『Abstract
Brucite (Mg(OH)2) dissolution rate was measured
at 25℃ in a mixed-flow reactor at various pH (5 to 11) and ionic
strengths (0.01 to 0.03 M) as a function of the concentration
of 15 organic and 5 inorganic ligands and 8 divalent metals. At
neutral and weakly alkaline pH, the dissolution is promoted by
the addition of the following ligands ranked by decreasing effectiveness:
EDTA≧H2PO4-＞catechol≧HCO3-＞ascorbate＞citrate＞oxalate＞acetate〜lactate
and it is inhibited by boric acid. At pH＞10.5, it decreases in
the presence of PO43-, CO32-, F-, oxine, salicylate,
lactate, acetate, 4-hydroxybenzoate, SO42-
and B(OH)4- with orthophosphate
and borate being the strongest and the weakest inhibitor, respectively.
Xylose (up to 0.1 M), glycine (up to 0.05 M), formate (up to 0.3
M) and fulvic and humic acids (up to 40 mg/L DOC) have no effect
on brucite dissolution kinetics. Fluorine inhibits dissolution
both in neutral and alkaline solutions. From F sorption experiments
in batch and flow-through reactors ad the analysis of reacted
surfaces using X-ray Photoelectron Spectroscopy (XPS), it is shown
that fluorine adsorption is followed by its incorporation in brucite
lattice likely via isomorphic substitution with OH. The effect
of eight divalent metals (Sr, Ba, Ca, Pb, Mn, Fe, Co and Ni) studied
at pH 4.9 and 0.01 M. concentration revealed brucite dissolution
rates to be correlated with the water molecule exchange rates
in the first hydration sphere of the corresponding cation.
The effect of investigated ligands on brucite dissolution rate
can be modelled within the framework of the surface coordination
approach taking into account the adsorption of ligands on dissolution-active
sites and the molecular structure of the surface complexes they
form. The higher the value of the ligand sorption constant, the
stronger will be its catalyzing or inhibiting effect. As for Fe
and Al oxides, bi- or multidentate mononuclear surface complexes,
that labilize Mg-O bonds and water coordination to Mg atoms at
the surface, enhance brucite dissolution whereas bi- or polynuclear
surface complexes tend to inhibit dissolution by bridging two
or more metal centers and extending the cross-linking at the solid
surface. Overall, results of this study demonstrate that very
high concentrations of organic ligands (0.01-0.1 M) are necessary
to enhance or inhibit brucite dissolution. As a result, the effect
of extracellular organic products on the weathering rate of Mg-bearing
minerals is expected to be weak.』
1. Introduction
2. Material and methods
2.1. Materials
2.2. Ligand sorption experiments
2.2.1. Batch experiments
2.2.2. Flow-through experiments
2.3. Analyses
2.4. Brucite dissolution experiments
3. Results
3.1. Ligand adsorption constants
3.2. Brucite steady-state dissolution kinetics in the presence
of ligands
3.3. Interaction of fluoride with the brucite surface
3.4. General rate equation for ligand-controlled brucite dissolution
3.5. Effect of dissolved metals on brucite dissolution rates
4. Concluding remarks
Acknowledgments
References
Appendix