1. Introduction

Open storage of phosphogypsum in waste dumps is common. Phosphogypsum dumps are located in open areas in close proximity to the enterprise, natural complexes, and even settlements, and occupy vast territories. Transportation of phosphogypsum and storage in dumps entails investment and operating costs. For example, up to 10% of the prime cost of phosphoric acid refers to the costs for transportation and storage of phosphogypsum [1]. Waste dumps of industrial processing of natural raw materials are continually being replenished and are taking on scales that threaten the sustainable function of biocenoses. The current growth rate of phosphogypsum is estimated at 200 million tons per year, with a mass utilization rate of 10–15% according to the most optimistic forecasts.

The problem of phosphogypsum utilization and storage is essential for many countries as it has environmental issues (pollution of water, land, and atmosphere). Currently, over 55 million tons of phosphogypsum are accumulated in Ukraine (Armyansk, Sumy, Rivne, and other cities), occupying large areas that may otherwise be suitable for agricultural activities. Specifically, over 15 million tons of phosphogypsum are accumulated in the Sumy region alone. Years of waste storage from the production of mineral fertilizers in Ukraine has resulted in the formation of anthropogenic phosphogypsum deposits, which amount to 15.2 million tonnes in Rivne region [2], and 421.11 tons of phosphogypsum has accumulated in Vinnitsa region of Ukraine [3]. The U.S. Environmental Protection Agency prohibits the use of phosphogypsum because of its radioactivity. For example, phosphogypsum from Central Florida has an average of 26 pCi/g radium, and that from North Florida has 10 pCi/g radium. An exception is made for phosphogypsum with an average concentration of less than 10 pCi/g radium, which can be used as an agricultural amendment, but not for other purposes [4]. It is important to note that the content of radioactive elements in phosphogypsum is related to the raw material used in industrial processes.

China also has significant production of phosphogypsum, and the five largest provinces in terms of production are Hubei, Yunnan, Guizhou, Sichuan, and Anhui [5]; hence, they these also have large storage sites. Poland also has significant phosphogypsum storage facilities. More than 5 million tons of apatite phosphogypsum has been accumulated in the waste dumps of the Wizowski Chemical Plant (Poland). These wastes contain rare-earth elements, which are listed as “critical” raw materials in the European Union (EU) [6]. In Russia, there is over 500 million tons of phosphogypsum accumulation at existing ammonium phosphorus production facilities, which accumulated over all the years of their operation [7].

The negative environmental impact of phosphogypsum dumps can be manifested in the contamination of groundwater and surface water, soil, and vegetation cover by substances seeping through the protective screen, as a result of their evaporation and washout from the dump by atmospheric precipitation as well as airborne emission under the influence of weathering and dusting. The dump is a source of hydrodynamic impact on the environment, causing changes in groundwater levels, which leads to negative phenomena in the residential area, such as alienation and pollution of land areas and the transformation of the natural landscape. The large dumping of phosphate fertilizer production waste has serious consequences for the marine environment, which was studied in [8]. For example, the dumping of phosphogypsum affects the phosphorus cycle with a high content of authigenic phosphorus, especially in the Gulf of Gabes [8]. As studied in [9], phosphogypsum consisting of calcium sulphate and other complementary salts enters seawater as particles. After the particles dissolve, the concentration of heavy metals may affect near the release point. This is a significant environmental problem, especially in the Atlantic Ocean around two locations: Safi and Yorf Lasfar, where the Moroccan phosphate industry emits large quantities of this waste [9]. A study [10] characterized the formed layers of foam from the discharge of phosphogypsum into coastal waters, which can float on the surface and be passively transported to remote areas to understand their role in the behavior of pollutants in the marine environment [10].

2. Overview of Studies on the Environmental Impact of Phosphogypsum Accumulation and Storage

These specific conditions should be considered when choosing a method for removing and storing phosphogypsum in dumps:

• the production capacity;
• the amount of phosphogypsum that must be removed;
• the remoteness of the extraction components from the phosphogypsum storage site;
• the availability of storage land (unsuitable for other uses);
• the dump topography;
• the climatic conditions;
• the geological and hydrogeological conditions at the phosphogypsum storage site.

To create dumps, it is necessary to allocate large areas. Often, these areas exceed the size of industrial sites of the production itself. Thus, P₂O₅ production requires an area of 1.2 × 1.2 km with a blade height of 15 m to store phosphogypsum for 20 years [11]. Storage of phosphogypsum in waste heaps poses an environmental hazard, even if the facility is operated correctly. The problem of the utilization and storage of phosphogypsum is relevant for many countries around the world.

There are known cases of soil, natural water pollution, and contamination of plant products due to interaction with phosphogypsum in different countries, such as Brazil, China, Greece, Jordan, Kazakhstan, Poland, Russia, Spain, Turkey, and United States of America ^[12][13][14][12,13,14].

When large areas are allocated for phosphogypsum storage sites, the natural landscape is transformed, the functioning of edaphotopes is disturbed, and the aesthetic appearance of the surrounding landscape is affected. This results in phosphogypsum storages being complex sources of pollution and environment deformation; they disturb the terrain, interrupt or change the natural flow of intrasoil migration of substance, pollute the landscape with technogenic substances, change the nature of surface air flows, and affect the humidity index of the site. The dump is a source of hydrodynamic impact on the environment, causes changes in groundwater levels, and leads to negative effects in the residential area. In addition, dry phosphogypsum dumps have a potential risk of erosion because of the content of more than 70% of particles with a diameter of less than 0.14 mm in the dump surface layer ^[11][15][11,15].

In the study done by Torres-Sánchez et al. (2020), hydrogen fluoride (HF) concentrations in an urban area (Huelva, southwest Spain) were associated with a nearby large phosphogypsum deposit. The geochemical anomaly of HF was discovered with average concentrations of up to 19.1 µg/m³ and concentrations of up to 1.6 µg/m³ were found in the nearest urban area. Concentrations were associated directly with the emission of HF from the phosphogypsum accumulation site [16].

The existing modern technologies of fertilizer production do not pay enough attention to the purification of raw materials from impurity elements. Therefore, the solid waste contains fluorine, rare-earth metals, arsenic, strontium, heavy metals (cadmium, lead, vanadium, and others), and radioactive elements. Thus, uranium in the structural lattice and isomorphously replaces calcium in phosphorites, which are sedimentary rocks. Hence, the content of uranium in phosphorites depends on the geochemical conditions of their origin [17].

During dry storage (without pre-neutralization), an average of 0.1% fluorine is released into the gas phase in terms of dry matter contained in phosphogypsum. Dust rising above the dumps contains an average of up to 10 g of fluorine per 1 ton of phosphogypsum (dust distribution radius is up to 1.5 km) [17]. Fluoride compounds are highly toxic to the flora. The harmless concentration of fluorine is 0.00017–0.00023 mg/m³, according to the study by Sharipov (2014), which is much lower than the maximum permissible concentration (MPC) and amounts to 0.005 mg/m³ [18]. Leaves of fruit trees get brown and fall off; fruits develop poorly under conditions of fluorine content in the atmosphere of 0.003–0.01%. A weakening of the growth of pine plantations up to 50 km away from the source of fluoride gases has been observed [19].

The impact of waste dumps on water pollution is caused by the leaching of phosphogypsum components during storage in open areas. The formation of runoff at the dry phosphogypsum dumps is associated with precipitation and with the loss of water under hydraulic forces [20].

Wastewater generated on the slopes of the dumps during precipitation contains up to 3.4 g/L Р₂О₅; this is hundreds of times higher than the natural content of phosphorus anion in the surface waters of humid zones. Approximately 10% of fluorine is washed out by precipitation [17]. Wet, freshly formed phosphogypsum has a low pH (because of the presence of water-soluble fluorine compounds: H₂SiF₆, Na₂SiF₆, K₂SiF₆, and HF), traces of unwashed phosphoric and sulfuric acid, and phosphoric salts, and shows high corrosive activity.

The issue of phosphogypsum disposal is still very relevant, for the following reasons:

• The storage of phosphogypsum on the territory of the enterprise deteriorates the sanitary condition of the site and the adjacent territory;
• The transportation and storage of phosphogypsum in dumps are connected with rather high costs—about 18% of the cost of construction of phosphoric acid production itself—and they significantly increase during the transition to more reliable hydrotransport for phosphogypsum. Operating costs are approximately 12% of the cost of raw material processing [21];
• The need to alienate large areas to create dumps. These areas may exceed the size of industrial sites of enterprises;
• The exploitation of the dumps poses a potential threat to the environment and residential landscapes adjacent to the dump.

It is noteworthy to mention that phosphogypsum valorization has still its limitations; particularly, because of the high cost of its removal, new alternatives must be found to reduce the use of land for phosphogypsum storage [22].