Oxidation Kinetics of Pyrite in Synthetic Seawater: Implications for Seafloor Mining Operations
As the demand for industrial materials rises, mining companies have become increasingly interested in exploiting unconventional metal resources. Seafloor massive sulfide (SMS) deposits, one such example of an unconventional resource, will be mined for their high ore grade and abundance along oceanic plate margins. As these deposits are mined, fresh and highly reactive surfaces of sulfide minerals will be exposed to seawater, causing them to immediately oxidize. Pyrite (FeS2) is the most common sulfide mineral and readily oxidizes under atmospheric conditions. Sulfuric acid is a product of the sulfide mineral oxidation process and is often responsible for the devastating effects of acid mine drainage at terrestrial mining sites.
Kinetics experiments have been conducted to determine a rate law for the abiotic rate of pyrite oxidation in synthetic seawater. Experiments run from pH 2-5, 0.995 or 0.10 atm O2, and temperatures of 285 – 303 K were used in rate law calculations. The experimentally derived molal specific rate law is:
Rsp = -10-11.02±0.03[H+]0.39±0.03[PO2]0.44±0.05
where [H+] and [PO2] represent the initial molal concentrations of protons and dissolved oxygen in the seawater, and the rate Rsp is in units of moles m-2 sec-1. The initial rate method was combined with the method of isolation to determine the effects that pH, dissolved oxygen concentration, and temperature have on the pyrite oxidation rate. Results show that the initial concentration of dissolved oxygen is more influential upon the initial pyrite oxidation rate than the initial pH of the seawater under acidic, low temperature conditions.
The pyrite oxidation rate in acidic seawater is the slowest of the sulfides pyrite, pyrrhotite, and chalcopyrite, with the reaction proceeding up to three orders of magnitude slower than that of pyrrhotite in synthetic seawater. The slow pyrite weathering rate (whether natural or anthropogenically induced) enhanced pyrite preservation in massive sulfide deposits. This may explain why VMS deposits are more enriched in pyrite than any other sulfide mineral.