Effects and mechanisms of nZVI on CO and CH emissions in uncontaminated and pentachlorophenol-contaminated soils.

With the increasing application of nanoscale zero-valent iron (nZVI) for in situ soil remediation, its effects on soil functionality and ecosystem need to be thoroughly evaluated. Herein, we investigated the effects of nZVI on CO and CH emissions from uncontaminated and pentachlorophenol (PCP)-contaminated soils and the underlying microbial mechanisms by designing a 68-day anaerobic soil culture experiment; thereafter, the effects of above aged nZVI on soil CO and CH emissions in the following 20 days were further studied. In the uncontaminated soil, 1-10 g/kg nZVI treatments reduced soil CO emission by 17.4-82.6% and increased soil CH emission by 10.8%-119.7%, but these effects disappeared after the nZVI was aged. The emissions of soil CO and CH were significantly inhibited by the PCP contamination (100 mg/kg) mainly due to the toxicity to related soil microorganisms. The applications of 1-10 g/kg nZVI significantly reduced CO emissions from the PCP-contaminated soil by 24.0-86.7%, while 10 g/kg nZVI markedly increased soil CH emission by 1875.4% and restored the methanogenic activity to the control level after the nZVI was aged. The 10 g/kg nZVI treatment enriched hydrogenotrophic methanogen (Methanobacterium) and organics-degrading bacteria by releasing H, increasing soil pH, and decreasing soil Eh; the abundance of genes encoding key enzymes (Mcr, Mtr, Hdr, Mta, and Mtb) in all methanogenic pathways significantly increased after the nZVI treatment, indicating that nZVI could have a broad promoting effects on soil methanogenic processes. The findings demonstrate that the addition of nZVI for in situ remediation of organochlorines-contaminated soils will affect soil greenhouse gas emissions and provide basic data for safe nZVI applications.