Enhanced organic-Cr(III) removal and stabilization in bioleaching wastewater through integrated denitrifying bacteria and zero-valent iron treatment

Chromium-contaminated soil remediation via bioleaching generates wastewater containing persistent organic-Cr(III) complexes, which pose significant challenges during the post-treatment process. This study developed an innovative ternary synergistic system integrating mixed heterotrophic/autotrophic denitrifying bacteria with zero-valent iron (Fe 0 ) for promoted organic-Cr(III) removal and stabilization. In the nitrate-modulated biotic-Fe 0 (NMB Fe 0 ) system, heterotrophic bacteria mediated organic acid degradation, resulting in Cr(III) ion liberation. Concurrently, autotrophic bacteria utilized cathodic hydrogen from iron corrosion to accelerate both Fe 0 corrosion and secondary mineral production (e.g., magnetite and green rust) for subsequent Cr(III) adsorption and immobilization. The integrated system achieved 78.5% organic-Cr(III) removal in 5 d, representing a 6.7-fold enhancement as compared to the abiotic-Fe 0 control. Statistical analysis revealed significant positive correlations between nitrate concentration, Fe 0 dosage, and removal efficiency within the biological adaptation range, confirming denitrification as the primary driving mechanism. Leaching toxicity and reoxidation assessments demonstrated superior environmental stability of immobilized Cr(III) by NMB Fe 0 . Mechanistic investigations indicated that the synergistic interplay between heterotrophic and hydrogen-autotrophic bacteria effectively inhibited both organic-Cr(III) complexation and Fe Cr passivation layer formation on Fe 0 surface. These findings provide a novel, efficient, and sustainable strategy for bioleaching wastewater treatment.