Effect of pH on Aqueous Se(IV) Reduction by Pyrite

Abstract

International audienceInteraction of aqueous Se(IV) with pyrite was investigated using persistently stirred batch reactors under O2-free (<1 ppm) conditions at pH ranging from 4.5 to 6.6. Thermodynamic calculations, an increase in pH during the experiments, and spectroscopic observation indicate that the reduction of aqueous Se(IV) by pyrite is dominated by the following reaction: FeS2 + 3.5HSeO3− + 1.5H+ = 2SO42− + Fe2+ + 3.5Se(0) + 2.5H2O. The released Fe(II) was partitioned between the bulk solution and pyrite surface at pH ≈ 4.5 and 4.8, with the Fe2+ density at pyrite-solution interface about 4 orders of magnitude higher than that in the bulk solution, while iron oxyhydroxide precipitated at pH ≈ 6.6, resulting in the decrease of dissolved iron. In the Se(IV) concentration range of the experiments, aqueous Se(IV) reduction rate follows the pseudofirst order which is in the form of ln mSe(IV) = −k′t + ln mSe(IV)0, where k′ is apparent rate constant combining the rate constant k and pyrite surface area to mass of solution ratio (A/M). And the aqueous Se(IV) reduction rate constant for a standard system (k) with 1 m2 pyrite surface area per 1 kg solution was obtained to be 1.65 × 10−4 h−1, 3.28 × 10−4 h−1, and 4.76 × 10−4 h−1 at pH around 4.5, 4.8, and 5.1, respectively. The positive correlation between reaction rate and pH disagrees with the theories that protons are consumed when HSeO3− is reduced to Se0, and negative charge density on pyrite surface increases as pH increases. Thus, a ferrous iron mediated electron transfer mechanism is proposed to operate during the reduction of aqueous Se(IV) by pyrite. pH and iron concentration affect significantly on Se(IV) reaction rate and reaction product

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

“…These facts are consistent with the reduction of Se(IV) to Se(0) (Kang et al, 2011 (Fig. 4) Descostes et al (2004) reported values ranging from 1.25 at pH 1.5 to 1.6 at pH 3, while Kamei and Ohmoto (2000) obtained values in the range 0.93-2.00 for the same reaction at pH 5.7 ± 0.3.…”

“…In a previous work we found that, in the presence of unreduced selenite, Se(0) rather than iron selenides can be formed on Fe(II)-bearing minerals (Kang et al 2013). Therefore, the iron decrease can be attributed to the formation of Fe(OH) 3 (Kang et al, 2011), in agreement with XPS results (Fig. A.6), while, at the same time, additional pyrite was decomposed through reaction (1) leading to the release of more aqueous S (see Section 3.4 in the following).…”

“…Commercial Na 2 SeO 4 (MERCK), Na 2 SeO 3 Á5H 2 O (Sigma Aldrich), crystalline grey Se(0) (Sigma Aldrich), and FeSe (Alfa Aesar) were used as Se(VI), Se(IV), Se(0), and Se(-II) references, respectively. FeSeO 3 reference was the same used in a previous work (Kang et al, 2011).…”

Selenium Speciation in Framboidal and Euhedral Pyrites in Shales

“…18−20 Framboids are difficult to synthesize in the laboratory, 39 and no experimental evidence on Se-framboidal interactions is available. Some experimental studies used iron sulfides (e.g., mackinawite, 42 pyrite [18][19][20]43 ) and sorbed Se onto them. When synthetic pyrite was used, the Se was coreduced leading to the formation of ferroselite FeSe(-I) x .…”

Sorption behavior of hydroselenide (HSe−) onto iron-containing minerals

“…Selenium had been considered to migrate as soluble SeO 3 2− /SeO 4 2− because of its slow reduction kinetics [53][54][55][56], in spite of thermodynamic stability of Se(−II) under reducing disposal conditions. Recent works have demonstrated the abiotic reduction of Se(IV) and Se(VI) to Se(0) by several Fe(II)-bearing minerals [24,[55][56][57][58][59][60][61][62] and to Se(−II) by Fe(0) nanoparticles [63]. Understanding the sorption behavior of Se(−II) species is important; however, the sorption data of Se(−II) are scarce because of the experimental difficulties of maintaining the reducing state of Se.…”