FIGURE 1.

Mechanism of microbial remediation. (A) Passive and active heavy metal uptake by biological materials. The uptake of heavy metals can be either passive (fast) through adsorption onto the cell surface or any extracellular components such as polysaccharides, or alternatively active (slow) through sequestration of the heavy metals via interaction with metallothioneins (MT) into the cell. Adapted from Scragg (2005). (B) Mechanisms of heavy metal biosorption by bacterial cells. Bacterial biosorption of heavy metals through (1) cell surface adsorption, (2) extracellular precipitation, (3) intracellular accumulation through special components, such as metallothioneins (MT) or, (4) intracellular accumulation into vacuoles. Adapted from Banik et al. (2014). (C) Heavy metal remediation via siderophore formation. Bacterial heavy metal remediation takes place through formation of the siderophore aided by membrane protein-mediated metal transport and the formation of siderophore-metal complexes. Adapted from Banik et al. (2014). (D) Mechanism of bacterial heavy metal remediation through biosurfactant production. The precipitation of heavy metals takes place through sorption and desorption at the soil–water-heavy metal matrix leading to heavy metal precipitation. Adapted from Banik et al. (2014).