ABIOTIC OXIDATION RATE OF CHALCOPYRITE IN SEAWATER: IMPLICATIONS FOR SEAFLOOR SULFIDE MINING

In situ mining of seafloor massive sulfide (SMS) deposits will have consequences thus far not quantified. On land, interaction of mined sulfide minerals with surface and groundwaters can yield acid mine drainage. Pulverization of SMS on the ocean floors will produce highly reactive sulfide mineral surface areas, leading to the localized potential for seafloor acid generation. Chalcopyrite (CuFeS₂) is one of several ore minerals found in SMS deposits whose oxidation kinetics need to be quantified to estimate the significance of acid production.

To constrain the oxidation rate of chalcopyrite in seawater, the initial rate experimental method was employed to derive a rate law in the form:

r = k(O₂)^x(H⁺)^y

where the rate of the reaction (r) in moles/cm²/second is a function of the rate constant (k), dissolved oxygen, and proton concentration of synthetic seawater. k is dependent on temperature as well as surface area. Reaction orders x and y were determined experimentally by evaluating changes in the rate corresponding to adjustments in oxygen concentration and pH, respectively.

Data collected from batch reactor experiments without abundant precipitates (most below pH 4.5), between 7^oC and 25^oC, and P_O2 from 0.1 to 0.995 were incorporated into the rate law. Two grain size fractions were examined with varying starting material mass to investigate the influence of surface area on the rate of oxidation.

Chalcopyrite oxidizes slowly in seawater relative to other sulfide minerals like pyrrhotite (Fe_1-xS), so data from this study will establish a minimum rate of abiotic SMS weathering by oxidation. Constraining the oxidation rates of individual sulfide mineral species will be useful in modeling SMS mining repercussions, as well as natural chemical cycling in the oceans over geologic time. The potential for local acid generation can be viewed as a microcosm of the global problem of ocean acidification caused by dissolution of anthropogenic atmospheric CO₂. Data show sulfide mineral oxidation rates increase with lower pH, implying that a worldwide drop in ocean pH may amplify the dissolution of SMS deposits, changing the marine ecosystem.