Effect of pH and organic ligands on the kinetics of smectite dissolution at 25 °C

Abstract

Forward dissolution rates of Na–Montmorillonite (Wyoming) SWy-2 smectite (Ca 0.06Na 0.56)[Al 3.08Fe(III) 0.38Mg 0.54] [Si 7.93 Al 0.07]O 20(OH) 4 were measured at 25 °C 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/cm 2/s] obtained in the present study at 25 °C, I = 0.01 M is given by R = 2.2 · 10 - 17 · a H + 0.21 + 1.0 · 10 - 20 + 6 · 10 - 17 · a OH - 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 HCO 3 −, CO 3 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.