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Dye removal by surfactant encapsulated polyoxometalates
Journal of Hazardous Materials 280 (2014) 428–435
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Dye removal by surfactant encapsulated polyoxometalates Lei Yao a,b , Shun Kuang Lua a , Lizhi Zhang b , Rong Wang b,∗ , ZhiLi Dong a,∗ a b
School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Singapore Membrane Technology Centre, Nanyang Technological University, 1 Cleantech Loop, Singapore 637141, Singapore
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• A novel surfactant encapsulated polyoxometalate (SEP) was prepared.
• The as-prepared SEP was found
to exhibit remarkable anionic dye removal activity. • The SEP was successfully incorporated into the PVDF membrane matrix (SEP-M). • The dye removal mechanisms of SEP and SEP-M were clarified. • The SEP-M can be regenerated without loss of dye removal efficiency.
a r t i c l e
i n f o
Article history: Received 26 May 2014 Received in revised form 23 July 2014 Accepted 6 August 2014 Available online 23 August 2014 Keywords: Polyoxometalate Dye removal Adsorption Polyvinylidene fluoride Membrane
a b s t r a c t A novel surfactant encapsulated polyoxometalate (SEP) has been synthesized by using a simple ionexchange reaction. The prepared SEP complex was found to self-assemble into nanospherical particles whose morphology and component were characterized by TEM and XPS. The SEP was further incorporated into polyvinylidene fluoride (PVDF) to fabricate SEP incorporated composite membrane (SEP-M). Both the SEP and SEP-M exhibited excellent dye removal activities, which is for the first time reported as an intriguing property of the SEP. A regeneration scheme for SEP-M was successfully proposed without any loss of dye removal efficiency. Detailed mechanism studies were carried out to elucidate the nature of dye decolorization. Ion exchange was revealed to play a dominant role in the dye removal process. The current research not only renders a new example for the simple and direct synthesis of SEP but more importantly provides an efficient dye removal methodology. © 2014 Published by Elsevier B.V.
1. Introduction Attention on polyoxometalates (POMs) that are well-defined early transition metal-oxygen macroanion cluster species has mushroomed since a rise in popularity of POMs started in the early 1990s. So far thousands of POMs with diverse structures in which the Keggin type ([XM12 O40 ]x−8 ), Wells-Dawson type ([X2 M18 O62 ]2x−16 ), Anderson-Evans type ([X6+ M6 O24 ]6− ) and Dexter-Silverton type ([XM12 O42 ]x−12 ) (X: the heteroatom; x: the oxidation state of X; M: the addenda atom) are the most common species, have been discovered [1]. POMs can possess fascinating properties in extensive fields such as catalysis, materials science, medicine and biology [2–7],
∗ Corresponding authors. Tel.: +65 67906727; fax: +65 67909081. E-mail addresses: [email protected] (R. Wang), [email protected] (Z. Dong). http://dx.doi.org/10.1016/j.jhazmat.2014.08.026 0304-3894/© 2014 Published by Elsevier B.V.
because of their versatile nature in terms of structure, size, and tunable properties such as redox potential, acidity, charge distribution, and solubility in various media, which can be manipulated by choosing proper constituent elements and counter cations [1]. In recent years, progress on POMs has been made in the synthesis of POMs nanoarchitectures [8–11]. Because POMs clusters usually aggregate to bulk crystalline solids that are hard to process, the development of POMs-based nanoarchitectures has been very slow. In this occasion, a technique of surfactant encapsulation involving introduction of surfactants as unique counter cations for POMs has emerged on the scene, and proved practical to synthesize POMs nanoarchitectures. Even though a few examples have been successfully reported [12–16], studies of surfactant encapsulated polyoxometalates (SEPs) are still rather limited, most of which have just stayed on the structural studies while only a few examples of catalytic properties have been reported [17]. In this paper, SEPs have been constructed by using a cationic surfactant dimethyldioctadecylammonium bromide (DODA·Br) and Keggin-type
L. Yao et al. / Journal of Hazardous Materials 280 (2014) 428–435 decamolybdodivanadophosphate (H5 [PV2 Mo10 O40 ], abbreviate to PV2 Mo10 ). Nanospherical structures were obtained. Interestingly, such a SEP was found to demonstrate remarkable dye (e.g. reactive black 5) adsorption activity. Reactive black 5 (RB5), as a diazo compound with two identical vinylsulphone reactive groups, exhibits high toxicity to aquatic life and mutagenicity to human. It is one of the most common dyes used in textile industries and typically resistant to biodegradation and recalcitrant to traditional treatment [18–20]. Strategies that currently exist for its removal from industrial effluents include chemical oxidation, reverse osmosis, biological treatment, etc., usually suffering from drawbacks such as high cost, secondary pollutants, regeneration problem, sludge generation, long retention times, and low efficiency [21]. The SEP may show great potential for the removal of dye. In order to overcome some issues such as separation and recycle of the as-prepared SEP in practical applications, SEP incorporated microfiltration membrane (SEP-M) was further developed for the first time in this study. SEP-M in its membrane form can present advantages such as ease of packaging and scale-up, feasibility to recycle, reduction of mass transfer resistance, lower pressure drops and higher flow rates [22]. Recently, membrane separation technology has been demonstrated as a promising alternative for dye removal from aqueous solutions, in which nanofiltration (NF) and reverse osmosis (RO) membranes were dominantly applied [23–25]. In contrast to NF and RO membranes with potential drawbacks such as high pressure supply and dramatic decline in flux [26], SEP-M as a novel functionalized microfiltration membrane is anticipated to enhance flow rate significantly without extra pressure supply as well as high energy consumption. A simple blending method was utilized to incorporate SEP into membranes. Polyvinylidene fluoride (PVDF) was selected as the polymeric matrix by virtue of its excellent processibility and chemical resistance [27]. Owing to the enhanced hydrophobicity after encapsulation of POMs by surfactants, it just offered a good opportunity for SEP to well incorporate and stabilize in the polymeric matrix through hydrophobic interactions. Dye removal mechanisms by the SEP and SEP-M were also well investigated. Additionally, the regeneration strategy of SEP-M has been proposed in this study so that the membrane may recycle in the dye removal process.
2. Experimental 2.1. Materials and methods All reagents used to synthesize SEP are of AR grade and used without further purification. H5 [PV2 Mo10 O40 ] was prepared according to the literature [1]. Dioctadecyldimethylammonium bromide (DODA·Br), chloroform, commercial granular active carbon (GAC, 0.9–2 mm), reactive black 5 (RB5), and Safranin T were purchased from Sigma. Commercial PVDF pellets (Kynar 761, Arkema) dried at 50 ◦ C under vacuum and 1-methyl-2-pyrrolidone (NMP, Merck) were used to fabricate membranes. The detailed nanostructure of the synthesized SEP was characterized by a transmission electron microscope (TEM, JEOL JEM-2010) operating at 200 kV. An X-ray photoelectron spectroscope (XPS, VG ESCA 220iXL) equipped with Mg K␣ X-ray source in twin-anode X-ray gun was used to analyze the chemical composition on materials surfaces precisely. The dimensions of SEP-M were investigated with a field emission scanning electron microscope (FESEM, JEOL JSM7600F) operating at 5 kV. Energy dispersive X-ray (EDX) analysis operating at 20 kV was performed in attachment in FESEM. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) spectra were recorded by IR Presitige-21 FTIR (Shimadzu). Nitrogen adsorption/desorption isotherms were measured at 77 K by ASAP 2000 adsorption apparatus (Micromeritics). The samples were degassed at 333 K for 24 h under vacuum before analysis. Thermogravimetric analysis (TGA) was carried out on TGA Q500 (TA Instruments) from 30 to 900 ◦ C at 10 ◦ C/min in air
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atmosphere. UV–vis spectroscope (UV-1800, Shimadzu) was used to measure the intensity of absorption peak (at 580 nm) of the sample solutions. The zeta potential of membranes was measured using a SurPASS electrokinetic analyzer (Anton Paar) in a 10 mM potassium chloride solution to analyze the surface charge. Total organic carbon (TOC) and total nitrogen (TN) were monitored using a Shimadzu TOC-VCSH analysis system. 2.2. Synthesis of SEP DODA·Br was dissolved in chloroform, followed by addition of H5 [PV2 Mo10 O40 ] powder under vigorous stirring. The molar ratio of DODA·Br and H5 [PV2 Mo10 O40 ] was controlled at 5:1. Ultrasonication was applied until the mixture turned clear. The final product was collected by evaporating the clear solution to dryness. 2.3. Fabrication of SEP-M SEP-M was fabricated with PVDF and SEP by the phase inversion technique [28]. First, SEP was added into NMP under stirring to get a homogenous solution. Then PVDF pellets were added under stirring at 60 ◦ C for at least 2 days to obtain a homogenous mixture. The resulting dope solution (PVDF/SEP/NMP at 15:7.5:77.5 weight ratio) was cooled down to room temperature prior to film casting. Then, the dope was casted onto a glass plate with a stainless steel casting knife with the gate height of 250 m. The casted film was immersed into tap water at room temperature immediately to form a membrane. After the membrane was fully separated from the glass plate, it was washed in tap water to remove the remaining NMP, rinsed by distilled water and finally dried in air. 2.4. Dye removal tests Batch experiments of dye removal by the SEP and SEP-M were conducted with 60 mL 15 ppm RB5 aqueous solution at 45 ◦ C. Contrast experiments were conducted with the as-synthesized SEP, DODA·Br and GAC respectively to analyze the dye removal activity of SEP. Then SEP-M (18 cm2 ) in which the loading amount of SEP is approximately the same with that in the previous contrast experiments was loaded into the dye solution under stirring or sealed into a convective flow membrane module driven by a peristaltic pump. A simple sketch of the circulation membrane system is shown in Fig. 1. Control experiment was also conducted with a pure PVDF membrane containing no SEP (PVDF/NMP at 15:85 weight ratio) fabricated in the same way. 2.5. Regeneration of SEP-M The used SEP-M was immersed in 50% HNO3 for 5 min followed by rinsing with distilled water. Then the dye removal activity was examined again. This regeneration process was performed twice. For the third time, 15% HCl was utilized and the membrane was treated for 20 min before reuse. 3. Results and discussion 3.1. Materials characterization
Fig. 1. The schematic of the circulation membrane setup.
The TEM images shown in Fig. 2 revealed the nanospherical structure of the synthesized SEP with diameter of approximately 120 nm. Fine parallel darker fringes related to the Keggin anions in the SEP sphere with a spacing around 2.9 nm can be clearly observed in Fig. 2b owing to the strong contrast between PV2 Mo10 and the organic component. It indicates that Keggin anions are encapsulated by DODA+ to form some lamellar arrangements in
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Fig. 2. (a) TEM image of as-prepared SEP and (b) the corresponding magnified TEM image of their edges.
the local structures. Many factors would account for the morphology of SEP assemblies attained, including the POM anion structure (e.g. size and charge density), the number of surfactant attached, the non-covalent interaction between the building blocks and the solvent system, as well as the external stimulus (e.g. temperature and photo-irradiation) [15,17]. Although it is a complicated process, the self-assembly mechanism can be rationally interpretated to some extent in the light of previous published studies and our experimental results [13,15–17]. Until now, the Keggin-type Si- and P-containing POM are the most versatile building blocks to form SEPs with controllable morphologies. Typically, through adjusting the mixed solvent system, and shape of SEPs can be tailored. [12,13] Interestingly, spherical SEP was obtained in our study while discal (DODA)3 PMo12 O40 was reported previously [12] in the same solvent system. Although PV2 Mo10 5− ion and PMo12 3− ion are in the same family with nearly the same size and Keggin structures, the resulting SEPs are completely different. On the other hand, (DODA)4 SiW12 O40 would form similar spherical structures as reported [16,29] though the addenda and hetero atoms are different. It is probable that the different number of surfactants attached on the surface of the POM clusters played the dominant role in the formation of spheres or disks. The increased negative charges of
Fig. 4. FTIR spectra of (a) as-prepared SEP and (b) SEP-M.
polyanionic clusters due to the replacement of two Mo atoms with V atoms without change of size could affect the electrostatic interaction between the inorganic and organic components as well as the electrostatic repulsion between inorganic components. Increased flexibility of the molecular arrangement driven by the repulsion between the polyanions may provide the possibility for the formation of spherical architectures to minimize the free energy. From the FESEM image of the cross-section of SEP-M (Fig. 3a), the most obvious characteristic of the composite membrane is its large macrovoids. Plenty of spherical particles smaller than 1 m were deposited on the walls of the macrovoids as seen in Fig. 3d. Sponge-like structure was developed beneath the macrovoids near the bottom layer. It can be observed clearly that spherical particles with particle size around 1 m were embedded in the cellular morphology of the sponge-like structure as shown in Fig. 3e. Both surfaces of the membrane were relatively smooth and no particles were observed on top. It was conjectured that the existence of the spherical particles was the evidence of SEP in the membrane. And the EDX spectrum (see Fig. 3f) further proved the existence of the SEP by the appearance of P, Mo, and V peaks.
Fig. 3. FESEM images of the composite membrane: (a) cross-section (scale bar 10 m), (b) top surface, (c) bottom surface, (d) and (e) detailed structure of the cross-section (scale bar 1 m). (f) EDX spectrum of the composite membrane.
L. Yao et al. / Journal of Hazardous Materials 280 (2014) 428–435
Fig. 5. Dye decolorization activities of different materials. Reaction conditions: dye (15 ppm, 60 mL), substrate (SEP, DODA·Br and GAC) 31 mg, atmospheric pressure and 45 ◦ C, stirring rate at 300 rpm.
The FTIR spectra shown in Fig. 4 further confirmed the successful incorporation and the maintaining of structural integrity of SEP in the membrane. Four sharp bands, corresponding to the typical skeletal vibrations of the Keggin structure of POM, were observed at 1058, 947, 869, and 800 cm−1 (marked by dashed circles in Fig. 4a) in the as-prepared SEP, indicating a good stability of PV2 Mo10 in the architecture [30]. A close inspection of the bands of SEP compared with pure PV2 Mo10 reveals that the (P–O) and (M–O) have red shifts while the (M–Oc –M) and (M–Oe –M) have blue shifts (M = Mo or V, Oe : edge-sharing oxygen, Oc : corner-sharing oxygen). The shifts of band positions probably results from the mutual effect of the presence of counter ions and the chemical environment [31,32]. Obvious bands appearing at 2918 and 2850 cm−1 , which are attributed to the asymmetric and symmetric stretching of CH2 bonds, indicate that the arrangement of hydrocarbon chains in SEP is highly ordered or crystalline (see Fig. 4a) [32]. After the SEP was incorporated into the PVDF membrane, the bands which represent the stretching vibrations of P–O and M–Oc –M species at 1058 and 869 cm−1 are covered by a series of intensive characteristic bands of PVDF that appear at 1402, 1277, 1234, 1171, 1072, 881, and 841 cm−1 [33]. The other two bands at 945 and 797 cm−1 (marked by orange dashed circles in Fig. 4b), which are attributed to the (M–O) and (M–Oe –M), manifest that PV2 Mo10 was quite stable in the membrane fabrication process and finally maintained in the form of SEP in the membrane. 3.2. Dye removal activities of SEP and SEP-M Contrast experiments were carried out with DODA·Br and commercial GAC to evaluate the decolorization activity of the asprepared SEP. Equal loading amount of 31 mg was added into 3 portions of 60 mL 15 ppm RB5 aqueous solution at 45 ◦ C with stirring at the same rate. The dye decolorization was monitored by UV–vis absorbance and the decolorization efficiency was calculated using the following formula: Decolorization efficiency (%) =
A0 − At × 100 A0
where A0 is the absorbance before decolorization, At is the absorbance after certain time t of decolorization. It can be seen in Fig. 5 that SEP exhibited the best decolorization activity toward RB5. 87.71% decolorization efficiency was achieved only in 10 min and 95.02% decolorization efficiency was attained in the testing period of 45 min. In contrast with the obvious decolorization of RB5 solution with SEP, the solution maintained turbid with DODA·Br powders suspended in the whole period of 45 min. RB5 solution was decolorized 36.51% in the first 5 min, which could be attributed to the ion exchange mechanism of DODA·Br toward
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Fig. 6. Dye decolorization activities of different substrate. (i) Pure PVDF membrane by stirring (at 200 rpm), (ii) SEP-M by circulating filtration (at 50 mL min−1 ) and (iii) SEP-M by stirring (at 200 rpm). Reaction conditions: dye (15 ppm, 60 mL), 18 cm2 SEP-M with SEP content 26 mg, 18 cm2 PVDF membrane, 45 ◦ C. The inset photograph shows the decolorization of dye with SEP-M.
RB5 in which anionic dye molecules were trapped by cationic DODA+ long chain. Then the dye concentration stayed fluctuating around 36.51% for the next 40 min, which is probably rendered by the remaining small amount of DODA·Br that cannot be centrifuged and separated from the testing samples. For the commercial GAC, it exhibited a relatively stable decolorization rate in the first 45 min, and a longer testing time of 120 min was thus adopted. The adsorption rate and capacity of GAC are limited by the exposed surface area and the diffusion rate of the dye. Finally, the decolorization efficiency of 50% was observed with the GAC. The above results indicate that a remarkable enhancement on the efficiency of dye removal has been achieved by the SEP. Given the limited application of SEP in the powder form in practical operations, SEP-M was developed with corresponding performances analyzed. 18 cm2 SEP-M was loaded into the dye solution under stirring or sealed into a convective low membrane module driven by a peristaltic pump. It is indicative in Fig. 6 that the SEP-M also exhibited excellent dye removal activity in which the adsorption capacity of PVDF toward dye can be ignored. Similar trends of dye decolorization with SEP-M by stirring and by circulating filtration were attained, while the SEP-M with stirring mode rendered a slightly better performance in 120 min reaching a decolorization efficiency of 97.5%. It is probable that both the stirring rate and the recirculation rate play an important and similar role in the process. In the light of the similar performances achieved, SEP-M operated in the membrane module under the convective flow mode is more promising in practical applications because it possesses the property of simple scale-up and operation by stacking membranes to form the membrane plant under traditional membrane separation protocols. The as-prepared SEP-M was found regenerable under the treatment with HNO3 or HCl. It can be seen in Fig. 7 that the SEP-M regenerated 3 times maintained very stable performance with the final dye removal achieved by nearly 100%. The physical appearance of the regenerated SEP-M remains constant after acid treatment, indicating that it is well chemical resistant even under highly acidic conditions with full regeneration of dye removal efficiency. These results suggest that the SEP-M is promising in practical applications and a continued optimization of this strategy may yield further progress, since little dye removal efficiency was sacrificed in the recovery process of the SEP-M. 3.3. Mechanism studies The adsorption effect may be the reasonable explanation considering the color change of the membrane from yellow to dark blue
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Fig. 7. Dye decolorization activities of SEP-M regenerated for different times. Reaction conditions: dye (15 ppm, 60 mL), 18 cm2 SEP-M with SEP content 26 mg, 45 ◦ C, stirring at 200 rpm. The inset photograph displays (a) the fresh SEP-M, (b) SEP-M after the dye removal test and (c) the regenerated SEP-M.
in the decolorization tests. To elucidate the existence of dye on the membrane, TGA was conducted. For the thermogravimetric curve of the original SEP-M (see Fig. 8a), the most obvious weight loss appears at 200–450 ◦ C, which is assigned to the decomposition of the SEP and PVDF. The corresponding derivative weight loss displays three distinct maxima of weight losses at 281.76, 398.74, and 408.37 ◦ C. By contrast, the TGA curve of the reacted SEP-M changes with the emergence of an obvious step of weight loss at 360–500 ◦ C (see Fig. 8b). The corresponding derivative weight loss, which is clearly different from the original one, indicates the presence of RB5. FTIR analysis was also conducted but valuable information was not obtained, because the characteristic N N stretching of the RB5 is very weak and overlapped by the C H deformation vibration [34]. Although the adsorption effect has been proved to some extent by the above TGA results, the catalytic degradation effect where the SEP functions as catalyst could not be excluded in the light of the redox nature of PV2 Mo10 [35,36]. To go a further step, contrast experiments were conducted under oxygen-free conditions and in air, respectively. It was found that the similar decolorization efficiencies were obtained in the two different conditions, meanwhile the final solution treated by the SEP in air turned light pink after 120 min (see Fig. 9). These results suggest that the adsorption effect of the SEP plays a dominant role in the dye decolorization process while partial oxidation of the dye also occurs. To investigate the roles of SEP and oxygen in the dye oxidation process, XPS
Fig. 9. Dye decolorization activities of (i) SEP-M with cationic dye, reaction conditions: safranin T (30 ppm, 60 mL), 18 cm2 SEP-M with SEP content 26 mg, 45 ◦ C, stirring at 200 rpm; (ii) SEP with N2 and (iii) SEP with air, reaction conditions: SEP 0.1 g, RB5 (80 ppm, 150 mL), 45 ◦ C. The inset photograph shows the decolorization of dye by SEP with air.
analysis was carried out on the original SEP and reacted SEP, respectively. The result indicated that both Mo and V showed unchanged valence, which suggests that the SEP plays a role as catalyst and at the same time oxygen is as oxidant. Consequently, it is deduced that the SEP mainly exhibits an excellent adsorption capacity for the dye RB5 followed by a limited catalytic activity toward the dye oxidation. In contrast with SEP, the catalytic property of SEP-M was hardly observed. For SEP-M, the SEP is trapped in the membrane matrix, which may inhibit the catalytic activity stemming from PV2 Mo10 clusters, therefore the treated dye solution would not turn pink even for a longer time. To further understand the adsorption mechanism of SEP toward the dye RB5, BET, XPS, zeta potential and TOC/TN measurements were also well carried out. The isotherms for nitrogen adsorption and desorption of PV2 Mo10 , DODA·Br and SEP conducted at 77 K were shown in Fig. 10. PV2 Mo10 and DODA·Br exhibited similar Brunauer–Emmett–Teller (BET) surface areas, which were 6.87 and 6.75 m2 /g, respectively. Interestingly, the BET surface area of SEP dropped significantly to 0.23 m2 /g. It implies that the nanoparticles formed by PV2 Mo10 and DODA consist of close packed layers with low porosity. Thus the excellent dye adsorption activity is not likely coming from the common physical adsorption, but the ion exchange mechanism. On the other hand, close packed lamellar structures of the SEP particles suggest large inner steric hindrance and the ion exchange should principally take place on the surface
Fig. 8. TGA of (a) original SEP-M and (b) reacted SEP-M with dye.
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Fig. 10. Nitrogen sorption isotherms of PV2 Mo10 , DODA·Br and SEP. The inset shows enlarged nitrogen sorption isotherm of SEP. Table 1 Element composition of the original SEP and reacted SEP observed by XPS analysis. Elements
Br Mo C N V O
Composition (At%) Original SEP
Reacted SEP
0.7 3.1 81.6 0.8 0.4 13.4
0.3 2.3 84.3 0.9 0.4 11.8
of these particles. In this case, the excellent dye adsorption activity benefited from the nano-sized nature of the SEP particles. It can be seen in Fig. 9 that the SEP-M just demonstrated adsorption activity toward anionic dye but not the cationic dye. Zeta potential analysis showed that after the addition of SEP into PVDF, the isoelectric point moved from pH 6.592 to 7.414, indicating that the SEP-M is positively charged compared with the pure PVDF membrane. So it is proposed that the ion exchange activity of SEP involves the trapping of anionic dye molecules with concomitant release of Br− ions. It is conjectured that SEP is not fully formulated as (DODA)5 PV2 Mo10 O40 even though the molar ratio of DODA·Br to H5 PV2 Mo10 O40 was controlled at 5:1. Traces of Br− ions were detected in SEP by XPS analysis as shown in Table 1. According to XPS results, the composition of SEP is approximately (DODA)6.41 H3.5 PV2 Mo10 O40 Br2.09 . Accurate calculation is not possible confined by the resolution and detection depth of the XPS analysis. N, V and Br can be weakly detected only from high resolution spectra, in which the values are not reliable to identify the formula of SEP. From Table 1, the atomic composition of Br in the SEP decreased from 0.7% to 0.3% after reacting with the dye, confirming the release of Br− ions into the environment in exchange with the anionic dye. The content of C and N increased slightly with the decrease of Mo and O, which implies the cover of dye onto the surface of the SEP. TOC/TN analysis further confirmed the supposition of ion exchange mechanism in the adsorption-desorption process. In the membrane regeneration treatment with acid, NO3 − or Cl− will exchange with anionic dye so as to desorb dye from SEP-M while keeping the valence balanced. In the following dye adsorption test, NO3 − or Cl− will leave SEP-M into the resultant solution. Evidence was provided by increased TN from the HNO3 regenerated SEP-M sample solution and disappearance of TN from the HCl regenerated SEP-M sample solution (see Fig. 11). On the other hand, it
Fig. 11. TOC and TN of dye feed solution (15 ppm, 60 mL) and dye solution treated with different materials (SEP, SEP-M, HNO3 regenerated SEP-M, and HCl regenerated SEP-M).
is noticed that the dye solution treated with SEP still displayed high TOC even though 95.02% decolorization was achieved, which may be attributed to the leaching of DODA into the solution. After calculation, only 0.28% leaching of SEP occurred to render the corresponding TOC enhancement. However, such a leaching phenomenon was not observed for the SEP-M system (see Fig. 11), as the SEP-M could possess a much more stable architecture where the SEP was confined in the polymer matrix, which suggests another advantage of SEP-M compared with SEP. The adsorption isotherm whose parameters express the surface properties and affinity of the dye RB5 on the SEP-M was also investigated. Langmuir and Freundlich isotherm models, which are the most commonly used, were utilized to fit the experimental data [37,38]. The Langmuir adsorption model is based on the assumption that maximum adsorption corresponds to a saturated monolayer of solute molecules without any interaction between molecules on the adsorbent surface. It can be described by the following equation: Ce 1 Ce = + qe Q0 Q0 b where qe is the amount adsored (mg/g), Ce is the equilibrium concentration of dye (ppm), and Q0 (mg/g) and b (L/mg) are Langmuir constants, which stand for the adsorption capacity and energy of adsorption, respectively.
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Table 2 Isotherm equation parameters of the dye RB5 on SEP-M. Parameters
Langmuir
SEP-M
Freundlich
Q0 (mg/g)
KL (L/g)
R2
RL
KF
n
R2
59.21
1.45
0.995
0.0115
47.18
19.32
0.820
The essential characteristics, the adsorption intensity, can be expressed by: RL =
1 1 + KL C0
where C0 (ppm) is the initial amount of dye. RL is reliably utilized to indicate the favorability of adsorption. The Freundlich adsorption model, which is an indicator of surface heterogeneity of the sorbent [39], can be expressed as: log qe = log KF +
1 log Ce n
where KF and n are the Freundlich constants. As listed in Table 2, Langmuir isotherm provides better linear regression (R2 = 0.995) than Freundlich equation (R2 = 0.820), indicating that the adsorption obeys the Langmuir model. The dimensionless equilibrium parameter, adsorption intensity, RL is used to predict the adsorption efficiency of the adsorption process. Values of RL < 1 represent favorable adsorption while values of RL > 1 represent unfavorable adsorption. The SEP-M showed RL = 0.0115, illustrating the favorable adsorption of dye on SEP-M. 4. Conclusions In this paper, a novel surfactant encapsulated polyoxometalate (SEP) has been synthesized by using cationic surfactant DODA·Br and Keggin-type H5 [PV2 Mo10 O40 ] through a simple ion-exchange reaction. The prepared SEP complex can self-assemble into welldefined nanospherical particles in the reaction solution. Anionic dye RB5 was utilized to evaluate the dye removal performances of SEP and an excellent dye removal activity was achieved. It was further incorporated into a PVDF membrane matrix to form the SEP incorporated composite membrane (SEP-M). SEP-M with 50 wt% SEP particles loading was successfully fabricated with SEP spherical particles smaller than 1 m embedded in the polymeric membrane matrix. It was found that desirable dye removal activity of SEP-M retaining the advantages of the membrane form was achieved. The used SEP-M can be easily regenerated by acid treatment without loss of dye removal efficiency. Detailed mechanism studies were carried out to elucidate the nature of dye decolorization by SEP and SEP-M. While the SEP mainly exhibits excellent adsorption capacity with potential catalytic ability coming from the redox nature of PV2 Mo10 , SEP-M follows Langmuir adsorption nearly without catalytic capability. The leaching problem was not observed with SEP-M. SEP-M is superior to the SEP because it can be easily separated, recycled, and scaled up by simply stacking into modules and operated without extra pressure supply. Acknowledgements We would like to thank the financial support from Singapore Membrane Technology Center (SMTC), Nanyang Technological University and NTU AcRF grant (RG76/12). We are also grateful to Singapore Economic Development Board for funding SMTC. The electron microscopy was performed at the Facility for Analysis, Characterization, Testing and Simulation (FACTS) in Nanyang Technological University, Singapore.
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Dye removal by surfactant encapsulated polyoxometalates Lei Yao a,b , Shun Kuang Lua a , Lizhi Zhang b , Rong Wang b,∗ , ZhiLi Dong a,∗ a b
School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Singapore Membrane Technology Centre, Nanyang Technological University, 1 Cleantech Loop, Singapore 637141, Singapore
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• A novel surfactant encapsulated polyoxometalate (SEP) was prepared.
• The as-prepared SEP was found
to exhibit remarkable anionic dye removal activity. • The SEP was successfully incorporated into the PVDF membrane matrix (SEP-M). • The dye removal mechanisms of SEP and SEP-M were clarified. • The SEP-M can be regenerated without loss of dye removal efficiency.
a r t i c l e
i n f o
Article history: Received 26 May 2014 Received in revised form 23 July 2014 Accepted 6 August 2014 Available online 23 August 2014 Keywords: Polyoxometalate Dye removal Adsorption Polyvinylidene fluoride Membrane
a b s t r a c t A novel surfactant encapsulated polyoxometalate (SEP) has been synthesized by using a simple ionexchange reaction. The prepared SEP complex was found to self-assemble into nanospherical particles whose morphology and component were characterized by TEM and XPS. The SEP was further incorporated into polyvinylidene fluoride (PVDF) to fabricate SEP incorporated composite membrane (SEP-M). Both the SEP and SEP-M exhibited excellent dye removal activities, which is for the first time reported as an intriguing property of the SEP. A regeneration scheme for SEP-M was successfully proposed without any loss of dye removal efficiency. Detailed mechanism studies were carried out to elucidate the nature of dye decolorization. Ion exchange was revealed to play a dominant role in the dye removal process. The current research not only renders a new example for the simple and direct synthesis of SEP but more importantly provides an efficient dye removal methodology. © 2014 Published by Elsevier B.V.
1. Introduction Attention on polyoxometalates (POMs) that are well-defined early transition metal-oxygen macroanion cluster species has mushroomed since a rise in popularity of POMs started in the early 1990s. So far thousands of POMs with diverse structures in which the Keggin type ([XM12 O40 ]x−8 ), Wells-Dawson type ([X2 M18 O62 ]2x−16 ), Anderson-Evans type ([X6+ M6 O24 ]6− ) and Dexter-Silverton type ([XM12 O42 ]x−12 ) (X: the heteroatom; x: the oxidation state of X; M: the addenda atom) are the most common species, have been discovered [1]. POMs can possess fascinating properties in extensive fields such as catalysis, materials science, medicine and biology [2–7],
∗ Corresponding authors. Tel.: +65 67906727; fax: +65 67909081. E-mail addresses: [email protected] (R. Wang), [email protected] (Z. Dong). http://dx.doi.org/10.1016/j.jhazmat.2014.08.026 0304-3894/© 2014 Published by Elsevier B.V.
because of their versatile nature in terms of structure, size, and tunable properties such as redox potential, acidity, charge distribution, and solubility in various media, which can be manipulated by choosing proper constituent elements and counter cations [1]. In recent years, progress on POMs has been made in the synthesis of POMs nanoarchitectures [8–11]. Because POMs clusters usually aggregate to bulk crystalline solids that are hard to process, the development of POMs-based nanoarchitectures has been very slow. In this occasion, a technique of surfactant encapsulation involving introduction of surfactants as unique counter cations for POMs has emerged on the scene, and proved practical to synthesize POMs nanoarchitectures. Even though a few examples have been successfully reported [12–16], studies of surfactant encapsulated polyoxometalates (SEPs) are still rather limited, most of which have just stayed on the structural studies while only a few examples of catalytic properties have been reported [17]. In this paper, SEPs have been constructed by using a cationic surfactant dimethyldioctadecylammonium bromide (DODA·Br) and Keggin-type
L. Yao et al. / Journal of Hazardous Materials 280 (2014) 428–435 decamolybdodivanadophosphate (H5 [PV2 Mo10 O40 ], abbreviate to PV2 Mo10 ). Nanospherical structures were obtained. Interestingly, such a SEP was found to demonstrate remarkable dye (e.g. reactive black 5) adsorption activity. Reactive black 5 (RB5), as a diazo compound with two identical vinylsulphone reactive groups, exhibits high toxicity to aquatic life and mutagenicity to human. It is one of the most common dyes used in textile industries and typically resistant to biodegradation and recalcitrant to traditional treatment [18–20]. Strategies that currently exist for its removal from industrial effluents include chemical oxidation, reverse osmosis, biological treatment, etc., usually suffering from drawbacks such as high cost, secondary pollutants, regeneration problem, sludge generation, long retention times, and low efficiency [21]. The SEP may show great potential for the removal of dye. In order to overcome some issues such as separation and recycle of the as-prepared SEP in practical applications, SEP incorporated microfiltration membrane (SEP-M) was further developed for the first time in this study. SEP-M in its membrane form can present advantages such as ease of packaging and scale-up, feasibility to recycle, reduction of mass transfer resistance, lower pressure drops and higher flow rates [22]. Recently, membrane separation technology has been demonstrated as a promising alternative for dye removal from aqueous solutions, in which nanofiltration (NF) and reverse osmosis (RO) membranes were dominantly applied [23–25]. In contrast to NF and RO membranes with potential drawbacks such as high pressure supply and dramatic decline in flux [26], SEP-M as a novel functionalized microfiltration membrane is anticipated to enhance flow rate significantly without extra pressure supply as well as high energy consumption. A simple blending method was utilized to incorporate SEP into membranes. Polyvinylidene fluoride (PVDF) was selected as the polymeric matrix by virtue of its excellent processibility and chemical resistance [27]. Owing to the enhanced hydrophobicity after encapsulation of POMs by surfactants, it just offered a good opportunity for SEP to well incorporate and stabilize in the polymeric matrix through hydrophobic interactions. Dye removal mechanisms by the SEP and SEP-M were also well investigated. Additionally, the regeneration strategy of SEP-M has been proposed in this study so that the membrane may recycle in the dye removal process.
2. Experimental 2.1. Materials and methods All reagents used to synthesize SEP are of AR grade and used without further purification. H5 [PV2 Mo10 O40 ] was prepared according to the literature [1]. Dioctadecyldimethylammonium bromide (DODA·Br), chloroform, commercial granular active carbon (GAC, 0.9–2 mm), reactive black 5 (RB5), and Safranin T were purchased from Sigma. Commercial PVDF pellets (Kynar 761, Arkema) dried at 50 ◦ C under vacuum and 1-methyl-2-pyrrolidone (NMP, Merck) were used to fabricate membranes. The detailed nanostructure of the synthesized SEP was characterized by a transmission electron microscope (TEM, JEOL JEM-2010) operating at 200 kV. An X-ray photoelectron spectroscope (XPS, VG ESCA 220iXL) equipped with Mg K␣ X-ray source in twin-anode X-ray gun was used to analyze the chemical composition on materials surfaces precisely. The dimensions of SEP-M were investigated with a field emission scanning electron microscope (FESEM, JEOL JSM7600F) operating at 5 kV. Energy dispersive X-ray (EDX) analysis operating at 20 kV was performed in attachment in FESEM. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) spectra were recorded by IR Presitige-21 FTIR (Shimadzu). Nitrogen adsorption/desorption isotherms were measured at 77 K by ASAP 2000 adsorption apparatus (Micromeritics). The samples were degassed at 333 K for 24 h under vacuum before analysis. Thermogravimetric analysis (TGA) was carried out on TGA Q500 (TA Instruments) from 30 to 900 ◦ C at 10 ◦ C/min in air
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atmosphere. UV–vis spectroscope (UV-1800, Shimadzu) was used to measure the intensity of absorption peak (at 580 nm) of the sample solutions. The zeta potential of membranes was measured using a SurPASS electrokinetic analyzer (Anton Paar) in a 10 mM potassium chloride solution to analyze the surface charge. Total organic carbon (TOC) and total nitrogen (TN) were monitored using a Shimadzu TOC-VCSH analysis system. 2.2. Synthesis of SEP DODA·Br was dissolved in chloroform, followed by addition of H5 [PV2 Mo10 O40 ] powder under vigorous stirring. The molar ratio of DODA·Br and H5 [PV2 Mo10 O40 ] was controlled at 5:1. Ultrasonication was applied until the mixture turned clear. The final product was collected by evaporating the clear solution to dryness. 2.3. Fabrication of SEP-M SEP-M was fabricated with PVDF and SEP by the phase inversion technique [28]. First, SEP was added into NMP under stirring to get a homogenous solution. Then PVDF pellets were added under stirring at 60 ◦ C for at least 2 days to obtain a homogenous mixture. The resulting dope solution (PVDF/SEP/NMP at 15:7.5:77.5 weight ratio) was cooled down to room temperature prior to film casting. Then, the dope was casted onto a glass plate with a stainless steel casting knife with the gate height of 250 m. The casted film was immersed into tap water at room temperature immediately to form a membrane. After the membrane was fully separated from the glass plate, it was washed in tap water to remove the remaining NMP, rinsed by distilled water and finally dried in air. 2.4. Dye removal tests Batch experiments of dye removal by the SEP and SEP-M were conducted with 60 mL 15 ppm RB5 aqueous solution at 45 ◦ C. Contrast experiments were conducted with the as-synthesized SEP, DODA·Br and GAC respectively to analyze the dye removal activity of SEP. Then SEP-M (18 cm2 ) in which the loading amount of SEP is approximately the same with that in the previous contrast experiments was loaded into the dye solution under stirring or sealed into a convective flow membrane module driven by a peristaltic pump. A simple sketch of the circulation membrane system is shown in Fig. 1. Control experiment was also conducted with a pure PVDF membrane containing no SEP (PVDF/NMP at 15:85 weight ratio) fabricated in the same way. 2.5. Regeneration of SEP-M The used SEP-M was immersed in 50% HNO3 for 5 min followed by rinsing with distilled water. Then the dye removal activity was examined again. This regeneration process was performed twice. For the third time, 15% HCl was utilized and the membrane was treated for 20 min before reuse. 3. Results and discussion 3.1. Materials characterization
Fig. 1. The schematic of the circulation membrane setup.
The TEM images shown in Fig. 2 revealed the nanospherical structure of the synthesized SEP with diameter of approximately 120 nm. Fine parallel darker fringes related to the Keggin anions in the SEP sphere with a spacing around 2.9 nm can be clearly observed in Fig. 2b owing to the strong contrast between PV2 Mo10 and the organic component. It indicates that Keggin anions are encapsulated by DODA+ to form some lamellar arrangements in
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Fig. 2. (a) TEM image of as-prepared SEP and (b) the corresponding magnified TEM image of their edges.
the local structures. Many factors would account for the morphology of SEP assemblies attained, including the POM anion structure (e.g. size and charge density), the number of surfactant attached, the non-covalent interaction between the building blocks and the solvent system, as well as the external stimulus (e.g. temperature and photo-irradiation) [15,17]. Although it is a complicated process, the self-assembly mechanism can be rationally interpretated to some extent in the light of previous published studies and our experimental results [13,15–17]. Until now, the Keggin-type Si- and P-containing POM are the most versatile building blocks to form SEPs with controllable morphologies. Typically, through adjusting the mixed solvent system, and shape of SEPs can be tailored. [12,13] Interestingly, spherical SEP was obtained in our study while discal (DODA)3 PMo12 O40 was reported previously [12] in the same solvent system. Although PV2 Mo10 5− ion and PMo12 3− ion are in the same family with nearly the same size and Keggin structures, the resulting SEPs are completely different. On the other hand, (DODA)4 SiW12 O40 would form similar spherical structures as reported [16,29] though the addenda and hetero atoms are different. It is probable that the different number of surfactants attached on the surface of the POM clusters played the dominant role in the formation of spheres or disks. The increased negative charges of
Fig. 4. FTIR spectra of (a) as-prepared SEP and (b) SEP-M.
polyanionic clusters due to the replacement of two Mo atoms with V atoms without change of size could affect the electrostatic interaction between the inorganic and organic components as well as the electrostatic repulsion between inorganic components. Increased flexibility of the molecular arrangement driven by the repulsion between the polyanions may provide the possibility for the formation of spherical architectures to minimize the free energy. From the FESEM image of the cross-section of SEP-M (Fig. 3a), the most obvious characteristic of the composite membrane is its large macrovoids. Plenty of spherical particles smaller than 1 m were deposited on the walls of the macrovoids as seen in Fig. 3d. Sponge-like structure was developed beneath the macrovoids near the bottom layer. It can be observed clearly that spherical particles with particle size around 1 m were embedded in the cellular morphology of the sponge-like structure as shown in Fig. 3e. Both surfaces of the membrane were relatively smooth and no particles were observed on top. It was conjectured that the existence of the spherical particles was the evidence of SEP in the membrane. And the EDX spectrum (see Fig. 3f) further proved the existence of the SEP by the appearance of P, Mo, and V peaks.
Fig. 3. FESEM images of the composite membrane: (a) cross-section (scale bar 10 m), (b) top surface, (c) bottom surface, (d) and (e) detailed structure of the cross-section (scale bar 1 m). (f) EDX spectrum of the composite membrane.
L. Yao et al. / Journal of Hazardous Materials 280 (2014) 428–435
Fig. 5. Dye decolorization activities of different materials. Reaction conditions: dye (15 ppm, 60 mL), substrate (SEP, DODA·Br and GAC) 31 mg, atmospheric pressure and 45 ◦ C, stirring rate at 300 rpm.
The FTIR spectra shown in Fig. 4 further confirmed the successful incorporation and the maintaining of structural integrity of SEP in the membrane. Four sharp bands, corresponding to the typical skeletal vibrations of the Keggin structure of POM, were observed at 1058, 947, 869, and 800 cm−1 (marked by dashed circles in Fig. 4a) in the as-prepared SEP, indicating a good stability of PV2 Mo10 in the architecture [30]. A close inspection of the bands of SEP compared with pure PV2 Mo10 reveals that the (P–O) and (M–O) have red shifts while the (M–Oc –M) and (M–Oe –M) have blue shifts (M = Mo or V, Oe : edge-sharing oxygen, Oc : corner-sharing oxygen). The shifts of band positions probably results from the mutual effect of the presence of counter ions and the chemical environment [31,32]. Obvious bands appearing at 2918 and 2850 cm−1 , which are attributed to the asymmetric and symmetric stretching of CH2 bonds, indicate that the arrangement of hydrocarbon chains in SEP is highly ordered or crystalline (see Fig. 4a) [32]. After the SEP was incorporated into the PVDF membrane, the bands which represent the stretching vibrations of P–O and M–Oc –M species at 1058 and 869 cm−1 are covered by a series of intensive characteristic bands of PVDF that appear at 1402, 1277, 1234, 1171, 1072, 881, and 841 cm−1 [33]. The other two bands at 945 and 797 cm−1 (marked by orange dashed circles in Fig. 4b), which are attributed to the (M–O) and (M–Oe –M), manifest that PV2 Mo10 was quite stable in the membrane fabrication process and finally maintained in the form of SEP in the membrane. 3.2. Dye removal activities of SEP and SEP-M Contrast experiments were carried out with DODA·Br and commercial GAC to evaluate the decolorization activity of the asprepared SEP. Equal loading amount of 31 mg was added into 3 portions of 60 mL 15 ppm RB5 aqueous solution at 45 ◦ C with stirring at the same rate. The dye decolorization was monitored by UV–vis absorbance and the decolorization efficiency was calculated using the following formula: Decolorization efficiency (%) =
A0 − At × 100 A0
where A0 is the absorbance before decolorization, At is the absorbance after certain time t of decolorization. It can be seen in Fig. 5 that SEP exhibited the best decolorization activity toward RB5. 87.71% decolorization efficiency was achieved only in 10 min and 95.02% decolorization efficiency was attained in the testing period of 45 min. In contrast with the obvious decolorization of RB5 solution with SEP, the solution maintained turbid with DODA·Br powders suspended in the whole period of 45 min. RB5 solution was decolorized 36.51% in the first 5 min, which could be attributed to the ion exchange mechanism of DODA·Br toward
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Fig. 6. Dye decolorization activities of different substrate. (i) Pure PVDF membrane by stirring (at 200 rpm), (ii) SEP-M by circulating filtration (at 50 mL min−1 ) and (iii) SEP-M by stirring (at 200 rpm). Reaction conditions: dye (15 ppm, 60 mL), 18 cm2 SEP-M with SEP content 26 mg, 18 cm2 PVDF membrane, 45 ◦ C. The inset photograph shows the decolorization of dye with SEP-M.
RB5 in which anionic dye molecules were trapped by cationic DODA+ long chain. Then the dye concentration stayed fluctuating around 36.51% for the next 40 min, which is probably rendered by the remaining small amount of DODA·Br that cannot be centrifuged and separated from the testing samples. For the commercial GAC, it exhibited a relatively stable decolorization rate in the first 45 min, and a longer testing time of 120 min was thus adopted. The adsorption rate and capacity of GAC are limited by the exposed surface area and the diffusion rate of the dye. Finally, the decolorization efficiency of 50% was observed with the GAC. The above results indicate that a remarkable enhancement on the efficiency of dye removal has been achieved by the SEP. Given the limited application of SEP in the powder form in practical operations, SEP-M was developed with corresponding performances analyzed. 18 cm2 SEP-M was loaded into the dye solution under stirring or sealed into a convective low membrane module driven by a peristaltic pump. It is indicative in Fig. 6 that the SEP-M also exhibited excellent dye removal activity in which the adsorption capacity of PVDF toward dye can be ignored. Similar trends of dye decolorization with SEP-M by stirring and by circulating filtration were attained, while the SEP-M with stirring mode rendered a slightly better performance in 120 min reaching a decolorization efficiency of 97.5%. It is probable that both the stirring rate and the recirculation rate play an important and similar role in the process. In the light of the similar performances achieved, SEP-M operated in the membrane module under the convective flow mode is more promising in practical applications because it possesses the property of simple scale-up and operation by stacking membranes to form the membrane plant under traditional membrane separation protocols. The as-prepared SEP-M was found regenerable under the treatment with HNO3 or HCl. It can be seen in Fig. 7 that the SEP-M regenerated 3 times maintained very stable performance with the final dye removal achieved by nearly 100%. The physical appearance of the regenerated SEP-M remains constant after acid treatment, indicating that it is well chemical resistant even under highly acidic conditions with full regeneration of dye removal efficiency. These results suggest that the SEP-M is promising in practical applications and a continued optimization of this strategy may yield further progress, since little dye removal efficiency was sacrificed in the recovery process of the SEP-M. 3.3. Mechanism studies The adsorption effect may be the reasonable explanation considering the color change of the membrane from yellow to dark blue
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Fig. 7. Dye decolorization activities of SEP-M regenerated for different times. Reaction conditions: dye (15 ppm, 60 mL), 18 cm2 SEP-M with SEP content 26 mg, 45 ◦ C, stirring at 200 rpm. The inset photograph displays (a) the fresh SEP-M, (b) SEP-M after the dye removal test and (c) the regenerated SEP-M.
in the decolorization tests. To elucidate the existence of dye on the membrane, TGA was conducted. For the thermogravimetric curve of the original SEP-M (see Fig. 8a), the most obvious weight loss appears at 200–450 ◦ C, which is assigned to the decomposition of the SEP and PVDF. The corresponding derivative weight loss displays three distinct maxima of weight losses at 281.76, 398.74, and 408.37 ◦ C. By contrast, the TGA curve of the reacted SEP-M changes with the emergence of an obvious step of weight loss at 360–500 ◦ C (see Fig. 8b). The corresponding derivative weight loss, which is clearly different from the original one, indicates the presence of RB5. FTIR analysis was also conducted but valuable information was not obtained, because the characteristic N N stretching of the RB5 is very weak and overlapped by the C H deformation vibration [34]. Although the adsorption effect has been proved to some extent by the above TGA results, the catalytic degradation effect where the SEP functions as catalyst could not be excluded in the light of the redox nature of PV2 Mo10 [35,36]. To go a further step, contrast experiments were conducted under oxygen-free conditions and in air, respectively. It was found that the similar decolorization efficiencies were obtained in the two different conditions, meanwhile the final solution treated by the SEP in air turned light pink after 120 min (see Fig. 9). These results suggest that the adsorption effect of the SEP plays a dominant role in the dye decolorization process while partial oxidation of the dye also occurs. To investigate the roles of SEP and oxygen in the dye oxidation process, XPS
Fig. 9. Dye decolorization activities of (i) SEP-M with cationic dye, reaction conditions: safranin T (30 ppm, 60 mL), 18 cm2 SEP-M with SEP content 26 mg, 45 ◦ C, stirring at 200 rpm; (ii) SEP with N2 and (iii) SEP with air, reaction conditions: SEP 0.1 g, RB5 (80 ppm, 150 mL), 45 ◦ C. The inset photograph shows the decolorization of dye by SEP with air.
analysis was carried out on the original SEP and reacted SEP, respectively. The result indicated that both Mo and V showed unchanged valence, which suggests that the SEP plays a role as catalyst and at the same time oxygen is as oxidant. Consequently, it is deduced that the SEP mainly exhibits an excellent adsorption capacity for the dye RB5 followed by a limited catalytic activity toward the dye oxidation. In contrast with SEP, the catalytic property of SEP-M was hardly observed. For SEP-M, the SEP is trapped in the membrane matrix, which may inhibit the catalytic activity stemming from PV2 Mo10 clusters, therefore the treated dye solution would not turn pink even for a longer time. To further understand the adsorption mechanism of SEP toward the dye RB5, BET, XPS, zeta potential and TOC/TN measurements were also well carried out. The isotherms for nitrogen adsorption and desorption of PV2 Mo10 , DODA·Br and SEP conducted at 77 K were shown in Fig. 10. PV2 Mo10 and DODA·Br exhibited similar Brunauer–Emmett–Teller (BET) surface areas, which were 6.87 and 6.75 m2 /g, respectively. Interestingly, the BET surface area of SEP dropped significantly to 0.23 m2 /g. It implies that the nanoparticles formed by PV2 Mo10 and DODA consist of close packed layers with low porosity. Thus the excellent dye adsorption activity is not likely coming from the common physical adsorption, but the ion exchange mechanism. On the other hand, close packed lamellar structures of the SEP particles suggest large inner steric hindrance and the ion exchange should principally take place on the surface
Fig. 8. TGA of (a) original SEP-M and (b) reacted SEP-M with dye.
L. Yao et al. / Journal of Hazardous Materials 280 (2014) 428–435
433
Fig. 10. Nitrogen sorption isotherms of PV2 Mo10 , DODA·Br and SEP. The inset shows enlarged nitrogen sorption isotherm of SEP. Table 1 Element composition of the original SEP and reacted SEP observed by XPS analysis. Elements
Br Mo C N V O
Composition (At%) Original SEP
Reacted SEP
0.7 3.1 81.6 0.8 0.4 13.4
0.3 2.3 84.3 0.9 0.4 11.8
of these particles. In this case, the excellent dye adsorption activity benefited from the nano-sized nature of the SEP particles. It can be seen in Fig. 9 that the SEP-M just demonstrated adsorption activity toward anionic dye but not the cationic dye. Zeta potential analysis showed that after the addition of SEP into PVDF, the isoelectric point moved from pH 6.592 to 7.414, indicating that the SEP-M is positively charged compared with the pure PVDF membrane. So it is proposed that the ion exchange activity of SEP involves the trapping of anionic dye molecules with concomitant release of Br− ions. It is conjectured that SEP is not fully formulated as (DODA)5 PV2 Mo10 O40 even though the molar ratio of DODA·Br to H5 PV2 Mo10 O40 was controlled at 5:1. Traces of Br− ions were detected in SEP by XPS analysis as shown in Table 1. According to XPS results, the composition of SEP is approximately (DODA)6.41 H3.5 PV2 Mo10 O40 Br2.09 . Accurate calculation is not possible confined by the resolution and detection depth of the XPS analysis. N, V and Br can be weakly detected only from high resolution spectra, in which the values are not reliable to identify the formula of SEP. From Table 1, the atomic composition of Br in the SEP decreased from 0.7% to 0.3% after reacting with the dye, confirming the release of Br− ions into the environment in exchange with the anionic dye. The content of C and N increased slightly with the decrease of Mo and O, which implies the cover of dye onto the surface of the SEP. TOC/TN analysis further confirmed the supposition of ion exchange mechanism in the adsorption-desorption process. In the membrane regeneration treatment with acid, NO3 − or Cl− will exchange with anionic dye so as to desorb dye from SEP-M while keeping the valence balanced. In the following dye adsorption test, NO3 − or Cl− will leave SEP-M into the resultant solution. Evidence was provided by increased TN from the HNO3 regenerated SEP-M sample solution and disappearance of TN from the HCl regenerated SEP-M sample solution (see Fig. 11). On the other hand, it
Fig. 11. TOC and TN of dye feed solution (15 ppm, 60 mL) and dye solution treated with different materials (SEP, SEP-M, HNO3 regenerated SEP-M, and HCl regenerated SEP-M).
is noticed that the dye solution treated with SEP still displayed high TOC even though 95.02% decolorization was achieved, which may be attributed to the leaching of DODA into the solution. After calculation, only 0.28% leaching of SEP occurred to render the corresponding TOC enhancement. However, such a leaching phenomenon was not observed for the SEP-M system (see Fig. 11), as the SEP-M could possess a much more stable architecture where the SEP was confined in the polymer matrix, which suggests another advantage of SEP-M compared with SEP. The adsorption isotherm whose parameters express the surface properties and affinity of the dye RB5 on the SEP-M was also investigated. Langmuir and Freundlich isotherm models, which are the most commonly used, were utilized to fit the experimental data [37,38]. The Langmuir adsorption model is based on the assumption that maximum adsorption corresponds to a saturated monolayer of solute molecules without any interaction between molecules on the adsorbent surface. It can be described by the following equation: Ce 1 Ce = + qe Q0 Q0 b where qe is the amount adsored (mg/g), Ce is the equilibrium concentration of dye (ppm), and Q0 (mg/g) and b (L/mg) are Langmuir constants, which stand for the adsorption capacity and energy of adsorption, respectively.
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Table 2 Isotherm equation parameters of the dye RB5 on SEP-M. Parameters
Langmuir
SEP-M
Freundlich
Q0 (mg/g)
KL (L/g)
R2
RL
KF
n
R2
59.21
1.45
0.995
0.0115
47.18
19.32
0.820
The essential characteristics, the adsorption intensity, can be expressed by: RL =
1 1 + KL C0
where C0 (ppm) is the initial amount of dye. RL is reliably utilized to indicate the favorability of adsorption. The Freundlich adsorption model, which is an indicator of surface heterogeneity of the sorbent [39], can be expressed as: log qe = log KF +
1 log Ce n
where KF and n are the Freundlich constants. As listed in Table 2, Langmuir isotherm provides better linear regression (R2 = 0.995) than Freundlich equation (R2 = 0.820), indicating that the adsorption obeys the Langmuir model. The dimensionless equilibrium parameter, adsorption intensity, RL is used to predict the adsorption efficiency of the adsorption process. Values of RL < 1 represent favorable adsorption while values of RL > 1 represent unfavorable adsorption. The SEP-M showed RL = 0.0115, illustrating the favorable adsorption of dye on SEP-M. 4. Conclusions In this paper, a novel surfactant encapsulated polyoxometalate (SEP) has been synthesized by using cationic surfactant DODA·Br and Keggin-type H5 [PV2 Mo10 O40 ] through a simple ion-exchange reaction. The prepared SEP complex can self-assemble into welldefined nanospherical particles in the reaction solution. Anionic dye RB5 was utilized to evaluate the dye removal performances of SEP and an excellent dye removal activity was achieved. It was further incorporated into a PVDF membrane matrix to form the SEP incorporated composite membrane (SEP-M). SEP-M with 50 wt% SEP particles loading was successfully fabricated with SEP spherical particles smaller than 1 m embedded in the polymeric membrane matrix. It was found that desirable dye removal activity of SEP-M retaining the advantages of the membrane form was achieved. The used SEP-M can be easily regenerated by acid treatment without loss of dye removal efficiency. Detailed mechanism studies were carried out to elucidate the nature of dye decolorization by SEP and SEP-M. While the SEP mainly exhibits excellent adsorption capacity with potential catalytic ability coming from the redox nature of PV2 Mo10 , SEP-M follows Langmuir adsorption nearly without catalytic capability. The leaching problem was not observed with SEP-M. SEP-M is superior to the SEP because it can be easily separated, recycled, and scaled up by simply stacking into modules and operated without extra pressure supply. Acknowledgements We would like to thank the financial support from Singapore Membrane Technology Center (SMTC), Nanyang Technological University and NTU AcRF grant (RG76/12). We are also grateful to Singapore Economic Development Board for funding SMTC. The electron microscopy was performed at the Facility for Analysis, Characterization, Testing and Simulation (FACTS) in Nanyang Technological University, Singapore.
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