Redox reaction of aqueous selenite with As-rich pyrite from Jiguanshan ore mine (China): Reaction products and pathway

Highlights

• Arsenic local environment in As-rich pyrite sample is very close to arsenopyrite.

• Se(0) was the main reduction product when Se(IV) reacted with As-rich natural pyrite at pH 5.05–8.65.

• Release of the As impurity via arsenopyrite oxidation is preferential for As-rich pyrite dissolution.

Abstract

The interaction of an As-rich natural pyrite (FeS_2.08As_0.043) with aqueous Se(IV) was investigated as a function of pH, ferrous iron concentration, and reaction time. Arsenic is often the most abundant minor constituent of natural pyrite, and is believed to substitute for S in the pyrite structure. EXAFS measurements confirmed the presence of AsS dianion group, with arsenic in the same local configuration as in the arsenopyrite. Speciation studies indicated that Se(0) was the unique reduction product in the pH range 5.05–8.65 over a reaction period of >1 month, while trace amounts of FeSeO₃ might be formed at pH ⩾ 6.10. At pH > 6.07, the formation of Fe(III)-(oxyhydr)oxide is kinetically favored, and it consumed nearly all the aqueous iron, including the extra added Fe²⁺, thereby inhibiting the formation of the thermodynamically most stable product: FeSe₂. After oxidation by Se(IV), the occurrence of surface S⁰, significant aqueous sulfur deficit, and excessive leaching of arsenic in solution, indicate the preferential release of As impurity via arsenopyrite oxidation. The data suggest that the polysulfide-elemental sulfur pathway, which prevails in acid-soluble metal sulfides, is an important pathway in the oxidation of As-rich pyrite, in addition to the thiosulfate pathway for acid-insoluble pyrite. Control experiments on As-free natural pyrite further support this mechanism. This study confirms the potential of reductive precipitation to attenuate the mobility of Se in the environment and demonstrates that minor elements commonly present in natural pyrite can play a significant role on its dissolution pathway.

Introduction

Selenium is essential to human life and diseases related to both its excess and deficiency occur all over the world (Fordyce et al., 2000, Klaasen, 2008, Rayman, 2008). In addition, the radioactive isotope ⁷⁹Se, with a half-life of 3.77 × 10⁵ years (Bienvenu et al., 2007), is presently considered as a key mobile fission product for the disposal of spent fuel and high-level radioactive waste (Chen et al., 1999, Grambow, 2008). For these reasons, Se mobility and bioavailability is of major interest in soil and environmental sciences, and an important concern for the safe disposal of radioactive waste.

The solubility of selenium is significantly controlled by its oxidation state, which in turn depends on the redox conditions of the surrounding environment. Tetravalent and hexavalent Se are much more soluble and mobile aqueous oxyanions than zero or lower oxidation states Se species (Scheinost and Charlet, 2008, Scheinost et al., 2008). Due to the limitations in physisorption of Se(IV) and Se(VI) on natural minerals (the sorption efficiency is relatively low and the sorption reaches saturation soon), in particular on granite and claystone minerals that are considered as host rock candidates for nuclear waste disposals (Baryosef and Meek, 1987, Guo et al., 2011), reductive precipitation is expected to be the most effective way to immobilize ⁷⁹Se.

On the other hand, pyrite (FeS₂) is the most widely distributed metal sulfide mineral in the geological environment, and is also present in claystones and granitic rocks (Baeyens et al., 1985, Metz et al., 2003, Gaucher et al., 2004). Owing to its strong reducing ability, and to its stability under anoxic conditions, pyrite is expected to be one of the major reductive components in candidates host rocks for radioactive waste disposal, and, therefore, to give an important contribution to limit the release of redox-sensitive radionuclides like ⁷⁹Se on a geological time-scale.

Selenium speciation on pyrite surfaces has been the subject of many studies, but conflicting results have been reported. For example, Breynaert et al. (2008) found crystalline Se(0) as the only Se-bearing reaction product of the pyrite-Se(IV) reaction between pH 6.5 and 8.5. Elemental Se was also observed as the reaction product of pyrite-HSe⁻ reaction at pH 6.6 (Liu et al., 2008), and Se(IV)-doped pyrite at pH values ranging from 3.7 to 5.0 (Diener et al., 2012). On the other hand, Naveau et al. (2007) excluded the formation of Se(0) and suggested either Se(-I), or Se(-II) solids were formed when aqueous Se(IV) and Se(-II) reacted with pyrite at pH 3.0. The different results observed in the above experiments suggest that Se speciation on pyrite surface is significantly affected by the physico-chemical condition under which the reactions occur. Thermodynamically, in nearly neutral to alkaline conditions, FeSe₂ should be the predominant product in pyrite-containing systems, and ferrous iron can favor its formation (Charlet et al., 2012).

The kinetics and mechanism of pyrite oxidation are of great concern in geochemistry and environmental science research, because of its main role in acid mine drainage (AMD) (Moses and Herman, 1991, Williamson and Rimstidt, 1994, Descostes et al., 2004). In addition to sulfate, elemental sulfur was also observed as an endproduct of pyrite oxidation (Sasaki et al., 1995). Luther (1997) proposed that the decomposition of sulfur intermediates (e.g., thiosulfate) could lead to these endproducts and the nonstoichiometric dissolution of pyrite. However, the influence factors on the pyrite oxidation pathway are still not well-documented.

Naturally occurring pyrite commonly contains minor substituted metals and metalloids (As, Se, Hg, Cu, Ni, etc.). Arsenian pyrites, for example, may contain up to 10% As (Rimstidt and Vaughan, 2003). Nevertheless, the studies linked to the influence of these minor or trace elements on pyrite oxidation are very few.

In this study, redox reaction between natural As-rich pyrite and aqueous Se(IV) were conducted in the pH range 5.05–8.65. The effects of adding ferrous ion and of the reaction time on the reduction products, as well as the effect of arsenic impurity on pyrite dissolution pathway, were investigated. The results were interpreted in light of thermodynamic calculations, X-ray Absorption Spectroscopy (XAS), X-ray Photoelectron Spectroscopy (XPS), and solution analyses.