Iodine dynamics in soils

Shetaya, W. H.; Young, S. D.; Watts, M. J.; Ander, E. L.; Bailey, E. H.

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 KH 2PO 4) 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 (k d) 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 (k d and D/r 2). 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 (E a) for 129IO3- transformation to organic forms was estimated to be ~43kJmol -1. The E a 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. © 2011 Elsevier Ltd.

Citation

Shetaya, W. H., Young, S. D., Watts, M. J., Ander, E. L., & Bailey, E. H. (2012). Iodine dynamics in soils. Geochimica et Cosmochimica Acta, 77, 457-473. https://doi.org/10.1016/j.gca.2011.10.034

• Journal Article Type: Article
• Acceptance Date: Oct 18, 2011
• Online Publication Date: Oct 28, 2011
• Publication Date: Jan 15, 2012
• Deposit Date: Feb 3, 2020
• Journal: Geochimica et Cosmochimica Acta
• Print ISSN: 0016-7037
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 77
• Pages: 457-473
• DOI: https://doi.org/10.1016/j.gca.2011.10.034
• Public URL: https://nottingham-repository.worktribe.com/output/3294824
• Publisher URL: https://www.sciencedirect.com/science/article/pii/S0016703711006326