Date of Award

1999

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Frank K. Cartledge

Abstract

A cement-based solidification/stabilization (S/S) system of Cr wastes treatment has been studied in this program. A combination of the two techniques; namely reduction of Cr(VI) to Cr(III) and simultaneous immobilization within a cement matrix, has been found to be effective and practical to treat Cr wastes. Cr(VI) can not be easily and effectively-immobilized in the S/S system unless being reduced to Cr(III). Ferrous compounds are used as reducing reagents. The reduction not only removes the toxic Cr(VI) species but also results in the low-solubility precipitation of (CrxFe 1-x) (OH)3(s) in the S/S system. Long-term and short-term samples were tested through TCLP procedure. For appropriate contents and time, the leachate concentrations can be brought below the EPA characteristic limit of 5 mg/L. Usually, the long-term samples show higher leaching concentration of Cr. Many metal ions have a profound influence on the cementitious matrix formed during the curing process of Portland cement. There is a consensus view, based on calorimetric studies, that Cr species are accelerators for early hydration of cement. This program has been investigating the system in most detail with 29Si MAS NMR, which shows retarded hydration by Cr salts alone, or by ferrous chloride (or sulfate) alone or in combination with Cr species. In addition to retarded hydration, the added Cr species and ferrous salts results in enhanced amounts of chain-lengthening and chain-branching silicates. X-ray absorption fine structure (XAS) was successfully applied in speciation of Cr(III) and Cr(VI) in this program. The ratio of Cr(VI) to total Cr in a cement-based S/S system was measured, and the effectiveness of different Fe(II) reductants is determined from XANES and compared to leachability results. EXAFS analyses revealed a complex situation in the structural picture of the product (or products) of the Cr wastes treatment system in the OPC cement matrix.

ISBN

9780599548831

Pages

173

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