Molecular Interaction of Aqueous Iodine Species with Humic Acid Studied by I and C K-Edge X-ray Absorption Spectroscopy

Iodine-129 is one of three key risk drivers at several US Department of Energy waste management sites. Natural organic matter (NOM) is thought to play important roles in the immobilization of aqueous iodide (I-) and iodate (IO3-) in the environment, but molecular interactions between NOM and iodine species are poorly understood. In this work, we investigated iodine and carbon speciation in three humic acid (HA)-I systems using I K-edge XANES and EXAFS and C K-edge XANES spectroscopy: (1) I- in the presence of laccase (an oxidase enzyme) and a mediator, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) in a pH 4 buffer, (2) I- in the presence of lactoperoxidase (LPO) and H2O2 in a pH 7 buffer, and (3) IO3- in a pH 3 groundwater. Both oxidase and peroxidase systems could oxidize I- to I-2 or hypoiodide (HOI) leading to organo-I formation. However, the laccase-ABTS mediator was the most effective and enhanced I- uptake by HA up to 13.5 mg/g, compared to 1.9 mg/g for the LPO-H2O2. IO3- was abiotically reduced to I-2 or HOI leading to an organo-I formation. Pathways for HA iodination include covalent modification of aromatic-type rings by I-2 / HOI or iodine incorporation into newly formed benzoquinone species arising from the oxidation of phenolic C species. This study improves our molecular-level understanding of NOM-iodine interactions and stresses the important role that mediators may play in the enzymatic reactions between iodine and NOM.