The kinetics of reactions between pyrite and O 2-bearing water revealed from in situ monitoring of DO, Eh and pH in a closed system

Abstract

Details of reactions between pyrite and water initially equilibrated with the atmosphere (pO 2 = 0.2 atm and pCO 2 = 10 −3.5 atm) were investigated in a closed-system, batch reactor at 25°C and 37°C. The values of dissolved O 2 content (DO), Eh, and pH of the experimental solutions were continuously monitored during the reactions that lasted from ∼30 h to ∼160 h; the SO 4 2− content was also determined for solutions periodically withdrawn from the experimental system. The changes with time in these variables of the experimental solutions suggest that pyrite decomposition proceeds through three major overall reactions. The first is the dissolution of iron monosulfide, commonly present on fractured pyrite surfaces, to generate Fe 2+, SO 4 2−, and H 2: “FeS”+4H 2 O⇒Fe 2+ +SO 4 2− +4H 2 Dissolved CO 2 facilitates this reaction, but dissolved O 2 is not involved. The second reaction is the oxidation of pyrite by dissolved O 2 to generate Fe 2+ and SO 4 2−: FeS 2+7/2O 2+H 2O⇒Fe 2++2SO 4 2−+2H + The third is the reaction to produce ferric hydroxide and SO 4 2−: FeS 2+15/4O 2+7/2H 2O⇒Fe(OH) 3(s)+2SO 4 2−+4H + Reactions (1) and (2) appear to be first-order with respect to [O 2] as suggested by Manaka (1998). The rate law for pyrite decomposition at pH = 5.7 ± 0.3 and T = 25°C is determined to be: − d[ py]/ dt = 10 −5.3±0.5 [O 2] (mol/m 2/s) for reaction (1) and − d[ py]/ dt = 10 −6.0±0.5 [O 2] (mol/m 2/s) for reaction (2). The pH dependency of the reaction rates was not determined in this study. In each stage of pyrite (and/or iron monosulfide) decomposition, the overall reactions can be divided into two principal reactions: (i) dissolution of minerals by acid (e.g., FeS to Fe 2+ and H 2S; FeS 2 to Fe 2+ and HS 2 −), and (ii) oxidation of aqueous sulfides, polysulfides, and Fe 2+ by H 2O and O 2. The rates of the dissolution reactions are generally faster than those of the redox reactions; the latter reactions only become significant when the reaction products from the former reactions have increased significantly. Previously, the rates of pyrite oxidation by dissolved O 2 were generally estimated from the rates of increases in Fe 2+ and/or SO 4 2− of solutions in the experimental systems that are open to O 2. However, those rates may be much higher or lower than the true rates: the true rates may be accurately determined from the rates of decreases in dissolved O 2 during reactions with pyrite in a closed system. Examination of the equilibrium relationships among the DO, Eh, pH, SO 4 2− and Fe 2+ of experimental solutions suggest that equilibrium is generally established among H +, OH −, e −, H 2, Fe 2+, and Fe(OH) 3, but not between these species and O 2, SO 4 2−, or pyrite, and that the Eh-pH-H 2 values of most groundwaters are generally buffered by ferric hydroxide (± clays ± carbonates).