Thermodynamic Concepts in Modeling Sorption at the Mineral-Water Interface

Abstract

Sorption phenomena on mineral-water interfaces (MWI) are known to control the retention of many trace elements and organic substances of environmental concern, as well as the kinetics of mineral dissolution/precipitation (Sposito 1984; Davis and Kent 1990; Dzombak and Morel 1990; Goldberg 1992; Stumm 1992; Morel and Hering 1993; Langmuir 1997; Zachara and Westall 1999; Benjamin 2002b; Ganor et al. 2009; and references therein). However, there is still no agreement about a uniform thermodynamic concept which can account for the relevant physicochemical interactions at MWI and can be implemented in popular geochemical modeling codes. This is a major obstacle for compiling a unified (ad)sorption thermodynamic database, also marred by disagreements in defining standard and reference states for (ad)sorbed species. In turn, confusion stems from the multiplicity of thermodynamic concepts used in the description of sorption phenomena on MWIs. The goal of this contribution is to summarize these concepts in a hope to illuminate ways along which they can eventually be made mutually consistent. A minor element (M) can be retained on mineral-water interfaces by a number of sorption mechanisms which also depend on time. On a kinetic basis, this can be summarized in the so-called sorption continuum . At short reaction times, aqueous complexation competes for M with ion exchange on permanently-charged surfaces (such as basal planes or interlayer of clay minerals), as well as the outer-sphere surface complexation on (hydr)oxides. These fast processes are (almost) reversible. At longer reaction times, specific ( multi-site ) adsorption on clay particle edges or (hydr)oxides takes place; this kind of binding into inner-sphere surface species often displays an energetic heterogeneity at low total M concentrations in the system and becomes progressively irreversible with time. At still longer reaction times, the incorporation of (ad)sorbed species deeper into …