Doctoral Dissertations

Date of Award

5-1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Civil Engineering

Major Professor

R. Bruce Robinson

Committee Members

J. L. Smoot, C. D. Cox, R. M. Counce

Abstract

Technetium (99Tc), an uranium fission product, is a waste material left over after reprocessing fuel rods for plutonium production. The nuclear tank waste present at all the Department of Energy (DOE) sites contains an estimated 1.8 metric tons of Tc mostly present as the pertechnetate anion, TcO4-. Technetium is environmentally mobile, has a half life of 2.13 E 05 years and is volatile upon vitrification. Existing techniques for Tc removal include solvent extraction, ion exchange, and reduction/precipitation. An alternative technique for the removal of pertechnetate** present in waste streams, is to employ supported liquid membranes (SLMs). This research aims at examining the feasibility of extracting Tc in SLMs using crown ethers as extractants. SLMs consist of a solvent contained in the pores of a polymeric membrane between two aqueous solutions (feed and strip). Crown ethers form a complex with sodium and technetium at the extraction interface. This complex diffuses across the membrane and is then stripped at the strip interface. SLM mass transfer models available in literature have either focussed only on the extraction side kinetics, or considered stripping side solute concentrations to be negligible, or have assumed identical solute distribution coefficients on the extraction and strip side. They have been unanimous in their claim that a solute cannot be transferred against its concentration gradient in cocurrent transport. The model developed by the author has eliminated all these deficiencies by considering finite concentrations of the solute in the strip, and different feed and strip side distribution coefficients. Also, as per the model technetium can be transported against its apparent concentration gradient even in cocurrent transport and without any pH effects. Experiments were conducted at the Oak Ridge National Laboratory, Oak Ridge, TN on flat sheet SLMs. The first set of experiments involved the determination of the optimum Extractant (E), Diluent (D), and Modifier (M) combination. Crown Ethers (CE) were selected as the broad extractant category. CEs selected for the study included bis-4, 4' (s')[(tert-butyl)cyclohexano]-18-crown-6 (Eichrom Industries, Inc.) abbreviated di-t- Bu CH18C6), dicycloxexano-18-crown-6 (Parish Chemical) abbreviated DCH18C6, and cis-sym-bis-(tert-octyl benzo)-14-crown-4-dial (synthesized at Oak Ridge) abbreviated BOB14C4dial. di-t-Bu CH18C6 was selected, for most studies, over DCH18C6 because of its higher lipophilicity and higher lipophilicity resulted in lower aqueous solvent losses. Diluents systems studied included blends of 2-octanone with Isopar M, blends of 4-(1-butyl pentyl) pyridine (4-BPP) with Isopar-M or Isopar-V Nitrophenyloctylether (NPOE), and Nitrophenylpentylether (NPPE). Only NPOE and NPPE showed technetium transport and hence were chosen as the desired diluent. The 4-BPP and 2- A set of experiments using d-t-Bu CH18C6 in NPOE was conducted to study the effect of initial CE concentration on Na and Re Flux. CE concentration was varied from 0.0 M (only NPOE) to 0.2 M. Na flux increased slowly between 0.0 and 0.1 M and then very rapidly between 0.1 and 0.2 M while the Re flux behaved in the exact opposite manner. Selectivity, defined as the ratio of Re over nitrate flux, reached a constant maximum value between 0.01 and 0.1 M. A second set of experiments involved studying the effect of flow rates on Na and Re flux. Fluxes increased and selectivity dropped with increasing flow rates. This shows that aqueous boundary layer resistances dropped with increasing flow rates, though not in a linear fashion, and the increase in nitrate flux was more than the increase in technetium flux. Na flux approximates nitrate flux as sodium nitrate is abundant over sodium perrhenate by over four orders of magnitude in the feed solution. A third set of experiments involved studying the effect of varying Re and Na concentrations in the feed solution on Na and Re flux. Re and Na fluxes increased by 20 and 6 times, respectively, and selectivity dropped by 15 times when the initial Re concentration in the feed was increased by 50 times. Na concentration in the feed and the strip and had no appreciable impact on either Na or Re flux. Experiments were conducted to demonstrate that Re can be transported against its concentration gradient. ** Sodium perrhenate was used as a surrogate for sodium perrhenate in all experiments as the former is not radioactive.

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