Fast and eco-friendly sulfidation of zero-valent iron using elemental sulfur and Geobacter sulfurreducens for groundwater remediation.

Sulfidated zero-valent iron (S-ZVI) with high electron selectivity and reactivity toward contaminants is a promising reagent for groundwater remediation. However, the widely-used chemical and mechanical sulfidation methods to prepare S-ZVI are often chemical- or energy-intensive. Biogenic sulfidation method relying on sulfate-reducing bacteria (SRB) are considered as green methods, yet are time-consuming and may release toxic and odorous hydrogen sulfide gas. Instead, elemental sulfur (S0) as sulfur precursor may avoid prolonged preparation time and hydrogen sulfide emission. However, elemental sulfur is difficult to be utilized by SRB so as to hinder the production rate of S-ZVI. In this study, therefore, we proposed a novel biogenic sulfidation method using Geobacter sulfurreducens (PCA) to achieve fast, low-cost and eco-friendly S-ZVI preparation with insoluble S0 and Fe0 powders. The results showed that the successful sulfidation of ZVI using PCA strains and S0 without the addition of organic carbon was achieved within 2 h, 13 times faster than SRB mediated sulfidation. The rate of hexavalent chromium removal by the PCA-synthesized S-ZVI was 68 % higher than that by SRB-synthesized S-ZVI. The fast sulfidation proceeded via the reductive dissolution of iron oxide passivation layer on ZVI by PCA and the enhanced electron transfer between ZVI and S0 mediated by conductive PCA cells. The PCA-mediated sulfidation minimized the potential secondary pollution associated hydrogen sulfide and chemical consumption for the S-ZVI preparation. This study demonstrated a fast, chemical-saving and eco-friendly approach for S-ZVI preparation, which could be an ideal solution for groundwater remediation.