Humin-accelerated microbial dechlorination of polychlorinated biphenyls in Taihu Lake sediment microcosms: Insights into congener and microbe shifts and the role of subsequently supplemented sulfidated nanoscale zero-valent iron.

Abstract

Microbial dechlorination is a promising route for the in situ detoxification of PCB-contaminated sediments, yet this is often limited by slow electron transfer and the progressive depletion of electron donors. Humin acts as a naturally occurring solid-phase redox mediator, whereas sulfidated nanoscale zero-valent iron (S-nZVI) can provide extra electrons. Both of them hold the potential to enhance PCB dechlorination. Herein, this study investigated the long-term (66 weeks) effect of humin on dechlorinating the most toxic commercial PCB mixture Aroclor 1254 in Taihu Lake sediment microcosms. Moreover, the triggering potential via supplemented S-nZVI were examined after Week 30, when initial dechlorination slowed down. The results showed that humin significantly accelerated PCB dechlorination during the rapid phase (Weeks 12-30), without altering the predominant meta- and para-dechlorination preferences. Total PCB mass decreased by approximately 32% while toxic equivalents declined by >99% by Week 66. Congener-specific analysis inferred three predominant humin-enhanced meta-dechlorination pathways of PCB110 (236-34)→PCB71 (26-34)→PCB32 (26-4), PCB99 (245-24)→PCB47 (24-24), and PCB92 (235-25)→PCB52 (25-25). Microbial analyses by multiple molecular biology tools suggest that humin (20 g·kg^-1 slurry basis) was associated with closer co-occurrence between putative dechlorinating and supporting anaerobic populations. Dehalococcoides abundance increased from ∼10⁷ to ∼10⁸ copies·mL^-1, accompanied by pronounced increases in the reductive dehalogenase (RDase) genes pcbA4 and pcbA5, which reached 1.00×10⁸ ± 3.75×10⁵ and 4.81×10⁸ ± 2.04×10⁷ copies·mL^-1, respectively. Supplementation with 0.1% (slurry basis) S-nZVI at Week 30, triggered further dechlorination and total PCB mass decreased by 44% at the end of Week 66. S-nZVI drove apparent increases of Dehalococcoides, Dehalogenimonas, as well as iron- and sulfate-respiration species. Predicted functional changes further displayed a wide community response accompanying sustained PCB dechlorination. Combining humin-enhanced rapid-phase dechlorination with S-nZVI-supported further dechlorination after dechlorination slows may provide a viable strategy for sustaining long-term PCB bioremediation.