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Tunisian phosphogypsum tailings: Assessment of leaching behavior for an integrated management approach

This study has been carried out to evaluate the leaching behavior of Tunisian phosphogypsum (PG) tailings in Skhira city (southern Tunisia). Two PG samples, including old and freshly deposited samples, were characterized in terms of physical, geotechnical, mechanical, chemical and mineralogical properties. Special attention was paid to their leaching behavior when subjected to standard leaching tests. Our results indicated that both samples are mainly composed of more than 31.85% CaO and 31.4% SO3, indicating the predominance of gypsum. This was further confirmed by XRD patterns that revealed the presence of characteristic reflections of gypsum, brushite, quartz and Maladrite. Compressive strength after 90 d exceeded 769 kPa, but still lower than that of natural sand (1,800 kPa). Leaching test was proposed as an appropriate method to determine the released contaminants from PG. The obtained results showed that Fluorine and Phosphorus are the most released elements from PG with 40 and 30%, respectively. The released Se, Cd, and Zn were the only trace elements that exceeded the threshold limits. It seemed that leached element concentrations were independent aging or particle size of the PG. Based on the assessment of leaching behavior, an integrated management approach of the PG deposits was proposed.


Introduction
Phosphogypsum (PG) is an industrial solid by-product generated during the production of phosphate fertilizers via chemical processing known as «wet process» by which apatite (mainly as fluorapatite Ca 10 (PO 4 ) 6 F 2 ) reacted with sulfuric acid (H 2 SO 4 ) [1,2] to produce phosphoric acid (H 3 PO 4 ). Wet process generates a large amount of PG [3], exceeding 280 million tons per year worldwide [4]. Of which, only 14% is being revalorized, 28% is dumped into the receiving water bodies and 58% is being stockpiled [5]. PG is composed mainly of calcium sulfate dehydrate (CaSO 4 2H 2 O) with numerous impurities like heavy metals, rare earths elements (REE) and naturally occurring radioactive materials [6,7]. Many studies have addressed trace elements contents of in PG from different locations worldwide (South Africa, Tunisia, Syria, Florida, Spain, Idaho); they showed ranges of 6-103 mg/kg for Copper (Cu); 6-315 mg/kg Zinc (Zn); 0.8-40 mg/kg Cadmium (Cd); 2-43 mg/kg Nickel (Ni); 6-135 mg/kg Uranium (U) and 3-7 mg/kg Lead (Pb) [8]. For instance, Pérez-López et al. [9] extensively studied the impact of Spanish PG stacks on the ecosystem. They found that salt-marshes play key role in the natural attenuation of contamination. In a further trial to evaluate the pollutant flows from PG area [10], the same author proposed to carry a preliminary restoration plan to overcome the tidal-induced leaching. Papageorgiou et al. [11] run a radiological study in the Shistos PG stack (Piraeus, Greece) using a combination of gamma ray spectroscopy and GIS based technique. They had posited that environmental risk by radioactivity in Shistos area would be minimized by simple thick soil cover and geomembrane. Kassir et al. [12] had attempted to valorize PG in soil amendment via the assessment of selected trace elements in Mediterranean red soil from Lebanon. They concluded that soil amendment by PG is feasible and viable because to low environmental impact of trace elements, that remained below the threshold limits required by relevant Lebanese regulations. Similarly, Tunisian chemical group (TCG), a series of factories for phosphoric acid production generated important tonnage 3 of PG. Phosphate industry is, thus, responsible for the generation of more than 12 million tons of PG that need to be correctly managed and stored [13]. This huge volume is expected to increase in the near future, especially with the exploitation of the new layers [13].
Management of these large quantities is one of the most challenging environmental problem in many countries [14]. To limit the potential impact of those by-products, several studies have been undertaken with an attempt to valorize PG in various industrial and agricultural applications, including building materials [15][16][17][18][19], agricultural fertiliser [20][21][22] and soil amendments [12,23,24]. The long-term management of PG is directly related to the problem of potentially toxic elements leaching. Therefore, monitoring the high volume of PG as embankment near the treatment unit needs a detailed study about its long-term behavior and/or evolution of heavy metals content. Unlike several wastes such as fly ash, sediments, up to know there are no specific regulations for the management of PG. Classification and characterization of PGs based on current legislations are not well known due to the fact that most studies on PG have focused on the environmental impact of this waste on the surrounding landfilling area, but not on the possible species leaching. Published studies, in Tunisia, have focused mainly on the spatial characterization of pollution near PG storage areas, by mapping the concentration levels of chemical elements and specifying contaminated areas [23,25,26]. But, no clear results on the quantification of released pollutants (trace elements, heavy metals, etc.) contained in Tunisian PGs. The assessment of the real potential release of trace elements, through PG leaching tests, is a viable tool to investigate the leaching behavior. In this context, the present study has been carried out to fill the gap caused by the lack of information about the possible time-dependent evolution of PG embankment.
Technological characterization (i.e., physical, geotechnical and mechanical properties) and evaluation of leaching behavior of the studied PG samples may serve as handy tools to draw an accomplished synopsis of this by-product for an integrated management of the PG 4 deposits. The effect of long term storage (> 20 y in outdoor stockpile) on the physical, mechanical and chemical properties of Tunisian PG was also advised. The innovative aspect discussed in the current study concerned early insights on the leaching behavior of trace elements from PG in the Skhira embankment. The main objective of the current work is, therefore, to (1) characterize the phosphate processing by-product (i.e., PG); (2) quantify the leached amount of pollutants from Tunisian PG stack and (3) propose an integrated management approach based on an UpToDate standards.

Samples Preparation
Two PG samples (fresh and aged) were collected from the TCG phosphoric acid production unit of Skhira lying 80 km to the south of Sfax city, southern Tunisia (34°20'50''N; 10°08'50''E). About 200 kg of PG were collected from each sampling point (Fig. S1). At the laboratory, PG samples were homogenized and stored in plastic bags; subsamples were taken from the raw PG for subsequent physico-chemical analysis. Aged PG sample was then fractionated into two different particle-sized subsamples (less than 45 µm and more than 125 µm sized-fractions) to assess the effect of particle size on PG leachability. Hereafter, we used PGF and PGA to refer to the fresh and aged PGs samples, respectively. We attributed acronyms of PGAf and PGAc for PGA subsamples of fine (i.e., less than 45 µm) and coarse fraction (more than 125 µm). Physical, geotechnical and mechanical performances of PG samples were compared to a natural sand (NS) commonly used in road material. 5 Physical characterization of both samples included the determination water content, bulk and real density, grain size distribution using the dry sieving method. Further information about particle size distribution was obtained using laser granulometry in wet suspensions [27].

Physico-Mechanical Characterization
Additionally, microstructure of PG was assessed by scanning electron microscope, (SU 8010 Hitachi) at 30 kV.
Geotechnical analyses through Proctor modified test and California Bearing Ratio (CBR) were carried out as described elsewhere [28][29][30][31]. The modified proctor compaction test in combination with CBR was used to determine the optimum moisture content (OMC) with the corresponding maximum dry density (MDD). Thereafter, cylindrical specimens (with diameter of 50 mm and height of 100 mm) were compacted using geotechnical results obtained from the modified compaction to determine mechanical behavior of PG studied samples. All specimens were prepared with the modified compaction method at the OMC and maximum density determined by the standard compaction test. The specimens were remolded and stored for testing after 28 and 90 curing days. To achieve this goal, an experimental setup was developed according to the diagram shown in

Chemical and Mineralogical Characterization
To ascertain the composition of PGA and PGF samples, about 200 mg of each PG sample was ground to less than 45 µm-sized fraction. Then, 10 mL of 5M HNO 3 was added to the Teflon 6 beaker containing the desired PG powder under heating temperature of 120C below a fume hood for 2 h. After the total dissolution of PG powder, the obtained solution was allowed to dry for 2 h. Finally, UPW was added to wash out the residual nitric acid; the whole was transferred to 100 mL volumetric flask and adjusted [34]. Chemical composition of PG was determined by ICP-AES (Jobin-Yvon Horiba Ultima-C2000). To measure the concentration of both major and trace elements, an acid digestion methodology was adopted as described above; H 2 O 2 was dropwise added to remove any organics.
Alkaline dissolution of the PG powder was also undertaken by the addition of LiBO 2 for comparative purposes. Mineralogical analysis of the PG samples (i.e., PGA and PGF) was performed by using an X-ray bruker diffractometer (X'Pert Pro MPD Panalytical) with Co Kα radiation (1.79 Å ), running at 35 kV and 40 mA with a total counting time of 10 h. The obtained XRD patterns were computer processed via the X'Pert High score plus software and compared to the international center of diffraction Data PDF-2 database to identify the main mineral phases.

Batch Leaching Tests
Batch leaching test was used as a handy tool to assess the mobility of major and trace elements according to the description of Papaslioti et al. [35]. Briefly, batch leaching test was performed by using ultra-pure water (UPW) as leachate. Samples were leached at a liquid solid (L/S) ratio of 10 L/kg in UPW at 25C for 24 h under continuous shaking. Then, about 10 mL from the supernatants were withdrawn through 0.45 µm syringe-driven millipore filters (Millex-LH, PTFE, Millipore Corp., France), acidified with HNO 3 acid (5 mol/L) and stored for subsequent chemical analysis by ICP-AES as shown in Bisone et al. [36].

Physical and geotechnical properties
Physical, geotechnical and mechanical properties of the studied Tunisian PG are summarized in Table 1. It was found that both samples contained somewhat high moisture contents exceeding 4% for PGF and 7% for PGA. These results showed that fresh PG (i.e., PGF) contained the highest water content (7%), when compared to NSs (1.01%; [37]) and natural gypsum (l-2%; [15]. Usually, PG from cake has a high free moisture contents ranging between 25-30% [33,38]. The free water content is largely variable depending on the production process, especially the filtration of PG; it depends also on how long the PG has been allowed to drain after stacking and on local weather conditions. Measured water contents of fresh and aged PG samples (i.e., PGA, PGF), varied between 4-7%, respectively, showing 8 lower values than those reported by Sfar Felfoul et al. [33] and Rouis [38] who studied PGs of Sfax area whose water contents varied between 19 to 30%. Likewise, El Cadi et al. [39] studied the PG from marrackech area-Morocco; they found 12 to 27% water contents. Those values are much higher than those reported in this study, probably because of the separation procedure adopted by the Skhira unit for phosphoric acid production (dry process) as well as climatic conditions during storage on tailings (high temperature in arid climate). It is therefore possible to conclude that the studied samples are continuously subjected to natural drying.
Both PG samples showed apparent densities of 0.83 and 0.81g/cm 3 ; the absolute densities ranged between 2.33 and 2.34 g/cm 3 indicating a typical range for natural gypsum Based on these results, it can be concluded that PG is a light material compared to NS (2.65 g/cm 3 ). The absolute and apparent densities of the studied PGs samples are comparable to those presented by Schaeffner [41] who studied PGs of sedimentary origin. However, they are lower than those of PG of magmatic origin [41]. Furthermore, the obtained results are close to those of natural gypsum [42, 43] but lower than NS, showing a very light and less dense material. reported a hydraulic conductivity range of 10 -3 -10 -5 cm/s, which is in agreement with the present PGA and PGF PGs.
Microstructural analysis, carried out by electron microscopy, revealed the abundance of particles with a diameter between 10 and 100 μm (Fig. 2). This is consistent with the particle size analyses carried out by dry sieving. Furthermore, the observations showed the tabular crystalline morphology of PGs, as was the case for most PGs from apatite of sedimentary origin [51].  As for other elements (i.e., Cr, Cs, Cu, Se and Sr), PGA presented lower concentrations with low enrichment factor (< 1) for only As, Cs and Sr (Table 3).  [43]. Brushite can be observed in the 3.047 Å and 3.78 Å reflections [55]. As for quartz, the only reflection appeared near 4.26 Å [56]. Maladrite (NaSiF 6 ) may appear near 4.435 Å ( Fig.   3(a)). The superposition of the different XRD diffractograms of the PGs indicated a mineralogical stability as the reflection trends were almost unchangeable due to the presence of the same mineralogical phases. Such composition is consistent with the results of chemical 15 analysis reported in Table 2. For Tunisian PGs, Ajam et al.

Leaching Behavior of Phosphogypsum Samples
Environmental risk assessment of a given waste mainly depended on the potential release of toxic chemical species, but not on the real contents. Indeed, the selection criteria of prospective industrial and/or environmental application of a by-product (e.g., PG waste) require a preliminary evaluation of its environmental impact before and after its introduction in an integrative management scenario. In this section, several leaching tests were performed to quantify the leaching behavior of Tunisian PG. slightly lower than that of PGA (2.7) without affecting the leaching abilities of both samples because of the fact that pH of the leaching solution is a key parameter in the process [36].

Leaching behavior
The results of leaching tests showed that the most available elements are Calcium (Ca), Fluorine (F), Sodium (Na), Phosphorus (P), Sulfur (S), and Silicon (Si) ( Table 4). These elements have the higher concentrations and higher leachability due to their availability. The most significant concentrations were related to Ca and sulfate; their presence in the studied samples depended mainly on gypsum solubility. The most mobile element was Na of which more than 90% was washed away from PGA and PGF; the lowest leachable elements are Aluminum (Al), Iron (Fe), Potassium (K), and Titanium (Ti).
Insert Table 4 here Elements of environmental interest are S, F, and P of which the detected values largely exceeded the threshold limits desired by relevant regulations. Because of those deleterious 18 elements, the studied PGA and PGF PGs can be classed as hazardous wastes. A special interest must be devoted to the management of those toxic elements before any possible valorization of PG to avoid its potential environmental impact.
F − species is of environmental concern because of more than 1.5 mg/L in drinking water causes dental and skeletal fluorosis [59,60]. pH is a critical parameter that significantly affect leaching behavior of trace elements [36]. In general, under acidic and oxide conditions, Zn is the most soluble and mobile cation which makes it the most phototoxic microelement after Al and manganese (Mn) as Zn can easily migrate to the sandy and slightly acidic soil [53]. Under acidic conditions, the solubility of Cd increases because of its predominate cationic species (i.e., Cd 2+ ).
Leaching tests revealed high environmental risks with potential release of As, Zn, Cd, Se and Sr, calling for immediate measures to properly address the disposal of PG by-products.

Effects of aging on leaching behavior
A quick look to the chemical composition of fresh PGF and aged PGA PG samples shed light on the similar content of almost all major elements, with the exception of F and P; both of them were released from the fresh sample with higher amounts. Fresh PGF sample have leached 22.9% P and 8.03% F, which are higher than the aged PGA sample release (3.29% P and 7.04% F).The released quantities are similar for Ca, S and Na.
The amounts of eluate concentrations of trace elements from fresh PG was slightly higher than from aged PG, except for Sr, Se and Zn which leached higher from aged PG. This trend agreed with the results of chemical analysis.

Effects of grain size distribution on element mobility
The effects of particle size distribution on leaching behavior of both PGA subsamples (i.e., PGAf and PGAc) shows similar solubilization level for both subsamples studied. In addition, the major element contents of both fractions are very close to each other and to the raw PGA sample.
As for trace element contents (i.e., As, Ba, Cu, Ni, Pb, Se, V, Y), fine fraction showed relatively higher than coarse PGAc fraction, but Cd, Cr, Sr and Zn kept similar concentrations; their release did not correlate with the particle size distribution. This is an obvious conclusion that particle size distribution cannot affect the leaching ability of the studied PG samples. No clear difference between the release of metal species from both subsamples (i.e., PGAf and PGAc) was observed, further confirming the absence of significant effect on leaching process.

Solid matrix alteration during leaching
To identify the evolution of the solid matrix of the PG sample before and after leaching, mineralogical analysis and electron microscope observation was carried out on PG samples. SEM images, shown in Fig. 4, revealed an altered surface of the PG grain surface that retained its tabular shape. This may plausibly explained by the effect of water erosion that contributes to the dissolution of the main adhering chemical elements associated with PG.

Potential Application for an Integrated Management Approach
This study provides an early insight on the leaching behavior of heavy metal from the PG embankment disposal of the Skhira treatment unit. Hitherto, PG management is limited to land disposal and controlled storage sites. We propose a management approach to valorize this by-product; save the cost of its storage and even make a financial profit as environmentally friendly process. The issue related to the reuse of PG is of great economic and environmental interest. Tunisian regulatory texts indicated that the previously adopted solution was limited to the installation of mono-dumps, managed by the producing industry.
In Tunisia, land-based disposal storage has led to the constitution of large stocks in the cities of Sfax and Gabes, causing deleterious environmental effects [13]. Understanding the risks of heavy metals leaching from PG stacks during its life cycle is necessary, as highlighted in the present work. Further integrated management plan includes the potential application in construction materials such as plaster [16], concrete and construction demolition waste [62], polymer concrete [63]. PG can be used as road base material, a developed approach to massively recycle PG for industrial application. It is to be mentioned that several attempts to sustainably valorise the high PG residues [15, 19,33,40,64]. Moreover, Pérez-López et al. [65] provided geochemical data about stack-piles of PG from Huelva, Spain, with an aim to route for restoration plan. They recommended to develop a treatment system for controlling outflow waters before the final discharge. Similarly, Papageorgiou et al. [11] proposed a photoremediation technique to mitigate the effect of trace element leaching. In this context, we propose to valorize PG for road base materials. Thus, the experimental design of the formulation mixtures will be optimized for better mechanical properties and strength performances.
The strength development (RC, RT as well as Young's modulus) at different curing times of the material will be studied and compared with conventional road base material. Our 22 preliminary results about the potential use of recycled PG as raw material for concrete were in perfect agreement with the findings of Manal [45], Rashad [66], Nizevičienė [64] and Chen [62]; it showed comparable properties as conventional road base materials. Hence, PG stacks can be engineered as road base material with competitive properties. The possible use of PG as raw material supply for application in road and construction materials may adhere with the strategic management plan that meets the desired environmental requirements.

Conclusions
This study provides an early insight on the leaching behavior of heavy metal from the PG embankment disposal of the Skhira treatment unit (southern Tunisia).
It was concluded that high amounts of fines (65 to 75% by weight) significantly affected the leaching behavior of the studied PG samples. Geotechnical and mechanical properties of PG samples showed similar plasticity and great sensitivity to water. Mechanical properties of old PG are better than the fresh one. CBR was found to be much lower than that required by most specifications.
Chemical analysis showed heterogeneous distribution of heavy metals with an abundance of Zn and Sr compared to worldwide PG. There is no real trend visible to evaluate the behavior of the major /minor elements according to the age. This may promote the quantitative consideration of environmental impacts of PG but this information does not allow an efficient evaluation of chemical elements mobility.
Both PGs reveal high potential release of As, Zn, Cd, Se and Sr further alarming the immediate adoption of suitable measures for safe disposal of PGs. Special care should be taken in the case of Se, Zn and Cd during the potential application of Tunisian PG. It is 23 possible to propose a robust and coherent model that reproduces the leaching behavior, based on several input parameters.