Facet-engineering strategy of phosphogypsum for production of mineral slow-release fertilizers with efficient nutrient fixation and delivery

Phosphogypsum (PG) presents considerable potential for agricultural applications as a secondary primary resource. However, it currently lacks environmentally friendly, economically viable, efficient, and sustainable reuse protocols. This study firstly developed a PG-based mineral slow-release fertilizer (MSRFs) by internalization and fixation of urea within the PG lattice via facet-engineering strategy. The molecular dynamics simulations demonstrated that the binding energy of urea to the (041) facet of PG surpassed that of the (021) and (020) facets, with urea's desorption energy on the (041) facet notably higher than on the (021) and (020) facets. Guided by these calculations, we selectively exposed the (041) dominant facet of PG, and then achieving complete urea fixation within the PG lattice to form urea-PG (UPG). UPG exhibited a remarkable 48-fold extension in N release longevity in solution and a 45.77% increase in N use efficiency by plants compared to conventional urea. The facet-engineering of PG enhances the internalization and fixation efficiency of urea for slow N delivery, thereby promoting nutrient uptake for plant growth. Furthermore, we elucidated the intricate interplay between urea and PG at the molecular level, revealing the involvement of hydrogen and ionic bonding. This specific bonding structure imparts exceptional thermal stability and water resistance to the urea within UPG under environmental conditions. This study has the potential to provide insights into the high-value utilization of PG and present innovative ideas for designing efficient MSRFs.

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.

Modified phosphogypsum whiskers for decontamination of mercury tailings

Mercury (Hg) tailings are hazardous solid wastes because of their high Hg concentrations. Modified phosphogypsum (PG) can decrease the bioactivity and mobility of heavy metals through chemisorption or electrostatic interactions. In this study, PG whiskers were modified by ZnCl2 and S, chitosan-hydrochloric acid, and thioglycolic materials; the resulting modified whiskers were used to decontaminate Hg tailings. Leaching tests and orthogonal experiments were conducted to optimize the modification parameters, including modifier quantity, pH, reaction temperature, and reaction time. The structure and physicochemical properties of the whiskers before and after modification were characterized through X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). The stabilization efficiency of the modified PG whiskers ranged from 93.05 to 97.50%, demonstrating excellent stabilization effects. The stabilization was achieved through chemisorption or complexation. The decontamination process using modified whiskers reduced the pH and total nitrogen of the tailings; increased the cation exchange, total phosphorus, organic carbon, and total carbon; and made the tailings suitable for planting. In addition, the modified PG promoted the morphological transformation of Hg in the tailings, thereby significantly decreasing the Hg content in the effective states and mitigating the risk of Hg contamination.

A review of organic and inorganic amendments to treat saline-sodic soils: Emphasis on waste valorization for a circular economy approach

Soil salinization poses a significant challenge to the sustainable advancement of agriculture on a global scale. This environmental issue not only hampers plant growth and soil fertility but also hinders the advancement of the national economy due to restrictions on plant development. The utilization of organic and/or inorganic amendments has demonstrated the ability to mitigate the detrimental impacts of salt stress on plant life. At the outset, this review, in addition to summarizing current knowledge about soil amendments for saline-sodic soils, also aims to identify knowledge gaps requiring further research. The organic or inorganic amendments modify soil conditions and impact plant development. For instance, organic amendments have the potential to improve the structure of the soil, augment its capacity to retain water, and stimulate microbial activity. As this occurs, salts gradually leach through the porous structure of the soil. Conversely, inorganic amendments, such as gypsum and phosphogypsum, displace sodium from soil-negative sorption sites reducing the salinity, they also increase base saturation, altogether positively impacting plant growth conditions. This review emphasizes that, under adequate rates, the combination of organic and inorganic amendment has a high potential to enhance the poor physicochemical properties of saline-sodic soils, thereby reducing their salinity. Consequently, an in-depth examination of the mineral composition, texture, and chemical composition of the soil is required to choose the most effective amendment to implement. Future research necessitates a thorough investigation of techno-economic and life cycle assessment, with active involvement from stakeholders, to enhance the decision-making process of the amendments in specific localities.

Synergistic preparation and application in PCU of α-calcium sulfate hemihydrate whiskers from phosphogypsum and electrolytic manganese residue

The α-calcium sulfate hemihydrate whiskers (α-CSHWs) were first prepared using phosphogypsum (PG) and electrolytic manganese residue (EMR) as raw materials for coating urea, demonstrating excellent controlled-release properties. The effects of different reaction conditions on α-CSHWs, achieved by optimizing the reaction time, the concentrations of NH4+, Mn2+, and other factors, were discussed. Results showed that when the EMR content was 25 wt%, the reaction temperature was 100 °C, and the reaction time was 3 h, α-CSHWs with a length-to-diameter ratio of 39 were obtained. Through experiments and density functional theory (DFT), the mechanism of α-CSHWs preparation was elucidated. The results show that the addition of EMR reduces the content of impurity ions PO43- and F- in PG while introducing NH4+ and Mn2+. Interestingly, both NH4+ and Mn2+ can reduce the nucleation time of α-CSHWs, while PO43-, Mn2+, and F- are more likely to adsorb on the (0 0 6) crystal plane of α-CSHWs, NH4+ readily adsorbs on the (4 0 0) crystal plane. The controlled-release performance of modified α-CSHWs incorporated into polyurethane-coated urea (PCU) was investigated, and it was found that the addition of Mα significantly prolonged the nutrient release period, with the period extending up to 116 days for coatings of 5wt% and above. This work not only enhances the efficiency of PG and EMR utilization but also serves as a reference for the straightforward synthesis and application of α-CSHWs.

Enhanced absorption of SO2 from phosphogypsum decomposition by phosphate slurry for phosphoric acid production

Phosphogypsum (PG) is a major industrial by-product of wet process phosphoric acid production, and untreated PG stockpiled on land will cause severe environmental pollution. Thermal treatment of PG is currently the mainstream treatment method PG can be thermally decomposed to produce CaO, and the decomposition process produces large amounts of SO2. In this paper, phosphate slurry was used to absorb SO2 generated during the PG decomposition to produce phosphoric acid. The effects of operating conditions such as pressure, inlet SO2 concentration, and additive content on the desulfurization efficiency, as well as phosphoric acid yield, were investigated. Under the optimal experimental parameters, the desulfurization efficiency was 100% in the first 3 h, and decreased to 67.42% after 5 h, the maximum phosphate concentration in the solution was 1445.92 mg/L. The Density functional theory (DFT) calculations showed that SO2 and O2 adsorbed on the surface of P2O5 underwent to generate SO3, which can react with H2O to produce H2SO4. Moreover, it was found that Fe3+ could enhance the catalytic oxidation process of SO2 and O2 by decreasing the reaction energy barrier. This study should be helpful for the recycling of phosphorus resources.

Staged purification of phosphogypsum using pH-dependent separation process

Phosphogypsum (PG) is an industrial by-product of the transformation of phosphate rocks. For decades, PG has been a source of environmental concern due to the massive amount produced thus far, i.e., 7 billion tons, with a current production rate of 200-280 million tons per year. Phosphate minerals contain various impurities that precipitate and concentrate within PG. These impurities hinder PG usability in various sectors. This paper aims to purify PG using an innovative process based on staged valorization of PG. Initially, PG dissociation by ethylenediaminetetraacetic acid (EDTA) was optimized. After screening of different parameters and monitoring the ionic conductivity of solutions, it was disclosed that a pH-dependent solubilization process in the presence of EDTA resulted in high solubility of PG, up to 11.82 g/100 mL at pH > 11. Subsequently, a recovery of the purified PG by selective precipitation of calcium sulfate dihydrate (CSD) from obtained filtrate through pH adjustment to 3.5 were investigated. An abatement of 99.34% Cr, 97.15% Cd, 95.73% P2O5, 92.75% Cu, 92.38% Al2O3, 91.16% Ni, 74.58% Zn, 72.75% F, 61.43% MgO, 58.8% Fe2O3, 56.97% K2O, and 55.41% Ba was achieved. The process relied on the variation of EDTA chelation properties towards monovalent, divalent, and trivalent cations at different pHs. According to the findings of this study, a staged purification process in the presence of EDTA is an effective method for removing impurities from the industrial PG.

Exploring the potential reuse of phosphogypsum: A waste or a resource?

Phosphogypsum (PG), the main industrial by-product of phosphate fertilizer industry, primarily consists of calcium sulfate dihydrate. However, it contains various impurities with variable quantities depending on the origin of the phosphate rock. These impurities can restrict the reuse of phosphogypsum as a secondary primary resource. Consequently, large quantities of produced PG are stored in surface stockpiles that occupy extensive land areas and may pose a significant risk of ecological contamination to the surroundings. Researchers have shown growing interest in addressing the worldwide accumulation of this waste material. To gain a comprehensive understanding of the environmental impact of phosphogypsum, it is crucial to explore its properties (e.g., chemistry, mineralogy, radioactivity), and how it interacts with the surrounding environment, enabling well-informed decisions decision regarding its management and its valorization. In this review, we will i) explore the chemical, radiological and mineralogical characteristics of PG; ii) discuss the environmental concerns related to land discharge and sea disposal; and iii) examine the latest advancements in various valorization techniques developed including agriculture, REE extraction, environmental application, chemical and thermal transformation, and also construction sector. Outlining their limitations and challenges restrict in the global variability of phosphogypsum (PG), technical and economic limitations, and the potential for secondary pollution in select valorization approaches. This requires a thorough assessment and comparison with conventional disposal alternatives.

Effect of alkaline leaching of phosphogypsum on sulfate reduction activity and bacterial community composition using different sources of anaerobic microbial inoculum

Phosphogypsum (PG), a by-product of the phosphate industry, is high in sulfate, (SO42-), which makes it an excellent substrate for sulfate-reducing bacteria (SRB) to produce hydrogen sulfide. This work aimed to optimize SO42- leaching from PG to achieve a high biological reduction of SO42- and generate high sulfide concentrations for subsequent use in the biological recovery of elemental sulfur. Five SRB consortia were isolated and enriched from: IS (Industrial sludges), MS (Marine sediments), WC (Winogradsky column), SNV (petroleum industry sediments) and PG (stored Phosphogypsum). The five consortia showed reduction activity when using PG leachate (with water) as source of SO42- and lactate, acetate, or glucose as the electron donor. The highest reduction rate (81.5 %) was registered using lactate and the IS consortium (81.5 %) followed by MS (79 %) and PG (71 %). To enhance the concentration of leached SO42- from PG for future utilization with the isolated consortia, PG was treated with NaOH solutions (2 % and 5 %). SO42- release of 97 % was achieved with a 5 % concentration and the resulting leachate was further diluted to target a SO42- concentration of 12.4 g·L-1 for utilization with the isolated consortia. Compared to water leachate, a significantly higher reduction rate was registered (2 g·L-1 of SO42) using the IS consortium, demonstrating limited inhibition effect of sulfide- concentration on SRB functionalities. Moreover, metagenomic analysis of the consortia revealed that using PG as a source of SO42- increased the abundance of Deltaproteobacteria, including known SRB like Desulfovibrio, Desulfomicrobium, and Desulfosporosinus, as well as novel SRB genera (Cupidesulfovibrio, Desulfocurvus, Desulfococcus) that showed, for the first time, significant potential as novel sulfate-reducers using PG as a SO42- source.

The mechanical properties and sustainability of phosphogypsum-slag binder activated by nano-ettringite

The cementitious material based on phosphogypsum (PG) and ground granulated blast furnace slag (GBFS) demonstrates good economy and sustainability, whereas its drawback of ultra-slow strength development seems unacceptable. In this study, an attempt to drive the hydration of PG-GBFS and further facilitate the strength development by introducing nano-ettringite (NE) was carried out. The impact of 1- 5 % NE on the compressive strength, hydration process, dissolution behavior, and microstructure evolution of PG-GBFS were investigated. The results showed that the incorporation of NE significantly increased the compressive strength of PG-GBFS. At 7 d, the strength grew from 0 MPa to a range of 7.6- 20.2 MPa, and at 28 d, it was enhanced from 22.9 MPa to a range of 45.6- 79.0 MPa. The reason was that the introduction of NE induced the formation of AFt, thereby accelerating the hydration process and promoting the development of the skeletal network, resulting in higher early strength. Besides, NE facilitated the formation of C-S(A)-H gel, which further refined the pore structure and led to continuous growth in later strength. Additionally, PG-GFBS with 5 % NE exhibited significantly lower total costs (35.0 % of NaOH-activated slag and 51.7 % of water glass-activated slag) and lower carbon emissions (30.8 % of NaOH-activated slag and 49.8 % of water glass-activated slag) at the same 28 d compressive strength, indicating its strong competitiveness in both sustainability and economy.

Recovery of Ce and La from phosphogypsum leachate by adsorption using grape wastes

The present research aimed to evaluate the use of grape stalk in the adsorption of lanthanum and cerium to identify the best operating conditions enabling the application of the bioadsorbent in REEs leached from phosphogypsum. The grape stalk was characterized and showed an amorphous structure with a heterogeneous and very porous surface. Also, it was possible to identify the groups corresponding to carboxylic acids, phenols, alcohols, aliphatic acids, and aromatic rings. The pH effect study showed that the adsorption process of La3+ and Ce3+ ions was favored at pH 5.0. The adsorption kinetics followed the pseudo-second-order model. In just 20 min, 80% saturation was reached, while equilibrium was reached after 120 min. The adsorption isotherms were appropriately adjusted to the Langmuir model, and the maximum adsorption capacities were obtained at 298 K, which were 35.22 mg g-1 for La3+ and 37.99 mg g-1 for Ce3+. Furthermore, the adsorption process was favorable, spontaneous, and exothermic. In the study's second phase, phosphogypsum was leached with a sulfuric acid solution. Then, the adsorption of REEs was carried out under the experimental conditions of pH after leaching and pH 5.0 (adjustment carried out with sodium hydroxide solution) at 298 K for 120 min and with adsorbent dosages of 1 and 5 g L-1. This process resulted in removal percentages above 95% for the most abundant REEs, such as neodymium, lanthanum, and cerium, at pH 5.0 and a dosage of 5 g L-1, demonstrating the effectiveness of the bioadsorbent used. These results indicate the potential of using grape residue as a promising bioadsorbent in recovering rare earth elements from phosphogypsum leachate.

Fluorides immobilization through calcium aluminate cement-based backfill: Accessing the detailed leaching characterization under torrential rainfall

Urbanization and economic development have increased the demand for fertilizers to sustain food crop yields. Huge amounts of by-products, especially phosphogypsum (PG), are generated during the wet processing of rock phosphate to produce fertilizers. Chronic exposure to fluoride in phosphogypsum in groundwater as a result of the weathering of fluoride-containing waste poses a significant health risk to millions of people. We propose a method for using calcium aluminate cement (CAC) to remediate high fluoride contents in solid waste. Column leaching tests under harsh rainfall conditions confirmed the efficient fluoride immobilization capacity of a CAC binder. Although the fluoride concentrations in leachates during the first 1-2 days (1.25 mg/L) slightly exceeded the threshold of 1.00 mg/L, the concentrations over 3-28 days (ranging from 0.98 to 0.83 mg/L) consistently remained well within the acceptable range. Furthermore, our characterization and geochemical modeling revealed the fluoride retention mechanisms of CAC-stabilized PG under laboratory-simulated conditions of torrential rainfall. During leaching, physical encapsulation prevents fluoride from contacting leachate. However, an unfavorable pH value can cause the release of fluoride from the cement matrix, which is subsequently captured by aluminate hydrate through adsorption or co-precipitation. We quantified the carbon footprint of CAC for immobilizing 1 mg of fluoride in PG, obtaining a remarkably low value of 4.4 kg of CO2, in contrast to the emissions associated with the use of ordinary Portland cement (OPC). The findings suggest a unique opportunity for extensive PG remediation. This opportunity extends the horizons of achieving zero-waste emissions in the phosphorus industry and has practical significance in the context of reducing carbon emissions.

Treating waste with waste: Adsorption of anionic dyes in wastewater with surfactant-modified phosphogypsum

Phosphogypsum (PG) is a solid waste generated during the wet process of phosphoric acid production. The environmental-friendly disposal and recycling of PG is vital in the field of environmental solid waste treatment. In this study, PG is used for adsorbent of dyes in wastewater to achieve the goal of recycling waste with waste. Surfactant-modified phosphogypsum (ODBAC@PG) was prepared using octadecyl dimethyl benzyl ammonium chloride (ODBAC) as modifier. ODBAC@PG exhibits high adsorption capability for anionic dyes (methyl blue (MeB) and indocyanine carmine (IC)). The pseudo-second-order kinetic model fits the kinetic experimental data for the adsorption of two organic anionic dyes. Langmuir adsorption isotherm fits the adsorption characteristics of MeB and IC on ODBAC@PG, exhibiting a monolayer adsorption pattern. Thermodynamic parameters indicate the spontaneous and exothermic properties of MeB and IC on ODBAC@PG. MeB and IC have antagonistic effects on each other in binary adsorption system. High adsorption capacity after six cycles of experiments demonstrates the high reusability of ODBAC@PG. The nature for the adsorption includes electrostatic interaction, hydrogen bond and hydrophobic interaction. Using ODBAC@PG for dyes wastewater treatment can accomplish the goal of treating waste with waste and turning waste into treasure.

Deep insight on mechanism and contribution of arsenic removal and heavy metals remediation by mechanical activation phosphogypsum

Arsenic-containing wastewater and arsenic-contaminated soil can cause serious environmental pollution. In this study, phosphogypsum with partial mechanical activation of calcium oxide was used to prepare a new phosphogypsum-based passivate (Ca-mPG), and its remediation performance on arsenic-contaminated soil was evaluated in terms of both effectiveness and microbial response. The results showed that the optimum conditions for the preparation of the passivate were optimized in terms of single factor and response surface with a ball milling speed of 200 r/min, a material ratio of 6:4 and a ball milling time of 4 h. Under these conditions, the adsorption capacity was 37.75 mg/g. The leaching concentration of arsenic (As) in the contaminated soil after Ca-mPG modification decreased from 25.75 μg/L to 5.88 μg/L, which was lower than the Chinese national standard (GB/T 5085.3-2007); Ca-mPG also showed excellent passivation effect on other heavy Metals (copper, nickel, cadmium, zinc). In addition, As-resistant bacteria and passivators work together to promote the stabilization effect of contaminants during the remediation of As-contaminated soil. The mechanisms of Cu, As(III)/As(V), Zn, Cd, and Ni removal were related to ion exchange, electrostatic adsorption of substances on heavy metals, calcium binding to other substances to produce precipitation; and microbially induced stabilization of HMs, oxidized. Overall, this study demonstrates an eco-friendly "waste-soil remediation" strategy to solve problems associated with solid waste reuse and remediation of HM-contaminated soils.

Integrated evaluation of the performance of phosphogypsum recycling technologies in China

The Chinese government is implementing policies, such as the "Guidance on comprehensive utilization of bulk solid waste for the 14th Five-Year Plan period", to stimulate phosphogypsum (PG) reduction and recycling. Thus, the comprehensive evaluation of PG recycling technologies for sustainable development is crucial. This study proposes a novel multi-criteria decision analysis (MCDA) method that considers the criteria of resources, environment, economy, and society and risk attitudes of decision-makers and integrates game theory (GT) and utility theory for criteria weighting and ranking to assess industrial-scale PG recycling technologies in China. The results demonstrate that GT provides more reasonable criteria weights than individual weighting methods. PG-based lightweight plaster is the top performer in the resource and environmental dimensions owing to its exceptional resource and energy efficiency. PG utilized for dry-mix mortar and organic fertilizer production exhibited the best utility performance of 0.74 and 0.73, respectively. Measures, such as subsidies and product publicity, should be implemented to promote these technologies. However, technologies with poor performance, such as PG used for the co-production of sulfuric acid and fertilizer or cement, may require optimization or substitution for the sustainable recycling of PG. The proposed MCDA method is robust and can serve as a reliable decision-making tool for other waste-recycling technologies. However, caution must be exercised when determining risk attitude using the MCDA method as it may vary with the number of technologies and affect the final rankings.

Reaction mechanism of thermal decomposition of Phosphogypsum

Phosphogypsum (PG) is a co-product of the phosphoric acid industry. To reduce the environmental impact and land occupation caused by PG disposal, researchers are trying to integrate it into building materials, agriculture, etc. Herein, PG decomposition with carbon monoxide (CO) was studied with i) thermogravimetric analysis (TGA), ii) induction heated fluidized bed reactor (IHFBR), and iii) thermodynamically using FactSageTM. Experimentally, PG starts decomposing around 600 °C and produces mainly calcium sulfide (CaS) at high CO partial pressure, above 50 %, and mainly to calcium oxide (CaO) at lower CO partial pressure (<20 %). At 1000 °C and above, CaSO4 was completely converted to CaS, CaO, and minor co-products due to the presence of impurities in PG. Elemental and XRD analyses were adopted to understand the reaction mechanisms of PG decomposition. Thermodynamic simulations confirmed the full conversion of calcium sulfate (CaSO4) above 600 °C for a CO/CaSO4 ratio above 6.81 (mol/mol), whereas only 60 % conversion would be achieved at 1500 °C and lower ratio (<0.49 (mol/mol)). As a result, CaS and CaO may be produced, depending on the temperature and CO partial pressure.

Study on application and environmental effect of phosphogypsum-fly ash-red mud composite cemented paste backfill

Phosphogypsum (PG) cementitious paste backfill (CPB) was prepared by using PG and fly ash (FA) as the main raw materials, red mud (RM) as the alkaline activator, Portland cement (OPC) as the binder, and silica fume (SF) as the additive, and its properties were investigated to achieve the objective of "treating harm with waste." The results showed that the addition of OPC facilitated the flowability of the slurry, while the addition of RM and SF had the opposite effect. The slurry presented ideal flowability when the water/binder ratio was 0.2 and the superplasticizer (SP) content was 0.7%. The mechanical properties and water resistance were improved significantly with increasing OPC, RM, and SF doping. The strength of the CPB material exceeded 22 MPa after curing at room temperature for 28 days, which met the mine filling requirements. Changes in the ion concentrations of the solution were first monitored during immersion. The dissolution rules of Ca2+ and SO42- at different immersion ages confirmed that RM promoted the continuous hydration of CPB, which was the key to improve water resistance. Microstructural analysis showed that the main hydration products were AFt and C-S-H, which played an important role in the strength development of the material. The leaching results demonstrated that the metal ion content satisfied the requirements of the III categories of Chinese environmental standards (GB/T 14848-2017), indicating that the technology is a reliable and environmentally friendly technology for PG, FA, and RM recovery that can simultaneously support safe mining.

Low temperature and highly-efficient one-step decomposition of phosphogypsum via biochar by Fe3+/Co2+/Ni2+ unitary/ternary catalyst

Phosphogypsum (PG), which has great environmental harm and restricts the sustainable development of phosphorus chemical industry, is a solid waste produced in phosphoric acid production. Thermal decomposition of PG is an extensive way to reutilization of resource, and the key point is to establish an appropriate decomposition path and catalyst system of PG. In the work, the strategy for low-temperature and highly-efficient decomposition of PG is established based on the thermodynamic analysis and the experimental research by metal ions to reduce the decomposition temperature. Meanwhile, SEM(Scanning Electron Microscope) is used to characterize the composition and morphology of PG in the various conditions, also the decomposition temperature is analyzed by TGA(Thermogravimetric Analysis). Then, the decomposition ratio via Fe3+/Co2+/Ni2+ unitary/ternary catalyst is obtained by precipitation method. Through kinetic analysis combined with XRF(X-ray Fluorescence Spectrometer) and EDX(Energy Dispersive X-Ray Spectroscopy) results, it is found that there is a reaction competition in the decomposition process by Fe3+/Co2+/Ni2+ ternary catalyst. Further the mechanism of catalytic system on PG is derived. The present work can be concluded that Fe3+/Co2+/Ni2+ can effectively reduce the decomposition temperature of PG, and the effect of ternary metal is more obvious than that of unitary metal. Finally, pomelo peel is used instead of coke to successfully decompose PG at low temperature by one step method. The establishment of low temperature decomposition system of PG has potential application in phosphorus chemical industry and is in line with sustainable development.

Enhanced removal of sulfonamide antibiotics from water by phosphogypsum modified biochar composite

Antibiotic pollution has become a global eco-environmental issue. To reduce sulfonamide antibiotics in water and improve resource utilization of solid wastes, phosphogypsum modified biochar composite (PMBC) was prepared via facile one-step from distillers grains, wood chips, and phosphogypsum. The physicochemical properties of PMBC were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), Zeta potential, X-ray diffraction (XRD), etc. The influencing factors, adsorption behaviors, and mechanisms of sulfadiazine (SD) and sulfamethazine (SMT) onto PMBC were studied by batch and fixed bed column adsorption experiments. The results showed that the removal rates of SD and SMT increased with the increase of phosphogypsum proportion, while decreased with the increase of solution pH. The maximum adsorption capacities of modified distillers grain and wood chips biochars for SD were 2.98 and 4.18 mg/g, and for SMT were 4.40 and 8.91 mg/g, respectively, which was 9.0-22.3 times that of pristine biochar. Fixed bed column results demonstrated that PMBC had good adsorption capacities for SD and SMT. When the solution flow rate was 2.0 mL/min and the dosage of PMBC was 5.0 g, the removal rates of SD and SMT by modified wood chips biochar were both higher than 50% in 4 hr. The main mechanisms of SD and SMT removal by PMBC are hydrogen bonding, π-π donor-acceptor, electrostatic interaction, and hydrophobic interaction. This study provides an effective method for the removal of antibiotics in water and the resource utilization of phosphogypsum.

Effect of phosphogypsum adding on setting kinetics and mechanical strength of geopolymers based on metakaolin or fly ash matrices

Our study aims to highlight the effects of the addition of phosphogypsum on certain fresh and hardened characteristics of geopolymer matrices based on metakaolin or fly ash. In the fresh state, workability and setting were studied by rheology and by the electrical conductivity measurement. The hardened state was characterized by XRD, DTA, SEM, and compressive strength measurement. Workability investigations reveal that the addition of phosphogypsum increases the viscosity, which limited the phosphogypsum addition rate to 15 wt% for metakaolin-based matrices and 12 wt% for fly ash-based matrices, with a setting retarding effect in both cases. Analyses of the matrices show dissolution of gypsum along with formation of sodium sulfate and calcium silicate hydrate. Moreover, the introduction of phosphogypsum to these matrices up to a mass rate of 6% has no significant effect on the mechanical strength. Beyond that rate, the compressive strength drops from a value of 55 MPa for the matrices without addition down to 35 MPa and 25 MPa when the addition rate is 12 wt% for the metakaolin-based and fly ash-based matrix, respectively. This degradation seems to be due to the increase in porosity created by addition of phosphogypsum.

Assessment of radiological impact on the surrounding environment and biota for phosphogypsum waste stockyard in Korean facility

In this study, the effects of deposited gypsum residues on the surrounding environment and radiation exposure in plants and animals were evaluated under various exposure situations. A waste stockyard in a Korean facility (surrounded by mountains and sea) was used to store phosphogypsum, a byproduct of phosphoric acid processes, in a slurry form in a large gypsum storage facility (provided separately on the facility site). The ERICA tool was used to evaluate the impact of radiation on nonhuman environments for mineral processing and waste storage for risk estimation. The impact of radiation on the environment due to the phosphogypsum stockyard was negligible with a screening dose of less than 10 μGy h^-1. However, to conservatively evaluate the environmental impact of rain and wind in the phosphogypsum stockyard, the soil at the interface of the stockyard, where plants could not grow, was considered as an input value, and the estimated dose rate of shrubs was found to be 45 μGy h^-1. The effects of the phosphogypsum stockyard on the surrounding environment accounted for 95-100% of the total dose for internal exposure in biota. In general, radium was found to be the highest contributor to biota, and the next lead and polonium were contributors to the dose. The findings contribute to an understanding of the radiological impact of waste stored and disposed of at the facility on the environment and biota (all routes of exposure) and to developing sustainable operations and pollution monitoring policies.

Examination of the environmental behavior of phosphogypsum with the application of lab-scale experiment

Phosphogypsum (PG) is a reject of the phosphoric acid production process in phosphate fertilizer industries. The process results in the production of relatively large quantities of PG that it might cause serious environmental and human health concerns. The data of a laboratory investigation of PG are presented here. Lab-scale experiments with lysimeters were conducted in order to simulate and examine the environmental characteristics and the temporal behavior of PG leachates in terms of physicochemical characteristics and chemical composition. Based on the results, leachates from already deposited for many years PG or its mixture with marble powder, seemed to have better pH and conductivity values and lower elemental concentrations compared to leachates from freshly disposed PG. However, the leachates characteristics improve and stabilize in both cases after four days of irrigation or of 1080-1240 mm of rain. Most major elements were found to have minimal leachability, and the material satisfied the environmental limits for its disposal at landfills for inert and non-hazardous wastes.

Optimizing calcium materials for minimizing arsenate phytoavailability in upland arable soil based on geochemical analysis

This study aimed to evaluate the reducing effects of calcite and phosphogypsum on arsenate [As(V)] availability to plants and elucidate the mechanisms of As(V) immobilization. The concentration of available As(V) to plants in upland arable soils with 1% calcite and phosphogypsum decreased to 17.4% and 36.9%, respectively, compared to the control. As(V) phytoavailability depends on the soil pH and calcium materials. The process of stabilizing As(V) (F3; anion exchange) with phosphogypsum is faster and easier compared to that with calcite (F4; bind to carbonate), but it results in a less stable form. New Ca-As(V) minerals (Ca₅2(HAsO₄)x(AsO₄)∙yH₂O, Ca₅H₂x(AsO₄)∙yH₂O, or Ca₃2(AsO₄)∙10 H₂O) were identified in X-ray diffraction (XRD) patterns with calcite treatment. Precipitation, the primary mechanism induced by calcite, was activated at a soil pH above 8.0. Based on the deconvolution of calcium and sulfur X-ray photoelectron spectroscopy spectra and the peak shift in the XRD pattern in phosphogypsum, the substitution in which SO₄^2- is exchanged with HAsO₄^2- is the primary mechanism for As(V) immobilization. Substitution induced by phosphogypsum is a suitable reaction in upland arable soils, the predominant form of As(V) in the soil, with a pH range of 5-7.

Fluoride immobilization and release in cemented PG backfill and its influence on the environment

Waste recycling must consider secondary pollution, which is affected by recycling methods. Cemented phosphogypsum (PG) backfill is a cost-effective method for PG recycling. However, due to dynamic environmental conditions, the impurity fluoride is challenging to stabilize. In this study, we investigated the immobilization and release of fluoride and its influence on backfill strength. The results showed that the fluoride was temporarily immobilized by PG. However, when the binder was mixed with PG to make the backfill, immobilized fluoride was re-released into the backfill slurry due to the increased pH caused by binder hydration. Therefore, simply converting fluoride into CaF precipitation cannot avoid the risk of fluoride exceeding the Chinese standard (GB8978-1996) (10 mg/L). Furthermore, fluoride deteriorated strength development by inhibiting binder hydration and weakening the backfill structure. The fluoride content in the slurry, rather than in PG, directly affected the backfill strength. Considering the recycling of PG as aggregate for backfill, fluoride should be removed in advance or immobilized in other low-solubility forms instead of CaF precipitation. These results were of great significance for the large-scale resource recycling and safety management of PG.

Continuous and simultaneous conversion of phosphogypsum waste to sodium sulfate and potassium sulfate using quaternary phase diagram

In this present work, the transformation of the Moroccan phosphogypsum (PG) waste, considered a potential source of sulfate, into potassium sulfate compound could help reduce environmental impact and create a new value chain for the phosphate industry. Generally, solid-liquid equilibria are frequently applied in chemical industries. They are a valuable aid in visualizing the precipitation, separation, and purification of a solid phase and the pathways by which crystallization can occur. This process aims to produce potassium sulfate (K₂SO₄), a high-value fertilizer, from sulfate solutions obtained after dissolving PG in a NaOH medium. The quaternary phase diagram Na⁺, K⁺//Cl^-, SO₄^2--H₂O at 25 °C was especially used to determine the operating conditions and the design of a crystallization process during the PG conversion into K₂SO₄. The Jänecke representation of this system enables the determination of the optimal trajectory in the phase diagram for the double decomposition reaction. X-ray fluorescent (XRF) and X-ray diffraction (XRD) techniques were conducted to identify the crystalline phases formed during our process. In summary, the results of this study could contribute to the development of a sustainable valorization PG. Furthermore, K₂SO₄ represents a good alternative to potassium chloride for chloride-sensitive crops.

In-situ remediation of phosphogypsum in a cement-free pathway: Utilization of ground granulated blast furnace slag and NaOH pretreatment

In-situ remediating phosphogypsum (PG) for cemented paste backfill (CPB) in the contaminated site is economic management for promoting sustainable developments in the phosphate industry. This study concerns the combined use of NaOH pretreatment and ground-granulated blast furnace slag (GGBFS) additives to promote the solidification/stabilization of PG with a lower carbon footprint pathway. According to physico-chemical analyses, the NaOH pretreatment effectively removed approximately 95% of F within the PG, which may originally be present as sparingly soluble fluorides or coexisting with silicates. The micro mineralogical characterization illustrates that the pretreatment can accelerate the early age hydration, with more hydration products observed, including calcium silicate hydrates and ettringite, effective F and P retention candidates. Whereas the incorporation of GGBFS plays an essential role in promoting the generation of additional cement hydrates at the following stages. The macro mechanical performance analysis indicates that the mixtures of pretreated-PG-OPC-GGBFS exhibit an excellent mechanical performance satisfying the design criteria. Subsequent elemental mapping and toxicity characteristic leaching procedures demonstrate that this combined approach has a competitive F and P immobilization ability compared to the typical OPC binder and individual GGBFS addition. The newly formed phases effectively controlled the concentration of F and P through adsorption, incorporation, or encapsulation. Objectively, the proposed methodology can be a promising candidate pathway for extrapolating the in-situ immobilization of PG. This study opens up new perspectives for synergetically recycling PG and GGBFS in a profitable and low carbon footprint way.

Geochemical and mineralogical characterization of phosphogypsum and leaching tests for the prediction of the mobility of trace elements

Phosphoric acid manufacturing generates large amounts of phosphogypsum (PG); a by-product generally disposed in the surface or evacuated in the seawater without any pretreatment. Phosphogypsum may host non-negligible amounts of valuable elements such as rare earth elements (REEs), which are critical elements on the global market. Surface disposal of PG may be a sustainable option to allow further processing in order to recover valuable elements. However, surface disposal exposes PG to atmospheric conditions (e.g., water, oxygen) which may increase their reactivity and accelerate the release rate of chemical species. This study aims to evaluate the trace element release rate from PG at atmospheric conditions. The studied PG samples were collected from a Moroccan phosphate treatment plant. The samples were characterized for their (i) chemical composition using inductively coupled plasma optical emission spectrometry (ICP-OES) for major elements and inductively coupled plasma mass spectrometry (ICP-MS) for trace elements; (ii) mineralogical composition by X-ray diffraction (XRD), scanning electron microscope equipped with energy-dispersive spectrometer (SEM-EDS), laser-induced breakdown spectroscopy (LIBS), and the mineral chemical composition was analyzed by electron probe microanalyzer (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS); and (iii) chemical species release rate using leaching tests over 24 h at 25 and 60 °C. Chemically, the PG samples were mainly composed of Ca (23.03-23.35 wt.%), S (17.65-17.71 wt.%), and Si (0.75-0.82 wt.%), and non-negligible amounts of trace elements: REE (344-349 ppm), Cd (3.5-7.4 ppm), U (9.3-27.4 ppm). Mineralogically, the PGs are mainly formed by gypsum (94.2-95.9 wt.%) and quartz (1.67-1.76 wt.%). In terms of chemical species release, the PGs showed a higher reactivity at 60 °C compared to room temperature with a higher release rate at the beginning of the leaching tests. Quantitatively, the PG samples released 3.57-4.11 µg/L/day of REE, 3.18-17.29 µg/L/day of U, and 1.67-5.49 µg/L/day of Cd. Based on the leaching results, we concluded that the trace elements (e.g., U, Cd, REE) are incorporated in PG crystal lattice, which may explain their low concentrations in the leachates. Consequently, total digestion of PG matrix is required to solubilize REE.

Preparation of phosphogypsum-copper smelting slag-based consolidating body with high compressive strength

Phosphogypsum (PG) is an industrial waste residue produced during the production of phosphoric acid through the wet process. With strong acidity and a large amount of toxic impurities, PG is difficult to reuse. In this study, the solidified body (PG-S) was made by mechanical compression of the mixture of PG, copper smelting slag (CSS), CaO, NaOH, and water. Results indicate that the composition of the material phases in the PG-S samples changed with hydrated calcium silicate and amorphous silicate derivatives were formed during the reaction; Fe and Ca in the material were transformed; and the prepared geopolymer material had a dense internal structure with the materials being cemented to each other. The highest compressive strength of PG-S cured for 28 days could reach 21.3 MPa with a fixation efficiency of PO₄^3-and F^-reaching 99.81 and 94.10%, respectively. The leaching concentration of heavy metals of the PG-S cured for 28 days met the requirements of the Comprehensive Wastewater Discharge Standard (GB 8978-1996). The simulation results of the geochemical model verified the feasibility of the whole immobilization process from the thermodynamic point of view. This work directly uses copper smelting slag and phosphogypsum for coupled immobilization/stabilization treatment not only to achieve the immobilization of pollutants in both solid wastes but also to obtain colloidal masses with certain compressive strength, which also provides a new option for resource utilization of phosphogypsum and copper smelting slag. This work also shows great potential in turning the actual mine backfill into cementitious material.

Economic losses related to the reduction of Posidonia ecosystem services in the Gulf of Gabes (Southern Mediterranean Sea)

In the early XXth century, the Gulf of Gabes in SE Tunisia used to host the most extended Posidonia oceanica seagrass beds in the Mediterranean basin and was a highly productive hotspot of benthic species. Since the 70's, >500 million t of wet toxic phosphogypsum discharges from a fertilizer industrial complex have led to the gradual loss of ∼90 % of its initial surface. This drastic shrinkage is accompanied by significant value losses originated from the direct and indirect-use services of which the most important ones are small scale fisheries and carbon storage function. Using market valuations of a number of services we estimate economic losses at 105 million € in 2014 (∼915€/ha), i.e., around 115 % of the added value of the gabesian fertilizer factories for the same year. Value losses should increase in the near future in relation with the COP26 agreements which boosted the open carbon credit market. Without actions to reduce negative production externalities caused by the fertilizer industry in the Gulf of Gabes it would not be possible to recover Posidonia ecosystems in this region leading to further economic, ecologic, and cultural losses.

Synergistic removal of phosphorus and fluorine impurities in phosphogypsum by enzyme-induced modified microbially induced carbonate precipitation method

Microbially induced carbonate precipitation (MICP) technology is difficult to be used for phosphogypsum (PG) treatment because the pH of PG is too low to be suitable for the growth of some bacteria. When acidophilus bacteria are used to treat PG, their low mineralization rate leads to low removal of the impurities. Based on the above problems, this study reports a new method that uses enzyme induced carbonate precipitation (EICP) modified acidophilus bacteria solution to remove phosphorus (P) and fluorine (F) in PG. Five kinds of mixtures of MICP and EICP (ME) were used to leach the PG column, and its mechanism was discussed. The results show that when the ratio of MICP to EICP is 2:1, the removal ratio of P and F is the highest, which reaches 72.87-74.92%. Compared with the single traditional bacillus solution or single acidophilic bacteria solution, the impurity removal ratio of the ME21 (MICP:EICP=2:1) mixture is increased by about 13%. The good acid resistance of the urease enzyme and acidophilic bacteria improves their growth and activity, thus increasing the biomineralization rate by about 22%. Additionally, the ME treatment is 30% cheaper than the traditional binder treatment. Therefore, this new treatment is a low-cost and environmentally friendly method.

Removal performance and mechanisms of Pb(II) and Sb(V) from water by iron-doped phosphogypsum: single and coexisting systems

The serious environmental risks caused by Pb(II) and Sb(V) co-contamination increase the need for their efficient and simultaneous removal. In this study, the remediation feasibility by Fe-doped phosphogypsum (FPG) was elucidated for single systems with Pb or Sb pollutant and coexisting systems with both from water. As for single systems, Fe doping effectively enhanced the Pb(II) removal performance by phosphogypsum (PG) at low Pb(II) concentrations of below 100 mg/L via the combination of precipitation and complexation. The optimal removal rate of Sb(V) by FPG increased by 2.08-3.31 times as compared to that of by PG (10-120 mg/L), mainly due to the strong affinity of iron hydroxyl (≡Fe-O-H) towards Sb(V). Compared with the single systems, the coexistence greatly enhanced the Pb(II) and Sb(V) removal performance by FPG, and the interaction behavior between Pb(II) and Sb(V) on the FPG was concentration dependent. Briefly, the sorption of FPG controlled the elimination of low coexisting concentrations of Pb(II) and Sb(V), whereas the co-precipitation process between Pb(II) and Sb(V) predominated with high ions concentration. The significant synergistic effects were found during the removal of Pb(II) and Sb(V) on FPG in the coexisting system, which mainly attributed to precipitation, bridging complexation and electrostatic attraction. Considering the advantages such as facile preparation, low cost and high removal capacity, FPG is a promising material to uptake Pb(II) and/or Sb(V) from contaminated water.

Highly-efficient fluoride retention in on-site solidification/stabilization of phosphogypsum: Cemented paste backfill synergizes with poly-aluminum chloride activation

Phosphogypsum (PG) is a massively generated hazardous by-product in the phosphorus industry. Large-scale, efficient, profitable on-site recycling is an emerging topic for promoting sustainable phosphorus circularity and mitigating potential human exposure. In this work, we integrated a green and low-cost additive polymeric aluminum chloride (PAC) into the binder design of PG immobilization. The overall experimental results illustrate that the incorporation of PAC can efficiently promote the cement hydration reaction, with amorphous phases increased from 25.9 wt% (control group) to 27.5 wt% (with 2 g/L PAC). The macro-investigations indicate that the PAC optimized the porosity and mechanical properties of specimens, facilitating a mechanically stable solidified matrix for extrapolating its field engineering application. The detailed micrographs and elemental mapping demonstrate that apart from co-existing with the hydration products, the PAC agent plays a role in the immobilization of fluoride. Herein, the combined optimization enhanced the fluoride retention capacity due to the precipitated additional hydration products, comparable encapsulation, and high adsorption ability of PAC agents. Therefore our design of PAC-augmented binders can open up a new field of PG on-site solidification/stabilization application that ensures efficient fluoride retention in a technically feasible and financially profitable methodology.

Effects of dicyandiamide, phosphogypsum and superphosphate on greenhouse gas emissions during pig manure composting

This study investigated the effects of dicyandiamide, phosphogypsum and superphosphate on greenhouse gas emissions and compost maturity during pig manure composting. The results indicated that the addition of dicyandiamide and phosphorus additives had no negative effect on organic matter degradation, and could improve the compost maturity. Adding dicyandiamide alone reduced the emissions of ammonia (NH₃), methane (CH₄) and nitrous oxide (N₂O) by 9.37 %, 9.60 % and 31.79 %, respectively, which was attributed that dicyandiamide effectively inhibited nitrification to reduce the formation of N₂O. Dicyandiamide combined with phosphogypsum or superphosphate could enhance mitigation of the total greenhouse gas (29.55 %-37.46 %) and NH₃ emission (18.28 %-21.48 %), which was mainly due to lower pH value and phosphoric acid composition. The combination of dicyandiamide and phosphogypsum exhibited the most pronounced emission reduction effect, simultaneously decreasing the NH₃, CH₄ and N₂O emissions by 18.28 %, 38.58 % and 36.14 %, respectively. The temperature and C/N content of the compost were significantly positively correlated with greenhouse gas emissions.

Characterisation of Zinc-bearing sulphate phases formed during the synthesis of phosphoric acid and Zinc removal by the ligands of Opuntia ficus-indica

Phosphogypsum (PG) is a solid waste generated from phosphate fertilisers industries. It represents a serious threat to the aquatic and terrestrial environment because of its acidity and its high content in heavy metals and radionuclides. The aim of this work is to describe the formation of PG during the synthesis of the phosphoric acid, the entrapment of Zinc (Zn) in PG and its lixiviation in presence of a natural organic matter extracted as powder from the cladodes of Opuntia ficus-indica (OFI) using physical and chemical characterisation techniques such as FTIR, XRD, SEM-EDX, laser diffraction, and AAS. The formation of PG mainly occurs in the pH range between 4.6 and 3 and it accompanies the transformation of H₂(PO₄)^- into phosphoric acid H₃PO₄. The maximal Zn incorporation within the PG was reached at pH 6 and decreased progressively with pH. Zinc was found to have a great tendency to migrate from PG particles to OFI's suspensions since a maximum Zn removal percentage of 93% was achieved.

Retention of phosphorus and fluorine in phosphogypsum for cemented paste backfill: Experimental and numerical simulation studies

The solidification/stabilization of phosphogypsum using cemented paste backfill (OCPB) provides a low-cost and alternative in-situ technique for recycling phosphogypsum stockpiles. But the OCPB is far from obtaining steady states in which the pollutants would redistribute as a response to dynamic environmental conditions. Further, the associated chemical interactions and the mineralogy information of the solubility-controlling phases of contaminants (fluorine and phosphorus) have not been thoroughly studied or fully understood. In this study, a framework coupling the chemical, mineralogical, and morphological analyses is used to determine the fluoride and phosphate retention mechanisms of immobilized OCPB. Then the pH-dependent leaching tests and numerical simulation is applied as a useful tool to identify the minerals controlling stabilized OCPB leaching behavior. The overall findings proved that aluminate-rich calcium silicate hydrates play an essential role in fluoride and phosphate retention. Both experimental and simulational acid neutralization and leaching curves indicate that the cementitious matrix works as a strong buffering material ensuring high pH conditions that are necessary for fluorine and phosphorus retention. Although discrepancies were observed in absolute fluorine and phosphorus leaching values at highly acidic conditions, the simulations are able to describe highly amphoteric leaching behavior. The simulation suggests that the aluminum species and calcium phosphates governed the solubility of fluorine and phosphorus, respectively. The results of this work would have implications for predicting the leaching behavior of OCPB in detrimental and multiple environments.

Flotation purification of waste high-silica phosphogypsum

High-silica phosphogypsum (PG) is a kind of industrial by-product with great utilization potential. However, it is difficult to reuse PG directly due to the related gangue minerals (e.g., SiO₂), and thus efficient purification is required to allow its further applications. Herein, a typical high-silica phosphogypsum waste was purified by a new "reverse-direct flotation" method. The organic matters and fine slimes were removed by reverse flotation, and then, the silica impurity was removed by direct flotation. Via the closed-circuit flotation process, the whiteness of the PG concentrate is improved from 33.23 to 63.42, and the purity of gypsum in the PG concentrate increases from 83.90% to 96.70%, with a gypsum recovery of 85%. Additionally, the content of SiO₂ is significantly reduced from 11.11% to 0.07%. In-depth investigations suggest that the difference in the floatability of gypsum and quartz is prominently intensified by flotation reagents at pH = 2-2.5, and thus leads to good desilication performance. Further characteristics of the PG concentrate prove that impurities have been well removed, and the PG concentrate meets the requirement of related standards for gypsum building materials. The flotation method reported here paves the way for the purification of high-silica phosphogypsum, which can be extended to the purification and value-added reutilization of other industrial solid wastes.

Evaluating the survival of Aspergillus niger in a highly polluted red soil with addition of Phosphogypsum and bioorganic fertilizer

Phosphate-solubilizing fungi (PSF) can enhance P release from phosphate minerals to immobilize heavy metals. However, this promotion substantially depends on their survival in highly polluted soils. The aim of this study was to investigate the survival of PSF after addition of phosphogypsum (PG) and bioorganic fertilizer (BF) in the soil with coexistence of multiple heavy metals, e.g., Pb, As, Cd, Sb, etc. Addition of typical PSF (Aspergillus niger) did not promote the formation of pyromorphite (the most stable form of Pb), possibly due to the buffering effect of the soil (the secreted oxalic acid was neutralized) and limited P supply. Meanwhile, despite that A. niger has high tolerance to heavy metal stress, its survival was significantly declined due to the deficiency of available P. It was also shown that PG, as the major by-product in phoschemical industry, still has relatively high available P compared with common natural soils. PG addition dramatically increased available P (up to 93.87 mg/kg) and the subsequent fungal growth. However, sole PG did not promote the formation of pyromorphite, probably as the abundant Fe²⁺ and Mn²⁺ prevented the contact between PO₄^3- and Pb²⁺ in the soil system. The enhanced soil respiration after addition of BF and PG confirmed the promoted microbial activity (elevated to 3465.58 μg C kg h^-1). This study showed PG's potential as P source for both microbial growth and heavy metal remediation in soil system. A combination of PG, A. niger, and BF can hence achieve long-term bioremediation of heavy metals.

Recycling of phosphogypsum and red mud in low carbon and green cementitious materials for vertical barrier

MgO activated slag and bentonite (MASB) slurry is a new and promising vertical barrier material along with excellent performances. Some solid wastes, such as phosphogypsum (PG), red mud (RM), fly ash and so on, show a positive effect on the performances of alkali activated slag. However, few studies focus on the recycling of these solid wastes in the system of MgO activated slag. The purpose of this paper is to study the incorporation of phosphogypsum and red mud on the mechanical property, permeability and hydration process of MASB slurry. The results showed that the addition of PG could significantly improve the mechanical strength and anti-permeability of the MASB slurry at early age (7 days), where the unconfined compressive strength (UCS) increased from 793.1 kPa to 1395.7 kPa and the permeability coefficient declined from 16.1 × 10^-7 cm/s to 1.7 × 10^-7 cm/s. In contrast, the introduction of RM had some negative effects on its macroscopic properties, resulting the UCS decreased to 580.4 kPa and the permeability coefficient rose to 25.9 × 10^-7 cm/s at 7 days. The ettringite formed in the PG blended MASB slurry led to a notable increase in the absolute solid volume, which could satisfactorily fill the pores and block the pore channels. The combined addition of RM and PG had a synergistic effect on the promotion of hydration process and optimization of the pore structure, contributing to establish a low permeability and high mechanical strength matrix. The overall findings indicate that the use of solid wastes in the MASB slurry can not only improve its engineering properties, but also promotes its sustainability and economical efficiency, holding a great potential for popularization and application.

Surface modification of phosphogypsum and application in polyolefin composites

Phosphogypsum (PG) is one of solid wastes with large amount of yield and serious pollution, which has attracted wide attention. The aim of this study is to investigate filling performance of PG on polypropylene (PP) or high-density polyethylene (HDPE) matrix. In this work, PG was calcined initially to improve whiteness and fix impurities. X-ray diffraction (XRD) results showed that after calcined at 500 °C, the PG phase changed from CaSO₄·2H₂O to CaSO₄. The modification effects of the three modifiers were evaluated by Fourier transform infrared spectra (FTIR), oil absorption value, water floatability, and contact angle analysis. The effects of weight fraction of PG in PP and HDPE matrix on mechanics and morphology were observed by tensile test, impact test, and scanning electron microscope. Scanning electron microscope (SEM) showed that modified PG can be dispersed uniformly in the matrix at low filling content. With the increase of PG filling content, the analysis of mechanical properties showed that the tensile strength of HDPE matrix increased, while the tensile strength of PP matrix decreased gradually. The impact strength of HDPE matrix would decrease, but the impact strength of PP matrix increased first and then decreased. Compared with calcium carbonate (CC), the mechanical properties of HDPE filled with PG performed better. The apparent density showed that polymer composites filled with PG have the characteristics of light weight.

Effects and microbial mechanisms of phosphogypsum and medical stone on organic matter degradation and methane emissions during swine manure composting

The degradation of organic matter (OM) and CH₄ emissions during composting greatly influence the composting efficiency and greenhouse effect. This study evaluated the effects of adding phosphogypsum (PPG) and medical stone (MS) on OM breakdown, CH₄ emissions, and their underlying mechanisms. MS accelerated the breakdown of OM in the early composting stage, whereas PPG increased it in the cooling and maturation periods. At the ending of composting, humification was also significantly promoted by PPG and MS (P < 0.05). Moreover, MS and PPG reduced CH₄ emissions by 27.64% and 23.12%, respectively, and significantly inhibited the activities of methanogens in terms of their abundance (mcrA) and composition (dominant genera such as Methanobrevibacter, Methanocorpusculum, and Methanothermus) (P < 0.05). Interestingly, MS enhanced the activity of enzymes and bacterial metabolism related to OM degradation in the early composting stage, whereas PPG promoted them during the cooling and maturity stages. MS and PPG inhibited the activities of enzymes related to CH₄ release during the cooling and maturity stages. Therefore, PPG and MS may have influenced OM degradation and CH₄ releases during composting via changes in bacterial metabolism and enzyme activity levels. PPG and MS could have altered the activities of methanogens to influence the transformation of carbon and CH₄ emissions according to network analysis and partial least-squares path modeling analysis. These findings provide insights at the molecular level into the effects of adding PPG and MS on OM degradation and CH₄ emissions during composting, thereby facilitating the application of PPG and MS in composting systems.

Leaching of rare earth elements from phosphogypsum

High amounts of phosphogypsum (PG) are generated in the production of phosphoric acid. Previous literature demonstrates that obtaining rare earth elements (REE) from PG is a promising alternative to managing this waste. However, the reported leaching efficiencies are low in most cases, or drastic leaching conditions are required. Therefore, this work aimed to study the leaching conditions of REE from PG to obtain high leaching efficiency values. Initially, a 2⁴ factorial experimental design investigated the factors that affect the conventional acid leaching of REE from PG (leaching acid (citric and sulfuric acid), solid/liquid ratio, acid concentration, and temperature). Better leaching efficiency values of the sum of all REE (62.0% and 89.7% for citric and sulfuric acid, respectively) were obtained using an acid concentration of 3 mol L^-1, solid/liquid ratio of 1/20 g mL^-1, and temperature of 80 °C. Subsequently, the experiments optimization, performed through a central composite rotational design, indicated that the maximum leaching efficiency was achieved using a sulfuric acid concentration of 2.9 mol L^-1, solid/liquid ratio of 1.7/20 g mL^-1, and 55 °C. Under these conditions, the leaching efficiency of the sum of all REE was 90.0%. Leaching kinetics results showed that the equilibrium was reached in about 20 min for most REE. The mechanism investigation suggested that surface chemical reaction and diffusion through the boundary layer controlled the leaching.

Effect of soil capping depth on phosphogypsum stack revegetation

Phosphogypsum is a by-product of the phosphorus fertilizer production process and is typically stacked at the production sites. These stacks can potentially pose environmental hazards, which can be substantially reduced through reclamation by capping with soil and revegetation upon decommissioning. We conducted a study on a phosphogypsum stack using five soil capping depths (8, 15, 30, 46, 91 cm), an uncapped treatment, and five vegetation treatments (monocultures of four grass species Agrostis stolonifera L., Festuca ovina L., Deschampsia caespitosa (L.) Beauv., Agropyron trachycaulum (Link) Malte ex H.F. Lewis and one mix of the four species with Trifolium hybridum L.) to assess plant growth, health, rooting characteristics, and trace element uptake. Cobalt and nickel concentrations in plant tissue from plots with ≥ 15 cm soil capping were within ranges found at reference sites, whereas fluorine was three times elevated. Vegetation cover was significantly greater on capped than uncapped plots, being greatest for Agropyron trachycaulum (26%) and Festuca ovina (26%). Capping depths ≥ 15 cm had greater cover, biomass, and healthy plants than the 8 cm cover. Soil water content was similar in the 15-46 cm capping depth, with the lowest in the 91-cm caps. Fluorine, cobalt, and nickel were elevated in select plant tissue samples on the research plots relative to references, and cap depth affected tissue fluorine and cobalt concentrations but not nickel. Concentrations of these trace elements were lower than maximum tolerable levels for animal consumption. From this 5-year study, Agropyron trachycaulum and Festuca ovina and a soil cover depth of ≥ 15 cm are recommended for reclamation of phosphogypsum stacks.

Industrial processing of phosphogypsum into organomineral fertilizer

The purpose of this study is to solve the problem of industrial waste storage. Waste from the production of mineral fertilizers is considered in this study (as an example-phosphogypsum LLC «Industrial group «Phosphorit»). Waste storages on a landfill have a significant negative impact on the environment. This fact has been confirmed by studies of wastewater from the dump (drainage water). Phosphogypsum utilization as a mineral component of an organic-mineral mixture without preliminary purification is the most promising approach. In this case, the joint utilization of lignin sludge as an organic component is required. The absence of excess content of toxic elements and the presence of plant nutrients in the required quantities was established as a result of laboratory studies of soil additive. In addition, the authors noted an increased content of stable strontium in phosphogypsum. Localization of strontium in the soil mixture is ensured by fractional application. Strontium is replaced by covalent calcium when the components are absorbed by the plants. The developed soil mixture should be considered as a single-use organomineral additive with prolonged action. Forestry activities, reclamation of disturbed lands, slopes of highways and landfills of solid municipal waste are promising areas of implementation.

Radioactivity in Future Phosphogypsum: New predictions based on estimates of 'Peak P' and rock phosphate resources

Global food supplies currently depend on producing inorganic P fertilisers from a finite reserve of rock phosphate (RP). P fertilisers are themselves significant pollutants but their production from RP also leaves a phosphogypsum (PG) by-product that is sufficiently radioactive that its reuse is restricted. PG is mostly accumulated in open 'stacks' that make up a significant proportion of all Technologically Enhanced Naturally-Occurring Radioactive Material (TENORM) waste. Using lower and upper estimates of current RP reserves, historic production, and Hubbert's logistic function-based 'peak theory', lower and upper boundaries for possible RP production were predicted to the year 2100. The 'low' boundary scenario had a production peak of c.350 Mt/a RP in c.2050 followed by a steep decline. The 'high' boundary scenario had a production peak of c.1200 Mt/a RP in about 2090. Future trends in P demand for food production were used to predict a possible, demand-driven, RP production scenario until 2100 which peaked at a demand of c.620 Mt/a RP and was within possible production boundaries. An RP:P ratio of 5.62:1 and PG:P fertiliser ratio of 4:1 was used to calculate that this predicted demand-driven scenario would ultimately produce nearly 350 Mt/a of PG and a cumulative total of c.30 Gt by 2100. Average PG activity concentrations of ²²⁶Ra (650 Bq/kg), ²¹⁰Po (300 Bq/kg) and ²³⁰Th (100 Bq/kg) give a total of c.30 PBq radioactivity in this by-product. Humanity is faced with a phosphorus dilemma - if the low production scenario unfolds it threatens food security but if predicted demand for P is met from RP the environmental challenges arising from P fertiliser use will be profound and exacerbated by a significant radioactive waste challenge. The estimates reported here show that studies of environmental radioactivity have a role to play in debates about P resources and global food security.

Exploring the microbial community inhabiting the phosphogypsum stacks of Huelva (SW SPAIN) by a high throughput 16S/18S rDNA sequencing approach

Around 100 Mt of phosphogypsum (PG) have been deposited in large stacks on the salt marshes of the Tinto River estuary in Huelva (SW Spain), covering about 1000 ha. These stacks contain extremely acidic water (pH < 2) with high concentrations of pollutants which can cause emissions into their surroundings, generating important environmental concerns. Despite many chemical, geological or hydrological studies have been conducted to characterize the PG stacks of Huelva, the microbial community inhabiting this extreme environment remains unexplored. Using a 16S/18S-rRNA-high throughput sequencing approach, we have uncovered the main taxonomic groups able to live in the acidic metal-contaminated water, which is in direct contact with the PG, demonstrating for the first time the existence of a huge diversity of microbial species in these extreme conditions. In addition, the physicochemical characteristics of the water sampled have been analyzed. These studies have revealed that the most abundant bacteria found in two different leachate samples of the PG stacks belong to the genera Acidiphilium, Pseudomonas, Leptosprillum, Acidithrix, or Acidithiobacillus, typically found in acid mine drainage (AMD) environments, which in total represent around 50% of the total bacterial community. Biodiversity of eukaryotes in PG water is lower than that of prokaryotes, especially in the water collected from the perimeter channel that surrounds the PG stacks, where the pH reaches a value of 1.5 and the activity concentrations exceed 300 Bq L^-1 for ²³⁸U or 20 Bq L^-1 for ²¹⁰Po, values which are from four to five orders of magnitude higher than those usually found in unperturbed surface waters. Even so, an unexpected diversity of algae, fungi, and ciliates have been found in the PG stacks of Huelva, where chlorophyte microalgae and basidiomycetes fungi are the most abundant eukaryotes. Additional bioinformatics tools have been used to perform a functional analysis and predict the most common metabolic pathways in the PG microbiota. The obtained data indicate that the extreme conditions of these PG stacks hide an unexpected microbial diversity, which can play an important role in the dynamics of the contaminating compounds of the PG and provide new strains with unique biotechnological applications.

Metabarcoding data of prokaryotes and eukaryotes inhabiting the phosphogypsum stockpiles on the salt marshes of Huelva (SW Spain)

Around 100 Mt of phosphogypsum (PG) of extreme acidity and with high concentrations of heavy metals and radionuclides have been deposited on the salt marshes of the Tinto River estuary in Huelva (SW Spain) for more than forty years. The microbial community able to thrive in these adverse conditions remains totally unknown, despite the fact that it can highly influence the biogeochemical cycle of the phosphogypsum components and include new species with biotechnological interest. High throughput sequencing of 16S/18S rRNA encoding genes is a potent tool to uncover the microbial diversity of extreme environments. This data article describes for the first time the prokaryotic and eukaryotic diversity of two water samples collected in the Huelva phosphogypsum stacks. The raw amplicons of the 16S/18S rRNA maker genes for the two phosphogypsum samples and two reference samples (seawater and the Tinto River water) obtained after sequencing on MiSeq platform are provided. The operational taxonomic units (OTUs) obtained after the treatment and clustering of the obtained reads with the QIIME2 pipeline and their taxonomic assignation performed by comparison with the SILVA database are also presented to complete the information of the article “Exploring the microbial community inhabiting the phosphogypsum stacks of Huelva (SW, Spain) by a high throughput 16S/18S rDNA Sequencing approach”.

Harmless treatment technology of phosphogypsum: Directional stabilization of toxic and harmful substances

Phosphogypsum is one of the typical by-products of phosphorus chemical industry. As a strategic industry related to the national livelihood of China, phosphorus chemical industry has accumulated and produced a significant amount of phosphogypsum. In general, phosphogypsum contains approximately 80%-95% calcium sulfate dihydrate, and less than 5% toxic and harmful elements. In this paper, toxic and hazardous components in phosphogypsum were efficiently solidified and stabilized by highly targeted solidification and stabilization technology. Calcium carbide slag or lime was used as an alkali-base neutralizer of phosphogypsum, and polymeric ferric sulfate or polymeric aluminum chloride as a directional solidification stabilizer to analyze the leaching toxicity of the mixed powder in 1, 3, 5 and 15 days. The experimental results demonstrate excellent solidification and stabilization effect with the leaching pH of 6-9, the leaching concentration of P, F and heavy metals of less than 0.5 mg/L, 10 mg/L and 0.1 mg/L, respectively, which meets the requirements of relevant international standards. Mechanistic analysis indicates that the solidification and stabilization of toxic and hazardous substances in phosphogypsum is perfectly achieved owing to the generation, adsorption and encapsulation of insoluble substances. This technology can reduce the costs and difficulty in the phosphogypsum treatment, and has extensive application potentials.

Recycling phosphogypsum as the sole calcium oxide source in calcium sulfoaluminate cement production and solidification of phosphorus

Because the disposal of phosphogypsum (PG) can lead to serious contamination of the air, soil, and water, recycling of PG has attracted wide attention. This study investigated the effect and solidification of phosphorus in the production of calcium sulfoaluminate (CSA) cement using PG as the sole CaO source. The effects of three phosphorus impurities (Ca₃(PO₄)₂, CaHPO₄, Ca(H₂PO₄)₂) on the decomposition of CaSO₄, formation of minerals, microstructure of the clinker, and the hydration and mechanical properties of the cement were studied. Experimental results show that Ca₃(PO₄)₂ and Ca(H₂PO₄)₂ promoted the decomposition of CaSO₄ and the formation of clinker minerals with the increase in P₂O₅ content, whereas CaHPO₄ showed a promoting effect only when the P₂O₅ content was more than 1.5 wt%. The increase in phosphorus incorporation in Ca₂SiO₄ leads to the transformation of β-Ca₂SiO₄ to α'-Ca₂SiO₄ and then to Ca₇Si₂P₂O₁₆. The presence of three phosphates in the clinker enhanced the growth of crystal grains and the generation of a liquid phase. Compared with Ca₄Al₆SO₁₆ without phosphorus, the hydration reaction of phosphorus-bearing Ca₄Al₆SO₁₆ started later and ended earlier, and the reaction time was shorter. The presence of phosphorus impurities reduces the 1-day strength of CSA cement but does not affect the development of the 3-day and 28-day strengths. Considering environmental aspects, the solidification of phosphorus in the production of CSA clinker were quantified by measuring the distribution of elements. The results indicated that phosphorus is solidified by Ca₄Al₆SO₁₆, Ca₂SiO₄, and Ca₄Al₂Fe₂O₁₀, and Ca₂SiO₄ has a stronger ability to solidify phosphorus than the other two minerals. Ca₃(PO₄)₂ is more difficult to solidify than CaHPO₄ and Ca(H₂PO₄)₂. This study is of great significant to guide the large-scale clean utilization of PG in the production of CSA cement.

Research hotspots and trends of comprehensive utilization of phosphogypsum: Bibliometric analysis

Phosphogypsum is a by-product of the phosphate fertilizer industry. It is generally treated by stacking, which not only causes environmental pollution, but also wastes resources. Therefore, the harmless, comprehensive, and high-value utilization of phosphogypsum has attracted more and more scholars around the world. From the perspective of bibliometrics, this paper systematically and comprehensively describes the research progress, trends and hot spots of phosphogypsum resource utilization. This article is based on a scientific network database, and a total of 1067 articles from 1985 to 2020 were collected. Then, use VOSViewer software to perform co-occurrence, co-citation analysis and cluster analysis. The visual analysis results demonstrate that the research on the resource utilization of phosphogypsum shows the characteristics of rapid growth. The active countries are mainly China, Brazil, Spain and the United States. Four of the top 10 active organizations are from China. It systematically expounds the changes in research hotspots in this field at different stages and possible future research hotspots. Including the gradual attention to the preparation of phosphogypsum building materials, phosphogypsum adsorption materials, and radioactive elements in phosphogypsum; the extraction of rare earth elements from phosphogypsum; the use of phosphogypsum to prepare fertilizer or soil improvement. Research shows that phosphogypsum can be reasonably designed, transformed, and used in different fields.However, the radioactivity contained in phosphogypsum should be paid attention to because it has an impact on humans and the environment. We believe that this research provides a comprehensive and systematic overview for future research on phosphogypsum resource utilization.

Adding an appropriate proportion of phosphogypsum ensured rice husk and urea composting to promote the compost as substrate utilization

To explore the effectiveness of urea replacing poultry manure as the nitrogen source in the rice husk composting system, and to promote the utilization of compost products as substrates, 0%, 10%, 20%, and 30% of phosphogypsum were added respectively in the urea composting system, and were compared with the chicken manure composting (RCP0). Finally, the fermentation and maturation of RCP0 were achieved, but high EC value limited the utilization of compost products as the substrate. Urea, as an N source, could lower the EC value, but the C/N ratio was uncoordinated during the initial stage of composting. Adding an appropriate proportion of phosphogypsum could ensure a proper C/N ratio to promote smooth fermentation and enable the products to be ideal substrates. When the added proportion was 30%, the thermophilic stage was shortened significantly but this may increase heavy metals. 10%-20% were concluded to be the recommended proportion.