Penicillium oxalicum induced phosphate precipitation enhanced cadmium (Cd) immobilization by simultaneously accelerating Cd biosorption and biomineralization

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 S2-, 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. Microbial induced phosphate (P) precipitation is an advancing and eco-friendly method to reduce bioavailability of cadmium (Cd) in environmental matrices. In this study, the combination of Penicillium oxalicum and hydroxyapatite (HAP) simultaneously boosted Cd2+ biosorption and biomineralization. More surface groups in P. oxalicum and HAP were involved to adsorb Cd2+ into Cd-apatite via ion exchange. Intracellular S2-, secreted organic acids and soluble P would simultaneously complex with Cd2+ into Cd-containing minerals. The simultaneously accelerated biosorption and biomineralization provided novel insights and theoretical basis for Cd2+ remediation using this method at metal contaminated environment.