Interactions between metal ions and biogeo-surfaces in soil and water : basis for quantitative risk assessment

 To provide the basis for an improved quantitative risk assessment of heavy metals in the environment, the interactions between the metal ions and the biogeo-surfaces in soil and water were studied using both experimental and modelling approaches.The Donnan membrane technique was developed and optimised for the measurement of metal speciation in soil solutions, in which a soil column was linked to the Donnan cell to provide buffering capacities. In acid natural waters and soils, aluminium is potentially a relatively strong competitor for the heavy metals to the binding on colloidal particles such as organic matter. Using the Donnan membrane technique, aluminium binding to purified humic acid and to the dissolved organic matter in soil solutions was measured. The ability of the NICA-Donnan model for the prediction of Al binding to humic substances was tested.A multi-surface model was developed for the prediction of metal speciation in soil and water. This model can also be used to estimate the relative contribution of the individual sorbents to the control of metal distribution. This multi-surface model considers soil as a set of independent sorption surfaces. Metal binding to soil organic matter (solid and dissolved), to clay mineral and to iron hydroxides was modelled using the NICA-Donnan, Donnan, DDL and CD-MUSIC models. This model approach was validated against the concentrations of total dissolved metal concentration and the activities of free metals in sandy soil samples measured using the Donnan membrane technique.The interactions between the metal ions and the natural organic matter are not only important for the metal distribution, but also for the solubility and mobility of organic matters. Using the NICA-Donnan model, the effects of the binding of protons and metal ions on the physico-chemical behaviour of humic acid was studied. It was found that the coagulation of humic acid started when the calculated electrostatic potential was less negative. The electrostatic potential of the dissolved organic matter in the soil solution of six forest soil profiles was calculated using the same model. The measured concentration of the dissolved organic matter also decreased in a soil profile, as the calculated potential became less negative. Humic and fulvic acids are the major components in the dissolved organic matter. The mobility and transport of humic and fulvic acids were studied in a column experiment. By comparing the breakthrough curves and the modelled physico-chemical behaviour, it was found that coagulation might be important in the control of the solubility and mobility of humic acid, while adsorption was more likely the mechanism that could explain the immobilisation of fulvic acid in this soil.The experimental and the modelling approaches developed are helpful in improving the quantitative risk assessment of the heavy metals. Pot experiments using three different soils and a solution culture experiment have been conducted to study the effects of pH and soil characteristics on the phytotoxicity of nickel. Using the model predictions the differences in the bioavailability of Ni in the three different soils could be explained. By comparing the toxicity of the free Ni 2+ion in the nutrient solution and in the soil solution, the toxicity of Ni in the soils could be predicted reasonably using the quantitative approach.The developedDonnan membrane technique is proved to be a useful analytical technique for the determination of metal speciation in soil and water samples. Further research is needed to improve the method to make it capable to measure lower concentrations. The multi-surface model approach proposed in this research is validated against the measurements of the samples from several different soils. Further validation and improvement of this model approach can be achieved after comparing the prediction with the measurement for more different soils, and by better understanding of the interactions between metal ions and the biogeo-surfaces.