experimental study of iron sulfides weathering under simulated calcareous soil conditions

In calcareous sites, hard rock mining activities release pyrite (FeS₂), pyrrhotite (Fe₁₋ₓ S) and other sulfides to soils. The sulfides then undergo weathering processes, generating acid rock drainage and secondary compounds. Despite the potentially important environmental impacts, very few studies have considered the mechanisms of pyrite and pyrrhotite weathering and the transformation of secondary compounds under neutral-alkaline carbonated conditions. In this study, we used an experimental approach combining electrochemical, microscopic and spectroscopic techniques to examine the interfacial processes involved in pyrite and pyrrhotite weathering under simulated calcareous soil conditions. The results showed an initial oxidation step with the formation of variable amounts of surface sulfur compounds (e.g., polysulfides, S ₙ ²⁻, and elementary sulfur, S⁰) and acid generation, leading to significant modification of the oxidative behavior of the minerals. The surface changes that occurred as a result of mineral weathering provoked transient enhancement of pyrite reactivity and progressive passivation in the pyrrhotite system. Iron sulfides weathering was found to involve the formation of an intermediate siderite (FeCO₃)-like compound, preceding the predominant formation of K-jarosite (K·Fe₃(SO₄)₂(OH)₆) and/or ferric oxyhydroxide (α, γ-FeOOH) compounds, depending on the surface acid condition reached in the systems. Mechanisms of pyrite and pyrrhotite weathering in calcareous soils are suggested on the basis of surface characterization and chemical analysis of the leachates generated, and the environmental implications are discussed.