Research article Spatial distribution and leaching behavior of pollutants from phosphogypsum stocked in a gypstack: Geochemical characterization and modeling Sara Bisone, Mathieu Gautier, Vincent Chatain, Denise Blanc * Universit e de Lyon, INSA-Lyon, DEEP (D echets Eaux Environnement et Pollutions e Wastes Water Environment Pollutions), 20 Avenue Albert Einstein, F- 69621, Villeurbanne, France article info Article history: Received 15 November 2016 Received in revised form 20 February 2017 Accepted 21 February 2017 Available online xxx Keywords: Phosphogypsum Geochemical modeling Leaching behavior Phosphates Fluoride abstract Phosphogypsum (PPG) is the byproduct of the production of phosphoric acid and phosphate fertilizers from phosphate rocks (PR) by acid digestion. Despite the technical feasibility, the impurities present in this waste make its reuse critical and large amounts of PPG are stockpiled, resulting in the production of polluted acid leachates. The aim of the present study was to characterize the spatial variability and evolution in time of a 20-year-old gypstack and to study the geochemical behavior of the waste in order to assess the best management options. Chemical and mineralogical analyses were performed on core samples taken from 4 different depths of the stack down to 13.5 m. Despite the high homogeneity shown by chemical and mineral characterization, leaching tests revealed a different chemical behavior with depth. pH-dependent leaching tests were also performed to measure the acid neutralization capacity of the studied matrices and to determine the leachability of the elements or pollutants of concern as a function of pH. The study was focused on Ca, Fe Na, Si, Cd and Sr and on F  , PO 4 3 and SO 4 2 anions. The geochemical modeling of these tests with PHREEQC enabled the identification of the minor phases controlling the solubilization of the elements analyzed. Validation of the model by the simulation of a column leaching test suggested that the model could be used as a predictive tool to assess different management scenarios. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Phosphogypsum (PPG) is the byproduct of the production of phosphoric acid and phosphate fertilizers from phosphate rocks (PR) by acid digestion. PR contain high concentrations of phosphate minerals, mostly from the apatite group Ca 5 (PO 4 ) 3 [F, OH, Cl]. Among the different production processes, the dihydrate (DH) process was the most widely used (IAEA, 2013). The acid digestion with sulfuric acid at high temperature (70e80  C) used in this process can be simplified with the following equation: Ca 5 ðPO 4 Þ 3 F þ 5H 2 SO 4 þ 10H 2 O/3H 3 PO 4 þ 5CaSO 4 ðH 2 OÞ 2 þ HF (1) The PPG is then separated from phosphoric acid by filtration, and later washed to reduce hydrofluoric acid (HF) concentration. Phosphoric acid recovery does not reach 100%, hence residual phosphorus and HF can be found in the PPG. Phosphorus is mostly present as phosphoric acid, but it can precipitate as Ca 3 (PO 4 ) 2 , Ca(H 2 PO 4 ) 2 and CaHPO 4 $2H 2 O(Aliedeh and Jarrah, 2012). HF can react with silica to form SiF 4 NaF, Na 2 SiF 6 , Na 2 FeF 6 and CaF 2 may also be found in PPG (Aliedeh and Jarrah, 2012). Beside the re- sidual acidity and F, some impurities may accumulate in PPG. Metals (Sr, Ba, Cu, Cd, etc.), and radio-elements ( 226 Ra, 238 U) are frequently found in PPG (Abril et al., 2008; Degirmenci et al., 2007; IAEA, 2013) and make the management of this waste a complex issue. These impurities come primarily from PR and to a lesser extent from sulfuric acid (Davister, 1998). The process used also de- termines the amount and the characteristics of PPG generated (Ghafoori and Chang, 1993). The DH process engenders about 4.9 t of PPG (dry mass) for every ton of phosphoric acid produced. PPG wet mass can reach 6.5 t (IAEA, 2013). In 2009, worldwide PPG * Corresponding author. E-mail address: denise.blanc@insa-lyon.fr (D. Blanc). Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman http://dx.doi.org/10.1016/j.jenvman.2017.02.055 0301-4797/© 2017 Elsevier Ltd. All rights reserved. Journal of Environmental Management xxx (2017) 1e9 Please cite this article in press as: Bisone, S., et al., Spatial distribution and leaching behavior of pollutants from phosphogypsum stocked in a gypstack: Geochemical characterization and modeling, Journal of Environmental Management (2017), http://dx.doi.org/10.1016/ j.jenvman.2017.02.055