Kinetics of inorganic arsenopyrite oxidation in acidic aqueous solutions

Abstract

In an effort to help evaluate the significance of common primary As-bearing minerals in releasing As into surface and ground waters, experiments were performed to determine rate laws for the irreversible inorganic aqueous oxidation of arsenopyrite by dissolved O 2 , Fe 3+ and NO 3 - in low temperature acidic solutions. Batch reactor run conditions varied from pH 2–4.5 and 10–40 °C at ionic strength 0.01 M. A major constraint on defining and measuring the rate of arsenopyrite oxidation is the non-stoichiometry (incongruency) of the reaction in acidic solutions; As and S are not released completely into solution, apparently remaining behind as more slowly-dissolving solids in an Fe-depleted lattice. Therefore the rate of mineral oxidation (destruction) at low pH is best defined and measured by rates of change in total dissolved Fe concentrations, not by changes in dissolved As or S concentrations or Eh. The measured mineral dissolution rate therefore places an upper limit on actual inorganic As-release rates, providing a conservative basis for geochemical modeling that may over-predict, but not underestimate, As concentrations observed in natural settings. The molal specific rate laws for the oxidation of arsenopyrite by dissolved Fe 3+ (pH 2) and O 2 (pH 2–4.5), respectively, are: R sp,Fe 3 + ( moles mineral m - 2 s - 1 ) = - 10 - 5.00 ( M Fe 3 + ) 1.06 ± 0.11 R sp,O 2 aq ( moles mineral m - 2 s - 1 ) = - 10 - 6.11 ( M O 2 aq ) 0.33 ± 0.18 ( M H + ) 0.27 ± 0.09 The results indicate that Fe 3+ oxidizes arsenopyrite at least 10 times faster than dissolved O 2 . Nitrate does not oxidize arsenopyrite. At low pH arsenopyrite oxidizes 3–4 orders of magnitude faster than (arsenical) pyrite and 4–5 orders of magnitude faster than realgar and orpiment. Therefore in rocks with low ratios of arsenopyrite to other As-bearing sulfides, including coals and ores, arsenopyrite oxidation can still be the dominant source of As-release from sulfide minerals. In natural settings undergoing long-term arsenopyrite oxidation at low pH (in the absence of secondary phases such as scorodite and Fe hydroxides), the rate of As-release may ultimately be controlled by the rate of dissolution of As–S compounds that form on the Fe-depleted arsenopyrite mineral surface. Rate data are therefore needed on native As, realgar and orpiment at low pH, as proxies for these compounds.