In the last several years, significant advancements have been made in understanding fundamental mechanisms controlling sorption, precipitation, dissolution, and heterogeneous redox reactions. This work points to a greater unification of our understanding of these surface processes by emphasizing similarities based on quantum mechanical, thermodynamic and kinetic approaches. As reviewed by Lasaga (1990), atomistic theory may ultimately explain the fundamental bonding and atomic dynamics of minerals and their interaction with hydrated ions and waters. For example, Lasaga and Gibbs (1990), performed ab-initio calculations of interatomic forces and activation energies for quartz that were in accord with independently-derived experimental results. At present the structures of most geochemical interfaces and the nature of the activated complex are too complicated for such treatments. As discussed by Sposito (1990), well established statistical mechanical models (e.g., Modified Gouy-Chapman theory and the Bragg-Williams approximation) underlay most current approaches to surface complexation theory. Recent advances include the incorporation of crystallochemical models that describe surface reaction sites not only in terms of the number and nature of metal ions bonded to them but also the nature of the hydrogen bonds that the surface can donate (Bleam, 1993).
Efforts to quantify observed sorption in both natural and perturbed geochemical systems have increasingly relied on numerical surface complexation models. In an extensive review of the subject, Davis and Kent (1990) point out that the basic tenets of surface complexation theory are that (1) the surface is composed of specific functional groups that react with dissolved solutes to form surface complexes, (2) equilibrium can be described via mass action laws incorporating electrostatic conditions, (3) surface charge is treated as a consequence of distribution of the functional groups and (4) the empirically derived equilibrium constants are related to thermodynamic activities of the surface species. Recently surface complexation models have also been expanded to include the description of ion solvation that permits more accurate description of specific interaction with complex oxides and silicates (Sverjensky, 1993). Most direct applications to environmental issues have been concerned with defining intrinsic parameters needed to fit experimental data to surface complexation models. The most extensive data base to date on such parameters has been that compiled by Dzombak and Morel (1990) for inorganic species on hydrous ferric oxides.
Significant advances have also defined the transition
between sorption processes and the subsequent development of
solid solutions in which the sorbate becomes incorporated into
the substrate structure. Such distinctions are important in
explaining the apparent irreversibility of many sorption
processes and the thermodynamic solubility controls on aqueous
systems. Several workers have investigated metal sorption/solid
solution transitions in Fe and Mn oxides, important substrates in
the environment (Fuller et al., 1993; Hem and Lind, 1991). Both
Wersin et al. (1989) and Stipp et al. (1992) documented
sorption/solid solution transitions for carbonate minerals. In
the first study, electron spin resonance (ESR) was used to
document a progressive transition from initial weak interaction
to solid solution formation as a function of increasing Mn
on the surface of FeCO
. In the second study, surface
sensitive techniques indicated initial surface Cd
sorption
that, with time, resulted in solid state diffusion and solid
solution formation within the calcite substrate. Theories
regarding stoichiometric saturation of such binary solid
solutions in carbonates have been developed by Glynn et al.
(1990) and may ultimately help to explain such microthermodynamic
processes.
Although surface complexation was originally developed to explain sorption processes, the same theory is applicable to describing the effects of pH and organic and inorganic ligands on the dissolution of oxides and silicates (Stumm and Wieland, 1990). Blum and Lasaga (1991) found the dissolution of albite to be directly proportional to the surface concentration of positively charged aluminum sites indicating kinetics controlled by the rate of hydrolysis of Al-O-Si bridging bonds. Wieland and Stumm (1992) demonstrated, using complexation modeling, that dissolution of kaolinite occurred from both edge sites and the gibbsite surface and that the relative rates varied differently as a function of pH. Finally van Cappellen et al. (1993) documented the role of protons, carbonic acid and ligand complexation on the dissolution rates of calcite.
When the surface free energy is zero, rates of both dissolution and precipitation reactions are equal, which is equivalent to macroscopic equilibrium. Based on transition state theory (Lasaga et al., 1994), microscopic reversibility or detailed balancing at small deviations from equilibrium should result in a linear relationship between the deviation in free energy and the dissolution or precipitation rate. Recent work investigating dissolution of gibbsite, kaolinite, and albite (Nagy et al., 1991; Nagy and Lasaga, 1992; and Burch et al., 1993) have generally validated this theory that close to equilibrium, a direct proportionality exists between free energy and dissolution rate. Far from equilibrium, rates were found to be independent of free energy. The most impressive feature of these experiments is the dramatic increase in the rate as a certain critical undersaturation is reached. This change in rate law is linked to the density of etch pits and surface defects, the formation of which have been predicted to form based on Monte Carlo simulations at critical unsaturations. Experimental work on a variety of minerals with varying dislocation densities has supported this theoretical prediction (Blum and Lasaga, 1990; MacInnis and Brantley, 1993). Most silicate dissolution experiments have been conducted under highly undersaturated conditions. However natural weathering may occur much closer to equilibrium, thus explaining commonly observed slower dissolution rates for natural systems (White and Peterson, 1990).
Significant advances have been made in the field of heterogeneous redox reactions involving the transfer of electrons between solid and aqueous phases. Such processes are environmentally important because the oxidation state of many aqueous species, such as transition metals, determine their solubility and sorption characteristics. Research in this field can be categorized as to whether oxidation or reduction occurs at surface substrate. The oxidation of Fe(II), V(IV) Mn(II), and Cu(I) by dissolved oxygen has been shown to be favored thermodynamically and kinetically by specific adsorption to hydrous ferric oxide and Al surfaces (Stumm, 1992). As discussed by Luther (1990) hydroxide ligands increase electron densities near the reduced metal ion and stabilize the increased oxidation state. Several studies have recently addressed the role of heterogeneous electron transfer related to reductive dissolution and precipitation of oxides both in the presence of inorganic and organic reducing agents. Redox studies related to hydrous ferric oxide include the works of Sulzberger et al., (1989) and Suter et al. (1991) and reactions related to Mn (III, IV) oxides have been characterized by Stone, (1987) and Hem et al., (1989).
Surfaces of mineral phases that contain reduced species such
as Fe(II) can also serve as electron donors during redox
reactions. White (1990) presented an extensive review of
heterogeneous reduction of aqueous species such as Cr(VI),
Cu(II), V(V), and Np (VI) by Fe(II) minerals and presented a
detailed characterization of Fe(II) oxide electrochemical
reactions in a later study (White et al., 1994). Important from
an environmental prespective is the role of Fe(II) oxides and
sulfides in the reductive reaction of aqueous organic compounds
as demonstrated for carbon tetrachloride degradation at the
surfaces of pyrite and mica (Kriegman-King and Reinhard, 1991).
Significant progress has also continued on understanding
photo-reduction reaction on mineral surfaces as recently reviewed by
Waite (1990). Siffert and Sulzberger (1991) experimentally
investigated light-induced dissolution of hematite in the
presence of dissolved oxalate and related these results to Fe(II)
production in natural lakes. Also photocatalytic oxidation of
organics such as chlorophenol has been demonstrated to occur on
substrate surfaces such as TiO
(Al-Ekabi et al., 1989).