Chemistry and Mechanisms of Low-Temperature Dissolution of Silicates by Organic Acids

Summary

Organic acids are important diagenetic agents in near-surface weathering processes. The mechanism of interaction, however, is complex, and their significance in a particular environment will depend on a variety of geo-chemical parameters. Organic acids in low-temperature aqueous systems can complex metals and metalloids in solution, thereby increasing their solubility and mobility. Organic acids are also implicated in mineral-surface interactions where they can act in ligand exchange reactions to increase the rate of mineral dissolution independent of solution concentration constraints. The manner in which organic acids accelerate dissolution rate can be examined by looking at how organic acids complex metals in solution, and by examining the fundamental mechanisms of silicate dissolution.

In the case of aluminosilicates, organic acids can complex aluminum, and to a lesser degree silica, in solution, thereby changing the total solubility of the mineral. Solution characteristics have a large influence on this interaction, in particular pH, ionic strength, the ligand concentration, and the stability (structure) of the resulting chelate. At neutral pH, aluminum complexes are less favored, while complexes of silica may be favored. At weakly acidic pH, in contrast, silica is not affected by the presence of organic acids, whereas aluminum complexes are stable. Aluminum is chelated by a variety of organic ligands, probably through a dissociative ligand exchange reaction (S^N1 reaction), whereas silica is chelated in an associative (S^N2 reaction). A system where aluminosilicates are dissolving and aluminum is chelated in solution by organic acids, for example, would increase the total aluminum solubility, thus shifting the equilibria with respect to secondary minerals such as gibbsite.

At the silicate surface, organic acids interact with both the silicon and aluminum metal centers in an associative ligand exchange reaction. This reaction acts to polarize and weaken framework crystal bonds, thus changing the energetics of the rate-limiting step of silicate dissolution. The result is an increase in dissolution rate independent of solubility constraints. Both surface and solution characteristics will directly influence this reaction by changing the speciation of the organic acid ligand, the speciation of the surface, and the distribution and characteristics of the surface metal-oxide sites.

The outcome of these interactions can be seen in field settings, especially where microbial activity in the subsurface produces high concentrations of organic acids. In an oil-contaminated aquifer, microbial degradation of the petroleum in a groundwater at neutral pH produces a variety of reactive organic compounds that mobilizes silica from quartz and feldspars, even in waters greatly supersaturated with respect to quartz. In this system, the quartz and feldspar dissolution rate is greatly increased, but only silica is mobilized, as aluminum is conserved in a solid phase. In a peat bog setting, in contrast, a low system pH and a high organic acid concentration result in enhanced aluminosilicate dissolution and both aluminum and silica transport. In both cases, the presence of organic acids dominates the control of silicate weathering in the subsurface.