The anthropogenic activities in agriculture, industrialization, mining, and metallurgy combined with the natural weathering of rocks, have led to severe contamination of soils by toxic metal(loid)s. In an attempt to remediate these polluted sites, a plethora of conventional approaches such as Solidification/Stabilization (S/S), soil washing, electrokinetic remediation, and chemical oxidation/reduction have been used for the immobilization and removal of toxic metal(loid)s in the soil. However, these conventional methods are associated with certain limitations. These limitations include high operational costs, high energy demands, post-waste disposal difficulties, and secondary pollution. Bioleaching has proven to be a promising alternative to these conventional approaches in removing toxic metal(loid)s from contaminated soil as it is cost-effective, environmentally friendly, and esthetically pleasing. The bioleaching process is influenced by factors including pH, temperature, oxygen, and carbon dioxide supply, as well as nutrients in the medium. It is crucial to monitor these parameters before and throughout the reaction since a change in any, for instance, pH during the reaction, can alter the microbial activity and, therefore, the rate of metal leaching. However, research on these influencing factors and recent innovations has brought significant progress in bioleaching over the years. This critical review, therefore, presents the current approaches to bioleaching and the mechanisms involved in removing toxic metal(loid)s from contaminated soil. We further examined and discussed the fundamental principles of various influencing factors that necessitate optimization in the bioleaching process. Additionally, the future perspectives on adding omics for bioleaching as an emerging technology are discussed.

The investigation of chemical speciation of primary toxic metal(loid)s (Cd, Pb, and As) in soil profile in nonferrous metal smelting site is a key to the assessment of their mobility characteristics and formulation of subsequent remediation strategy. In this study, 74 soil samples were collected at 12 different soil profiles; soil physio-chemical properties and total content of Cd, Pb and As and corresponding chemical speciation were also determined. The results showed that the mean total concentration followed the order of Pb > As > Cd. A large proportion of Pb, Cd and As were accumulated in upper soil profiles (depth < 3 m). Heavy pollution of Pb, Cd and As were observed in the whole soil profile at the area of fuel oil storage tank (ZY6) and lead smelting area (ZY8). The dominant fraction of Cd was exchangeable fraction (F1); Pb was dominant in Fe/Mn oxides-bound fraction (F3) in most cases; Crystallized Fe/Al hydrous oxides bound fraction (F4) generally accounted for a large proportion of As. Mobility factor (MF) followed the order Cd > As > Pb, indicating that Cd was the most mobile element in soil profiles. Pearson correlation analysis found that MF_Cd was significantly positively correlated to soil silt; the F4 fraction percentage of As was significantly positively correlated to soil redox potential (Eh). Additionally, MF_Cd/Pb was found to be positively correlated to crystalline iron (Fe_c), while negatively correlated to amorphous iron (Fe_o). The findings reported in this study, on the basis of distribution characteristics of chemical speciation could provide a new solution for future soil remediation at the site. Long-term solutions to metal(loid)s pollution might be offered by microbial-assisted soil washing technique that promotes the transformation of Fe/Mn oxides-bound fraction and organic/sulfide-bound fraction.

Cadmium (Cd) pollution poses a serious risk to human health and ecological security. Bioremediation can be a promising and effective remediation technology for treating Cd contaminated soils. In this study, seven heterotrophic strains were isolated from Cd contaminated soil and 7 autotrophic strains were isolated from acid mine drainage. Cd removal efficiencies were compared after leaching with autotrophic bacteria (Att-sys), heterotrophic isolates (Htt-sys) and cooperative leaching systems (Co-sys) in laboratory agitating reactors. The results indicated that Cd removal efficiency of Co-sys (32.09%) was significantly higher than that of Att-sys (23.24%) and Htt-sys (0.74%). By analyzing the soil microbial community in different bioleaching systems, we found that the addition of heterotrophic isolates significantly promoted the growth of some heavy metal resistant inhabitants (Massilia, Alicyclobacillus, Micromonospora, etc.), and Co-sys had a minor effect on the growth of soil indigenous microbes. In Co-sys, the content of the four Cd fractions all decreased compared with other leaching systems. The analysis of soil physicochemical parameters during the leaching process showed that pH and ORP (oxidation reduction potential) were not the only determinants for Cd removal efficiency in Co-sys, synergistic metabolic activities of autotrophic and heterotrophic strains may be other determinants. This study demonstrated that cooperative bioremediation may prove to be a safe and efficient technique for field application in heavy metal soil pollution.