Applied Geochemistry 110 (2019) 104435 Available online 30 September 2019 0883-2927/© 2019 Published by Elsevier Ltd. Characterization of phosphate coating formed on pyrite surface to prevent oxidation Konstantinos Kollias a, * , Evangelia Mylona a , Katerina Adam b , Maria Chrysochoou c , Nymphodora Papassiopi a , Anthimos Xenidis a a Laboratory of Metallurgy, School of Mining & Metallurgical Engineering, National Technical University of Athens, 15780, Zografou, Greece b Laboratory of Excavation Engineering, School of Mining & Metallurgical Engineering, National Technical University of Athens, 15780, Zografou, Greece c Department of Civil and Environmental Engineering, University of Connecticut, Storrs, CT, 06269, USA A R T I C L E INFO Editorial handling by Dr. R. Seal Keywords: Acid mine drainage Sulfdic wastes Pyrite Phosphate coating ABSTRACT The sulfdic wastes produced from mining activities when exposed to atmospheric oxidizing conditions during disposal and in the absence of suffcient alkaline minerals can result in the generation of acidity as well as the release of toxic metals and metalloids. An emerging technique to prevent acid generation is the formation of protective coating layers on the surface of sulfde grains (mainly pyrite) in order to inhibit the oxidation process. The composition of phosphate coating and its effectiveness in inhibiting pyrite oxidation under oxidizing con- ditions is investigated in the present study. A combination of wet-chemical, microscopic (SEM/EDS) and spec- troscopic techniques (FTIR, XPS) was used. The experiments involved treatment of pyritic tailings with solutions containing PO 4 3 and H 2 O 2 at pH 5.5. Based on the results, the protective coating layer around pyrite particles is composed mainly of Fe(II)-phosphates followed by Fe(III)-phosphate, iron (hydr)oxides and iron oxy- hydroxysulfate phases. The optimum conditions for the development of an effcient protective layer on pyrite particles correspond to treatment with 0.01 M PO 4 3 for 48 h. In this case, the oxidative dissolution of sulfur was reduced by 66%, as compared to the non-treated pyrite sample. 1. Introduction Iron disulfdes and other sulfde minerals, contained in mine wastes, are oxidized in the presence of oxygen and water through a complex process catalyzed by acidophilic bacteria. The chemistry and the geo- microbiology behind the sulfde oxidation process is quite complex and still the subject of extensive research (Nordstrom, 2011a). The resulting aqueous discharge is known as acid mine drainage (AMD). The levels of acidity and heavy metals in solution are closely related to the type and content of sulfde phases as well as the presence or absence of alkaline materials, such as calcite (Blowes et al., 2014; Courtin-Nomade et al., 2009; Huang et al., 2016; Johnson and Hallberg, 2003). Pyrite (FeS 2 ), the most abundant sulfde phase in the earth crust (Rickard, 2012), plays a dominant role in the AMD generation (Blowes et al., 2014; Singer and Stumm, 1970). AMD can negatively affect the sur- rounding terrestrial and aquatic ecosystems (Amos et al., 2015; Betrie et al., 2016; Luís et al., 2011). The development of effective technologies for the stabilization of pyritic tailings, before their fnal disposal is of primary importance in sulfde mining activities. A promising approach to environmentally safe management of potentially acid generating waste is the development of artifcial coatings on the sulfde mineral surfaces to prevent oxidation (Garbarino et al., 2018). Phosphate coating technology involves leach- ing of the sulfdic material with a solution containing an oxidant (usu- ally H 2 O 2 ) to produce Fe(III) ions in the presence of a phosphate source (KH 2 PO 4 , NaH 2 PO 4 ) and form an iron phosphate layer on the surface of pyrite grains. The pH of the coating solution must be adjusted to 4.0–6.0, which can be achieved by using an appropriate buffer, e.g. sodium ac- etate (CH 3 COONa) (Evangelou and Huang, 1994). The formation of iron phosphate precipitates on the pyrite surface is described by the following reaction (Evangelou, 1995a, 1995b): FeS 2 þ 7.5H 2 O 2 þ H 2 PO 4  → FePO 4 þ 2SO 4 2 þ 3H þ þ 7H 2 O (1) An alternative approach involves the use of solid phosphate com- pounds, i.e. apatite, as the source of PO 4 3 (Flynn, 1969; Kalin and Harris, 2005; Spotts and Dollhopf, 1992; Stiller et al., 1986; Ueshima * Corresponding author. E-mail address: kkollias@metal.ntua.gr (K. Kollias). Contents lists available at ScienceDirect Applied Geochemistry journal homepage: http://www.elsevier.com/locate/apgeochem https://doi.org/10.1016/j.apgeochem.2019.104435 Received 3 March 2019; Received in revised form 27 September 2019; Accepted 28 September 2019