Distinct impacts of ferric iron and sulfate on anaerobic microbial dechlorination of polychlorinated biphenyls (PCBs) : insights from congener-specific and microbial community analyses.

Anaerobic microbial dechlorination contributes to natural attenuation of polychlorinated biphenyls (PCBs) in sediments. However, the influence of competing electron acceptors in iron- and sulfate-rich sediments remains unclear. This study compared the effects of FeOOH (40 mmol/kg slurry, T-Fe) and sulfate (16 mmol/kg slurry, T-S) amendments on PCB dechlorination in Taihu Lake sediment microcosms spiked with a defined PCB mixture (50 mg/kg slurry). An unamended control (T-1) was included. After 66-week incubation, residual total PCB mass was 4.68% higher in T-Fe and 14.15% higher in T-S than in T-1. Under the tested amendment conditions, sulfate produced stronger and more persistent inhibition of PCB dechlorination than FeOOH in the Taihu Lake sediment microcosms. Ortho-dechlorination was limited in both treatments. Sulfate further suppressed meta- and para-dechlorination and maintained higher dioxin-like toxicity. T-Fe exhibited dechlorination pathways similar to T-1, whereas T-S altered pathways, with congeners bearing two ortho-chlorines on a single ring (PCB 64, 71, 149) remaining largely undechlorinated, and meta-/para-dechlorination of highly chlorinated congeners (PCB 153, 170) hindered. The two electron acceptors reshaped microbial communities along distinct trajectories. Under Fe(III)-reducing conditions, Fe(II) accumulated rapidly during the early incubation, while Chloroflexi increased during the later incubation period. Dehalogenimonas was enriched, and reductive dehalogenase (RDase) genes (ardA, rdh12, pcbA4, and pcbA5) increased by 1-2 orders of magnitude. Under sulfate-reducing conditions, acetate-utilizing Desulfococcus and Desulfobacca outcompeted dechlorinators for carbon sources and electrons. Dehalococcoides enrichment was minimal, and RDase genes increased by only ∼1 order of magnitude. These findings advance the understanding of how competing electron acceptors affect PCB dechlorination rate, extent and pathway selectivity.