Application of surface complexation modelling:Nickel sorption on quartz, manganese oxide, kaolinite and goethite, and thorium on silica

This study is a follow-up to a previous modelling task on mechanistic sorption. The experimental work has been carried out at the Laboratory of Radiochemistry, University of Helsinki (HYRL), and the sorption modelling was performed using the HYDRAQL code. Parameters taken from the open literature were employed in the modelling phase. The thermodynamic data for aqueous solutions were extracted from the EQ3/6 database and subsequently modified for HYDRAQL where necessary. The experimental data were obtained from five different experiments, four of which concerned the adsorption of nickel. The first experimental system was a mixture of Nilsia quartz and manganese dioxide. In the second experiment, quartz was equilibrated with a fresh and saline groundwater simulant instead of an electrolyte solution. The third and fourth experiments dealt with nickel adsorption from an electrolyte solution onto goethite and kaolinite surfaces respectively. In the fifth experiment, adsorption of thorium onto a quartz surface was investigated.The modelling of the first experimental system was successful provided an updated set of sorption parameters for both quartz and manganese dioxide was adopted. For simulated groundwaters, the pre-modelling correctly predicted the experimentally observed lower adsorption of nickel in the saline simulant than in the fresh one. The sorption of nickel onto a goethite surface over the entire pH range was successfully predicted in the pre-modelling and no further modelling effort was needed. The sorption of nickel onto a kaolinite surface was considerably more difficult to model than the previous system. The pre-modelling results provided a poor fit to experimental results at lower ionic strengths (0.001 M, 0.01 M). The phenomenon is similar to that observed earlier for quartz. It was not possible to improve the fit in the final modelling using the existing sorption data. The experiments on the adsorption of thorium on quartz were of a preliminary nature. The predictions from pre-modelling were fair enough and could be slightly improved in the final modelling