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Journal of Hazardous Materials 480 (2024) 135732 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Unveiling the organic nature of phosphogypsum foam: Insights into formation dynamics, pollution load, and contribution to marine pollution in the Southern Mediterranean Sea Radhouan Belgacem El Zrelli a,* , Sébastien Fabre b, Sylvie Castet c, Michel Grégoire c , Oussema Fersi d, Claudie Josse e, Anne-Marie Cousin c , Pierre Courjault-Radé c a International Environmental Consultant, Toulouse, France Institut de Recherche en Astrophysique et Planétologie (IRAP), Université de Toulouse, 14 Avenue Edouard Belin, 31400 Toulouse, France c Géosciences Environnement Toulouse (GET), Université de Toulouse, UMR 5563 CNRS/UPS/IRD/CNES, 14 Avenue Edouard Belin, 31400 Toulouse, France d National Institute of Oceanography and Applied Geophysics (OGS), Borgo Grotta Gigante 42/C, 34010 Sgonico, TS, Italy e UAR Raimond CASTAING, Université de Toulouse, 31400 Toulouse, France b H I G H L I G H T S G R A P H I C A L A B S T R A C T • This is the first detailed organic characterization of PGF. • PGF is artificially induced, surfactantstabilized, and ephemeral aqueous foam. • Short-chain amphiphilic OM and gypsum crystals are pivotal in PGF formation. • PGF’s amphiphilic nature and specific OM groups cause its high contaminant load. • PGF accumulates, carries, and disperses PG radiochemical contaminants in the sea. A R T I C L E I N F O A B S T R A C T Keywords: Phosphogypsum foam Organic characterization Amphiphilic organic matter Surfactant Southern Mediterranean Sea The foamability of dissolved phosphogypsum from the phosphate fertilizer factories of Gabes (SE Tunisia) is a spectacular phenomenon that has not yet been thoroughly studied. The main objective of this research was to investigate the organic properties of phosphogypsum foam (PGF) to understand its formation process, determine the origin of its enhanced radiochemical contaminants load, and identify its role in pollutants dispersion in marine environment of the Southern Mediterranean Sea. This study identified PGF as an unnatural, surfactantstabilized, and ephemeral aqueous foam. PGF-forming process comprises three main steps: (i) formation (through phosphogypsum dissolution), (ii) stabilization (facilitated by organic surfactants and gypsum crystals), and (iii) destabilization (geochemical (involving the dissolution of the PGF skeleton gypsum) and/or mechanical (influenced by wind and wave action)). The amphiphilic nature of PGF organic matter and the presence of specific organic groups are responsible for its high toxic contaminants load. PGF contributes, through its elevated * Corresponding author. E-mail address:

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(R.B. El Zrelli). https://doi.org/10.1016/j.jhazmat.2024.135732 Available online 5 September 2024 0304-3894/© 2024 Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies. Journal of Hazardous Materials 480 (2024) 135732 R.B. El Zrelli et al. pollutants content and its ability to migrate far from its source, to the marine dispersion of industrial toxic radiochemical contaminants. It is therefore recommended to mitigate the environmental and health risks associated with PGF, including banning the discharge of untreated phosphogypsum and other industrial wastes into the coastal environment of Gabes. 1. Introduction small fraction recycled (̴ 15 %; [43]) in specific sectors (agricultural fertilizers, soil amendment for saline soils, road construction, building materials…; [46-50,43,51]). However, the majority of the produced PG (̴ 255 Mt.y−1) is stored in ‘wilderness’ stacks (e.g., Florida (USA; [52, 53]), Cubatão (Brazil; [54]), Sfax and Gafsa (Tunisia; [55,56]), Tébessa (Algeria; [57]), Nea Karvali (Greece; [58]), Qattinah (Syria; [59]), Huelva (Spain; [60,61]), Vasiliko (Cyprus; [62])) or directly dumped into aquatic environments without any treatment (sea or watercourses; e.g., Gabes (Tunisia; [63]), Jorf Lasfar and Safi (Morocco; [64]; Belahbib et al., [65]; [66]), Annaba (Algeria; [67]), Batroun (Lebanon; [68,69]), and São Paulo State (Brazil; [70])). Ecological, health, economic, and even social disasters/catastrophes are observed worldwide wherever the phosphate industry operates (e.g., Tunisia, Spain, Algeria, Lebanon, Greece, USA, Morocco, China…; [71,57,72,38,73–86,42,50,54,43,87]). Consequently, PG is not solely a problem of developing countries (Tunisia, Morocco, Algeria, Lebanon, Syria…) but also a global issue affecting even well-developed countries such as Spain, Canada, USA, Finland, Greece and China. Ultimately, relocating this polluting industry to third world ‘lawless’ countries or to countries with lax regulations will not resolve the environmental and health challenges associated with this industry in a world where pollution is a global concern. Since the 1970′s, the Central Gulf of Gabes acts as a repository for nearly all untreated solid and liquid industrial wastes of the Tunisian Chemical Group (Groupe Chimique Tunisien; GCT) factories (as well as other Gabesian chemical factories). This marine environment is characterized by a minimal agitation [88] with a moderate hydrodynamic regime [89]. These natural conditions significantly reduce the dissolution of gypsum, which represents the main component of phosphogypsum (more than 95 wt%; [90,41-43]). Knowing that PG constitutes the primary industrial waste released into the Gabesian littoral environment accumulating to over 500 million tons of untreated wet PG since 1972 (with daily quantities of ~ 30 ×103 tons; [39,91]). To facilitate the disposal of these substantial quantities into the Gabesian coastal environment, GCT dissolves the PG wastes in seawater, leading to the formation of the phosphogypsum foam ([92]; Fig. 1). The foamability of the dissolved PG, known as gypseous water, from the phosphate fertilizer factories in Gabes, is remarkable and visually striking, as depicted in Fig. 1. El Zrelli et al. [93] conducted comprehensive chemical and mineralogical characterizations of GCT’s PGF, nevertheless numerous aspects of this man-made foam remain unexplored and require further investigation. In this context, we employ chemical organic proxies for the first time to elucidate the PGF formation process, its elevated pollutants content, and its contribution to marine environmental pollution in the Central Gulf of Gabes (Southern Mediterranean Sea). Additionally, we discuss the health and the environmental impacts of PGF. For a long time, natural foams (e.g., foam nests of frogs [1,2], fish [3], insects [4,5], sea foams [6,7]), and man-made foams (e.g., soap foams [8,9], fire-fighting [10,11], beer [12,13] have intrigued the international scientific community. Consequently, numerous studies have focused on foams (e.g., [14-17]); each providing its own definition (Table 1). Generally, foam is defined as a colloidal dispersion [18] in the form of a gas cells enclosed in a liquid [19]. It forms at the surface of a liquid (water, oil, milk…) due to the dispersion of gas bubbles in a continuous liquid phase [18] or solid matrices [20]. Today, these foams find several important industrial applications (food processing, petrochemicals, fire protection, mineral flotation…; [21-24]) and constitute an entire science discipline known as ‘Aqueous Foam Science’, dating back to the 19th century, with its founding father being the blind Belgian physicist Joseph Plateau [25]. In spite of its great economic importance (agriculture, food industry, pharmaceutical industry…), the phosphate fertilizer industry is widely regarded as one of the most polluting industries globally [35-37]. This industrial activity is characterized by its diverse, hazardous, and substantial atmospheric emissions (e.g., Sulfur oxides (SO2), fluorinated gases, nitrogen oxides (NOx), ammonia gas (NH3), malodorous gases, dust…; [38]), liquid discharges (seawater used for the acid cooling system and gas washing…; EC, 2017; [39]), and solid waste outputs (phosphogypsum, sulfur dross, spent catalysts…; [40]). Phosphogypsum (PG) remains the primary mismanaged radiochemical waste by-product of the phosphate fertilizer industry globally [41-44]. Annually, approximately 300 Mt of PG are produced worldwide [45], with only a Table 1 Some definitions of foam. Foam definitions References Liquid foam is an example of soft matter (or a complex fluid) with a very well-defined structure. Agglomeration of gas bubbles separated from each other by thin liquid films. An impermanent form of matter in which a gas, often air, is dispersed in an agglomeration of bubbles that are separated from one another by films of liquid that is almost but not entirely water. Two-phase colloidal system in which the gas bubbles are dispersed in the continuous liquid phases. Temporary dilute dispersion of bubbles in the liquid, but on ageing, the structure gradually changes, and the bubbles transform into polyhedral gas cells with thin flat walls. Highly concentrated dispersions of gas (dispersed phase) in a liquid (continuous phase). Dispersion of gas bubbles in liquid or solid matrices. Dispersion of gas in a liquid or a solid, whereas the volume fraction of gas in the foam is mostly between 0.5 and 0.9. Structures of bubbles in contact. Liquid with gas cells enclosed. Dispersion in which a large proportion of gas by volume in the form of bubbles is dispersed in a liquid, solid or gel, hence forming closed cell structures. Dispersion of a high-volume fraction of gas bubbles is a small volume of liquid with some additives. Dispersion of gas in water. Bingham pseudoplastic fluids, which have yield stress, and the viscosity of foams decreases with increasing shear rate. Fluid mixtures of gas and liquid. Foam is a colloidal dispersion in the form of gas cells enclosed on a liquid. It’s formed on the surface of a liquid (water, oil, milk…) by a gas bubbles dispersion in a continuous liquid phase or solid matrices. [25] [14] [26] [27] [28] [17] [20] [29] 2. Materials and methods [30] [19] [15] Five samples of phosphogypsum foam, each weighing approximately 200 g, were collected from the littoral industrial discharge channel adjacent to the GCT factories in Gabes (Fig. 1). These samples were combined to create a composite sample of approximately 1 kg. The latter was then dried, homogenized, and stored according to the sampling protocol outlined by El Zrelli et al. [93]. In the laboratory, the PGF composite sample underwent successive extractions of its organic compounds. To ensure maximum extraction efficiency, we employed three organic solvents of varying polarities simultaneously: cyclohexane (Cyc, a nonpolar solvent), tetrahydrofuran (THF, a low-polarity solvent), and methanol (MeOH, a high polar [31] [32] [33] [34] This study 2 R.B. El Zrelli et al. Journal of Hazardous Materials 480 (2024) 135732 Fig. 1. Phosphogypsum foam layers of Tunisian Chemical Group in Chatt Sidi Abd Essalam Beach. solvent). For this purpose, 10 g of PGF was treated with 200 ml of each solvent (Cyc, THF, and MeOH). Then, the organic analyses of the different extraction products from the PGF sample (PGF-Cyc, PGF-THF, and PGF-MeOH) underwent separate analyses using five complementary techniques: Proton Nuclear Magnetic Resonance spectroscopy (1H NMR), High Performance Liquid Chromatography with Ultraviolet detection (HPLC/UV), Gas Chromatography-Mass Spectrometry (GCMS), Fourier Transform Infrared spectroscopy (FTIR), and Size Exclusion Chromatography (SEC) equipped with UltraViolet (UV) and Differential Refractometric (DR) detectors. (pH=2.65) with pure acetic acid, alongside a water/acetonitrile mixture (20:80 v/v) at the same pH (2.65). Ensuring the detection of the majority of compounds, the three PGF extracts were standardized to a concentration of 20 mg.ml−1 in acetonitrile. This technique is generally used to detect compounds with a molar mass of less than 3000 g.mol−1. 2.3. Gas chromatography-mass spectrometry The Gas chromatography-mass spectrometry was used to identify volatile and thermostable organic compounds. GC-MS analysis was performed using a Varian Saturn 2000 ion trap GC-MS system, coupled with a CP-3800 GC system equipped with a fused silica capillary DB-5MS column (30 ×0.25 mm, 5 % phenyl methylpolysiloxane, film thickness 0.25 µm). The analytical chromatographic conditions involved a temperature increase from 60 to 260 ◦ C at a gradient of 5 ◦ C.min−1, followed by 15 min of isothermal conditions at 260 ◦ C. A secondary gradient was applied, raising the temperature to 340 ◦ C at a rate of 40 ◦ C.min−1. The trap temperature was maintained at 250 ◦ C, while the transfer line temperature was set at 270 ◦ C. Mass scanning ranged from 40 to 650 m.z−1. Extracts were dissolved in their respective extraction solvents (Cyc, THF, and MeOH) at a concentration of 3 mg.ml−1, with 2 µl injected. We used two methods to identify the compounds in each extract. The first identification method involves comparing their retention index (RI) relative to C5-C24 n-alkanes obtained on a nonpolar DB-5MS column with literature values, while the second method entails comparing their mass spectra with those recorded in NIST/EPA/NIH Mass Spectral Library (NIST 08). 2.1. Proton nuclear magnetic resonance spectroscopy The Proton nuclear magnetic resonance spectroscopy technique involves the application of nuclear magnetic resonance principles, specifically targeting the hydrogen-1 nuclei within a substance’s molecules [94]. Its primary objective is to elucidate the molecular structure of the substance under investigation [94]. The 1H NMR analyses were carried out on a Bruker NMR-300 spectrometer operating at 300 MHz. This spectrometer uses the commonly used deuterated NMR solvents (CDCl3, THF-d8, and CD3OD) which allow to identify the most important molecules (polar or nonpolar) present in each PGF extract. A 5 mm diameter tube was filled with each sample, comprising 20 mg, along with 500 µl of deuterated solvent. 2.2. High performance liquid chromatography with ultraviolet detection The High performance liquid chromatography is a method using Ultraviolet detection (UV) to determine the chromatographic profile of UV–visible absorbing compounds. The HPLC analysis involved the utilization of a Thermo Scientific Dionex UltiMate 3000 Pump (Model HPG-3400RS), and an ultraviolet detector (UV-150 model detector) with a 280 nm detection wavelength. An RP-C18 column (Phenomenex) measuring 25 cm × 4.6 mm with 5 µm particle size, was used for separation at ambient temperature. Elution proceeded at a flow rate of 1.2 ml.min−1, utilizing a mobile phase composed of acidified water 2.4. Fourier transform infrared The Fourier transform infrared technique is used to determine the characteristic function of organic and inorganic compounds. The FT-IR analyses were obtained by using an Alpha-P spectrometer (Bruker). The measurements were performed on a Diamond Attenuated total Reflectance (ATR) Crystal, which was covered with a flow-through cell to facilitate sample analysis. The instrument was equipped with OPUS 3 R.B. El Zrelli et al. Journal of Hazardous Materials 480 (2024) 135732 Table 2 Extraction yields (% (w/w)) of cyclohexane (Cyc), tetrahydrofuran (THF), and methanol (MeOH) extracts from the phosphogypsum foam (PGF). PGF (%) Cyc THF MeOH 0.19 5.18 2.47 Table 3 Identification of 1H NMR chemical shifts functional group in cyclohexane (Cyc), tetrahydrofuran (THF), and methanol (MeOH) extracts from phosphogypsum foam (PGF). (Sm: small peak; Md: Medium peak; Gr: Great peak). Type of protons Chemical shift δ (ppm) Proton Formula Methyl CH3 0.8 – 2 Methyl, Methylene, Methine CH3, CH2, CH 2–5 Hydroxy, amine, thiol Alkene Aromatic Aldehydes Carboxylic acid OH, NH, SH 1.5 – 5 –C H-C– HCHO COOH 5.5 – 7 7–9 9 – 10 10 – 12 PGF Cyc THF MeOH Sm (1.7, 1.5, 1.1) Md (0.8) Gr (1.28) Sm (4.05, 2.3, 2, Md (0.9) Sm (1.1, 1.6, 1,8, 1,9, 1.95) Gr (1.32) Sm (1.82) Md (1.63) Sm (2.1, 2.3, 2.4, 3.32, 3.40, 3.42, 3.7, 3.8, 3.9, 4.3,5.1) Sm (2.18, 2.26,) Md (2.02) Gr (4.23, 3.96) Md (5.1) Fig. 2. (A) HPLC chromatograms of cyclohexane (Cyc), tetrahydrofuran (THF), and methanol (MeOH) extracts from the phosphogypsum foam (PGF). (B) Infrared spectrums of cyclohexane (Cyc), tetrahydrofuran (THF), and methanol (MeOH) extracts from the phosphogypsum foam (PGF). 3. Results Sm (5.4, 5.3) Sm (8.1) Sm (9.8) 3.1. Extraction yields software (version 7.5, Bruker). All PGF extracts spectra were recorded from 400 to 4000 cm−1 with a resolution exceeding 4 cm−1, co-adding 32 scans, achieving a frequency data accuracy better than 0.01 cm−1, and at room temperature. The extraction yields of organic solvents (Cyc, THF, and MeOH) from PGF samples are presented in Table 2. THF exhibited a significantly higher extraction yield (5.18 %) compared to both polar (MeOH) and nonpolar solvents (Cyc). The order of decreasing performance in extraction yield is as follows: THF˃MeOH˃Cyc (Table 2). Furthermore, PGF contains a higher proportion of polar compounds (2.47 %) compared to nonpolar compounds (0.19 %). 2.5. Size exclusion chromatography 3.2. Proton nuclear magnetic resonance spectroscopy analysis The Size exclusion chromatography was used for separating compounds based on their size, specifically their hydrodynamic volume, which was determined by how effectively they pass through the pores of the stationary phase [95]. The SEC system that was used included an HXL PROD01N pre-column mounted in series with three Waters Styragel columns (HR1 (separation range 100–5000), HR3 (separation range 500–30000), and HR4 (separation range 5000–600000)) with tetrahydrofuran as an eluent, at a flow rate of 1 ml.mn−1, and an injection volume of 100 µl. It was equipped with a differential refractometric detector (Optilab Rex Wyatt, T = 35 ◦ C) and a Varian UV detector (λ = 254 nm). To determine the average molar weights of sample compounds, mass calibration was done with increasing mass polymethyl methacrylate (PMMA) standards from Agilent (960, 1780, 4640, 10640, 26080, 66650, 265300, 675500 and 1568000 g.mol−1). All samples were dissolved in THF, filtered through 0.2 µm PTFE filters, and then analyzed in the SEC system under the same conditions as the PMMA standards. The molecular sample masses were calculated using the retention volume of compounds by ASTRA V Software (Wyatt Technology). For each extract, the weight average molecular weight (Mw), the number average Molecular weight (Mn), and the poly-dispersity index (Đ=Mw/Mn) were determined. Table 3 illustrates the identification outcomes of 1H NMR functional group chemical shifts in the three PGF extracts. The table reveals that traces of aromatics (8.1 ppm) and alkenes (5.3 and 5.4 ppm) are only detectable in the PGF-THF extract. Additionally, a broad peak at 5.1 ppm is solely present in the PGF-MeOH extract, indicating the likely presence of a hydroxy, amine, or thiol group. Nevertheless, for the methyl, methylene and methine functions (2–5 ppm), traces are observed in all three PGF extracts, with higher concentrations characterizing the PGF-MeOH extract (2.02, 3.96 and 4.23 ppm). While all three PGF extracts contain traces and medium quantities of methyl groups (0.8–2 ppm), major quantities are found only in the PGF-THF extract. 3.3. High performance liquid chromatography analysis The HPLC analysis of PGF-Cyc, THF, and MeOH extracts reveals detection by UV at 280 nm, as illustrated in Fig. 2.A. The HPLC spectra of these three extracts indicate the presence of few polar compounds (retention times: 1–10 min), and a larger quantity of nonpolar compounds (retention time: 40–60 min; Fig. 2). Furthermore, the chromatograms of the three PGF extracts exhibit distinct appearances between 40 and 60 min, with MeOH showing more absorbable compounds compared to the Cyc and THF. This discrepancy may stem from 4 Journal of Hazardous Materials 480 (2024) 135732 R.B. El Zrelli et al. Table 4 List of compounds identified in Gas Chromatography-Mass Spectrometry (GC-MS) analysis of cyclohexane (Cyc), tetrahydrofuran (THF), and methanol (MeOH) extracts from phosphogypsum foam (PGF). N◦ TR (min) Name Molecular formula Class/ Family 1 8.77 m-Cymene C10H14 Aromatic 2 9.14 3.3.5Trimethylcyclohexanone C9H16O Cyclic ketone Structure PGF Cyc THF MeOH £ £ 3 12.3 1.1′-Bicyclohexyl C12H22 Bicyclic saturated 4 13.68 3-Ethyl−5-methyl−1.2.4trithiolane C5H10S3 Sulfide £ 5 14.60 Hexathiane S6 Sulfide £ 6 15.72 2.6.10-Trimethyltetradecane C17H36 Alkane £ 7 15.95 Tetradecanoic acid C14H28O2 Fatty acid £ 8 16.84 Phthalic acid, butyl tetradecyl ester C26H42O4 Phtalate £ £ (continued on next page) 5 R.B. El Zrelli et al. Journal of Hazardous Materials 480 (2024) 135732 Table 4 (continued ) N◦ TR (min) Name Molecular formula Class/ Family 9 17.1 10 Structure PGF Heptadecanoic acid, 16methyl, methyl ester C19H38O2 Fatty acid ester 17.39 Hexadecanoic acid C16H30O4 Fatty acid £ 11 18.34 Cyclic-octaatomic sulfur S8 Sulfide £ 12 18.39 Methyl stearate C19H38O2 Fatty acid ester 13 22.96 Ethanediamide, N-(2ethoxyphenyl)-N′-(2ethylphenyl)- C18H20N2O3 Alcaloids Cyc THF MeOH £ £ £ £ differences in the chemical structures of their nonpolar compounds. albeit with different chemical functionalities: PGF-Cyc (1705 cm−1), PGF-THF (1702 cm−1), and PGF-MeOH (1698 cm−1). 3.4. Gas chromatography-mass spectrometry analysis 3.6. Size exclusion chromatography analysis Table 4 provides a summary of the compounds identified by GC-MS analysis of the three PGF extracts. Generally, the majority of identified compounds are nonpolar, with the presence of a few polar compounds. In total, 13 compounds were identified: 7 compounds for Cyc (1.1′bicyclohexyl; 2.6.10-trimethyltetradecane; tetradecanoic acid; phthalic acid, butyl tetradecyl ester; hexadecanoic acid; cyclic-octaatomic sulfur; and ethanediamide, N-(2-ethoxyphenyl)-N′-(2-ethylphenyl)-), 4 for THF (m-cymene; 3.3.5-trimethylcyclohexanone; 3-ethyl-5-methyl-1.2.4-trithiolane; and hexathiane), and 3 for MeOH (Heptadecanoic acid, 16methyl, methyl ester; hexadecanoic acid; and methyl stearate). However, the detection of hexadecanoic acid in both Cyc and MeOH GC-MS lists (Table 4) is attributed to its primary nonpolar function (majority), and its polar function (minority). Table 5 presents the calculated weight average molecular mass (Mw), number average molecular mass (Mn), and polydispersity index or dispersity (Đ= Mw/Mn) of the three studied PGF extracts. The PGF-THF extract contains compounds with higher Mw (3498 g.mol−1) and Mn (4694 g.mol−1) compared to those extracted by Cyc (777 and 230 g. mol−1, respectively) and MeOH (834 and 250 g.mol−1, respectively). The THF extract molecule size is around 20 times (Mn ratio) larger than the two other PGF extracts (Cyc and MeOH). Despite the high ratio of Mn, the PGF-THF molecules remain relatively small compared to polymers. The results obtained from all these five complementary techniques (1H NMR, HPLC/UV, GC-MS, FTIR, and SEC) provide an initial understanding of the nature of the PGF organic matter. In fact, it appears to be an amphiphilic (polar and nonpolar) organic matter composed of various organic families including aromatic, cyclic ketone, bicyclic saturated, sulfide, alkane, fatty acid, phthalate… Moreover, it is primarily formed by oligomers with molecular weights generally less than 5000 g.mol−1. 3.5. Fourier transform infrared spectroscopy analysis Fig. 2. B depicts the infrared spectra of Cyc, THF and MeOH extracts from the PGF. In the 3000 to 3500 cm−1 part of these spectra, the PGFMeOH extract exhibits the highest concentration of OH, NH, or SH, followed by PGF-THF, and finally the least concentrated, PGF-Cyc. Between 2800 and 3000 cm−1, the spectra of the three extracts are comparable, showing the presence of CH, CH2 and CH3. Lastly, in the 1650 to 1800 cm−1, carbonyl functions (C– –O) are observed in all samples, 6 R.B. El Zrelli et al. Journal of Hazardous Materials 480 (2024) 135732 Fig. 3. (A and B) Organic matter deposited on the surface of gypsum crystals in phosphogypsum (PG) wastes. (C) The beginning of phosphogypsum foam (PGF) formation in the form of the blackish-brown color organic suspension in gypseous water in the inside part of industrial channel. (D) The end of formation of PGF on the surface of gypseous water at the exit of the outside part of the industrial channel. 4. Discussion 4.1. Role of organic matter in the PGF formation process To prevent gypsum deposition at the entrance of the industrial littoral discharge channel and subsequent mouth clogging, PG is initially dissolved in seawater (utilized in sulfuric acid cooling process) forming gypseous water after its vacuum separation from phosphoric acid [93]. Then, the gypseous water is dumped into an artificial channel ⁓820 m long (within the GCT’s factories), where it is mixed with seawater used for sulfuric acid cooling process and gas washing (40 ×103 m3.h−1; [38]). Following this, the mixture is discharged into the sea via the extension of the channel outside the GCT’s factories (⁓665 m). In general, PG solubility increases with temperature and stirring time in the solvent medium [96]. Thereby, the dissolution of PG is facilitated by the strong artificial current within the industrial channel and the elevated temperature of the discharged seawater originated from sulfuric acid cooling process. The dissolution of gypsum releases the organic matter deposited on the surface of PG crystals (Fig. 3. A and B), and thus leads to the formation of an organic suspension in gypseous water which gives it this blackish-brown color (Fig. 3. C). Furthermore, the artificial agitation within the industrial channel generates air bubbles on the surface of the gypseous water solution (Fig. 3. C). As the solution moves through the channel towards its mouth, the PGF’s inter-bubbles liquid drains rapidly due the gravity difference between air and foam liquid [16], and thus a second less wet foam (Fig. 3. D) is progressively obtained from the first more wet foam (Fig. 3. C). Typically, during this foam formation stage (after the PGF drainage), and in the absence of stabilizing agents (e.g., proteins, organic matter, salts, solid particles…; [97-100]), the formed surface bubbles will burst when the thin liquid film separating them ruptures, causing to bubbles coalescence [16,101]. However, in our case, the amphiphilic nature of PGF organic matter allows it to adhere to the bubbles’ surface, rigidify them, accumulate them against each other, and thus forming a first wet foam on the surface of gypseous water (Fig. 3. C). Therefore, the PGF amphiphilic organic matter (surfactant) decreases the surface tension at Fig. 4. (A) Gypsum crystals skeleton at the bubble interfaces and fine gypsum crystals at the surface of phosphogypsum foam (PGF), in both backscattered (B) and secondary (C) electron image. bubbles’ interface [102], resulting in what we refer to as surfactant-stabilized foams [15,97]. The fine gypsum crystals within the PGF (Fig. 4. A) also synergistically contribute with surfactant organic molecules to the PGF stabilization, resembling other instances of foam stabilization by solid particles [103,98,104,100]. Fig. 4. B and C clearly illustrate that, in addition to the surfactant organic matter, the fine gypsum crystals on the PGF thin film surfaces contributes to consolidating the bubble interfaces, reduce the surface tension to avoid their rupture, and therefore the destruction of the PGF. At the end of this process, the foam gradually evolves from a less stable wet form to a much more stable less wet one (Fig. 3. D). However, as the PGF transitions from a gypsum saturated environment (industrial channel) to an undersaturated one (the open sea), the gypsum PGF skeleton crystals (Fig. 4. A) begin to dissolve, marking the onset of PGF geochemical destabilization. Moreover, waves and winds actively contribute to this destruction, hence we refer to it as PGF mechanical destabilization (Fig. 5). 7 R.B. El Zrelli et al. Journal of Hazardous Materials 480 (2024) 135732 Fig. 5. The various stages of geochemical and mechanical phosphogypsum foam (PGF) destruction during its marine migration: (A) Well- formed PGF on the seawater surface, (B) PGF degradation initiation, (C) Advanced PGF degradation, and (D) Complete PGF degradation. (Fig. 1). Kruglyakov et al. [104] elucidate the formation of such spectacular foams through the utilization of a mixture of nano/micro solid particles (small gypsum crystals in the PGF case; Fig. 4) and short-chain amphiphiles (oligomers in the PGF case; Table 5). The low density of the PGF’s organic matter (0.96 ± 0.01 g.cm−3) is less than the seawater average density (1.02–1.07 g.cm−3; [109]). The lightness allows PGF to float on the surface of seawater, to be carried long distances, and thus to disperse its elevated toxic pollutant load (trace elements, radionuclides, rare earth elements…) in the marine environment, after its degradation (geochemical and/or mechanical). Upon exiting the industrial waste discharge channel, the PGF can reach a thickness of ~8.5 to < 12 cm. Once in the marine environment, the PGF evolution depends primarily on the direction of the winds and, consequently, the amplitude of the waves. Specifically, if the winds blow (slowly) from the western sector, the PGF gradually migrates across the sea surface (owing to weak waves) over distances ranging from a few to several tens of kilometers, occasionally, reaching the Island of Djerba, as reported by numerous fishermen in Gabes and Zarzis (following a field survey carried out in 2013). During this marine migration, the PGF lightness allows its aerial part to function as a ‘sail’, facilitating forward movement aided by wind action. At this step, the two main antifoam agents (winds and waves) start to act, by breaking the thin films occurring between PGF bubbles, initiating mechanical foam destruction (destabilization). Occasionally, waves generated by fishermen’s boats can also contribute to this PGF mechanical destruction as they traverse the floating foam layers (Fig. 5. A). Throughout the PGF marine migration, the synergy between mechanical and geochemical foam destruction (Fig. 5. A to D) is the driving force behind the release of PGF’s radiochemical pollutant load into the marine environment of the Southern Mediterranean Sea. Thus, PGF plays the role of the main accumulating, transporting, and dispersing agent of PG radiochemical pollutants in the Central Gulf of Gabes [93]. Table 5 Calculated weight average molecular mass (Mw), number average molecular mass (Mn), and poly-dispersity index (Đ) of cyclohexane (Cyc), tetrahydrofuran (THF), and methanol (MeOH) extracts from the phosphogypsum foam (PGF). Samples PGF Cyc THF MeOH Mw (g.mol−1) Mn (g.mol−1) Đ 777 3482 834 230 4694 250 3.4 0.7 3.3 4.2. Role of organic matter in the PGF pollution load PGF stands out from other natural and man-made foams by its richness in organic matter (24.7 %; [92]). In addition to its main role in the PGF formation process, this organic matter also plays another role in relation with its strong pollutants load. In fact, previous studies have revealed that PGF contains high concentrations of trace elements (Zn, Cr, Cu, Cd, U, As…; [92]), radioelements (226Ra, 238U, 40K, 232Th…; [93]), and rare earth elements (Ce, Y, La, Nd, Pr, Sm…; [105]). The amphiphilic organic matter found in PGF (Table 2), is known for its strong affinity for toxic environmental pollutants [106-108]. Indeed, Chuang et al. [106] link this high affinity to the strong chelating binding sites associated to some of the hydrophilic (or charged) groups of amphipathic organic matter (e.g., R-N-R, R-S-R, -OH, -COOH, -O=POOO-, and -CONHO- groups). For instance, Pb prefers S (or N) over O binding sites, unlike Th, which prefers O over S and N binding sites [106]. In our study, we identified several of these sites in the three PGF extracts (e.g., -OH (tetradecanoic acid (in PGF-Cyc), hexadecanoic acid (in PGF-Cyc and -MeOH), and methyl stearate (in PGF-MeOH)), R-S-R (3-Ethyl-5-methyl-1.2.4-trithiolane (in PGF-THF), hexathiane (in PGF-THF), and cyclic-octaatomic sulfur (in PGF-Cyc), and R-N-R (Ethanediamide, N-(2-ethoxyphenyl)-N′-(2-ethylphenyl)- (in PGF-Cyc)); Table 4). Therefore, the amphiphilic nature of PGF organic matter and the presence of these organic groups most likely explain the potential PGF toxic contaminant load. 4.4. Health and environmental impacts of PGF Firstly, it’s important to state that the health and environmental impacts of PGF are closely related to its high contaminant load and the potential release of this toxic load, particularly after PGF geochemical and/or mechanical destruction. In Gabes, the coastal harbor developments play a key role in the spatial limitation of the environmental and health impacts of PGF, and other untreated industrial wastes discharged into the littoral 4.3. Role of organic matter, wind and wave in the diffusion of pollutants in the marine environment We have already noted the excessive foamability of the GCT’s PGF 8 R.B. El Zrelli et al. Journal of Hazardous Materials 480 (2024) 135732 Fig. 6. (A) Phosphogypsum foam in the beach of Casemate (July 2019). (B) Coastal artisanal fishing on the beach of Chatt Sidi Abd Essalam and in the middle of phosphogypsum foam waves (October 2022), with a crate of fish caught before (C) and after seawater washing (D). environment ([63,84] and 2019b). Confronted with the escalating concerns regarding industrial pollution in Gabes following the post-revolutionary events [110], authorities have opted to contain its impact geographically, instead of permanently halting coastal discharges of PG and other industrial wastes. Indeed, the piers of commercial harbor already ensure the protection of the northern beaches (Ghannouche, Ouedhref, Metouia…) against GCT marine pollution impacts. However, following the completion of the fishing harbor developments in 2018, the PGF seldom reaches the southern beaches of Gabes (Corniche, Casmate, M’tourrech, Tebelbou…; Fig. 6. A). Really, these works have transformed the inter-harbor zone (Chatt Sidi Abd Essalam Beach; Fig. 1) into a marine dumping ground for various industrial effluents (gypseous water, seawater used for the acid cooling process and gas washing (hydrogen fluoride (HF)) …; [39]), and solid wastes (phosphate grains, sulfur dross, spent catalysts…; [40]). As a result, PGF is often trapped in the inter-port area (Fig. 1). Consequently, at Chatt Sidi Abd Essalam Beach, local fishermen catch PGF-soaked fish normally inedible by humans, as depicted in Fig. 6. B, C and D. Through various and different pathways, people living in the vicinity of this coastal zone are chronically exposed to the toxic radiochemical pollutants of PGF (trace elements, radionuclides, rare earth elements…). These exposure routes include the dermal contact with seawater through fishing and bathing in the sea (Fig. 6. A and B), ingestion of contaminated fish (Fig. 6. C and D), inhalation of dried volatile PGF microfragments stranded on the beach, and direct exposure to radiation emitted by PGF ([93] and c). The occurrence of several diseases in the area, such as congenital malformations, cancers (e.g., lung, kidney, prostate, liver…), respiratory and cardiovascular diseases, infertility, bone fluorosis, and premature death ([40,41] and 2021a), may be closely related to such exposure. Nevertheless, it is worth to note that the PGF toxic pollutants act synergistically with the other untreated industrial toxic pollutants from GCT (e.g., sulfide oxides (SO2; 21.02 ×103 t. y−1), fluorinated gases (131.4 t.y−1), nitrogen oxides (NOx; 876 t.y−1), industrial dusts (1.6 ×106 t.y−1), sulfide dross, used catalysts…; [38, 40];). This cocktail of potentially toxic pollutants is likely exacerbating the health situation of Gabesian people [71,73,111,112]. However, it should be noted that only an epidemiological analysis could confirm with certainty the origin of the various illnesses suffered mainly by the people living near Chatt Sidi Abd Essalam beach. Similarly, marine animals inhabiting on this polluted coastal zone are also constantly exposed to the various toxic pollutants contained in the PGF via contact with polluted seawater, ingestion of contaminated organisms, inhalation of PGF volatile micro-fragments, and direct exposition to radiation emitted by PGF [92,93]. This is why, Chatt Sidi Abd Essalam Beach is where the majority of paralyzed (e.g., Larus michahellis, Sterna hirundo…), malformed (e.g., Liza aurata, Portunus segnis…), and dead marine animals (e.g., Caretta caretta (mass mortality), Phalacrocorax carbo (mass mortality), Liza saliens…; Fig. 7) are discovered. Locals have aptly dubbed this beach ‘the cemetery’ of the Mediterranean Sea. 9 R.B. El Zrelli et al. Journal of Hazardous Materials 480 (2024) 135732 Fig. 7. Photographs of some of paralyzed (A: Chroicocephalus ridibundus; B: Charadrius alexandrinus; C: Larus michahellis), malformed (D: Liza aurata; E: Portunus segnis; F: Belone belone), and dead animals (E, F, and G: Cartetta caretta); H: Sterna hirundo and its chick; I: Mola mola; J; Liza saliens) found in Chatt Sidi Abd Essalam Beach. 5. Conclusions may contribute to the dispersion of radiochemical pollutants of phosphate fertilizer factories in the Southern Mediterranean Sea. The dispersion of PGF toxic pollutants has significant impacts on human health and wildlife, especially in Chatt Sidi Abd Essalam littoral communities. One may note that other chemical and physical aspects pertaining to phosphogypsum foam formation warrant further investigation. These include its structure (bubbles shape, and arrangement…), rheology, gravity and capillary driven drainage, bubbles coarsening and coalescence… Additionally, the dynamic evolution of phosphogypsum foam from wet to dry forms involves myriad unknown processes occurring across several scales of length (molecular, mesoscopic, and In summary, phosphogypsum foam represents an artificial, surfactant-stabilized, and ephemeral aqueous foam. The formation process of this synthetic foam can be outlined in three steps: (i) formation (by dissolution of phosphogypsum wastes), (ii) stabilization (by organic surfactants and small gypsum crystals), and (iii) destabilization (geochemical (skeleton gypsum dissolution by seawater) and/or mechanical (resulting from the action of waves and winds)). The amphiphilic nature of PGF organic matter and its richness in specific organic groups (e.g., -OH, R-S-R, and R-N-R) are responsible of its high toxic pollutant load. 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Indeed, it plays the role of the main accumulating, transporting, and dispersing agent of phosphogypsum radiochemical pollutants in the Gabes Gulf. This research is an important part of the response to the global pollution problem caused by the phosphate fertilizer industry. CRediT authorship contribution statement Pierre Courjault-Radé: Writing – review & editing, Supervision, Methodology, Conceptualization. Claudie Josse: Writing – review & editing, Formal analysis, Data curation. Anne-Marie Cousin: Writing – review & editing, Resources. Michel Grégoire: Writing – review & editing, Supervision, Funding acquisition, Conceptualization. Oussema Fersi: Writing – review & editing, Formal analysis, Data curation. Sébastien Fabre: Writing – review & editing, Formal analysis, Data curation. Sylvie Castet: Writing – review & editing, Formal analysis, Data curation. Radhouan Belgacem El Zrelli: Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. Acknowledgments In loving memory of my beloved father, Belgacem Ben Mohamed Ben Amor Zrelli (22.11.1946/05.05.2023), this work is dedicated. The authors extend heartfelt appreciation to all individuals whose support was invaluable in conducting fieldwork sampling and laboratory analyses. Special gratitude is expressed to the editor, Dr. Joao Pinto da Costa, and the three anonymous reviewers whose insightful comments and feedback greatly contributed to enhancing the quality of this manuscript. 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