Integrated Summary of Scientific Studies on the Prahovo PG Stack

Next to the town of Prahovo, by the Danube River in eastern Serbia, there is a large phosphogypsum (hereafter “PG”) stack created by the fertilizer and phosphoric acid industry operating in the area for decades. The industrial complex (today associated with the company “Elixir – Prahovo”) produced phosphoric acid and fertilizers from phosphate rock, generating large quantities of PG as a by-product. As in many other countries, this waste was deposited in open stacks close to the production site, creating a long-term environmental legacy and often considered environmental hot spots that require careful scientific assessment before any decisions about reuse, remediation, or long-term management can be made.

In the last years, several scientific studies have investigated the Prahovo PG stacks from different perspectives: its chemical and mineralogical composition, its radiological properties, the distribution of radioactivity in different particle-size fractions, and the possibility of purifying and reusing the material. Together, these studies provide the most complete picture so far of what the Prahovo stack contains and what risks and opportunities it presents.

1. Why these studies matter

These studies matter because the Prahovo PG stack represents one of the largest industrial waste deposits of this type in Serbia, so before any decisions can be made about monitoring, reuse, or remediation, it is essential to understand what exactly is in the material and how it behaves in the environment.

PG stacks are a global issue as they can generate dust and release radon gas. At the same time, PG, if properly treated, could potentially be reused, reducing the need for new raw materials and shrinking waste deposits. Current studies around Prahovo examine not only the composition of phosphogypsum and options for its transformation and reuse, but also strategies to reduce its impact on human health and on the surrounding environment near the stack.

2. What the researchers found

Across the different studies, one result is consistent: the Prahovo phosphogypsum contains elevated levels of natural radioactivity, dominated by radium-226 (²²⁶Ra), with much smaller contributions from thorium-232 (²³²Th) and potassium-40 (⁴⁰K). An important detail is that radioactivity is not evenly distributed in the material. When the PG is separated into different particle-size fractions, the finest particles show significantly higher concentrations of radionuclides than the coarser ones. This matters because fine particles are more easily dispersed as dust and are also the fraction most likely to be used in industrial processing.

Radiological risk indices commonly used for building materials (such as radium equivalent activity and gamma indices) indicate that raw, untreated phosphogypsum from Prahovo is not suitable for indoor use as a construction material. In some cases, the calculated annual effective dose for hypothetical indoor use would exceed the recommended reference level of 1 mSv per year for the general population. This means that using untreated phosphogypsum directly in walls or boards inside buildings would not be acceptable from a radiation protection point of view.

At the same time, the studies show that the main radiological concern is external gamma radiation from ²²⁶Ra and its decay products, rather than extremely high radon release. Measurements of radon emanation indicate relatively low emanation coefficients, and model calculations suggest that, under normal ventilation conditions, indoor radon concentrations from gypsum boards made of phosphogypsum would remain moderate. Nevertheless, because the gamma dose is too high, raw material use indoors is still not recommended.

A very important finding comes from studies that tested purification by recrystallization. By dissolving phosphogypsum in water under controlled temperature conditions and recrystallizing gypsum, researchers were able to separate a much purer gypsum product from an insoluble residue. This process significantly reduces the concentration of radionuclides in the purified gypsum. For example, ²²⁶Ra activity can drop by a factor of five or more, reaching values comparable to or only slightly above those of natural gypsum. Radiological indices calculated for the purified product fall within acceptable limits for use as a construction material.

In parallel, chemical and mineralogical analyses show that the main impurities in raw PG from Prahovo are minerals such as quartz and iron-bearing phases. These impurities concentrate in the insoluble residue during purification, while the recrystallized gypsum becomes whiter, chemically cleaner, and more homogeneous.

3. How the work was carried out

The researchers used a wide range of scientific methods to study the Prahovo phosphogypsum.

• Field sampling was carried out on the stack to collect representative material. In the laboratory, samples were dried, crushed, and sieved into different grain-size fractions to study how composition and radioactivity change with particle size.
• Radiological analysis was performed mainly by high-resolution gamma spectrometry, measuring radionuclides such as ²²⁶Ra, ²³²Th, ²¹⁰Pb, uranium isotopes, and ⁴⁰K. From these data, standard radiological indices and dose estimates were calculated. Radon emanation was also measured using dedicated instruments.
• Mineralogical and chemical characterization relied on X-ray diffraction (XRD), scanning electron microscopy with EDS (SEM-EDS), and chemical analysis techniques such as ICP-OES. These methods allowed researchers to identify the mineral phases present, quantify major and minor elements, and observe changes in crystal morphology after purification.
• For purification experiments, controlled dissolution and recrystallization cycles were used, followed by filtration and washing, to separate purified gypsum from insoluble residues and to evaluate yields and material quality.

4. What it means for Prahovo

Taken together, the studies show that the Prahovo PG stack is not an immediate radiation emergency, but it is clearly an environmental hot spot that requires careful management. The raw material contains elevated natural radioactivity and is not suitable for direct use in indoor construction. Fine particles are especially enriched in radionuclides and deserve particular attention because of dust and processing issues.

At the same time, the research also shows a promising pathway for risk reduction and resource recovery. Through relatively simple purification by recrystallization, it is possible to obtain a gypsum product with much lower radioactivity and improved chemical and physical properties, potentially suitable for industrial use. This could reduce the volume of waste while creating a usable raw material, provided that the radioactive residue is safely managed.

From a policy and environmental management perspective, the results point to the need for continued monitoring, dust control, and long-term planning for the stack. The scientific evidence suggests that the problem is manageable, as FICFighters project is contributing to, but not negligible.

5. Want to know more?

Several scientific publications provide detailed information on the Prahovo phosphogypsum stack, its composition, radioactivity, and purification options. These studies form the scientific basis for any future decisions about monitoring, reuse, or remediation of the site. You can read the full version of the studies (see references below) or read more about PG worldwide in the FICfighters Virtual Forum.

Key References

• Jovanović, S., et al. (2007). Ispitivanje mogućnosti primene fosfogipsa iz IHP Prahovo u proizvodnji gips-kartonskih ploča [Study on the possibility of using phosphogypsum from IHP Prahovo in plasterboard production] (in Serbian).
• Životić, D., Pavlović, Z., Simić, V., Miladinović, D., & Kovačević, D. (2018). Characterisation of purified gypsum and insoluble impurities from waste phosphogypsum. Conference paper / Proceedings.
• Milenković, B., Todorović, D., Nikolić, J., & Pantelić, G. (2020). Radiological characterization of phosphogypsum produced in Serbia. Radiation Physics and Chemistry.
• Životić, D., Pavlović, Z., Simić, V., Miladinović, D., & Kovačević, D. (2020). Refinement of waste phosphogypsum from Prahovo, Serbia. Clay Minerals.
• Classification of phosphogypsum. Technical / scientific report.