Production of hydroxyl radicals by the oxygenation of FeS and the effect on sulfide removal in sewer systems.

Ferrous-based iron salts (e.g., FeCl2) have been commonly used for sulfide control in sewers, where they chemically generate ferrous sulfide (FeS) particles to remove dissolved sulfide. This study demonstrates that oxidation of FeS particles in sewer sediments can further enhance sulfide removal, likely due to the formation of hydroxyl radical (•OH) during FeS oxygenation. For the first time, an integrated kinetic model was developed to describe •OH production and consumption by FeS oxygenation in sewer environment. The model was validated against three independent sets of batch experiments, including (1) dynamics of •OH production from FeS oxygenation, (2) dynamics of •OH production from sewer sediment oxygenation, and (3) sulfide removal under various conditions. Model-based analysis identified three key sulfide chemical oxidation pathways (•OH, Fe (III), O2), with the •OH pathway significantly contributing 63-97 % of sulfide oxidation in gravity sewers with surface reaeration. An empirical function (a × CFeSb) was further introduced to quantify the influence of FeS on sulfide oxidation rates, which can be integrated into existing sewer management models for improved control strategies. Overall, this work identifies •OH oxidation as a dominant pathway for sulfide removal in ferrous-dosed sewers, challenging the traditional view that oxygen is the primary driver of sulfide oxidation.