A kinetic approach on hexavalent chromium removal with metallic iron

doi:10.1016/j.jenvman.2017.03.031

Journal of Environmental Management

Volume 203, Part 3, 1 December 2017, Pages 937-941

https://doi.org/10.1016/j.jenvman.2017.03.031 Get rights and content

Highlights

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Chemical reduction is generally the main path of Cr(VI) removal with Fe(0) at pH 2.5.
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Experimental parameters may, however, affect kinetics and mechanism of Cr(VI) removal.
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Adsorption seems to be the main path of Cr(VI) removal at low temperature.
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Adsorption seems to be the dominant path of Cr(VI) removal with nano-Fe(0).
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Relevance of indirect reduction with Fe(II) was emphasized in presence of SO₄²⁻/Cl⁻.

Abstract

This paper examines the mechanism of Cr(VI) removal with Fe(0), and the possible effect of various experimental parameters, from a kinetic perspective. The experimental data was analyzed using five different kinetic models: three for chemical reactions and two for adsorption processes. It was found that the process fitted well to the zero-order kinetic model for all investigated systems, excepting experiments conducted at 6 °C and those with nano-Fe(0), when the process followed the Ho's pseudo second-order model. Therefore, even though, under acidic conditions, chemical reduction can be generally considered as the main mechanism of Cr(VI) removal with Fe(0), under some experimental conditions (e.g. when working with nano-Fe(0) or at low temperatures), adsorption seems to be the dominant removal path. The enhanced Cr(VI) removal noticed in co-presence of SO₄²⁻ and Cl⁻ anions reiterates the significance of the secondary reductant Fe(II) within the process of Cr(VI) removal with Fe(0).

Introduction

Nowadays, contamination of water environments has become a significant concern, especially in the industrialized countries, due to increasing anthropogenic inputs after the industrial revolution (Chrysochoou and Dermatas, 2015). Because metallic iron (Fe(0)) is a relatively low cost material, readily available, with low toxicity (Btatkeu et al., 2016), important efforts have been focused on the use of Fe(0) for the removal of a wide range of pollutants, both inorganic (e.g. heavy metals (Hashim et al., 2011), metalloids (Vitkova et al., 2017)) and organic (e.g. dyes (Raman and Kanmani, 2016), phenols (Nakatsuji et al., 2015), estrogens (Jarosova et al., 2015)). Heavy metals are particularly problematic contaminants because they are highly toxic, non-biodegradable, and persistent (Pehlivan and Altun, 2008). Chromium is an important metal with widespread use in various industries; as a result, large quantities of this metal have been discharged into the environment due poor storage practices, improper disposal or leakage of chromium waste. In natural environments, chromium can exist mainly in two oxidation states: (+III) and (+VI). Among these two, Cr(VI) exerts the most toxic effects on living organisms, having also the highest mobility in the environment (Gheju, 2011, and references therein). Over the last decades, Fe(0) has been demonstrated to represent a highly efficient reagent for the removal of Cr(VI) from contaminated waters; however, there is yet no consensus at this time in what regards the mechanism of Cr(VI) removal with Fe(0). The first mechanism, proposed in the nineties (the ”reductive precipitation” mechanism) (Cantrell et al., 1995), and widely accepted until our days (Kong et al., 2016), attributed the efficiency of Fe(0)-systems mainly to the direct electron transfer from Fe(0) surface to Cr(VI), coupled with (co-)precipitation of resulted Cr(III). It was probably suggested in agreement with the direct reductive dechlorination mechanism, previously proposed as the most likely cause of chlorinated organics removal with Fe(0) (Gillham and O'Hannesin, 1994). Subsequent studies have, however, acknowledged the importance of another process, Cr(VI) adsorption, as intermediate step within the mechanism of Cr(VI) removal with Fe(0) (Powell et al., 1995). Moreover, it has been indicated that adsorption on some types of Fe(0) (e.g. nano-sized) can be regarded not only as intermediate step, but also as a dominant Cr(VI) removal mechanism by itself (Ai et al., 2008). Recent studies also suggested that, in Fe(0)-H₂O systems, along with co-precipitation (Noubactep, 2015a) and size-exclusion (Yoon et al., 2011), adsorption is one of the main contaminant removal mechanisms, while reduction, when possible, occurs mainly indirectly via Fe(0) corrosion products (Noubactep, 2015b). Even though numerous studies investigated the kinetics of Cr(VI) removal with Fe(0) (Gheju, 2011, and references therein), to authors knowledge, the assessment of the kinetic model was not yet used to evaluate the role of different mechanisms within the process of Cr(VI) removal with Fe(0). Therefore, the goal of the present paper was to investigate the importance of different possible removal paths within the mechanism of Cr(VI) removal with Fe(0), as well as the effect of several important parameters, by means of kinetic analysis of experimental data.

Section snippets

Materials and methods

Commercially available Fe(0) from Alfa Aesar (≥99%, ∼1–2 mm) and from Merck (≥99%, ∼10 μm) (hereinafter referred to as milli-Fe(0) and micro-Fe(0), respectively) was used as received. In addition, nano-Fe(0) was synthesized via the liquid-phase reduction method with sodium borohydride, following a procedure described by Xi et al. (2010). Cr(VI) removal experiments were carried out in a 1.5 L Berzelius flask, by introducing a mass of 0.5 g Fe(0) into 1000 mL of Cr(VI) solution. The mixture was

Effect of pH

The influence of solution pH was investigated at 20 °C, within the range of 1.1–3.5, using a 2 mg/L Cr(VI) solution and micro-Fe(0). It is shown that Cr(VI) removal significantly decreased with increasing pH, being already almost totally inhibited at pH 3.1 (Fig. 1). This observation can be ascribed to involvement of H⁺ ions in processes contributing to Cr(VI) removal in Fe(0)-H₂O system. Cr(VI) removal with Fe(0) is the result of a complex interplay of processes such as adsorption, reduction

Conclusion

In this work, the variation of the kinetic model was used to evaluate the role of different mechanisms within the process of Cr(VI) removal with Fe(0). The reported results have shown that temperature and nature of Fe(0) can significantly affect both kinetics and mechanism of Cr(VI) removal with Fe(0). While over the temperature range of 20–33 °C the kinetics was described by a zero-order model, at 6 °C the Ho's pseudo second-order model exhibited the highest correlation with experimental data.

Acknowledgments

This work was supported by a grant of the Romanian National Authority for Scientific Research and Innovation, CNCS – UEFISCDI, project number PN-II-RU-TE-2014-4-0508. The manuscript was improved by the insightful comments of anonymous reviewers from JEMA.

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Research articleA kinetic approach on hexavalent chromium removal with metallic iron

Highlights

Abstract

Introduction

Section snippets

Materials and methods

Effect of pH

Conclusion

Acknowledgments

J.Hazard. Mater.

J. Hazard. Mater.

J. Environ. Manage.

J. Environ. Manage.

J. Hazard. Mater.

J. Environ. Manage.

J. Environ. Manage.

J. Environ. Manage.

J. Hazard. Mater.

Water Res.

J. Hazard. Mater.

J. Environ. Manage.

Chem. Eng. J.

Nano Today

J. Environ. Manage.

Mater. Res. Bull.

Research article
A kinetic approach on hexavalent chromium removal with metallic iron