Date of Award

1-2011

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Environmental Engineering and Science

Advisor

Carraway, Elizabeth R.

Committee Member

Schlautman , Mark A.

Committee Member

Powell , Brian A.

Committee Member

Brumaghim , Julia

Abstract

This work examined several reductants for remediation of low pH aqueous uranium contamination by reductive precipitation. Reductants included sodium dithionite, L-ascorbic acid, and nanosized zero-valent iron (nZVI). Additionally, the reductive capacity of nZVI successfully entrapped within a silicate matrix (cryogel) was also examined. This supported nZVI is expected to increase nZVI stability in the subsurface, and to provide a convenient in situ delivery mechanism for other remediation systems which may combine entrapped reductant and catalyst.
Fluorescence and absorbance spectroscopy were used to investigate the interaction between hexavalent uranium (U(VI), uranyl) and both sodium dithionite and L-ascorbic acid at pH 2. Results suggest complex formation between U(VI) and dithionite decomposition products and between U(VI) and L-ascorbic acid. Some evidence of U(VI) reduction by both sodium dithionite and L-ascorbic acid is also presented; however, neither are suggested for in situ remediation due to complex formation with U(VI), and due to the aqueous instability of the dithionite ion.
Analytical difficulties prevented an accurate study of the U(VI) interaction with nZVI and supported nZVI. However, data presented indicate some U(VI) removal by supported nZVI at pH 2, and because U(VI) has a low sorption affinity for silica at pH 2, this removal was attributed to reduction. The presence of fluoride resulting from synthesis of the supported nZVI, may also have negatively impacted uranyl reduction by forming stable aqueous U(VI)-fluoride complexes.

The reaction of hexavalent chromium (Cr(VI)) with nZVI and supported nZVI was also examined. Reaction with both nZVI and supported nZVI resulted in Cr(VI) reduction at pH 2 and 4. Calculated rate constants were 0.055, 0.22, and 7.86 hr-1 for supported nZVI at pH 4 and 2, and free nZVI at pH 4, respectively. Although too rapid to accurately calculate, the rate constant for the reaction of nZVI with Cr(VI) at pH 2 was greater than 50 hr-1. Rate constants for Cr(VI) reduction by supported nZVI and free nZVI at pH 4 were compared to results for a resin supported nZVI material and free nZVI as reported by Ponder et al. (2000).
This is the first report of successful Cr(VI) reduction by nZVI entrapped within a cryogel. This work also presents the first known report of U(VI) reduction by dithionite. Additionally, this work provides the initial framework needed for further studies that aim to demonstrate the full potential of supported nZVI for in situ subsurface remediation of metal (and other) contaminants.

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