In-situ reactivation and reuse of micronsized sulfidated zero-valent iron using SRB-enriched culture: A sustainable PRB technology.

Sulfidated zero-valent iron (S-ZVI) is an attractive material of permeable reactive barriers (PRBs) for the remediation of contaminated groundwater. However, S-ZVI is prone to be passivated due to the oxidation of reactive and conductive iron sulfide (FeSx) shell and the formation of inactive and non-conductive ferric (hydr)oxides, which serve as electron transfer barriers to hinder the electron flow from Fe° core to contaminants. This study thus proposed a novel approach for in-situ reactivation and reuse of micronsized S-ZVI (S-mZVI) in PRB using sulfate-reducing bacteria (SRB) enriched culture to realize long-lasting remediation of Cr(VI)-contaminated groundwater. S-mZVI were passivated after reactions with Cr(VI) due to the formation of electron transfer barriers (mainly inactive and non-conductive Fe(III) (hyd)oxides, which increased the polarization resistance from 16.38 to 27.38 kΩ cm2 and hindered the electron transfer from the Fe° core. Interestingly, the passivated S-mZVI was efficiently reactivated by providing the SRB-enriched culture and organic carbon within 12 h, and the Cr(VI) removal capacity of S-mZVI in the three use cycles increased to 37.4 mg Cr/g, which was 2.1 times higher than that of the virgin S-mZVI. After biological reactivation, the Rp of reactivated S-mZVI decreased to 12.30 kΩ cm2. SRB-mediated reactivation removed the electron transfer barriers via biotic and abiotic reduction of Fe(III) (hyd)oxides. Especially, the microbial Fe(III) reduction mediated by FmnA-dmkA-fmnB-pplA-ndh2-eetAB-dmkB protein family enhanced the Fe2+ release from the surface and the subsequent re-formation of reactive and conductive FeSx shell. A long-term PRB column test further demonstrated the feasibility of in-situ biological reactivation and reuse of S-mZVI for enhanced Cr(VI)-contaminated groundwater remediation. Within 64 days, the Cr(VI) removal capacity of S-mZVI in the four use cycles increased by 3.2 times, compared to the virgin one. The bio-reactivation using the SRB-enriched culture and sulfate locally-available in groundwater will reduce the chemical and maintenance costs associated with the frequent replacement of reactive ZVI-based materials. The PRB technology based on the bio-renewable S-mZVI can be a sustainable alternative to the conventional PRBs for the long-lasting and low-cost remediation of groundwater contaminated by oxidative pollutants.

Long-term stable and efficient degradation of ornidazole with minimized by-product formation by a biological sulfidogenic process based on elemental sulfur.

Conventional biological treatment processes cannot efficiently and completely degrade nitroimidazole antibiotics, due to the formation of highly antibacterial and carcinogenic nitroreduction by-products. This study investigated the removal of a typical nitroimidazole antibiotic (ornidazole) during wastewater treatment by a biological sulfidogenic process based on elemental sulfur (S0-BSP). Efficient and stable ornidazole degradation and organic carbon mineralization were simultaneously achieved by the S0-BSP in a 798-day bench-scale trial. Over 99.8 % of ornidazole (200‒500 µg/L) was removed with the removal rates of up to 0.59 g/(m3·d). Meanwhile, the efficiencies of organic carbon mineralization and sulfide production were hardly impacted by the dosed ornidazole, and their rates were maintained at 0.15 kg C/(m3·d) and 0.49 kg S/(m3·d), respectively. The genera associated with ornidazole degradation were identified (e.g., Sedimentibacter, Trichococcus, and Longilinea), and their abundances increased significantly. Microbial degradation of ornidazole proceeded by several functional genes, such as dehalogenases, cysteine synthase, and dioxygenases, mainly through dechlorination, denitration, N-heterocyclic ring cleavage, and oxidation. More importantly, the nucleophilic substitution of nitro group mediated by in-situ formed reducing sulfur species (e.g., sulfide, polysulfides, and cysteine hydropolysulfides), instead of nitroreduction, enhanced the complete ornidazole degradation and minimized the formation of carcinogenic and antibacterial nitroreduction by-products. The findings suggest that S0-BSP can be a promising approach to treat wastewater containing multiple contaminants, such as emerging organic pollutants, organic carbon, nitrate, and heavy metals.