Selective Sorption of Technetium
from Groundwater
Gilbert Brown
Chemical and Analytical Sciences Division
Oak Ridge National Laboratory
P. O. Box 2008
Oak Ridge, Tennessee 37831-6119
Published in the Proceedings of the Efficient Separations and Processing Crosscutting Program 1997 Technical Exchange Meeting (PNNL-SA-28461), January 28-30, 1997, Gaithersburg, Maryland, pp. 2.5-2.8.
EM Focus Area: contaminant plume containment
and remediation
Technology Needs
Groundwater at DOE sites in which uranium or
plutonium has been processed is frequently contaminated with the
radionuclide Tc-99. DOE's Paducah and Portsmouth sites are typical
of the contamination problem. Solutions contaminated with radionuclides
were poured into lagoons and burial pits, which created a plume
that has seeped into the sandy aquifers below the vados zone.
Technetium is the principal radioactive metal-ion contaminant
in the groundwater at the Paducah site, and it is present at a
concentration of about 25 ng/L. Tc is present in the groundwater
at Portsmouth at a concentration which varies a great deal with
distance from the source, and concentrations of >400 ng/L have
been reported. Under the oxidizing conditions of near-surface
groundwater, the principal form of the element Tc is expected
to be the pertechnetate anion, TcO4-
[1]. Pertechnetate salts are highly water-soluble and quite mobile
in underground aquifers, and, when coupled with the long half-life
of Tc-99 (213,000 years), the resulting probable transport into
the biosphere makes the presence of this radioisotope in groundwater
a great concern. A related problem exists at other DOE sites
where the processing of uranium or plutonium resulted in the release
of technetium to the surrounding groundwater. Commercially available
anion-exchange resins are capable of removing TcO4-
ion in the presence of typical anions found in groundwater, but
improving the selectivity will result in substantial cost savings
in terms of the quantity of resin needed and the scale of the
equipment required to treat huge flow rates.
Technology Description
The pertechnetate anion is strongly sorbed
on commercially-available strong-base anion-exchange resins, but
in view of the low (typically nanomolar) concentrations of Tc
involved, enhanced selectivity for the pertechnetate anion over
other anions commonly found in groundwater such as chloride, sulfate,
and nitrate will be needed. We have prepared and evaluated new
anion-exchange resins which were designed to be highly selective
for pertechnetate. The technology involves building those features
which are known to enhance the selectivity of pertechnetate over
other anions into the exchange sites of the resin (hydrophobicity),
while at the same time maintaining favorable exchange kinetics.
A resin bed of this material will be used either
as part of a coupled treatment-recirculation system for the in
situ remediation of groundwater contaminated with technetium or
in a once-through treatment scheme.
Benefits to DOE/EM
While commercially available strong-base anion-exchange
resins are effective in removing pertechnetate from groundwater,
improvements in selectivity can result in substantial cost savings
due to less resin required to treat a given volume of groundwater.
The use of exchangeable resin modules is expected to lead to
a low-maintenance easily-cared-for technology for technetium removal.
Technology Transfer/Collaborators
Collaborators: Department of Chemistry, University of Tennessee, Knoxville; Environmental Sciences Division, ORNL.
Technology Transfer: Eichrom Industries, Inc.,
Darien, IL
Scientific Background
Strong base anion exchange resins can remove TcO4- ion with varying degrees of selectivity from waste streams containing an excess of other competing anions. Ashley et al. [2] at LANL have demonstrated that Reillex™-HPQ resin is effective in removing pertechnetate from tank waste simulants that have high concentrations of nitrate ion. At ORNL, Dowex™
Other schemes have been proposed for removing
technetium from groundwater. One method under examination will
reduce the technetium to a lower oxidation state that is insoluble
or that precipitates from solution. This method may not be effective
if oxygen in the aquifer or from other sources can oxidize the
technetium back to the soluble TcO4-
ion. Use of a resin to sorb the anion will have advantages over
reductive schemes if the resin can be made selective. Air stripping
is the currently favored technology to remove trichloroethylene
and other volatile organic compounds from groundwater, and this
process necessarily puts a high concentration of oxygen in the
aquifer.
We are developing resins to selectively sorb
technetium from groundwater that can be used with a remediation
scheme involving recirculation of the water through a decontamination
station within the aquifer. These resins will also be effective
with once-through treatment schemes such as that being demonstrated
at Paducah.
Technical Approach
The pertechnetate anion has a high affinity
for strong-base anion-exchange resins such as those made from
quaternary amines. Most commercially-available strong-base resins
do not have as high a selectivity for TcO4-
over chloride, sulfate, or nitrate ion
as is desirable or attainable. These latter anions can be present
in the groundwater at concentrations 106
times that of pertechnetate.
On theoretical grounds, the microenvironment
of the exchange sites within the resin is expected to play a major
role in ionic selectivity, and the synthesis of a new series of
resins in which the properties of the resin were systematically
varied was undertaken. These resins were part of a program to
modify the microenvironment of the exchange sites and to thus
enhance the selectivity for pertechnetate ion over the other anions
commonly found in groundwater. During the past two years, we
have prepared and evaluated over 80 laboratory resins. These
were tested along with 7 commercial resins for sorption of pertechnetate
from a "Groundwater Test Solution" consisting of pertechnetate
at a concentration of 6.0 micromolar in a matrix of sodium chloride,
sodium nitrate, and sodium sulfate (each at 60 millimolar), as
previously described [3]. The affinity of a resin for pertechnetate
was determined by measuring the batch distribution coefficient
(K'd)
for TcO4-
sorption on the resin following a given equilibration period (e.g.,
1, 4, 24, and 168 hour time periods). The batch equilibrium testing
method for measuring pertechnetate uptake provided a useful means
for comparing the affinity of a resin for pertechnetate as a function
of time. Early results were instrumental in alerting us to the
key structural and chemical properties of the resin that enhanced
both the overall pertechnetate selectivity and the rate of pertechnetate
uptake.
We have applied our knowledge of the properties
of existing anion-exchange resins in making both the chemical
and physical modifications necessary to improve both the K'd
and the K'd(eq)
(the K'd
per exchange site) for pertechnetate. The iterative process of
resin synthesis, evaluation of Tc selectivity, and further resin
synthesis in a close collaboration allowed us to rapidly converge
on a novel class of resins with superior selectivity for the pertechnetate
ion. These resins were studied more thoroughly under flow-through
column conditions using our Groundwater Test Solution, with the
objective of determining selectivity under conditions of solution
flow in a column where mass-transport limitations become important.
The best resin from this study was then tested using actual Paducah
groundwater, followed by a field test, in which pertechnetate-contaminated
groundwater from monitoring well #106 in the Pits and Trenches
area of ORNL at the DOE Oak Ridge Reservation was pumped through
a column containing the resin for a month. We are presently evaluating
different methods of preparing the best resins, to determine the
most economical synthetic procedures that can be used for preparing
bulk quantities of the best resins, while maintaining optimum
pertechnetate sorptive performance.
Accomplishments
Best Candidate Resins Identified.
A new class of anion exchange resins with improved selectivity
and sorptive capacity for the pertechnetate anion as compared
to commercially available resins have been prepared and evaluated,
both in batch-equilibrium and flow-through column testing conditions
using a groundwater test solution. When evaluated in column flow-through
experiments, the best laboratory resin, code-named VP02-217, can
process more than 30 times the volume of groundwater test solution
before reaching the same level of break-through as the commercial
resin Purolite®
A-520E (see Table 1), which is currently being used to remove
pertechnetate from groundwater at DOE's Paducah, KY facility.
Invention Disclosure Submitted.
An invention disclosure was submitted describing the synthesis
and characterization of the new class of anion exchange resins.
Eichrom Industries, Inc., has expressed interest in this resin
to complement other Tc-selective materials in their product lines.
Field Tests at Monitoring Well #106 in the
Pits and Trenches Area.
Evaluation of the Tc selectivity of our synthetic resin under
actual field conditions using contaminated groundwater was begun
during late July and continued into August, 1996. This small-scale
field test was designed for the optimization and eventual engineering
scale-up for a pilot-scale or a full-scale field demonstration
in FY 1997 and FY 1998. We determined the breakthrough of Tc-99
using our best synthetic resin VP02-217 at a relatively high flow
rate which is representative of the rate that may be used in field
pump-and-treat operations. In this test, the system fouling,
clogging, and hydraulic head loss during the experiment were also
evaluated. The test was performed with the VP02-217 resin contained
in a small column (10 x 40 mm) at a constant flow rate of 17.8
mL/min. The volume of the resin bed ("bed volume")
was 3.1 mL, with an effective void fraction for the resin of ~31%
(making the "pore volume" for this resin bed equal to
0.96 mL). Therefore, the interstitial linear velocity was calculated to be ~73 cm/min, which is comparable to the value (~80 cm/min) in use at the Northwest Plume pump-and-treat
facility at DOE's Paducah Gaseous Diffusion Plant (PGDP) site.
Table 1. Column breakthrough data for pertechnetate sorption from
groundwater test solution for Purolite
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*
Not determined due to the variation
of the background data.
This field test was conducted in the Pits and Trenches area at the DOE Oak Ridge Reservation. Groundwater in this area is contaminated with Tc-99 (as TcO4-). Monitoring well #106 was selected as our test site because the groundwater contains a relatively high Tc-99 concentration but relatively low concentrations of other contaminants such as Co-60, H-3, Sr-90, and Cs-137. However, the groundwater contains a relatively high concentration of NO3- and SO42- (>100 mg/L). The TcO4- concentration in the groundwater varied from ~1000 to 5000 pCi/L during the test period because of rainfall events.
Results indicated that no significant breakthrough
of Tc-99 was observed during the first 5 days of operation. At
day 7 (or after ~56,000 bed volumes of groundwater passed through
the column), we observed ~2% breakthrough of TcO4-.
The column was continuously operated for 31 days, and by an observed
color change, the column appeared to pick up a coating of organic
materials on the resin which did not appear to significantly reduce
the performance of the synthetic resin. The performance of this
resin in this test (see data in Table 1) indicates that it is
at least an order of magnitude more selective than any other sorbant
which has been tested for Tc sorption including iron filings [4],
other resins, and activated carbon [5].
Acknowledgment: This research was sponsored by the Efficient Separations and Processing Crosscutting Program, Office of Science and Technology, Office of Environmental Management, U. S. Department of Energy, under contract number DE-AC05-96OR22464 with Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp.
For further information, please contact:
LITERATURE CITED
[1] Pourbaix, M., Atlas of Electrochemical
Equilibria, p.24, Pergamon Press, Oxford (1966).
[2] Ashley K. R.; Ball, J. R.; Pinkerton, A.
B.; Abney, K. D.; Schroeder, N. C. "Sorption behavior of
99TcO4-
on Reillex-HPQ anion exchange resin from nitric acid solution."
Solvent Extraction and Ion Exchange, 1994, 12,
239-259.
[3] (a) Brown, G. M.; Bates, L. M.; Bonnesen,
P. V.; Moyer, B. A.; Alexandratos, S. D.; Hussain, L. A.; Patel,
V.; Liang, L.; Siegrist, R. L. "Selective Resins for Sorption
of Technetium from Groundwater, FY 1995 Letter Report", Oak
Ridge National Laboratory, September 27, 1995. (b) Brown, G.
M.; Presley, D. J.; Bonnesen, P. V.; Bates, L. M.; Moyer, B. A.;
Alexandratos, S. D.; Hussain, L. A.; Patel, V.; Gu, B.; Liang,
L.; Siegrist, R. L. "Column Tests of Resins for Selective
Sorption of Technetium from Groundwater", Oak Ridge National
Laboratory, July 31, 1996.
[4] Liang, L.; Gu, G.; Yin, X. Separations
Technol., 1996, 6, 111-122.
[5] Gu, B.; Dowlen, K. E.; Liang, L.; Clausen,
J. L. Ibid, 1996, 6, 123-132.
Gilbert M. Brown
Principal Investigator
Oak Ridge National Laboratory
Bldg. 4500S, MS-6119, P.O. Box 2008
Oak Ridge, TN 37831-6119
(423) 576-2756, fax (423) 574-4939
E-mail: GBN@ornl.gov
TTP Number: OR-16C3-11 [3TEU]