Application of Mechanosynthesized Azine-Decorated Zinc(II) Metal−
Organic Frameworks for Highly Efficient Removal and Extraction of
Some Heavy-Metal Ions from Aqueous Samples: A Comparative
Elham Tahmasebi,† Mohammad Yaser Masoomi,† Yadollah Yamini, and Ali Morsali*
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, Tehran, Islamic Republic of Iran *S Supporting Information
ABSTRACT: The three zinc(II) metal−organic frameworks [Zn2(oba)2(4-bpdb)]·(DMF)x (TMU-4), [Zn(oba)(4bpdh)0.5]n·(DMF)y (TMU-5), and [Zn(oba)(4-bpmb)0.5]n· (DMF)z (TMU-6) [DMF = dimethylformamide, H2oba = 4,4′-oxybisbenzoic acid, 4-bpdb = 1,4-bis(4-pyridyl)-2,3-diaza1,3-butadiene, 4-bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene, and 4-bpmb = N1,N4-bis((pyridin-4-yl)methylene)benzene-1,4-diamine], which contain azine-functionalized pores, have been successfully synthesized by mechanosynthesis as a convenient, rapid, low-cost, solventless, and green process.
These MOFs were studied for the removal and extraction of some heavy-metal ions from aqueous samples, and the effects of the basicity and void space of these MOFs on adsorption efficiency were evaluated. The results showed that, for trace amounts of metal ions, the basicity of the N-donor ligands in the MOFs determines the adsorption efficiency of the MOFs for the metal ions. In contrast, at high concentrations of metal ions, the void space of the MOFs plays a main role in the adsorption process. The studies conducted revealed that, among the three MOFs, TMU-6 had a lower adsorption efficiency for metal ions than the other two MOFs. This result can be attributed to the greater basicity of the azine groups on the TMU-4 and TMU-5 pore walls as compared to the imine groups on the N-donor ligands on the TMU-6 pore walls. Subsequently, TMU-5 was chosen as an efficient sorbent for the extraction and preconcentration of trace amounts of some heavy-metal ions including Cd(II),
Co(II), Cr(III), Cu(II), and Pb(II), followed by their determination by flow injection inductively coupled plasma optical emission spectrometry. Several variables affecting the extraction efficiency of the analytes were investigated and optimized. The optimized methodology exhibits a good linearity between 0.05 and 100 μg L−1 (R2 > 0.9935) and detection limits in the range of 0.01−1.0 μg L−1. The method has enhancement factors between 42 and 225 and relative standard deviations (RSDs) of 2.9− 6.2%. Subsequently, the potential applicability of the proposed method was evaluated for the extraction and determination of target metal ions in some environmental water samples. ■ INTRODUCTION
Metal−organic frameworks (MOFs), as a new class of crystalline porous materials, have received great attention in the past decade because of their intriguing structures.1 The unique characteristics of MOFs include high surface area, good thermal stability, uniform structured nanoscale cavities, uniform but tunable pore size, controllable particle dimensions and morphology, accessible cages and tunnels, specific adsorption affinities, and the availability of in-pore functionality and outersurface modification.2 These features make MOFs very promising materials for recognition,3,4 separation,5−7 gas storage,8−10 sensing,11 drug delivery,12,13 biomedical imaging,14 and catalysis.15−17 The diverse structures and unique properties also make MOFs attractive for analytical applications.18 MOFs have been successfully explored as sorbents for sampling,19,20 solid-phase extraction (SPE),21,22 and solid-phase microextraction23−25 and as stationary phases for gas chromatography26−30 and liquid chromatography.31−36 However, the exploration of MOFs as efficient sorbents for sample preparation is still controversial. Indeed, problems can arise in the application of MOFs as sorbents in aqueous matrixes or in their exposure to even very small amounts of moisture.
Water stability is a key property for MOFs in many applications, especially in sample preparation techniques, as most biological and environmental samples contain water.
However, few efforts have been made in this area.
Heavy metals as persistent environmental contaminants are of great importance among chemical pollutants. Potential sources of heavy-metal-ion pollution include various effluents
Received: July 2, 2014
Article pubs.acs.org/IC © XXXX American Chemical Society A DOI: 10.1021/ic5015384
Inorg. Chem. XXXX, XXX, XXX−XXX emanating from industrial facilities, domestic activities, and erosion of natural deposits. Recently, the toxicity and effects of trace elements that are dangerous to public health and the environment have attracted increasing attention in the fields of pollution and nutrition.37 At trace levels, several heavy metals such as chromium, copper, and cobalt are essential micronutrients for plants, living organisms, and the human body, whereas in large amounts, the same elements are toxic. On the other hand, lead and cadmium are well-recognized to be highly toxic and hazardous to human health even at low concentrations.37 As a consequence, contamination levels in urban and industrial wastewaters need to be controlled, and strict regulations have been drawn up and proposed in this regard. Accordingly, the removal and determination of heavy metals in different samples is desired, and achieving a fast, simple, sensitive, and accurate method of analysis is necessary.
In various publications, different analytical methods have been reported for the determination of metal ions, such as flame or electrothermal atomic absorption spectroscopy (FAAS or
ETAAS, respectively), inductively coupled plasma optical emission spectrometry (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS).38−40
Because of the low concentrations of metal ions and the high number of interfering species present in complicated matrixes, the direct determination of such ions at trace levels is limited.
Therefore, a sample preparation step prior to final analysis is intended to improve the sensitivity and accuracy of the assay by removing the majority of the matrix interference while concentrating the analyte. Among sample preparation techniques, solid-phase extraction (SPE) has become well-established for preconcentrating the desired components from a sample matrix because of its many obvious advantages, such as high extraction efficiency, low consumption of organic solvents, rapidity, and convenience of operation.41 Given that, in the SPE procedure, the sorbent plays a very prominent role in the analytical performance (i.e., analytical sensitivity, selectivity, and precision), most of the current studies on SPE focus on the development of new sorbents.