Sorption of selected radionuclides to clay in the presence of humic acid

Within the framework of the FUNMIG programme, Loughborough University is performing work to increase understanding of the sorption behaviour of selected radionuclides with various minerals in the absence and presence of competing complexing ligands, such as humic acid (HA). The determination of the distribution ratios (Rd) of binary (metaland humicsolid), and ternary (metal-solid-humic) systems using a batch adsorption technique is reported. Four radionuclides have been used; Cs, Ni, Eu and Cd, to facilitate modelling. Montmorillonite, kaolinite and α-goethite have been used as solids. Humic acid concentrations (2 300 ppm (w/v)) were determined using UV spectrophotometry. Radiometric analysis was used for radionuclide measurement. Construction of sorption isotherms using the Langmuir and Freundlich Equations has allowed characterisation of sorption types, and has provided maximum sorption capacities of the solid surfaces for each of the metals in the binary systems. Distribution relationships between metal and humic acid for each ternary system have been established and correlated. Introduction Several attempts have been made to predict the speciation of radionuclides in the presence of humic substances. Reliable predictions are dependent on accurate models. It is well documented that humic substances can significantly affect the sorption of radionuclides on natural solids. Murphy et al. postulated that mineral bound humic substances modify inorganic surfaces, changing the nature and number of complexation sites for contaminants [1], therefore providing a natural sink for the removal of contaminants from aquifers. In general, the suppression of radionuclide and heavy metal sorption by humic and fulvic substances at low pH and enhancement of sorption at high pH has been observed. It has been proposed that the formation of a surface mineral-humate complex with a high affinity for metals occurs [2]. The work reported here examines the effect of increasing humic acid concentration on the distribution of four metals on three mineral surfaces. Warwick et al. 1st. Annual Workshop Proceedings 6 EC FP FUNMIG IP Saclay (France) 28 Nov.1. Dec. 2005 Objectives To measure the distribution ratios of metal-ligand, metal-solid, ligand-solid and metal-ligand-solid interactions using the batch experiment method. To fit the distribution data of binary and ternary systems to a sorption model.