Application and uncertainty of a geochemical speciation model for predicting oxyanion leaching from coal fly ash under different controlling mechanisms.

Three primary mechanisms (adsorption to iron oxides or analogous surfaces, co-precipitation with Ca, and substitution in ettringite) controlling oxyanion retention in coal fly ashes (CFAs) were identified by differentiating the leaching behavior of As, B, Cr, Mo, Se, and V from 30 CFAs. Fidelity evaluation of geochemical speciation modeling focused on six reference CFAs representing a range of CFA compositions, whereby different leaching-controlling mechanisms of oxyanions were systematically considered. For three reference CFAs with low Ca and S content, calibration of adsorption reactions for the diffuse double-layer model for hydrous ferric oxides improved the simultaneous prediction of oxyanion leaching, which reduced uncertainties in Se and V predictions caused by nonideal adsorption surfaces and competitive adsorption effects. For two reference CFAs with intermediate Ca content, the solubility constants for Ca-arsenates from literature and postulated phases of B, Cr, Se, and V were used to describe co-precipitation of oxyanions with Ca-bearing minerals under alkaline conditions. For the reference CFA with high Ca and S content, an ettringite solid solution was used to capture the simultaneous retention of all oxyanions at pH> 9.5. Overall, the simultaneous leaching predictions of oxyanions from a wide range of CFAs were improved by calibration of adsorption reactions and controlling solid phases.

The influence of redox conditions on aqueous-solid partitioning of arsenic and selenium in a closed coal ash impoundment.

A closed coal ash impoundment case study characterized the effects of field redox conditions on arsenic and selenium partitioning through monitoring of porewater and subsurface gas in conjunction with geochemical speciation modeling. When disposed coal ash materials and porewater were recovered for testing, oxidation led to lower arsenic and higher selenium concentrations in leaching test extracts compared to porewater measurements. Multiple lines of evidence suggest multiple mechanisms of arsenic retention are plausible and the concurrent presence of several redox processes and conditions (e.g., methanogenesis, sulfate reduction, and Fe(III)-reduction) controlled by spatial gradients and dis-equilibrium. Geochemical speciation modeling indicated that, under reducing field conditions, selenium was immobilized through the formation of insoluble precipitates Se(0) or FeSe while arsenic partitioning was affected by a progression of reactions including changes in arsenic speciation, reduction in adsorption due to dissolution and recrystallization of hydrous ferric oxides, and precipitation of arsenic sulfide minerals.