Sorption and Migration of Radioiodine in Saturated Sandy Soil

Abstract

Using ¹²⁵I 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 at 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 (¹²⁵IO₃-) more strongly than iodide (¹²⁵I-), and that, for the same chemical form, more effective sorption is shown at lower than at 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.

KEYWORDS:

• sorption
• radionuclide migration
• iodine 125
• iodides
• iodates
• sandy soils
• column experiments
• tailing
• numerical simulations
• equilibrium model
• non-equilibrium model
• pH value
• concentration dependence