Kinetics of sulfide mineral oxidation in seawater: Implications for acid generation during in situ mining of seafloor hydrothermal vent deposits

Growth in global metal demand has fostered a new age of unconventional mining on the seafloor. In situ pulverization and extraction of seafloor massive sulfide (SMS) deposits is economically attractive due to minimal overburden and high ore grades. However, important environmental questions remain on the significance of localized acid generation via irreversible sulfide mineral oxidation. Data on the reaction kinetics are necessary to estimate anthropogenic acid production during seafloor mining.Laboratory experiments were performed to evaluate the effects of pH, temperature, dissolved oxygen, and surface area on the oxidation rate of pyrrhotite and chalcopyrite in seawater. These minerals were chosen to constrain the range of reaction rates because pyrrhotite oxidizes relatively quickly while chalcopyrite is kinetically slow. The rate laws for the abiotic oxidation of pyrrhotite and chalcopyrite in seawater at 22 °C are given in the form:Rsp=K(mO2)a(mH+)bwhere Rsp is the specific rate (moles m⁻² sec⁻¹), k is the rate constant, oxygen and proton concentrations are expressed in molalities (m), and their reaction orders as a and b, respectively. The specific rate laws obtained for each sulfide studied are:Rsp(pyrrhotite)=−10−7.27(mO2(aq))0.51±0.08(mH+)0.08±0.03Rsp(chalcopyrite)=−10−9.38(mO2(aq))1.16±0.03(mH+)0.36±0.09When used to quantitatively predict maximum acid generation rates, these rate laws indicate that acid production from in situ SMS mining is insufficient to exceed the buffering capacity of advecting seawater. We also calculated the residence times of crushed sulfides in seawater with low PO2 (0.10 atm, pH of 8, 23 °C) and find that, depending on grain size, mining waste may persist near the seafloor for years. The implications are positive in terms of slow acid production, but potentially problematic considering the potential ecological effects of an unnatural influx of particulates.