Removal of chromium(VI) from aqueous solution using manganese oxide nanofibers

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Abstract

Manganese oxide nanofibers (MONFs) were synthesized, characterized, and successfully used for the removal of the toxic Cr(VI) ions from a model and the real solution. The results showed that the MONFs were fibers, whose shape consisted of an average diameter of 10 nm to 16 nm and a specific surface area equal to 94.1 m2 g−1, that was mainly composed of crystalline α-MnO2 (i.e., as was confirmed with the XRD measurement). The effect of different parameters that affected the adsorption process was studied and optimized. The results showed an efficient removal at pH 2.0, within 60 min, and by using 150 mg MONFs at the ambient temperature. The adsorption of Cr(VI) ions from an aqueous solution was studied kinetically at different temperatures. The results showed that the removal process followed the pseudo-second-order model, with an adsorption capacity of 14.6 mg g−1 at ambient temperature. The thermodynamic parameters were calculated and the results showed that the adsorption of Cr(VI) ions from aqueous solution by MONFs is physical, spontaneous, and exothermic (ΔH = −39.3 kJ mol−1) in nature, with negative entropy. Finally, MONFs were used for the removal of Cr(VI) ions from a real sample, and the result indicated that the MONFs are highly efficient at removing the toxic Cr(VI) ions.

Introduction

Water pollution that results from rapid technological development and lax environmental oversight is considered to be one of the most important problems around the world. Heavy metals such as chromium are major pollutants of some ground and surface waters and are often present in industrial or urban waste waters. Chromium is usually generated by different anthropogenic activities, such as paper and dye manufacturing, electroplating, leather tanning, and wood preservation techniques [1]. Chromium exists in the environment in both trivalent [Cr(III)] and hexavalent [Cr(VI)] forms. However, hexavalent chromium is much more toxic than the trivalent form; thus, it causes many health problems for people (e.g., skin irritations; lung cancer; or kidney, liver, and gastric damage) [2]. Although many methods are used for the removal and elimination of Cr(VI) ions from the environment, adsorption is considered to be an efficient, cost-effective, and versatile method for the removal of Cr(VI) ions [3], [4], [5]. However, there is a need to identify new types of adsorbent approaches that ate characterized by a high adsorption capacity and rapid process kinetics. Manganese oxides are usually used in different applications, including energy storage [6], the use of electrochemical capacitors [7], and catalysis [8]. Many studies have indicated that manganese oxides could be used for the adsorption and elimination of different heavy metals from aquatic environment. For example, manganese oxide-coated zeolite was used for the removal of copper(II) and lead(II) from aqueous solutions [9], manganese oxide-coated carbon nanotubes were used for the removal of lead(II) from aqueous solution [10], [11], ferric and manganese binary oxides were used for the removal of arsenic from an aqueous solution [12], silver ions were removed from an aqueous solution by manganese oxide-modified vermiculite [13], and manganese oxide-coated graphene oxide achieved the defluoridation of an aqueous solution [14]. However, the application of manganese oxide for the removal and remediation of toxic chromium ions from within the aquatic environment is scarce in extant literature [15], [16]; additional work is required to explore the application of manganese oxide nanoparticles as an efficient adsorbent for the removal of hexavalent chromium.

In the present work, the synthesis of manganese oxide nanofibers (MONFs) was studied. The prepared MONFs were characterized in accord with different techniques, such as evaluation with a scanning electron microscope, a surface area analyzer, and X-ray diffraction. The prepared MONFs were used for the removal of the toxic hexavalent chromium from an aqueous model and a real solution. The effect of different parameters; such as removal time, adsorbent mass, and solution pH, all of which affect the adsorption/removal process, was researched. Further, the adsorption of the toxic hexavalent chromium from aqueous solution was studied kinetically to explore and understand the adsorption mechanism.

Section snippets

Chemical

All chemicals were analytical grade and obtained from Sigma-Aldrich. The experiments were performed by using de-ionized water with resistivity that did not exceed 18.2  cm and that was obtained with a Millipore Milli-Q system (Billerica, USA). A stock solution was prepared by dissolving a known quantity of potassium dichromate (K2Cr2O7) in de-ionized water. The stock solution was further diluted to the desired concentration of hexavalent chromium ions.

Synthesis of manganese oxide nanofibers

A sulfuric acid solution (3.0 M) was heated

Characterization of the manganese oxide nanofibers

Scanning electron microscope imaging was used to study the morphological structure of the prepared MONFs and representative images are presented in Fig. 1. The MONFs were fibers whose shape consisted of an average diameter of 10 nm to 16 nm and that were a couple of 100 nm in length. The nitrogen adsorption/desorption isotherms were measured at 77 K and the specific surface area, which was calculated from those isotherms according to the Brunauer–Emmett–Teller equation, was found to equal 94.1 m2 g−1

Conclusion

MONFs were synthesized and characterized and the results showed that the MONF fibers exhibited an average diameter of 10 nm to 16 nm. Their specific surface area, which was equal to 94.1 m2 g−1, was mainly composed of crystalline α-MnO2 according to the XRD measurement. The MONFs were used for the removal of the toxic Cr(VI) ions from model and actual water samples, and the effect of different parameters that affected the adsorption process was explored. The results revealed that the adsorption

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