Surface complexation of protons and sulfate by kaolinite Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ww72bf381

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  • The emission of sulfur oxides from anthropogenic sources represents a potential threat to terrestrial ecosystems. Sulfate, the most highly oxidized form of sulfur, is the most common form of inorganic sulfur in most soil environments. The sorption of sulfate may influence the alkalinity of the soil solution and as a consequence the pH of surface waters. Batch experiments were performed to determine the influence of pH, total sulfate concentration and solubility of the adsorbent on the sorption of sulfate by kaolinite. Proton surface charge density for kaolinite was determined over a solution pH range from 3.5 to 11.0 in the absence and presence of sulfate (10.99 to 526.3 μM) in 0.1 M NaCl0₄ at 298 K. Total aluminum (Al[subscript TS]) and silica (Si[subscript TS]) concentrations were measured as a function of pH after varying reaction times (2 to 60 h). The dissolution of kaolinite was incongruent (Al[subscript TS]/Si[subscript TS] ≠ 1) over the entire pH range. Proton adsorption by edge surface functional groups of kaolinite, corrected for proton consuming dissolution reactions, showed no influence of the reaction time. Titration data were modeled by the nonlinear least squares fitting routine FITEQL to obtain intrinsic proton complexation constants. Aluminol, aquo and silanol groups were found to undergo complexation reactions. Average values over all reaction times and total sulfate concentrations for log K₋ (int) and log K₊ (int) were -9.3 and 7.3 for aluminol and aquo groups, and -5.2 and 4.9 for silanol groups, respectively. The total concentration of surface sites was 1.74 cmol kg⁻¹. This higher value, in comparison to the theoretically determined concentration, could be explained by differences in the particle size of kaolinite. Kinetics of sulfate sorption by kaolinite KGa-1, determined in a stirred batch reaction in 0.1 M NaCl0₄ for a suspension density of 33.33 g L⁻¹ at pH 4.5 and 6.5 were rapid. No sorption reaction could be observed after 1 min reaction time. Sulfate sorption was independent of pH and increased linearly with the free sulfate concentration. Linear regressions resulted in an overall partition coefficient for sulfate of 7.8 L kg⁻¹ (R² - 0.99) between solid and solution. A multiple regression model showed that pH was not a significant variable in determining the amount of sulfate sorbed. Specific adsorption by edge surface groups via ligand exchange could be excluded, mainly due to the complete lack of a pH influence on sulfate sorption. Precipitation of a sparingly soluble aluminum sulfate mineral was considered a possible mechanism controlling sulfate concentration in solution. The solubility of several aluminum and aluminum sulfate minerals was computed using the solution speciation program MICROQL. Precipitation of basaluminite, a basic aluminum sulfate mineral present in acid sulfate soils, was found to be possible within the pH range from 5.0 to 6.0. However, no conclusive evidence can be presented for precipitation being the mechanism of sulfate sorption in presence of kaolinite.
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