Microbial Response Following a Carboxymethyl Cellulose-Stabilized Sulfidated Nano Zero-Valent Iron Injection: A Long-Term Field Study

Sulfidated nano zero-valent iron (S-nZVI), recognized for its high remediation efficiency, was evaluated in a field trial to assess its impact on microbial communities in subsurface environments contaminated with chlorinated volatile organic compounds (cVOCs). Over a 1.5 year monitoring period, groundwater samples were collected to track microbial responses following the injection of carboxymethyl cellulose (CMC)-stabilized and dithionite-sulfidated nZVI (CMC-S-nZVI). The study observed pronounced and prolonged biostimulatory effects at locations receiving notable amounts of CMC-S-nZVI, as evidenced by increased DNA and bacterial concentrations. 16S rRNA gene sequencing revealed the selective enrichment of organohalide-respiring bacteria (OHRB), including Dehalococcoidaceae, which played a crucial role in cVOCs biotic dechlorination. Numerical ecological analyses indicated a sustained shift in microbial community structures post injection. Additionally, Fe3+/sulfur-reducing OHRB, including some strains of Geobacter, Sulfurospirillum, and Desulfitobacterium, emerged as major components of the total bacterial population. These microorganisms likely utilized both injected and native sulfur and iron species to form iron sulfides, potentially contributing to abiotic dechlorination. This microbially mediated process may explain the complete dechlorination of cVOCs with minimal vinyl chloride accumulation. This study demonstrates the long-term adaptation of microorganisms to the injected CMC-S-nZVI and its effectiveness in removing cVOCs through a combination of abiotic and biotic processes.