Nano Research & Applications

Keywords

Heavy metals; Cyclometalated thiolated bridged dimer; Cyclometalated thiolated bridged dimmers; Nano-hybrid materials

Introduction

Wastes containing traces of toxic metal ions such as chromium, copper, cobalt lead and zinc etc., when discharged in the environment through chemical manufacturing, power generation, battery industry and welding etc., can cause serious environmental problem and pollution. Furthermore, contaminated waste can also cause a variety of diseases that would threaten human life as well as other animals [1-3].

A different number of methods are available for the removal of heavy metals from wastewater including, electrochemical treatment, chemical precipitation, ion-exchange, solvent extraction membrane technology, ultrafiltration, adsorption on activated carbon etc. However, most of these methods are ineffective, expensive and inapplicable to a wide range of pollutants [4-8].

In recent years, the synthesis of novel adsorbents for removal of toxic heavy metals from contaminated water has been a subject of interest for industrial research. There are several different types of adsorbents currently being used and proposed for adsorption of heavy metals ions such as clays, ion-exchange resins and activated carbon. However, these adsorbents have shown several disadvantages like poor mechanical stability, low thermal stability as well as low adsorption capacities and poor selectivity [9-11]. Recently, organic-inorganic hybrid materials have emerged as promising adsorbents for heavy metals overcoming the limitations of current adsorbents. This is due to their strong affinities, high adsorption capacities towards selected metal ions, in addition, organic–inorganic hybrid containing silica has high chemical stability and high heat resistance [12,13].

Nanostructure organic-inorganic hybrid materials which contain both organic compounds and inorganic oxides were developed and have been widely used in numerous fields including catalysis, optics and in the manufacture of photoelectric components and electrochemical biosensors [14-17]. For example, Franville et al. prepared europium(III)-containing hybrid materials based on dipicolinic acids by hydrolysis and condensation, europium complex was covalently bonded into silica framework. They have shown that, the europium (III) complexes in the silica hybrid matrix has good thermal stability and optical performance [18-20]. Li Junfu et al. also have reported a series of organically modified silicates doped with red-emitting europium complex. Jingxia, has reported a pH sensing mesoporous material containing covalently bonded Ru(II) complex in the silicate [21,22].

In the present paper, a new nano-hybrid adsorbents have been designed and synthesized namely, cyclometalated thiolated bridged dimmers [(L)₂ IrS(CH₂)₃OH]₂, where L is a bidentate cyclometalated ligand, 2,5-di(thiophen-2-yl) pyridine (dtp) (1) and 2-benzo [4,5-]thienylpyridine (btp) (2) were covalently bonded into silicate framework by hydrolysis and condensation reaction involving the silanol groups (Si-OH) and the hydroxyl groups of the cyclometalated thiolated bridged dimer (4). The resulted phosphorescent amorphous nano-hybrid materials 1 and 2 were fully characterized by Fourier-transform infrared (FTIR) spectra, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images. Their application on removal of heavy metals such chromium, copper, cobalt, lead and zinc ions from aqueous solution were also investigated.

Materials and Methods

2-methoxyethanol, 3-(triethoxysilyl) propane-1-thiol and IrCl3.3H₂O were obtained from Aldrich and used as received. Chloride-bridged dimers [Ir [(L)₂ (μ-Cl)]₂ and the thiolated bridged iridium dimers [(L)₂ IrS(CH₂)₃OH]₂, where L is a bidentate cyclometallated ligand, 2,5-di(thiophen-2-yl) pyridine(dtp) (1) and 2-benzo[4,5-]thienylpyridine (btp) (2) were prepared as described in literature Nonoyama [23] and Kotera et al. [24].

Method for synthesis of compound 1

The thiolated bridged iridium dimer [(dtp)₂ IrS(CH₂)₃OH]₂ (1 eqv) (4) were added to a solution of deionized water/ EtOH (25.0 mL, 50% v/v). The resulting suspension was sonicated for 30 min. The 3-(triethoxysilyl) propane-1-thiol (2.5 eqv) (6) was then dissolved in a deionized water/EtOH (25.0 mL, 50% v/v) solution and added to the thiolated bridged iridium dimer. The mixture was stirred for 24 h at 60°C. The resulting yellow precipitation was filtered, washed several times with distilled water and EtOH and dried overnight at about 105°C.

Nano-hybrid characterization

Infrared spectra were recorded in the range 400 cm^-1 to 4000 cm^-1 on a Shimadzu 8400S instrument using KBr pellets. The crystalline phases were identified using a powder X-ray diffractometer (XRD P-6000-ShimadzuX-ray diffractometer) using Cu Kα radiation. Transmission electron microscopy (TEM) was performed on a JEOL 1400 microscope. Agilent Inductively coupled plasma mass spectrometry (ICP-MS) Model 7500 Ce was used to measure the concentration of the metal ions in aqueous solution.

Results and Discussion

The complete synthesis tactic for the (EtO)₃Si-SH@thiolate bridged iridium chloride dimer is shown in Scheme 1. The preparation involves two steps. First, the thiolated bridged iridium dimer (4) was prepared as described in literature by Kotera et al. [24]. In a typical reaction, addition of 2-mercaptoethanol and KOtBu in 2-methoxyethanol to a solution of [Ir [(btp₂) (μ-Cl)]₂ (3) in dichloromethane afforded the cyclometalated thiolated bridged dimer (4) as a yellow solid in a good yield. Second, the iridium (III)-doped hybrid material (1) was prepared by the reaction of hydroxyl groups on the cyclometalated thiolated bridged dimmer (4) with the hydrolyzed molecules of 3-(triethoxysilyl) propane-1-thiol (6) in ethanol/water mixture (50% v/v) at 50 °C for 24 h. Further hydrolysis of silicon ethoxide groups (Si-OEt) of compound 6 produces more silanol groups (Si-OH). Subsequent polycondensation reactions, involving the silanol groups (Si-OH) produced compound 1 as shown in Scheme 1.

Scheme 1 Schematic representation of the strategy for the preparation 1 and 2.

For the synthesis of compound 2, the cyclometalated thiolated bridged dimmer [(btp)₂ IrS(CH₂)₃OH]₂ was mixed with 3-(triethoxysilyl) propane-1-thiol (6) under the similar conditions as that carried out for the preparation of (1). Both compounds 1 and 2 are insoluble in water and all organic solvents and have been fully characterized by Infra-red spectroscopy FTIR, TEM and SEM and XRD.

Figures 1 and 2 show the FT-IR absorption spectra of compounds 1 and 2, both figures display typical infrared spectra of hybrid materials, with both components, organic and inorganic phase. The inorganic components have strong absorption bands at 1068 cm^-1, 1066 cm^-1 for 1 and 2 respectively due to ν(Si-O-Si). The broad bands around 3500 cm^-1 to 3000 cm^-1 assigned to ν(Si- OH). SH stretching frequency appeared at 2561 cm^-1 for both compounds 1 and 2. The organic components have absorption bands at 1244 cm^-1 to 1477 cm^-1 corresponding to aromatic rings stretching of pyridine and thiophene, bands at 2934 cm^-1 and 29255 cm^-1 due to aliphatic CH stretching modes for compounds I and 2 respectively [25-27].

Figure 1 The FT-IR absorption spectrum of compound 1.

Figure 2 The FT-IR absorption spectrum of compound 2.

Figure 3 shows the XRD spectra of compounds 1 and 2. XRD results indicated that both hybrid materials have no clear crystals phase peaks and exhibited typical silica amorphous structures with diffraction peaks at 2θ values around 21.5° [28].

Figure 3 2 XRD spectra of samples 1 and 2.

The TEM images of compounds 1 and 2 are shown in Figures 4 and 5 respectively. Both hybrid materials have particle sizes over a small range approximately from 3 nm to 7 nm in diameter.

Figure 4 TEM images s of compound 1.

Figure 5 TEM images s of compound 2.

Figures 6-8 show SEM microphotographs obtained for both hybrid materials 1 and 2. The SEM results showed a different morphology, compound 1 has a rough surface and showed sponges of composite with pores, while compound 2 has a rough surface and dense body with pores. However, SEM images showed both nanocomposites had an amorphous structure which may be beneficial for adsorption of heavy metals. EDS spectra in Figures 7 and 9 confirmed the formation of compounds 1 and 2.

Figure 6 SEM images of compound 1.

Figure 7 An EDS spectrum obtained from the region imaged in Figure 6.

Figure 8 SEM images of compound 2.

Figure 9 An EDS spectrum obtained from the region imaged in Figure 8.

Adsorption capacity

The metal ion uptake capacity of Cr²⁺, Cu²⁺, Co²⁺and Pb²⁺ and Zn²⁺ metal ions by ICPMs was determined by mixing 50 mg of compounds 1 or 2 with buffered solutions of the divalent metal ions (1 ml of 1000 ppm) in 50 ml volumetric flask. Volume was made up to 50 ml with 2% nitric acid. With the help of stirrer, capacity by both adsorbents 1 and 2.

Conclusion

In this work, phosphorescent cyclometalated thiolated bridged iridium dimmers were immobilized in silicates by hydrolysis and condensation reactions. The new nanohybrid materials were fully characterized by IR, XRD and TEM. The results confirmed successful doping of the silicates with phosphorescent iridium organometallic complex. The powder XRD and TEM studies have suggested the nanoparticles nature. The prepared nanohybrids were found to be effective adsorbents for the removal of metal ions such as Cr²⁺, Cu²⁺ and Pb²⁺ and Zn²⁺ from aqueous solutions. samples were kept for 48 hours at speed of 50 rpm speed at room temperature. Samples were filtered with 0.45 mu syringe filter and analyzed by ICPMS Agilent 7500. Results obtained in ppm levels are given in Table 1 and Figure 10 below.

Figure 10 Percentage adsorption (% ads) of the metal ions from aqueous solutions by a. adsorbent 1 and b. adsorbent 2.

Table 1 Adsorption capacities (qt) and percent adsorption (% ads) of Cr²⁺, Cu²⁺, Co²⁺, Pb²⁺ and Zn²⁺ ions from aqueous solutions by 1 and 2.

The amount of each (II) ions adsorbed per unit weight of adsorbent (qt) and the percent adsorption (% ads) are calculated by using respectively the following equations [29].

(% ads) = (C₀ – C_t)/ C₀ x 100 and qt = [(C₀ – C_t)] *V/W

Where C₀ and C_t are the initial concentration and concentration at time contact of interaction adsorbate-adsorbent of each (II) ions. V is the volume of each metals solutions and W is the weight of Nano compounds.

The results show that Nano-hybrid materials 1 and 2 exhibit good potential for extraction of chromium, copper, cobalt, lead and zinc metal ions. This could be attributed to the change of morphology and increased in the surface area of the silica by the organometallic iridium doping. In addition, the presence of the immobilized thioyl group (SH) on the silica surface enhances the ion exchange process through acid-base interactions [30]. The results in Table 1 and Figure 10 show that zinc ions have the largest maximum adsorption capacity by both adsorbents 1 and 2.

Acknowledgment

The author is grateful to Dr Awad Asuhaimi and Mr. Manssor Sheik for the ICP-MS measurements.

Graphical Abstract

New phosphorescent amorphous nano-hybrid materials 1 and 2 were designed, synthesized and fully characterized by Fourier-transform infrared (FTIR) spectra, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images. The new nanohybrids were found to be effective adsorbent for the removal of metal ions such as chromium (Cr²⁺), copper (Cu²⁺), cobalt (Co ²⁺), lead (Pb²⁺) and zinc (Zn²⁺) ions and from aqueous solutions.

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