Abstract
A five-year, humidity-cell experiment was used to study the weathering evolution of a low-sulfide, granitic waste rock at 5 and 22 °C. Only the rock with the highest sulfide content (0.16 wt %) released sufficient acid to overcome a limited carbonate acid-neutralization capacity and produce a substantial decline in pH. Leached SO4 and Ca quickly increased then decreased during the first two years of weathering. Sulfide oxidation continued to release acid and SO4 after carbonate depletion, resulting in an increase in acid-soluble elements, including Cu and Zn. With the dissolution of Al-bearing minerals, the pH stabilized above 4, and sulfide oxidation continued to decline until the end of the experiment. The variation in activation energy of sulfide oxidation correlates with changes in sulfide availability, where the lowest activation energies occurred during the largest releases of SO4. A decrease in sulfide availability was attributed to consumption of sulfide and weathered rims on sulfide grains that reduced the oxidation rate. Varying element release rates due to changing carbonate and sulfide availability provide identifiable geochemical conditions that can be viewed as neutralization sequences and may be extrapolated to the field site for examining the evolution of mineral weathering of the waste rock.
Highlights
The oxidation of sulfide minerals in waste rock can produce acid rock drainage (ARD), which is characterized by low pH, elevated SO4 concentrations, and the mobilization of elements such Co, Cu, Fe, Ni, and Zn that are typically less mobile under neutral pH conditions [1,2,3]
Waste rock samples were collected through a random selection method using power equipment and/or a shovel per standard practice for sampling aggregates [22], and the samples were subsampled for laboratory use per American Society for Testing and Materials (ASTM) standards [23,24,25]
Type III rock produced the lowest slake durability value (Table 1) indicating it is the weakest of the three rock types; the standard deviation for the three replicate samples of each rock type (35.3 ± 2.1 for Type I, 34.2 ± 0.5 for Type II, 33.5 ± 0.3 for Type III) indicates a variability that lessens the distinction between the rock types
Summary
The oxidation of sulfide minerals in waste rock can produce acid rock drainage (ARD), which is characterized by low pH, elevated SO4 concentrations, and the mobilization of elements such Co, Cu, Fe, Ni, and Zn that are typically less mobile under neutral pH conditions [1,2,3]. A significant part of the project was a long-term (years), humidity-cell experiment to examine the generation of ARD, acid neutralization, and element release rates from the accelerated weathering of small-scale samples of waste rock under different temperatures. Insight into the mobilization/immobilization of oxidation products assists in understanding the generation of ARD, which may occur over periods of years to decades to centuries [2,4,5,6] The utility of this long-term, humidity-cell experiment is the identification of possible changes in leachate chemistry due to changes in the availability of acid-generating and acid-neutralizing minerals that provide a temporal trend of neutralization sequences for evaluation of field conditions.
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