Removal of mercury (II) and chromium (VI) from wastewater using a new and effective composite: Pumice-supported nanoscale zero-valent iron

doi:10.1016/j.cej.2014.02.011

Chemical Engineering Journal

Volume 245, 1 June 2014, Pages 34-40

https://doi.org/10.1016/j.cej.2014.02.011 Get rights and content

Highlights

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The properties of NZVI were obviously enhanced using pumice as the support.
•
P-NZVI was more effective to remove heavy metals from wastewater.
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P-NZVI is a regenerated material.
•
P-NZVI was suitable for applications to in situ environmental remediation.

Abstract

Nanoscale zero-valent iron successfully supported on pumice (P-NZVI) was used to remove heavy metals from wastewater with a higher removal capacity and efficiency. NZVI particles with a mean diameter of 30.6 nm are distributed uniformly on the surface of P-NZVI. The thermal stability and mechanical strength of P-NZVI were also obviously enhanced. P-NZVI with a 7.7% NZVI mass fraction had a specific surface area (S_BET) of 32.2 m²/g. The removal capacity of Hg (II) and Cr (VI) by P-NZVI was 332.4 mg Hg/g Fe and 306.6 mg Cr/g Fe, respectively. As an increase of pH, the removal rates of Hg (II) increased but those of Cr (VI) decreased gradually. P-NZVI is a regenerated material. The X-ray photoelectron spectroscope analysis (XPS) results indicated that Hg (II) and Cr (VI) were removed by a rapid physical adsorption in the first 0.5 min and predominantly by reduction. In terms of the efficiency and speed, P-NZVI was a promising candidate for applications to in situ environmental remediation, especially to the heavy metals pollution incidents with an extremely high concentration of heavy metals.

Introduction

Mercury (II) and chromium (VI) are two toxic metals found in various industrial wastewaters. Several industrial activities, such as electroplating, leather tanning, metal finishing and petroleum refining, can cause water pollution by these ions [1], [2]. Their levels in the wastewater are much higher than the action level of 0.001 mg/L for Hg (II) and 0.1 mg/L for Cr (VI) [3]. Mercury (II) and chromium (VI) contaminate the environment, affect aquatic life and cause several health problems. Hence, these heavy metals must be removed from wastewaters before being discharged to the environment.

Various methods, including chemical precipitation, membrane filtration, ion exchange and adsorption, were used to remove heavy metals from wastewaters [4]. Recently, the use of nanoscale zero-valent iron (NZVI) to remove heavy metals has become one of the most promising and effective remediation technologies because of its extremely small particle size, large surface area and high reactivity [5], [6], [7]. However, NZVI was usually agglomerated in conventional systems along with a decrease of its reactivity and mechanical strength, which limited the practical application of NZVI [8]. In recent years, porous materials, including zeolite, kaolinite, bentonite and pillared clay, have been widely used as mechanical supports to enhance the dispersibility of NZVI particles [9], [10], [11]. More recently, NZVI was stabilized by chitosan and supported by chitosan beads to increase the dispersibility and stability in air [12], [13], [14]. Nevertheless, the mechanical strength of chitosan beads needed to be improved [15].

Pumice is a porous volcanic rock and has a large surface area and skeleton structure, which contains open channels allowing water and ions to travel into and out of the crystal structure [16]. Furthermore, pumice can be processed easily and used as a low-cost heavy metals adsorbent [17]. Pumice–iron granular mixtures perform well in removing contaminants and maintaining the long-term hydraulic conductivity [18]. However, only a few studies have focused on using the pumice as the support material for NZVI.

In the study, in order to enhance its dispersibility and stability in the air, NZVI was supported on pumice (P-NZVI) for the removal of Hg (II) and Cr (VI) from aqueous solution. The main objectives are to: (1) synthesize and characterize the new and stable P-NZVI composite, (2) evaluate the removal efficiency of Hg (II) and Cr (VI) by P-NZVI under different experimental conditions, (3) assess the reuse of P-NZVI and (4) test the role of pumice during remediation process and investigate the elemental composition of final products to reasonably conclude the removal mechanism of Hg (II) and Cr (VI) by P-NZVI.

Section snippets

Materials and chemicals

Pumice with a diameter of 0.5–1.0 mm was provided by Dahe Building Materials Co., Ltd. (Hebei, China) and the chemical composition was 67.2% SiO₂, 22.1% Al₂O₃, 2.98% Fe₂O₃, 2.28% CaO and small amounts of Mn, Mg, P and S. Iron (III) chloride hexahydrate (FeCl₃·6H₂O) and sodium borohydride (NaBH₄) were purchased from Fuchen Chemical Reagent Manufactory (Tianjin, China). K₂CrO₄, HgCl₂ and absolute alcohol were provided by First Chemical Reagent Manufactory (Tianjin, China). All other chemicals were

Characterization of P-NZVI

The morphology of P-NZVI is presented in Fig. 1. It can be seen from Fig. 1(a) that pumice is porous with open channels and rugged surfaces [16], which is favorable for the load and support of NZVI particles on overall framework and all of its surfaces. Other researchers reported that pumice particles consisted of a network of irregular or oval shape internal voids/pores or vesicles, some of which were interconnected and open to the external surface [20]. Pumice was a promising porous support

Conclusions

In this study, P-NZVI was successfully prepared with a higher capacity and efficiency to remove heavy metals from wastewater. Based on the results, the major findings are summarized as follows:

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NZVI particles are nearly spherical in shape with a mean diameter of 30.6 nm. NZVI particles are distributed uniformly on the surface of pumice demonstrating that pumice is effective to prevent NZVI particles from agglomerating. P-NZVI with a 7.7% NZVI mass fraction had a S_BET of 32.2 m²/g.
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The thermal

Acknowledgments

The authors thank Zhigang Zhang, Qian Wang and Fei He for their support with analyses. This work was financially supported by National Science & Technology Pillar Program (2012BAC07B00), Program for New Century Excellent Talents in University (NCET-10-0954), National Natural Science Foundation of China (21307090), the National Science & Technology Pillar Program (2012BAC07B02) and the University Science & Technology Development Project of Tianjin (20110528).

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Removal of mercury (II) and chromium (VI) from wastewater using a new and effective composite: Pumice-supported nanoscale zero-valent iron

Highlights

Abstract

Introduction

Section snippets

Materials and chemicals

Characterization of P-NZVI

Conclusions

Acknowledgments

J. Hazard. Mater. B

J. Hazard. Mater.

Water Res.

React. Funct. Polym.

Appl. Clay Sci.

Chem. Eng. J.

Chemosphere

J. Hazard. Mater.

Chem. Eng. J.

Chem. Eng. J.

J. Hazard. Mater. B

J. Environ. Manage.

Catal. Commun.

Chemosphere

Water Res.

Water Res.

Biomaterials

J. Colloid Interface Sci.

Desalination

J. Hazard. Mater.

J. Contam. Hydrol.

Chem. Eng. J.