Experimental study of the effect of organic ligands on diopside dissolution kinetics

Diopside (CaMgSi 2O 6) dissolution rates were measured at 25 °C in a mixed-flow reactor as a function of concentration of nine organic ligands in circumneutral (pH = 5.3–7.0) and basic (pH = 10.4–10.9) solutions. At neutral pH, EDTA, citrate, oxalate, acetate, gluconate and 2,4-DHBA lead to an increase of the diopside dissolution rate. Alginic acids and glucosamine have no detectable effect on the dissolution rate, while the glucuronic acid weakly decreases the diopside dissolution rates. At basic pH, citrate and acetate do not affect the dissolution while the EDTA leads to an increase of the rates in a much smaller degree than at neutral pH. This is consistent with overall negative surface charge of diopside in basic solutions. Ligand-affected rates were rationalized using a phenomenological equation which postulates the Langmurian adsorption of a negatively-charged or neutral ligand on rate-controlling surface sites, presumably > Mg(Ca)OH 2 +. This model was qualitatively confirmed by electrophoretic measurements in the presence of organic ligands: a decrease of pH IEP and zeta-potential occurs upon adsorption of ligands on diopside surface. Results of this study demonstrate that very high concentrations (0.01–0.1 M) of organic ligands, whether they are originated from enzymatic degradation of organic matter, or bacterial metabolic activity are necessary to appreciably affect diopside dissolution. Thus, the effect of natural organic ligands on the weathering of diopside in soil environment is likely to be weak.