Prediction of ligand-promoted dissolution rates from the reactivities of aqueous complexes

Abstract

EARTH scientists have long recognized^1á€-4 that the soluble organic acids excreted by soil biota enhance rates of mineral weathering, thereby chemically stratifying the soil and affecting the biodegradation pathways of organic matter, including pollutants⁵. Multidentate organic ligands^6,7 also exist in industrial waste waters⁸ and can enhance the mobility of heavy elements, including radionuclides⁹. Here we examine whether rate coefficients for ligand-promoted disolution of minerals can be predicted from existing studies of dissolved metal complexes. We have performed dissolution experiments on bunsenite (NiO) to compare with published studies of ligand exchange around dissolved Ni(_II)á€-ligand complexes^10á€-12. The hypothesis is confirmed with surprising detail: the dissolution rate coefficient increases with the number of ligand functional groups coordinated to the surface metal, as do the exchange rate coefficients^10á€-12. Furthermore, we find that the dissolution rate coefficients can be predicted from the equilibrium constants for metal complexation in solution, indicating that the activated surface complexes resemble the corresponding dissolved complexes in important ways.