Design and optimization of sustainable passive treatment systems for phosphogypsum leachates in an orphan disposal site

The treatment of phosphogypsum leachate is more urgent than phosphogypsum

Phosphogypsum(PG) is one of the typical bulk industrial solid wastes generated in the phosphate chemical industry. Due to its huge production volume and immature resource treatment technology, a large amount of PG can only be stored and disposed in slag yards, and its impact on the ecological environment is becoming increasingly significant during long-term storage. Up to now, many researchers have focused their research on PG, with less attention paid to the PG leachate(PG-L). On the basis of the resource utilization of PG, this article analyzed the migration and transformation of pollutants and their impact on the ecological environment during long-term storage of PG. The content of pollutants in PG-L and PG was compared, and it was found that the content of toxic and harmful substances in PG-L was significantly higher than that in PG itself, and the pollution diffusion ability was greater than that of PG, the pollution of PG to the ecological environment is mainly caused by PG-L, indicating that the harmless treatment of PG-L is more urgent than PG. On the basis of traditional leachate treatment methods, a new technology of valuable element recovery and electrochemical synergistic treatment is proposed to achieve high value-added treatment of PG-L.

Valorisation diagnosis of waste from the decontamination of phosphogypsum leachates through a combined calcium carbonate/hydroxide process

Phosphogypsum is an industrial waste considered as naturally occurring radioactive material. Stack disposal and exposure to the environmental condition involve the production of acid leachates with high potential pollutant loads as heavy metals and radionuclides. In this study, a sequential neutralisation process was applied for cleaning the generated releases, and the two obtained residues were characterised from the physical-chemical and radiological point of view before their valorisation. The cleaning process was made up of two steps: the first one using calcium carbonate until pH = 3.5, and the second one using calcium hydroxide until pH = 12. The residue obtained in the first step was mostly calcium fluoride, while in the second step most phosphates were precipitated, mainly as hydroxyapatite. The final liquid was treated to reduce pH lower than 9, which is the limit included in the current directive for discharges of liquid effluents into coastal waters. The main conclusion was that the solids from the first step could be valorised as an additive in the manufacture of commercial Portland cements and ceramics, while the solids from the second step could be used as raw material for the phosphoric acid manufacture.

Environmental assessment of phosphogypsum: A comprehensive geochemical modeling and leaching behavior study

Understanding the variations in the geochemical composition of phosphogypsum (PG) destined for storage or valorization is crucial for assessing the safety and operational efficacy of waste management. The present study aimed to investigate the environmental behavior of PG using different leaching tests and to evaluate its geochemical behavior using geochemical modeling. Regarding the chemical characterization, the PG samples were predominantly composed of Ca (23.03-23.35 wt%), S (17.65-17.71 wt%), and Si (0.75-0.82 wt%). Mineralogically, the PG samples were primarily composed of gypsum (94.2-95.9 wt%) and quartz (1.67-1.76 wt%). Moreover, the automated mineralogy revealed the presence of apatite, fluorine and malladrite phases. The overall findings of the leaching tests showed that PG could be considered as non-hazardous material according to US Environmental Protection Agency limitations. However, a high leachability of elements at a L/S of 2 under acidic conditions ([Ca] = 166.52-199.87 mg/L, [S] = 207.9-233.59 mg/L, [F] = 248.62-286.65 mg/L) is observed. The weathering cell test revealed a considerable cumulative concentration over 90 days indicating potential adverse effects on the nearby environment (S: 8000 mg/kg, F: 3000 mg/kg, P: 700 mg/kg). Based on these results, it could be estimated that the surface storage of PG could have a serious impact on the environment. In this context, a simulation model was developed based on weathering cell results showed encouraging results for treating PG leachate using CaO before its disposal. Additionally, PHREEQC was used to analyze the speciation of major elements and calculate mineral phase saturation indices in PG leaching solutions. The findings revealed pH-dependent speciation for Ca, S, P, and F. The study identified gypsum, anhydrite, and bassanite as the key phases governing the dissolution of these elements.

Simplification of the pretreatment method for phosphate oxygen isotope measurement in phosphogypsum leachate

The stacking of phosphogypsum has caused considerable phosphorus pollution in water bodies near phosphogypsum yards through surface runoff and underground infiltration. The phosphate oxygen isotope (δ18Op) tracing method has served as a valuable tool for tracing phosphorus pollution in water. However, the existing δ18Op enrichment and purification methods are complex, costly, and inefficient for phosphate recovery, particularly for phosphogypsum leachate with complex compositions. Herein, a simplified and optimized pretreatment method for δ18Op measurement in phosphogypsum leachate was developed. Zirconium/polyvinyl alcohol (Zr/PVA) gel beads showed good selectivity for phosphate enrichment from water at different initial phosphate concentrations with appropriate Zr/PVA dosage. The optimal enrichment pH value was <7, and the concentrated phosphate on the Zr/PVA gel beads could be effectively eluted in an alkaline environment. Compared with the traditional Fe or Mg coprecipitation enrichment methods, impurities in the solution showed no obvious adverse effects on the phosphate enrichment process. Further, the phosphate solution eluted from the Zr/PVA gel beads was purified by a simple adjustment of the pH instead of cation exchange in the traditional purification process. Magnesium ions in the solution could be completely removed when the pH ranged from 3.17 to 6.15, and the phosphate recovery rate could reach 98.66% when the eluent pH was 5.02. Fourier-transform infrared spectroscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy revealed that similar to traditional pretreatment method, the proposed method can obtain high-purity Ag3PO4 solids for δ18OP measurement and no isotope fractionation of δ18OP was observed. Therefore, this study provides a promising and reliable pretreatment method for δ18OP measurement, especially in complex phosphogypsum leachate.

Simultaneous and efficient removal of fluoride and phosphate in phosphogypsum leachate by acid-modified sulfoaluminate cement

The high concentration of fluoride and phosphate in phosphogypsum leachate is harmful to the environment and ecosystem. Thus, there is a need to develop feasible materials or technologies to remove both fluoride and phosphate in acidic phosphogypsum leachate. In this study, sulfoaluminate cement (SC) was used to simultaneously remove fluoride and phosphate in wastewater based on its moderate alkalinity and rich content of metal elements (Ca, Al and Fe, etc). The acidized sulfoaluminate cement (ASC) composite was prepared through modifying SC with hydrochloric acid, which can increase the specific surface areas of the raw SC, as well as the activity of the metal elements in SC. Compared with other coagulants, ASC showed excellent removal performance for fluoride and phosphate, such as higher removal efficiency, better effluent quality, and accelerated settling rate. The fluoride and phosphate removal performances of ASC herein were investigated at different dosages, pH values, coexisting substances, and initial concentrations. As a result, ASC exhibited wide pH adaptability and satisfactory selectivity for fluoride and phosphate. The possible removal mechanisms of fluoride and phosphate by ASC included chemisorption, ion exchange, and precipitation. The main end products associated with fluoride were fluorite (CaF₂), aluminum fluoride (AlF₃), and iron trifluoride (FeF₃). The main final products amid phosphate removal, on the other hand, were brushite (CaHPO₄·2H₂O), aluminophosphate ((H₃O)·AlP₂O₆(OH)₂), silicocarnotite (Ca₂SiO₄·Ca₃(PO₄)₂) and iron phosphate (Fe(H₂PO₄)₃). More importantly, ASC can effectively treat the phosphogypsum leachate at a wide range of concentrations, and the concentrations of phosphate and fluoride in the effluents were lower than 0.5 mg P L^-1 and 4 mg L^-1, respectively. To sum up, ASC is a competitive candidate to treat wastewater with high fluoride and phosphate content, such as phosphogypsum leachate.

Predicting the leachate generation from wet phosphogypsum stack using a water-balance-analysis based model

Leachate from wet phosphogypsum (PG) stack should be properly managed to mitigate the negative environmental impact of phosphoric industry. Accurate prediction of leachate amount is the prerequisite for efficient leachate management. In this study, a model using water balance analysis to predict leachate production from wet PG stack is established. The extruded water, which is related to PG deformation, is innovatively introduced as a variable in the model to account for the porewater's contribution. Model simulation suggested that at the early stage, fresh water need to be added to PG to facilitate the transfer or PG slurries; however, as the leachate accumulates in the tailings pond, a net discharge of PG is required starting at the fourth year for the studied PG stack. Model simulation also indicated that the leachate generation increased gradually over time and that the leachate generation in each month could deviate from the average leachate generation during the life cycle of the stack. The model output matches with measured values reasonably well, which confirmed the model's accuracy. Sensitivity analysis indicated that average precipitation and evaporation are the two most important factors that determine leachate generation rate. Monthly leachate generation rates vary significantly within the year, as the precipitation and evaporation vary in different seasons. The highest leachate generation rates were reached in rainy seasons and the lowest rates were reached in wintery months. This study could be used to optimize the PG leachate managements and to mitigate the PG related pollution to the environment.

Environmental management and potential valorization of wastes generated in passive treatments of fertilizer industry effluents

A phosphogypsum stack located in SW Spain releases highly acidic and contaminated leachates to the surrounding estuarine environment. Column experiments, based on a mixture of an alkaline reagent (i.e., MgO or Ca(OH)₂) dispersed in an inert matrix (dispersed alkaline substrate (DAS) technology), have shown high effectiveness for the treatment of phosphogypsum leachates. MgO-DAS and Ca(OH)₂-DAS treatment systems achieved near total removal of PO₄, F, Fe, Zn, Al, Cr, Cd, U, and As, with initial reactive mass:volume of leachate treated ratios of 3.98 g/L and 6.35 g/L, respectively. The precipitation of phosphate (i.e., brushite, cattiite, fluorapatite, struvite and Mn₃Zn(PO₄)₂·2H₂O) and sulfate (i.e., despujolsite and gypsum) minerals could control the solubility of contaminants during the treatments. Therefore, the hazardousness of these wastes must be accurately assessed in order to be properly managed, avoiding potential environmental impacts. For this purpose, two standardized leaching tests (EN-12457-2 from the European Union and TCLP from the United States) were performed. According to European Union (EN-12457-2) regulation, some wastes recovered from DAS treatments should be classified as hazardous wastes because of the high concentrations of SO₄ or Sb that are leached. However, according to United States (US EPA-TCLP) legislation, all DAS wastes are designated as non-hazardous wastes. Moreover, the solids generated in the DAS systems could constitute a promising secondary source of calcite and/or P. This research could contribute to worldwide suitable waste management for the fertilizer industry.

Bibliometric analysis of phosphogypsum research from 1990 to 2020 based on literatures and patents

The demand together with the urgency of phosphogypsum (PG) treatment will pose significant challenges for many countries. This research aims to explore the research progress of PG, including basic status, cooperation situation, research fields, and development trends, based on the Web of Science database through bibliometric analysis of publications (articles and patents) from 1990 to 2020. The results show that academic research on PG originated early, but the number of patents grew quickly. China is a global leader in terms of the number of publications and plays a significant role in international cooperation. The knowledge of PG has remained concentrated in the fields of natural radioactivity, cement paste backfilling, soil, crystal morphology, and synthetic gas. However, academic hotspots focus on the microstructure of chemical processes and various environmental impacts; patents and hot technologies are based on the production of refractory materials, ceramics, surface materials, cement mortar, and composite materials. The academic frontiers of PG will be centered on exploiting the methods of recovering rare earth elements from PG, the conditions of ion solidification/stabilization in PG, the impact of reaction conditions on product quality, and the reaction mechanism at the micro-level. The frontiers of patents need to focus on the improvement of manufacturing equipment, new wall materials, and chemically modified polymer materials. Envisaging the number of articles and patents to be published in the future, architectural research has a large room for improvement. This paper conducts an in-depth analysis of PG and provides information on the technological development prospects and opportunities, which is helpful for researchers engaged in PG management.