Strategy for Enhancing Cr(Vi)-Contaminated Soil Remediation and Safe Utilization by Microbial-Humic Acid-Vermiculite-Alginate Immobilized Biocomposite

Bioreduction is an efficient approach to remediate Cr(VI)-contaminated soil, but further strengthening methods are still urgently needed. Herein, humic acid (HA) combined with vermiculite (VE) embedded an efficient strain Bacillus sp. CRB-7 was first used to investigate its performance and enhancement mechanism in remediating Cr(VI)-contaminated soil by analyzing whole-genome, Cr detoxification, soil micro-ecology variation, and subsequent crop response. Genomic annotation demonstrated that CRB-7 contained multiple genes contributed to Cr metabolism, and other metals resistance. The embedded CRB-7 biocomposites resulted in more efficient Cr(VI) reduction than control and free CRB-7, especially B-HA-VE-SA achieved the highest Cr(VI) removal efficiency (96.18%) because the mechanisms including carrier effect, nutrient sustained-release, and electron-shuttle effect enhanced the bioremediation process. Furthermore, the synergies of CRB-7 and immobilizers (HA-VE-SA) significantly improved soil microecology (enzyme activities, microbial quantity and diversity), and engendered the evolution of microbial community composition and functional pathways. Consequently, the plant growth experiment (Brassica napus L.) verified the plant-growth-promoting (12.00%–18.00% and 43.82%–69.00% higher in emergence rate and biomass) and Cr-accumulation-reducing effects (19.47%–91.09% and 29.11%–89.80% lower in root and aerial parts) of free and immobilized CRB-7. These findings highlighted the superiority of B-HA-VE-SA in remediation, micro-ecological improvement and safe utilization of Cr(VI)-contaminated soil