# Phosphogypsum Recycling: A Review of Environmental Issues, Current Trends, and Prospects
## 2 Analysis of Trends in Phosphogypsum Use
### Phosphogypsum Recycling in Agriculture  
Phosphogypsum contains residues of phosphate, phosphoric acid (up to 4%, including water-soluble form up to 1.5%), sesquioxide, silicon compounds, rare-earth elements trace contaminants as impurities [60]. Consequently, phosphogypsum is an ameliorant and a multicomponent fertilizer [61]. On the bacterial complex side, phosphogypsum has a positive effect on the growth of streptomyces and root bacteria. The most common types of bacteria are proteobacteria, acidophilic bacteria, and actinobacteria. These groups account for over 78% of the total number of microbial groups, followed by _Firmicutes_ and _Chlorofexi_[62]. [PERSON] [63] confirms that phosphogypsum does not interfere with plant growth in general. During storage in a dump, the surface of moist phosphogypsum can be covered with lower plants, mainly algae, while the surface of stale phosphogypsum, relatively dry, slowly overgrows with flowering plants [63]. The presence of calcium, phosphorus, and sulphur in phosphogypsum makes the substance beneficial for use as a fertilizer additive in agriculture. The advantage of using phosphogypsum in agriculture is that it does not need to be purified from P\({}_{2}\)O\({}_{5}\); this impurity plays a positive role in suppressing the impact of fluorine, which is a component of phosphogypsum, and additives can be used to form insoluble compounds in the soil. However, phosphogypsum, as it comes to dump (without neutralization), is not applied to fertile soil because of high acidity, poor physico-chemical, and material handling properties (which are important for the final user). There is a well-known process of converting phosphogypsum into complex fertilizer [64]. The process involves processing of the initial phosphogypsum by phosphate solution, filtration, and drying of the finished product. The process is carried out commonly in two stages: at the first stage, the pre-calcined phosphogypsum is treated with alkaline carbonate reagent to pH 9-10, and at the second stage, the obtained carbonate pulp is mixed with a semi-product of phosphoric acid production--washing water--to pH 5.4-6. Thus, the processing of phosphogypsum allows the obtainance of complex KPCA5-mineral fertilizer and dicalcium phosphate. Another way of complex conversion of phosphogypsum in fertilizers includes preparation of a solution of ammonium carbonate; conversion of phosphogypsum by ammonium carbonate solution on the reaction of the exchange decomposition of salts: CaSO\({}_{4}\cdot 2\)H\({}_{2}\)O + ((NH\({}_{4}\))\({}_{2}\)CO\({}_{3}\))solution = CaSO\({}_{3}\) + (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) + 2H\({}_{2}\)O; separation on the filter of the converted suspension into ammonium sulphate solution, which is evaporated to extract crystalline or granulated ammonium sulfate; and carbonate residue mixed with ammonium nitrate alloy, granulated, and dried [65]. However, both methods have several disadvantages: the technological complexity of process and sewage generation, which require neutralization. In agronomy, phosphogypsum can be effectively used for the reclamation of acidic soils (together with lime), for the reclamation of saline soils, and as fertilizer meliorants (1 ton of phosphogypsum contains about 10 kg of phosphorites) for composting with biopreparation and organic fertilizers. The use of phosphogypsum for plastering and liming soils may be as effective as natural raw gypsum. There are known methods for treating manure with phosphogypsum to stop losses of ammonia nitrogen [66] and methods for producing slow-release nitrogen fertilizers from phosphogypsum, such as CaSO\({}_{4}\)-CO(NH\({}_{2}\))\({}_{2}\). The advantages of this type of fertilizer are reduction of solubility of carbamide, which reduces washout losses [48]. However, it is necessary to take into account the different characteristics of phosphogypsum depending on the conditions of its formation and the initial raw materials.