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Kinetics of Pyrrhotite Oxidation in Seawater: Implications for Mining Seafloor Hotsprings

  • Author(s): Romano, Gina Yolanda
  • Advisor(s): McKibben, Michael A
  • et al.

Laboratory experiments have been performed to evaluate the effects of pH, temperature, dissolved oxygen, and mineral surface area on the rate of oxidation of pyrrhotite in seawater. Experiments employed to determine these effects utilized temperature-controlled circulation baths, Teflon reaction vessels, synthetic seawater, and pure, hand-sorted natural pyrrhotite crystals. Both batch and flow-through reactor methods were used and reaction products were analyzed using inductively coupled plasma-mass spectrometry (ICP-MS). The rate law takes the following form:

Rsp (Fe(1-x)S) = k (MH+)a(MO2(aq))b

where R is the specific oxidation rate of pyrrhotite (moles/L sec), k is the rate constant (a function of temperature and surface area), and a and b are reaction orders for reactant concentrations (M), determined experimentally. The isolation method was used to obtain the reaction order of each reactant on the basis of initial rates. The rate law for the oxidation of pyrrhotite in seawater derived in this study is:

Rsp(Fe(1-x)S) = -5.38 x 10-8 (MH+)0.08±0.03 (MO2(aq))0.30±0.07

The value for k is averaged from runs at 22.0oC, which was used at the primary run temperature for convenience. Values for k from all runs are in Appendix A. Data from batch reactor experiments indicated positive influences of oxidant concentration, surface area, temperature, and [H+] on the initial rate. Pyrrhotite oxidizes significantly faster than chalcopyrite, providing an upper limit to the anthropogenic and natural inorganic weathering rates of seafloor massive sulfide (SMS) deposits. Inorganic rates are most relevant to rapid seafloor mining timespans (minutes to days), within which significant bacterial colonization of freshly ground sulfide mineral surfaces is not likely to occur (e.g., McBeth et al., 2011). In the future, microbial studies will be needed in order to quantify the catalyzing or inhibitory effects of bacteria on natural, in situ seafloor pyrrhotite oxidation using this study as a baseline.

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