CHEMICAL-COMPOSITION OF OCHREOUS PRECIPITATES FROM OHIO COAL-MINE DRAINAGE

Ochreous precipitates and associated solutions were collected from waters draining abandoned coal works at 28 locations in eastern Ohio. Major and trace element contents of the precipitates were determined, and comparisons were made with source solutions to evaluate the effects of pH, organic C, and ionic strength on solid-phase composition and trace metal distribution. The pH and calculated ionic strength of the effluents ranged from 2.6 to 7.8 and 6 to 106 mmol L-1, respectively. Associated precipitates consisted primarily of poorly crystallized oxides and oxyhydroxides of Fe with jarosite occuring as a major constituent of only one specimen. Mean Fe, SO4, Al, and Si contents of the precipitates were 8.7, 1.34, 1.11, and 0.25 mol kg-1, respectively. Silicon content increased with increasing pH of the source solution suggesting pH-dependent adsorption. In contrast, SO4 and Al appeared to be influenced both by precipitation and adsorption reactions. Partition coefficients for As, B, Ba, Co, Cu, Cr, Mn, P, and Zn showed a preferential accumulation of all elements by the solid phase with mean log K(d) values ranging from 2.9 for Cu to 6.3 for As. Manganese, Co, and Zn all showed increased partitioning into the solid phase with increasing pH and decreasing ionic strength of the drainage waters. Copper appeared to be strongly associated with organic matter, and a weaker but positive relationship was also observed with Co and Zn. No effects of pH, ionic strength, or organic C were observed for any other trace elements. Solubility equilibrium calculations suggested that the solid phase concentrations of Co, Cr, Cu, Mn, Zn, and As were primarily controlled by adsorption and/or coprecipitation phenomena whereas Ba and P activities in solution exceeded the solubilities of barite and strengite, respectively.