Production of hydroxyl radicals from abiotic oxidation of pyrite by oxygen under circumneutral conditions in the presence of low-molecular-weight organic acids

In the absence of LMOWAs, OH is produced mostly from the oxidation of adsorbed Fe(II) by O2. In the presence of citrate, OH production is attributed mainly to the oxidation of Fe(II)-citrate− by O2 and secondarily to the oxidation of H2O on surface-sulfur defects. The acceleration of pyrite oxidation by Fe(III)-citrate increases OH production. Fe(II)-citrate− is generated mainly from the complexation of adsorbed Fe(II) by citrate and the reduction of Fe(III)-citrate, and the generation is suppressed by the oxidation of adsorbed Fe(II). Fe(III)-citrate is generated predominantly from Fe(II)-citrate− oxidation. Most soluble Fe3+ that is produced from pyrite oxidation hydrolyzes to Fe(III) hydroxide. Kinetic models that were developed according to the proposed mechanisms identified the relative importance of each reaction for OH production. Using the kinetic model, the oxidation efficiencies of the redox-active substances that react with OH at different reaction-rate constants were estimated in the presence of 1 mM citrate. This study presents on the overlooked role of LMWOAs in enhancing OH production from pyrite oxidation by O2 in pyrite-rich circumneutral environments, such as marine and estuarine sediments and salt marshes.