Synthesis and application of Fe₃O₄@SiO₂@TiO₂ for photocatalytic decomposition of organic matrix simultaneously with magnetic solid phase extraction of heavy metals prior to ICP-MS analysis

Highlights

• Fe₃O₄@SiO₂@TiO₂ is used for magnetic solid phase MSPE of Cu(II), Zn(II), Cd(II) and Pb(II) prior to ICP-MS.

• Fe₃O₄@SiO₂@TiO₂ acts as magnet, photocatalyst and acid resistant adsorbent nanoparticle.

• The process includes enrichment of metals simultaneously with degradation of interfering organic matrix.

• LOQs were 0.20, 0.15, 0.12, 0.25 µg L⁻¹ for Cu(II), Zn(II), Cd(II) and Pb(II).

Abstract

Interference of organic compounds in the matrix of heavy metal solution could suppress their pre-concentration and detection processes. Therefore, this work aimed to develop simple and facile methods for separation of heavy metals before ICP-MS analysis. Fe₃O₄@SiO₂@TiO₂ core-double shell magnetic adsorbent was prepared and characterized by TEM, SEM, FTIR, XRD and surface area, and tested for Magnetic Solid Phase Extraction (MSPE) of Cu(II), Zn(II), Cd(II) and Pb(II). TEM micrograph of Fe₃O₄@SiO₂@TiO₂ reveals the uniform coating of TiO₂ layer of about 20 nm onto the Fe₃O₄@SiO₂ nanoparticles and indicates that all nanoparticles are monodispersed and uniform. The saturation magnetization from the room-temperature hysteresis loops of Fe₃O₄ and Fe₃O₄@SiO₂@TiO₂ was found to be 72 and 40 emu g⁻¹, respectively, suggesting good separability of the nanoparticles. The Fe₃O₄@SiO₂@TiO₂ showed maximum adsorption capacity of 125, 137, 148 and 160 mg g⁻¹ for Cu(II), Zn(II), Cd(II) and Pb(II) respectively, and the process was found to fit with the second order kinetic model and Langmuir isotherm. Fe₃O₄@SiO₂@TiO₂ showed efficient photocatalytic decomposition for tartrazine and sunset yellow (consider as Interfering organic compounds) in aqueous solution under the irradiation of UV light. The maximum recovery% was achieved at pH 5, by elution with 10 mL of 2 M nitric acid solution. The LODs were found to be 0.066, 0.049, 0.041 and 0.082 µg L⁻¹ for Cu(II), Zn(II), Cd(II) and Pb(II), respectively while the LOQs were found to be 0.20, 0.15, 0.12 and 0.25 µg L⁻¹ for Cu(II), Zn(II), Cd(II) and Pb(II), respectively.

Introduction

Environmental pollution by heavy metal ions is increased due to the rapid industrialization worldwide which may distribute through food chain ending with human body [1], [2]. The accurate determination of traces metal ions is necessary for environmental protection as well as maintaining the water and food quality [3], [4]. With the effective analytical instruments, most of heavy metal ions can be detected, even at trace levels. However, in some environmental samples, the heavy metals concentrations cannot be accurately detected due to the ultra-trace levels or the presence of highly complex matrix [5]. To overcome these difficulties, sample pretreatment is required [6].

Sample pretreatments involve digestion and extraction to isolate the analytes and purify them before the analysis, which involve complex, time consuming and tedious multistep to perform the complete analysis [7]. The purpose of the digestion step is to degrade the complicated organic matrix, and it is usually conducted using concentrated acids [8], [9]. In addition, there are various processes which can be applied to treat organic contamination including chemical treatment by chlorination or ozonation [10], [11], electrochemical treatment [12], adsorption [13], [14], [15], [16], [17], [18]. In some cases, silica, zinc oxide (ZnO) or titania (TiO₂) is used for the decomposition of dyes [19]. However, if TiO₂ nano-powder is applied independently, it tends to disperse in the solution and due to the nano-size their separation will be quite difficult. On the other hand, the extraction process aimed to enrich the analyte and it can be achieved by different procedures such as Liquid Liquid Extraction (LLE) [20], Solid Phase Extraction (SPE) [5], Cloud Point Extraction (CPE) [21] and Dispersive Liquid Liquid Microextraction (DLLME) [22]. Among these methods, SPE is the most efficient due to its repeatability and applicability for various heavy metals [23], [24]. Many traditional adsorbents have been used for SPE such as silica-immobilized formylsalicylic acid [25], chitosan/natural zeolites [26] activated carbon [5]. Typically, nanomaterials based adsorbents showed promising performance for heavy metal adsorption such as amino-functionalized hollow core-mesoporous shell silica spheres [27], but still these nanomaterials limited for wide industrial application because of the difficultness of their solid/liquid separation [28]. To resolve solid/liquid separation issue, magnetic based nanomaterials have been utilized for removal of heavy metals under external magnetic field [29].

Magnetic core-porous shell system can provide facile separation and isolation of metal ions from the solution, where the heavy metal cations are absorbed into the porous shell, while the magnetic core facilitates the separation process [30], [31]. In addition, by constructing photocatalytic TiO₂ shell, core-shell system can decompose organic molecules [32], [33]. Magnetic iron oxide nanoparticles are commonly used for the core-shell nanostructures as it is nontoxic and simple in preparation [34]. However, during the preparation of magnetic core-TiO₂ shell photocatalyst, the direct combination of TiO₂ with the magnetic cores lead to a decrease in the photocatalytic efficiency due to transferee of charges from TiO₂ shell to the core where it works for the combination of negative electrons and positive holes [35]. Thus, the presence SiO₂ layer between Fe₃O₄ and TiO₂ is necessary to keep the efficiency of the core-shell photocatalyst [36]. In addition, the SiO₂ shell can enhance the stability and dispersity of the magnetic core and support the thermal stability as well as increase the resistance for the highly acidic solution [32]. However, the simultaneous removal of some dyes with heavy metals by adsorption was studied by Mazaheri et al. [13], and Asfaram et al. [14], the application of core-double shell photocatalyst for organic matrix degradation simultaneously with heavy metals ion recovery by magnetic solid phase extraction has not reported yet. Therefore, this work aim to fabricate tri-functional Fe₃O₄@SiO₂@TiO₂ core-double shell nanoparticles capable of photocatalytic decomposition of interfering organic molecules, adsorption of heavy metal as well as facile separation under magnetic field that in turn reduce the sample preparation steps during analysis of heavy metal ions. Based on our survey, the advantages and novelty of this work lies on combining, for the first time, UV- irradiation photocatalytic activity by Fe₃O₄@SiO₂@TiO₂ with SPE and ICP-MS detection for fast and accurate detection of Cu(II), Zn(II), Cd(II) and Pb(II). In addition, the easy separation of Fe₃O₄@SiO₂@TiO₂ adsorbent by applying external magnetic field facilitates the preconcentration process and reuse of the adsorbent. Factors controlling the process are optimized including pH, eluent type and concentration, presence of coexisting ions, sample volume and addition/recovery tests.