The Bacau (Romania) phosphogypsum stacks as a source of radioactive threat: a case study

For a detailed characterization of the 5.7 10⁶ mt phosphogypsum (PG) stack in the vicinity of Bacau city, Romania, the air dose rate (ADR) was measured in 72 points covering the stack surface, while 10 samples of stack material were collected for future analysis. Radiometric determinations showed for the ADR values varying between 364 ± 53 and 489 ± 8 nSv/h, with some extreme values of 2775 ± 734 nSv/h, significantly exceeding 90 nSv/h, the average value reported for the Romanian territory. High-resolution gamma-ray spectroscopy (HRGS), performed on 10 samples collected from the entire PG stack evidenced only the presence of ²²⁶Ra as the major radioactive element with a specific activity varied between 820 ± 150 and 5278 ± 264 Bq/kg for hot spots. Further analysis performed on a similar number of samples by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX), evidenced, beside gypsum as the main component, traces of brushite (CaHPO₄·2H₂O) and ardealite (Ca₂(PO₃OH)(SO₄)·4H₂O), as well as the presence of small acicular celestine (SrSO₄) agglomerates. XRF determinations of the mass fractions of major elements evidenced values such as SiO₂ (2.31 ± 0.73 %), TiO₂ (0.07 ± 0.01 %), Al₂O₃ (0.17 ± 0.04 %), Fe₂O₃ (0.87 ± 0.18 %), MnO (0.01 ± 0.01 %), MgO (0.17 ± 0.02 %), CaO (32.5 ± 0.82 %), Na₂O (0.04 ± 0.04 %), K₂O (0.05 ± 0.01 %), P₂O₅ (2.12 ± 0.51 %), LOI (20.2 ± 0.3 %), i.e. closer to literature reported data for PG of different provenience while the data concerning the distribution of 20 trace elements, including incompatible Sc, La, Ce, and Th were relatively closer to the upper continental crust (UCC).