Oak Ridge National Lab Test Results


Removal of Trace Concentrations of Heavy Metals
Using Complexing Ion-Exchange Resins

Paul Taylor
Oak Ridge National Laboratory
14th Symposium on Separation Science and Technology

For Energy Applications
October 24-27, 2005

ABSTRACT

Oak Ridge National Laboratory is examining potential new technologies for treating radiologically-contaminated process wastewater. The current treatment facility is aging and is optimized to remove 90Sr, but future wastewaters are likely to contain mostly activated metals, such as 51Cr and 64Cu. Other low-volume wastewaters may contain trace concentrations of Hg and U. Complexing ion-exchange resins and other specialized sorbents were tested for removing trace concentrations of heavy metals. Short-term column tests and batch loading tests were conducted using a surrogate wastewater and various sorbents. These tests showed that metal uptake was very rapid, and that good removal and relatively high loadings could be achieved from a very dilute wastewater surrogate. Forager M-TU (Dynaphore, Inc.) showed the best overall results, removing 91.9% of the Cr, 99.3% of the Cu, >99.7% of the Hg, and >99.9% of the U with a contact time of 120 seconds in a short-term column test. .

INTRODUCTION

Oak Ridge National Laboratory (ORNL) is considering building a new treatment facility for radiologically-contaminated process wastewater. The current facility, the Process Wastewater Treatment Complex, is aging, is very large (~150 gpm) and is optimized to remove 90Sr. Most of the current wastewater comes from contaminated groundwater, which future environmental remediation work should greatly reduce. The primary future sources of radiological process wastewater will be the High Flux Isotope Reactor (HFIR) and the Spallation Neutron Source (SNS), which will produce water containing trace quantities of activated metals, such as 51Cr and 64Cu. The flow rate from these sources is expected to be in the range of 5-10 gpm. Other low-volume wastewaters, such as burial ground leachate, may contain trace concentrations of Hg and U. Complexing ion-exchange resins and other specialized sorbents were tested for removing trace concentrations of heavy metals.

MATERIALS AND METHODS

Surrogate wastewater was prepared by adding small amounts of analytical standard solutions to tap water, to give nominal concentrations of 10 mg/L of Cu, Cr and Hg, and 300 mg/L U. These concentrations correspond to measured or predicted concentrations for wastewaters that will be generated in the future at ORNL. Four different sorbents were initially tested, including ion-exchange resins with iminodiacetic and methylenethiol active sites, and specialized sorbents with polyamine/polyamide, and dithiocarbamate groups. A fifth sorbent with humic acid groups was tested later (See Table 1.).

Table 1. Description of Sorbents Tested

Sorbent Active Site Company
Amberlite IRC-718 Iminodiacetate Rohm & Haas
Forager M Polyamine/Polyamide Dynaphore,Inc.
Forager M-TU Dithiocarbamate Dynaphore,Inc.
Ionac SR-4 Methylenethiol Sybron Chemicals
Humasorb Humic Acid Arctech, Inc.


Short-term column tests were performed using a 1.3-cm I.D. by 25-cm tall clear PVC column. About 7 cc of sorbent was weighed, slurried in water, and poured into the column. Surrogate solution was pumped down through the column for several minutes, and then the sorbent height was measured and the settled volume was calculated. The flow rate was measured by collecting effluent in a graduated cylinder. A sample was taken for metals analysis and then the pump speed was adjusted and a new sample was taken after several bed volumes of surrogate had been pumped through the column. These short-term column tests were designed to determine the relative performance of the sorbents, and to determine the minimum residence time required to meet treatment requirements. Because of the low metal concentrations in the surrogate solution, running the columns to breakthrough would have required very long run times, and were beyond the scope of these tests.

Batch loading tests were conducted using Forager M-TU sorbent, which showed the best results in the column tests, and Humasorb, which was received after the column tests were completed. Various ratios of surrogate solution and sorbent were measured into polyethylene bottles, which were placed on a roller for 4 days to gently mix the slurries. Then samples of the solution were removed, filtered using 0.2mm syringe filters, and submitted to the lab for analysis. Chromium, copper and uranium were analyzed by inductively-coupled plasma – mass spectroscopy (ICP-MS), and mercury was analyzed by cold-vapor atomic adsorption (CV-AA)..

RESULTS AND DISCUSSION

Table 2 shows the results of the short-term column tests. A sample of the unused surrogate wastewater contained 10.8 mg/L Cr, 13.9 mg/L Cu, 8.6 mg/L Hg, and 380 mg/L U. All of the sorbents were very good at removing mercury from the water, with all of the effluent samples below the detection limit of 0.2 mg/L. All of the sorbents except Ionac SR-4 were also very good at removing uranium. Chromium, which was present as the chromate ion, showed the poorest results for all of the sorbents. Overall, the Forager M-TU sorbent showed the best results, with Forager M a close second. The actual contact time of the surrogate with the sorbents was very short (30 – 120 seconds); however, longer contact times would probably be required to fully utilize the capacity of the sorbents for long-term use.


Table 2. Summary of Analytical Results for Short-Term Column Tests


Sorbent Flow Rate (BV/min*) Percent Removal
Cr Cu Hg U
Amberlite IRC-718 1.03 3.7 80.9 >97.7 99.9
0.51 10.7 81.0 >97.7 99.7
0.27 10.1 77.6 >97.7 97.3
Forager M 1.00 61.7 93.5 >97.7 >99.9
0.54 76.5 86.5 >97.7 >99.9
0.26 90.5 86.9 >97.7 >99.9
Forager M-TU 1.03 73.1 91.0 >97.7 >99.9
0.51 88.3 97.5 >97.7 >99.9
0.27 91.9 99.3 >97.7 >99.9
Ionac SR-4 1.06 36.1 86.9 >97.7 52.6
0.52 51.1 91.2 >97.7 62.4
0.28 65.9 93.0 >97.7 68.9

*Bed volumes per minute, based on the empty-bed volume of the sorbent.

Tables 3, 4, and 5 show the results of the batch loading tests using the Forager M-TU sorbent. A batch loading test using 0.101 g Humasorb in 10 mL of surrogate showed final liquid concentrations of 8.2 mg/L Cr, 4.1 mg/L Cu, 0.2 mg/L Hg, and 185 mg/L U, which are much higher than the corresponding results for the Forager M-TU.


Table 3. Summary of Batch Loading Test Results Using Forager M-TU Sorbent


SolutionVolume (mL) Sorbent Weight (g) Liquid Concentrations (mg/L)
Cr Cu Hg U
100 0.0 11.1 27.8 5.02 286
100 0.001 8.09 8.63 0.226 167
100 0.0107 6.02 5.12 <0.2 1.48
10 0.0094 2.09 5.01 <0.2 0.10
10 0.1006 1.046 1.20 <.02 0.01


Table 4. Sorbent Loading Results for Batch Loading Tests Using Forager M-TU Sorbent

Solution/Sorbent Ratio (mL/g) Sorbent Loading (mg / g sorbent)
Cr Cu Hg U
100,000 301 1917 1217 11900
9346 47 212 106 2659
1063 9.6 24 12 304
99.4 1.0 2.6 1.2 28


Table 5. Distribution Coefficient Results for Batch Loading Tests Using Forager M-TU Sorbent


Solution/Sorbent Ratio (mL/g) Distribution Coefficient (L/kg)
Cr Cu Hg U
100,000 37206 222132 2830233 71257
9346 7886 41399 258537 1796666
1063 4586 4839 62000 3041489
99.4 656 2203 6126 2842843


The test results using the highest ratio of surrogate to sorbent show that fairly high loadings can be obtained from a very dilute wastewater. The uranium concentration in the surrogate solution was at least 10 times higher than for the other metals, and the maximum uranium loading was also much higher on the sorbent. The large increase in soluble uranium concentration and the drop in distribution coefficient for the test with the highest solution-to-sorbent ratio, compared to the second highest, suggests that the sorbent is reaching its maximum loading capacity for uranium