THE ROLE OF ORGANIC-ACIDS IN MINERAL WEATHERING

Organic acids and their anions (for brevity we shall use the term ''ac ids'' to include both) may affect mineral weathering rates by at least three mechanisms: by changing the dissolution rate far from equilibri um through decreasing solution pH or forming complexes with cations at the mineral surface; by affecting the saturation state of the solutio n with respect to the mineral; and by affecting the speciation in solu tion of ions such as Al3+ that themselves affect mineral dissolution r ate. In this paper we review the effects of organic acids on the disso lution rates of silicate minerals, particularly feldspars, under condi tions approximating the natural weathering environment - 25 degrees C, pH 4-7 - and with concentrations of organic acids comparable to those measured in soil solutions. Feldspar dissolution rates far from equil ibrium increase with decreasing pH below pH 4-5. They appear to be ind ependent of pH between pH 4-5 and about 8, and above pH 8 feldspar dis solution rates increase with increasing pH. Small chelating ligands su ch as oxalate appear to accelerate feldspar dissolution through comple xation of Al at the surface of the mineral. Feldspar dissolution rates in the presence of 1 mM oxalic acid show effects ranging from no enha ncement 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 sc atter in the available data. In general, concentrations of oxalate of the order of 1 mM are necessary to cause a significant effect. Humic a cids do not appear to increase feldspar dissolution rates significantl y. Dissolution rates must decrease as the solution approaches saturati on with respect to the primary phase (the chemical affinity effect). O rganic acids will influence chemical affinity by complexing Al (and po ssibly other elements) in solution and hence decreasing the chemical a ctivity of Al3+. There are essentially no data on the effect of chemic al affinity on feldspar dissolution rate at 25 degrees C and mildly ac id pH, so it is hard to evaluate the importance of organic acids in ac celerating silicate dissolution through the chemical affinity effect. The effect of complexation of dissolved Al does not appear to be an im portant determinant of silicate dissolution rates in nature. Observed rates of silicate weathering in the field are typically much slower th an predicted from laboratory experiments far from equilibrium, suggest ing control by transport of solutes between ''micropores'' and ''macro pores'' (''micropores'' include fractures and crystal defects within g rains). 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.