TRACE-METAL COMPLEXATION AT HETEROGENEOUS BINDING-SITES IN AQUATIC SYSTEMS

A critical review of the existing theories of trace metal complexation in aquatic systems is presented. It was demonstrated that original ti tration curves, the corresponding log-log plot as well as different li nearizing plots are not suitable for accurate determination of complex ation parameters without additional correction procedures. Models of d iscrete and continuous distribution of binding strengths were compared . In both cases, the complexing capacity can be determined using the s ame method from the van den Berg-Ruzic-Lee plot. The complexation of t race metals in aquatic systems is equivalent to the adsorption at hete rogeneous binding sites (surface complexation). Exact numerical and an alytical solutions for surface complexation at heterogeneous binding s ites have been proposed for the analysis of titration data, based on t he Langmuirian type of local isotherm and the Boltzmann distribution f unction of binding energies. This exact solution was compared with dif ferent semi-empirical isotherms, derived in literature and proposed fo r the analyses of titration data as well. These isotherms are mainly b ased on different quasi-Gaussian distribution functions. Disadvantages of these isotherms and their multicomponent versions are discussed. T he generalized Langmuir and van Riemsdijk isotherms have a completely different shape (at large additions of trace metals during titration) from the shape of rigorous solution which agrees well with titration e xperiments of a wide titration window. In addition, the generalized Fr eundlich isotherm which has a shape similar to the shape of the rigoro us solution is shifted from the rigorous solution in order to predict well the complete monolayer. It is concluded that the Boltzmann distri bution function of binding energies should be used instead of the quas i-Gaussian type of distribution functions which are not suitable for t he interpretation of titration experiments. The rigorous solution can predict the Langmuir, Freundlich, Dubinin-Radushkewich and Henry isoth erms observed experimentally at different regions of surface coverages (trace metal concentrations).