『Abstract
　Forward dissolution rates of Na-Montmorillonite (Wyoming) SWy-2 smectite (Ca0.06Na0.56)[Al3.08Fe(III)0.38Mg0.54] [Si7.93Al0.07]O20(OH)4 were measured at 25℃ in a mixed-flow reactor equipped with interior dialysis compartment (6-8 kDa membrane) as a function of pH (1-12), dissolved carbonate (0.5-10 mM), phosphate (10^-5 to 0.03 M), and nine organic ligands (acetate, oxalate, citrate, EDTA, alginate, glucuronic acid, 3,4-dihydroxybenzoic acid, gluconate, and glucosamine) in the concentration range from 10^-5 to 0.03 M. In organic-free solutions, the Si-based rates decrease with increasing pH at 1≦pH≦8 with a slope close to -0.2. At 9≦pH≦12, the Si-based rates increase with a slope of 〜0.3. In contrast, non-stoichiometric Mg release weakly depends on pH at 1≦pH≦12 and decreases with increasing pH. The empirical expression describing Si-release rates [R, mol/cm²/s] obtained in the present study at 25℃, I =0.01 M is given by

　　　R = 2.2・10^-17・aH+^0.21 + 1.0・10^-20 + 6・10^-17・aOH-^0.33

At circum-neutral pH, the Si-release-based dissolution is promoted by the addition of the following ligands ranked by decreasing effectiveness: EDTA＞3,4-DHBA＞citrate≧oxalate. Phosphate, glucuronate, glucosamine, gluconate, alginate, and acetate act as inhibitors of dissolution and HCO3^-, CO3^2- exhibit no effect on dissolution rate. Non-stoichiometric, non-steady-state Mg release was very weakly affected by the presence of ligands. Analysis of reacted solid products using XRD, FT-IR, and XPS revealed no major change in structure, surface chemical composition or specific surface area as a function of pH, ligand concentration, and duration of experiments. Ligand-affected rates re-calculated to constant pH were interpreted using a phenomenological equation which postulates the Langmurian adsorption of a ligand on surface sites. Overall, results of this study demonstrate that very high concentrations (0.001-0.01 M) of organic ligands, whether they are originated from organic matter enzymatic degradation or bacterial metabolic activity are necessary to appreciably affect smectite dissolution. As a result, the effect of natural organics on the weathering rate of smectite is expected to be weak.』

1. Introduction
2. Materials and methods
　2.1. Starting clay
　2.2. Dissolution kinetics experiments
　2.3. Solution analysis
　2.4. Solid phase preparation for the analyses
　2.5. X-ray photoelectron spectroscopy (XPS)
　2.6. Microwave digestion
　2.7. Infra-red spectroscopy
　2.8. X-ray diffraction
　2.9. Scanning electron microscopy
　2.10. Specific surface area measurements
3. Results and discussion
　3.1. Solid phase characterization
　3.2. Steady-state attainment and the stoichiometry of dissolution reaction
　3.3. Effect of pH on smectite dissolution rates
　3.4. Effect of ligands on smectite dissolution rates
　3.5. Implications for smectite weathering in natural environment
4. Concluding remarks
Acknowledgments
Appendix A
References

Fig. 3. Molar Mg/Si (diamonds) and Al:Si (squares) ratio in the outlet fluid after 1000-1500 h of reaction as a function of pH. Stochiometric dissolution is observed only in strongly acidic and alkaline solutions. Adsorption of Al on the interlayer positions at pH < 4.5 and proton-octahedral magnesium exchange at 2 < pH < 10 are suggested to explain this non-stoichiometry (see Section 3.2). Fig. 4. Summary of smectite dissolution at 25℃ as a function of pH in CO2-free and CO2-bearing solutions. Diamonds, Si-based rates; squares, non-stoichiometric Mg release; circles, Al-based rates. The solid line represents the fit of experimental data to Eq. (3) (see explanation in the text). Thick dash line corresponds to Amram and Ganor (2005) data. Fig. 5. Silica-release-based dissolution rates of smectite SWy-2 in organic-bearing solutions at 25℃, I = 0.01 M and constant pH for each ligand (average pH = 6.08 ± 0.45, see Table 1 and Table 2). The symbols represent the experimental data and the solid lines are for guiding purposes. For acetate and oxalate it is postulated that ligand-free experiments rates are equivalent to 10-6 M oxalate or acetate since no effect is observed till 10-4 M. Fig. 6. Rates of non-stoichiometric magnesium release from SWy-2 smectite in organic-bearing solutions at 25 ℃, I = 0.01 M and pH = 6.08 ± 0.45. The symbols represent the experimental data and the solid lines are for guiding purposes. 〔Golubev,S.V., Bauer,A. and Pokrovsky,O.S.(2006): Effect of pH and organic ligands on the kinetics of smectite dissolution at 25℃. Geochimica et Cosmochimica Acta, 70, 4436-4451.から〕

Fig. 7. Effect of added ligands on smectite Si-based steady-state dissolution rates. Triangles represent Si-release-based rates and the solid lines represent the fit to the data using Eq. (8a) and (8b) with parameters listed in Table 2. 〔Golubev,S.V., Bauer,A. and Pokrovsky,O.S.(2006): Effect of pH and organic ligands on the kinetics of smectite dissolution at 25℃. Geochimica et Cosmochimica Acta, 70, 4436-4451.から〕