Iodine dynamics in soils

Abstract

We investigated changes in iodine ( 129 I) 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 129 I sorption was greater in soils with large organic carbon contents (%SOC), low pH and at higher temperatures. Loss of iodide (I À ) from solution was extremely rapid, apparently reaching completion over minutes-hours; iodate (IO À 3 ) 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 IO À 3 . For iodide a faster overall reaction meant that discrimination between the two processes was less clear. Instantaneous sorption of IO À 3 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 (0.15 M KH 2 PO 4 ) of soils, $100 h after 129 I spike addition, indicated that concentrations of sorbed inorganic iodine ( 129 I) 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 129 IO À 3 and 129 I À to sorbed organic forms was modelled using a range of reactionand diffusion-based approaches. Irreversible and reversible first order kinetic models, and a spherical diffusion model, adequately described the kinetics of both IO À 3 and I À 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 129 IO À 3 transformation to organic forms was estimated to be $43 kJ mol À1 . 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.

Comparison of two methods to assess heterogeneity of water flow in soils

“…Recent research confirmed older findings that iodide cannot be considered as a nonreactive tracer because it is immobilized in soils in the form of organically bound iodine (Behrens, 1982) through the laccasecatalyzed oxidation (Seki et al, 2013). It was found that iodide was fully transformed into organic forms after 1 day of incubation in highly organic soils and was fully transformed in the studied soils after 60 days (Shetaya, 2011). Therefore, the NaI solution was added in our experiment as a carrier to prevent 131 I from incorporation into organic compound and field microorganisms.…”

Kinetic study of time-dependent fixation of UVI on biochar

“…The spherical diffusion model is based on the assumption that sorption is controlled by diffusion into uniform spherical particles of radius (r) controlled by a diffusion coefficient (D) [16][17][18]. It has been successfully applied to describe time-dependent sorption processes in materials, including soil [19,20], minerals [21] activated carbon [22] and waste materials e.g.…”

Transport of iodide in structured soil under spring barley during irrigation experiment analyzed using dual-continuum model

“…Iodide is immobilized in soils in the form of organically bound iodine through the laccasecatalyzed oxidation (Seki et al 2013). It was found that iodide was fully transformed into organic forms after 1 day of incubation in highly organic soils and was fully transformed in the studied soils after 60 days (Shetaya 2011).…”

“…Thus, field irrigation experiment combined with radioactive tracer technique may provide valuable data (e.g., spatial and temporal variations of applied tracer), which serve for model calibration and validation (Alaoui et al 1997). Shetaya (2011) presented the distribution coefficient K d of iodide for woodland and arable soil equal to 3.8 and 5.3 L kg −1 , respectively, and Mikolajków (2003) found that K d of nitrate for brown, podsolic, and boggy soil were 5.22, 4.29, and 6.25 L kg −1 , respectively. As iodide and nitrate retardation in soils are similar, we used radioactive iodide as a tracer for nitrate movement in clay loam soil.…”