Insights into biostimulation-enhanced microbial detoxification of chromium ore processing residue-contaminated soil: The critical role of Cr(VI) key host-phase transformation and soil microbiota shifts.

The continuous and slow release of Cr(VI) from chromium ore processing residue contaminated soil (COPR-soil) poses a substantial threat to soil and groundwater. Despite microbial reduction is considered as an effective approach for the remediation of Cr(VI)-contaminated soil, the efficiency and rate of Cr(VI) reduction in COPR-soil, especially Cr(VI) embedded in minerals (e.g., vaterite, Ca/Al-Cr layered double hydroxide (Ca/Al-Cr LDH)) remain low. Here, a biostimulation-enhanced microbial detoxification strategy was developed, utilizing the strong electron transfer properties of FeSx. The removal efficiency of Cr(VI) from COPR-soil reached 99.9 %, with a 9-fold increase in the reduction rate of dissolved Cr(VI) compared to microbial remediation. FeSx semiconductor nanoparticles adhered tightly to the surface of the electroactive bacterium Pannonibacter phragmitetus BB (BB), facilitating mineral-microbial interactions that increased protein concentration by 35.8 % and Cr(VI) tolerance by 23.0 %. Biostimulation with FeSx significantly enhanced the biochemical dissolution capacity and electron shuttle potential of BB, accelerating the transformation of Cr(VI) host-phases. Vaterite was completely converted to calcite with a 22 % increase in transformation degree, while the interlayer nanoconfined Ca-Cr coordination in Ca/Al-Cr LDH shifted to a more accessible outer nonconfined structure. This transformation reduced the Cr(VI) binding capacity by 68.6 % and 79.4 %, respectively, effectively releasing Cr(VI) from mineral. Soluble Fe(III) emerged as a critical electron shuttle, enabling indirect electron transfer from BB to Cr(VI) via the Fe(III)/Fe(II) redox cycle. Additionally, biostimulation enhanced soil fertility and stability, fostering microbial consortia with improved resistance to environmental stresses through Cr(VI) efflux and intracellular translocation of Fe-Fe carrier complexes. This study provides a promising strategy to promote effective microbial remediation of COPR-soil.