# Phosphogypsum Recycling: A Review of Environmental Issues, Current Trends, and Prospects
## 2 Analysis of Trends in Phosphogypsum Use
### Phosphogypsum Recycling in Agriculture
As can be seen from the highlighted fragment of visualization (Figure 6), the main components that are important for the implementation of certain technological solutions of phosphogypsum conversion are calcium sulfates in a semi-hydrate or dihydrate form, undecomposed residues of phosphates (Table 1), and the potential presence of heavy metals and radionuclides. There is a variation in the content of CaSO4 2H2O in phosphogypsum, which is associated with the technological process, while in the feedstock (apatite, phosphate rock), CaSO4 2H2O content is not less than 90%. The biggest environmental hazard relates to the inflow of chemical impurities contained in phosphogypsum in the form of water-soluble and volatile compounds. The content of radioactive, rare-earth elements, and heavy metals is in direct proportion to their \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Origin of Raw Materials**} \\ \cline{2-4}
**Phosphogypsum** & **Phosphorite** & **Apatite–Phosphorite** & **Apatite** \\ \cline{2-4}
**Sample** & **(Tatarstan)** & **(South African)** & **(Colsky Peninsula)** \\ \cline{2-4}  & **Aktyubinsky** & **Apatite–Phosphorite** & **Apatite** \\ \hline CaO & 24.4 & 30.0 & 32.4 \\ \hline SO3 & 34.8 & 39.8 & 46.2 \\ \hline P2O5 (total) & 1.9 & 6.2 & 1.3 \\ \hline P2O5 (water soluble) & 1.1 & 3.3 & 0.7 \\ \hline Fe2O3 & 0.9 & 0.7 & 0.1 \\ \hline Al2O3 & 0.8 & 0.8 & 0.3 \\ \hline F & 0.1 & 0.3 & 0.3 \\ \hline Insoluble residue & 21.7 & 4.3 & 0.7 \\ \hline Crystalline water & 15.6 & 17.4 & 18.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The chemical composition of phosphogypsum converted to dihydrate depending on the type of raw material (in %) [20]. Figure 6: The fragment of overlay visualization (constructed via VOSViewer v.l.6.15). content in phosphate raw materials. Figure 7 shows the spectrum of elemental composition of phosphogypsum taken from the production line obtained by means of X-ray fluorescence analysis. The percentage of samples in fresh phosphogypsum content of Strontium is not less than 11.0%, and that of Yttrium is not less than 1.3% by weight [70]. Magmatic phosphates (Colsky and South African apatite) have a higher content of rare-earth elements (REEs) than sedimentary phosphates (Morocco, Florida, Senegal, and so on) and cadmium has a lower content of rare-earth elements (REEs) [13; 71; 72; 73]. The overlay visualization shown in Figure 8 has been chosen as a more effective tool for checking the latest trends in time scale studies. The size of the circles corresponds to the predominance of terms when publishing studies in this area. The distribution of colors depends on the year of publication (average for the cluster); the last ones are yellow. Thus, the most intensive use of terminology for the clusters under study occurred in 2016. The main core clusters are defined by terms that include the main components of phosphogypsum use. Figure 8: Overlay visualization of most frequent terms (constructed via VOSViewer v.1.6.15), where the score of the item is the time since publication. Figure 7: The element spectrum of phosphogypsum sample [70]. Among the well-known areas of recycling and disposal of phosphogypsum, it is quite new to consider the use of phosphogypsum in environmental protection technologies. [PERSON] et al. [74] conducted a study on the development of an energy-efficient methodology of pure calcium carbonate production and the binding of CO2 minerals by hydrolyzate of phosphogypsum decomposition products. Pure calcium carbonate was obtained by extraction and mineralization of CO2 using the hydrolyzate solution of phosphogypsum decomposition products. The carbonization rate of over 90% was obtained when the absorption of the liquid phase reached saturation. This method can be referred to as an alternative solution to the environmentally friendly use of phosphogypsum [74], which is consistent with the data obtained earlier by [PERSON] et al. [75]. A study by [PERSON] et al.