The societal impacts of As in water resources in the arid western USA are potentially acute as a consequence of the combined effects of limited water supplies and the pervasive occurrence of naturally occurring As in subsurface geologic formations, including the carbonate-hosted, disseminated gold-bearing formations of the Carlin Trend. The prevalence of As in secondary minerals in gold-bearing carbonate-hosted ores is of interest because of the potential for As release as a result of ore development. A key component to gold mining is the engineering and construction of large-scale heap-leach and waste-rock containment structures that are characterized by variably saturated hydrology. Estimating As release behavior from these structures with a variably saturated reactive flow and transport numerical model requires the quantification of the significant differences in the sorption behavior for the stable redox states for As. Therefore, the objective of this study was to quantify this sorption behavior and to represent the observed behavior with an isotherm formulation. The pH-dependent sorption behavior of arsenite, As(III), and arsenate, As(V), onto two carbonate-hosted gold ores is presented. The experimentally determined pH-dependent sorption behavior for both As(III) and As(V) is consistent with sorption on metal oxides as reported in studies on rock and soils with similar bulk mineralogical properties. The experimental sorption data are represented with two modified isotherm formulations. Modified formulations of the Langmuir isotherm and of the Sips isotherm are presented that include the pH of the sorbate solution as an additional model parameter. These formulations are applied to both As(III) and As(V) sorption data to generate an isotherm surface. The pH-dependent isotherm methodology can be incorporated readily into numerical models for the purposes of estimating As transport behavior in field-scale, variably saturated environments.

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