The role of organic acids in mineral weathering

Organic acids and their anions (for brevity we shall use the term “acids” to include both) may affect mineral weathering rates by at least three mechanisms: by changing the dissolution rate far from equilibrium through decreasing solution pH or forming complexes with cations at the mineral surface; by affecting the saturation state of the solution with respect to the mineral; and by affecting the speciation in solution of ions such as Al³⁺ that themselves affect mineral dissolution rate. In this paper we review the effects of organic acids on the dissolution rates of silicate minerals, particularly feldspars, under conditions approximating the natural weathering environment ?25°C, pH 4–7 and with concentrations of organic acids comparable to those measured in soil solutions.

Feldspar dissolution rates far from equilibrium increase with decreasing pH below pH 4–5. They appear to be independent of pH between pH 4–5 and about 8, and above pH 8 feldspar dissolution rates increase with increasing pH.

Small chelating ligands such as oxalate appear to accelerate feldspar dissolution through complexation of Al at the surface of the mineral. Feldspar dissolution rates in the presence of 1 mM oxalic acid show effects ranging from no enhancement to enhancements of a factor of 15, depending upon the data set, pH, and aluminum content of the mineral: there is a great deal of scatter in the available data. In general, concentrations of oxalate of the order of 1 mM are necessary to cause a significant effect. Humic acids do not appear to increase feldspar dissolution rates significantly.

Dissolution rates must decrease as the solution approaches saturation with respect to the primary phase (the chemical affinity effect). Organic acids will influence chemical affinity by complexing Al (and possibly other elements) in solution and hence decreasing the chemical activity of Al³⁺. There are essentially no data on the effect of chemical affinity on feldspar dissolution rate at 25°C and mildly acid pH, so it is hard to evaluate the importance of organic acids in accelerating silicate dissolution through the chemical affinity effect. The effect of complexation of dissolved Al does not appear to be an important determinant of silicate dissolution rates in nature.

Observed rates of silicate weathering in the field are typically much slower than predicted from laboratory experiments far from equilibrium, suggesting control by transport of solutes between “micropores” and “macropores” (“micropores” include fractures and crystal defects within grains). If such transport is rate-controlling, analysis of the effect of organic acids on weathering rates in nature in terms of dissolution rates far from equilibrium may be misleading.