Modeling of the Association of Metal Ions with Heterogeneous Environmental Sorbents
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MODELING OF THE ASSOCIATION OF METAL IONS WITH HETEROGENEOUS ENVIRONMENTAL SORBENTS
JOHN C. WESTALL Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, OR 97331-4003, USA.
ABSTRACT The use of mechanistic (surface-complexation, electric-double layer) and semi-empirical (affinity spectrum) models for representation of the association of metal ions with heterogeneous environmental materials, such as humic acids and soil particle surfaces, is compared. It is seen that mechanistic models are not nearly as mechanistic as one generally assumes, and that semiempirical models are much more valuable than one might assume by comparison to simple K, models. A semi-empirical discrete-log-K-spectrum model was used to describe the binding of Co(II), as a function of pH and NaC10 4 concentration, to two environmental substrates: leonardite humic acid and a kaolinitic subsoil. Excellent agreement of the model and the data was obtained over a wide range of solution composition. These models appear to be the most promising among several alternatives for modeling interactions of metal ions with complex heterogeneous environmental materials over a wide range of solution composition. INTRODUCTION Models are needed for estimation of speciation of metals in surface waters, ground waters, soils, and sediments.
Speciation controls not only transport and fate of metals, but also their
biological availability and environmental effects. Speciation models are thus needed in virtually all aspects of managing metal contamination of the environment, including risk assessment, site remediation, and waste disposal management. Although metal speciation models based on chemical equilibrium have been applied in the environmental sciences for at least twenty five years [1,2], the number of cases is still very small in which either (i) the output of the model has been "validated" by field measurements or (ii) the output of the model has actually been used as the primary basis for a decision in environmental management. Thus, there is an apparent inconsistency among the need for the model, the apparent availability of the model, and the absence of applications of the model to the needs. This apparent inconsistency arises from the fact that the simple equilibrium models based on laboratory data just don't work well in many field systems. Since metal speciation models are presumed to be based on fundamental processes and inviolable laws of nature, one might assume that one can have complete confidence in the output of the model. While this assumption might be approximately correct for some well defined laboratory systems, it is certainly not correct for most complex environmental systems. One of the greatest obstacles to the use of equilibrium models to predict metal speciation in field systems is the heterogeneity of environmental sorbents, such as humic substances and the surfaces of rocks and minerals. (Another major source of uncertainty is slow rates of chemical reactions, which we shall not address). Thus, while one can
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