PREDICTION OF LIGAND-PROMOTED DISSOLUTION RATES FROM THE REACTIVITIESOF AQUEOUS COMPLEXES

EARTH scientists have long recognized(1-4) that the soluble organic ac ids excreted by soil biota enhance rates of mineral weathering, thereb y chemically stratifying the soil and affecting the biodegradation pat hways of organic matter, including pollutants(5). Multidentate organic ligands(6,7) also exist in industrial waste waters(8) and can enhance the mobility of heavy elements, including radionuclides(9). Here we e xamine whether rate coefficients for ligand-promoted disolution of min erals can be predicted from existing studies of dissolved metal comple xes. We have performed dissolution experiments on bunsenite (NiO) to c ompare with published studies of ligand exchange around dissolved Ni(l I)-ligand complexes(10-12). The hypothesis is confirmed with surprisin g detail: the dissolution rate coefficient increases with the number o f ligand functional groups coordinated to the surface metal, as do the exchange rate coefficients(10-12). Furthermore, we find that the diss olution rate coefficients can be predicted from the equilibrium consta nts for metal complexation in solution, indicating that the activated surface complexes resemble the corresponding dissolved complexes in im portant ways.