C. J. Coleman
Westinghouse Savannah River Company
Aiken, SC 29808

The Sludge Batch 3 macrobatch feed to the DWPF is expected to contain a relatively high concentration of oxalate. A simple acid addition at room temperature has been shown to be effective for dissolving forms of oxalate likely to be in high-level radioactive sludge. This sample preparation requires only about 5 minutes and yields solutions suitable for oxalate determinations by ion chromatography.

The data and observations support the following conclusions:

• A step-wise addition of 2 ml concentrated HCl and 2 ml concentrated HNO₃ to about 1 g of sludge slurry, followed by dilution of this mixture to 250 ml, yields on average about 90-95% recovery of oxalate spiked into Sludge Batch 3 simulant. This treatment resulted in near complete dissolution of the sludge so that no filtration was required to remove solids prior to IC determinations. For this reason, we recommend that this acid treatment be used to support DWPF Sludge Batch 3 development studies and to analyze actual Tank 7 sludge.

• Dilution of the Sludge Batch 3 simulant with de-ionized water followed by acidification and dilution to 250 ml (essentially the inverse of the procedure discussed in the first bulleted item) yielded oxalate values that were statistically identical to those obtained by adding the acids to the undiluted slurry. However,this treatment leaves considerable amounts of solids that should be removed by filtration prior to IC determinations.

• Both acid treatments discussed above resulted in complete digestion of calcium oxalate. This was confirmed by both visual observation and by analysis of Sludge Batch 3 simulant that had calcium added at a much higher concentration than expected in the Tank 7 sludge. All experiments showed that the oxalate determinations were independent of the calcium in the sample when sufficient acid was added to ensure dissolving the calcium oxalate.

• The oxalate concentration of some acidified solutions dropped dramatically within a few days of the initial preparation. A possible mechanism of the oxalate destruction is photo-decomposition of the iron-oxalato complex to form CO₂ and formate. No formal studies were done to measure the kinetics of decomposition or to absolutely determine the mechanism of the decomposition. Oxalate decomposition is insignificant if acidified solutions are kept away from light and analyzed by IC within 12 hours after acidification.

• Oxalate in dilute acid solution without the presence of Mn, Fe, or other metals is stable indefinitely.

• An analytical round robin between the SRTC Mobile Laboratory and the ADS IC Laboratory showed that both labs produced comparable results. Blind standards were analyzed accurately by both labs as part of the QC program.

• Follow the procedure in this report to analyze subsequent Sludge Batch 3 development samples and actual Tank 7 waste.

• Prepare and analyze a sodium oxalate solution of known concentration by IC to serve as a blind standard.

• Prepare, acidify, and analyze a matrix standard consisting of a known amount of sodium oxalate added to a Sludge Batch 3 simulant.

Sludge Batch 3 macrobatch feed to the DWPF will be composed primarily of Tank 7 sludge. The concentration of oxalate in Tank 7 is relatively high, estimated at about 0.13 g of sodium oxalate/g of slurry ¹. It is expected that sludge washing will reduce the amount of oxalate in the sludge, but significant quantities of oxalate will still be processed in the DWPF. Accurate oxalate determinations in sludge are required for at least three reasons:

1. The fate of oxalate is being studied in SRAT tests with Sludge Batch 3 simulant.

Oxalate measurements are needed for material balances at various stages of the SRAT cycle to elucidate the chemical behavior with this sludge feed.

2. Sludge washing of actual Tank 7 sludge will require accurate oxalate values to measure the degree of oxalate removal and to obtain material balances of the wash water and residual oxalate concentrations in the sludge.
3. Oxalate is an organic molecule that will effect the safety margin for lower explosive limits. Also, the oxalate will have some effect on the redox potential of the DWPF melter. The effect of oxalate on both lower explosive limits and redox is much less than the coal in Tank 7 on a weight basis. However, the amount of oxalate even after washing to remove 50% of the oxalate is expected to be about 50 times that of coal on weight basis, so its effect on DWPF processing cannot be ignored.

Most experiments to measure oxalate in sludge consisted of spiking in about 0.13g of sodium oxalate to 1 g of Sludge Batch 3 simulant (Tank 8 sludge obtained from C.C. Herman of Immobilization Technology Section). To determine the recoveries of oxalate when calcium was present, about 0.065 g of Ca(OH)₂ was added. For some experiments, the calcium hydroxide was first dissolved by acid adjustment to pH 2. This solution was then added to a separate solution of sodium oxalate. The immediate formation of the white precipitate confirmed that water-insoluble calcium oxalate was present in the sludge before the acid addition.

The most commonly used acidification procedure was to weigh out 1 g of slurry in a plastic bottle and then add 2 ml of concentrated HCl to the slurry. This mixture was swirled for 2-3 minutes to dissolve the iron complexes in the sludge. Then 2 ml of concentrated HNO₃ was added and this mixture swirled for an additional 2-3 minutes. This mixture was then completely transferred to a 250-ml plastic volumetric flask and diluted to the mark with de-ionized water. For experiments that used only HCl or HNO₃, 4 ml of concentrated acid was used. Detailed instructions for analyzing an unknown sludge sample for oxalate are provided at the end of this report.

Solutions containing oxalate were analyzed using a standard anion chromatography method (ADS-2306). Instrumental conditions are listed below:

DX-120 Ion Chromatograph, Conductivity Mode
Column(s): AG-14 guard column (4 x 50 mm), AS-14 analytical column (4 x 250 mm)
Flow Rate: isocratic, 1 mL/min
Run Time: 18 min.
Auto Suppression Mode: ASRS II
Eluent: 0.75 mM carbonate/2.63 mM bicarbonate
Eluent pH: 9 -10
Diluent: 0.5 mM carbonate/1.75 mM bicarbonate
Sample Injection Loop: 50 L
Column Pressure: 1600 psi
Calibration Points: 10 g/mL, 25 g/mL, 50 g/mL, Relative Standard Deviation (RSD) < 7%, Linear Curve Fit
Method Detection Limit: oxalate = 1.0 g/mL
Method Uncertainty: +10%
Reagent Water: 18 MOhm Resistivity

The acid treatment is required to ensure that any calcium oxalate present in the sludge is dissolved. A large percentage of the oxalate in Tank 7 is expected to be in the form of sodium oxalate that has a maximum solubility of 3.7 g/ 100 g of water. A simple water dilution would be sufficient for measuring the oxalate if it were all sodium oxalate. However, calcium oxalate is soluble at a concentration of only 0.0007 g/ 100 g of water. The acid treatment was pursued as a conservative pre-treatment step to dissolve the calcium oxalate and other water-insoluble oxalates. Tests were performed with the highest expected concentration of sodium oxalate (the unwashed scenario) and with calcium hydroxide spiked in at 50% of the weight of sodium oxalate. This concentration of calcium is much higher than actually expected in Tank 7 sludge, but this strategy also provides a conservative test of the acidification method.

Initial tests were performed with no sludge present to visually determine the acid conditions needed to dissolve calcium oxalate. Calcium oxalate was formed in situ by mixing calcium hydroxide and sodium oxalate at pH 2. A white precipitate of insoluble calcium oxalate formed. Dropwise addition of concentrated HNO₃ completely dissolved the calcium oxalate at about pH 1.

Table 1 shows the results of a series of tests to measure the recovery of oxalate after spiking sodium oxalate and calcium hydroxide into a Sludge Batch 3 simulant. Several acid combinations provide 90-100% recoveries of oxalate added to the sludge. The acid combination of 2 ml HCl followed by 2 ml HNO3 was found to dissolve 1 ml of sludge. For this reason, this method is recommended in this report for oxalate determinations of Tank 7 sludge (and other high-level sludge samples).

Complete digestion of the sludge is not required as shown by the good recoveries of oxalate after a large initial dilution of the sludge followed by acidification. Comparing analyses (Table 2) of SRAT simulant samples after acid addition to both the concentrated sludge and the dilute sludge confirmed that it is not necessary to completely dissolve the sludge to obtain accurate oxalate values.

One of the experimental goals was to perform the acid treatment on a number of sludge simulants and split these solutions for analysis by both the ADS IC Lab and the SRTC Mobile Lab. The main goal of this round robin was to confirm that both labs were in control and to help separate sample preparation errors from instrumental errors. A series of solutions consisting of standards and actual samples produced by the SRAT cycle tests with Sludge Batch 3 were analyzed by both labs. Table 3 compares the results of this round robin. In general, the agreement between the labs was excellent. Both labs accurately measured blind standards prepared from sodium oxalate and matrix-matched blind standards prepared by spiking sodium oxalate into Sludge Batch 3 simulant.

D.R. Best of Immobilization Technology Section observed that oxalate concentrations decreased with time as he performed the analyses required to support the SRAT cycle tests with Sludge Batch 3 simulant. Therefore, another goal of the analytical round robin was to measure the rate of oxalate decomposition. Four acidified solutions were analyzed by both the ADS IC lab and the Mobile Lab (Table 4). Both labs agreed well on analyses performed a few hours after the acidification step. The solutions were then set aside and re-analyzed after 72 hours. The ADS IC Lab measured nearly the same oxalate concentrations as on the first day of the tests. In contrast, the Mobile Lab measured oxalate values that were about 50% of the initial value. The solutions were then exchanged and re-analyzed to rule out an instrumental problem.

This experiment confirmed that oxalate decomposition had occurred only with the solutions that were initially sent to the Mobile Lab. Further discussions with R.J. Ray and D.R. Best identified the only difference in the way the samples were treated: the Mobile Lab left their solutions on the bench top exposed to lab light while the ADS IC Lab stored their solutions in a cabinet away from lab light.

A literature search and further experiments conducted by D.R. Click showed that photo-decomposition of the Fe-oxalato complex is a plausible mechanism for oxalate concentrations decreasing with time ². The Fe-oxalato complex decomposes to yield formic acid and CO₂. An experiment was performed by splitting a solution and then measuring the oxalate after leaving part of the solution in the dark overnight and exposing the other part of the solution to a UV light source overnight. The solution exposed to UV light lost about 20% of the initial oxalate whereas the solution kept in the dark suffered no measurable oxalate decomposition.

To minimize the problem of oxalate decomposition, we offer two low-tech recommendations: (1) immediately protect acidified solutions from light; and, (2) analyze the solutions as soon as possible (preferably 12 hours or less) after acidification.

Submission of analytical blind standards is always a good practice. Creation of blind standards for oxalate determinations is straightforward and analysis of these standards should be part of the QC program for high-level sludge samples. Sodium oxalate solutions should be analyzed to assess the accuracy of the IC calibration. A more rigorous way to assess the accuracy of the entire method is to spike sodium oxalate into sludge simulants and perform the entire acidification, dilution, and IC analysis. We recommend use of both standards. Procedures for preparing these standards are included in this report.

A simple room temperature acidification of sludge samples yields solutions that can be analyzed accurately for oxalate even when significant concentrations of Ca are present. The acidified solutions are unstable and must be protected from light and analyzed within a few hours after the acidification. The QC program for measuring oxalate in sludge should include both sodium oxalate and matrix-matched standards.

Table 1
Recovery of Oxalate Spiked into Sludge Batch 3

Acids Used	Ca(OH)2 Added (g)	Na2C2O4 Added (g)	Expected Oxalate (mg/L)	Measured Oxalate (mg/L)	% Recovery
HCl-HNO3	0	0.073	192	174	91
HCl-HNO3	0	0.077	203	182	90
HCl-HNO3	0.068	0.120	315	309	98
HCl-HNO3	0.065	0.131	344	321	93
HCl-HNO3	0.065	0.127	334	327	98
HCl-HNO3	0.066	0.125	328	309	94
HNO3	0.073	0.125	328	323	99
HNO3	0.062	0.126	331	317	96
HNO3	0.057	0.099	260	246	94
HCl	0.069	0.118	310	282	91
HCl	0.052	0.132	347	359	103
HCl	0.063	0.121	318	293	92

Table Notes: The Ca(OH)2 and Na2C2O4 were added to about 1 gram of sludge slurry. After acidification, the solution was diluted to 250 ml. The HCl-HNO3 acidifications were done by adding acid directly to 1 gram of slurry plus the spikes. This treatment appeared to essentially dissolve the sludge so no filtration was required. The HNO3 only acidifications were done by diluting the sludge slurry plus spikes to about 100 ml with de-ionized water, then adding 4 ml of HNO3. The solution was then diluted to 250 and filtered to remove any un-dissolved sludge particles. The HCl only acidifications were done by diluting the sludge slurry plus spikes to about 100 ml with de-ionized water, then adding 4 ml of HCl. The solution was then diluted to 250 and filtered to remove any un-dissolved sludge particles.

Table 2
Comparison of Oxalate Determinations in Sludge Batch 3 Simulant after
Acid Addition to Concentrated As-Received Sludge versus Diluted Sludge

Sample I.D	Oxalate Concen. (ug/g of slurry) Acid Addition to concen. sludge	Oxalate Concen. (ug/g of slurry) Acid Addition to diluted sludge	% Difference in oxalate concen.
Herman SB-7	28,500	29,800	4
Herman SB-10	23,330	22,200	5
Herman SB-13	21,800	18,950	13
Herman SB-17	21,730	22,250	2

Table Notes: Samples of Sludge Batch 3 simulants containing oxalate were submitted by C.C. Herman of ITS. A 3 ml HNO3-1ml HCl acid mixture used to dissolve the sludge by adding the acid to the concentrated or undiluted sludge. 4 ml of HNO3 was used to treat the sludge after 1 g of sludge was diluted to 100 ml with de-ionized water

Table 3
Round Robin Results For Oxalate Determinations:
Comparison of ADS IC Lab and Mobile Lab on the Same Acidified Solutions

Table 4
Time Dependency of Oxalate Measurements in Sludge Batch 3 Simulant

Sample I.D	Initial Oxalate Concentration (mg/oxalate per Kg sludge)	Oxalate Concentration (mg/oxalate per Kg sludge) after 72 hours in the dark	Oxalate Concentration (mg/oxalate per Kg sludge) after 72 hours exposed to lab light
13-A	25,119	27,357	12,800
16 A	26,393	27,350	14,500
SB3-5	32,282	34,058	18,800
SB3-14	27,614	25,105	12,700

1. Perform the balance calibration check and record the data in the balance user log.
2. Place the sample on a magnetic stirrer, add a stir bar and start the stirring at a vigorous rate.
3. Tare a plastic bottle (60-125 ml is recommended) without the cap on the balance pan.
4. Using a slurry pipette transfer 1.0 ± 0.1g of the slurry to the plastic bottle. Record the sample weight in mg in the lab notebook.
5. In a fume hood, add 2 ml of concentrated HCl to the bottle.
6. Cap the bottle.
7. Swirl the bottle for about 2 minutes until the bulk of the sludge solids have dissolved.
8. Uncap the bottle and add 2 ml concentrated HNO3.
9. Cap the bottle.
10. Swirl the bottle for 1 minute.
11. Immediately uncap the bottle and add about 50 ml de-ionized water to the bottle.
12. Transfer the solution to a 250-ml volumetric flask.
13. Rinse the dissolution bottle with several 10-20 ml portions of de-ionized water and transfer these rinses to the volumetric flask.
14. Add de-ionized water to the mark on the volumetric flask.
15. Cap the flask and mix by inverting several times.
16. Transfer the solution in the volumetric flask to a labeled 250-ml bottle.
17. Label the bottle with the sample weight and final dilution to enable the IC lab to report the oxalate values in m g/g of sample.

Important: The oxalate will decompose fairly rapidly when in contact with acid and some transition metals and also exposed to light. To minimize the oxalate decomposition, immediately protect the solution from lab light until ready for analysis. The IC determinations should be performed within 12 hours to further minimize errors brought on by oxalate decomposition.

1. Perform the balance calibration check and record the data in the balance user log.
2. Tare a wide mouth plastic bottle or beaker on the balance.
3. Weigh out 190 mg (0.190 g) of ACS grade sodium oxalate into the bottle. Record the exact weight in mg in the lab notebook.
4. Add about 50 ml de-ionized water, add a stir bar, and stir gently on the magnetic stirrer until all the white sodium oxalate particles dissolve.
5. Remove the stir bar with a clean magnetic stir bar retriever, rinse off the stir bar and

retriever with a squirt of de-ionized water.

6. Transfer the solution in the bottle to a 250-ml volumetric flask.
7. Rinse the bottle twice and transfer the rinses to the volumetric flask.
8. Cap the flask and mix by inverting several times.
9. Pour this solution into a 250-ml plastic bottle.
10. Calculate the exact concentration of oxalate from the weight of sodium oxalate weighed out in Step 3.

mg/L oxalate= mg sodium oxalate x 88 mg oxalate/134 mg sodium oxalate
------------------------------------------------------------------------
0.25 L

11. Record the calculated concentration in mg/L of oxalate ion on the plastic bottle.

1. Perform the balance calibration check and record the data in the balance user log.
2. Tare a wide mouth plastic bottle or beaker on the balance.
3. Weigh out 190 mg (0.190 g) of ACS grade sodium oxalate into the bottle. Record the exact weight in mg in the lab notebook.
4. Pipette 1 ml of a well-mixed Sludge Batch 3 simulated sludge into the plastic bottle. It is not necessary to record the weight.
5. Swirl the bottle to mix the sodium oxalate with the sludge slurry.
6. Add 2 ml concentrated HCl and swirl the bottle for about 2 minutes.
7. Add 2 ml concentrated HNO₃ and swirl the bottle for about 2 minutes.
8. Add about 50 ml of de-ionized water to the bottle.
9. Transfer this solution to a clean 250-ml volumetric flask.
10. Rinse the bottle several times with de-ionized water and pour the rinses into the 250- ml volumetric flask.
11. Cap the flask and mix by inverting several times.
12. Pour this solution into a 250-ml plastic bottle.
13. Calculate the exact concentration of oxalate from the weight of sodium oxalate weighed out in Step 3.

mg/L oxalate= mg sodium oxalate x 88 mg oxalate/134 mg sodium oxalate
-----------------------------------------------------------------------
0.25 L

14. Record the concentration of oxalate ion in mg/L on the plastic bottle.

Important: The oxalate will decompose fairly rapidly when in contact with acid and some transition metals and also exposed to light. To minimize the oxalate decomposition, immediately protect the solution from lab light until ready for analysis. The IC determinations should be performed within 12 hours to further minimize errors brought on by oxalate decomposition.

1. D.C. Koopman to C.J. Coleman, personal communication, 6/13/2002.
2. Yuegang Zuo and Jurg Holgne, "Formation of Hydrogen Peroxide and Depletion of Oxalic Acid in Atmospheric Water by Photolysis of Iron (III)-Oxalato Complexes", Environmental Science Technology, 1992, vol. 26, pp. 1014-1022