Evaluation of the function of polymeric bentonite adsorbent to retain strontium and cobalt radionuclides in OPC matrices

Abstract

The effect of polyacrylamide-bentonite (PAAmB) as an adsorbent/cement-enhancement material was examined for the retention of strontium (Sr2+) and cobalt (Co2+) ions from aqueous system. PAAmB composite was synthesized and analyzed by diverse characterization techniques. The acquired experimental data revealed that the addition of bentonite to polyacrylamide polymer (PAAmP) enhanced the adsorption of strontium and cobalt ions capacities. To examine the adsorption mechanism onto PAAmB, the experimental data were fitted using hyperbolic and Langmuir equations at different temperatures. It was found that the adsorption nature was chemisorption and the adsorbed amount of the studied ions increased with increasing the temperature. Immobilization of spent polyacrylamide-bentonite composite loaded with 85Sr and 60Co radionuclides in different extents with Ordinary Portland Cement (OPC) was investigated. The results showed that the optimal compressive strength was attained at OPC blended with 5% PAAmB composite. The leaching patterns of the studied radionuclides from the immobilized waste form were compared with the standard static long-term leaching approach of International Atomic Energy Agency (IAEA). The cumulative leaching fraction (CLF) of the studied radionuclides from PAAmB-OPC was found to be < 5% in all scrutinized conditions, which suggests the applicability of the IAEA-recommended approach for assessing the diffusion coefficient. The acquired leaching data were nonlinearly regressed with numerous mathematical kinetic models to determine the leaching parameters and to evaluate the prevailing leaching mechanism. The results from the regression scrutiny showed that 85Sr and 60Co leaching resulted from three succeeding leaching mechanisms that were the first-order reaction exchange among the waste matrix surface and the leaching solution, diffusion and dissolution. Compliant with IAEA, the calculated leachability indices values implied that the studied matrices performance was within a suitable range and the immobilized spent PAAmB matrices had an adequate mechanical efficacy.