『Abstract
Diopside (CaMgSi2O6)
dissolution rates were measured at 25℃ 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)OH2+. This model was qualitatively confirmed
by electrophoretic measurements in the presence of organic ligands:
a decrease of pHIEP and zeta-potential occurs
upon adsorption of ligands on diopside surface.
Results of this study demonstrate that very high concentrations
(0.01-0.1M) 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.
Keywords: Diopside; Kinetics; Dissolution; Organic ligands』
1. Introduction
2. Materials and methods
2.1. Mineral samples
2.2. Dissolution kinetics experiments
2.3. Solution analysis
2.4. Electrokinetic measurements
3. Results and discussion
3.1. Steady-state attainment and the stoichiometry of dissolution
reaction
3.2. Effect of ligands on diopside dissolution rates
3.3. Electrokinetic study
4. Concluding remarks
Acknowledgements
Appendix A. Plots of element concentrations in outlet solutions
as a function of reaction time. Experimental conditions are
listed in Table 1.
References