Comparison of rate laws for the oxidation of five pyrites by dissolved oxygen in acidic solution

Abstract

Pyrites from five localities were oxidized with dissolved oxygen (DO) in acidic solution at 298 K. The objective of this study was to compare the oxidation rates of pyrite samples with different impurity contents. Initial conditions were a solution pH adjusted to approximately 4 and air saturation. DO, Fe2+, Fe(III) (as Fe3+ and Fe(III)-OH complexes), and SO42− concentrations in the experimental solutions were periodically measured along with pH and redox potential (Eh). On the basis of the relationship between DO concentration and reaction duration, the rate laws for the oxidation of the pyrites were obtained. The rate laws differed for the various pyrites and can be expressed as follows: Rate = −(1/fDO) (V/A) (d[DO]/dt) = k[DO]nwhere the unit of oxidation rate is mol m−2 s−1, fDO is the stoichiometric coefficient for DO in the pyrite oxidation reaction, V is the volume (L) of the solution, A is the total surface area (m2) of pyrite, [DO] is the DO concentration (mol L−1), t is time (s), k is a rate constant at pH ∼ 4, and n is the reaction order (0.5-1). The differences in the rate laws were due to differences in the oxidation rates of Fe2+ during the oxidation of the pyrites, and the differences in the oxidation rates of Fe2+ were related to the occurrence of impurities in the pyrites. Oxidation of Fe2+ was accelerated by increases in the total amount of impurities in the pyrite. Precipitation of the Fe3+ produced during the reaction was promoted by increased amounts of arsenic in the pyrites.