Parameter Estimation for the Triple Layer Model. Analysis of Conventional Methods and Suggestion of Alternative Possibilities.

The 2-pK one-site triple layer model (TLM) is one of the most popular surface complexation models (SCMs). The present paper deals with parameter estimation for this model. It is shown that there is a need for new, reliable estimation techniques for TLM parameters. The existing methods are not convincing. In the present paper this becomes evident from application to theoretical data, which unfortunately never yields the expected parameters, be it with graphical or numerical approaches. A possible alternative method is proposed, which uses systems with differences in points of zero charge to determine the inner layer capacitance and the pristine point of zero charge for theoretical and experimental data. This method is based on a simple equation for the isoelectric point. Application of the approach to three sets of experimental surface charge data showed that it is possible to decide if the TLM is a reasonable model for a certain system. The equation for the point of zero net proton charge could in principle be applied in a similar way, but would involve some assumption(s) about the results of electrokinetic measurements. No knowledge about site density is required in such approaches. An equation for the point of zero surface potential was found to be at present too complex to be used in a similar way since it involves the site density parameter. Reliable methods might have a promising perspective insofar as they would in principle allow the prediction of parameters for a wide range of sorbents. The alternative approach suggested in this paper can in principle be applied to other SCMs (both single site 1-pK and 2-pK), which account for electrolyte adsorption. Copyright 1998 Academic Press.