metal ions. Figure 1: Biosorption mechanisms of microorganisms. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type** & **Method** & **Application** & **Advantages (\(\ast\)) and Disadvantages (\(\ast\))** \\ \hline \multirow{4}{*}{Soll washing} & Physical Separation & & (\(\ast\)) Thorough and permanent [5] \\  & Chemical Extraction & Large area (a large volume of soil) & (\(\ast\)) Both soil and metal can be recycled [9] \\  & Integrated process & Large area (a large volume of soil) & (\(\ast\)) Fine-grained soil may be treated [16] \\  & Integrated process & Large area (a large volume of soil) & (\(\ast\)) Inboxerine (need large equipment) [16] \\  & Integrated process & Large area (a large volume of soil) & (\(\ast\)) Inboxerine and chemical agents [5] \\  & & & (\(\ast\)) Inboxerine (need large equipment) [5]