Sulfate-rich wastewater poses an ecological threat to freshwater ecosystems and is highly regulated in the wild rice lakes of Minnesota. Biological sulfate reduction is one of several nascent remediation technologies that uses sulfate-reducing bacteria to convert sulfate to sulfide (H₂S, HS^-, S^2-). After reduction, the sulfide can then be immobilized as a solid phase material and removed from the aqueous phase via reaction with iron-bearing minerals that are abundant in waste materials from Minnesota’s mining industry. While the sulfidation of iron-bearing substrates has been studied before, the focus has largely been on ferric minerals, zero-valent iron and steels. We studied the reaction of aqueous sulfide with siderite (FeCO₃), an abundant ferrous mineral in the “Mesabi Select” aggregate, in a geochemically engineered system with emphasis on its accessible capacity for sulfide immobilization for the purpose of water remediation. In acidic, buffered batch reactors, siderite was exposed to a concentrated sulfide solution via constant-volume spiking. Fe²⁺ and sulfide concentrations were monitored after sulfide exposures, producing reaction rates and sulfide immobilization capacities. These data showed the accessible sulfide immobilization capacity of siderite was typically limited by the Fe²⁺ concentration of the aqueous phase immediately before the first sulfide exposure. This capacity could be marginally tuned by varying the mass ratio of siderite to sulfide. The reaction products were characterized using powder X-ray diffraction, scanning and transmission electron microscopy, and energy-dispersive spectroscopy. Mackinawite (FeS) appeared most abundant, and evidence of greigite (Fe₃S₄) and elemental sulfur (S⁰) was also observed. The formation of iron monosulfides, rather than disulfides, was promising due to their preferential oxidation to S⁰ rather than sulfate. Preliminary oxidation studies showed the persistence of mackinawite and greigite after months in ambient conditions as well as the formation of lepidocrocite (γ-FeO(OH)) and S⁰ in greater abundance. These results improve our understanding of the sulfidation of siderite and its viability for novel use in a sulfate remediation technology.

R Lee Penn

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