Effects of low-molecular-weight organic acids/thiols on hydroxyl radical production from natural siderite oxidation

Abstract Oxygenation of siderite produces hydroxyl radicals (•OH) that are capable of transforming the pollutants. Yet, the influence of low-molecular-weight organic acids (LMWOAs)/ thiols (LMWOTs) which are widely occurring in soils and waters on •OH production from the oxygenation process of siderite is poorly understood. Results of this study show that the presence of LMWOAs/LMWOTs significantly facilitated •OH production from siderite oxidation by O2 at pH 6–8. For instance, at pH 7, the cumulative •OH concentration within 24 h was only 1.3 μM for oxygenation of 1 g/L siderite, while it boosted to 19.7 and 7.2 μM, respectively, with the respective addition of 1 mM ethylenediaminotetraacetate and cysteine. The mechanisms responsible for the enhanced •OH production are associated with the complexation ability and redox activity of LMWOAs/LMWOTs. In the absence of LMWOAs/LMWOTs, •OH is mainly produced from the oxidation of adsorbed Fe(II) by O2, followed by the oxidation of structural Fe(II) on siderite surface. In the presence of LMWOAs, LMWOAs react with adsorbed Fe(II) and structural Fe(II) to produce complexed Fe(II). Therefore, the oxidation of LMWOAs complexed Fe(II) by O2 predominantly contributed to •OH production. Although the complexation ability of LMWOTs is weaker than that of LMOWAs, LMWOTs can reduce O2 to generate •O2− and H2O2, thus facilitating •OH production. Based on the proposed mechanisms, a kinetic model was developed to quantify the relative contribution of each pathway to •OH production. The addition of 1 mM citrate and cysteine in siderite suspension could increase the removal of trichloroethylene from