230 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) ISSN (Print) 2313-4410, ISSN (Online) 2313-4402 © Global Society of Scientific Research and Researchers http://asrjetsjournal.org/ Characterization and pH Dependent Leaching Behavior of Tunisian Phosphogypsum Raja Zmemlaa*, Perrine Chaurandb, Mounir Benjdidiac, Boubaker Elleuchd, Jean Yves Botteroe a,c,dNational Engineering School of Sfax, LaboratoryWater, Energy and Environment L3E, Route de Soukra Km4.5 BP W, 3038 Sfax, Tunisia b,eCEREGE, UMR 6635 CNRS Aix-Marseille University, 13545 Aix-En-Provence, France aEmail: zmemla@yahoo.fr Abstract The current study aims to characterize and as well as to investigate the leaching behavior of Tunisian Phosphogypsum (PG). The results of the physical characterization studies showed that, as worldwide Phosphogypsum, the Tunisian PG behaves like fine silty sand with important initial water content. Relevant attention was given to the leaching behavior of the sample which was subjected to two leaching tests according EN 12457-2 (2002) at liquid to solid ratios (L/S) of 10 and 100 as well as pH stat leaching test according to the CEN/TS 14997 (2006). The progressive release of major elements as well as the metals indicates high mobility of the most analysis elements. Calcium, sulfate and phosphorus were the major elements having the highest leaching concentrations. Broadly, mobility of trace metals in PG was classified into three degrees: elements with high mobility were Sr, Zn, those with moderate mobility were As, Ba, Cd and Cr and those with low mobility were Cu, Ni, Pb, Se, V, Y and Zr. The highest concentrations of the most part of the metals were obtained from L/S 100. Based on the pH dependent leaching experiments, results show that the PG has a maximum susceptibility to leaching out metals when exposed to a strongly acidic condition (2-4). While, alkaline condition appears to be the most stable for the analyzed material. This paper offers a data base which is useful when later recycling actions are taken. Indeed, acidic conditions should be avoided in order to prevent metal leaching from PG. Keywords: Phosphogypsum; Heavy metals; Leaching behavior; characterization; Batch leaching test. ------------------------------------------------------------------------ * Corresponding author. http://asrjetsjournal.org/ American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 24, No 1, pp 230-244 231 1. Introduction Phosphogypsum (PG) is an industrial solid by-product formed during the production of phosphate fertilizers. It is produced by chemical processing called « wet process », when phosphate ores apatite (mainly as Fluorapatite Ca10 (PO4)6F2) is reacted with sulfuric acid (H2 SO4) [1-3]. The wet process generates a large amount of PG which is filtered off and pumped as slurry to nearby ponds [4]. PG is composed mainly of calcium sulfate dehydrate (CaSO4 2H2O) and partially contains impurities, as well as many trace elements, heavy metals, rare earths elements and naturally occurring radioactive materials [5-7]. Actually the worldwide production of PG is estimated to be around 280 million tons per year [8].Only 14% of the worldwide production is reprocessed, while 28% is dumped into water bodies and 58% is being stockpiled [9]. Tunisia is the fifth important phosphate producer in the world. Five phosphate industries are responsible for the annual production of 10 million tons of Phosphogypsum [10], which is stored in stacks near the factories. A PG stockpile occupies so large land areas that cause severe pollution to the surrounding environment; in Sfax (containing 30MT of PG which occupy approx.48 Ha up 50 meters in height) and Skhira (40 MT of PG covering approx. 112 Ha up 60 meters in height). This is a large volume which is expected to be increased in the event of exploitation of the new layers of « Sra Ouertana » in the future [11]. The management of these large quantities, which have been continuously generated, is one of the most serious problems in many countries, as well as in Tunisia. Several studies have been recommended to use Phosphogypsum as building materials [12], agricultural fertilizers [13] and as an amendment to the soil [14]. However, only a few of them were put into practice due to the absence of information about long term behavior of PG, indeed The leaching potential of Tunisian PG has received little attention. A useful tool for investigative waste and assessing the environmental soundness of the waste PH stat leaching test can be used to predict the mobility of major and trace elements in waste samples in a wide pH range [15-17]. The aim of this study is to evaluate the behavior of heavy metals present in Tunisian PG; indeed less research has been published regarding the leaching behavior of Tunisian PG and mobility of these elements. It is within this context that the present study has been undertaken aiming both at the characterization of Tunisian PG and the determination of its heavy and major leaching behavior. The main objective of this work is to gain basic information about distribution of heavy metals and its leachabilty as a first step previous to modulate its behavior. Physical, chemical, mineralogical, and leachable characteristics are determined in order to draw up an accomplished synopsis of studied PG. In the first section, materials and performed experimental protocol are described; then the physical characteristics and the results of leaching test are presented and analyzed according to literature. 2. Materials and methods 2.1. Material PG sample was collected from a phosphoric Tunisian plant: Industrial society of phosphoric acid and fertilizers of Skhira, situated in the Gulf of Gabes in south-eastern Tunisia (34°20’50’’N; 10°08’50’’E) as shown in Figure1. Around 20 kg of PG were collected from each sampling point. Later on, material is homogenized and American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 24, No 1, pp 230-244 232 stored in plastic bags. Figure 2: Location of the PG pile, with sampling point indicated 2.2. Experimental program • Physical characterization Physical characterization of Tunisian PG includes the determination of classical soil properties as water content NF P94-050, grain size distribution using the dry sieving method NF P94-056, bulk density NF P94-060, real density NF P94-061. To add more information, particle size analysis was determined also by laser granulometry in wet suspensions with water saturated of gypsum NF ISO13320-1. Furthermore, morphology and microstructures of PG were performed by scanning electron Microscopy, SEM (SU 8010 Hitachi) at 30 kV. • Chemical and mineralogical characterization The chemical composition of PG major and trace elements was determined by ICP-AES (Jobin-Yvon Horiba Ultima-C2000), after either acid digestion by HNO3 and H2O2, alkaline hydrolysis by LiBO2 or microwave assisted digestion in HF/HNO3/HCL. The major mineral phases of the PG were identified using X ray powder method with bruker diffractometer (X’Pert Pro MPD Panalytical)with Co Kα radiation ( λ 1.79 Å) and running at 35 kV and 40 mA, the total counting time during XRD measurements was 10h . The international Center of diffraction Data PDF-2 database and the X’Pert High score plus software Panalytical were used to identify the mineral phases from the obtained X- ray diffraction XRD patterns. • Leaching procedure and conditions PG was subjected to a pH static experiment according to European standard CEN/TS 14997 (2006) [18].The pH American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 24, No 1, pp 230-244 233 dependence leaching test with continuous pH control can be useful for simulating different scenarios including in particular the worst-case scenarios. The pH-static experiments were conducted under ambient conditions for 48 h at a liquid-to-solid (L/S) ratio of 10. Values were selected within the wide pH range from 3 to 11 (3, 4, 5, 6, 8, 9, 10, and 11); including experiment under natural pH (i. e., without addition). The leachants were prepared by adding sodium hydroxide (NaOH) solution to deionized water. In addition, a 24-h leaching test in deionised water at a L/S ratio of 10 and 100 according to European standard EN 12457-2 (2002) [19] was performed to assess the hazardous properties of the studied PG according to EU legislation (EU, 1999; EU, 2003) [20]. All the experiments were performed in duplicate. 3. Results and discussion 3.1. Physical characterization The basic data of Physical properties of PG are summarized in Table 1. Results allow to conclude that the PG was characterized by a high water content (6.5%) higher than natural gypsum (approx 1-2%). The bulk (apparent) density was 0.85 g/cm3. The absolute (real) density of PG is in the same range and comparable to that of natural gypsum (2.3 g/cm3), the real density is a parameter that depends mainly on the mineralogical composition of the material. Based on the results, it can be concluded that PG is a light material compared to natural sand (approx 2.6g/cm3); also it is characterized by high initial water content. These results are similar to those reported in other studies [21]. Table 1: Physical properties of Phosphogypsum Water content (%) 6.5 Bulk density (g/cm3) 0.85 Real density (g/cm3) 2.33 Particle size D<80µm 65 Laser distribution D50 µm 40-250 The main granulometric results obtained are compiled also in Table 1. It is clear that PG was composed of a very fine material. Indeed 65 % of a PG fraction under 80µm. Laser granulometric was used to better understand the particular distribution of PG. Figure 2 shows the percentage of particles size of PG sample, by volume, the granulometric curve of PG denotes t two populations of the particles with large size range; the first population has a significant particle number of around 40µm of diameter, is the greatest fraction, the second population has a particle number above 400µm. The grain size distribution of the samples was found to be very similar and consistent with the data in the literature [22]. Microstructural of samples was shown in Figure3. PG particles are regular in form and small in size, which confirms the results obtained by the laser granulometry analysis. PG has a clear crystalline structure, a crystals American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 24, No 1, pp 230-244 234 exhibited shapes in the form of a tabular structure associated with crystals in needle. The morphology of PG is characterized by the abundance of needle crystals, which illustrates the fact that PG is submitted to several physical processes of erosion and dissolution [23]. Figure 2: Particle- size distribution of PG Figure 3: SEM images of PG sample (A: x250; B: X500) 3.2. Chemical and Mineralogical composition • Major and trace elements distribution Chemical composition of the PG is shown in Table 2. This provides also a comparison of those components in PG (max-min) from literature [24]. Data on the major content as oxides indicate that PG is mainly composed of CaO and SO3, indeed their mass is approximately close to 80% of the total weight. There is also appreciated quantity of SiO2, P2O5 and Al2O3 and a lesser extent of Fe2O3, K2O, MgO and TiO2. Compared to literature values, major elements were present the same order of concentrations of other PG reported in the literature. Analyses also show a higher content of organic material (19.691%) by the weight. Concerning the trace elements (expressed in ppm), results show that the most abundant concentrations in the PG are: Sr, Cs, Zn, Cu, Y, Ni, Ba, As and Cd, in order of abundance. The lowest concentration values observed were for Cr, Pb, Zr, V, Se and Hg. The amount of the selected elements has a similar concentration to those reported by authors in PG American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2016) Volume 24, No 1, pp 230-244 235 obtained from sedimentary phosphate rock, except the concentrations of Zn and Sr, which exceeded maximum concentrations reported in the literature ranges. Table 2: Major and trace element concentrations in PG sample Literature Trace elements Literature Free water (%) 12.19 - As 21 1.3-42 Organic material MO (%) 19.691 - Ba 29.33 32-236 Loss on ignition (LOI-MO) (%) 1.158 - Cd 12.23 0.8-25 Major elements Cr 6.12 1.6-75 Al2O3 0.649 0.05-0.60 Cs 186.56 - CaO 38.785 27.8-34 Cu 69.08 2-195 Fe2O3 0.05 0.01-0.25 Hg 0.03 0.005-10 K2O 0.02 - Ni 34.82 1.7-250 MgO 0.01 0.01-0.54 Pb 6.67 0.5-17 MnO Zn>Ba>As>Cd>Cr>Ni>Cu>Se>Y>V>Pb>Zr. Higher concentrations of several elements of potential environmental significance are found in leachates giving evidence that special attention should be taken in the case of later valorization. (4) Considering the results obtained from pH-dependent leaching experiments, in general, the leaching of PG trends increase at low pH, maximum leached concentration showed at pH 2-4. Indeed, PG shows a maximum susceptibility to leaching out metals when exposed to a strongly acidic condition, whereas, alkaline condition appear to be the most stable for the analyzed material. In fact, the results show a higher immobilization of the heavy metals. The findings of this study promote the quantitative consideration of environmental impacts of long-term leaching of Tunisian PG and it can serve as guidance in different waste management scenarios to control leaching behavior of PG when later recycling actions are taken. Acknowledgements The authors would like to thank Ms Helene Miche for technical assistance with the ICP-AES analyses. References [1] Becker.P, Phosphates and phosphoric acid: raw materials, technology and economics of the wet process, in fertilizer Science and technology series, Marcel Dekker, Inc.,New York, 1989,p.752 [2] Koopman.C., Witkamp. G. 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