Chromium(VI) removal by maghemite nanoparticles

Abstract

Maghemite nanoparticles were prepared by a co-precipitation method and characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, nitrogen adsorption and desorption isotherms. The Brunauer–Emmett–Teller surface area, average particle size, pore volume and porosity of maghemite were 73.8 m² g⁻¹, 17.2 ± 4.4 nm, 0.246 cm³ g⁻¹, and 56.3%, respectively. Removal of Cr(VI) by the maghemite nanoparticles follows a pseudo-second-order kinetic process. Intraparticle diffusion kinetics implies the adsorption of Cr(VI) onto the maghemite occurs via two distinct phases: the diffusion controlled by external surface followed by an intra-particle diffusion. The equilibrium data was nicely fit to the Langmuir and Langmuir–Freundlich (L–F) models and indicates the adsorption of Cr(VI) is spontaneous and highly favorable. The heterogeneity index, 0.55, implies heterogeneous monolayer adsorption. The adsorption Cr(VI) is favorable under acidic and neutral conditions with maximum removal observed at pH 4. The adsorption of Cr(VI) is modestly inhibited by the presence of ⩾5 ppm humic acid. In summary, the adsorption of Cr(VI) by maghemite nanoparticles is rapid, can be accurately modeled, and is effective under a variety of conditions. Our results indicate these magnetic materials have promising potential to cleanup Cr(VI) contaminated waters to acceptable drinking water standards.

Introduction

Chromium is a common drinking water contaminant in the USA because of its wide spread use in industrial processes [1]. The use of chromium in wood preservatives, leather tanning, paint formulation, steel fabrication, and metal finishing are the main sources of chromium based pollution. The toxicity and mobility of chromium are strongly dependent on the oxidation state. In nature, chromium exists primarily in two oxidation states (III and VI). Cr(III), an essential trace element for human beings, may play a role in the metabolism of glucose [2]. Cr(VI) is a more toxic and soluble specie, compared to Cr(III) which is toxic only at a high concentrations. ${\text{CrO}}_4^{2-}$ and ${\text{Cr}}_2{\text{O}}_7^{2-}$ are the primary forms of Cr(VI) with ${\text{Cr}}_2{\text{O}}_7^{2-}$ being predominant under strongly acidic conditions and at high Cr(VI) concentrations in aqueous solutions [3]. Cr(VI) is a human carcinogen and poses a significant threat to the environment and human beings [4]. The World Health Organization (WHO) recommends a maximum allowable level of 50 ppb total chromium for drinking water. The US Environmental Protection Agency established a guideline of 100 ppb maximum contaminant level for total chromium in drinking water [5], while California’s office of Environmental Health Hazard Assessment proposed in 1999 a public health goal of ⩽2.5 ppb for total chromium [6].

Unlike many organic pollutants, chromium species are not removed and/or degraded through typical environmental and biological processes, thus it is critical to develop and identify an effective method for the removal of chromium from industrial wastewater. Water purification technologies must be capable of reducing the level of chromium considered safe for human consumption. A number of conventional methods have been employed for the removal of Cr(VI) from wastewater [7]. Adsorption processes can offer significant advantages including availability, profitability, ease of operation and efficiency, in comparison with many conventional methods. A variety of natural and synthetic materials have been used as Cr(VI) sorbents, including activated carbons, biological materials, zeolites, chitosan, and industrial wastes. Unfortunately, these sorbents can also suffer from a number of disadvantages, including high cost, low adsorption capacity and/or difficulties associated with separation and removal following treatment. The application of magnetic nanoparticles for adsorption is attractive because of their high surface area, easy separation and recovery [8], [9]. Iron based materials are especially attractive because they are inexpensive and environmentally friendly [10], [11]. The magnetite form of iron can be oxidized to maghemite under aerated conditions [12]. Maghemite, a common magnetic material, is a promising adsorbent for heavy metals removal because it is inexpensive, readily available and can be easily separated and recovered [13], [14]. While maghemite nanomaterials appear to be promising for Cr(VI) removal, detailed kinetic and adsorption studies have yet to be reported. Herein we report the synthesis of maghemite nanoparticles by a co-precipitation method. The observed adsorption of Cr(VI) by maghemite nanoparticles is rapid, accurately model and effective under a variety of conditions. Our results demonstrate these maghemite nanoparticles with high adsorption capacity and magnetic properties are promising materials for the Cr(VI) removal from aqueous solution.