Iron phyllosilicates: Targeted removal by acid treatment and their differing potential to immobilize aqueous sulfide

Abstract Iron-bearing silicates present in mining waste rock are candidates for use in sulfide immobilization as part of a sulfate remediation process. A mineral aggregate derived from taconite overburden (referred to as MS) contains quartz, Fe-bearing carbonates (siderite), and Fe phyllosilicates, including greenalite, stilpnomelane, and minnesotaite. The extent and rate of sulfide immobilization by the Fe-rich silicates of MS were investigated. A series of fractions were formed by treating MS with different acids in which target minerals were selectively removed from the total mixture. Acetic acid targeted siderite, and aqua regia targeted siderite and greenalite. Hydrochloric acid targeted siderite, greenalite, and stilpnomelane. Condensed phosphoric acid targeted all Fe-bearing minerals. Mössbauer spectroscopy showed that the Fe in the minerals was mostly Fe2+ except for stilpnomelane, which had much more Fe3+ that could affect reactivity. The acid-treated fractions were exposed to aqueous sulfide in mildly acidic, anaerobic batch reactors. Sulfidation experiments showed that MS reactivity mirrored the behavior of the sulfidation of siderite alone, with rapid FeS precipitation facilitated by aqueous Fe2+ and increased pH from carbonate dissolution. Acetic acid-treated MS, lacking siderite, and aqua regia-treated MS, lacking siderite and greenalite, still exhibited significant capacity for immobilization due to greenalite and/or stilpnomelane, whereas hydrochloric acid-treated MS, containing only quartz and minnesotaite, was not reactive toward sulfide. Possible reasons for the different reactivity are discussed. These findings demonstrate the potential of Fe phyllosilicate components of taconite waste rock in sulfide immobilization and sulfate remediation.