# Phosphogypsum Recycling: A Review of Environmental Issues, Current Trends, and Prospects
## 2 Analysis of Trends in Phosphogypsum Use
### Phosphogypsum Recycling in Agriculture
The impurities it contains may have a negative impact on the quality of the resulting product and its safe use in agriculture. Therefore, it is necessary to consider possible ways of binding toxic components of phosphogypsum. There are methods of obtaining calcium-phosphoric additive from phosphogypsum by the processing of extraction phosphoric acid, which is used in animal husbandry, as feed dicalcium phosphate (precipitate) [67]. Feed precipitate is a mineral feed for different animal and bird species. It is used in animal husbandry as a veterinary supplement to conventional feed as premixes and compound feeds. The disadvantages of this method are high energy costs of the first stage, the use of productive phosphoric acid, and a high fluorine content in the finished product. Furthermore, as in previous engineering solutions, large amounts of wastewater are generated in the process of treatment, which can lead to secondary pollution of the environment. In Brazil, major phosphate fertilizer producers generate around 12 million tons of phosphogypsum per year. Brazilian phosphogypsum has been used in agriculture as a soil improver for many years. For its safe long-term use, it is necessary to characterize the impurities present in phosphogypsum and to study the leaching or distribution of such impurities in soil water or other elements of the ecosystem. For this purpose, an experiment was conducted by [PERSON] et al. [68], in which columns filled with typical Brazilian sandy and clay soils mixed with phosphogypsum were washed with water to gently remove these elements. The results obtained within the study [68] showed that the addition of phosphogypsum to the soil does not significantly increase the number of rare-earth elements in water. It should be noted that, during any technological processes of phosphogypsum use in agriculture, it is necessary to conduct field studies with accurate calculation of the dosage of phosphogypsum. Moreover, as phosphogypsum may contain different impurities (such as heavy metals) that are toxic to plants, the preliminary treatment is appropriate. It is necessary to consider that repeated application of high doses of phosphogypsum (12-24 t/ha) leads to a significant increase of total and water-soluble fluorine content in soil [69]. Besides, the phosphogypsum application in the soil can enhance the vertical and horizontal migration of heavy metals, which must also be considered for dosing. Visualization of the Areas of Phosphogypsum Research and Identification of Alternative Solutions for Its Use  
Through the VOSviewer programme, a visualization of cluster interconnections by keyword \"phosphogypsum\" was carried out in research publications of various fields of phosphogypsum utilization (Figure 5). Five clusters were formed: red cluster--phosphogypsum recycling in the construction industry; green cluster--radiation pollution problem of phosphogypsum and phosphate fertilizers; yellow cluster--monitoring migration of phosphogypsum components in the ecosystem, with the mobile forms of heavy metals and their inflow into aquifers from phosphogypsum dumps; blue cluster--use of phosphogypsum in agriculture as an ameliorant and a component of complex fertilizer; and purple cluster--the impact of phosphogypsum on microorganisms, particularly in bioremediation processes. The analysis of phosphogypsum component composition is important in assessing the environmental safety of phosphogypsum use (Figure 6). Phosphogypsum of different genesis and storage time has a variation in the composition of impurities, which significantly affects the scope of its possible application. The composition of phosphogypsum is determined by the initial phosphoric raw material from which phosphate fertilizers are produced. Figure 5: General network visualization (constructed via VOSViewer v.1.6.15): 5 clusters, 94,846 links, 209,564 total link strength.