KINETICS OF AQUEOUS ARSENIC RELEASE FROM SULFIDE MINERALS

Pyrite and arsenopyrite are the most abundant arsenic-bearing minerals in the crust, making them important natural primary sources of arsenic release to waters and soils.  Although the kinetics of aqueous pyrite oxidation are well known, those of arsenopyrite are not.  

A high flow-rate batch reactor experimental method was used to measure rates of inorganic arsenopyrite oxidation in 0.01 M NaCl aqueous solutions as a function of pH, oxidant concentration and temperature.  Natural arsenopyrite crystals were ground, sieved, and cleaned ultrasonically and chemically to produce fresh, bulk grain surfaces free of fine mineral powder for the rate measurements.  The initial rate method was used to determine reaction rate order dependencies on pH (from 2-5) and oxidant concentrations (O_2(aq), Fe³⁺).  B.E.T surface area measurements of the grains were used to calculate specific rate constants, and temperature dependence of the rates were measured over the range 10-40^oC.  For systems in which ferric ion is initially absent, the specific rate law at 25^oC is:

        R_sp = -k (O₂) ^0.33 (H+) ^0.27

where dissolved oxygen and proton concentrations are expressed in molar units, the specific rate in units of moles arsenopyrite m^-2 s^-1, and the rate constant is 10^-6.11 moles^0.5 L^0.5 m^-2 s^-1.  Arsenopyrite oxidation by aqueous ferric iron is significantly faster and shows a first-order dependence on Fe³⁺ concentration.  

Arsenopyrite oxidation by aqueous O₂ and Fe³⁺ under these conditions is nearly three orders of magnitude faster than pyrite oxidation, indicating that arsenopyrite will release As much faster even in rocks with high pyrite/arsenopyrite ratios.  In the absence of initial ferric iron the release of As into solution is non-stoichiometric, with about half of it apparently remaining behind with sulfur on mineral surfaces.  The presence of initial ferric iron or pH values above 4.5 cause As release to become stoichiometric.  This behavior suggests that ferric iron either inhibits As retention on arsenopyrite surfaces or rapidly oxidizes As to a more mobile aqueous species.