Environ Sci Pollut Res DOI 10.1007/s11356-014-3430-6
RESEARCH ARTICLE
High-efficient mercury removal from environmental water samples using di-thio grafted on magnetic mesoporous silica nanoparticles Ali Mehdinia & Maryam Akbari & Tohid Baradaran Kayyal & Mohammad Azad
Received: 11 April 2014 / Accepted: 7 August 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract In this work, magnetic di-thio functionalized mesoporous silica nanoparticles (DT-MCM-41) were prepared by grafting dithiocarbamate groups within the channels of magnetic mesoporous silica nanocomposites. The functionalized nanoparticles exhibited proper magnetic behavior. They were easily separated from the aqueous solution by applying an external magnetic field. The results indicated that the functionalized nanoparticles had a potential for high-efficient removal of Hg2+ in environmental samples. The maximum adsorption capacity of the sorbent was 538.9 mg g−1, and it took about 10 min to achieve the equilibrium adsorption. The resulted adsorption capacity was higher than similar works for adsorption of mercury. It can be due to the presence of di-thio and amine active groups in the structure of sorbent. The special properties of MCM-41 like large surface area and high porosity also provided a facile accessibility of the mercury ions into the ligand sites. The complete removal of mercury ions was attained with dithiocarbamate groups in a wide range of mercury concentrations. The recovery studies were also applied for the river water, seawater, and wastewater samples, and the values were over of 97 %.
Keywords Dithiocarbamate functionalization . Mesoporous silica . Magnetic nanoparticles . Mercury removal . Water samples
Responsible editor: Bingcai Pan A. Mehdinia (*) Department of Marine Science, Iranian National Institute for Oceanography and Atmospheric Science, Tehran, Iran e-mail: [email protected] M. Akbari : T. Baradaran Kayyal : M. Azad Department of Chemistry, Faculty of Science, K. N. Toosi University of Technology, Tehran, Iran
Introduction A wide variety of toxic inorganic and organic chemicals discharged to the environment as industrial wastes cause serious water, air, and soil pollution. Heavy metals found in wastewaters are originated from chemical manufacturing, nuclear, and other industries (Fenglian and Wang 2011; Barakat 2011). Mercury is one of the most toxic and dangerous heavy metals because of its high affinity for thiol groups in proteins and enzymes, which leads to dysfunction of cells and consequently many health problems in the brain, kidney, and central nervous system (Tchounwou et al. 2003). Several adsorptive compounds were used to adsorb metal ions from the environmental samples containing zeolites (Chojnacki et al. 2004; Deng 2006), activated charcoal (Kurniawan et al. 2006), and clays (Senevirathna et al. 2011). An intrinsic defect of these materials was low adsorption capacities and relatively poor affinity of metal ions to the substrate. To overcome these limitations, a variety of modifications was utilized by application of some chelating ligands with thiol, amine, or crown ether functions to support matrices including of inorganic oxides or organic polymers. Silica-based mesoporous structures such as MCM-41 (Hakami et al. 2012; Li et al. 2011), HMS (Evangelista et al. 2007), SBA-15 (Dana and Sayari 2011), and SBA-1 (Kanel et al. 2005) have been functionalized by various groups to afford strong interaction with metallic cations, particularly mercury (Hakami et al. 2012; Li et al. 2011), or metallic anions such as chromate and arsenate (Kanel et al. 2005). MCM-41 has some prominent features such as the following: hexagonal pores with the low size distributions and low blocking network effects; pores with tailoring and finetuning dimensions (1.5–20 nm); pores with large volumes (>0.6 cm3 g−1); high loading capacity; high surface area (∼700–1,500 m2 g−1); large amount of internal hydroxyl groups (∼40–60 %); high surface reactivity; ease of
Environ Sci Pollut Res
modification and excellent thermal, hydrothermal, chemical, and mechanical stability (Selvam et al. 2001). Therefore, the development of functionalized mesoporous silica as adsorptive material was generated much research attention (Savage et al. 2009; Shahbazi et al. 2011). In particular, the trapping was identified as a promising method for environmental cleanup applications (Savage et al. 2009). An important advantage of these kinds of adsorbents is their comparatively large (nanometer-scale) open-framework pore structure which allows large access of the metal ions to each site in their structure (Feng et al. 1997). On the other hand, the utilization of magnetic nanoparticles is widely employed for preparation of composites in the analytical systems. In this regard, the incorporation of iron oxide nanoparticles into the structure of mesoporous materials can provide a convenient method based on magnetic separation (Aguilar-Arteaga et al. 2010). Therefore, an efficient composite can be achieved by combination of the advantages of mesoporous silica and magnetic nanoparticles (Dong et al. 2008). It is well known that the dithiocarbamate (DT) group has a strong tendency for reaction with the mercury ions. It is due to the presence of two thio groups in the structure of DT which can create strong interaction with the mercury ions because of the strong bonding of Hg and S. In addition, the chelates of DT are remarkably stable in aqueous solutions. Therefore, DT can be qualified as a functionalizing group for adsorption of the mercury ions. In this work, we reported the synthesis of highly efficient mercury ion adsorbent via grafting of DT groups onto the inner surface of the magnetic mesoporous silica nanoparticles. The proposed method is a convenient approach for highly sensitive separation of Hg2+ from aqueous solutions.
transform infrared (FT-IR) (Bruker VERTEX 70 spectrometer). X-ray powder diffraction (XRD) was performed on X’Pert pro MPD diffractometer equipped with a PIXcel detector (PANalytical Company). The sample was scanned in step size 0.01°, time per step of 170 s, and the acquisition time of 15 min. The N2 adsorption–desorption isotherm was determined using a Belsorp mini II (Japan Co.) at −196 °C; after that, the adsorbent was dehydrated and degassed at 120 °C for 13 h. The specific surface area was determined in relative pressure of 0.05–0.2, and cross-sectional area was also 0.162 nm2. The surface area and the pore distribution were obtained by Brunauer–Emmett–Teller (BET) and Barret– Joyner–Halenda (BJH) equations, respectively. Chemicals and materials Iron(III) chloride hexahydrate (FeCl3·6H2O), iron(II) chloride tetrahydrate (FeCl 2 ·4H 2 O), ammonium hydroxide, cetyltrimethyl ammonium bromide (CTAB), sodium silicate, 3-aminopropyltriethoxysilane (APTES), carbon bisulfide (CS2), triethyl amine (Et3N), toluene, acetone, and dichloromethane were purchased from Merck (Darmstadt, Germany). Acetonitrile was obtained from Caledon (Ontario, Canada). The Hg2+ working solutions were prepared daily from mercury standard solution (1,000 mg L−1) (Merck, Germany). The standard solution for the selectivity studies were prepared from salts of lead(II) nitrate, cadmium(II) oxide, and zinc(II) nitrate (Merck, Darmstadt, Germany). Tin(II) chloride dihydrate (SnCl2·2H2O) was used as a reducing agent for cold-vapor analyzing, and sodium hydroxide (NaOH) (from Merck) and diluted nitric acid (HNO3) (Scharlau company, Spain) were used for adjusting pH of the solutions. Preparation of sorbent
Experimental Synthesis of magnetic MCM-41 Instrumentation The GBC 932 plus (Australia) atomic absorption spectrophotometer equipped with a mercury hallow cathode lamp (GBC, Australia) at the wavelength of 253.7 nm was applied for the determination of mercury in the sample solutions. The GBC vapor generation accessory model HG3000 (Australia) with a T-quartz cell was utilized for the cold generation of mercury vapor and its determination in water samples, respectively. In addition, the graphite furnace atomic absorption (GBC GF3000) spectrometry was applied for lead, cadmium, and zinc ion determination. The resulting sorbent was characterized by scanning electron microscopy (VEGA TESCAN) equipped with energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (Philips CM30 TEM), dynamic light scattering (DLS, Malvern, MAL1001767), and Fourier
For preparation of colloidal suspension magnetic nanoparticles, FeCl3·6H2O (2 g) and FeCl2·4H2O (0.8 g) were dissolved in 10 mL of water and was added dropwise to a 100-mL solution of 1.0-M ammonia solution containing 0.4 g of CTAB under nitrogen atmosphere while increasing the temperature up to 80 °C. The black solution was acquired and sonicated. The resultant solution (20 mL) dropwise was added to 840-mL solution of distilled water containing 140 mL of ammonia (1 M) and 4 g of CTAB. Then, 16 mL of sodium silicate was slowly added and maintained for 24 h in a closed container under stirring. The resulting magnetic MCM-41 was washed by distilled water for several times. The surfactant template was then removed from the synthesized material by calcination at 450 °C for 4 h.
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Fig. 1 The schematic representation of the synthesis steps for the prepared sorbent
Synthesis of magnetic NH2-MCM-41 After synthesis of magnetic MCM-41, 1.024 mL of APTES was added for modifying the surface of MCM-41 and the solution was refluxed with toluene under nitrogen atmosphere during 120 min for three times to prepare the magnetic NH2MCM-41. The amine functionalized MCM-41 was also soxhleted and washed with solution of dichloromethane/ acetone (1:1, v/v) for 12 h.
series of 30-mL solutions containing Hg2+ with concentrations of 0, 0.005, 0.010, 0.025, 0.050, 0.075, 0.100, 0.250, 0.500…and 500.00 mg L−1 at pH of 6.0. The resulting solutions were stirred at 25 °C for 10 min to study the adsorption isotherm of the adsorbent.
Synthesis of magnetic DT functionalized MCM-41 (DT-MCM-41) For grafting of DT onto the surface of sorbent, 100 mg of CS2 was added in 25 mL of acetonitrile containing 1 % ET3N and 100 mg of magnetic NH2-MCM-41. The resulting mixture was stirred at the room temperature for 24 h, and then rotary evaporator was used for drying. The schematic diagram for synthesis of magnetic DT-MCM-41 was also represented in Fig. 1. Adsorption isotherm and experimental conditions The adsorption capacity of the sorbent was measured three times by adding of 10 mg of magnetic DT-MCM-41 into a
Fig. 2 Dispersion and magnetic separation of magnetic DT-MCM-41 sorbent
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Fig. 3 The XRD patterns of magnetic MCM-41 (a), the N2 adsorption–desorption isotherms of the prepared sorbent (b), and the BJH plot of the prepared sorbent (c)
The adsorption capacity was calculated as Eq. 1. Qe ¼
ðC 0 −C e Þ V m
ð1Þ
In Eq. 1, Qe (mg g−1) is the absorbed amounts of mercury, and C0 and Ce (mg L−1) are the initial and equilibrium concentrations of mercury solution, respectively. V (L) is the volume of the initial solution, and m (g) is the mass of adsorbent. In this work, two kinds of three-parameter isotherms were studied for fitting of the experimental data. The Sips model, presented as Eq. 2, is capable of modeling both homogeneous and heterogeneous binding surfaces (Lopes et al. 2013). Besides, the Redlich–Peterson is investigated as other equilibrium model for this purpose and it can be described in Eq. 3. Qe ¼
Qmax aC m e 1 þ aC m e
ð2Þ
Qe ¼
k RP C e 1 þ aRP C βe
ð3Þ
In Eq. 2, Qmax (mg/g) represents saturation capacity, a is related to the median binding affinity, and m is the heterogeneous index, which varies from 0 to 1. On the other hand, kRP (L/g) and aRP are the Redlich–Peterson constants in the Eq. 3 and β is the Redlich–Peterson exponent. For selection of more favorable model, the average relative error (ARE) of the fittings between the experimental Qe value and the predicted value of model is also calculated.
ARE ¼
1 0 X yi−byi @ A yi n
100
ð4Þ
Base on Eq. 4, n is the sample size,yi and byi also represent the experimental values and predicted or modelled values, respectively.
Blank experiments on non-functionalized magnetic MCM41 and NH2-MCM-41 were carried out using 10 mg sorbent and 30 mL of Hg 2+ solutions in the concentration of 100 μg L−1. Hg2+ analyses were also performed on the supernatant solution to confirm Hg2+ uptakes by the adsorbent materials. For desorption procedure, 10 mg of adsorbent was suspended with 30 mL of mixture of HNO3 1 M and 2 % thiourea at mercury concentration level of 1 mg L−1. The procedure was performed in batch system, and magnetic separation was applied for this purpose. In addition, the real samples were sampled from Gulf of Oman, Karun River, and wastewater of a casting factory. The samples were collected in glass bottles and maintained in darkness and cool place until analysis. Figure 2 shows the dispersion and separation process of the prepared sorbent. It could be seen that when an external magnetic field was applied, the particles were attracted to the wall of vial. Therefore, a facile separation was performed by a magnet in aqueous media and functionalization of magnetic MCM-41 did not show significant effect on magnetic properties of the sorbent.
Results and discussion Characterization of magnetic DT-MCM-41 The magnetic DT-MCM-41 was characterized using a number of techniques. The XRD pattern of the prepared magnetic MCM-41 is presented in Fig. 3a. The result showed relatively Table 1 The surface area and rp,peak of magnetic MCM-41 and magnetic DT-MCM-41 Adsorbent
Surface area (m2 g−1)
rp,peak (nm)
Magnetic MCM-41 Magnetic DT-MCM-41
1,207 830
2.49 1.23
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Fig. 4 FT-IR spectrums of magnetic MCM-41 (a), NH2-MCM-41 (b), and DT-MCM-41 (c)
well-defined XRD patterns for MCM-41 materials, with one strong peak around 2.45 and two small peaks at 4.25 and 4.83 that were assigned to (100), (110), and (200) planes, respectively. Moreover, the fourth peak at 6.44 can be observed and indexed as (210) in the hexagonal system. The observed wellresolved diffraction peaks come from the typical MCM-41. Fig. 5 TEM (left) and SEM (right) images of magnetic DTMCM-41 sorbent
The N2 adsorption–desorption isotherm and BJH plot were exhibited as Fig. 3b, c, respectively. In this regard, the surface area and rp,peak of BJH plot that expressed mode of the obtained pore radius in the structure of the synthesized magnetic MCM-41 and magnetic DT-MCM-41 are also summarized in Table 1. Although, the surface area of DT-MCM-41
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DLS analysis proved that nanoparticles have diameters between 122 and 255 nm with the mean particle size of 187 nm. Elemental analysis also demonstrated that the sulfur content of the adsorbent was 3.84 % wt. Effect of DT functionalization of adsorbent
Fig. 6 The removal comparison of magnetic MCM-41, NH2-MCM-41, and DT-MCM-41. Experimental conditions—sample volume 30 mL, initial concentration of Hg2+ 100 μg L−1, amount of adsorbent 10 mg
was partially decreased, it was predominantly suitable for interaction and chemisorption of the mercury ions via DT groups. FT-IR analysis of magnetic MCM-41 is illustrated in Fig. 4, the peaks of 781.7, 957.85, and 1,078.79 cm−1 are attributed to the stretching of Si–O, Si–OH, and Si–O–Si, respectively. In addition, the wide peak in region of 3,000–3,600 is related to the OH stretching. The absorption bands below 580 cm−1 are usually attributed to the Fe–O stretches (Yantasee et al. 2007). Figure 4b also shows the symmetry and asymmetry stretching bonds of methyl group in the structure of modifier at the range of 2,800– 3,000 cm−1 which expressed amine functionalization of mesoporous substrate. In addition, FT-IR of magnetic DT-MCM-41 (Fig. 4c) shows the double peaks of S–H at 2,171 and 2,099 cm−1, which are typically very weak due to the aggregation of mercapto groups within hydrogen-binding effects. Both SEM and TEM images (Fig. 5) illustrate the spherical morphology and nanostructure for magnetic DT-MCM-41. Size estimation by
For investigation of the functionalization effect on the adsorption efficiency of MCM-41, a comparison was made between magnetic MCM-41, magnetic NH2-MCM-41, and magnetic DT-MCM-41. The results are shown in Fig. 6, which represent significant superiority of magnetic DT-MCM-41 than the other sorbents. Due to the high surface area and regular porous structure of MCM-41, it is expected that it can also remove somewhat mercury from the sample. In magnetic NH2-MCM41, not only the structure of the silica substrate can adsorb the target analyte but also the presence of hydrophilic NH2 functional groups can cause a greater ability to remove the mercury ions from the aqueous sample. On the other hand, due to the presence of two thio groups in magnetic DT-MCM-41, the sorbent can remove Hg2+ ions from the aqueous sample completely and removal efficiency is close to 100 %. Optimization Effect of CS2 amount A study was performed to evaluate the amount of DT modifier groups. It was known from Fig. 7a that the CS2/NH2-MCM-41 ratios of 2:1 and 1:2 caused great reduction of adsorption
Fig. 7 The effect of CS2 amount for modification of magnetic MCM-41 (a) and effect of solution pH (b), different mass of magnetic DT-MCM-41 (c), and extraction times (d) for quantitative removal of Hg2+ ions
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from 1 to 20 mg were exposed to the 30-mL solutions of Hg2+ (20 mg L−1). As shown in Fig. 7c, the removal efficiency was reached to ∼100 % at the adsorbent mass of 10 mg and no significant increase was observed at 20 mg. Therefore, 10 mg was used in the subsequent experiments. Effect of contact time
Fig. 8 The plot of Qe (mg g−1) versus Ce (mg L−1) at temperature of 25 °C and pH of 6.0
efficiency. The reduction of adsorption efficiency at the 2:1 ratio can be attributed to the filling of adsorbent pores by the modifier with increasing the amount of it. The elevation of hydrophobic properties of the prepared sorbent can be another reason. Besides, lack of the sufficient thio groups on the sorbent can cause the reduction in adsorption capacity at 1:2 ratio. Therefore, a 1:1 ratio of CS2/NH2-MCM-41 was selected for synthesis of the adsorbent. Effect of pH In order to investigate the effect of pH on the adsorption efficiency of the mercury ions, magnetic DT-MCM-41 was mixed with solutions containing 1 mg L−1 of Hg2+ ions at different pH values. The concentrations of mercury ions in the solutions were detected after adsorption, and the results are shown in Fig. 7b. It was observed that the removal efficiency of mercury ions by adsorbent at pH of 6.0 was slightly higher than the other pH values. High pH values may cause the formation of stable hydroxyl complexes or hydroxides for some heavy metals which inhibit the removal of them from the aqueous solutions (Yu et al. 2001). On the other hand, the decreasing of adsorption efficiency in the low pH values can be attributed to the competition of H+ ions with mercury ions for available surface sites of the adsorbent. Effect of mass of adsorbent The optimized amount of the adsorbent was required for quantitative removal of the metal ion from the solutions. In this regard, different amounts of magnetic sorbent ranging Table 2 The Sips and Redlich– Peterson fitting models for magnetic DT-MCM-41
The different contact times between magnetic DT-MCM-41 (10 mg) and Hg2+ ions in the solutions were studied from 1 to 20 min. The results in Fig. 7d demonstrated that contact time of 10 min was sufficient for the efficient removal of mercury ions from 30-mL aqueous solution of Hg2+ (1 mg L−1). The short contact time can be due to the strong tendency of the DT groups of the synthesized sorbent to mercury ions, which can uptake it during the short times. It should be mentioned that for the larger solution volumes (>100 mL), higher contact times would be needed for complete removal of Hg2+ ions with the same amount of adsorbent. Adsorption isotherm Adsorption capacity is the main factor for determining the adsorption behavior of a sorbent. Figure 8 shows the adsorption isotherm of Hg2+ ions at pH of 6.0. The isotherm curves demonstrated the equilibrium concentration (Qe) as the function of Hg2+ concentrations in the solution. The relative standard deviations (RSDs) of the data in all studied concentrations were below 9.1 %. In addition, the Sips and Redlich– Peterson models were investigated for fitting of experimental data and the related constants are shown in Table 2. It can be seen that fitting of the data on the Sips equation did not only yield a good fitting with R2 of 0.9978, but also the related constant of this model like Qmax was adjusted and appropriate to the empirical Qmax. The Sips isotherm also provided lower ARE compared to Redlich–Peterson model. As exhibited in Fig. 8, the resulting maximum adsorption capacity (Qmax) of the prepared magnetic sorbent was obtained as 538.9 mg g−1, which is more than other silica-based sorbents used previously for removal of mercury (Mercier and Pinnavaia 1998; Feng et al. 1997; Brown et al. 2000; Bibby and Mercier 2002; Parham et al. 2012; Soleimani et al. 2011; Dong et al. 2008; Hakami et al. 2012; Kim et al. 2011). The use of mercaptopropyl-trimethoxysilaneas linker for
Sips model
Redlich–Peterson model
Sips constant Qmax (mg g−1) 713.04
m 0.37
a 0.38
Goodness of the fit
Redlich–Peterson constant
Goodness of the fit
R2 0.9978
kRP (Lg−1) 2034.74
R2 0.9936
ARE (%) 3.63
aRP 8.55
β 0.85
ARE (%) 5.13
Environ Sci Pollut Res Table 3 Comparison of the different removal parameters of magnetic DT-MCM-41 sorbent and other silica-based sorbents used in the literature
MTZ 2-mercaptothiazoline, TFSCMNPs thiol-functionalized silica-coated magnetite nanoparticles
Maximum loading capacity (mg g−1)
Contact time (min)
Removal efficiency (%)
Reference
MP-HSM-C12 MTZ-MCM-41-Het
310.0 50.12
1,080 240
– 90
Mercier and Pinnavaia (1998) Brown et al. (2000)
MTZ-MCM-41-Hom Octadecyl silica cartridge
140.40 210
240 240
90 96
Bibby and Mercier (2002) Parham et al. (2012)
Mesoporous magnetic sorbents TF-SCMNPs
14
–
–
Feng et al. (1997)
207.7
–
–
Soleimani et al. (2011)
Mag-SBA-15
357.1
420
97
Kim et al. (2011)
SH-mSi@Fe3O4
260.77
20
97
Hakami et al. (2012)
Magnetic DT-MCM-41
538.9
10
>99
This work
modification of magnetic mesoporous was currently applied for the mercury removal (Dong et al. 2008; Li et al. 2011), but in the present study, the DT functionalized MCM-41 was used and it provided proper Qmax than conventional linker. It can be owing to the presence of two groups of thio on each silicon atom that it makes higher distribution of thio groups on the adsorbent and a strong tendency for reaction with mercury. The presence of amino groups in the structure of the sorbent can also cause hydrophilic property of the sorbent and increase removal efficiency. On the other hand, the selection of MCM-41 as qualified substrate with large surface area and high porosity leads to efficient removal of mercury ions. Therefore, DT functionalized mesoprous silica was a potential sorbent which could create stable chelates in aqueous media. Table 3 also shows a comparison between the main factors of the previously reported adsorbents and the magnetic adsorbent introduced in the present work. As can be seen from this table, magnetic DT-MCM-41 showed >99 % removal efficiency and loading capacity, at short contact time compared with the other similar adsorbents reported in the literature.
Fig. 9 Effect of interfering ions at 25 °C and pH of 6.0. Experimental conditions— different concentrations of each ion 0.5, 1, and 5 mg L−1, sample volume 50 mL, amount of adsorbent 20 mg, pH of 6.0
Effect of competing ions on adsorption The competitive adsorption of analogous ions on the binding sites is often an essential problem when the conventional adsorbents are used for removal of heavy metals. To study the availability and effectiveness of magnetic DT-MCM-41 on the uptake of mercury ion, the removal of Hg2+ in the presence of other ions such as Pb2+, Zn2+, and Cd2+ was examined. For this propose, solutions with different concentrations of Hg2+, Pb2+, Zn2+, and Cd2+ were prepared and then mixed with the adsorbent. After analysis of the supernatant of each solution, it was found that magnetic DT-MCM-41 was more selective for mercury ion than the other ion metals (Fig. 9). The results showed that the adsorption capacities for the Hg2+ were the highest among the other ions, and Pb2+ was also a good competitive ion for Hg2+ in the experiments. The interfering effect of Cd2+ was also comparable with Pb2+, but Zn2+ showed the lowest interference with Hg2+. The high affinity of magnetic sorbent for mercury ions can be attributed to the strong interaction between sulfur groups of the sorbent and mercury ions.
Environ Sci Pollut Res Table 4 The characterization of the real water samples Analysis
Seawater
River water
Wastewater
Na (mg L−1) K (mg L−1) Li (mg L−1) Ca (mg L−1) Mg (mg L−1) Al (mg L−1) Fe (mg L−1) Mn (mg L−1) Co (mg L−1) Cr (mg L−1) Cd (mg L−1) Ni (mg L−1) Cu (mg L−1) Ag (mg L−1) As (mg L−1) Ba (mg L−1) B (mg L−1)
11,700 710.68 0.20751 464.27 1,460.8 N.D. 0.08215 0.00491 0.00127 N.D. 0.00003 0.00805 0.02965 N.D. 0.03329 0.00740 4.5008
362.76 11.203 0.17570 219.07 276.55 N.D. 0.4642 N.D. N.D. N.D. N.D. 0.00700 N.D. N.D. N.D. 0.00694 N.D.
155.78 6.3506 N.D. 283.49 195.00 N.D. 1070.1 0.53052 N.D. N.D. N.D. 0.53076 N.D. N.D. N.D. N.D. N.D.
Be (mg L−1) Mo (mg L−1) Se (mg L−1) Ti (mg L−1) V (mg L−1) Sr (mg L−1) Sn (mg L−1) Si (mg L−1) P (mg L−1) Pb (mg L−1) Hg (mg L−1) Zn (mg L−1) Cl (mg L−1) SO4 (mg L−1)
N.D. 0.01205 N.D. 0.01632 N.D. 8.2805 N.D. 0.34300 N.D. 0.00247 N.D. N.D. 18143 381.07
N.D. 0.02908 N.D. 0.04711 N.D. N.D. N.D. 0.26887 N.D. N.D. N.D. N.D. 619.92 158.61
N.D. 0.01926 N.D. N.D. N.D. N.D. 0.13257 N.D. N.D. N.D. N.D. 80.007 293.51 172.27
N.D. not detected
Table 5 Recovery amounts of Hg2+ from different water samples using magnetic DT-MCM-41
Stability and reuse When strong acids with high concentrations were used for desorption of the metal ions from the ordered mesoporous silica, a partial destruction would occur for the meso structures. Moreover, the mercury uptake capacity can be dropped to ca. 40–60 % of their original values after leaching by strong acids (Li et al. 2011). Therefore, desorption of mercury ions was carried out by using a mixture of 1 M HNO3 and 2 % thiourea. The addition of thiourea as complexing agent obtained high recovery value (∼100 %) for mercury ions. The recovery efficiency of mercury ions was also >98 % at concentration level of 1 mg L−1 after using for three times. Real sample analysis Natural waters comprise complex matrices via the presence of different inorganic and organic materials. To study the availability and the effectiveness of magnetic DT-MCM-41 on the uptake of mercury ion in natural water resources, three water samples including river water, seawater, and wastewater were utilized. The composition of the real water and wastewater samples was listed in Table 4. In addition, we prepared artificial media to assure that bulk precipitation of mercury ion was excluded at the optimal pH in the matrix of the studied real samples. The recovery values were calculated by determination
Water sample
Added (μg L−1)
Found (μg L−1)
Recovery (%)
River water
0 5.00 10.0 20.0 0 5.00 10.0 20.0
N.D. 5.01 9.91 19.86 N.D. 4.94 9.94 19.45
– 100 99.1 99.3 – 98.8 99.4 97.2
0 5.00 10.0 20.0
N.D. 4.95 9.87 19.82
– 99.0 98.7 99.1
Seawater
Wastewater
N.D. not detected
The influence of electrolyte concentration (adjusted by sodium chloride) on the adsorption and removal of Hg2+ ions (30 mL, 100 μg L−1) was also investigated. It was demonstrated that the removal of Hg2+ ions was almost constant in a 37 % solution of sodium chloride. It confirmed that the complex formation between the sulfide groups on the magnetic DT-MCM-41 and Hg2+ ions in the test solutions was not significantly affected even by high salt concentration under the applied conditions for real sample analysis.
RSD (%) (n=3)
3.82 2.17 3.58 1.68 4.51 1.84 2.53 3.20 3.26
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of Hg2+ concentrations in the eluted solutions. It was found from Table 5, that magnetic DT-MCM-41 showed good recoveries for Hg2+ in different water samples, with recoveries of over 97 %. This indicated that the interaction between sulfide groups on the present sorbent and Hg2+ ions was not affected by high ionic strength of the real samples matrices.
Conclusions The new magnetic adsorbent was successfully prepared for mercury removal by DT-functionalized mesoporous silica. The DT-functionalized mesoporous silica is expected to use as an adsorbent for soft acids such as Hg2+, because the thiol group is a soft base. The magnetic sorbent exhibited proper properties for removal of mercury with respect to the conventional used linkers. It can be attributed to the presence of two thio and amino groups in structure of mesoporous sorbent. The Sips isotherm was more appropriate for the study of adsorption behavior. The presence of coexisting heavy metal ions did not influence significantly on the mercury adsorption. The resulted adsorbent was used to extract Hg2+in different water resources (river water, industrial wastewater, and seawater). The high removal efficiency in ppb level and short adsorption time were also obtained. The proposed method showed high capability for fast removal of Hg2+ ion from natural waters with high efficiency. Acknowledgment We gratefully acknowledge the financial support of this work by the Iranian National Science Foundation with grant number 92001925.
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Deng S (2006) Sorbent technology. Encycl Chem Process. doi:10.1081/ E-ECHP-120007963 Dong J, Xu Z, Wang F (2008) Engineering and characterization of mesoporous silica-coated magnetic particles for mercury removal from industrial effluents. Appl Surf Sci 254:3522– 3530 Evangelista SHM, Deoliveira E, Castro GR, Zara LF, Prado AGS (2007) Hexagonal mesoporous silica modified with 2mercaptothiazoline for removing mercury from water solution. Surf Sci 601:2194–2202 Feng X, Fryxell GE, Wang LQ, Kim AY, Liu J, Kemner KM (1997) Functionalized monolayers on ordered mesoporous supports. Science 5314:923–926 Fenglian F, Wang Q (2011) Removal of heavy metal ions from wastewaters: a review. J Environ Manage 92:407–418 Hakami O, Zhang Y, Banks CJ (2012) Thiol-functionalised mesoporous silica-coated magnetite nanoparticles for high efficiency removal and recovery of Hg from water. Water Res 46:3913–3922 Kanel S, Manning B, Charlet L, Choi H (2005) Removal of arsenic (III) from ground water by nanoscale zero-valent iron. Environ Sci Technol 39:1291–1298 Kim BC, Lee J, Um W, Kim J, Joo J, Lee JH, Kwak JH, Kim JH, Lee C, Lee H, Addleman RS, Hyeon T, Gu MB, Kim J (2011) Magnetic mesoporous materials for removal of environmental wastes. J Hazard Mater 192:1140–1147 Kurniawan TA, Chan G, Lo W, Babel S (2006) Comparisons of low-cost adsorbents for treating wastewaters laden with heavy metals. Sci Total Environ 366:409–426 Li G, Zhao Z, Liu J, Jiang G (2011) Effective heavy metal removal from aqueous systems by thiol functionalized magnetic mesoporous silica. J Hazard Mater 192:277–283 Lopes CB, Figueira P, Tavares DS, LinZ D-d-S AL, Duarte AC, RochaJ TT, Pereira E (2013) Core–shell magnetite-silica dithiocarbamatederivatised particles achieve the water framework directive quality criteria for mercury in surface waters. Environ Sci Pollut Res 20: 5963–5974 Mercier L, Pinnavaia TJ (1998) Heavy metal ion adsorbents formed by the grafting of a thiol functionality to mesoporous silica molecular sieves: factors affecting Hg (II) uptake. Environ Sci Technol 32: 2749–2754 Parham H, Zargar B, Shiralipour R (2012) Fast and efficient removal of mercury from water samples using magnetic ironoxide nanoparticles modified with 2-mercaptobenzothiazole. J Hazard Mater 205:94–100 Savage N, Diallo M, Duncan J, Street A, Sustich R (2009) Nanotechnology applications for clean water, Elsevier, Amsterdam, The Netherlands Selvam P, Bhatia S, Sonwane C (2001) Recent advances in processing and characterization of periodic mesoporous MCM-41 silicate molecular sieves. Ind Eng Chem Res 40: 3237–3261 Senevirathna U, Zhang W, Gu H (2011) Effect of carboxylic and thiol ligands (oxalate, cysteine) on the kinetics of desorption of Hg (II) from kaolinite. Water Soil Pollut 215:573–584 Shahbazi A, Younesi H, Badiei A (2011) Functionalized SBA-15 mesoporous silica by melamine-based dendrimer amines for adsorptive characteristics of Pb (II), Cu (II) and Cd (II) heavy metal ions in batch and fixed bed column. Chem Eng J 168:505–518 Soleimani M, Mahmodi MS, Morsali A, Khani A, Ghahraman-Afshar M (2011) Using a new ligand for solid phase extraction of mercury. J Hazard Mater 189:371–376 Tchounwou PB, Ayensu WK, Ninashvili N, Sutton D (2003) Review: Environmental exposure to mercury and its toxic opathologic implications for public health. Environ Toxic 18:149–175 Yantasee W, Warner CL, Sangvanich T, Addleman RS, Carter TG, Wiacek RJ, Fryxell GE, Timchalk C, Warner MG (2007) Removal
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RESEARCH ARTICLE
High-efficient mercury removal from environmental water samples using di-thio grafted on magnetic mesoporous silica nanoparticles Ali Mehdinia & Maryam Akbari & Tohid Baradaran Kayyal & Mohammad Azad
Received: 11 April 2014 / Accepted: 7 August 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract In this work, magnetic di-thio functionalized mesoporous silica nanoparticles (DT-MCM-41) were prepared by grafting dithiocarbamate groups within the channels of magnetic mesoporous silica nanocomposites. The functionalized nanoparticles exhibited proper magnetic behavior. They were easily separated from the aqueous solution by applying an external magnetic field. The results indicated that the functionalized nanoparticles had a potential for high-efficient removal of Hg2+ in environmental samples. The maximum adsorption capacity of the sorbent was 538.9 mg g−1, and it took about 10 min to achieve the equilibrium adsorption. The resulted adsorption capacity was higher than similar works for adsorption of mercury. It can be due to the presence of di-thio and amine active groups in the structure of sorbent. The special properties of MCM-41 like large surface area and high porosity also provided a facile accessibility of the mercury ions into the ligand sites. The complete removal of mercury ions was attained with dithiocarbamate groups in a wide range of mercury concentrations. The recovery studies were also applied for the river water, seawater, and wastewater samples, and the values were over of 97 %.
Keywords Dithiocarbamate functionalization . Mesoporous silica . Magnetic nanoparticles . Mercury removal . Water samples
Responsible editor: Bingcai Pan A. Mehdinia (*) Department of Marine Science, Iranian National Institute for Oceanography and Atmospheric Science, Tehran, Iran e-mail: [email protected] M. Akbari : T. Baradaran Kayyal : M. Azad Department of Chemistry, Faculty of Science, K. N. Toosi University of Technology, Tehran, Iran
Introduction A wide variety of toxic inorganic and organic chemicals discharged to the environment as industrial wastes cause serious water, air, and soil pollution. Heavy metals found in wastewaters are originated from chemical manufacturing, nuclear, and other industries (Fenglian and Wang 2011; Barakat 2011). Mercury is one of the most toxic and dangerous heavy metals because of its high affinity for thiol groups in proteins and enzymes, which leads to dysfunction of cells and consequently many health problems in the brain, kidney, and central nervous system (Tchounwou et al. 2003). Several adsorptive compounds were used to adsorb metal ions from the environmental samples containing zeolites (Chojnacki et al. 2004; Deng 2006), activated charcoal (Kurniawan et al. 2006), and clays (Senevirathna et al. 2011). An intrinsic defect of these materials was low adsorption capacities and relatively poor affinity of metal ions to the substrate. To overcome these limitations, a variety of modifications was utilized by application of some chelating ligands with thiol, amine, or crown ether functions to support matrices including of inorganic oxides or organic polymers. Silica-based mesoporous structures such as MCM-41 (Hakami et al. 2012; Li et al. 2011), HMS (Evangelista et al. 2007), SBA-15 (Dana and Sayari 2011), and SBA-1 (Kanel et al. 2005) have been functionalized by various groups to afford strong interaction with metallic cations, particularly mercury (Hakami et al. 2012; Li et al. 2011), or metallic anions such as chromate and arsenate (Kanel et al. 2005). MCM-41 has some prominent features such as the following: hexagonal pores with the low size distributions and low blocking network effects; pores with tailoring and finetuning dimensions (1.5–20 nm); pores with large volumes (>0.6 cm3 g−1); high loading capacity; high surface area (∼700–1,500 m2 g−1); large amount of internal hydroxyl groups (∼40–60 %); high surface reactivity; ease of
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modification and excellent thermal, hydrothermal, chemical, and mechanical stability (Selvam et al. 2001). Therefore, the development of functionalized mesoporous silica as adsorptive material was generated much research attention (Savage et al. 2009; Shahbazi et al. 2011). In particular, the trapping was identified as a promising method for environmental cleanup applications (Savage et al. 2009). An important advantage of these kinds of adsorbents is their comparatively large (nanometer-scale) open-framework pore structure which allows large access of the metal ions to each site in their structure (Feng et al. 1997). On the other hand, the utilization of magnetic nanoparticles is widely employed for preparation of composites in the analytical systems. In this regard, the incorporation of iron oxide nanoparticles into the structure of mesoporous materials can provide a convenient method based on magnetic separation (Aguilar-Arteaga et al. 2010). Therefore, an efficient composite can be achieved by combination of the advantages of mesoporous silica and magnetic nanoparticles (Dong et al. 2008). It is well known that the dithiocarbamate (DT) group has a strong tendency for reaction with the mercury ions. It is due to the presence of two thio groups in the structure of DT which can create strong interaction with the mercury ions because of the strong bonding of Hg and S. In addition, the chelates of DT are remarkably stable in aqueous solutions. Therefore, DT can be qualified as a functionalizing group for adsorption of the mercury ions. In this work, we reported the synthesis of highly efficient mercury ion adsorbent via grafting of DT groups onto the inner surface of the magnetic mesoporous silica nanoparticles. The proposed method is a convenient approach for highly sensitive separation of Hg2+ from aqueous solutions.
transform infrared (FT-IR) (Bruker VERTEX 70 spectrometer). X-ray powder diffraction (XRD) was performed on X’Pert pro MPD diffractometer equipped with a PIXcel detector (PANalytical Company). The sample was scanned in step size 0.01°, time per step of 170 s, and the acquisition time of 15 min. The N2 adsorption–desorption isotherm was determined using a Belsorp mini II (Japan Co.) at −196 °C; after that, the adsorbent was dehydrated and degassed at 120 °C for 13 h. The specific surface area was determined in relative pressure of 0.05–0.2, and cross-sectional area was also 0.162 nm2. The surface area and the pore distribution were obtained by Brunauer–Emmett–Teller (BET) and Barret– Joyner–Halenda (BJH) equations, respectively. Chemicals and materials Iron(III) chloride hexahydrate (FeCl3·6H2O), iron(II) chloride tetrahydrate (FeCl 2 ·4H 2 O), ammonium hydroxide, cetyltrimethyl ammonium bromide (CTAB), sodium silicate, 3-aminopropyltriethoxysilane (APTES), carbon bisulfide (CS2), triethyl amine (Et3N), toluene, acetone, and dichloromethane were purchased from Merck (Darmstadt, Germany). Acetonitrile was obtained from Caledon (Ontario, Canada). The Hg2+ working solutions were prepared daily from mercury standard solution (1,000 mg L−1) (Merck, Germany). The standard solution for the selectivity studies were prepared from salts of lead(II) nitrate, cadmium(II) oxide, and zinc(II) nitrate (Merck, Darmstadt, Germany). Tin(II) chloride dihydrate (SnCl2·2H2O) was used as a reducing agent for cold-vapor analyzing, and sodium hydroxide (NaOH) (from Merck) and diluted nitric acid (HNO3) (Scharlau company, Spain) were used for adjusting pH of the solutions. Preparation of sorbent
Experimental Synthesis of magnetic MCM-41 Instrumentation The GBC 932 plus (Australia) atomic absorption spectrophotometer equipped with a mercury hallow cathode lamp (GBC, Australia) at the wavelength of 253.7 nm was applied for the determination of mercury in the sample solutions. The GBC vapor generation accessory model HG3000 (Australia) with a T-quartz cell was utilized for the cold generation of mercury vapor and its determination in water samples, respectively. In addition, the graphite furnace atomic absorption (GBC GF3000) spectrometry was applied for lead, cadmium, and zinc ion determination. The resulting sorbent was characterized by scanning electron microscopy (VEGA TESCAN) equipped with energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (Philips CM30 TEM), dynamic light scattering (DLS, Malvern, MAL1001767), and Fourier
For preparation of colloidal suspension magnetic nanoparticles, FeCl3·6H2O (2 g) and FeCl2·4H2O (0.8 g) were dissolved in 10 mL of water and was added dropwise to a 100-mL solution of 1.0-M ammonia solution containing 0.4 g of CTAB under nitrogen atmosphere while increasing the temperature up to 80 °C. The black solution was acquired and sonicated. The resultant solution (20 mL) dropwise was added to 840-mL solution of distilled water containing 140 mL of ammonia (1 M) and 4 g of CTAB. Then, 16 mL of sodium silicate was slowly added and maintained for 24 h in a closed container under stirring. The resulting magnetic MCM-41 was washed by distilled water for several times. The surfactant template was then removed from the synthesized material by calcination at 450 °C for 4 h.
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Fig. 1 The schematic representation of the synthesis steps for the prepared sorbent
Synthesis of magnetic NH2-MCM-41 After synthesis of magnetic MCM-41, 1.024 mL of APTES was added for modifying the surface of MCM-41 and the solution was refluxed with toluene under nitrogen atmosphere during 120 min for three times to prepare the magnetic NH2MCM-41. The amine functionalized MCM-41 was also soxhleted and washed with solution of dichloromethane/ acetone (1:1, v/v) for 12 h.
series of 30-mL solutions containing Hg2+ with concentrations of 0, 0.005, 0.010, 0.025, 0.050, 0.075, 0.100, 0.250, 0.500…and 500.00 mg L−1 at pH of 6.0. The resulting solutions were stirred at 25 °C for 10 min to study the adsorption isotherm of the adsorbent.
Synthesis of magnetic DT functionalized MCM-41 (DT-MCM-41) For grafting of DT onto the surface of sorbent, 100 mg of CS2 was added in 25 mL of acetonitrile containing 1 % ET3N and 100 mg of magnetic NH2-MCM-41. The resulting mixture was stirred at the room temperature for 24 h, and then rotary evaporator was used for drying. The schematic diagram for synthesis of magnetic DT-MCM-41 was also represented in Fig. 1. Adsorption isotherm and experimental conditions The adsorption capacity of the sorbent was measured three times by adding of 10 mg of magnetic DT-MCM-41 into a
Fig. 2 Dispersion and magnetic separation of magnetic DT-MCM-41 sorbent
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Fig. 3 The XRD patterns of magnetic MCM-41 (a), the N2 adsorption–desorption isotherms of the prepared sorbent (b), and the BJH plot of the prepared sorbent (c)
The adsorption capacity was calculated as Eq. 1. Qe ¼
ðC 0 −C e Þ V m
ð1Þ
In Eq. 1, Qe (mg g−1) is the absorbed amounts of mercury, and C0 and Ce (mg L−1) are the initial and equilibrium concentrations of mercury solution, respectively. V (L) is the volume of the initial solution, and m (g) is the mass of adsorbent. In this work, two kinds of three-parameter isotherms were studied for fitting of the experimental data. The Sips model, presented as Eq. 2, is capable of modeling both homogeneous and heterogeneous binding surfaces (Lopes et al. 2013). Besides, the Redlich–Peterson is investigated as other equilibrium model for this purpose and it can be described in Eq. 3. Qe ¼
Qmax aC m e 1 þ aC m e
ð2Þ
Qe ¼
k RP C e 1 þ aRP C βe
ð3Þ
In Eq. 2, Qmax (mg/g) represents saturation capacity, a is related to the median binding affinity, and m is the heterogeneous index, which varies from 0 to 1. On the other hand, kRP (L/g) and aRP are the Redlich–Peterson constants in the Eq. 3 and β is the Redlich–Peterson exponent. For selection of more favorable model, the average relative error (ARE) of the fittings between the experimental Qe value and the predicted value of model is also calculated.
ARE ¼
1 0 X yi−byi @ A yi n
100
ð4Þ
Base on Eq. 4, n is the sample size,yi and byi also represent the experimental values and predicted or modelled values, respectively.
Blank experiments on non-functionalized magnetic MCM41 and NH2-MCM-41 were carried out using 10 mg sorbent and 30 mL of Hg 2+ solutions in the concentration of 100 μg L−1. Hg2+ analyses were also performed on the supernatant solution to confirm Hg2+ uptakes by the adsorbent materials. For desorption procedure, 10 mg of adsorbent was suspended with 30 mL of mixture of HNO3 1 M and 2 % thiourea at mercury concentration level of 1 mg L−1. The procedure was performed in batch system, and magnetic separation was applied for this purpose. In addition, the real samples were sampled from Gulf of Oman, Karun River, and wastewater of a casting factory. The samples were collected in glass bottles and maintained in darkness and cool place until analysis. Figure 2 shows the dispersion and separation process of the prepared sorbent. It could be seen that when an external magnetic field was applied, the particles were attracted to the wall of vial. Therefore, a facile separation was performed by a magnet in aqueous media and functionalization of magnetic MCM-41 did not show significant effect on magnetic properties of the sorbent.
Results and discussion Characterization of magnetic DT-MCM-41 The magnetic DT-MCM-41 was characterized using a number of techniques. The XRD pattern of the prepared magnetic MCM-41 is presented in Fig. 3a. The result showed relatively Table 1 The surface area and rp,peak of magnetic MCM-41 and magnetic DT-MCM-41 Adsorbent
Surface area (m2 g−1)
rp,peak (nm)
Magnetic MCM-41 Magnetic DT-MCM-41
1,207 830
2.49 1.23
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Fig. 4 FT-IR spectrums of magnetic MCM-41 (a), NH2-MCM-41 (b), and DT-MCM-41 (c)
well-defined XRD patterns for MCM-41 materials, with one strong peak around 2.45 and two small peaks at 4.25 and 4.83 that were assigned to (100), (110), and (200) planes, respectively. Moreover, the fourth peak at 6.44 can be observed and indexed as (210) in the hexagonal system. The observed wellresolved diffraction peaks come from the typical MCM-41. Fig. 5 TEM (left) and SEM (right) images of magnetic DTMCM-41 sorbent
The N2 adsorption–desorption isotherm and BJH plot were exhibited as Fig. 3b, c, respectively. In this regard, the surface area and rp,peak of BJH plot that expressed mode of the obtained pore radius in the structure of the synthesized magnetic MCM-41 and magnetic DT-MCM-41 are also summarized in Table 1. Although, the surface area of DT-MCM-41
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DLS analysis proved that nanoparticles have diameters between 122 and 255 nm with the mean particle size of 187 nm. Elemental analysis also demonstrated that the sulfur content of the adsorbent was 3.84 % wt. Effect of DT functionalization of adsorbent
Fig. 6 The removal comparison of magnetic MCM-41, NH2-MCM-41, and DT-MCM-41. Experimental conditions—sample volume 30 mL, initial concentration of Hg2+ 100 μg L−1, amount of adsorbent 10 mg
was partially decreased, it was predominantly suitable for interaction and chemisorption of the mercury ions via DT groups. FT-IR analysis of magnetic MCM-41 is illustrated in Fig. 4, the peaks of 781.7, 957.85, and 1,078.79 cm−1 are attributed to the stretching of Si–O, Si–OH, and Si–O–Si, respectively. In addition, the wide peak in region of 3,000–3,600 is related to the OH stretching. The absorption bands below 580 cm−1 are usually attributed to the Fe–O stretches (Yantasee et al. 2007). Figure 4b also shows the symmetry and asymmetry stretching bonds of methyl group in the structure of modifier at the range of 2,800– 3,000 cm−1 which expressed amine functionalization of mesoporous substrate. In addition, FT-IR of magnetic DT-MCM-41 (Fig. 4c) shows the double peaks of S–H at 2,171 and 2,099 cm−1, which are typically very weak due to the aggregation of mercapto groups within hydrogen-binding effects. Both SEM and TEM images (Fig. 5) illustrate the spherical morphology and nanostructure for magnetic DT-MCM-41. Size estimation by
For investigation of the functionalization effect on the adsorption efficiency of MCM-41, a comparison was made between magnetic MCM-41, magnetic NH2-MCM-41, and magnetic DT-MCM-41. The results are shown in Fig. 6, which represent significant superiority of magnetic DT-MCM-41 than the other sorbents. Due to the high surface area and regular porous structure of MCM-41, it is expected that it can also remove somewhat mercury from the sample. In magnetic NH2-MCM41, not only the structure of the silica substrate can adsorb the target analyte but also the presence of hydrophilic NH2 functional groups can cause a greater ability to remove the mercury ions from the aqueous sample. On the other hand, due to the presence of two thio groups in magnetic DT-MCM-41, the sorbent can remove Hg2+ ions from the aqueous sample completely and removal efficiency is close to 100 %. Optimization Effect of CS2 amount A study was performed to evaluate the amount of DT modifier groups. It was known from Fig. 7a that the CS2/NH2-MCM-41 ratios of 2:1 and 1:2 caused great reduction of adsorption
Fig. 7 The effect of CS2 amount for modification of magnetic MCM-41 (a) and effect of solution pH (b), different mass of magnetic DT-MCM-41 (c), and extraction times (d) for quantitative removal of Hg2+ ions
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from 1 to 20 mg were exposed to the 30-mL solutions of Hg2+ (20 mg L−1). As shown in Fig. 7c, the removal efficiency was reached to ∼100 % at the adsorbent mass of 10 mg and no significant increase was observed at 20 mg. Therefore, 10 mg was used in the subsequent experiments. Effect of contact time
Fig. 8 The plot of Qe (mg g−1) versus Ce (mg L−1) at temperature of 25 °C and pH of 6.0
efficiency. The reduction of adsorption efficiency at the 2:1 ratio can be attributed to the filling of adsorbent pores by the modifier with increasing the amount of it. The elevation of hydrophobic properties of the prepared sorbent can be another reason. Besides, lack of the sufficient thio groups on the sorbent can cause the reduction in adsorption capacity at 1:2 ratio. Therefore, a 1:1 ratio of CS2/NH2-MCM-41 was selected for synthesis of the adsorbent. Effect of pH In order to investigate the effect of pH on the adsorption efficiency of the mercury ions, magnetic DT-MCM-41 was mixed with solutions containing 1 mg L−1 of Hg2+ ions at different pH values. The concentrations of mercury ions in the solutions were detected after adsorption, and the results are shown in Fig. 7b. It was observed that the removal efficiency of mercury ions by adsorbent at pH of 6.0 was slightly higher than the other pH values. High pH values may cause the formation of stable hydroxyl complexes or hydroxides for some heavy metals which inhibit the removal of them from the aqueous solutions (Yu et al. 2001). On the other hand, the decreasing of adsorption efficiency in the low pH values can be attributed to the competition of H+ ions with mercury ions for available surface sites of the adsorbent. Effect of mass of adsorbent The optimized amount of the adsorbent was required for quantitative removal of the metal ion from the solutions. In this regard, different amounts of magnetic sorbent ranging Table 2 The Sips and Redlich– Peterson fitting models for magnetic DT-MCM-41
The different contact times between magnetic DT-MCM-41 (10 mg) and Hg2+ ions in the solutions were studied from 1 to 20 min. The results in Fig. 7d demonstrated that contact time of 10 min was sufficient for the efficient removal of mercury ions from 30-mL aqueous solution of Hg2+ (1 mg L−1). The short contact time can be due to the strong tendency of the DT groups of the synthesized sorbent to mercury ions, which can uptake it during the short times. It should be mentioned that for the larger solution volumes (>100 mL), higher contact times would be needed for complete removal of Hg2+ ions with the same amount of adsorbent. Adsorption isotherm Adsorption capacity is the main factor for determining the adsorption behavior of a sorbent. Figure 8 shows the adsorption isotherm of Hg2+ ions at pH of 6.0. The isotherm curves demonstrated the equilibrium concentration (Qe) as the function of Hg2+ concentrations in the solution. The relative standard deviations (RSDs) of the data in all studied concentrations were below 9.1 %. In addition, the Sips and Redlich– Peterson models were investigated for fitting of experimental data and the related constants are shown in Table 2. It can be seen that fitting of the data on the Sips equation did not only yield a good fitting with R2 of 0.9978, but also the related constant of this model like Qmax was adjusted and appropriate to the empirical Qmax. The Sips isotherm also provided lower ARE compared to Redlich–Peterson model. As exhibited in Fig. 8, the resulting maximum adsorption capacity (Qmax) of the prepared magnetic sorbent was obtained as 538.9 mg g−1, which is more than other silica-based sorbents used previously for removal of mercury (Mercier and Pinnavaia 1998; Feng et al. 1997; Brown et al. 2000; Bibby and Mercier 2002; Parham et al. 2012; Soleimani et al. 2011; Dong et al. 2008; Hakami et al. 2012; Kim et al. 2011). The use of mercaptopropyl-trimethoxysilaneas linker for
Sips model
Redlich–Peterson model
Sips constant Qmax (mg g−1) 713.04
m 0.37
a 0.38
Goodness of the fit
Redlich–Peterson constant
Goodness of the fit
R2 0.9978
kRP (Lg−1) 2034.74
R2 0.9936
ARE (%) 3.63
aRP 8.55
β 0.85
ARE (%) 5.13
Environ Sci Pollut Res Table 3 Comparison of the different removal parameters of magnetic DT-MCM-41 sorbent and other silica-based sorbents used in the literature
MTZ 2-mercaptothiazoline, TFSCMNPs thiol-functionalized silica-coated magnetite nanoparticles
Maximum loading capacity (mg g−1)
Contact time (min)
Removal efficiency (%)
Reference
MP-HSM-C12 MTZ-MCM-41-Het
310.0 50.12
1,080 240
– 90
Mercier and Pinnavaia (1998) Brown et al. (2000)
MTZ-MCM-41-Hom Octadecyl silica cartridge
140.40 210
240 240
90 96
Bibby and Mercier (2002) Parham et al. (2012)
Mesoporous magnetic sorbents TF-SCMNPs
14
–
–
Feng et al. (1997)
207.7
–
–
Soleimani et al. (2011)
Mag-SBA-15
357.1
420
97
Kim et al. (2011)
SH-mSi@Fe3O4
260.77
20
97
Hakami et al. (2012)
Magnetic DT-MCM-41
538.9
10
>99
This work
modification of magnetic mesoporous was currently applied for the mercury removal (Dong et al. 2008; Li et al. 2011), but in the present study, the DT functionalized MCM-41 was used and it provided proper Qmax than conventional linker. It can be owing to the presence of two groups of thio on each silicon atom that it makes higher distribution of thio groups on the adsorbent and a strong tendency for reaction with mercury. The presence of amino groups in the structure of the sorbent can also cause hydrophilic property of the sorbent and increase removal efficiency. On the other hand, the selection of MCM-41 as qualified substrate with large surface area and high porosity leads to efficient removal of mercury ions. Therefore, DT functionalized mesoprous silica was a potential sorbent which could create stable chelates in aqueous media. Table 3 also shows a comparison between the main factors of the previously reported adsorbents and the magnetic adsorbent introduced in the present work. As can be seen from this table, magnetic DT-MCM-41 showed >99 % removal efficiency and loading capacity, at short contact time compared with the other similar adsorbents reported in the literature.
Fig. 9 Effect of interfering ions at 25 °C and pH of 6.0. Experimental conditions— different concentrations of each ion 0.5, 1, and 5 mg L−1, sample volume 50 mL, amount of adsorbent 20 mg, pH of 6.0
Effect of competing ions on adsorption The competitive adsorption of analogous ions on the binding sites is often an essential problem when the conventional adsorbents are used for removal of heavy metals. To study the availability and effectiveness of magnetic DT-MCM-41 on the uptake of mercury ion, the removal of Hg2+ in the presence of other ions such as Pb2+, Zn2+, and Cd2+ was examined. For this propose, solutions with different concentrations of Hg2+, Pb2+, Zn2+, and Cd2+ were prepared and then mixed with the adsorbent. After analysis of the supernatant of each solution, it was found that magnetic DT-MCM-41 was more selective for mercury ion than the other ion metals (Fig. 9). The results showed that the adsorption capacities for the Hg2+ were the highest among the other ions, and Pb2+ was also a good competitive ion for Hg2+ in the experiments. The interfering effect of Cd2+ was also comparable with Pb2+, but Zn2+ showed the lowest interference with Hg2+. The high affinity of magnetic sorbent for mercury ions can be attributed to the strong interaction between sulfur groups of the sorbent and mercury ions.
Environ Sci Pollut Res Table 4 The characterization of the real water samples Analysis
Seawater
River water
Wastewater
Na (mg L−1) K (mg L−1) Li (mg L−1) Ca (mg L−1) Mg (mg L−1) Al (mg L−1) Fe (mg L−1) Mn (mg L−1) Co (mg L−1) Cr (mg L−1) Cd (mg L−1) Ni (mg L−1) Cu (mg L−1) Ag (mg L−1) As (mg L−1) Ba (mg L−1) B (mg L−1)
11,700 710.68 0.20751 464.27 1,460.8 N.D. 0.08215 0.00491 0.00127 N.D. 0.00003 0.00805 0.02965 N.D. 0.03329 0.00740 4.5008
362.76 11.203 0.17570 219.07 276.55 N.D. 0.4642 N.D. N.D. N.D. N.D. 0.00700 N.D. N.D. N.D. 0.00694 N.D.
155.78 6.3506 N.D. 283.49 195.00 N.D. 1070.1 0.53052 N.D. N.D. N.D. 0.53076 N.D. N.D. N.D. N.D. N.D.
Be (mg L−1) Mo (mg L−1) Se (mg L−1) Ti (mg L−1) V (mg L−1) Sr (mg L−1) Sn (mg L−1) Si (mg L−1) P (mg L−1) Pb (mg L−1) Hg (mg L−1) Zn (mg L−1) Cl (mg L−1) SO4 (mg L−1)
N.D. 0.01205 N.D. 0.01632 N.D. 8.2805 N.D. 0.34300 N.D. 0.00247 N.D. N.D. 18143 381.07
N.D. 0.02908 N.D. 0.04711 N.D. N.D. N.D. 0.26887 N.D. N.D. N.D. N.D. 619.92 158.61
N.D. 0.01926 N.D. N.D. N.D. N.D. 0.13257 N.D. N.D. N.D. N.D. 80.007 293.51 172.27
N.D. not detected
Table 5 Recovery amounts of Hg2+ from different water samples using magnetic DT-MCM-41
Stability and reuse When strong acids with high concentrations were used for desorption of the metal ions from the ordered mesoporous silica, a partial destruction would occur for the meso structures. Moreover, the mercury uptake capacity can be dropped to ca. 40–60 % of their original values after leaching by strong acids (Li et al. 2011). Therefore, desorption of mercury ions was carried out by using a mixture of 1 M HNO3 and 2 % thiourea. The addition of thiourea as complexing agent obtained high recovery value (∼100 %) for mercury ions. The recovery efficiency of mercury ions was also >98 % at concentration level of 1 mg L−1 after using for three times. Real sample analysis Natural waters comprise complex matrices via the presence of different inorganic and organic materials. To study the availability and the effectiveness of magnetic DT-MCM-41 on the uptake of mercury ion in natural water resources, three water samples including river water, seawater, and wastewater were utilized. The composition of the real water and wastewater samples was listed in Table 4. In addition, we prepared artificial media to assure that bulk precipitation of mercury ion was excluded at the optimal pH in the matrix of the studied real samples. The recovery values were calculated by determination
Water sample
Added (μg L−1)
Found (μg L−1)
Recovery (%)
River water
0 5.00 10.0 20.0 0 5.00 10.0 20.0
N.D. 5.01 9.91 19.86 N.D. 4.94 9.94 19.45
– 100 99.1 99.3 – 98.8 99.4 97.2
0 5.00 10.0 20.0
N.D. 4.95 9.87 19.82
– 99.0 98.7 99.1
Seawater
Wastewater
N.D. not detected
The influence of electrolyte concentration (adjusted by sodium chloride) on the adsorption and removal of Hg2+ ions (30 mL, 100 μg L−1) was also investigated. It was demonstrated that the removal of Hg2+ ions was almost constant in a 37 % solution of sodium chloride. It confirmed that the complex formation between the sulfide groups on the magnetic DT-MCM-41 and Hg2+ ions in the test solutions was not significantly affected even by high salt concentration under the applied conditions for real sample analysis.
RSD (%) (n=3)
3.82 2.17 3.58 1.68 4.51 1.84 2.53 3.20 3.26
Environ Sci Pollut Res
of Hg2+ concentrations in the eluted solutions. It was found from Table 5, that magnetic DT-MCM-41 showed good recoveries for Hg2+ in different water samples, with recoveries of over 97 %. This indicated that the interaction between sulfide groups on the present sorbent and Hg2+ ions was not affected by high ionic strength of the real samples matrices.
Conclusions The new magnetic adsorbent was successfully prepared for mercury removal by DT-functionalized mesoporous silica. The DT-functionalized mesoporous silica is expected to use as an adsorbent for soft acids such as Hg2+, because the thiol group is a soft base. The magnetic sorbent exhibited proper properties for removal of mercury with respect to the conventional used linkers. It can be attributed to the presence of two thio and amino groups in structure of mesoporous sorbent. The Sips isotherm was more appropriate for the study of adsorption behavior. The presence of coexisting heavy metal ions did not influence significantly on the mercury adsorption. The resulted adsorbent was used to extract Hg2+in different water resources (river water, industrial wastewater, and seawater). The high removal efficiency in ppb level and short adsorption time were also obtained. The proposed method showed high capability for fast removal of Hg2+ ion from natural waters with high efficiency. Acknowledgment We gratefully acknowledge the financial support of this work by the Iranian National Science Foundation with grant number 92001925.
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