Iodine dynamics in soils

Abstract

We investigated changes in iodine (129I) solubility and speciation in nine soils with contrasting properties (pH, Fe/Mn oxides, organic carbon and iodine contents), incubated for nine months at 10 and 20°C. The rate of 129I sorption was greater in soils with large organic carbon contents (%SOC), low pH and at higher temperatures. Loss of iodide (I−) from solution was extremely rapid, apparently reaching completion over minutes–hours; iodate (IO3-) loss from solution was slower, typically occurring over hours–days. In all soils an apparently instantaneous sorption reaction was followed by a slower sorption process for IO3-. For iodide a faster overall reaction meant that discrimination between the two processes was less clear. Instantaneous sorption of IO3- was greater in soils with high Fe/Mn oxide content, low pH and low SOC content, whereas the rate of time-dependent sorption was greatest in soils with higher SOC contents. Phosphate extraction (0.15M KH2PO4) of soils, ∼100h after 129I spike addition, indicated that concentrations of sorbed inorganic iodine (129I) were very low in all soils suggesting that inorganic iodine adsorption onto oxide phases has little impact on the rate of iodine assimilation into humus. Transformation of dissolved inorganic 129IO3- and 129I− to sorbed organic forms was modelled using a range of reaction- and diffusion-based approaches. Irreversible and reversible first order kinetic models, and a spherical diffusion model, adequately described the kinetics of both IO3- and I− loss from the soil solution but required inclusion of a distribution coefficient (kd) to allow for instantaneous adsorption. A spherical diffusion model was also collectively parameterised for all the soils studied by using pH, soil organic carbon concentration and combined Fe+Mn oxide content as determinants of the model parameters (kd and D/r2). The kinetic model parameters were not directly related to a single soil parameter; inclusion of pH, SOC, oxide content and temperature was necessary to describe the observed behaviour. From the temperature-dependence of the sorption data the activation energy (Ea) for 129IO3- transformation to organic forms was estimated to be ∼43kJmol−1. The Ea value was independent of %SOC and was consistent with a reaction mechanism slower than pore diffusion or physical adsorption, but faster than most surface reactions.