Enhancement and modelling of caesium and strontium adsorption behaviour on natural and activated bentonite
Alaaeldine Sh. Saleh, Oluwasola O.D. Afolabi
Abstract
This study investigates the efficacy of bentonite in its natural and activated form via a newly developed activation process to sequester Cs and Sr – two hazardous radiotoxic pollutants – from aqueous solution. The elemental and mineralogical composition, morphology, and surface area of natural and activated bentonite were studied. Then, batch adsorption experiments of Cs and Sr on bentonite were conducted as a function of solid:liquid ratio, pH, Cs and Sr concentrations. Sr adsorption was largely affected by pH, while Cs was slightly influenced. At environmentally relevant pH, neutral to weakly alkaline, the distribution ratios, R d , of both Cs and Sr adsorption on activated bentonite were mostly larger than those on natural bentonite by ∼50 %. Spectroscopic characterisation and batch studies were combined to discuss the observed experimental results. Cs and Sr showed different adsorption mechanisms. Cs was predominantly adsorbed on bentonite via ion exchange, while Sr adsorption could be mainly attributed to electrostatic interactions. Activated bentonite showed a larger adsorption capacity than natural bentonite, and the data were well-fitted to adsorption isotherm models. The model adsorption capacities of Cs and Sr on activated bentonite were 7.28 and 8.51 mg/g, respectively. Finally, the adsorption performance of both bentonite forms was found to decrease in saline solutions, with activated bentonite persistently showing more adsorption than natural bentonite. The findings of this study contribute towards developing effective activated bentonite with improved adsorption capacities. This bears significance in limiting radionuclide migration from storage repositories, reducing transportable radioactive material volumes, and minimising pollution risk. Distribution of Cs on natural and activated bentonite in the presence of competing ions ([Na]:[K]:[Ca] = 1:1:1); [Cs] initial = 10 −6 mol/L, pH= 8.2, solid: liquid = 5 g/L • Bentonite was successfully activated with NaCl to efficiently sequester Cs and Sr. • Physicochemical characterisation demonstrated good adsorption potential. • Activation enhanced bentonite adsorption affinity of Cs and Sr by ∼50 %. • Adsorption models predicted, to a high degree, the Cs and Sr adsorption behaviour. • Activated bentonite has a higher adsorption capacity in presence of competing ions.