John V. Walther

Controls on silicate dissolution rates in neutral and basic pH solutions at 25°C

The pH-dependence of far from equilibrium steady-state dissolution rates of silicates can be understood by considering the detachment rates of their oxide components through surface protonation-deprotonation reactions. At high pH, where Si surface sites are deprotonated and therefore carry negative charge, detachment of silicon appears to control overall silicate dissolution rates. At low pH, near the zero point of charge of SiO2 (where surface charge is dominated by the other oxide components), detachment of the non-silicon structure-forming oxides apparently controls dissolution rates of multi-oxide silicates. Correlations between metal-oxygen site potentials and pH-dependent surface detachment reactions permit estimation of dissolution rates of a large number of silicates from pH 5 to pH 12.

A surface complex reaction model for the pH-dependence of corundum and kaolinite dissolution rates

Abstract Comparison of experimental and theoretical potentiometric titrations of kaolinite at 25°C indicates that the adsorption of H+ and OH− ions to the mineral surface are metal cation specific and that the net adsorption can be modeled in terms of the constituent oxide components. We conclude, from the potentiometric, electrophoretic, and dissolution rate data presented in this paper, that the pH-dependence of the dissolution rates of slightly soluble oxide and silicate minerals is controlled by the adsorption of H+ and OH− ions to specific metal cation surface reaction sites. The dissolution kinetics of corundum and γ-Al2O3 at 2 °C and of kaolinite at 25°C and 60°C were studied as functions of solution pH. Corundum dissolution rates decrease from pH l to 8, are pH-independent from pH 8 to 9, and increase from pH 9 to 11. The dissolution rates are directly proportional to the adsorption of H+ and OH− ions to the oxide surface. The rate equations are: k = 10 −12.75 C H s and k = 10 +24.50 ¦C H s ¦ 4 at pH pKa2(intr) respectively, where pKa1(intr) and pKa2(intr) are the γ-Al2O3 intrinsic acidity constants. Three consecutive periods of dissolution are observed for kaolinite at 25°C and 60°C: an initial period of incongruent dissolution where A1 is preferentially removed over Si at pH 9.3 at 25°C and at pH 12 at 60°C, and the reverse occurs at pH > 6.0 and pH 4.5 and pH 8. Kaolinite dissolution is a complex function of H+ and OH− ion adsorption to > AlOH and > SiOH surface reaction sites.

Olivine dissolution at 25°C: Effects of pH, CO2, and organic acids

Abstract The dissolution rates of Fo100 and Fo91 in aqueous solutions in the pH range 2–12.4 at 25°C have been measured using fluidized bed and batch reactors. Rates depend upon the pH, the partial pressure of CO2, and the presence of organic ligands. At low PCO2 (≤10−4.5 atm) with no organic ligands present, the rate of olivine dissolution, R, is given by R ( mol cm −2 s −1 ) = 9.07 × 10 −12 a H + 0.54 + 5.25 × 10 −15 + 2.33 × 10 −17 a H + −0.31 , where aH+ is the activity of H+ in solution. However in basic solutions, when the partial pressure of CO2 is equal to atmospheric levels (PCO2 = 10−3.5 atm), olivine dissolution rates are nearly pH independent throughout the pH range 6–12 and are about equal to the minimum rate of dissolution under CO2 purged conditions. At pH 11 the presence of atmospheric levels of CO2 reduces the dissolution rate by over an order of magnitude (to 10−14.1 mol cm−2 s−1). Apparently, positive charge on the olivine surface can be neutralized by increasing PCO2. In contrast, experiments conducted in the acidic and near neutral pH ranges indicate that organic ligands chelate surface Mg causing an increase in the olivine dissolution rate when present. Organic ligand effects are greatest in the near neutral pH domain. For example, at pH 4 dissolution rates are increased by 0.75 log units (to 10−12.25 mol cm−2 s−1) in solutions of 10−3 molar ascorbic acid or 0.05 molar potassium acid phthalate over rates measured in organic free solutions. The chelation effect becomes less important as pH decreases. Rates at pH 2 in the presence of these organic ligands are indistinguishable from those measured without organics.

Kinetics of quartz dissolution at low temperatures

Quartz dissolution rates have been measured as a function of pH and ionic strength at 25° and 60°C and can be modeled by a rate law which takes into account speciation at the quartz-solution interface. Dissolution rates far from equilibrium in basic solutions are directly proportional to negative silica surface charge, changing with ionic strength and pH as surface charge changes. At pH 7.5 at 25°C and pH 7.0 at 60°C the logarithm of quart dissolution rates are −16.1 and −15.2 mol cm−2 s−1, respectively. Dissolution rates decrease slightly from pH 7 as pH is lowered, to pH 3, where they begin to increase with decreasing pH. The calculated activation enthalpy changes as a function of pH, being ∼11 kcal. mol−1 at near-neutral pH but increasing from 13 kcal. mol−1 at pH 8 to 23 kcal. mol−1 at pH 11.

Olivine dissolution kinetics at near-surface conditions

Fluidized bed dissolution experiments have been conducted as a function of pH at 25°C with fayalitic olivine (Fo6) in HCl solutions and with forsteritic olivine (Fo91) in solutions containing the organic ligand potassium hydrogen phthalate (KHP). At 25°C the dissolution rate (R) of fayalite as a function of pH is: R(mol cm −2s−))=1.1·10−10aH+0.69 +3.2·10−14+1.2·10−16aH+−0.31 where aH+ is the activity of H+ in solution. The dissolution rate at 25°C of Fo6 at a given pH is a factor of 6 greater than that of forsteritic olivine. The assumption that the rates increase on a molar basis with Fe content allows calculation of dissolution rates of Fe-Mg solid solution olivines of intermediate compositions. Batch-type dissolution experiments were completed with Fo91 at 65°C in solutions at pH 1.8, 6.0 and 9.8. The rate equation obtained from these experiments is: R (mol cm−2s−1)=3.5·10−10aH+0.5+1.0·10−13aH+0.5 When combined with previously published data for Fo91 at 25°C, the 65°C experiments indicate that the activation energy of the olivine dissolution reaction in acidic, organic-free solutions is ∼19 ± 2.5 kcal. mol−1. Dissolution experiments with forsteritic olivine in solutions containing KHP at 25°C demonstrate that the rate of dissolution is increased in these solutions relative to rates measured in KHP-absent HCl-H2O solutions. Apparently, the increase in rate is caused by Mg complexation at the olivine surface. Ligand-promoted dissolution is thought to occur in parallel with proton-promoted dissolution. Therefore, the net rate, Rnet, is the sum of the two rates: Rnet (mol cm−2s−1)=0.8·10−12[LP]0.45+RH+ where [LP] denotes the concentration of KHP; and RH+ denotes the proton-promoted dissolution rate.

Rates of Hydrothermal Reactions

The rates of reactions of silicates and aqueous fluids follow zero-order kinetics controlled by the reacting surface area with the rate constant given by the equation: log k[unknown] -2900/T -6.85, where T is temperature and where k has the units gram-atoms of oxygen per square centimeter per second. This expression appears to hold for all silicates and for reactions involving dissolution, fluid production, or solid-solid transformations in the presence of a fluid of moderate to high pH.

Quartz solubilities in NaCl solutions with and without wollastonite at elevated temperatures and pressures

The effect of NaCl on the solubility of quartz with and without wollastonite in aqueous solutions was investigated at temperatures between 200 and 600°C and pressures of 0.5, 1.0, 1.75, and 2.0 kbars. At 0.5 kbar and temperatures of 300°C and greater, quartz solubility increases with increasing NaCl concentration up to 1.86 molal; that is, it salts in. The extent of salting-in increases with increasing temperature. At 500°C, quartz solubility in 0.83 m NaCl solution is about 0.2 log units higher than in pure water. At 1 kbar, quartz does not start to salt-in until the temperature is above 400°C. Below 400°C at 1 kbar, quartz tends to salt-out in NaCl solutions up to 4.0 molal and the extent of salting-out increases with decreasing temperature. Above 400°C at 1 kbar, the degree of salting-in also increases with temperature with quartz solubility in 0.83 m NaCl solution being about 0.15 log units greater than in pure H2O. Within experimental error, at 1.75 and 2 kbars and between 300 and 600°C, there is no significant saltingin or salting-out effect on quartz solubility in NaCl solutions up to 0.83 molal. Our solubility data combined with experimental measurements of previous investigators were interpreted with a Setchenow-type equation which accounts for both solvent effects and short-range interactions between charged solute species and aqueous silica. The calculation indicates that Born-type dipole-dipole interactions between NaCl solutes and the aqueous silica monomer are not strong enough to cause the quartz salting-in in NaCl solutions that is observed at supercritical conditions.

Incongruent dissolution and surface area of kaolinite

Abstract The surface of kaolinite is modeled as a combination of Al and Si surface sites in order to understand its aqueous dissolution behavior. The pH dependence of the initial nonstoichiometric dissolution of kaolinite at 25°C can be related to the pH dependence of protonation and deprotonation of the surface Si and Al sites. The analysis indicates Al dissolution rates are greater than Si at pH pH 11. An analysis of the surface site density of kaolinite from surface potentiometric titration data is presented. This indicates that the surface area of kaolinite and possibly other clay minerals available for forming surface complexes and sites for cation detachment is significantly larger than that measured by standard gas adsorption methods. It is believed that H + and OH - can penetrate between layers in the sheet silicates which is not possible for the gas, Kr, used in the BET analysis.

Experimental determination of corundum solubilities in pure water between 400–700°C and 1–3 kbar

Experimentally reversed corundum solubilities in pure water at 400° to 700°C and 0.7 to 3 kbar yield values of dissolved aluminum that range from 1–4 ppm Al. At constant pressure the solubility shows a sigmoidal behavior with a slight maximum at 500°C and minimum at 600°C. Corundum solubility increases with increasing pressure at constant temperature. The dissolved aluminum appears to form an uncharged, but polar species under these conditions probably of the form Al(OH)30.

Dissolution kinetics of silica glass as a function of pH between 40 and 85°C

Anhydrous vitreous silica glass was reacted in solutions buffered from pH 1.1 to pH 10.9 between 40 and 85°C for up to 1500 h. The initial release of silicon to solution is proportional to the square root of time, consistent with a surface diffusion process. At later times, the release becames linear with time, indicating surface reaction control. The reproducibility of square root of time release rates with previously used glass can be explained within the context of current models of vitreous silica structure. At 65°C, above pH 6.5 the log diffusional rate constants increase with pH such that δ log k/δpH = 0.4, while below pH 6.5 the diffusional rate constants are nearly pH-independent. The surface reaction release rate constants show a similar pH dependence with δ log k/δpH = 0.5 above pH 7, while below pH 7 the rate constants are nearly pH-independent. The activation energy of surface reaction is 22.7 ± 3.7 kcal mol−1 at pH 4 for vitreous silica. This value is consistent, but slightly higher than that of Rimstidt and Barnes [Geochim. Cosmochim. Acta 44 (1980) 1683] for amorphous silica.