Sorption behavior of radioiodine on organic rich soil, alumina and clay minerals

Abstract

Sorption behavior of radioiodine on organic rich soil, alumina, chlorite- illite clay mixture, kaolinite and bentonite have bc;en studied using the batch method. was used as tracer. Characterization of samples were done by FTIR, X-ray diffraction spectrometry and particle size distribution by Andreason pipette method. The grain size of the samples used were all below 38/xm and the experiments were performed at room temperature. The aqxreos phase used in all experiments were synthetic ground water with a composition similar to the underground waters of the regions where the samples were obtained. Stock solutions containing tracer, were prepared using synthetic ground- water. Initial concentration of inactive iodide ion in the solutions ranged from 10~®to 10“^ mmole/ml. The influence of contact time, solution/solid ratio, pH of synthetic ground- water, biomass of soil and I~ ion concentration on the sorption of radioiodine on the organic rich soil were investigated. The effect of I~ ion concentration and complexing agents on the sorption behavior of radioiodine on alumina and clay minerals were also studied. A rather slow kinetics was observed for tlie adsor])tion of radioiodine on the organic rich soil, tending saturation within 14 da.ys. It was observed that distribution ratio increased with increasing ratio of the volume of solution to the mass of soil(V/m ratio) and reached a platee.u after V/m=50. Distribution ratio did not change very much by changing the pH of the solution. Comparison of Eh-pH behavior of the solutions with the standard Eh-pH graph of iodine-water system indicated that in the whole pH rcinge, iodine is mostly preseirt in I~ form. To observe the effect of soil biomass soil samples were sterilized by heat and gamma-ray source and the results were found to be much lower than the control sample. Increasing the concentration of iodide ion in the .solution, caused a decrease in Rd. values. Desorption Wcvs ol)served only for low initial iodine concentrations < 1 X I0~^7nmol/ml), for organic rich soil which suggests that at least two sites and/or mechanisms are involved in the sorption. The sorption data were fitted to Freundlich and Dubinin-Radushkevich isotherms. The slope of Freundlich isotherm was found to be less than 1 which indicates the non-linearity of the isotherm. The mean free energy of adsorption was calculated from the slope of Dubinin-Radushkevich isotherm and was found as about 11 k.J/mol. The distribution of sites on organic rich soil was calculated using the Freundlich and Dubinin-Radushkevich constants, considering Cl~ as the competing ion with I~ in the solution. The affinities of sites were found to be about three times higher for Cl~ than I~. Among the clay minerals, the highest Rd value was found for chlorite-illite cla}^ mixture but tlie Rd values found for alumina and the other sanrples were well below those found for the organic rich soil. The sorption data were fitted to Freundlich and Dubinin-Radushkevich isotherms. All the Freundlich isotherms were non-linear (slope<l), except for alumina (slope=0.98), since the alumina sample used was 99% AI2O3. The mean energies of adsorjition calculated for these samples, were in the range of 9-11 kJ/ntol. The .site disiribulion curvois were also obtained for tl samples and the affinities of sites were found to be at least three times higher for Cl~ than I~. Clay samples were ])retreated with comjilexing agents EDTA (Ethelene Diamine Tetra Acetic acid) cuid TPAI (Trimethyl Phenyl Ammonium Iodide). Very low or zero Rii values were found for treated samples.