Date of Graduation

1997

Document Type

Dissertation/Thesis

Abstract

Five clay minerals (Well Crystallized Kaolinite (KWC), Poorly Crystallized Kaolinite (KPC), Illite, Attapulgite, and Montmorillonite) with a range of surface characteristics were selected to investigate the adsorption of three oxyanions (molybdate, selenite, and selenate). The minerals were coated with five different levels of each of Fe and Al hydrous oxides. The charge characteristics were determined using potentiometric titration. Adsorption was studied by batch experiment at 11 different concentrations of each of the oxyanions at three different pH levels. Coating with both Fe as well as Al hydrous oxide coatings raised the pH{dollar}\\sb{lcub}\\rm o{rcub}{dollar} (condition where mineral surface is electrically neutral) of all the clay minerals. The pH{dollar}\\sb{lcub}\\rm o{rcub}{dollar} of montmorillonite was below the lowest pH level achieved during the investigation. There was a general increase in pH{dollar}\\sb{lcub}\\rm o{rcub}{dollar} with coating, but not the same for all minerals. Coating material seemed to be deposited dominantly on the edge surfaces of the clay minerals. Coating with both Fe as well as Al hydrous oxides increased anion retention by all the clay minerals studied. Significant adsorption of anions was observed at equilibrium anion concentrations as low as 0.4 {dollar}\\mu{dollar}M. Adsorption curves had a very steep slope at low equilibrium concentrations indicating a high affinity between anions and the mineral surface. The slope decreased with increase in equilibrium concentration and became negative at higher concentrations in some of the samples indicating the presence of only a limited number of sites. Uncoated samples of all the clay minerals exhibited negative adsorption of all the oxyanions at higher pH (5.5 and 6.5) and higher equilibrium concentrations. As the levels of hydrous oxide coatings increased, anion retention by all the clay minerals also increased. Although molar concentration of Al hydrous oxide coating was higher than that of Fe hydrous oxide coating, adsorption was higher for Fe hydrous oxide coated samples. Among the three oxyanions, adsorption was lowest for selenate. In general, higher amount of molybdate than selenite was adsorbed at low pH. However, there was a more drastic decrease with increase in pH in adsorption of molybdate than that of selenite. Consequently, adsorption of selenite exceeded that of molybdate at higher pH values. Adsorption capacity of various minerals was related to the total proportion of octahedrally coordinated cations (Al, Fe, Mg) and their position within the mineral structure. Both charge as well as anion adsorption characteristics minerals seemed to be dependent upon edge surface characteristics. Hydrous oxide coated samples were prepared by neutralizing acidic solutions of Fe and Al simulating the conditions during treatment of acidic mine soils with alkaline fly ash. Results of this investigation indicate that hydrous oxide coatings are important in controlling the aqueous concentrations of toxic oxyanions in fly ash treated mine soils that could reduce the environmental pollution. (Abstract shortened by UMI.).

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