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National Engineering Laboratory for Vacuum Metallurgy (Kunming University of ... of Complex Nonferrous Metal Resources Clear Utilization, Kunming 650093, ...

J. Cent. South Univ. (2014) 21: 3575−3579 DOI: 10.1007/s11771-014-2338-0

Selective removal of heavy metal ions from aqueous solutions with surface functionalized silica nanoparticles by different functional groups KONG Xiang-feng(孔祥峰)1, 2, 3, YANG Bin(杨斌)1, 3, XIONG Heng(熊恒)1, 3, ZHOU Yang(周阳)1, 3, XUE Sheng-guo(薛生国)2, XU Bao-qiang(徐宝强)1, 3, WANG Shi-xing(王世兴)4 1. National Engineering Laboratory for Vacuum Metallurgy (Kunming University of Science and Technology), Kunming 650093, China; 2. School of Metallurgy and Environment, Central South University, Changsha 410083, China; 3. State Key Laboratory of Complex Nonferrous Metal Resources Clear Utilization, Kunming 650093, China; 4. Yunnan Institute of Product Quality Supervision and Inspection, Kunming 650223, China © Central South University Press and Springer-Verlag Berlin Heidelberg 2014 Abstract: The industrial silica fume pretreated by nitric acid at 80 °C was re-used in this work. Then, the obtained silica nanoparticles were surface functionalized by silane coupling agents, such as (3-Mercaptopropyl) triethoxysilane (MPTES) and (3-Amincpropyl) trithoxysilane (APTES). Some further modifications were studied by chloroaceetyl choride and 1,8-Diaminoaphalene for amino modified silica. The surface functionalized silica nanoparticles were characterized by Fourier transform infrared (FI-IR) and X-ray photoelectron spectroscopy (XPS). The prepared adsorbent of surface functionalized silica nanoparticles with differential function groups were investigated in the selective adsorption about Pb2+, Cu2+, Hg2+, Cd2+ and Zn2+ ions in aqueous solutions. The results show that the (3-Mercaptopropyl) triethoxysilane functionalized silica nanoparticles (SiO2-MPTES) play an important role in the selective adsorption of Cu2+ and Hg2+, the (3-Amincpropyl) trithoxysilane (APTES) functionalized silica nanoparticles (SiO2-APTES) exhibited maximum removal efficiency towards Pb2+ and Hg2+, the 1,8-Diaminoaphalene functionalized silica nanoparticles was excellent for removal of Hg2+ at room temperature, respectively. Key words: industrial silica fume; surface functionalization; heavy metal ions; selective removal

1 Introduction Industrial and chemical processes have worsened the contamination of surface water and groundwater resulting from heavy metal ions, and have increased concern for the high toxic effect of such contamination to the environment and public health. The accumulative of heavy metal in the body can lead to many serious human afflictions. Unlike some organic pollutants, heavy metals are not biodegradable and cannot be decomposed. Therefore, effective methods are needed to remove and detect heavy metals in environmental and biological samples. In recent decades, a large number of researches have been focused on the effective removal of heavy metal ions. The traditional methods commonly used for removal from aqueous solutions include ion-exchange [1], solvent extraction [2−3], chemical precipitation [4], membrane separation [5−6], molecular imprinting [7],

adsorption [8−12]. The adsorption process is arguably one of the most popular methods for removal and has attracted greater interests because of its simplicity and efficiency. Recently, many research groups have explored several nanoparticles for removal because of the ease of modifying their surface functionality and their high surface area-to-volume ratio for increased adsorption capacity and efficiency. The chelating nanomaterial is a functional material that can form coordination complexes with heavy metal ions, which is a kind of new adsorption material developed in recent years. The unbonding isolated electrons existing in chelating groups can form a stable chelate with heavy metal ions. Therefore, adsorption of chelating nanomaterials is more specific than traditional nanomaterials because of their higher selectivity combination with metal ions. Previous literature reported the use of chelating ligands functionalized onto silica for heavy metal extraction and removal. For example, 5formyl-3-(1’-carboxyphenylazo) salicylic acid was used

Foundation item: Project(2012CB722803) supported by the Key Project of National Basic Research and Development Program of China; Project(U1202271) supported by the National Natural Science Foundation of China; Project(IRT1250) supported by the Program for Innovative Research Team in University of Ministry of Education of China Received date: 2013−05−17; Accepted date: 2013−10−06 Corresponding author: XIONG Heng, PhD; Tel: +86−871−65163583; E-mail: [email protected]

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for Cu2+ extraction from natural water [13]. 4-amino3,5,6-trichloropicolinic acid has been used for divalent cations (Cu2+) adsorption from aqueous solutions [14]. Quinolinol was immobilized on silica surface for enrichment of trace metal ions like Cu2+, Ni2+, Co2+, Cr3+, Pb2+ and Hg2+ [15]. Hydroxyquinoline functionalized silica was reported for preconcentration of Cu2+, Zn2+, Pb2+, Mn2+, Ni2+ and Cd2+ from seawater samples [16]. Herein, we report a highly selective and efficient method of surface-functionalized silica nanoparticles for heavy metal ions removal from aqueous solutions. (3Mercaptopropyl) triethoxysilane (MPTES) and (3Amincpropyl) trithoxysilane (APTES) were synthesized and grafted onto the surface of silica nanoparticles, chloroaceetyl choride and 1,8-Diaminoaphalene was further modificated for amino modified silica, which could be used to selectively adsorb Pb2+, Cu2+, Hg2+, Cd2+ and Zn2+ ions.

2 Methods and materials 2.1 Materials (3-Mercaptopropyl) triethoxysilane (MPTES), chloroacetyl chloride, 1,8-Diaminoaphalene were purchased from Sigma-Aldrich Chemical Corporation. 3-aminopropyltriethoxysilane (APTES) were endowed by Dowcorning. Nitric acid, ammonium hydroxide, ethanol, toluene, acetonitrile, dichloromethane (DCM), triethylamine, were obtained from Sinopharm Chemical Reagent Tianjing Co., Ltd.. Lead nitrate, copper sulfate, mercuric chloride, cadmium chloride, zinc nitrate were purchased from Chengdu Chemical Reagent Factory. Distilled water was prepared by our own laboratory. Silica fume was donated from a smelter in Yunnan. 2.2 Characterization Fourier transform infrared (FT-IR) spectra were obtained using Spectrum One FT-IR spectrometer (Perkin Elmer) with a resolution of 4 cm−1. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al K radiation. In curve fitting, the line width for the Gaussian peaks was maintained constant for all components in a particular spectrum. The X-ray photoelectron spectroscopy analyses were performed on a Kratos AXIS Ultra with a monochromatic Al X-ray source at 150 W. Each analysis started with a survey scan from 0 to 1200 eV with a dwell time of 100 ms, pass energy of 160 eV at steps of 1 eV with one sweep. 2.3 Surface modification of silica nanoparticles Nitric acid pretreated silica fume (2 g, SiO2) was added into 50 mL of ethanol and 50 mL of distilled water,

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ultrasonic dispersing 30 min, 4 mL of (3-Mercaptopropyl) triethoxysilane (MPTES) and 4 mL of ammonium hydroxide were added in dropwise. The mixture was vigorous mechanically stirred in darkness for 12 h at 65 °C under nitrogen atmosphere. Then, the crude product was further purified by washing with toluene (2× 50 mL) and distilled water (2×50 mL) and centrifugal separated. APTES functionalized silica nanoparticles were synthesized according to our previous report [17]. Chloroacetyl chloride was synthesized based on Lee’s method. APTES surface modified silica nanoparticles (2 g, SiO2-APTES) and 4 mL of triethylamide in 50 mL of DCM were mixed in a round flask for 2 h at 0 °C. Then, 4 mL of chloroacetyl chloride was added in dropwise. The mixture was vigorously mechanically stirred in darkness for 48 h at room temperature. Chloroacetyl chloride modified SiO2-APTES was further purified by washing with toluene (2×50 mL) and acetonitrile (2×50 mL), then dispersed in acetonitrile. Finally, 1,8- Diaminoaphalene (1 g), K2CO3 (1 g) were added. The mixture was refluxed for 12 h under nitrogen atmosphere with vigorous mechanical stirring. After reaction, the mixture was cooled down to room temperature and centrifugally separated. The 1,8Diaminoaphalene functionalized silica nanoparticles were washed with DCM (2×50 mL) and ethanol (2×50 mL). The chemical reaction is shown in Fig. 1.

Fig. 1 Synthesis process of functionalized silica nanoparticles.

1,8-Diaminoaphalene

3 Results and discussion 3.1 Adsorbent characterization FT-IR spectra of SiO2 and SiO2-MPTES nanoparticles are presented in Fig. 2. The peaks at 478, 800 and 1116 cm−1 correspond to the Si — O — Si vibration of the silica phase. The peak at 3468 cm−1

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should be ascribed to — OH on the silnaol based. Compared FT-IR spectra of SiO2 nanoparticles Fig. 2(a) with SiO2-MPTES nanoparticles Fig. 2(b), we can see that the new peaks appeared at 2553, 2929 and 694 cm−1 may be attributed to the — SH stretching vibration, —CH2— asymmetric stretching and rocking vibration, respectively. It implies that the MPTES was successfully chemical covalent bonded on the surface of SiO2 nanoparticles. Figure 3 shows XPS spectra of MPTES functionalized SiO2 nanoparticles. As illustrated in Fig. 3(a), the S 2p core-level spectrum of SiO2-MPTES can be curve-fitted into two peak components with

Fig. 2 FT-IR spectra of SiO2 nanoparticles (a) and SiO2MPTES nanoparticles (b)

Fig. 3 S 2p core-level (a) and C 1s core-level (b) spectra of SiO2-MPTES nanoparticles

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binding energies at about 163.8 and 164.8 eV, attributed to the R—CH and S—H species, respectively. C 1s core-level spectrum (Fig. 3(b)) presented three peak components having binding energies at about 282.8, 285.0 and 287.0 eV, assigned to the C—Si, C—H and C—C species, respectively. The results confirmed the formation of MPTES on the surface of SiO2. To confirm the effectiveness of APTES grafting onto the silica nanoparticles and 1,8-Diaminoaphalene grafting onto the SiO2-APTES nanoparticles, FT-IR spectra of SiO2 nanoparticles (a), SiO2-APTES nanoparticles (b), chloroacetyl chloride modified SiO2 nanoparticles (c) and 1,8-Diaminoaphalene functionalized SiO2 nanoparticles (d) are presented in Fig. 4. The peaks at 476, 799 and 1117 cm−1 correspond to the Si—O—Si vibration of the silica phase. The peak at 3446 cm−1 should be ascribed to —OH on the silnaol based. Comparing FT-IR spectra of SiO2 nanoparticles (a) and SiO2-APTES nanoparticles (b), we can see that the new peaks appeared at 2936 and 694 cm−1 may be attributed to — CH2 — asymmetric stretching and rocking vibration, —NH2— vibration peak appears in the 3000−3500 cm−1, which is overlapped with —OH on the silnaol based. It implies that the APTES has been grafted onto the surface of SiO2 nanoparticles. Comparing FT-IR spectra of SiO2-APTES nanoparticles (b) and chloroacetyl chloride modified SiO2 nanoparticles (c), we can see that the new peak appeared at 1653 cm−1 may be attributed to — CONH — stretching vibration. Comparing FT-IR spectra of chloroacetyl chloride modified SiO2 nanoparticles (c) and 1,8-Diaminoaphalene functionalized SiO2 nanoparticles (d), we can see that the new peak appeared at 1595 cm−1 may be attributed to naphthyl vibration. It possibly implies that the 1,8- Diaminoaphalene was successfully chemical covalent bonded on the surface of SiO2 nanoparticles.

Fig. 4 FT-IR spectra of SiO2 nanoparticles (a), SiO2-APTES nanoparticles (b), chloroacetyl chloride modified SiO2 nanoparticles (c) and 1,8-Diaminoaphalene functionalized SiO2 nanoparticles (d)

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Figure 5 shows XPS spectra of 1,8Diaminoaphalene functionalized SiO2 nanoparticles. As illustrated in Fig. 5(a), the N 1s core-level spectrum of 1,8-Diaminoaphalene functionalized SiO2 nanoparticles can be curve-fitted into three peak components with binding energies at about 398.9, 400.3, 402.0 eV, attributed to the N—H, C—N and naphthalene-N species, respectively. C 1s core-level spectrum (Fig. 5(b)) presented three peak components having binding energies at about 285.0, 286.3 and 287.7 eV, assigned to the C—H, naphthyl and C=O species, respectively. The results confirmed the formation of 1,8-Diaminoaphalene on the surface of SiO2.

Fig. 6 Pb2+, Cu2+, Hg2+, Cd2+, Zn2+ removal efficiency histograms of SiO2-MPTES in 10 mL of Pb2+, Cu2+, Hg2+, Cd2+, Zn2+ aqueous solution (Conditions: SiO2-MPTES, 10 mg; initial concentration of Pb2+, Cu2+, Hg2+, Cd2+ and Zn2+, 2×10−4; pH=7; time, 4 h; temperature, 25 °C)

in Fig. 7. In this case, the Pb2+, Cu2+, Hg2+, Cd2+, Zn2+ accurate adsorption capacities of SiO2-MPTES are 180, 198, 198, 166, 130 mg/g, respectively. It presented that the APTES functionalized SiO2 exhibited maximum removal efficiency towards Pb2+ and Hg2+ at room temperature.

Fig. 5 N 1s core-level (a) and C 1s core-level (b) spectra of 1,8-Diaminoaphalene functionalized SiO2 nanoparticles

3.2 Adsorption study of SiO2-MPTES The efficiency of SiO2-MPTES nanoparticles for Pb2+, Cu2+, Hg2+, Cd2+, Zn2+ removal is demonstrated in Fig. 6. In this case, the Pb2+, Cu2+, Hg2+, Cd2+, Zn2+ accurate adsorption capacities of SiO2-MPTES are 169, 150, 166, 140, 130 mg/g (mass of metal ions/mass of adsorbent), respectively. It implies that the MPTES functionalized SiO2 would play an important role in the efficiency of Pb2+ and Hg2+ removal at room temperature. 3.3 Adsorption study of SiO2-APTES The efficiency of SiO2-MPTES nanoparticles for Pb 2+ , Cu 2+ , Hg 2+ , Cd 2+ , Zn 2+ removal is demonstrated

Fig. 7 Pb2+, Cu2+, Hg2+, Cd2+, Zn2+ removal efficiency histograms of SiO2-APTES in 10 mL of Pb2+, Cu2+, Hg2+, Cd2+, Zn2+ aqueous solution (Conditions: SiO2-APTES, 10 mg; initial concentration of Pb2+, Cu2+, Hg2+, Cd2+ and Zn2+, 2×10−4; pH=7; time, 4 h; temperature, 25 °C)

3.4

Adsorption study of 1,8-Diaminoaphalene functionalized SiO2 The efficiency of 1,8-Diaminoaphalene functionalized SiO2 nanoparticles for Pb2+, Cu2+, Hg2+, Cd2+, Zn2+ removal is demonstrated in Fig. 8. In this case, the Pb2+, Cu2+, Hg2+, Cd2+, Zn2+ of accurate adsorption capacities of 1,8-Diaminoaphalene functionalized SiO2 nanoparticles are 220, 299, 320, 199, 133 mg/g, respectively. It shows that the 1,8-Diaminoaphalene functionalized SiO2 nanoparticles are excellent for removal of Hg2+ at room temperature. The coming investigation will be carried out to further examine the resuability and selective removal of heavy metal ions in the presence of the other interfering cations.

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Fig. 8 Pb2+, Cu2+, Hg2+, Cd2+, Zn2+ removal efficiency histograms of 1,8-Diaminoaphalene functionalized SiO2 in 10 mL of Pb2+, Cu2+, Hg2+, Cd2+, Zn2+ aqueous solution (Conditions: 1,8-Diaminoaphalene functionalized SiO2 nanoparticles, 10 mg; initial concentration of Pb2+, Cu2+, Hg2+, Cd2+ and Zn2+, 5×10−4; pH=7; time, 4 h; temperature, 25 °C)

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4 Conclusions 1) We describe the preparation and characterization of silica nanoparticles modified with MPTES, APTES and 1,8-Diaminoaphalene and re-use industrial silica fume based on its property. 2) The adsorption capacity of SiO2-MPTES nanoparticles for removal of heavy metal ions follows the order of Pb2+Hg2+>Cu2+>Cd2+>Zn2+; the adsorption capacity of SiO2-APTES nanoparticles for removal of heavy metal ions follows the order of Cu2+Hg2+>> Pb2+>Cd2+>Zn2+; the adsorption capacity of 1,8Diaminoaphalene functionalized SiO2 nanoparticles for removal of heavy metal ions follows the order of Hg2+> Cu2+ >Pb2+ >Cd2+>Zn2+. 3) The differential function groups synthesized and functionalized onto the surface of silica nanoparticles exhibit significant selective ability to heavy metal ions. Further onwards, the design of differential function groups and results presented here would offer a new approach to construct high efficient and selective adsorbent. It may also be re-used to fabricate a nanosensor to directly remove or sense specific targets in wastewater containing heavy metal ions.

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