Surface complexation in natural particle suspensions

Abstract: This thesis is a summary of our studies on (i) the proton and copper adsorption at aqueous illite surfaces, (ii) the proton and copper complexation of humic substances (fulvic acid FA and humic acid HA), (iii) the interaction between illite and fulvic acid, and (iv) the copper complexation behaviors in illite-FA bi-complexant systems. These particular studies were designed to help us develop a framework for modelling heavy metal complexation/adsorption mechanisms at natural particle surfaces. Our data were mainly generated from potentiometric titration, batch adsorption and FI-IR experiments; the model calculations and simulations were accomplished using computer programs FITEQL and SOLGASWATER. To account for the complexation behavior in illite/water suspensions, we analyzed the experimental data using the constant capacitance surface complexation model. Our analysis showed that (i) the proton reactions in the supernatants of illite suspensions can be represented by those of Al(H20)6 3+ and Si(OH)4 in water solutions, (ii) the acid-base properties of illite surfaces can be illustrated using only deprotonation reactions, which suggests that the surface acidic behavior of illite is similar to that of amorphous SiO2, and (iii) the uptake of copper in illite suspensions can be interpreted by the surface adsorption of different Cu(II) aqueous species, followed by the formation of copper precipitates, which were found to be hydroxide precipitates in carbonate-free illite suspensions and hydroxylcarbonate precipitates in carbonate-containing illite suspensions. To account for the complexation properties of humic substances, we used a nonelectrostatic, discrete site distribution model. Model analysis showed that (i) the acid-base properties of FA and HA can be modelled as those of a mixture of three monoprotic acids (R1H, R2H and R3H), (ii) the complexation of copper can be explained by assuming that copper binds mainly with the strongest and weakest acid sites (R1H and R3H) to form monodentate complexes, and (iii) FA has a stronger acid strength, but a weaker copperbinding potential, than HA. As to the adsorption of FA by illite, we found that it decreases with increases in pH values and its pH adsorption edge resembles those of SiO2-FA and montmorillonite-FA systems. The adsorption mechanism might be explained by assuming that the functional groups of FA react with the water molecules held by electrostatic forces near the illite surfaces to form H-bonds. In FA-illite bi-complexant systems, the complexation of Cu2+ ions can be effectively simulated by taking it to be a combination of individual bindings by the illite surfaces and the FA functional groups. This suggests that in natural water systems, fulvic acid can inhibit the retention of heavy metals at solid surfaces by forming soluble complexes with metal ions.