Oxalate-promoted forsterite dissolution at low pH

Abstract

We ran a series of 124 semi-batch reactor experiments to measure the dissolution rate of forsterite in solutions of nitric and oxalic acid solutions over a pH range of 0–7 and total oxalate concentrations between 0 and 0.35 m at 25 °C. We found that the empirical rate law for the dissolution of forsterite in these solutions is r = 10 - 7.03 a H + 0.46 + 10 - 5.44 m ox 2 - 0.40 a H + 0.47 where a H + is the activity of hydrogen ions, m ox is the concentration of oxalate in molal, and the rate is given in (mol/m 2 s). The first term of the rate law expresses the dissolution rate for solutions with no oxalate and the second term expresses the additional rate produced by the presence of oxalate ions. Our experiments show that oxalate-promoted dissolution rates depend upon both oxalate concentration and pH. Based on this, we propose a reaction mechanism in which a hydrogen ion and an oxalate ion are simultaneously present in the activated complex for the reaction that releases H 4SiO 4 into solution. By analogy, we propose that water acts as a ligand in the absence of oxalate, so the first term of our rate law has a H 2 O m (=1) as a hidden term. Thus, our results suggest that both a proton and a ligand (either an anion or a water molecule) must be present in the activated complex for forsterite dissolution, suggesting that proton-promoted and ligand-promoted dissolution are simply two aspects of the same process, and that the distinction between the two is artificial. The increased dissolution rate of forsterite produced by oxalate ions at a concentration of 0.001 m, a concentration comparable to the amount of organic acids in typical soils, translates into an approximately a 6-fold increase in forsterite dissolution rates at pH > 4.2. This suggests that organic acids have a measurable, but not profound, effect on chemical weathering.