EXPERIMENTAL STUDIES OF SPHALERITE (ZNS) DISSOLUTION: RELATIVE SIGNIFICANCE OF PH, DISSOLVED OXYGEN, AND FERRIC IRON ON RATES OF ZINC SOLUBILIZATION

Pyrite (FeS₂) weathering often dominates the aqueous chemistry of acid drainage, but dissolution of monosulfide ore minerals such as sphalerite (ZnS) can contribute significant amounts of dissolved metals to effluent waters in metal-mining wastes where pyrite is low in abundance or metals content.  Sphalerite dissolution occurs via 1) nonoxidative hydrolysis, 2) oxidation by dissolved oxygen [O₂(aq)], 3) oxidation by ferric iron [Fe(III)], and 4) the catalytic effect of microbes on the ferrous-to-ferric oxidation rate.  The combined rate of ZnS dissolution attributable to the first three (abiotic) processes has been evaluated by experimental leaching of ZnS in acid solution (pH 2 to 4) at 25 °C.

Sphalerite dissolution rates (expressed as rates of Zn²⁺ solubilization) range from 10^-9 to 10^-11 mol Zn²⁺ L^-1 s^-1.  The slowest rate of ZnS dissolution is via nonoxidative hydrolysis that is weakly dependent upon pH.  Faster rates of ZnS dissolution are dependent primarily upon the iron content of the solid.  In the presence of O₂(aq) in an abiotic system, low-iron (<0.2 wt %) ZnS is oxidized at a slower rate than high-iron (12.1 wt %) ZnS because of limitations of available total iron and the rate of abiotic conversion of Fe(II) to Fe(III).  Low-iron ZnS dissolution is initially mediated primarily by O₂(aq) oxidation; this process is faster than the nonoxidative process but slower than Fe(III) oxidation.

High-iron ZnS is solubilized by a combination of the O₂(aq) and Fe(III) oxidative processes, and because of higher available total iron, is less-limited by the conversion of Fe(II) to Fe(III).  Faster iron solubilization rates (10^-9 to 10^-13 mol Fe(tot) L^-1 s^-1) result in faster ZnS dissolution rates, indicating Fe(III) oxidation is the most rapid abiotic ZnS dissolution mechanism.

Previous ZnS dissolution rate laws used in some modeling scenarios were based on only Fe(III) oxidation or only O₂(aq) oxidation, and generally led to underestimation of field Zn concentrations.  This suggests that a revised rate law that incorporates ZnS dissolution mediated by all three species (Fe(III), O₂(aq) and H⁺) is needed.  Additional work will address the specific effects of each dissolution process on the rate law.