Biochar assists phosphate solubilizing bacteria to resist combined Pb and Cd stress by promoting acid secretion and extracellular electron transfer

Microorganisms have difficulty surviving and performing remediation functions in mixed systems with high concentrations of Pb and Cd. Biochar has the potential to assist microorganism remediation as an excellent adsorbent for heavy metals. In this study, pig manure biochar (PMB) was used to assist phosphorus solubilizing bacteria (PSB) to explore the mineralization protection and biofeedback mechanism of biochar on PSB under mixed stress of 1000 mg/L Pb2+ and 500 mg/L Cd2+. The adsorption results showed that the removal of Pb2+ and Cd2+ by PMB+PSB was 148.77% and 72.27% higher than that by PSB. Meanwhile, the non-bioavailable fraction of Cd2+ and acid-soluble fraction of Pb2+ in PMB+PSB were increased by 9% and 3%, respectively. Mineralogical and microbial secretion results confirm that showed that the acidic soluble fraction and non-bioavailable fraction were mostly Pb/Cd-carbonate and Pb/Cd-phosphate. The pore adsorption and precipitation (carbonate) of biochar were able to reduce the exposure of PSB to Pb/Cd and the background stress concentration, thus stimulating the biological positive feedback effect of PSB and forming a microenvironment in the cell periphery. The vesicle detoxification and extracellular polymeric substance protection mechanism of PSB were improved under biochar protection, and the individual size and activity of PSB cells were enhanced. Besides, citric acid release from PSB (28.85% increase) accelerated the dissolution of unstable Cd-carbonate, thereby releasing a large amount of Cd2+ to compete with Pb2+ for PO43-. Thus, the protection of biochar and the positive feedback effect of PSB could reduce the biotoxicity of Cd2+ in the stress system by preferentially forming a stable Cd-phosphate. In addition, the excellent electrical conductivity and organic material adsorption of biochar increased the extracellular electron transport rate of microorganisms, which further accelerated the mineralization and immobilization of Pb2+ and Cd2+, so as to ensure the repair effect of PSB on heavy metals.