# Remediation of Metal/Metalloid-Polluted Soils: A Short Review
## 4 Removal Techniques
### Biological Processes
#### 4.3.1 Microbial Bioremediation  
The removal of heavy metals and metalloids can be actuated by fostering the growth and development of specific microorganisms in contaminated soil, namely through bioaugmentation and/or biostimulation processes. Heavy metals/metalloids can be oxidized, reduced, immobilized, and metabolized by microorganisms. Different from organic pollutants, heavy metals/metalloids are not degraded by microorganisms, but simply accumulate and are adsorbed at the binding sites present in the cellular structure, for example into microbial tissue.  
The main microbial processes are biosorption and bioleaching, and are described in the study by [PERSON] and [PERSON] [135].  
Figure 4: Electrochemical remediation scheme.  
Figure 5: Biological processes to remove heavy metals and metalloids.  
Biosorption  
In the biosorption process, the heavy metals are immobilized onto the cellular structure of microorganisms. This is possible through the extracellular binding created between the cell surface (anions) and metal ions (cations). Extracellular materials have active functional groups that promote the binding mechanisms. The biosorption mechanism is complex and may include physical adsorption, ion exchange, complex formation, reduction, and precipitation. The efficiency of biosorption is influenced by several factors: metal ion properties, process conditions, density of sorption centers, and types of immobilization agents, as shown in the study by [PERSON] et al. [136].  
Typical bios adsorbents are bacteria [91; 92; 93; 94], fungi [95; 96; 97; 98], and algae [99]:  
* Bacteria: heavy metal ions can be bound and accumulated on polysaccharide slime layers of bacteria through functional groups, such as carboxyl, amino, phosphate, or sulfate groups.
* Fungi: are used to adsorb the heavy metals and metalloids through ion exchange and coordination in the chitin-chitosan complex, glucuronic acid, phosphate, and polysaccharides present in their cells.
* Algae: the absorption of heavy metals /metalloids occurs since algae form peptides as a defense mechanism. The functional groups (carboxyl, amino, sulfhydryl, and sulfonate) are among the constituents of the algal cell wall, and ion exchange promotes the adsorption of metal ions.  
Bioleaching  
The bioleaching process reduces mobility and stabilizes pollutants using the capacity of microorganisms to produces secretions, such as low molecular weight organic acids, that can dissolve heavy metals and soil particles containing heavy metal ones. In this way, heavy metals are directly solubilized by the metabolism of microorganisms or indirectly by their metabolites [101]. The agents that promote the leaching are biosurfactants, namely polysaccharides, lipids, and lipoproteins, produced by microorganisms, with a high surface activity that permits the formation of chelating metals and binding of metal ions [100]. This method has been extensively studied since it is an environmentally-friendly and inexpensive technique. For example, [PERSON] et al. [100] showed the capability of a biosurfactant produced by a microbial strain isolated from cafeteria sewer sludge.  
In general, all microbial biodegradation is low cost and eco-friendly. However, there are some limits:  
* several microorganisms cannot bind toxic metals into harmless metabolites;
* the process is slower than others;
* it is efficient if the environmental conditions are suitable for microbial metabolism.