Evaluating the protection of bacteria from extreme Cd (II) stress by P-enriched biochar

Cadmium cations (Cd2+) are extremely toxic to organisms, which limits the remediation of Cd by microorganisms. This study investigated the feasibility of applying biochar to protect bacteria from extreme Cd2+ stress (1000 mg/L). An alkaline biochar (RB) and a slightly acidic biochar (SB) were selected. SB revealed a higher Cd2+ removal than RB (15.5% vs. 4.8%) due to its high surface area. Addition of Enterobacter sp. induced formation of Cd phosphate and carbonate on both SB and RB surface. However, Cd2+ removal by RB enhanced more evidently than SB (78.9% vs. 30.2%) due to the substantial microbial regulation and surficial alkalinity. Thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and geochemical modeling (GWB) all confirmed that the formation of stable Cd phosphate on RB was superior to that in SB. These biomineralization, together with biochar pore structure, protect bacterial cells from Cd stress. Moreover, the alkalinity of biochar promoted the formation of carbonate, which strengthened the decline of Cd2+ toxicity. The protection by RB was also confirmed by the intense microbial respiration and biomass (PLFA). Furthermore, this protection induced a positive feedback between P-abundant biochar and Enterobacter sp.: biochar provides P source (the most common limiting nutrient) to support microbial growth; bacteria secrete more organic acids to drive P release. This study therefore elucidated the protection of bacteria by P-enriched biochar based on both physic-chemical and microbial insights. [Display omitted] •Pore structure contributes to isolation of bacterial cells from toxic Cd2+.•Surficial alkalinity causes Cd biomineralization, reducing Cd toxicity.•GWB model confirms that Cd-phosphate dominates mineralization after adding bacteria.•The abundant P induces a positive feedback between the bacteria and biochar. There are three main pathways (pore structure, carbonate and phosphate biomineralization) for biochar to assist bacteria to resist heavy metal stress. The data regarding bioactivities of bacteria cells (PLFA and microbial respiration) and GWB modeling confirmed the protection function of biochar.