2.7.2.1 - NORM in Phosphogypsum

Introduction

Phosphogypsum is a by-product from the wet process production of phosphoric acid. It represents one of the most serious problems for the phosphate fertilizer industry. The material contains relatively high levels of natural radionuclides that are typically derived from the Uranium decay series. Its use as building materials and in agriculture is therefore restriced and needs to be monitored.

The combined procedure as described below, is for the sequential determination of 210Pb, 210Po, 226Ra, Th- and U-isotopes in phosphogypsum. The procedure is adapted from IAEA’s Environmental Programme Activities and described in more detail in [IAEA 2010, 2012]. The part on Radium separation and analysis is derived and makes use of Radium RAD Disks. Phosphogypsum reference material is available by IAEA as IAEA-434.

The method is based on the dissolution of small samples by microwave digestion, followed by sequential separation with extraction chromatography using SPEC, TEVA and UTEVA resins. 226Ra is isolated by Radium Rad Disk and measured like 210Pb by LS counting. The α-sources of Th and U are prepared by electrodeposition on stainless steel platelets, while 210Po is auto-deposited on a silver plate. They are measured by isotope dilution α-particle spectrometry. The flow chart of the combined procedure is given in figure 41.

The method should give an example for a comprehensive dissolution procedure, which could be used as well for similar non aqueous sample materials. Moreover, it demonstrates the fruitful combination of Liquid Scintillation with other counting techniques.

Materials and Equipment

  • Microwave oven

  • Vacuum pump

  • Auto-deposition cell

  • Electrodeposition apparatus (Pt wire anode) equipped with DC power supply (up to 1A) (see 2.3.13. and fig. 34)

  • Hot plate with magnetic stirrer

  • Filtering set with filter paper (e.g. Whatman 41)

  • Chromatography column with stopper, control valve, funnel and holder

  • Teflon beakers

  • Centrifuge and tubes

 

Chemical Reagents:

  • 30 mg/mL Pb2+ solution (Pb(NO3)2

  • Tracer: 209Po, 232U, 226Ra or 225Ra

  • HNO3 (65 %)

  • HF (40 %)

  • H3BO3

  • HCl (32 %)

  • SPEC, (Ø 10 mm, L 100 mm) TEVA and UTEVA (Ø 8 mm, L28 mm each) (EICHROM)

  • Ascorbic acid

  • NH4SCN

  • EDTA

  • NH4OH

  • (NH4)2SO4 (10 %)

  • Oxalic acid

  • Ethanol

  • Gelating cocktail (OptiPhase HiSafe III)

 

Procedure

(see [IAEA 2010] and [IAEA 2012] for Pb-, Po-, U- and Th-analysis)

(a) Sample Dissolution

  1. Weigh 0.5 g of phosphogypsum into a microwave container and add 15 mL conc. HNO3 (65 %)

  2. Add 30 mg Pb2+ and about 0.05 (232U, 225Ra) to 0.5 Bq (209Po) tracer

  3. Digest at 120 °C for 15 min and at 150 °C for 25 min at 1000 W

  4. Centrifuge and transfer the supernatant into a Teflon beaker; the residue is digested again in the microwave container with 3 mL of conc. HNO3 and 2 mL of 40 % HF.

  5. Combine the solutions, add 0.1 g H3BO3 and evaporate repeatedly to dryness with three portions of conc. HNO3 (removal of HF)

  6. Dissolve the residue in 30 mL of 2 M HCl

(b) Polonium and Lead Separation

  1. Precondition the Sr resin column with 2 M HCl

  2. Load the sample solution onto the resin (flow rate 5-6 drops/min) and rinse the beaker

  3. Wash the column with 100 mL of 2 M HCl followed by 25 mL 6 M HNO3 (removal of non-retained ions)

  4. Combine effluent and washing solution (analysis of U and Th)

  5. Strip Po with 60 mL 6 M HNO3

  6. Elute Pb with 60 mL 6 M HCl into a centrifuge tube

(c) Thorium Separation

  1. Precondition the TEVA column with 3 M HNO3

  2. Evaporate the combined effluents from Sr resin and dissolve the residue in 20 mL 3 M HNO3

  3. Load the sample on the TEVA column and wash it twice with 10 mL 3 M HNO3 (effluents for U analysis)

  4. Elute Th with 20 mL 8 M HCl

(d) Uranium Separation

  1. Precondition the UTEVA column with 3 M HNO3

  2. Load the effluent from TEVA column onto UTEVA and wash it twice with 20 mL 3 M HNO3 (effluents for Ra analysis)

(e) Radium Separation and Analysis

  1. Precondition the Radium RAD Disk filter with 2 M HNO3

  2. Dilute the effluents from UTEVA column with distilled water to 2 M with respect to HNO3

  3. Extract the Ra containing sample through the filter (< 50 mL/min) and wash according to procedure 2.2.1.5

  4. Elute Radium dropwise twice with 5 mL 2 M alkaline EDTA, add 16 mL OptiPhase HiSafe III cocktail and measure by LSC (α-channel: 226Ra; β-channel: 228Ra) according to 2.2.1.5.

Alternatively, Radium isotopes might be measured by α-spectrometry after BaSO4 micro co-precipitation [IAEA 2012]

(f) Polonium-210 Source Preparation

  1. Evaporate the Po solution carefully to dryness (< 150 °C)

  2. Dissolve the residue in 10 mL 0.5 M HCl and add 0.05 g ascorbic acid (reduction of Fe3+)

  3. Place a cleaned Ag disc into a deposition cell (Teflon) (fig. 40)

  4. Transfer the solution to the deposition cell, rinse the beaker with 2 x 2 mL 0.5 M HCl and adjust the pH of the solution to pH 2 with 6 M NaOH for 2 to 3 hours

  5. Deposit Po at 85 to 90 °C with mechanical stirring for 2 to 3 hours

  6. Remove the Ag disk, wash with acetone, dry it at room temperature and measure by a-spectrometry

 

(g) Uranium and Thorium Source Preparation

  1. Evaporate the purified U or Th solution three times to dryness with a few mL conc. HNO3 (distraction of organic matter)

  2. Dissolve the residue in 10 mL 10 % (NH4)2SO4 at pH 1.5 – 2

  3. Transfer the solution into an electrodeposition cell containing a clean (absolute ethanol) polished stainless steel disc

  4. Electrodeposit at 1 A for 90 minutes

  5. At the end of the deposition time fill the cell with 1.5 M NH4OH; continue deposition for 1 min, discard the solution, and then turn off the voltage (important sequence!)

  6. Disassemble the cell, rinse with dist. water, dry and measure by α-spectrometry

 

(h) Lead-210 Source Preparation

  1. Evaporate the Pb solution from step a), re-evaporate three times with 5 mL conc. HNO3 each time, and dissolve the residue in 10 mL 1 M HNO3

  2. Add 40 mg oxalic acid, and precipitate Pb-oxalate by adjusting to pH 2 - 3 with 25 % NH4OH

  3. Filtrate the cold solution on a filter paper (weigh after drying!); wash three times with 1 mL water and 2 mL 80 % ethanol

  4. Dry (40-50 °C), weigh the precipitate for chemical recovery and transfer precipitate and filter into a LS vial

  5. Add 1 mL 6 M HNO3 to dissolve the precipitate and mix with 15 mL scintillation cocktail for measurement in a 210Pb adjusted low energy β-window

Evaluation

For LS evaluation, 100 % a-counting efficiency for 226Ra can be taken unless α/β-PSD is applied, for 210Pb (Eβmax 17 keV, 80 %; Eβmax 63 keV, 20 %) 60 % or less according to quenching and equipment.

The chemical yield for Ra separation has to be determined using a 226Ra standard solution or using a 133Ba tracer with final γ-measurement. The chemical yield for 210Pb is determined gravimetrically.

Po, U, and Th are determined by isotope dilution analysis with tracers; thus, the counting efficiency calibration is avoided. In case of 232U, a freshly purified solution should be used in order to provide interferences from ingrown daughter nuclides (see chapter Quality Assurance 3.1. and fig. 47)

Be aware that if separation and measurement are not done in timely coherence, the formation of daughter nuclides has to be taken into consideration (210Pb/210Bi, 232U daughters). For validation of the procedure and uncertainty consideration, see [IAEA 2010, 2012].

Figure 41: Analytical scheme for NORM in Phosphogypsum (derived from [IAEA 2012])

IAEA 2010 International Atomic Energy Agency, Reference Material IAEA-434: Naturally Occuring Radionuclides in Phosphogypsum; IAEA Analytical Quality in Nuclear Applications, Series No. 17, IAEA/AQ/17, IAEA, Vienna (2010)

IAEA 2012: A Procedure for the Sequential Determination of Po-210, Pb-210, Ra-226, Thorium and U isotopes in Phosphogypsum by Liquid Scintillation Counting and Alpha Spectrometry; 2012, C.-K. Kim

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2.7.2.2 Rn in Crude Oil and Oil Fractions