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Fe3+ and amino functioned mesoporous silica: Preparation, structural analysis and arsenic adsorption
Journal of Hazardous Materials 235–236 (2012) 336–342
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Fe3+ and amino functioned mesoporous silica: Preparation, structural analysis and arsenic adsorption Jun-Chao Zuo a , Sheng-Rui Tong a,∗ , Xiao-Lin Yu a , Ling-Yan Wu a , Chang-Yan Cao b , Mao-Fa Ge a,∗∗ , Wei-Guo Song b a State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Beijing National Laboratory for Molecular Science (BNLMS), Institute of Chemistry, Chinese Academy of Science, Beijing 100190, PR China b Laboratory for Molecular Nanostructurals and Nanotechnology, Institute of Chemistry, Chinese Academy of Science, Beijing 100190, PR China
h i g h l i g h t s
Materials were prepared by grafting amino and then coordinating Fe3+ on silica gel. The materials had mesoporous structure, large surface area, abundant amino and Fe3+ . The materials had high removal ability and adsorption speed for As(V) and As(III). Langmuir and Freundlich model were used to fit the adsorption isotherms. The effect of chloride and sulfate for As(V) and As(III) removal was investigated.
a r t i c l e
i n f o
Article history: Received 24 April 2012 Received in revised form 3 August 2012 Accepted 3 August 2012 Available online 10 August 2012 Keywords: Silica gel Amino Iron Arsenic Adsorption
a b s t r a c t Two novel adsorbents to remove excess arsenate and arsenite in the drinking water were prepared for the first time by grafting monoamine and diamine, respectively, and then coordinating Fe3+ on silica gel that was obtained using sol–gel method with two-step acid-base catalysis. It was found that both adsorbents had mesoporous structure, large specific surface, and high amino and iron content according to N2 adsorption isotherms, FTIR, XPS, and NMR analysis. The removal ability and adsorption rate of the adsorbents were very high for both As(V) and As(III). Langmuir and Freundlich models were used to fit the adsorption isotherm and investigate the adsorption mechanism. The effects of chloride and sulfate anion on the removal of arsenate and arsenite for the two adsorbents were also studied. © 2012 Elsevier B.V. All rights reserved.
1. Introduction High concentration of arsenic in drinking water can cause many diseases, such as lung, liver, kidney, bladder cancer and keratosis [1–3]. A maximum arsenic content of 10 ppb in the drinking water has been established by WHO and adopted by most countries [1,4,5]. However, due to the limits of economic and technical conditions, millions of people suffer from drinking water with arsenic content far above the standard of 10 ppb in Bengal basin of Bangladesh and India, Chianan plain of Taiwan, western USA, Huhhot basin of Inner Mongolia (China), Red River basin of
∗ Corresponding author. Tel.: +86 10 62558682; fax: +86 10 62559373. ∗∗ Corresponding author. Tel.: +86 10 62554518; fax: +86 10 62559373. E-mail addresses: [email protected] (S.-R. Tong), [email protected] (M.-F. Ge). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.08.009
Vietnam and so on [6,7]. Therefore, it is important to develop efficient technologies for the removal of arsenic in the water. Compared with other methods for arsenic removal, such as precipitation–coagulation, membrane separation and ion exchange, more and more attention was attracted on adsorption in recent years, for it is cheap and facile [8,9]. Active carbon, alumina, zeolite, magnesia and iron are the adsorbents most widely used to treat the water contaminated by arsenic [9–14]. Among these materials, iron was more popular for its high affinity for both arsenate(V) and arsenite(III), the two arsenic forms mainly existing in nature water [15,12,16–19]. The latter form is usually hard to be removed by other adsorbents. Recently, nano-iron particles are used as candidate of adsorbents, since they have larger surface area and higher ability for arsenic removal [20,21]. However, the nanoparticles are difficult to separate from water and easy to block the filtrate [22]. In order to solve these problems, alumina, iron and its oxides were loaded on materials with high surface area, such
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as activated carbon, zeolite, and mesoporous silica. These methods showed promising results in the adsorption of arsenic in water [1,15,22–25]. Feng et al. put forward a novel synthetic route by grafting functionalized monolayers on ordered mesoporous supports with silylation, then some researchers used this approach to bring useful groups, mainly amino, on mesoporous silica for the removal of arsenic in water [26–28]. Fe3+ in hydrated reactive form, mainly nanoparticles of ferrihydrite, is an excellent adsorbent, and has been widely used in water treatment plants for arsenic and other contaminants removal [29,30]. However, it usually takes long time for ferrihydrite to precipitate and easy to block the filter when filtration was used. So it is not suitable for water treating in remote area. On the contrary, to incorporate Fe3+ into supporting materials with high surface area is more preferable. Silica gel is a good selection for the incorporation of Fe3+ . As SiO2 aerogel attracted more and more attention in recent year, the preparation of silica gel with large block and high surface area has been developed [31]. However, Fe3+ can be easily dissolved from SiO2 without specific interaction. To joint SiO2 and Fe3+ together, the strategy of grafting amino on the SiO2 first, and then coordinating Fe3+ with amino was used here [28,29]. To the best of our knowledge, the load of functional groups on mesoporous silica gel prepared by the sol–gel technologies for arsenic removal has rarely been reported. In this work, mesoporous silica gel with high network skeleton strength and large block was prepared by using sol–gel method with two-step acid–base catalysis. This sample was expected to have larger surface area and higher hydroxyl density, as no calcination was used to decompose the template. Amino was introduced on silica gel, and then Fe3+ was coordinated with amino to remove the arsenic in the water. N2 adsorption isotherm, Fourier transformed infrared spectroscopy, X-ray photoelectron spectroscopy and 29 Si NMR spectra analytic techniques were used to characterize the materials. The effect of pH, and the kinetic and thermodynamic properties of adsorption were studied to investigate the removal ability of adsorbents for arsenic. The influence of coexistent anion was also investigated. 2. Methods 2.1. Chemicals Tetraethyl orthosilicate (TEOS), ethanol, oxalic acid, and ammonia were used as the silica source, solvent, acid catalyst, and base catalyst, respectively. All of them were bought from Sinopharm Chemical Reagent Co. Ltd. Aminopropyltriethoxysilane (H2 N(CH2 )3 Si(OC2 H5 )3 ) and N-[3-(trimethoxysilyl)propyl] ethylenediamine ((CH3 O)3 Si(CH2 )3 NHCH2 CH2 NH2 ) were purchased from Aladdin reagent as monoamine and diamine source, respectively. FeCl3 ·6H2 O was also purchase from Sinopharm Chemical Reagent Co. Ltd. Sodium hydrogen arsenate (Na2 HAsO4 ) was purchased from J&K. Sodium arsenite (NaAsO2 ) was purchased from Sigma–Aldrich. All chemicals were of reagent grade and used as received. Deionized water (Barnstead Easypure II D7411, Thermo Scientific) was used in all the experiments. 2.2. Synthesis of silica gel 22.4 ml TEOS and 35 ml ethanol were mixed together first, and then 1.8 ml water of 11.2 mg oxalic acid was dipped dropwise into the mixture with agitation. After stirring continuously for 48 h, 5.4 ml water and 5.8 ml ethanol of 0.075 ml 25–28 wt% ammonia were added with violent stirring. Then the mixture was poured into petri dish. The gel formed several hours later, and strengthened
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with aging. All the above steps were carried out at 25 ◦ C. Then the gel was cut into pieces with several millimeters. 10 ml gel and 20 ml ethanol were put into the hydrothermal reaction filter, reacting 2.5 h at 120 ◦ C for further condensation. 2.3. Functionalization of amino and Fe3+ 10 ml gel obtained in Section 2.2 was put into the hydrothermal reaction filter again with 20 ml ethanol, and then 6 ml aminopropyltriethoxysilane or 8 ml N-[3(trimethoxysilyl)propyl]ethylenediamine were added into the filter, reacting at 120 ◦ C for 2.5 h to graft monoamine or diamine on silica gel. After cooling down to room temperature, the gel silanizated with monoamino or diamino was washed with ethanol twice. After that, the gel was put into 20 ml ethanol of 6 g FeCl3 ·H2 O and stewed at 30 ◦ C overnight. Then the gel was washed twice with ethanol and dried at 75 ◦ C for 24 h to get the final product. The adsorbents silanizated with monoamine and diamine were named as Fe1N and Fe2N, respectively. In order to compare the adsorbents, the pure SiO2 gel and the gels only silanizated with monoamine and diamine were also dried at 75 ◦ C, and the dried samples were named as SiO2 , 1NSiO2 , and 2NSiO2 , respectively. 2.4. Instrument The Fourier transformed infrared spectroscopy (FTIR) was recorded using a Thermo Nicolet 6700 with detector in the absorption mode with a resolution 2 cm−1 in the range of 4000–650 cm−1 . The samples were crushed, dried, and mixed with spectroscopy grade KBr before tested. N2 adsorption isotherms were collected at 77 K on an Autosorb IQ-Chemi system (Quantachrome Instruments). Before operation, the samples were degassed at 155 ◦ C and −0.1 MPa for 400 min to remove the contamination that may be present on the surface. The specific surface area, pore volume, and median pore size were calculated with the built-in software of the Quantachrome instrument. BET surface area was calculated in the relative pressure range of p/p0 = 0.05–0.3. Pore size distribution was calculated using desorption isotherm curve. The X-ray photoelectron spectroscopy (XPS) was tested using monochromatic Al X-ray source (1486.6 eV) at 350 W, 14 kV, and 25 mA at a vacuum of 10−8 Torr (ESCALab220I-XL). Both regular and high-solution scans were conducted, and the data were plotted with respect to the binding energy. Proton-decoupled 29 Si MAS NMR spectra were recorded on a BRUKER AVANCE III 400 MHz spectrometer at 79.3 MHz and a sample spinning frequency of 10 kHz. 2.5. Sorption experiments The standard solutions of arsenate(V) and arsenite(III) were prepared by dissolving Na2 HAsO4 ·7H2 O and NaAsO2 in deionized water, respectively. The sorption experiments were carried out on a constant temperature shaker (HZ-9511K) at 200 rpm and 25 ◦ C in a conical flask. The solution pH was adjusted to certain level by adding 0.1 M NaOH or 0.1 M HCl solution and measured with pH meter (PHSJ-3F, Shanghai Lei-Ci). The solution was filtered with 0.45 m nylon and the concentration of arsenic solutions was analyzed using an inductively coupled plasma optical emission spectrometer (Shimadzu ICPE-9000). The adsorption capacity qe (mg/g) was calculated by the following equation: qe =
(C0 − Ce )V m
(1)
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Fig. 1. FTIR spectra of SiO2 , 1NSiO2 and Fe1N.
Fig. 2. Nitrogen adsorption isotherms of 1NSiO2 , Fe1N and Fe2N adsorbents.
is shown in Eq. (2): where C0 (mg/l) is the initial arsenic concentration, Ce (mg/l) is the arsenic equilibrium concentration, V (l) is the volume of the arsenic anion solution and m (g) is the mass of adsorbent. The effect of pH on arsenic adsorption by Fe1N and Fe2N samples was investigated at pH 3–9 for arsenate and pH 5–10 for arsenite, respectively. 50 mg adsorbents were mixed in 50 ml arsenic solution of 200 mg/l concentration. The pH was adjusted to certain value once an hour. After 6 h, the final concentration was measured. The sorption kinetics study was carried out at pH 4. 100 mg adsorbents were mixed in 100 ml 200 mg/l arsenic solution for 10 min, 30 min, 1 h, 2 h, 4 h, and 6 h. About 6 ml mixture was taken out and filtrated every time. The pH was adjusted once or twice in the interval. The concentration of filtrate was measured. The sorption isotherm study of arsenic was carried out by mixing 50 mg adsorbent in 50 ml arsenic solution with various As(V) and As(III) concentrations. The pH was adjusted to 7 ± 0.05 once an hour to ensure the final solution is neutral. After 6 h, the concentration of arsenic was measured. The effect of coexistent anion on arsenic was carried out by mixing 10 mg adsorbent and 0.165 g NaCl or 0.074 g Na2 SO4 in 50 ml arsenic solution. The pH of solution was adjusted once an hour to ensure the solution is 7 ± 0.05. After 6 h, the final arsenic concentration was measured. It should be noticed that although the hydrolysis of FeCl3 and the adding of HCl solution can also introduce Cl− , the concentration is very low compared to the coexistent Cl− used.
3. Results and discussion 3.1. Characterization of Fe3+ /silica gel 3.1.1. FTIR result To investigate the change of functional group during synthesis process, the infrared spectra of pure SiO2 , 1NSiO2 , and Fe1N were tested and the results are given in Fig. 1. The peaks at 2984, 2940, 2902, 1478, 1450, and 1396 cm−1 are assigned to characteristic banding of CH , while the peaks at 1086 and 800 cm−1 are assigned to the characteristic banding of Si O Si [27,33,34]. The peak at 1630 cm−1 is probably attributed to the bending of water residual on the surface. Compared with SiO2 , the peaks at 3742 and 950 cm−1 that are attributed to Si OH adsorption almost disappeared for 1NSiO2 , while a new peak at 1600 cm−1 and two small peaks at 3360 and 3286 cm−1 which are attributed to the characteristic banding of NH2 appeared [23,26,35,36]. It indicates that the amino was grafted on the silica gel successfully, as the reaction
≡ Si OH + Si(OC2 H5 )3 (CH2 )3 NH2 → ≡ Si O Si(OC2 H5 )2 (CH2 )3 NH2 + C2 H5 OH
(2)
A new peak at 1509 cm−1 which is attributed to NH3 + appeared in the infrared spectrum of Fe1N, indicating that primary amino is protonated by Fe3+ [28]. 3.1.2. N2 adsorption isotherm To investigate the solid surface and pore structure of the samples, the nitrogen adsorption isotherms of SiO2 , 1NSiO2 , Fe1N, and Fe2N adsorbents were tested and the results are shown in Fig. 2. All the isotherms have obvious hysteresis loop, indicating mesoporous structure of the samples. For the order of 1NSiO2 , Fe1N, and Fe2N, the initial slope of the isotherm decreases, the height of hysteresis loop reduces and the loop moves to lower pressure, which indicate that the specific surface area decreases, the specific pore volume reduces, and the average pore diameter diminishes. It is confirmed by the data listed in Table 1. The surface area of 1NSiO2 is 563.4 m2 /g, decreases to 379.8 m2 /g when coordinated with Fe3+ , and a further decrease to 272.0 m2 /g when grafted with diamine instead of monoamine. The average pore diameter and the specific volume of the samples show the similar trends. It can be attributed to the exclusion volume of the larger groups on the surface, as is widely observed for the situation of grafting [27,28,32,33]. The pore diameter distribution of the four samples is shown in Fig. 3. The average diameter diminishes from 1NSiO2 to Fe1N and then Fe2N. However, the SiO2 sample is an exception. Although SiO2 have the largest surface area and pore volume, its pore size is smaller that of 1NSiO2 and Fe1N. This can be explained that the surface of SiO2 is covered by Si OH, so large surface tension generates during drying, causing the shrinkage of pores. The problem is not obvious for 1NSiO2 , Fe2N and Fe2N because the surface of these samples is mainly alkyl with low surface tension [33]. It should be noticed that although shrinkage occurred for the matrix, SiO2 still have a high Table 1 BET surface area, specific pore volume and average diameter of 1NSiO2 , Fe1N and Fe2N adsorbents. Materials
SBET (m2 /g)
DP (nm)
VP (cm3 /g)
SiO2 1NSiO2 Fe1N Fe2N
985.6 563.4 379.8 272.0
5.426 9.04 5.825 3.943
1.525 1.352 0.6 0.379
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Fig. 3. The pore diameter distribution of 1NSiO2 , Fe1N and Fe2N adsorbents.
Fig. 5.
29
Si NMR spectra of 1NSiO2 adsorbent.
lower than that of Fe1N (7.1 wt%). The reason is that Fe3+ has a better selectivity for primary amine than for secondary amine.
Fig. 4. N1s XPS spectra of Fe1N and Fe2N adsorbents.
specific pore volume of 1.525 cm3 /g, which is much higher than that of MCM-41 (0.284 cm3 /g) and SBA-1 (0.281 cm3 /g) [27], also higher than M41 (1.07 cm3 /g) and S15 (0.99 cm3 /g) [28]. The high specific pore volume is beneficial to the function of amino and Fe3+ . 3.1.3. XPS To investigate the chemical composition on the surface of the samples, the XPS spectra of Fe1N and Fe2N were tested. The N1s spectra are plotted in Fig. 4, and the content of Fe and N element is listed in Table 2. For Fig. 4, the peak at 400 eV is attributed to the amino group, and the peak at 401.9 eV is attributed to amino perturbed by a positive cation ( NH2 · · ·Fe3+ ), which was also observed by Chen et al. [33]. The peak at 401.9 eV is much higher than the peak at 400 eV for Fe1N, while the two peaks are almost the same for Fe2N, indicating that most amino in Fe1N is coordinated with Fe3+ , while only half amino in Fe2N is coordinated with Fe3+ . As shown in Table 2, the N content of Fe2N (3.2 wt%) was higher than that of Fe1N (2.7 wt%), while the Fe content of Fe2N (6.7 wt%) is
Table 2 The surface Fe and N element analysis of Fe1N and Fe2N. Materials
N (wt%)
Fe (wt%)
Fe1N Fe2N
2.7% 3.2%
7.1% 6.7%
3.1.4. NMR results To investigate the chemical state of Si in the sample, the 29 Si NMR spectrum of 1NSiO2 adsorbent was tested and the result is shown in Fig. 5. The peaks at −58.4, −64.8, −102.3 and −110.2 ppm are assigned to Si(OR )R(OSi)2 (T2 ), SiR(OSi)3 (T3 ), Si(OR )(OSi)3 (Q3 ), and Si(OSi)4 (Q4 ), respectively [27,28]. However, Si(OR )2R(OSi) (T1 ), which is expected to appear around −50 ppm, is not observed. It indicates that silane molecule mainly reacts with two or three Si OH on the surface. The proportion of integrated area for T2 and T3 is 17.4% and 6.8%, respectively. The ratio of silicon bonded with aminopropyl to the total silicon is 24.2%, which is higher than NNMCM-41 (14.4%), NN-SBA-1 (19.0%), ClPrM41 (10.8%) and ClPrS15 (4.2%) [27,28]. 3.2. Arsenic adsorption 3.2.1. The effect of pH To evaluate the influence of pH on the removal abilities of materials, the adsorption capabilities of Fe1N and Fe2N adsorbents for arsenate and arsenite were studied at different pH values and the results are shown in Figs. 6 and 7. Both Fe1N and Fe2N materials have very high removal abilities, 131.8 mg/g and 124.6 mg/g for As(V) at pH 4, 85 mg/g and 87 mg/g for As(III) at pH 7 and 8, respectively. It is attributed to the abundant Si OH on the silica gel, as the Si OH was generated in situ with the hydrolysis of TEOS, and no calcination was adopted to remove the template that was used in most ordered mesoporous silica materials to generate pore [27,28]. With more Si OH, the silica gel can be grafted with more amino and then coordinated with more Fe3+ . The large specific pore volume is also beneficial to the function of amino and Fe3+ . So the adsorbents show better arsenic removal ability. It should be noticed that the initial concentration of arsenic used here is 200 ppm, and 65.9% of the arsenate was adsorbed on Fe1N at the high adsorption ability of 131.8 mg/g. If the initial concentration increased, the removal ability of arsenate would be higher for Fe1N. It can also be found that Fe1N material has better removal ability for As(V) than Fe2N, which is attributed to the higher iron content of Fe1N, as was mentioned in Section 3.1.3.
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Fig. 6. The effect of pH on Fe1N and Fe2N adsorbents for arsenate.
It was shown in Fig. 4 that both the two materials have the highest arsenate removal ability at pH 4. It can be explained by the following reasons. First, arsenate mainly existed as H2 AsO4 − at pH 4. The proportion of H3 AsO4 would increase as the pH below this value, while the proportion of HAsO4 2− would increase as the pH above this value [37]. It is unfavorable for arsenic removal in both situations. Second, the iron can dissolve in the water at low pH which leads to the removal ability depressed. This phenomenon has been mentioned by An et al. [38]. Third, the negative charge on the surface of materials increases as the pH increases when pH > 4, so the repulsive force between the arsenate and the absorbents strengthens, which has been reported by Jeon et al. [1]. As shown in Fig. 7, the highest removal ability for arsenite appears at pH = 7 for Fe1N and pH = 8 for Fe2N, respectively. It can be explained that electrostatic attraction plays an important role on arsenite removal. When the pH is below 7, arsenite mainly exists as H3 AsO3 , and few As(III) exists as anion. When the pH is above 8, the material surface becomes negative and has a repulsive force with arsenite [37]. Fe2N material has better removal ability for As(III) than Fe1N here, which may be attributed to the reason that arsenite can be adsorbed not only by iron, but also by amino, as the protonated amino is also an adsorbent for arsenite [39]. 3.2.2. The kinetics study To evaluate the kinetics of adsorbents, the adsorption of Fe1N and Fe2N for arsenate was tested at different reaction time and the results are shown in Fig. 8. The adsorption rates of both adsorbents
Fig. 8. The adsorption change of Fe1N and Fe2N adsorbents for arsenate with time. Table 3 The results obtained from pseudo-second-order model for arsenate adsorption. Materials
k2 (g/(mg h))
qe (mg/g)
r2
Fe1N Fe2N
0.0634 0.194
131.9 126.1
0.9997 0.9998
for As(V) are very fast, and reaches a high amount that close to saturation value in just 10 min. It can be attributed to three aspects. First, the hydrolysis of Fe3+ in adsorbents is very fast. Second, the fresh hydrolysis product has a fast adsorption for arsenate. Third, the diffusion in silica pores is very fast and has little inhibition effect for arsenic removal [35]. The fast adsorption rate of the two adsorbents was also observed in arsenite adsorption experiments and at other pH values. Due to the hydrolysis of Fe3+ , pH was not constant at other values in the initial stages, so kinetics studies that were carried at pH = 4 were investigated. To investigate the kinetics study, we used pseudo-first-order and pseudo-second-order model to fit the date. We found that the pseudo-first-order was not suitable to fit the date, while the pseudo-second-order fit the date well. The mathematic representation of the pseudo-second-order model is given in 1 t t = + qt qe k2 q2e
(3)
where qe and qt are the adsorption capacities (mg/g) of the adsorbent at equilibrium and at any time t (h), respectively; and k2 (g/(mg h)) is the related adsorption rate constant. The results are shown in Table 3. Both the adsorption of arsenate with Fe1N and Fe2N fit pseudo-second-order model well with r2 > 0.999. The result was in accordance with the previous results published [40–42]. 3.2.3. The isotherm study Using appropriate correlation models for the experiment equilibrium data is of great importance to understand the mechanism of adsorption for arsenic. Langmuir and Freundlich isotherm models are the two most important models used [41,42]. The two models were used to correlate the adsorption behavior in present study. The arsenic adsorption equilibrium data was obtained at pH 7. The Langmuir isotherm model and Freundlich isotherm model are given by the following equation: q=
qm × b × Ce 1 + b × Ce 1/n
Fig. 7. The effect of pH on Fe1N and Fe2N adsorbents for arsenite.
q = K × Ce
(4) (5)
J.-C. Zuo et al. / Journal of Hazardous Materials 235–236 (2012) 336–342
Fig. 9. The adsorption isotherms of arsenate for both the Fe1N and Fe2N materials with Langmuir (solid line) and Freundlich (dashed line) model.
Table 4 The parameters of arsenate for both the Fe1N and Fe2N materials with Langmuir and Freundlich model. Materials
Fe1N Fe2N
Langmuir
Freundlich 2
qm (mg/g)
Ka (l/mg)
R
KF (mg/g)
n
R2
101.74 78.42
0.093 1.028
0.971 0.912
20.91 33.74
2.93 4.69
0.938 0.951
where q (mg/g) is the amount of arsenic adsorbed at equilibrium, Ce (mg/l) is the arsenic equilibrium concentration, qm (mg/g) and b (l/mg) are the Langmuir constants related to the capacity and energy of the adsorption, respectively, K and n are the Freundlich constants related to the adsorbents capacity and adsorption intensity, respectively [1,25,40]. The adsorption isotherms of arsenate for both the Fe1N and Fe2N materials with Langmuir and Freundlich models are shown in Fig. 9. The parameters for the Langmuir and Freundlich models are presented in Table 4. For the arsenate isotherm adsorption, Langmuir model is more suitable for Fe1N material, while Freundlich model fits Fe2N material better. It can be explained by that the surface of the Fe1N material is mainly covered with by Fe3+ . Therefore, there is only one kind of acting force for arsenate adsorption. As arsenate form inner-sphere complexes with Fe3+ hydrated form, the arsenate covers the surface in monolayer style [1,25,43–45]. For Fe2N material, only a part of amino on the surface is coordinated with Fe3+ . So the surface of the adsorbent is not homogeneous and both the iron and the amino participate in the adsorption of arsenate. As the amino and iron have different acting force to arsenate adsorption, Langmuir model does not fit well for Fe2N. The adsorption isotherms of both the Fe1N and Fe2N for arsenite with Langmuir and Freundlich models are shown in Fig. 10. The parameters of Langmuir and Freundlich models are presented in Table 5. For arsenite adsorption, both Fe1N and Fe2N materials fit Freundlich model better. It can be explained that iron oxide adsorbs arsenite in both inner-sphere and outer-sphere complexes, and forms multilayer adsorption at high concentration [25,43–46].
341
Fig. 10. The adsorption isotherms of arsenite for both the Fe1N and Fe2N materials with Langmuir (solid line) and Freundlich (dashed line) model.
3.2.4. The effect of coexistent anion The main anions in natural water are mainly chloride and sulfate ions, which may have an effect on the adsorption of arsenic. The removal abilities of Fe1N and Fe2N for As(III) and As(V) were investigated with the existent of these two kinds of anion. The concentrations of coexistent anion investigated here are 2 g/l for Cl− and 1 g/l for SO4 2− , which are higher than the concentration in natural water. The initial arsenic concentration is 2 mg/l, and the adsorbents employed here is 0.2 g/l. The results are shown in Fig. 11. The adding of chloride and sulfate lowered the removal abilities of the two adsorbents. Chloride has a more serious effect for the adsorption of arsenic than sulfate in all the four cases, which can be attributed to the stronger attraction between the adsorbent surface and chloride, as was mentioned by Yoshitake et al. [32,36], and the sulfate has little effect on the adsorption, which has been manifested by Partey et al. [43]. Both the chloride and sulfate have greater effect on arsenic adsorption for Fe1N than for Fe2N in general, which can be explained that the combined force of iron and amino to arsenic for Fe2N is stronger at low concentration. Arsenate is more easily to be removed than arsenite in almost all the cases, due to the strong chemical interaction between arsenate and adsorbents and the weak interaction between arsenite and adsorbents, as inner-sphere complexes form between iron oxide and arsenate, while both inner-sphere and outer-sphere complexes form between iron oxide and arsenite [28,43–46].
Table 5 The parameters of arsenite for both the Fe1N and Fe2N materials with Langmuir and Freundlich model. Materials
Fe1N Fe2N
Langmuir
Freundlich
qm (mg/g)
Ka (l/mg)
R2
KF (mg/g)
n
R2
142.6 132.7
0.0088 0.0133
0.969 0.948
4.44 6.71
1.72 1.93
0.993 0.982
Fig. 11. The effect of chloride and sulfate on Fe1N and Fe2N adsorbents for arsenate and arsenite.
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4. Conclusions In this work, two adsorbents for arsenic removal were prepared by grafting monoamine and diamine on the silica gels obtained using sol–gel method with two-step acid–base catalysis, and then coordinating amino with Fe3+ . The adsorbents have mesoporous structure, large surface area and pore volume, abundant amino and Fe3+ , thus have high adsorption capacity for both arsenate and arsenite. The Fe1N adsorbent has the highest adsorption capacity of 132 mg/g at pH 4 and a high adsorption capacity of 95 mg/g at neutral condition for As(V), while Fe2N adsorbent has the highest adsorption capacity of 87 mg/g at pH 8 and a high adsorption capacity of 85 mg/g at neutral condition for As(III). The adsorbents show fast rate for arsenic removal, close to saturation in 10 min. The isotherm study showed that the adsorption of Fe1N for As(V) fitted Langmuir model better, while the adsorption of Fe2N for As(V) and the adsorption of both materials for As(III) fitted Freundlich model better. The existence of anion inhibits the adsorption of Arsenic. The chloride has greater inhibition effect than sulfate, and Fe1N is more easily to be affected by the coexistence of anion. Overall, the two materials are excellent adsorbents for arsenic removal and useful to prepare other materials for environmental applications. Acknowledgment This work was supported by the National Basic Research Program of China (973 Program, No. 2011CB933700) of Ministry of Science and Technology of China. References [1] C.S. Jeon, K. Baek, J.K. Park, Y.K. Oh, S.D. Lee, Adsorption characteristics of As(V) on iron-coated zeolite, J. Hazard. Mater. 163 (2009) 804–808. [2] I. Katsoyiannis, A. Zouboulis, H. Althoff, H. Bartel, As(III) removal from groundwaters using fixed-bed upflow bioreactors, Chemosphere 47 (2002) 325–332. [3] H. Guo, D. stuben, Z. Berner, Arsenic removal from water using natural iron mineral-quartz sand columns, Sci. Total Environ. 377 (2007) 142–151. [4] I.A. Katsoyiannis, A.I. Zouboulis, Removal of arsenic from contaminated water sources by sorption onto iron-oxide-coated polymeric materials, Water Res. 36 (2002) 5141–5155. [5] C.L. Chuang, M. Fan, M. Xu, R.C. Brown, S. Sung, B. Saha, C.P. Huang, Adsorption of arsenic(V) by activated carbon prepared from oat hulls, Chemosphere 61 (2005) 478–483. [6] A. Mukherjee, P. Bhattacharya, F. Shi, A.E. Fryar, A.B. Mukherjee, Z.M. Xie, G. Jacks, J. Bundschuh, Chemical evolution in the high arsenic groundwater of the Huhhot basin (Inner Mongolia, PR China) and its difference from the western Bengal basin (India), Appl. Geochem. 24 (2009) 1835–1851. [7] S. Fendorf, H.A. Michael, A.V. Green, Spatial and temporal variations of groundwater arsenic in south and southeast Asia, Science 328 (2010) 1123–1127. [8] W. Li, C.Y. Cao, L.Y. Wu, M.F. Ge, W.G. Song, Superb fluoride and arsenic removal performance of highly ordered mesoporous aluminas, J. Hazard. Mater. 198 (2011) 143–150. [9] J. Pattanayak, K. Mondal, S. Mathew, S.B. Lalvani, A parametric evaluation of the removal of As(V) and As(III) by carbon based adsorbents, Carbon 38 (2000) 589–596. [10] M. Szlachta, V. Gerda, N. Chubar, Adsorption of arsenite and selenite using an inorganic ion exchanger based on Fe–Mn hydrous oxide, J. Colloid Interface Sci. 365 (2012) 213–221. [11] J. Wang, S.J. Zhang, B.C. Pan, W.M. Zhang, L. Lv, Hydrous ferric oxide–resin nanocomposites of tunable structure for arsenite removal: effect of the host pore structure, J. Hazard. Mater. 198 (2011) 241–246. [12] T.F. Lin, J.K. Wu, Adsorption of arsenite and arsenate within activated alumina grains: equilibrium and kinetics, Water Res. 35 (2001) 2049–2057. [13] P. Chutia, S. Kato, T. Kojima, S. Satokawa, Arsenic adsorption from aqueous solution on synthetic zeolites, J. Hazard. Mater. 162 (2009) 440–447. [14] X.Y. Wu, T. Luo, Y. Jia, Y.X. Zhang, J.H. Liu, X.J. Huang, Porous hierarchically micro-/nanostructured MaO: morphology control and their excellent performance in As(III) and As(V) removal, J. Phys. Chem. C 115 (2011) 22242–22250. [15] L. Zeng, A method for preparing silica-containing iron(III) oxide adsorbents for arsenic removal, Water Res. 37 (2003) 4351–4358. [16] Y.H. Xu, T. Nakajima, A. Ohki, Adsorption and removal of arsenic(V) from drinking water by aluminum-loaded shirasu-zeolite, J. Hazard. Mater. B 92 (2002) 275–287.
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Fe3+ and amino functioned mesoporous silica: Preparation, structural analysis and arsenic adsorption Jun-Chao Zuo a , Sheng-Rui Tong a,∗ , Xiao-Lin Yu a , Ling-Yan Wu a , Chang-Yan Cao b , Mao-Fa Ge a,∗∗ , Wei-Guo Song b a State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Beijing National Laboratory for Molecular Science (BNLMS), Institute of Chemistry, Chinese Academy of Science, Beijing 100190, PR China b Laboratory for Molecular Nanostructurals and Nanotechnology, Institute of Chemistry, Chinese Academy of Science, Beijing 100190, PR China
h i g h l i g h t s
Materials were prepared by grafting amino and then coordinating Fe3+ on silica gel. The materials had mesoporous structure, large surface area, abundant amino and Fe3+ . The materials had high removal ability and adsorption speed for As(V) and As(III). Langmuir and Freundlich model were used to fit the adsorption isotherms. The effect of chloride and sulfate for As(V) and As(III) removal was investigated.
a r t i c l e
i n f o
Article history: Received 24 April 2012 Received in revised form 3 August 2012 Accepted 3 August 2012 Available online 10 August 2012 Keywords: Silica gel Amino Iron Arsenic Adsorption
a b s t r a c t Two novel adsorbents to remove excess arsenate and arsenite in the drinking water were prepared for the first time by grafting monoamine and diamine, respectively, and then coordinating Fe3+ on silica gel that was obtained using sol–gel method with two-step acid-base catalysis. It was found that both adsorbents had mesoporous structure, large specific surface, and high amino and iron content according to N2 adsorption isotherms, FTIR, XPS, and NMR analysis. The removal ability and adsorption rate of the adsorbents were very high for both As(V) and As(III). Langmuir and Freundlich models were used to fit the adsorption isotherm and investigate the adsorption mechanism. The effects of chloride and sulfate anion on the removal of arsenate and arsenite for the two adsorbents were also studied. © 2012 Elsevier B.V. All rights reserved.
1. Introduction High concentration of arsenic in drinking water can cause many diseases, such as lung, liver, kidney, bladder cancer and keratosis [1–3]. A maximum arsenic content of 10 ppb in the drinking water has been established by WHO and adopted by most countries [1,4,5]. However, due to the limits of economic and technical conditions, millions of people suffer from drinking water with arsenic content far above the standard of 10 ppb in Bengal basin of Bangladesh and India, Chianan plain of Taiwan, western USA, Huhhot basin of Inner Mongolia (China), Red River basin of
∗ Corresponding author. Tel.: +86 10 62558682; fax: +86 10 62559373. ∗∗ Corresponding author. Tel.: +86 10 62554518; fax: +86 10 62559373. E-mail addresses: [email protected] (S.-R. Tong), [email protected] (M.-F. Ge). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.08.009
Vietnam and so on [6,7]. Therefore, it is important to develop efficient technologies for the removal of arsenic in the water. Compared with other methods for arsenic removal, such as precipitation–coagulation, membrane separation and ion exchange, more and more attention was attracted on adsorption in recent years, for it is cheap and facile [8,9]. Active carbon, alumina, zeolite, magnesia and iron are the adsorbents most widely used to treat the water contaminated by arsenic [9–14]. Among these materials, iron was more popular for its high affinity for both arsenate(V) and arsenite(III), the two arsenic forms mainly existing in nature water [15,12,16–19]. The latter form is usually hard to be removed by other adsorbents. Recently, nano-iron particles are used as candidate of adsorbents, since they have larger surface area and higher ability for arsenic removal [20,21]. However, the nanoparticles are difficult to separate from water and easy to block the filtrate [22]. In order to solve these problems, alumina, iron and its oxides were loaded on materials with high surface area, such
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as activated carbon, zeolite, and mesoporous silica. These methods showed promising results in the adsorption of arsenic in water [1,15,22–25]. Feng et al. put forward a novel synthetic route by grafting functionalized monolayers on ordered mesoporous supports with silylation, then some researchers used this approach to bring useful groups, mainly amino, on mesoporous silica for the removal of arsenic in water [26–28]. Fe3+ in hydrated reactive form, mainly nanoparticles of ferrihydrite, is an excellent adsorbent, and has been widely used in water treatment plants for arsenic and other contaminants removal [29,30]. However, it usually takes long time for ferrihydrite to precipitate and easy to block the filter when filtration was used. So it is not suitable for water treating in remote area. On the contrary, to incorporate Fe3+ into supporting materials with high surface area is more preferable. Silica gel is a good selection for the incorporation of Fe3+ . As SiO2 aerogel attracted more and more attention in recent year, the preparation of silica gel with large block and high surface area has been developed [31]. However, Fe3+ can be easily dissolved from SiO2 without specific interaction. To joint SiO2 and Fe3+ together, the strategy of grafting amino on the SiO2 first, and then coordinating Fe3+ with amino was used here [28,29]. To the best of our knowledge, the load of functional groups on mesoporous silica gel prepared by the sol–gel technologies for arsenic removal has rarely been reported. In this work, mesoporous silica gel with high network skeleton strength and large block was prepared by using sol–gel method with two-step acid–base catalysis. This sample was expected to have larger surface area and higher hydroxyl density, as no calcination was used to decompose the template. Amino was introduced on silica gel, and then Fe3+ was coordinated with amino to remove the arsenic in the water. N2 adsorption isotherm, Fourier transformed infrared spectroscopy, X-ray photoelectron spectroscopy and 29 Si NMR spectra analytic techniques were used to characterize the materials. The effect of pH, and the kinetic and thermodynamic properties of adsorption were studied to investigate the removal ability of adsorbents for arsenic. The influence of coexistent anion was also investigated. 2. Methods 2.1. Chemicals Tetraethyl orthosilicate (TEOS), ethanol, oxalic acid, and ammonia were used as the silica source, solvent, acid catalyst, and base catalyst, respectively. All of them were bought from Sinopharm Chemical Reagent Co. Ltd. Aminopropyltriethoxysilane (H2 N(CH2 )3 Si(OC2 H5 )3 ) and N-[3-(trimethoxysilyl)propyl] ethylenediamine ((CH3 O)3 Si(CH2 )3 NHCH2 CH2 NH2 ) were purchased from Aladdin reagent as monoamine and diamine source, respectively. FeCl3 ·6H2 O was also purchase from Sinopharm Chemical Reagent Co. Ltd. Sodium hydrogen arsenate (Na2 HAsO4 ) was purchased from J&K. Sodium arsenite (NaAsO2 ) was purchased from Sigma–Aldrich. All chemicals were of reagent grade and used as received. Deionized water (Barnstead Easypure II D7411, Thermo Scientific) was used in all the experiments. 2.2. Synthesis of silica gel 22.4 ml TEOS and 35 ml ethanol were mixed together first, and then 1.8 ml water of 11.2 mg oxalic acid was dipped dropwise into the mixture with agitation. After stirring continuously for 48 h, 5.4 ml water and 5.8 ml ethanol of 0.075 ml 25–28 wt% ammonia were added with violent stirring. Then the mixture was poured into petri dish. The gel formed several hours later, and strengthened
337
with aging. All the above steps were carried out at 25 ◦ C. Then the gel was cut into pieces with several millimeters. 10 ml gel and 20 ml ethanol were put into the hydrothermal reaction filter, reacting 2.5 h at 120 ◦ C for further condensation. 2.3. Functionalization of amino and Fe3+ 10 ml gel obtained in Section 2.2 was put into the hydrothermal reaction filter again with 20 ml ethanol, and then 6 ml aminopropyltriethoxysilane or 8 ml N-[3(trimethoxysilyl)propyl]ethylenediamine were added into the filter, reacting at 120 ◦ C for 2.5 h to graft monoamine or diamine on silica gel. After cooling down to room temperature, the gel silanizated with monoamino or diamino was washed with ethanol twice. After that, the gel was put into 20 ml ethanol of 6 g FeCl3 ·H2 O and stewed at 30 ◦ C overnight. Then the gel was washed twice with ethanol and dried at 75 ◦ C for 24 h to get the final product. The adsorbents silanizated with monoamine and diamine were named as Fe1N and Fe2N, respectively. In order to compare the adsorbents, the pure SiO2 gel and the gels only silanizated with monoamine and diamine were also dried at 75 ◦ C, and the dried samples were named as SiO2 , 1NSiO2 , and 2NSiO2 , respectively. 2.4. Instrument The Fourier transformed infrared spectroscopy (FTIR) was recorded using a Thermo Nicolet 6700 with detector in the absorption mode with a resolution 2 cm−1 in the range of 4000–650 cm−1 . The samples were crushed, dried, and mixed with spectroscopy grade KBr before tested. N2 adsorption isotherms were collected at 77 K on an Autosorb IQ-Chemi system (Quantachrome Instruments). Before operation, the samples were degassed at 155 ◦ C and −0.1 MPa for 400 min to remove the contamination that may be present on the surface. The specific surface area, pore volume, and median pore size were calculated with the built-in software of the Quantachrome instrument. BET surface area was calculated in the relative pressure range of p/p0 = 0.05–0.3. Pore size distribution was calculated using desorption isotherm curve. The X-ray photoelectron spectroscopy (XPS) was tested using monochromatic Al X-ray source (1486.6 eV) at 350 W, 14 kV, and 25 mA at a vacuum of 10−8 Torr (ESCALab220I-XL). Both regular and high-solution scans were conducted, and the data were plotted with respect to the binding energy. Proton-decoupled 29 Si MAS NMR spectra were recorded on a BRUKER AVANCE III 400 MHz spectrometer at 79.3 MHz and a sample spinning frequency of 10 kHz. 2.5. Sorption experiments The standard solutions of arsenate(V) and arsenite(III) were prepared by dissolving Na2 HAsO4 ·7H2 O and NaAsO2 in deionized water, respectively. The sorption experiments were carried out on a constant temperature shaker (HZ-9511K) at 200 rpm and 25 ◦ C in a conical flask. The solution pH was adjusted to certain level by adding 0.1 M NaOH or 0.1 M HCl solution and measured with pH meter (PHSJ-3F, Shanghai Lei-Ci). The solution was filtered with 0.45 m nylon and the concentration of arsenic solutions was analyzed using an inductively coupled plasma optical emission spectrometer (Shimadzu ICPE-9000). The adsorption capacity qe (mg/g) was calculated by the following equation: qe =
(C0 − Ce )V m
(1)
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Fig. 1. FTIR spectra of SiO2 , 1NSiO2 and Fe1N.
Fig. 2. Nitrogen adsorption isotherms of 1NSiO2 , Fe1N and Fe2N adsorbents.
is shown in Eq. (2): where C0 (mg/l) is the initial arsenic concentration, Ce (mg/l) is the arsenic equilibrium concentration, V (l) is the volume of the arsenic anion solution and m (g) is the mass of adsorbent. The effect of pH on arsenic adsorption by Fe1N and Fe2N samples was investigated at pH 3–9 for arsenate and pH 5–10 for arsenite, respectively. 50 mg adsorbents were mixed in 50 ml arsenic solution of 200 mg/l concentration. The pH was adjusted to certain value once an hour. After 6 h, the final concentration was measured. The sorption kinetics study was carried out at pH 4. 100 mg adsorbents were mixed in 100 ml 200 mg/l arsenic solution for 10 min, 30 min, 1 h, 2 h, 4 h, and 6 h. About 6 ml mixture was taken out and filtrated every time. The pH was adjusted once or twice in the interval. The concentration of filtrate was measured. The sorption isotherm study of arsenic was carried out by mixing 50 mg adsorbent in 50 ml arsenic solution with various As(V) and As(III) concentrations. The pH was adjusted to 7 ± 0.05 once an hour to ensure the final solution is neutral. After 6 h, the concentration of arsenic was measured. The effect of coexistent anion on arsenic was carried out by mixing 10 mg adsorbent and 0.165 g NaCl or 0.074 g Na2 SO4 in 50 ml arsenic solution. The pH of solution was adjusted once an hour to ensure the solution is 7 ± 0.05. After 6 h, the final arsenic concentration was measured. It should be noticed that although the hydrolysis of FeCl3 and the adding of HCl solution can also introduce Cl− , the concentration is very low compared to the coexistent Cl− used.
3. Results and discussion 3.1. Characterization of Fe3+ /silica gel 3.1.1. FTIR result To investigate the change of functional group during synthesis process, the infrared spectra of pure SiO2 , 1NSiO2 , and Fe1N were tested and the results are given in Fig. 1. The peaks at 2984, 2940, 2902, 1478, 1450, and 1396 cm−1 are assigned to characteristic banding of CH , while the peaks at 1086 and 800 cm−1 are assigned to the characteristic banding of Si O Si [27,33,34]. The peak at 1630 cm−1 is probably attributed to the bending of water residual on the surface. Compared with SiO2 , the peaks at 3742 and 950 cm−1 that are attributed to Si OH adsorption almost disappeared for 1NSiO2 , while a new peak at 1600 cm−1 and two small peaks at 3360 and 3286 cm−1 which are attributed to the characteristic banding of NH2 appeared [23,26,35,36]. It indicates that the amino was grafted on the silica gel successfully, as the reaction
≡ Si OH + Si(OC2 H5 )3 (CH2 )3 NH2 → ≡ Si O Si(OC2 H5 )2 (CH2 )3 NH2 + C2 H5 OH
(2)
A new peak at 1509 cm−1 which is attributed to NH3 + appeared in the infrared spectrum of Fe1N, indicating that primary amino is protonated by Fe3+ [28]. 3.1.2. N2 adsorption isotherm To investigate the solid surface and pore structure of the samples, the nitrogen adsorption isotherms of SiO2 , 1NSiO2 , Fe1N, and Fe2N adsorbents were tested and the results are shown in Fig. 2. All the isotherms have obvious hysteresis loop, indicating mesoporous structure of the samples. For the order of 1NSiO2 , Fe1N, and Fe2N, the initial slope of the isotherm decreases, the height of hysteresis loop reduces and the loop moves to lower pressure, which indicate that the specific surface area decreases, the specific pore volume reduces, and the average pore diameter diminishes. It is confirmed by the data listed in Table 1. The surface area of 1NSiO2 is 563.4 m2 /g, decreases to 379.8 m2 /g when coordinated with Fe3+ , and a further decrease to 272.0 m2 /g when grafted with diamine instead of monoamine. The average pore diameter and the specific volume of the samples show the similar trends. It can be attributed to the exclusion volume of the larger groups on the surface, as is widely observed for the situation of grafting [27,28,32,33]. The pore diameter distribution of the four samples is shown in Fig. 3. The average diameter diminishes from 1NSiO2 to Fe1N and then Fe2N. However, the SiO2 sample is an exception. Although SiO2 have the largest surface area and pore volume, its pore size is smaller that of 1NSiO2 and Fe1N. This can be explained that the surface of SiO2 is covered by Si OH, so large surface tension generates during drying, causing the shrinkage of pores. The problem is not obvious for 1NSiO2 , Fe2N and Fe2N because the surface of these samples is mainly alkyl with low surface tension [33]. It should be noticed that although shrinkage occurred for the matrix, SiO2 still have a high Table 1 BET surface area, specific pore volume and average diameter of 1NSiO2 , Fe1N and Fe2N adsorbents. Materials
SBET (m2 /g)
DP (nm)
VP (cm3 /g)
SiO2 1NSiO2 Fe1N Fe2N
985.6 563.4 379.8 272.0
5.426 9.04 5.825 3.943
1.525 1.352 0.6 0.379
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Fig. 3. The pore diameter distribution of 1NSiO2 , Fe1N and Fe2N adsorbents.
Fig. 5.
29
Si NMR spectra of 1NSiO2 adsorbent.
lower than that of Fe1N (7.1 wt%). The reason is that Fe3+ has a better selectivity for primary amine than for secondary amine.
Fig. 4. N1s XPS spectra of Fe1N and Fe2N adsorbents.
specific pore volume of 1.525 cm3 /g, which is much higher than that of MCM-41 (0.284 cm3 /g) and SBA-1 (0.281 cm3 /g) [27], also higher than M41 (1.07 cm3 /g) and S15 (0.99 cm3 /g) [28]. The high specific pore volume is beneficial to the function of amino and Fe3+ . 3.1.3. XPS To investigate the chemical composition on the surface of the samples, the XPS spectra of Fe1N and Fe2N were tested. The N1s spectra are plotted in Fig. 4, and the content of Fe and N element is listed in Table 2. For Fig. 4, the peak at 400 eV is attributed to the amino group, and the peak at 401.9 eV is attributed to amino perturbed by a positive cation ( NH2 · · ·Fe3+ ), which was also observed by Chen et al. [33]. The peak at 401.9 eV is much higher than the peak at 400 eV for Fe1N, while the two peaks are almost the same for Fe2N, indicating that most amino in Fe1N is coordinated with Fe3+ , while only half amino in Fe2N is coordinated with Fe3+ . As shown in Table 2, the N content of Fe2N (3.2 wt%) was higher than that of Fe1N (2.7 wt%), while the Fe content of Fe2N (6.7 wt%) is
Table 2 The surface Fe and N element analysis of Fe1N and Fe2N. Materials
N (wt%)
Fe (wt%)
Fe1N Fe2N
2.7% 3.2%
7.1% 6.7%
3.1.4. NMR results To investigate the chemical state of Si in the sample, the 29 Si NMR spectrum of 1NSiO2 adsorbent was tested and the result is shown in Fig. 5. The peaks at −58.4, −64.8, −102.3 and −110.2 ppm are assigned to Si(OR )R(OSi)2 (T2 ), SiR(OSi)3 (T3 ), Si(OR )(OSi)3 (Q3 ), and Si(OSi)4 (Q4 ), respectively [27,28]. However, Si(OR )2R(OSi) (T1 ), which is expected to appear around −50 ppm, is not observed. It indicates that silane molecule mainly reacts with two or three Si OH on the surface. The proportion of integrated area for T2 and T3 is 17.4% and 6.8%, respectively. The ratio of silicon bonded with aminopropyl to the total silicon is 24.2%, which is higher than NNMCM-41 (14.4%), NN-SBA-1 (19.0%), ClPrM41 (10.8%) and ClPrS15 (4.2%) [27,28]. 3.2. Arsenic adsorption 3.2.1. The effect of pH To evaluate the influence of pH on the removal abilities of materials, the adsorption capabilities of Fe1N and Fe2N adsorbents for arsenate and arsenite were studied at different pH values and the results are shown in Figs. 6 and 7. Both Fe1N and Fe2N materials have very high removal abilities, 131.8 mg/g and 124.6 mg/g for As(V) at pH 4, 85 mg/g and 87 mg/g for As(III) at pH 7 and 8, respectively. It is attributed to the abundant Si OH on the silica gel, as the Si OH was generated in situ with the hydrolysis of TEOS, and no calcination was adopted to remove the template that was used in most ordered mesoporous silica materials to generate pore [27,28]. With more Si OH, the silica gel can be grafted with more amino and then coordinated with more Fe3+ . The large specific pore volume is also beneficial to the function of amino and Fe3+ . So the adsorbents show better arsenic removal ability. It should be noticed that the initial concentration of arsenic used here is 200 ppm, and 65.9% of the arsenate was adsorbed on Fe1N at the high adsorption ability of 131.8 mg/g. If the initial concentration increased, the removal ability of arsenate would be higher for Fe1N. It can also be found that Fe1N material has better removal ability for As(V) than Fe2N, which is attributed to the higher iron content of Fe1N, as was mentioned in Section 3.1.3.
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Fig. 6. The effect of pH on Fe1N and Fe2N adsorbents for arsenate.
It was shown in Fig. 4 that both the two materials have the highest arsenate removal ability at pH 4. It can be explained by the following reasons. First, arsenate mainly existed as H2 AsO4 − at pH 4. The proportion of H3 AsO4 would increase as the pH below this value, while the proportion of HAsO4 2− would increase as the pH above this value [37]. It is unfavorable for arsenic removal in both situations. Second, the iron can dissolve in the water at low pH which leads to the removal ability depressed. This phenomenon has been mentioned by An et al. [38]. Third, the negative charge on the surface of materials increases as the pH increases when pH > 4, so the repulsive force between the arsenate and the absorbents strengthens, which has been reported by Jeon et al. [1]. As shown in Fig. 7, the highest removal ability for arsenite appears at pH = 7 for Fe1N and pH = 8 for Fe2N, respectively. It can be explained that electrostatic attraction plays an important role on arsenite removal. When the pH is below 7, arsenite mainly exists as H3 AsO3 , and few As(III) exists as anion. When the pH is above 8, the material surface becomes negative and has a repulsive force with arsenite [37]. Fe2N material has better removal ability for As(III) than Fe1N here, which may be attributed to the reason that arsenite can be adsorbed not only by iron, but also by amino, as the protonated amino is also an adsorbent for arsenite [39]. 3.2.2. The kinetics study To evaluate the kinetics of adsorbents, the adsorption of Fe1N and Fe2N for arsenate was tested at different reaction time and the results are shown in Fig. 8. The adsorption rates of both adsorbents
Fig. 8. The adsorption change of Fe1N and Fe2N adsorbents for arsenate with time. Table 3 The results obtained from pseudo-second-order model for arsenate adsorption. Materials
k2 (g/(mg h))
qe (mg/g)
r2
Fe1N Fe2N
0.0634 0.194
131.9 126.1
0.9997 0.9998
for As(V) are very fast, and reaches a high amount that close to saturation value in just 10 min. It can be attributed to three aspects. First, the hydrolysis of Fe3+ in adsorbents is very fast. Second, the fresh hydrolysis product has a fast adsorption for arsenate. Third, the diffusion in silica pores is very fast and has little inhibition effect for arsenic removal [35]. The fast adsorption rate of the two adsorbents was also observed in arsenite adsorption experiments and at other pH values. Due to the hydrolysis of Fe3+ , pH was not constant at other values in the initial stages, so kinetics studies that were carried at pH = 4 were investigated. To investigate the kinetics study, we used pseudo-first-order and pseudo-second-order model to fit the date. We found that the pseudo-first-order was not suitable to fit the date, while the pseudo-second-order fit the date well. The mathematic representation of the pseudo-second-order model is given in 1 t t = + qt qe k2 q2e
(3)
where qe and qt are the adsorption capacities (mg/g) of the adsorbent at equilibrium and at any time t (h), respectively; and k2 (g/(mg h)) is the related adsorption rate constant. The results are shown in Table 3. Both the adsorption of arsenate with Fe1N and Fe2N fit pseudo-second-order model well with r2 > 0.999. The result was in accordance with the previous results published [40–42]. 3.2.3. The isotherm study Using appropriate correlation models for the experiment equilibrium data is of great importance to understand the mechanism of adsorption for arsenic. Langmuir and Freundlich isotherm models are the two most important models used [41,42]. The two models were used to correlate the adsorption behavior in present study. The arsenic adsorption equilibrium data was obtained at pH 7. The Langmuir isotherm model and Freundlich isotherm model are given by the following equation: q=
qm × b × Ce 1 + b × Ce 1/n
Fig. 7. The effect of pH on Fe1N and Fe2N adsorbents for arsenite.
q = K × Ce
(4) (5)
J.-C. Zuo et al. / Journal of Hazardous Materials 235–236 (2012) 336–342
Fig. 9. The adsorption isotherms of arsenate for both the Fe1N and Fe2N materials with Langmuir (solid line) and Freundlich (dashed line) model.
Table 4 The parameters of arsenate for both the Fe1N and Fe2N materials with Langmuir and Freundlich model. Materials
Fe1N Fe2N
Langmuir
Freundlich 2
qm (mg/g)
Ka (l/mg)
R
KF (mg/g)
n
R2
101.74 78.42
0.093 1.028
0.971 0.912
20.91 33.74
2.93 4.69
0.938 0.951
where q (mg/g) is the amount of arsenic adsorbed at equilibrium, Ce (mg/l) is the arsenic equilibrium concentration, qm (mg/g) and b (l/mg) are the Langmuir constants related to the capacity and energy of the adsorption, respectively, K and n are the Freundlich constants related to the adsorbents capacity and adsorption intensity, respectively [1,25,40]. The adsorption isotherms of arsenate for both the Fe1N and Fe2N materials with Langmuir and Freundlich models are shown in Fig. 9. The parameters for the Langmuir and Freundlich models are presented in Table 4. For the arsenate isotherm adsorption, Langmuir model is more suitable for Fe1N material, while Freundlich model fits Fe2N material better. It can be explained by that the surface of the Fe1N material is mainly covered with by Fe3+ . Therefore, there is only one kind of acting force for arsenate adsorption. As arsenate form inner-sphere complexes with Fe3+ hydrated form, the arsenate covers the surface in monolayer style [1,25,43–45]. For Fe2N material, only a part of amino on the surface is coordinated with Fe3+ . So the surface of the adsorbent is not homogeneous and both the iron and the amino participate in the adsorption of arsenate. As the amino and iron have different acting force to arsenate adsorption, Langmuir model does not fit well for Fe2N. The adsorption isotherms of both the Fe1N and Fe2N for arsenite with Langmuir and Freundlich models are shown in Fig. 10. The parameters of Langmuir and Freundlich models are presented in Table 5. For arsenite adsorption, both Fe1N and Fe2N materials fit Freundlich model better. It can be explained that iron oxide adsorbs arsenite in both inner-sphere and outer-sphere complexes, and forms multilayer adsorption at high concentration [25,43–46].
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Fig. 10. The adsorption isotherms of arsenite for both the Fe1N and Fe2N materials with Langmuir (solid line) and Freundlich (dashed line) model.
3.2.4. The effect of coexistent anion The main anions in natural water are mainly chloride and sulfate ions, which may have an effect on the adsorption of arsenic. The removal abilities of Fe1N and Fe2N for As(III) and As(V) were investigated with the existent of these two kinds of anion. The concentrations of coexistent anion investigated here are 2 g/l for Cl− and 1 g/l for SO4 2− , which are higher than the concentration in natural water. The initial arsenic concentration is 2 mg/l, and the adsorbents employed here is 0.2 g/l. The results are shown in Fig. 11. The adding of chloride and sulfate lowered the removal abilities of the two adsorbents. Chloride has a more serious effect for the adsorption of arsenic than sulfate in all the four cases, which can be attributed to the stronger attraction between the adsorbent surface and chloride, as was mentioned by Yoshitake et al. [32,36], and the sulfate has little effect on the adsorption, which has been manifested by Partey et al. [43]. Both the chloride and sulfate have greater effect on arsenic adsorption for Fe1N than for Fe2N in general, which can be explained that the combined force of iron and amino to arsenic for Fe2N is stronger at low concentration. Arsenate is more easily to be removed than arsenite in almost all the cases, due to the strong chemical interaction between arsenate and adsorbents and the weak interaction between arsenite and adsorbents, as inner-sphere complexes form between iron oxide and arsenate, while both inner-sphere and outer-sphere complexes form between iron oxide and arsenite [28,43–46].
Table 5 The parameters of arsenite for both the Fe1N and Fe2N materials with Langmuir and Freundlich model. Materials
Fe1N Fe2N
Langmuir
Freundlich
qm (mg/g)
Ka (l/mg)
R2
KF (mg/g)
n
R2
142.6 132.7
0.0088 0.0133
0.969 0.948
4.44 6.71
1.72 1.93
0.993 0.982
Fig. 11. The effect of chloride and sulfate on Fe1N and Fe2N adsorbents for arsenate and arsenite.
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4. Conclusions In this work, two adsorbents for arsenic removal were prepared by grafting monoamine and diamine on the silica gels obtained using sol–gel method with two-step acid–base catalysis, and then coordinating amino with Fe3+ . The adsorbents have mesoporous structure, large surface area and pore volume, abundant amino and Fe3+ , thus have high adsorption capacity for both arsenate and arsenite. The Fe1N adsorbent has the highest adsorption capacity of 132 mg/g at pH 4 and a high adsorption capacity of 95 mg/g at neutral condition for As(V), while Fe2N adsorbent has the highest adsorption capacity of 87 mg/g at pH 8 and a high adsorption capacity of 85 mg/g at neutral condition for As(III). The adsorbents show fast rate for arsenic removal, close to saturation in 10 min. The isotherm study showed that the adsorption of Fe1N for As(V) fitted Langmuir model better, while the adsorption of Fe2N for As(V) and the adsorption of both materials for As(III) fitted Freundlich model better. The existence of anion inhibits the adsorption of Arsenic. The chloride has greater inhibition effect than sulfate, and Fe1N is more easily to be affected by the coexistence of anion. Overall, the two materials are excellent adsorbents for arsenic removal and useful to prepare other materials for environmental applications. Acknowledgment This work was supported by the National Basic Research Program of China (973 Program, No. 2011CB933700) of Ministry of Science and Technology of China. References [1] C.S. Jeon, K. Baek, J.K. Park, Y.K. Oh, S.D. Lee, Adsorption characteristics of As(V) on iron-coated zeolite, J. Hazard. Mater. 163 (2009) 804–808. [2] I. Katsoyiannis, A. Zouboulis, H. Althoff, H. Bartel, As(III) removal from groundwaters using fixed-bed upflow bioreactors, Chemosphere 47 (2002) 325–332. [3] H. Guo, D. stuben, Z. Berner, Arsenic removal from water using natural iron mineral-quartz sand columns, Sci. Total Environ. 377 (2007) 142–151. [4] I.A. Katsoyiannis, A.I. Zouboulis, Removal of arsenic from contaminated water sources by sorption onto iron-oxide-coated polymeric materials, Water Res. 36 (2002) 5141–5155. [5] C.L. Chuang, M. Fan, M. Xu, R.C. Brown, S. Sung, B. Saha, C.P. Huang, Adsorption of arsenic(V) by activated carbon prepared from oat hulls, Chemosphere 61 (2005) 478–483. [6] A. Mukherjee, P. Bhattacharya, F. Shi, A.E. Fryar, A.B. Mukherjee, Z.M. Xie, G. Jacks, J. Bundschuh, Chemical evolution in the high arsenic groundwater of the Huhhot basin (Inner Mongolia, PR China) and its difference from the western Bengal basin (India), Appl. Geochem. 24 (2009) 1835–1851. [7] S. Fendorf, H.A. Michael, A.V. Green, Spatial and temporal variations of groundwater arsenic in south and southeast Asia, Science 328 (2010) 1123–1127. [8] W. Li, C.Y. Cao, L.Y. Wu, M.F. Ge, W.G. Song, Superb fluoride and arsenic removal performance of highly ordered mesoporous aluminas, J. Hazard. Mater. 198 (2011) 143–150. [9] J. Pattanayak, K. Mondal, S. Mathew, S.B. Lalvani, A parametric evaluation of the removal of As(V) and As(III) by carbon based adsorbents, Carbon 38 (2000) 589–596. [10] M. Szlachta, V. Gerda, N. Chubar, Adsorption of arsenite and selenite using an inorganic ion exchanger based on Fe–Mn hydrous oxide, J. Colloid Interface Sci. 365 (2012) 213–221. [11] J. Wang, S.J. Zhang, B.C. Pan, W.M. Zhang, L. Lv, Hydrous ferric oxide–resin nanocomposites of tunable structure for arsenite removal: effect of the host pore structure, J. Hazard. Mater. 198 (2011) 241–246. [12] T.F. Lin, J.K. Wu, Adsorption of arsenite and arsenate within activated alumina grains: equilibrium and kinetics, Water Res. 35 (2001) 2049–2057. [13] P. Chutia, S. Kato, T. Kojima, S. Satokawa, Arsenic adsorption from aqueous solution on synthetic zeolites, J. Hazard. Mater. 162 (2009) 440–447. [14] X.Y. Wu, T. Luo, Y. Jia, Y.X. Zhang, J.H. Liu, X.J. Huang, Porous hierarchically micro-/nanostructured MaO: morphology control and their excellent performance in As(III) and As(V) removal, J. Phys. Chem. C 115 (2011) 22242–22250. [15] L. Zeng, A method for preparing silica-containing iron(III) oxide adsorbents for arsenic removal, Water Res. 37 (2003) 4351–4358. [16] Y.H. Xu, T. Nakajima, A. Ohki, Adsorption and removal of arsenic(V) from drinking water by aluminum-loaded shirasu-zeolite, J. Hazard. Mater. B 92 (2002) 275–287.
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