Comparative study for measurement of environmental natural radioactivity in the crude phosphoric acid using nuclear and non-nuclear techniques

• The majority of natural radioactivity of CPA is mainly due to γ-emissions of Th-234, Pa-234m and U-235.
• The total Unat concentration was ∼81 and 83 ppm by the γ-emissions of its Th-234 daughter and U-235 isotope, respectively.
• Activity ratio of Th-234/U-235 was (21.0) close to U-238/U-235 ratio (21.7) as in the most ecosystems.
• Total Unat in CPA was 72.3 ppm by UV–vis (as U-arsenazo III complex) and not quantified by ICP-OES.
• γ-Emission of U-235 is the best choice to assess total Unat in fresh or aged CPAs and its environmental impacts.

The present work was attempts to develop rapid and accurate alternative analytical method for measuring naturally occurring radioactivity in the crude phosphoric acid (CPA). The measurements were achieved using nuclear method based on the non-destructive γ-ray measurements; and non-nuclear methods, including the ICP-OES and UV–vis colorimetric method. The results showed that, radionuclides of Th-234, Pa-234m (daughters of U-238) and U-235 were identified and quantified. The activity ratio of Th-234:U-235 was 21.0 ± 0.86. This value is significant because it is close and agrees with the reference naturally occurring activity ratio of 21.7 between U-238 and U-235. Total U in four different brands of the CPA (I–IV) was quantified using γ-emissions of its progeny (Th-234) for the aged samples only, or its natural isotope (U-235) for both fresh and aged samples. It was found that, the total U concentration in different four brands CPA (I–IV) of CPA using γ-emissions of Th-234 was 100.2, 68.7, 67.9 and 87.6 ppm for the brands I, II, III and IV, respectively with small coefficient of variance (CV,%) does not exceed 4%. For the same brands (I–IV) using γ-emissions of U-235 isotope, the total U concentration was 98.3, 75.5, 70.2 and 89.6 ppm, respectively with CV(%) below 5.3%. By ICP-OES method, U determination is negatively interfered with the phosphorous in all brands of CPA(I-IV). Alternative UV–vis spectrophotometry analytical method has been developed to detect the total U concentration using 10% oxalic acid as an efficient masking agent for Fe. By UV–vis method, the total U concentration in CPA brands was 102.3, 58, 56 and 73 ppm for I, II, III and IV brands, respectively with CV (%) less than 3.3%. Finally, it is concluded that the nondestructive γ-ray spectrometry may be the best choice for uranium measurement, especially in crude phosphoric acid (CPA) with relatively high P and Fe concentration.

Activity ratio of Th-234/U-235 in different brands of crude phosphoric acid (I–IV) via U-238/U-235 in ecosystems.Figure optionsDownload as PowerPoint slide