Biochar-immobilized Bacillus spp. for heavy metals bioremediation: A review on immobilization techniques, bioremediation mechanisms and effects on soil

Heavy metals contamination present risks to ecosystems and human health. Bioremediation is a technology that has been applied to minimize the levels of heavy metals contamination. However, the efficiency of this process varies according to several biotic and abiotic aspects, especially in environments with high concentrations of heavy metals. Therefore, microorganisms immobilization in different materials, such as biochar, emerges as an alternative to alleviate the stress that heavy metals have on microorganisms and thus improve the bioremediation efficiency. In this context, this review aimed to compile recent advances in the use of biochar as a carrier of bacteria, specifically Bacillus spp., with subsequent application for the bioremediation of soil contaminated with heavy metals. We present three different techniques to immobilize Bacillus spp. on biochar. Bacillus strains are capable of reducing the toxicity and bioavailability of metals, while biochar is a material that serves as a shelter for microorganisms and also contributes to bioremediation through the adsorption of contaminants. Thus, there is a synergistic effect between Bacillus spp. and biochar for the heavy metals bioremediation. Biomineralization, biosorption, bioreduction, bioaccumulation and adsorption are the mechanisms involved in this process. The application of biochar-immobilized Bacillus strains results in beneficial effects on the contaminated soil, such as the reduction of toxicity and accumulation of metals in plants, favoring their growth, in addition to increasing microbial and enzymatic activity in soil. However, competition and reduction of microbial diversity and the toxic characteristics of biochar are reported as negative impacts of this strategy. More studies using this emerging technology are essential to improve its efficiency, to elucidate the mechanisms and to balance positive and negative impacts, especially at the field scale.