Efficient selenate removal by zero-valent iron in the presence of weak magnetic field

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Highlights

  • Efficient Se(VI) removal was achieved by ZVI at very low dosage coupling with WMF.

  • The rate of Se(VI) removal by ZVI with WMF increased with increasing ZVI dosage.

  • Se(VI) adsorption to the ZVI surface was the first step of Se(VI) removal by ZVI.

  • The adsorbed Se(VI) was rapidly reduced to Se(IV) and then slowly to Se(0).

  • Employing WMF to enhance Se(VI) removal by ZVI under oxic conditions is promising.

Abstract

Se(VI) was very refractory to be removed by zero-valent iron (ZVI), therefore weak magnetic field (WMF) was employed to achieve efficient Se(VI) removal by ZVI. Batch experiments showed that negligible Se(VI) (<4%) was removed by ZVI without the application of WMF within 72 h. The presence of WMF dramatically enhanced Se(VI) sequestration by ZVI and complete removal of 10.0 mg L−1 Se(VI) was achieved by ZVI in 90 min. The main portion of kinetics of Se(VI) removal by ZVI in the presence of WMF followed zero-order rate law and the rate constants of Se(VI) sequestration by ZVI increased progressively with increasing the ZVI dosage. Fe K-edge XAFS spectra and synchrotron radiation-XRD analysis revealed that ZVI was transformed to X-ray amorphous Fe3O4 before finally transformed to lepidocrocite (γ-FeOOH). The LCF analysis of Se K-edge XANES spectra indicated that adsorptive removal of Se(VI) was minor but adsorption of Se(VI) to the corroded ZVI surface was the first step of Se(VI) removal. Se(VI) was rapidly reduced to Se(IV), which could be further transformed to Se(0). Comparison of the performance of Se(VI) removal in the literature suggested that employing WMF to enhance Se(VI) removal by ZVI under oxic conditions be a promising method.

Introduction

Selenium is an environmental pollutant and ranks 147th on the Superfund Priority List of Hazardous Substances of the U.S. Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) [1]. Depending on its concentration, selenium can act as an essential micro-nutrient protecting against reactive oxygen species damages, or as a toxic compound [2]. Se pollution is a worldwide problem and mainly originates from agricultural practices, manufacture processes, coal combustion and mining processes [3]. Se was found to be present in elevated concentrations in acid mine drainage which varied from 1 to 7000 μg L−1 [4].

Selenium (Se) is a metalloid that exists in a variety of oxidation states including selenide (Se(-II)), elemental Se (Se(0)), selenite (Se(IV)), and selenate (Se(VI)) [5]. The oxidized forms of Se, Se(VI) and Se(IV), are soluble and mobile and thus are potentially toxic [6]. Selenite is similar to phosphate in terms of mobility in the environment and tends to be adsorbed more strongly than selenate onto adsorbents such as goethite and hematite [7], [8]. However, selenate, similar to sulfate, is very difficult to be adsorbed on various minerals. Thus, it is the most mobile selenium species and very difficult to be removed by the conventional methods including coagulation, lime softening and adsorption [9]. Compared to the above Se(VI) removal methods, reductive removal by zero-valent iron (ZVI) should be more favored since ZVI is a readily available, inexpensive, and moderately strong reducing agent and can transform Se(VI) to the more immobile species, Se(IV), Se(0) or Se(-II) [3].

Several studies had been carried out to examine the performance of ZVI toward Se(VI) removal. However, the iron filings or microscale iron powder had low reactivity toward Se(VI) removal. A huge dosage of micron-sized ZVI (50–100 g L−1) was necessary to sequester Se(VI) and the removal capacity of ZVI for Se(VI) was very low [2], [10]. To improve the removal rate of Se(VI) by ZVI, nano-sized ZVI (nZVI) and NiFe bimetal were used [11]. Moreover, Tang et al. [12], [13] proposed to apply Co2+, Mn2+ or Fe2+ to improve Se(VI) removal by ZVI. However, both methods bear some demerits. Although iron is inexpensive in bulk form, nZVI and nano-sized NiFe bimetal were much more expensive because the costly precursor reagents and complicated processes are needed to synthesize them [14]. Furthermore, the toxicity of nanomaterial has arisen much concern [15] and Lee et al. [16] reported that nZVI showed a strong bactericidal activity comparable to that of silver nanoparticles. Although Tang et al. [12], [13] did show that the application of Co2+, Mn2+ or Fe2+ could greatly enhance Se(VI) by ZVI, a ZVI dosage as high as 50.0 g L−1 was employed in their study to removal 20.0 mg L−1 Se(VI) and the dosing of Co2+ or Mn2+ may induce secondary pollution. Therefore, it is critical to explore an environmentally friendly method that can significantly improve the reactivity of ZVI to remove Se(VI).

Our recent studies reported that the application of a weak magnetic field (WMF) could greatly accelerate ZVI corrosion and sequestration of Se(IV), As(III), and As(V) [17], [18], [19]. The primary role of WMF in the process of contaminants removal by ZVI was to enhance mass transfer [19]. Up to now, no study had been performed on the influence of WMF on Se(VI), which was much more refractory than Se(IV), removal by ZVI. However, two studies [11], [20] on Se(VI) removal by ZVI, which employed magnetic stirrer to offer mixing, showed much more efficient Se(VI) removal by ZVI than other studies. The magnetic field generated by the magnetic stirrer was stronger than the WMF applied in our previous studies [18], thus it was expected that WMF had promoting effect on Se(VI) removal by ZVI. Therefore, the current work was aimed at investigating the kinetics and mechanisms of Se(VI) removal from water by ZVI in the presence of WMF.

Section snippets

Materials

All chemicals were analytical grade and used as received. The high purity ZVI powders (99.8–99.9% Fe0), with d50 of 7.4 μm and BET specific surface area (as) of 0.3015 m2 g−1, were purchased from Beijing Dk Nano technology Co., LTD, and used in this study without further treatment. Chemicals including Na2SeO4⋅10H2O, HCl, NaOH and 2-(N-morpholino)ethanesulfonic acid (MES) were purchased from Shanghai Qiangshun Chemical Reagent Company and were used as received in this study. The stock solutions

Effect of initial Se(VI) concentrations

As illustrated in Fig. 1(a), negligible Se(VI) (<4%) was removed by ZVI without the application of WMF within 72 h, regardless of the initial Se(VI) concentration varying from 10.0 to 100.0 mg L−1. Similar phenomenon had been reported by Tang et al. [13] although ZVI dosage as high as 50 g L−1 was employed in their study. In our previous study [18], it took only 60 min to achieve almost complete removal of Se(IV) of 40.0 mg L−1 by the same ZVI sample under similar conditions. Obviously, the reductive

Conclusions

This study showed that Se(VI) could be effectively removed by ZVI in the presence of WMF at pH 6.0. With the application of WMF, 10.0 mg L−1 Se(VI) could be completely removed by 1.0 g L−1 ZVI in 90 min and 40.0 mg L−1 Se(VI) could be completely removed by 2.5 g L−1 ZVI in 4 h under oxic conditions. Se(VI) removal by ZVI in the presence of WMF was much more favored under oxic conditions than that under anoxic conditions. Fe K-edge and Se K-edge XAFS spectra indicated that Se(VI) was sequestered by

Acknowledgments

This work was supported by the National Natural Science Foundation of China – China (21277095 and 51478329), the Zhejiang Province Natural Science Foundation (LQ15E0 80003), the Shaoxing University of Research Startup Project (20145033), the Specialized Research Fund for the Doctoral Program of Higher Education (20130072110026), and the Tongji University Open Funding for Materials Characterization (2013080). The authors thank beam-lines BL14W1 and BL14B1 (Shanghai Synchrotron Radiation

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