Review on the physical chemistry of iodine transformations in the oceans

Abstract

The transformation between iodate (IO3−), the thermodynamically stable form of iodine, and iodide (I-), the kinetically stable form of iodine, has received much attention because these species are often dependent on the oxygen concentration, which ranges from saturation to non-detectable in the ocean. As suboxic conditions in the ocean’s major oxygen minimum zones indicate that IO3− is minimal or non-detectable, the incorporation of IO3− into carbonate minerals has been used as a redox proxy to determine the O2 state of the ocean. Here, I look at the one and two electron transfers between iodine species with a variety of oxidants and reductants to show thermodynamics of these transformations. The IO3− to IO2− conversion is shown to be the controlling step in the reduction reaction sequence due to thermodynamic considerations. As IO3− reduction to IO2− is more favorable than NO3− reduction to NO2− at oceanic pH values, there is no need for nitrate reductase for IO3− reduction as other reductants (e.g. Fe2+, Mn2+) and dissimilatory IO3− reduction by microbes during organic matter decomposition can affect the transformation. Unfortunately, there is a dearth of information on the kinetics of reductants with IO3−; thus, the thermodynamic calculations suggest avenues for research. Conversely, there is significant information on the kinetics of I- oxidation with various oxygen species. In the environment, I- oxidation is the controlling step for oxidation. The oxidants that can lead to IO3− are reactive oxygen species with O3 and •OH being the most potent as well as sedimentary oxidized Mn, which occurs at lower pH than ocean waters. Recent work has shown that iodide oxidizing bacteria can also form IO3−. I- oxidation is more facile at the sea surface microlayer and in the atmosphere due to O3.

Early Diagenetic Controls on Sedimentary Iodine Release and Iodine‐To‐Organic Carbon Ratios in the Paleo‐Record

“…While IO 3 − production has been demonstrated in sediments (Kennedy & Elderfield, 1987a, 1987b), the intermediates I 2 and HOI are important precursors (Luther, 2023). These precursors are short‐lived but can both form bonds to fresh organic material, thus removing iodine from solution (Francois, 1987; Harvey, 1980; Kennedy & Elderfield, 1987b; Truesdale & Luther, 1995).…”

“…The redox potential of the half reaction is in between the oxygen‐H 2 O and nitrate‐nitrite redox couples, meaning that IO 3 − reduction takes place in oxygen‐depleted water masses (Cutter et al., 2018; Wong & Brewer, 1977). Conversely, I − oxidation requires oxygen (O 2 ) and cannot proceed under nitrogenous, manganous or ferruginous conditions (Cutter et al., 2018; Luther, 2023). Accordingly, I − enrichment at the expense of IO 3 − is observed in the oxygen minimum zones (OMZs) of the northeastern and southeastern equatorial Pacific and the Arabian Sea (Cutter et al., 2018; Farrenkopf et al., 1997; Moriyasu et al., 2020; Rapp et al., 2020; Rue et al., 1997), where oxygen concentrations drop to very low values (<2 μM).…”

“…In the absence of oxygen in the surface sediment and bottom water, I − can leak out of the sediment, which can explain the presence of excess dissolved iodine in the water column of OMZs. By contrast, under oxic conditions in the surface sediment, I − (valence state of −I) is oxidized to diatomic iodine (I 2 , valence state of 0), hypoiodous acid (HOI, valence state of +I) and eventually IO 3 − (valance state of +V) (see Luther (2023) for a recent review of the underlying thermodynamics).…”

Characterizing the marine iodine cycle and its relationship to ocean deoxygenation in an Earth System model

“…A recent thermodynamic review indicates that the reactive oxygen species (ROS) such as hydrogen peroxide and OH radicals can fully oxidize Ito IO3 -. Iodide oxidation to IO3is a 6-electron transfer and other ROS, such as superoxide, are only thermodynamically favorable to catalyze partial oxidation to intermediates (Luther, 2023). These ROS species have heterogenous distributions and ambient ocean concentrations that are typically relatively low compared to iodine, supporting the likelihood of temporally or spatially isolated high Ioxidation rates despite of overall extremely slow rates (Schnur et al, 2024).…”

Freezing-Enhanced Photoreduction of Iodate by Fulvic Acid

“…In particular, the transformation of inorganic iodine species in the absence or presence of natural organic matter (NOM) provides various abiotic mechanisms within the natural iodine cycle (e.g., the oxidation of iodide by natural metal oxides, the novel transformation of iodide in aqueous microdroplets, and the interaction between iodine and soil). − The transformation of iodine species can also occur in water treatment processes, potentially inducing the formation of iodinated disinfection byproducts (I-DBPs). , The occurrence of I-DBPs, exhibiting higher cytotoxic and genotoxic attributes compared to their chlorinated or brominated counterparts, has been frequently reported and has raised public health concerns. , In general, iodide, iodine-based sanitizers, and iodinated X-ray contrast media (ICM) are recognized as the primary iodine sources of organoiodine compounds (OICs) during disinfection or oxidation processes . Iodate, the thermodynamically favorable form of iodine, becomes a desired sink of iodine species during water treatment since the oxidation of unstable iodine species to iodate can mitigate the production of I-DBPs. , Recently, the reduction of iodate back to iodide or reactive iodine species (RI) (precursors of I-DBP) during UV disinfection (∼254 nm) has been reported . However, the abiotic reduction of iodate in natural environments might occur under specific circumstances, including high concentration of NOM (over g/L), long-term reaction (days–months), high temperature, or sunlight irradiation. ,,− NOM-photosensitized reduction of iodate was reported under terrestrial sunlight irradiation .…”