Reinforcing natural attenuation of Cr(VI) in groundwater through single- and composite BDOM-NEVO reagents: Performance contrast and mechanistic insights

The natural attenuation by microbes is crucial for removing residual Cr(VI) in remediation processes. However, insufficient natural attenuation can lead to pollutant rebound or tailing phenomena, affecting remediation efficiency. To address this issue, this study explored the enhancement effects and mechanisms of action of environmentally friendly carbon sources (NEVO), soluble biochar (BDOM), and their coupling (BDOM-NEVO) on the removal of residual Cr(VI). The results showed that BDOM-NEVO achieved the best removal performance, significantly outperforming NEVO and BDOM. This superiority is attributed to BDOM-NEVO's smaller particle size (262.2 nm), lower viscosity (1.2 mPa·s), richer oxygen-containing functional groups, as well as higher electron transfer activity (1.2-3.6 times higher). In the cyclic experiments (I-VI), the higher removal efficiency (1.9-2.4 times higher than NEVO) and shorter half-life (half of NEVO's) ensure the long-term effectiveness of BDOM-NEVO. The microbial physiological results indicate that BDOM-NEVO exhibits superior performance, the primary mechanism involves promoting the growth of microbial communities like Sphingomonas containing chromium-resistant genes (ChrA, ChrR, and AzoR), significantly enhancing metabolic pathways and synergistic interactions among microbial communities, thereby improving the natural attenuation efficiency of Cr(VI). These findings offer valuable insights into improving the remediation efficiency of residual Cr(VI) pollution. • BDOM-NEVO improved low-concentration Cr(VI) attenuation, surpassing single reagents • Optimized droplet size, viscosity, and functional groups enhanced BDOM-NEVO's longevity • Enhancing microbes with chromium-resistant genes and physiology was primary mechanism • The study proposes a novel composite reagent strategy for Cr(VI) remediation.