Effect of HA-Fe(III) coprecipitation on •OH production in Fe(III)/O2 systems perturbed by sulfide

Abstract

Dark •OH production from sulfide perturbation of Fe(III)/O₂ and NOM/O₂ systems has been recently documented. In spite of the ubiquity of natural organic matter (NOM)-Fe(III) coprecipitates in the environment, the effect of NOM-Fe coprecipitation on •OH production in such systems remains unexplored. This study showed that •OH production from sulfide reaction with oxic HA-Fe(III) coprecipitates was influenced by sulfide concentration and C/Fe molar ratios. With increasing sulfide concentration from 0 to 5 mM, the cumulative •OH concentration within 120 min increased to 71.8 μM for 11.2 mM Fe in HA-Fe(III) coprecipitates (C/Fe molar ratio = 1.4). At this C/Fe ratio, the cumulative •OH concentrations at this C/Fe ratio in the presence of ≤2.5 mM S(-II) were lower than those produced from lepidocrocite control mineral, while it was higher at 5 mM S(-II). •OH production was attributed to enhanced HA-Fe(II)/FeS production coupled to HA desorption and reduction. These reduced species could activate O₂ to generate •OH. In the presence of 2.5 mM S(-II), the cumulative •OH concentration decreased with C/Fe molar ratios of the HA-Fe(III) coprecipitates. Depending on the carbon content of the NOM-Fe coprecipitates, •OH production might be affected by the interaction of NOM and sulfide. The addition of 2.5 mM S(-II) into 100 g/L of field sediments also produced significant levels of •OH, and it was more efficient with clayed sediments than sandy sediments. The results of this study suggest that the influence of HA-Fe(III) coprecipitation should be cautiously considered for •OH production in sulfide-perturbed Fe(III)/O₂ systems.