The microbe-clay mineral system is widely known to reduce the fluidity of heavy metals through biomineralization, thus mitigating soil pollution stemming from heavy metals. Here, we investigated the effect of mineral distinction on the solidification of cadmium (Cd) using sulfate-reducing bacteria (SRB) to construct symbiotic systems with purplish soil, clay-sized fraction of purple soil (Clay-csp), clay particles of amorphous iron (Fe) oxide (Clay-ox), clay particles removing crystalline Fe oxide (Clay-CBD), and residues of Clay-CBD treated by hydrochloric acid (Clay-HCl). The difference in Cd morphology among purplish soil, Clay-csp, and Clay-ox indicated that the fixation of Cd in soil was largely determined by Fe oxides. The content of Cd in Clay-csp decreased by 66.7% after the removal of amorphous Fe, confirming that clay easily adsorbed infinitive Fe oxides in purple soil. In the system of SRB and Clay-ox, carbonate-bound Cd (F2) decreased by 14.85% and residual Cd (F5) increased by 14% from the retardation to late decline phase, eventually forming iron-sulfur (Fe-S) compounds. Based on the correlation analyses of Cd and Fe in amorphous-bound state and Fe-manganese (Mn) oxidation state in simulation experiments, it is demonstrated that Fe-Mn oxides control the behavior of Cd in soil clay, and SRB-mediated Fe-bearing minerals promote the transformation of Cd from activated to stable state.
Keywords: Clay minerals; Fe-bearing minerals; Fixation of cadmium; Purplish soil; Sulfate reducing bacteria.
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