Synergistic remediation of PCB-contaminated soil with nanoparticulate zero-valent iron and alfalfa: targeted changes in the root metabolite-dependent microbial community

Phytoremediation is a cost-effective and environmentally sustainable remediation technology for many types of contaminants, but has low efficacy and applicability for persistent organic pollutant- (POP-) contaminated soils. Reactive nanoparticulate zero-valent iron (nZVI) can enhance plant growth and synergistically promote the remediation of POP-contaminated soil. Here, we investigated the efficacy and mechanisms by which nZVI (0, 10, 100, and 1000 mg kg(-1)) and alfalfa (Medicago sativa) synergistically remediate polychlorinated biphenyl- (PCB-) contaminated agricultural soil for 120 days. The results show that the alfalfa remediation efficiencies of PCB28- and PCB180-contaminated soil are significantly elevated from 66.7% and 38.5% to 93.1% and 52.3% with the increase of nZVI from 0 to 1000 mg kg(-1), respectively. These values are significantly greater than those of nZVI and phytoremediation strategies alone, and the synergistic effect is induced by the alterations of microecological environment of the alfalfa rhizosphere. Plant root metabolomic analysis and rhizosphere microbial community profiling reveal that nZVI influences amino acid metabolism and this change in metabolites could be an energy source for reshaping the rhizosphere microbial community towards PCB degradation. The capabilities of the microbiota to utilize carbon sources are facilitated in PCB-contaminated soil with nZVI and alfalfa co-treatment. In addition to significant microbial degradation, root exudates are shown to promote the production of hydroxyl radicals in the simulated nZVI-alfalfa system. This work provides a new strategy using nanomaterial-facilitated phytoremediation to promote the restoration of POP-contaminated soils.