Penicillium oxalicum induced phosphate precipitation enhanced cadmium (Cd) immobilization by simultaneously...

Soil cadmium (Cd) is immobilized by the progressing biomineralization process as microbial induced phosphate precipitation (MIPP), which is regulated by phosphate (P) solubilizing microorganisms and P sources. However, little attention has been paid to the implications of Cd biosorption during MIPP. In this study, the newly isolated Penicillium oxalicum could immobilize 5.4-12.6 % of Cd²⁺, while the presence of hydroxyapatite (HAP) considerably enhanced Cd²⁺ immobilization in P. oxalicum and reached over 99 % Cd²⁺ immobilization efficiency within 7 days. Compared to P. oxalicum mono inoculation, MIPP dramatically boosted Cd biosorption and biomineralization efficiency by 71 % and 16 % after 96 h cultivation, respectively. P. oxalicum preferred to absorbing Cd²⁺ and reaching maximum Cd²⁺ biosorption efficiency of 87.8 % in the presence of HAP. More surface groups in P. oxalicum and HAP mineral involved adsorption which resulted in the formation of Cd-apatite [Ca₈Cd₂(PO₄)₆(OH)₂] via ion exchange. Intracellular S^2-, secreted organic acids and soluble P via HAP solubilization complexed with Cd²⁺, progressively mineralized into Cd₅(PO₄)₃OH, Cd(H₂PO₄)₂, C₄H₆CdO₄ and CdS. These results suggested that Cd²⁺ immobilization was enhanced simultaneously by the accelerated biosorption and biomineralization during P. oxalicum induced P precipitation. Our findings revealed new mechanisms of Cd immobilization in MIPP process and offered clues for remediation practices at metal contaminated sites.