Dissolution mechanisms of silicate minerals yielded by intercomparison with glasses and radiation damage studies

Abstract

The chemical and mineralogical forms of As, Pb, Cr, and Cd were studied in a metal-contaminated organic soil (Histosol) that received runoff and seepage water from a site that was once occupied by a lead smelter. Soil samples were collected from different depth intervals during both wet and dry seasons and analyzed using bulk powder X-ray diffraction (XRD), synchrotron-based micro X-ray diffraction (μ-XRD), and micro X-ray fluorescence (μ-SXRF) spectroscopy. There was a clear pattern of mineral distribution with depth that indicated the presence of an intense redox gradient. The oxidized reddish brown surface layer (0–10 cm) was dominated by goethite (α-FeOOH) and poorly crystalline akaganeite (β-FeOOH). Lead and arsenic were highly associated with these Fe oxides, possibly by forming inner–sphere surface complexes. Gypsum (CaSO4·2H2O) was abundant in the layer as well, particularly for samples collected during dry periods. Fe(II)-containing minerals, such as magnetite (Fe3O4) and siderite (FeCO3), were identified in the intermediate layers (10–30 cm) where the reductive dissolution of Fe(III) oxides occurred. A number of high-temperature minerals, such as mullite (3Al2O3·2Si2O), corundum (α-Al2O3), hematite (α-Fe2O3), and wustite (FeO) were identified in the subsurface and they probably formed as a result of a burning event. Several sulfide minerals were identified in the most reduced layers at depths > 30 cm. They included realgar (AsS), alacranite (As4S4), galena (PbS), and sphalerite (Zn, Fe2+)S, and a series of Fe sulfides, including greigite (Fe2+Fe3+2 S4), pyrrhotite (Fe1 − xS), mackinawite (FeS), marcasite (FeS2), and pyrite (FeS2). Most of these minerals occurred as almost pure phases in sub-millimeter aggregates and appeared to be secondary phases that had precipitated from solution. Despite the elevated levels of Cd in the soil, no specific Cd phases were identified. The complex mineralogy has important implications for risk assessment and the design of in-situ remediation strategies for this and similar metal-contaminated sites.