Remediation of trichloroethylene-contaminated groundwater using bio-sulfidized micrometer-sized iron： roles of particle size and sulfate concentration

Trichloroethylene （TCE）， a typical halogenated organic pollutant in groundwater， poses a serious threat to groundwater ecosystem and human health. Micrometer-sized zero-valent iron （mZVI） has attracted considerable attention for its ability in removing TCE. To further enhance the reactivity of mZVI， sulfidation modification is often employed. While traditional chemical sulfidation can improve the reactivity of mZVI， it suffers from high chemical reagent consumption and environmental pollution. In contrast， biological sulfidation， which utilizes sulfide produced by microbial metabolism to modify mZVI， is more environmentally friendly and cost-effective. However， the mechanisms underlying the effects of mZVI particle size and sulfate concentration on the sulfidation efficiency and dechlorination performance of biologically sulfidated mZVI remain unclear. Therefore， this study investigated the effects of mZVI particle size and sulfate concentration on the efficiency of TCE removal by biologically sulfidated mZVI with different particle sizes， and explored the underlying mechanisms. The results showed that the mZVI particle size significantly affected the biological sulfidation efficiency and TCE removal rate. 7 μm ZVI exhibited the fastest removal rate （0.16 d-1） due to its larger specific surface area. However， it was prone to agglomeration and might inhibit microbial growth， leading to the lowest degree of biological sulfidation. In contrast， 40 μm ZVI showed the highest content of reductive sulfur （81.92%） after biological sulfidation， and its reactivity was significantly better than that of the chemically sulfidated group with the same particle size， indicating that biological sulfidation was more effective for larger-sized mZVI. Additionally， sulfate concentration had a significant impact on the dechlorination performance of mZVI. The increase in sulfate concentration created a favorable environment for the growth of sulfate-reducing bacteria， increased the content of reductive sulfur on the mZVI surface， and thus enhanced the dechlorination efficiency of S-mZVI. When the sulfate concentration reached 700 mg/L， S-mZVI could completely remove TCE within 24 days. The dechlorination product results showed that the sulfate concentration was positively correlated with the degree of dechlorination， and high sulfate concentrations promoted the complete removal of chlorine atoms from TCE molecules. The findings can provide theoretical guidance and technical support for the bio-sulfidated mZVI remediation of TCE-contaminated groundwater.