The phase transformation of microbial induced struvite and its Cd(II) immobilization mechanism

The research and development of high-efficiency biological mineral materials for the treatment of heavy metal pollution is key to the remediation of heavy metal pollution. Biogenic struvite (BS) was synthesized using microbial culture technique, and a batch adsorption experiments and mineral characterization methods were performed to evaluate the immobilization effect of BS on Cd(II) and the phase transformation in this process. The results show that Bacillus velezensis can regulate the synthesis of irregular BS containing about 14.21% organic matter in Luria-Bertani medium. The adsorption of BS on Cd(II) can be better fitted by the Langmuir equation (R2 = 0.9990), Qmax = 282.96 mg/g. The adsorption kinetics and hermodynamics show that the adsorption of Cd(II) by BS is a spontaneous exothermic chemisorption process with increased chaos. The adsorption capacity of BS on Cd(II) is always maintained at about 174.35 mg/g with pH ≥ 3. Furthermore, the part of Cd(II) combines with organic functional groups on the BS to form Cd-containing complexes or chelates, and another part of Cd(II) partially displaces Mg2+ and NH4+ from struvite crystals to form Cd-bearing phosphate minerals. In summary, the superior immobilization capacity of BS on Cd(II) is attributed to the organic-inorganic structure of BS, complexation or chelation effect, crystal ion exchange, phase transformation, and the formation of secondary phosphate minerals, which makes it difficult to desorb Cd(II). The results offer new ideas for the research and development of new materials for heavy metal pollution remediation.