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

Besides acidic environments, pyrite oxidation also occurs in circumneutral environments, such as well-buffered marine and estuarine sediments and salt marshes where low-molecular-weight organic acids (LMWOAs) (e.g., citrate and oxalate) prevail. However, the production of hydroxyl radicals ( OH) from pyrite oxidation by oxygen (O 2 ) in these circumneutral environments is poorly understood. In this study, OH production was measured during the abiotic oxidation of pyrite by O 2 under circumneutral conditions. A pyrite suspension (50   g/L pyrite) that was buffered at pH 6–8 was exposed to air for oxygenation in the dark. Benzoate (20   mM) was added into the suspension to trap OH. At pH 7, the cumulative OH reached 7.5   μM within 420   min in the absence of LMWOAs, whereas it increased to 14.8, 12 and 11.2   μM in the presence of 1   mM ethylenediaminotetraacetate, citrate and oxalate, respectively. When the citrate concentration, which serves as a LMWOAs model, was increased from 0.5 to 5   mM, the cumulative OH increased from 10.3 to 27.3   μM within 420   min at pH 7. With the decrease in pH from 8 to 6, the cumulative OH increased from 2.1 to 23.3   μM in the absence of LMWOAs, but it increased from 8.8 to 134.9   μM in the presence of 3   mM citrate. The presence of LMWOAs enhanced the OH production from pyrite oxidation under circumneutral conditions. In the absence of LMOWAs, OH is produced mostly from the oxidation of adsorbed Fe(II) by O 2 . In the presence of citrate, OH production is attributed mainly to the oxidation of Fe(II)-citrate − by O 2 and secondarily to the oxidation of H 2 O 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 Fe 3+ 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 O 2 in pyrite-rich circumneutral environments, such as marine and estuarine sediments and salt marshes.