RESEARCH ARTICLE

 

Radioactive nuclides in phosphogypsum from the lowveld region of South Africa

 

 

Xolani Msila^I; Frans Labuschagne^I; Werner Barnard^II; David G. Billing^III

^ISasol Dyno Nobel, Bronkhorstspruit, South Africa
^IISasol Technology Research and Development, Sasolburg, South Africa
^IIISchool of Chemistry, University of the Witwatersrand, Johannesburg, South Africa

Correspondence

 

 

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ABSTRACT

We evaluated the suitability of phosphogypsum from the Lowveld region of South Africa (LSA), for the manufacturing of building materials, with reference to (1) the National Nuclear Regulator Act 47 of 1999 and (2) the radioactivity associated risks as quantified in terms of the external and internal hazard indices, the activity concentration index and the radium equivalent. The distribution of radioactive nuclides in the LSA phosphogypsum was also examined. Analyses of 19 samples of the phosphogypsum show that phosphogypsum contains lower activity concentrations of naturally occurring radioactive nuclides of uranium and thorium and their progeny than the 500 Bg/kg limit set for regulation in South Africa. The potassium-40 (⁴⁰K) activity concentration was below the minimum detectable amount of 100 Bq/kg. The values obtained for external and internal hazard indices and the activity concentration index were: 2.12 + 0.59, 3.44 + 0.64 and 2.65 + 0.76 respectively. The calculated radium equivalent Ra_eq was 513 + 76Bq/kg. The final decision regarding phosphogypsum's suitability for use as a building material should consider scenarios of use.

Keywords: apatite; radium equivalent; radiation hazard indices; Anderson-Darling; secular equilibrium

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Introduction

A phosphoric acid production facility has been in operation in the Lowveld region of South Africa (LSA) since the 1960s. The process of producing phosphoric acid involves the digestion of fluoro-apatite ore (Ca₁₀(PO₄)₆F₂) with sulfhuric acid as shown in Equation 1:

Ca₁₀(PO₄)₆F₂ + 10H₂SO₄ + 20H₂O → 10CaSO₄.2H₂O + 6H₃PO₄ + 2HF Equation 1

Each year, tons of calcium sulfate dihydrate (CaSO₄.2H₂O) or gypsum, or specifically phosphogypsum, is produced as a by-product. The LSA phosphogypsum is stored in waste stacks alongside the phosphoric acid factory. In a country like South Africa, that has a challenge to provide low cost or affordable housing,¹ the use of phosphogypsum to manufacture building material is an attractive option. The manufacturing of low cost prefabricated building material from gypsum has in the recent past been demonstrated by Rajkovic and Toskovic². But the solubility of uranium, thorium and their daughter products that exist in apatite ores^3,4 result in these radioactive nuclides partitioning between the phosphoric acid and the phosphogypsum^5,6 during the processing of the ore. Rajkovic and Toskovic², Al-Jundi et al.⁶ and Hussein⁷ studied phosphogypsum from Serbia, Jordan and Egypt respectively. Hussein did not report on the activity concentrations of potassium-40 (⁴⁰K), thorium-232 (²³²Th) and radium-226 (²²⁶Ra) but the results of radioactive nuclides activities from Serbia and Jordan are given in Table 1.

 

 

The worldwide average activities of ²²⁶Ra, ²³²Th and ⁴⁰K on the earth's crust are about: 40; 40 and 400 Bq/kg, respectively.⁸ Humans are therefore exposed to this naturally occurring radioactive material (NORM). If the material used for the construction of human dwellings contributes additional radioactivity to the naturally occurring radioactive material, radiation exposure increases. The presence of these radioactive nuclides impurities in the LSA phosphogypsum can limit its use as a building material.

In this study we evaluated the suitability of the LSA phosphogypsum for the manufacturing of building materials, with reference to (1) the National Nuclear Regulator Act 47 of 1999⁹ and (2) the radioactivity associated risks as quantified in terms of the external (H_ex) and internal (H_i) hazard indices, the activity concentration index (I_y) and the radium equivalent (Ra_eq). The effect of ore processing on the distribution of the radioactive nuclides was also examined.

 

Materials and method

During the production of phosphoric acid for analysis, 19 grab samples of LSA phosphogypsum were taken.

Sample preparation

The phosphogypsum was dried overnight in an oven at 80 °C. About 100 g of the dried phosphogypsum was transferred to a grinding vessel (containing stainless steel grinding balls and block) with a swing mill. Milling was performed at 960 rpm. for one min to obtain a fine powder. The process was repeated until 0.5 kg of homogeneous fine powder was generated.

Gamma analysis

A standard glass container was filled with phosphogypsum and the mass recorded. The container was closed with a lid and sealed airtight with the aid of epoxy resin. The prepared sample was allowed to stand for 3 weeks so that radon-222 (and some of its progeny) reached radioactive equilibrium with radium-226. The γ-spectrum was acquired on a high purity germanium detector calibrated for the standard glass container geometry. The detector was housed in a lead shield to reduce background radiation. Gamma spectrum analysis was performed with Genie™ 2000 Model S501 Gamma Analysis software. The weighted average activity of bismuth-214 and lead-214 was calculated and reported for radium. Thorium-228 was calculated from lead-212 and thalium-208. Radium-228 was determined from actinium-228. Potassium-40 was measured directly.

Uranium and thorium analysis by activation analysis

Aliquots of the powder were transferred into irradiation capsules. Uranium and thorium standards were prepared by transferring known amounts of these elements from certified reference solutions into capsules and evaporated to dryness. Samples and standards were sequentially transferred with a pneumatic system into an in-core position of the SAFARI-1 reactor and irradiated with neutrons for a fixed time. After irradiation, the sample was transferred to a detector to measure neutrons emitted by products formed from fission of uranium-235.

The emission of neutrons between samples and standards was compared to calculate the uranium concentration in the sample. After a prolonged period, the irradiated samples and standard were measured on a gamma detector to determine neptunium-239 and protactinium-231. These nuclides formed from the neutron activation of uranium-238 and thorium-232 respectively. By comparing the activities of the nuclides in the standards and samples, the uranium-238 and thorium-232 sample activities were calculated.

Radiation indices

From the activity concentrations of the radioactive nuclides, H_ex, H_i, I_y and Ra_eq were calculated using the formulae in Equations 2-5 respectively. The formulae and their applications are comprehensively defined by the European Commission for Radiation Protection⁸.

where C_Ra, C_Th and C_K are activities of ²²⁶Ra, ²³²Th and ⁴⁰K in Bq/kg, respectively.^10,11

 

Results and discussion

Radionuclide concentrations in phosphogypsum

The average activity concentrations of the radioactive nuclides: uranium-23 8 (²³⁸U), lead-210 (²¹⁰Pb), uranium-235 (²³⁵U), radium-228 (²²⁸Ra), thorium-2 38 (²³⁸Th), ²²⁶Ra, ²³²Th and ⁴⁰K in the LSA phosphogypsum are presented in Table 2. According to the National Nuclear Regulator Act, 47 of 1999,⁹ an operation or material is excluded from regulation if activity concentration of the naturally occurring radioactive nuclides of uranium, thorium and their progeny are each below 500 Bq/kg. The limit for ⁴⁰K is 10 000 Bq/kg.

The results confirm that the activity concentrations measured in the 19 LSA phosphogypsum samples are all below the regulation limits set by the National Nuclear Regulator Act 47 of 1999⁹for ²³⁸U, ²³⁵U, ²¹⁰Pb, ²²⁸Ra, ²²⁶Ra, ²²⁸Th, and ²³²Th. The activity concentration of ⁴⁰K in the LSA phosphogypsum was determined to be below the minimum detectable limit of 100 Bq/kg. Numerous studies have revealed that building materials contain appreciable activity concentrations of radioactive nuclides ²²⁶Ra, ²³²Th and ⁴⁰K. There are limited data available on the radiological safety of phosphogypsum use as a building material beyond the study by Rajkovic and Toskovic² and therefore to put the results obtained for LSA phosphogypsum into perspective, some of the results obtained for bricks from several countries are presented in Table 3.

 

 

The average activity concentration of ²²⁶Ra in the bricks listed in Table 3 range from 18 Bq/kg to 104 Bq/kg. This is lower than the average activity concentration in the LSA phosphogypsum (109 Bq/kg ± 18 Bq/kg). Similarly, the average activity concentration of ²³²Th in the bricks ranges from 14 Bq/kg to 89 Bq/kg which is lower than the average in the LSA phosphogypsum, where ²³²Th is 253 Bq/kg ± 160 Bq/kg. The activity concentration of ⁴⁰K (<100 Bq/kg) is lower in the LSA phosphogypsum compared to the range of activity concentrations reported for the bricks from other countries (204-1058 Bq/kg).

Radiation hazard indices

The LSA phosphogypsum is rich in ²²⁸Ra compared to ²³²Th and as a result the activity concentration of the former is used in the place of the activity of the latter to calculate the hazard indices in Equations 2-4. The calculated hazard indices H_ex, H_in, I_y and the Ra are plotted in Figure 1.

 

 

The error bars in Figure 1 represent the standard deviation of the results of the 19 LSA phosphogypsum samples. The values obtained for H_ex, H_in and I are 2.12 ± 0.59, 3.44 ± 0.64 and 2.65 ± 0.76 respectively. The calculated Ra_eq is 513 ± 76 Bq/kg.

The assessment of a material's suitability for use as a building material should be based on scenarios where the material is used. The scenarios for use at different dose criteria⁸ are given in Table 4. The activity concentration index,I_y, should be evaluated against these criteria. If the I_v is 1 or less, the material can be used as building material, without restriction, as far as radioactivity is concerned, whereas if the I_v is above 1 and less or equal to 6, the material should be used superficially.

 

 

The Ra_eq as defined in Equation 5, is calculated from the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K. Equation 2 is based on the estimation that 1 Bq/kg of ²²⁶Ra, 0.7 Bq/kg of ²³²Th and 13 Bq/kg of ⁴⁰K generate the same γ-rays dose rate.^10,21,22 According to Mondal et al.²³ and supported by El-Taher and Makluf²⁴, I_v=1 is equivalent to the Ra_eq of 370 Bq/kq. The calculated Ra_eq of 513 + 76 Bq/kg for LSA phosphogypsum is consistent with the activity concentration (1<I_y<6).

Radioactive nuclides distribution in LSA phosphogypsum

A simplified definition of secular equilibrium is found in Zhang et al.²⁵ Activity of daughter radionuclides build up to that of the parent in about seven half-lives and thereafter, parent activity (A₁) is the same as the activity of its progeny (A₂). It follows therefore that at the state of secular equilibrium, the ratio of the activity of the parent to that of the daughter is 1 (^A/A₂= 1). The secular equilibrium observed in natural ores can be disturbed by mineral processing. Al-Jundi et al.⁷ showed that concentration of ²³⁸U and its decay products ²¹⁰Pb and ²²⁶Ra originating from apatite ore are partitioned during processing in such a way that ²³⁸U accumulates in the phosphate fertiliser while ²²⁸Ra, ²²⁶Ra and ²¹⁰Pb accumulate in the phosphogypsum. This partitioning behaviour can be attributed to the very low aqueous solubility of radium sulfate and lead sulfate relative to that of uranium sulfate as almost all the ²²⁸Ra and ²²⁶Ra reported by Van der Westhuizen²⁶ as present in the phosphate rock is observed in the LSA phosphogypsum.

The ratios of activity concentrations in the LSA phosphogypsum for ²³⁸U, ²³⁵U, ²³²Th and their progeny in the natural decay series are plotted Figure 2. The ²²⁶Ra and ²¹⁰Pb belong to the ²³⁸U natural decay series and are not in secular equilibrium in the LSA phosphogypsum. Although the linear trend of the ratio of these two radionuclides is close to unity, there is a poor correlation between their activities as shown by the regression coefficient (R²) of 0.133. The uranium radioactive nuclide ²³⁸U is also in disequilibrium with its progeny ²²⁶Ra. Similarly, ²³⁵U is in disequilibrium with ²¹⁰Pb and disequilibrium is also observed between ²³²Th versus ²²⁸Ra and ²²⁸Ra versus ²²⁸Th.

 

 

The Anderson-Darling test has been shown to give appropriate empirical distribution function statistics for detecting departure from normality, even with small samples (n<25).²⁷ The test was performed on a set of data to evaluate their fit to a chosen distribution pattern. For the Gaussian distribution test, the statistics are based on the squared difference between the normal and the empirical data. If the calculated p-value is less than a chosen alpha (one minus the confidence interval), the null hypothesis, that the data come from that distribution, is rejected. The Anderson-Darling (AD) test was performed on the radioactive nuclide activity concentration data using the Minitab® 16 statistical software to evaluate the distribution pattern of the activity concentration in the LSA phosphogypsum. The results obtained are presented in the probability plot in Figure 3.

At 95% confidence level, a p value of less than 0.05 indicates no deviation from the Gaussian distribution. This behaviour (normal distribution) is observed for the activity concentration of the radioactive nuclides ²³⁸U ²³⁵U, ²²⁸Th and ²³²Th in the LSA phosphogypsum. The contrary is observed for the ²²⁸Ra and ²²⁶Ra activity concentrations. The calculated p values are 0.318 and 0.434 respectively and the Anderson-Darling values are 0.406 and 0.350 respectively. This can be attributed to the low solubility of radium sulfate which results in accumulation in the phosphogypsum rather than the phosphoric acid.

 

Conclusion and recommendation

The LSA phosphogypsum contains appreciable amounts of the radioactive nuclides ²³⁸U; ²¹⁰Pb; ²³⁵U; ²²⁸Ra; ²²⁸Th; ²²⁶Ra; ²³²Th and below-detectable levels of ⁴⁰K. The average activities are below the limits set by the National Nuclear Regulator Act 47 of 1999.⁹The phosphogypsum is therefore excluded from regulatory control. The calculated results of hazard indices: H_ex, H_in, I_Y and Ra_eq indicate that the LSA phosphogypsum can be utilised as building material if used superficially or with restriction. A final decision on the usability of the material can be made when the scenario of use is known and a more representative sample of the aged bulk material is analysed. The radioactive nuclides and their progenies are not in secular equilibrium in the LSA phosphogypsum and the distribution ²³⁸U, ²³⁵U, ²³²Th and ²²⁸Th deviates from Gaussian.

Authors' contributions

D.G.B. and W.B. were the project leaders, F.L. was responsible for sampling and arranging for analysis. X.M. handled the arrangement for sample analysis, performed the calculations and wrote the manuscript.

 

Acknowledgements

Many thanks to Deon Kotze (NECSA) for performing some of the radioactive nuclides analyses.

 

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Correspondence:
Xolani Msila
Sasol Dyno Nobel, 486 Brandbach Road, Ekandustria, Bronkhorstspruit 1021, South Africa
xolani.msila@sasol.com

Received: 11 Mar. 2015
Revised: 13 July 2015
Accepted: 01 Aug. 2015