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Effect of organic ligands and heterotrophic bacteria on wollastonite dissolution kinetics

Wollastonite (CaSiO3) dissolution rates were measured at 25°C in 0.01 M NaCl using a mixed-flow reactor as a function of pH (5 to 12) and concentration of forty organic ligands. Mostly stoichiometric dissolution was observed at these conditions. For seven ligands (acetate, citrate, EDTA, catechol, glutamic acid, 2,4-dihydroxybenzoic acid, glucuronic acid), batch adsorption experiments and electrokinetic measurements performed as a function of pH and ligand concentration confirmed the interaction of ligands with >CaOH2+ sites and allowed quantification of their adsorption constants. The effect of investigated ligands on wollastonite dissolution rate was modeled 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. A positive correlation between surface adsorption constant and the stability constant of the corresponding reaction in homogeneous solution was observed. At neutral and weakly alkaline pH, the following total dissolved concentrations of ligands are necessary to double the rate of wollastonite dissolution: EDTA (10−4 M), phosphate (1.5 · 10−4 M), catechol (3 · 10−4 M), 8-hydroxyquinoline, gallic acid or adipate (5 · 10−4 M), 3,4-DHBA (7 · 10−4 M), PO3− (7.5 · 10−4 M), glutamate (0.002 M), citrate (0.003 M), malate or 2,4-DHBA (0.004 M), phthalate or succinate (0.005 M), tartrate (0.006 M), thioglycolate (0.008 M), aspartame (0.01 M), gluconate, ascorbate (> 0.01 M), malonate, diglycolate or lactate at pH 8.4 (0.02 M), formate or fumarate (0.05 M), oxalate (>0.05 M), bicarbonate (0.075 M), lactate at pH 5.6 (0.1 M), acetate (> 0.1 M), salicylate (0.15 M), humic acids (> 54 mg/L of dissolved organic carbon, DOC), gum xanthan (1.5-2.0 g/L). Sorbitol, mannitol, glucose, glucosamine, saccharose, fulvic acids and silica at pH ∼ 7 exhibit weakly inhibiting or no effect up to concentration of 0.1 M. The presence of the following ligands leads to a decrease of dissolution rates by a factor of 2: silica at pH 10.7 (2 · 10−4 M), glucuronic acid (0.001 M), algae exudates (30 mg/L DOC), mannit (0.02 M), urea (>0.05 M), pectin (>15 g/L), alginic acid (> 2 g/L). Overall, results of this study demonstrate that 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 enhance wollastonite dissolution. This is further corroborated by batch experiments on live and dead cultures of soil bacteria Pseudomonas aureofaciens interaction with wollastonite. The release rates of both Ca and Si are only weakly affected by the presence of live or dead bacterial cells in inert electrolyte solution and in nutrient media: there is only ∼20 percent-increase of dissolution rate in experiments with live cultures compared to dead cultures. However, the reproducibility of rate measurements in ligand-free solutions at 7 ≤ pH ≤ 8 achieves ± 30 percent. Therefore, the effect of extracellular organic products on the weathering rate of Ca-bearing minerals is expected to be weak and the acceleration of “basic” silicate rocks dissolution in natural settings in the presence of soil bacteria is likely solely due to the pH decrease.