Clay-assisted protection of Enterobacter sp. from Pb (II) stress

Clay minerals can adsorb both microorganisms and heavy metals. In this study, typical soil bacterium, Enterobacter sp. was applied to investigate the potential protection of the bacterial cells from Pb²⁺ stress by clay minerals. The sorption by two representative types of montmorillonite (Mt) were contrasted, i.e., Mt_s/Mt_w with strong/weak CEC. There was no significant difference between the two clay minerals regarding their adsorption of Pb²⁺ cations in water (i.e., ~55 mg L^-1). However, the sorption of bacterial cells on the two clay minerals showed evident contrasts, which resulted in the different capacity of Pb sorption. Mt_s with high CEC preferentially adsorbed abundant bacterial cells (rather than Pb²⁺) on its surface. The residual Pb²⁺ concentration in solution actually raised by 7.5% after the addition of Enterobacter sp. In addition, both the Pb-contaminated cells and "healthy" cells (with low Pb contamination) could be adsorbed onto Mt surface, whereas the latter dominated the adsorbents on Mt_s. This was due to that the Mt_s with high CEC could provide more exchangeable cations, building more cation bridging ligands between the microbial cells (whatever the types of cells) and clay surface. Furthermore, the adsorbed "healthy" bacterial cells might escape from clay surface via "self-liberating" mechanism, i.e., increasing electrostatic repulsion between the bacteria and clay during microbial decomposition of the medium. This study hence elucidated the protection of microorganisms from Pb²⁺ stress by Mt.