Effects of pH on the Arrhenius Paradigm

摘要

Kinetic weathering reactions are temperature sensitive, and most industry standard kinetic tests are conducted at room temperature. The temperature effect on sulphide oxidation rates measured in room temperature experiments is generally scaled using the Arrhenius equation. Activation energies are only available for a limited number of mineral phases, therefore, temperature scaling is generally done by assuming metal leaching rates are controlled by oxidation of the dominant sulphide mineral, which is often pyrite. In order to evaluate this approach, 12 humidity cell tests were operated at room temperature until geochemical loading rates stabilized. The tests were then moved to a cold room at 4°C where the tests were continued. This temperature change would theoretically result in pyrite oxidation rates decreasing by a factor of 0.37 to 0.11 based on activation energies available in the literature. The decrease in sulphate loading rates generally fell within this range. However, distinct differences are observed in metal loading rates depending on the pH of the kinetic test. In acidic samples, most metal loading rates decreased to a similar degree as sulphate. Conversely, in pH-neutral tests declines in metal cation loading rates showed little change, or in some instances produced higher loading rates at colder temperatures. While temperature effects on individual elements may vary depending on the activation energy of the host mineral, increases in metal loading rates at lower temperatures is not consistent with the Arrhenius equation. These results demonstrate that under acidic conditions the Arrhenius relationship accurately predicts temperature effects on rates of metal cation release, however, this equation cannot accurately predict metal cation loading behavior in pH-neutral kinetic tests. The inability to accurately predict metal cation behavior in pH-neutral kinetic tests is interpreted to result from metal cation release being more closely tied to pH related solubility constraints than to weathering rates of primary sulphides.