Role of EDTA, citrate and phthalate on Cs, Sr, Eu and U adsorption by smectite and evaluation by geochemical modeling

Abstract

The presence of organic ligands may exacerbate the mobility of contaminants by stabilizing them in the aqueous phase and reducing their retention on the solid phases, representing an additional environmental hazard. We investigated the impact of EDTA, citrate and phthalate on the adsorption of four different radionuclides (RN = 137Cs, 85Sr, 152Eu and 233U) by a well-characterized smectite clay and evaluated the capability of sorption modeling to incorporate the impact of these organic ligands. We analyzed the effect of ligand concentration (from 1⋅10-6 to 1⋅10-3 M) on RN retention at two different pH values (approximately 5 and 9) to contemplate the occurrence of different adsorption mechanisms in clays (cation exchange or surface complexation) as well as the RN and organic speciation. The results suggested that phthalate had a minimal effect on the retention of the studied RNs, while EDTA and citrate significantly decreased the adsorption of Eu, Sr and U, even making it drop to zero in some cases. The simulation outcomes demonstrated that, in most cases, the variation in the distribution coefficient (Kd) as a function of the organic concentration could be accurately replicated with the available thermodynamic data, but few relevant discrepancies were identified. The importance of the possible competitive effects of trace ions in the equilibrium water (notably Ca) for the overall assessment of the role of organic matter on RN retention was highlighted.