Elsevier

Chemosphere

Volume 215, January 2019, Pages 739-745
Chemosphere

Enhanced removal of Cr(VI) by silicon rich biochar-supported nanoscale zero-valent iron

https://doi.org/10.1016/j.chemosphere.2018.10.030Get rights and content

Highlights

  • Silicon particles in biochar served as a support site for nZVI.

  • SiO2 in biochar promotes the oxidation of iron.

  • Cr(VI) removal was enhanced by silicon-rich biochar.

  • NZVI-RS700 showed the highest Cr(VI) removal due to reduction and coprecipitation.

  • It was proved that Cr(VI) removal by nZVI-RS700(-Si) was significantly decreased.

Abstract

Silicon-rich biochar-supported nanoscale zero-valent iron (nZVI) was studied to evaluate enhanced removal of hexavalent chromium (Cr(VI)) in solution. The compositional structures of the nZVI and biochar-supported nZVI were analyzed by Fourier transform infrared spectroscopy, X-ray diffraction and X-ray photoelectron spectra before and after Cr(VI) reaction. The removal amount of Cr(VI) by nZVI-RS700 (rice straw pyrolyzed at 700 °C) was considerably greater than that by nZVI and other biochar-supported nZVI samples. Upon the silicon was removed from RS700 (nZVI-RS700(-Si)), a significant decreased removal of Cr(VI) was observed. It was revealed that nZVI supported by silicate particles of biochar and the promotion of iron oxidation by SiO2 both contribute to the enhanced Cr(VI) removal. We found that the reduction and adsorption both contributed to the removal of Cr(VI), ferrous chromite (FeCr2O4) was observed on the surface of the nZVI-RS700 composite. The formation of FeCr2O4 is attributed to the reduction of Cr(VI) by nZVI and the adsorption of chromium oxide with iron on the surface of RS700. Therefore, RS700-supported nZVI can be used as a potential remediation reagent to treat Cr(VI)-contaminated groundwater.

Graphical abstract

The mechanism of Cr(Ⅵ) removal by biochar support nanoscale zero-valent iron.

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Introduction

Nanoscale zero-valent iron (nZVI) has received increasing attention for its high activity and environmental friendliness (Chen et al., 2018, Laumann et al., 2013, Suanon et al., 2016, Zhang, 2003). As an effective reducing agent, nZVI can be used to remove a variety of heavy metal and chlorinated organic contaminants through chemical reduction (Bruton et al., 2015, Elliott and Zhang, 2001, Zou et al., 2016). However, the rapid loss of reactivity in groundwater due to aggregation severely limits environmental application of nZVI (Phenrat et al., 2007, Su et al., 2016a, Su et al., 2016b). Recent studies have shown that the activity can be dramatically improved using zeolite, biopolymers, Fe3O4, resin, carbon materials and many other materials (Chen et al., 2018, Fu et al., 2015, Fu et al., 2017, Lei et al., 2018, Kim et al., 2014). These supported material prevent nZVI from agglomeration, thereby prolonging the reactivity of the particles.

Among such supporting materials, porous carbon is one of the most commonly used support for nZVI (Stefaniuk et al., 2016, Tian et al., 2018). This material is widely employed primarily due to its large specific surface area, porous structure and the adsorption effect of pollutants (Cao et al., 2009, Chen et al., 2008; Lv et al., 2011; Yan et al., 2015). As one of typical porous carbon materials, biochar can serve as an effective supporter of nZVI, and thus enhance the removal of organic pollutants and hexavalent chromium (Chen et al., 2011, Lyu et al., 2017, Qian et al., 2017). Typically, many biochars, such as rice straw biochar, contain a number of silicon minerals (Liu et al., 2015, Qian and Chen, 2013, Qian and Chen, 2014). Our previous report has assumed that the silicon in biochar may serve as one of the support sites for nZVI (Qian et al., 2017). However, the comprehensive mechanism for such support needs to be further studied. It is proposed that silicon can be a supporter of nZVI, with the crystallization and adsorption properties of ferric oxides (Anderson and Benjamin, 1985, Zheng et al., 2008). To the best of our knowledge, few studies have described the influence of silicon in biochar on nZVI support and the mechanism for Cr(VI) removal.

Chromium exists primarily in the form of Cr(III) and Cr(VI), with Cr(VI) being approximately 1000 times more toxic than Cr(III), leading to classification as a priority pollutant in China (He et al., 2012, Norseth, 1981). The primary objective of this study was to evaluate enhancement in Cr(VI) removal by silicon-rich biochar-supported nZVI. The various biochars were derived from rice straw pyrolyzed under varying temperatures (300, 500 and 700 °C). The removal of Cr(VI) by nZVI and biochar-supported nZVI (nZVI-RS300, nZVI-RS500 and nZVI-RS700) was investigated for pH values ranging from 3.0 to 4.5. The effect of the initial Cr(VI) concentration was analyzed to identify the optimal concentration for Cr(VI) removal by nZVI and biochar-supported nZVI. Then, the structures of nZVI and biochar-supported nZVI before and after interaction with Cr(VI) were analyzed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS). Finally, a potential mechanism was proposed for Cr(VI) removal by the biochar-supported nZVI.

Section snippets

Preparation of biochars and biochar-supported nZVI

Rice straw was prepared as the precursor for biochar, which was collected from Changshu Agroecological Experiment Station of the Institute of Soil Science, Chinese Academy of Sciences. The precursor was air-dried and ground before being passed through a 0.154-μm sieve. Biochars were produced through pyrolysis of the rice straw at 300, 500 and 700 °C in oxygen-limited conditions, with the production method being modified in accordance with previous reports (Qian et al., 2013, Qian et al., 2017).

Effect of pH and initial Cr(VI) concentration on Cr(VI) removal by nZVI and biochar-supported nZVI

The effect of pH on Cr(VI) removal by nZVI and biochar-supported nZVI is shown in Fig. 1. When the initial Cr(VI) concentration was 50 mg/L, the Cr(VI) removal rate was dependent on pH and the pyrolysis temperature. The Cr(VI) removal rate decreased rapidly as the initial pH was increased from 3.0 to 4.5. At the initial pH of 3.0, the removal rate of Cr(VI) by nZVI, RS300-nZVI, RS500-nZVI and RS700-nZVI was 78.5%, 67.2%, 71.5%, and 89.0%, respectively. When the pH increased, the Cr(VI) removal

Conclusions

Rice straw biochar-supported nZVI composite was successfully synthesized by an adsorption-reduction method and used to remove Cr(VI) in solution. The biochar was demonstrated to hinder the agglomeration and strengthen the dispersibility of nZVI particles and thus dramatically increase the reaction activity of nZVI and Cr(VI) removal efficiency. The biochar-supported nZVI showed better Cr(VI) removal compared to nZVI. The Cr(VI) removal efficiency was influenced by the pH value and initial

Acknowledgments

This article is financially supported by the National Natural Science Foundation of China (Grants No. 21507138 and 51309214); the Natural Science Foundation of Jiangsu Province, China (Grants No. SBK2015041561); the Frontier Fields during the Thirteenth Five-Year Plan Period of the Institute of Soil Science, Chinese Academy of Sciences (ISSASIP1656).

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