A layered magnetic iron/iron oxide nanoscavenger for the analytical enrichment of ng-L−1 concentration levels of heavy metals from water
Graphical abstract
Highlights
► A layered magnetic Fe/Fe2O3 nanomaterial is prepared. ► Analytical enrichment of selected heavy metals on Fe/Fe2O3 nanoparticles. ► The mechanism of enrichment involves the adsorption and reduction. ► The applicability of the nanomaterial is verified using a real sample matrix.
Introduction
Heavy metal ions are toxic and tend to accumulate in human organs, causing damages. Heavy metals occur in the ecosystems naturally or due to anthropogenic resources with large variations in concentrations [1]. Motivations for controlling heavy metal concentrations in the environment are diverse. In any case, due to the complexity of sample matrixes and the frequently low concentrations of metals, there is a need for extraction and preconcentration of toxic metals before their analysis using atomic absorption spectrometry (AAS) or other techniques.
The development of materials at nanoscale has been a major area of focus in the fields of chemistry, materials science and engineering over the past fifteen years [2]. The size, surface structure and interparticle interaction of nanomaterials determine their properties and make them applicable in many research areas. Nanoparticles, due to their intrinsic surface reactivity and high surface areas strongly chemisorb substances. Because of the above-mentioned properties, nanotechnology has recently become one of the most exciting fields being at the forefront of analytical chemistry [3]. The homogeneous distribution of dispersed nanoparticles in solutions causes favorable mass transport to surfaces and the dispersed adsorbents overcome problems such as occluding in filtration and fouling in packed columns and membranes. Therefore, the unique properties of nanoscale materials offer excellent prospects for designing new methods and instrumentation for chemical analysis [4], [5].
Among nanomaterials magnetic nanoparticles (MNPs) have attracted much attention because of their unique physical and chemical properties and high potential applications in separation and preconcentration of target analytes from environmental and food samples [6], [7], [8]. Silica-modified magnetite NPs functionalized with cetylpyridinium bromide has been used as adsorbent for microextraction and determination of trace amounts of Cu(II), Ni(II), Co(II), Cd(II), Pb(II) and Mn(II) from environmental water samples [9]. In addition, MNPs with special functional groups bound onto the surface have been synthesized and applied to the extraction of different analytes from water samples [7].
Many nanomaterials also show tremendous promise for use in the field of contaminant remediation. Maghemite NPs as adsorbent have offered an attractive and inexpensive option for the removal of heavy metals considering its simple synthesizing method, high surface area and magnetic properties [10], [11]. In most cases, the reactivity of nanomaterials is strictly a function of the increased surface area associated with small particles but, nevertheless, the enhancement rates observed when employing some nanomaterials are substantial. This has been the case for the iron oxide and zero-valent iron (nZVI) NPs to contaminant remediation. The physicochemical properties of nZVI and its reductive capacity allow its application in the rapid decontamination of many aqueous pollutants [12]. Its application to remove varieties of pollutants has been demonstrated with halogenated hydrocarbons [13], pesticides [14], [15], anions [16] and heavy metals [17]. In this context, Zhang and his co-workers have reported the simple and tuneable fabrication of Fe@Fe2O3 core/shell nanowires and nanonecklaces and used the former for the removal of chromium(VI) from aqueous solutions [18].
The objective of this work is to develop an enrichment method for selected heavy metal ions (i.e. Pb, Cd, Ni, As and Cr) and obtain the generic parameters that describe their enrichment onto unmodified layered magnetic iron/iron oxide NPs. Although the adsorption of metal ions onto nZVI has been researched extensively, the retention preference of the nanoscale Fe/Fe2O3 material for heavy metal ions at analytical scale has not been reported up to now. Apart from combining the dual properties of (hydrous) iron oxides (as efficient sorbents) and zero-valent iron (as a reductant), the other advantages of using Fe/Fe2O3 as nanoscavenger are: (1) the adsorption capacity is high due to its large surface area, (2) the separation of metal-loaded magnetic adsorbent from water samples is simple and rapid and can be achieved via an external magnetic field, (3) the amount of chemicals used is diminished (no complexing agent is required) and (4) it is cost-effective and can be synthesized very easily.
Section snippets
Materials and methods
Stock standard solutions of Pb(II), Cd(II), Ni(II), As(V) (1000 ± 2 mg L−1 each) were purchased from Merck (Darmstadt, Germany). Potassium dichromate ≥99.5% from Sigma-Aldrich was used for the preparation of the standard solution of Cr(VI). The suprapur grade HNO3 used in the experiments was obtained from Merck (Darmstadt, Germany). Diluted solutions of each metal were prepared in 2% HNO3. Sodium borohydride 99.99% and anhydrous iron(III) chloride powder ≥99.99% trace metals basis, were from
Characterization of the synthesized magnetic NPs
The X-ray diffraction pattern of the synthesized NPs (Fig. S1 of Supplementary Material) reveals the presence of γ-Fe2O3 on the basic phase, which is the iron. The broad peaks of low intensity suggest the synthesis of NPs at low nanometers size.
The representative plate-like particles H3 hysterisis loop of the isotherms in Fig. 1, proves that the Fe/Fe2O3 is a layered material [20]. The calculated BET surface area is found to be 286 m2 g−1, despite the aggregation of the layers and the estimated
Conclusions
The versatility of nanometer-scale zero-valent iron material in association with the sorption properties of (hydrous) iron oxides has been demonstrated for the extraction of heavy metals. The dual properties of sorption and reduction may yield new and unique applications of Fe/Fe2O3 NPs for the separation and transformation in analytical chemistry. The enrichment behavior of several heavy metal ions onto Fe/Fe2O3 NPs at analytical scale was studied systematically and a simple, sensitive and
Acknowledgements
The use of the XRD unit of the Network of Research Units of the University of Ioannina is gratefully acknowledged. Many thanks are due to Assoc. Prof. T. Vaimakis for the thermogravimetric analysis.
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