Release of Aluminum, Arsenic, Cadmium, Chromium, Copper, Iron, Lead, and Zinc in a Coal Leachate, and their Removal from Solution Undergoing Neutralization

Abstract

Whole coal contains significant amounts of iron pyrite which is oxidized ultimately to ferric acid sulfate. As a result, trace elements are released from the coal and other minerals in potentially hazardous concentrations. The purpose of this research was to: 1) study the release and mobility of selected trace elements during the weathering of coal; 2) seek to understand factors controlling solubility of trace elements in a synthetic, acidic leachate undergoing gradual neutralization; and 3) develop a chemical thermodynamic computer model to predict the effects of dilution and neutralization of leachate on trace element mobility and speciation. Samples collected periodically from a slurry of whole ground coal in water were filtered and analyzed for dissolved sulfate (by ion chromatrography), iron (by flame atomic absorption spectrophotometry), and Al, Zn, Cd, Cu, Cr, Pb, As, and Se (by graphite furnace AAS). Iron, copper, and probably arsenic tracked the production of sulfate, while aluminum, zinc, chromium, and cadmium concentrations were stable or rose slightly. A synthetic leachate of ferric sulfate and sulfuric acid was doped with trace levels of Al, Zn, Cu, Cd, Cr, Pb, As, and Se. Slow injection of sodium bicarbonate solution neutralized the stirred system, though hydrolysis of iron buffered the pH near 2.5. Computer modeling of the sample analyses indicated that sulfate complexes dominated the speciation of iron and the trace elements. The other findings were used to develop a thermodynamic equilibrium model based on the aqueous geochemistry computer model PHREEQE. Iron and sulfate removal were best modeled by the precipitation of Fe16O16 (OH) 12 (SO4 ) 2. Aluminum solubility was modeled by precipitation of jurbanite below pH 4, of bayerite and basaluminite for pH 4 - 5, and of gibbsite at pH above 5. Chromium, copper, and lead removal was modeled by solid solution formation with the ferric oxyhydroxysulfate precipitates. Program convergence failures above pH 5 precluded the modeling of zinc and cadmium, but it is hypothesized that their ions are adsorbed onto suspended particles of hydrous ferric oxyhydroxides. The model was tested with our laboratory data, and field data from a creek system contaminated with acid sulfate mine drainage.