MOBILITY OF METALS FROM WEATHERED BLACK SHALE: THE ROLE OF SALT EFFLORESCENCES

Weathering of black shale recently has been a focus of research on the global carbon cycle. In addition to carbon, black shale typically is enriched in trace metals. Therefore, black-shale weathering may be significant in cycling of metals, many of which are environmentally sensitive.

This study found high concentrations of As, Co, Cr, Mo, Ni, V, and Zn in 60 of 1200 stream sediments from across the central Appalachian basin. Distribution of the metal-rich sediment correlates with outcrops of pyrite-rich Devonian black shale. To investigate metal dispersion from black shale and to better understand metal mobility during weathering, we sampled unweathered and weathered shale, salt efflorescences, and soil. This abstract focuses on salt efflorescences and their role in metal cycling.

Efflorescent salts from a Kentucky New Albany shale outcrop were analyzed. Salts span the recognized paragenetic sequence of variably hydrated ferrous/ferric sulfates associated with pyrite oxidation--melanterite, szomolnokite, copiapite, coquimbite, and jarosite. These salts contain high metal concentrations (average ppm concentrations for selected metals are As, 139; Ce, 33; Co, 151; Mo, 64; Ni, 350; Th, 17; U, 76; and Zn, 295). Most metals were originally contained in sulfides, but Ce, Th, and U are tentatively attributed to diagenetic phosphate minerals that dissolve in low pH solutions produced from sulfide oxidation. In water, salts produced a solution with initial pH of 3.0±0.1. The solution then was titrated to pH 8.0, the average river pH in our study area. Metals initially dissolved were quantitatively coprecipitated with amorphous iron oxyhydroxides.

Salt efflorescences can have a significant transitional role in releasing metals from black shale into the surface environment. Oxidation of shale pyrite forms the salts that temporarily store acidity and metals. During rainfall events, salts dissolve and metals are transported in acidic solutions until entering a higher pH environment. There, the metals are captured by oxyhydroxides precipitating on grain surfaces.