Quantification of mineral dissolution rates and applicability of rate laws: Laboratory studies of mill tailings

Reliable quantification of mineral weathering rates is a key to assess many environmental problems. In this study, the authors address the applicability of pure mineral laboratory rate laws for dissolution of mill tailings samples. Mass-normalised sulfide and aluminosilicate mineral dissolution rates, determined in oxygenated batch experiments, were found to be different between two samples from the same ∼50-year-old, carbonate-depleted mill tailings deposit. Consideration of difference in particle surface area and mineralogy between the samples resolved most of this discrepancy in rates. While the mineral surface area normalised dissolution rates of pyrite in a freshly crushed pure pyrite specimen and a sulfide concentrate derived from the tailings were within the range of abiotic literature rates of oxidation by dissolved molecular O2, as were rates of sphalerite and chalcopyrite dissolution in the tailings, dissolution rates of pyrite and aluminosilicates in the tailings generally differed from literature values. This discrepancy, obtained using a consistent experimental method and scale, is suggested to be related to difficulties in quantifying individual mineral reactive surface area in a mixture of minerals of greatly varying particle size, possibly due to factors such as dependence of surface area-normalised mineral dissolution rates on particle size and time, or to non-proportionality between rates and BET surface area.