Sorption and Migration of Radioiodine in Saturated Sandy Soil.

Abstract

Using lZ51 as tracer, the migration of iodide and iodate in a solvent flowing steadily through saturated sandy soil was examined in column experiment and analyzed theoretically, with particular reference to the effect of differences in pH value. Runs a t pH 3, medial pH and pH 10 were performed to determine breakthrough data from radioactivity measurements on the effluent solution.'The results obtained from column experiments indicated that, for the same pH, sandy soil sorbed iodate ('25103-) more strongly than iodide (lZSI-), and that, for the same chemical form, more effective sorption is shown a t lower than a t high pH. This indicated that the sorption behavior of iodine in sandy soil depended significantly on their chemical form and on pH-i.e. the hydrogen ion concentration in the influent solution.The strong sorption of iodate under medial pH and pH 3-evidenced by the breakthrough data-was simulated numerically, based on mathematical expressions representing a combination of instantaneous (equilibrium) and limited-rate (non-equilibrium) sorption processes. Unknown parameters incorporated in the model were determined by a procedure of fitting the mathematically derived curves to the observed plots for different experimental conditions, as a result of which good agreement was obtained of the calculated curves with the observed breakthrough plots.

Conceptual Model of Iodine Behavior in the Subsurface at the Hanford Site

“…Yoshida et al (1992) concluded that the difference was that iodide sorption was likely purely electrostatic attraction, while iodate was chemically adsorbed or exchanged by the mineral allophane or sesquioxides. Nishimaki et al (1994) observed the same behavior of iodide versus iodate sorption, with the iodate sorption showing a two-step mechanism: an initial rapid equilibrium sorption, followed by slow non-equilibrium sorption. The conclusions of Fukui et al (1996) were consistent with Yoshida et al (1992), except they reported that iodide sorption seemed to be more complicated than pure electrostatic attraction.…”

Groundwater Radioiodine: Prevalence, Biogeochemistry, and Potential Remedial Approaches

“…Yoshida et al (1992) concluded that the difference was that iodide sorption was likely purely electrostatic attraction, while iodate was chemically adsorbed or exchanged by the mineral allophane or sesquioxides. Nishimaki et al (1994) observed the same behavior of iodide versus iodate sorption, with the iodate sorption showing a two-step mechanism -an initial rapid equilibrium sorption, followed by slow non-equilibrium sorption. The conclusions of Fukui et al (1996) were consistent with Yoshida et al (1992), except they concluded that iodide sorption seemed to be more complicated than pure electrostatic attraction.…”

“…Most researchers (e.g. Whitehead, 1973;Whitehead, 1974;Yoshida et al, 1992;Nishimaki et al, 1994;Yu et al, 1996;Fukui et al, 1996;Kaplan, 2003) have found that increased pH values are generally accompanied by decreased sorption of iodine. This is consistent with adsorption theory, that anions such as iodide and iodate, will adsorb less at pH values above the zero point of charge of the sorbent because the negatively charged surface tends to repel the negatively charged aqueous ions.…”

Evaluation of a Radioiodine Plume Increasing in Concentration at the Savannah River Site

“…Neal and Truesdale observed that there was little sorption of iodide by ferric hydroxide or kaolinite, whereas iodate sorbed strongly to ferric hydroxide. Nishimaki et al observed the same behavior of iodide versus iodate sorption on to a sandy soil, with the iodate sorption showing a two-step mechanism: an initial rapid equilibrium sorption, followed by slow nonequilibrium sorption. The single strongest natural complexant of iodine is dissolved organic carbon , .…”