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

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.