Doctoral Dissertations

Date of Award

3-1988

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major Professor

Gary Stacey

Committee Members

Charles Garten, Gordon Blaylock, Jeff Wolt, Gary Sayler

Abstract

Technetium-99 (99Tc) is a long-lived radionuclide (half-life of 2.1x105 years) produced by nuclear industries. In case of release, its likely form in the environment is the pertechnetate anion TcO4-. The behavior of TcO4- in soil is important to assess Tc short-term and long-term geomobility and plant availability. Behavior of Tc in soil has been the object of many studies, however, results of laboratory experiments, indicating low Tc sorption in soil, conflict with the observed behavior of Tc in the field (high sorption and reduction of the soil to plant transfer factor with time). Microbial activity has been suggested to play a role in Tc sorption; however, the process involved and the extent of the bacterial involvement are unknown.

The objectives of the first part of this thesis are to determine if soil bacteria affect TcO4-, and if so, by which mechanism. Bioaccumulation of Tc and chemical modification of TcO4- were studied in mixed cultures of aerobic and anaerobic soil bacteria, and in pure culture of sxilfate-reducing bacteria. Bacteria were grown in liquid media and Tc association with organics was determined by gel permeation chromatography. Mixed aerobic bacteria had no apparent effect on TcO4-. Mixed anaerobic bacteria reduced TcO4-, enabling its association with the organics of the growth medium. Reduction was a metabolic process and not merely the result of reducing conditions. Sulfate-reducing bacteria efficiently removed Tc from solution (up to 70% of the total Tc was bioaccumulated and/or precipitated as Tc-sulfide) and promoted its association with organics. Pertechnetate was not reduced by the same mechanism as dissimilatory sxilfate reduction but by a reducing agent released in the growth medium. Technetium associated with bacterial polysaccharides only in the anaerobic cultures.

The second part of the thesis has for objectives to (1) provide data on the kinetics and the extent of Tc sorption and desorption under conditions realistic for the field, (2) determine the importance of microbial activity, soil water content, Tc concentration, and organic matter on Tc sorption, and (3) describe the behavior of Tc under fluctuating water tension. Technetium sorption was studied in 2 contrasting horizons of an Ultisol: Ap horizon (2.73% organic matter, high microbial activity) and Bt horizon (0.61% organic matter, low microbial activity). Fresh samples of the 2 horizons were incubated in the laboratory under various soil water contents and Tc concentrations. At time intervals, soil water was collected from the samples by centrifugation and sorption was calculated from the change of Tc concentration in the soil water. Microbial activity was reduced by gamma-irradiation of the soil. Under all experimental conditions, little sorption took place in the Bt horizon. In the Ap horizon, sorption was rapid (equilibrium was achieved in a few days) and the Tc soil-water equilibrium distribution was primarily a function of the soil water content and microbial activity. A reduction of microbial activity delayed and diminished sorption. As the soil water content increased, Tc sorption augmented. At field capacity, 50% of the Tc was sorbed, whereas under flooded conditions, sorption was complete. As Tc concentration in the soil water increased, the amount of Tc sorbed by Ap at field capacity increased, but the fraction of Tc sorbed decreased. Under flooded conditions, sorption was complete at all concentrations. Mathematical parameters describing Tc sorption as a function of time and Tc equilibrium distribution as a function of Tc concentration are provided in the text.

Organic matter was extracted from contaminated soil samples in 3 successive fractions of increasing pH. Technetium association with these fractions and the soil residue was determined as a function of time and Tc concentration. A large fraction of the Tc sorbed was associated with the soil organic matter indicating that complexation with soil organics was the predominant mechanism of Tc sorption. Technetium distribution pattern among fractions did not vary with time or To concentration ; Tc was distributed among fractions proportionally to the amount of functional groups they carried. Consequently, Tc was predominantly associated with the low molecular weight fraction of the soil organic matter. Technetium in the soil water was not associated with soluble organics but remained as TcO4-.

Release of Tc from Ap samples upon air-drying was rapid and important; up to 75% of the sorbed Tc was released within a few days. No measurable additional release took place in the following 85 days of the experiment. Under conditions of fluctuating soil water content, an expected sequence of sorption, desorption, and resorption was observed. In conclusion, the Bt horizon that is poor in organic matter and has a low bacterial population has also a low sorption potential for Tc. The Ap horizon did sorb Tc, the proposed conceptual model of Tc behavior in Ap is that (1) Tc is used by anaerobic soil bacteria as terminal electron acceptor for the respiratory chain and reduced, (2) once reduced, Tc is complexed with the carboxyl groups of the soil organic matter, (3) when oxygen is present in the soil, there is an equilibrium between reduction-complexation and oxidation of Tc that determines the partitioning of Tc between sorbed Tc and TcO4-, (4) sorbed Tc is partially released from the Tc-organic matter complexes when the soil air-dries. Technetium behavior in soil is a dynamic process determined primarily by soil type and fluctuations in soil water content.

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