Determination of the parameters for the 1-pK triple-layer model of ion adsorption onto oxides from known parameter values for the 2-pK TLM

Prior knowledge of the model parameters describing the metal oxide/simple electrolyte interface is frequently crucial to successful application of surface complexation modeling. On the basis of the parameter values for the 2-pK triple-layer model recently published by Sverjensky, the parameters for the 1-pK TLM were determined. This was done by fitting (using the 1-pK model) the proton surface charge and ion adsorption isotherms generated by the 2-pK TLM. It was assumed that inner-layer capacitance was the same for both models. A strong correlation was found between ion adsorption constants of different electrolytes on the same oxide predicted by both models. Additionally, the 1-pK and 2-pK models predict different electrokinetic potentials in spite of very similar values of calculated surface charge and ion adsorption isotherms when the same value of outer-layer capacitance is assumed. Because the value of outer-layer capacitance practically does not influence surface charge and electrolyte adsorption isotherms, it can be treated as a freely adjusted parameter, the value of which can be estimated if we have measured the electrokinetic potential.

Correlation between values of adsorption constants of monovalent anions sorbing on different oxides obtained by using the 1-pK and the 2-pK triple-layer model.Figure optionsDownload as PowerPoint slide