Parameterization of adsorption onto minerals by Extended Triple Layer Model

Abstract

Adsorption is the most important geochemical process that regulates trace element concentrations in natural water. The behavior of solutes adsorbed onto mineral surfaces depends on conditions in the solution, including pH, ionic strength, and concentrations of competing components. Surface complexation modeling (SCM) is the commonly accepted way to model the effect of variable solution conditions on adsorption. Extended Triple Layer Model (ETLM) is a SCM for ion adsorption to minerals developed by Prof. Sverjensky group. It considers adsorption reaction on mineral surfaces that contain one type of amphoteric surface hydroxyl, in combination with a triple layer model for the electrostatic interaction. The amphoteric surface hydroxyl represents an average of the actual surface sites. The ETLM implements the site occupancy standard states for the activities of surface species, which enables the direct comparison of the adsorption constants. ETLM has the minimum complexity required to realistically represent adsorption processes with the least number of parameters. The ETLM can provide predictions of adsorption behavior and surface speciation that are consistent with independent spectroscopic observations. Nowadays, ETLM can be used in many conventional geochemical codes. Practical application of ETLM in geochemical modeling will require an expansion of the inter-consistent database for the adsorption of various solutes on minerals. This review describes the theoretical background and conventions needed to estimate inter-consistent ETLM parameters. We review the surface charge characteristics of minerals in general, the experimental acquisition of their surface charges, the theory and conventions associated with the modeling of surface charges in the ETLM, and the procedure used to parameterize them. The acquisition of adequate adsorption data, the theory needed to model solute adsorption, and the procedure for parameterizing adsorption are then described. Finally, we introduce the features of a newly developed program, ETL(MIN)2, that facilitates the application of the ETLM, including model parameterization.