Studies on metal-humic interactions

The favoured option for disposal of low and intermediate level radioactive waste is burial deep underground. In the safety assessment of deep disposal facilities a possibility which has to be considered is the eventual penetration of the engineered and natural barriers by ground water. Unfortunately humic and fulvic acids, present in most natural waters, can combine with radionuclides to produce metal complexes which are potentially more mobile in the environment than the original radionuclides from which they are formed. Migration of such complexes via groundwater and/or subsequently surface water routes could result in accelerated release of radionuclides to the biosphere. Quantitative knowledge of the extent of possible metal humate and fulvate interactions is therefore of considerable importance. In this study the complexation of Ni, Eu and Ce by humic and fulvic acids present in both ground and surface waters has been investigated. In most studies, to simplify the chemistry involved, humics and fulvics are separated from the water in which they occur before determination of their complexation properties. However, the severe conditions employed in the extraction process could conceivably alter the very properties to be measured. Accordingly the complexation properties of humics and fulvics present in a surface water have been studied both before and after extraction. The major part of this study was undertaken using a High Performance Size Exclusion Chromatographic technique especially developed to facilitate determinations on nonextracted materials. A fluorescence technique was also investigated as a more rapid alternative but was found to be of limited applicability. Complexation parameters were derived using a strong and weak site model. The effects of side reactions, ionic strength, pH, ligand type and nature of the cation were taken into account. With respect to measured stability constants, no major differences between ground and surface waters or extracted and non extracted materials were found. However, measured capacities of humics and fulvics for metals showed some variation according to the nature of the material and the system.