Semester

Summer

Date of Graduation

2004

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Horticulture

Committee Chair

Louis M. McDonald, Jr.

Abstract

The effect of mixed aqueous-organic solvents on exchange and adsorption reactions onto clay minerals is examined in this study while accounting for the effect of cosolvents on metal solution properties, ionic activity and complexation. In the absence of published values, a determination of the primary association constant of selected ions in solution was conducted using conductometric methods. These stability constant values were used to correct for ion-pairing in the exchange and the adsorption experiments. A spectrophotometric determination of pH was accomplished through the quantification of the conditional dissociation constants, sKI, of two indicators (methyl red and phenol red) at constant ionic strength and at 25°C +/- 2.0 in ethanol and methanol-water mixtures. In these mixtures and under these conditions, solution pH was determined from measurements of the indicator absorbance ratios at two wavelength lambdaHIn, and lambdaIn- within the range 4.5 ≤ pH ≤ 8.5. Calcium-sodium exchange on Wyoming bentonite in methanol, ethanol and acetone-water systems were investigated at constant total chloride concentration and at room temperature. In all treatments, Ca-Na exchange in cosolvents was a surface-controlled phenomenon involving electrostatic and specific solvent-surface interactions. The effect of cosolvents on Cd and Zn sorption to Ca-saturated bentonite and illite in mixed alcohol-water systems at low ionic strength and low initial metal concentration in the presence of nitrate varied between metals. Cd2+ sorption to bentonite and illite was independent of the solvent dielectric constant suggesting a specific mechanism for Cd2+ sorption involving inner-sphere complexes with the surface edge sites. Zn2+ sorption to both clay minerals was strongly dependent on epsilonr with both increases (illite) and decreases (bentonite) in retention observed with decreased epsilon r. Although it is not yet possible to predict the effects of cosolvents on metal-surface reactions, it is clear that cosolvents can affect metal concentrations in solution, and therefore contaminant transport (increased hydraulic conductivity resulting from flocculation as Ca2+ replaces Na+, and hence greater risk of clay liners failure). Additional study is needed on the effects of cosolvents on surface charge density, changes in the interlayer spacing, surface acidity and pH changes before and after sorption.

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