RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The influence of electrolytes on the dissolution kinetics of samples of commercial Fontainebleau sand consisting of >99.6% silica are studied at 25°C and over an ionic strength range of 0.01 M to 1.0 M at pH 10. The dissolution rates are studied in NaCl, Na₂SO₄, and CaCl₂ solutions and in a stabilized "natural" water using an automated titrimetric method. The dissolution results are analyzed using an electrical double layer model containing options to account for cation complexation of the silica surface and ion-pair formation in the bulk solution. Simplified expressions are given for the surface potential calculated for 1:1, 2:1, and 1:2 electrolytes which are used to determine the activities of the ions in the surface plane. The results demonstrate similar effects of NaCl and Na₂SO₄ salts on the dissolution rates with a greater than twofold increase over the range of ionic strength studied. In contrast, the results from the dissolution of silica in CaCl₂ solutions show no influence at calcium concentrations between ca. 2 and 70 mM but the dissolution rate increases at high calcium concentrations. The dissolution rate in the natural water is similar to that found in CaCl₂ solutions containing the same calcium concentration. The results support the conclusion that the complexation of Na⁺ with the surface of silica is not an important influence on the dissolution reaction, whereas it is necessary to invoke the surface complexation of Ca²⁺ to explain the observed increase of the dissolution rate at higher calcium concentrations. No difference could be determined between the effects of NaCl and Na₂SO₄ electrolytes on the dissolution rates expressed as a function of ionic strength or sodium ion concentration. The optimum parameters for the hydrolysis of the ionized and cation-complexed silica, both normalized with respect to the hydrolysis of the neutral silanol groups, are given together with the calculated values of the surface density of the terminal groups. CaCl₂ is shown to enhance the dissolution rate for ionic strengths of <0.1 M relative to NaCl and Na₂SO₄.