Rapid removal of Cr(VI) from aqueous solution by novel sepiolite/Fe3O4/nZVI nanocomposite: Material characterizations, enhanced performance in Cr(VI) removal and mechanism

Abstract

The novel magnetic sepiolite/Fe3O4/zero-valent iron (nZVI) nanocomposite (nZVI@SepH-Mag) was prepared and used to achieve the removal of Cr(VI) in this work. The nZVI@SepH-Mag composites were characterized by XRD, FTIR, BET, SEM and TEM. The characterization results indicated that the structure of the composite consisted of small nanoscale nZVI and magnetite (Mag) particles uniformly anchoring on the surface of acid-activated sepiolite (SepH). Batch experiments were used to analyze the effects of main factors on Cr(VI) removal. A 100% removal efficiency in 60 min and enhanced reaction ratio were reached by the composite comparing other existing materials. The kinetic of the adsorption and possible Cr(VI) removal mechanism of the hybrids were also evaluated and proposed. Based on the removal products identified by Raman, XRD and XPS, a reduction mechanism was proposed. The results indicated that the SepH and Mag can inhibit the agglomeration and enhance the dispersibility of nZVI, and Mag and nZVI displayed good synergetic effects.

1. Introduction

Chromium, like some of its chemical cousins, can be either beneficial or harmful to living things, depending on its form, concentration, and method of absorption. Chromium(III), for example, the metal’s natural state, is stable and an essential nutrient. On the other hand, under oxidizing conditions, Cr is highly soluble and mobile as the Cr(VI) anions chromate (CrO4^2−) and bichromate (HCrO4^−). Cr(VI) is highly reactive and a suspected carcinogen, mutagen and teratogen if inhaled or consumed in contaminated fish or polluted water [1]. Therefore, under reducing conditions, Cr(VI) may convert to Cr(III), which is insoluble, strongly adsorbed onto solid surfaces, and less toxic [2, 3].

Cr(VI) can be removed from solution artificially through traditional methods, such as adsorption [4], electrochemical precipitation [5], ion exchange [6], membrane separation [7], bioremediation [8], photocatalysis [9−11], in situ chemical reduction [12] and their synergy effect [13], among which zero valent iron (ZVI) is the most commonly used and researched reductant [14]. Nevertheless Fe- and Cr-precipitates on granular ZVI surface decrease the performance and efficiency during reaction process [15]. Hence nano zero valent iron (nZVI) was synthesized to remedy those weaknesses. Meanwhile, nZVI modified with surfactants and hybrid nZVI with other materials sprang up to further strengthen the nZVI family [16, 17].

Sepiolite (Sep) is a fibrous clay mineral with a 2:1 layer structure, formed by an alternation of blocks and tunnels that grow up in the fiber direction with a particle size of 0.2−2 μm in length, 100−300 nm in width and 50−100 nm in thickness range [18]. It has captured much attention because of its low cost, high chemical stability, large specific surface area and large thermal stability [19]. Sep as a carrier is used in variety of industries, such as in dyes, pharmaceuticals, papers, drilling fluids, ceramics and detergents [20]. DANESHKHAH et al synthesized sepiolite-supported nZVI (SPT-nZVI), which reduced the agglomeration of nZVI and thereby provided a large reactive surface area. The composite was successfully applied to metoprolol removal. HABISH et al [22] prepared nature Sep and partially acid-activated Sep (AAS) for loading nZVI at different (Sep or AAS)/nZVI ratios in order to achieve the best nZVI dispersibility and the highest adsorption capacity for Cd2+. FU et al [23] imm