The oxidation of pyrite at pH 7 in the presence of reducing and nonreducing Fe(III)-chelators

The oxidation rate of pyrite at pH 7, 25 degrees C and at constant partial pressure of oxygen (0.21 and 0.177 atm) was measured in the presence of the Fe(III)-chelators NTA, oxalate, leucine, EDTA, citrate, IDA and the Fe(III )-reductant ascorbic acid. With the exception of leucine and EDTA, non-redu cing Fe(III)chelators increased the oxidation rate relative to the referenc e state of formation of the Fe(OH)(2)(+) complex at pH 7. The rate increase was proportional to the logarithm of the conditional stability constant of the ligands for the complexation of Fe3+. No effect on the oxidation rate was observed in the presence of EDTA, which shifted the redox potential of the redox couple Fe2+/Fe3+ to a Value below that in the absence of any Liga nd at pH 7. Ascorbic acid decreased the pyrite oxidation rate by a factor o f 5 at ascorbic acid concentrations between 10(-4) and 10(-2) mol L-1. Comp arison of the rate constants for the oxidation of ascorbic acid by surface bound Fe(III) in the absence and presence of pyrite shows that the pyrite s urface accelerates this reaction by a factor of 10. The oxidation of both p yrite and ascorbic acid is of fractional order with respect to ascorbic aci d (HAsc): r(py) = 0.55 c(HAsC)(-0.35) rHAsc = 3.6 c(HAsc)(0.59) Both the results from experiments with Fe(III)-chelating ligands and the Fe (III)-reductant, suggest a very efficient interference in the electron cycl ing between Fe(II) and Fe(III) at the pyrite surface. The interference seem s to be mainly related to the reductive side of the iron cycling. It is the refore concluded that the electron transfer between ferric iron and pyritic sulfur limits the pyrite oxidation rate at pH 7. Copyright (C) 1999 Elsevi er Science Ltd.