Benzene carboxylic acid derivatized graphene oxide nanosheets on natural zeolites as effective adsorbents for cationic dye removal

Journal of Hazardous Materials 260 (2013) 330–338

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Benzene carboxylic acid derivatized graphene oxide nanosheets on natural zeolites as effective adsorbents for cationic dye removal Yang Yu a , Bandaru N. Murthy a , Joseph G. Shapter a , Kristina T. Constantopoulos a , Nicolas H. Voelcker b , Amanda V. Ellis a,∗ a Flinders Centre for Nanoscale Science & Technology, School of Chemical and Physical Sciences, Flinders University, Sturt Road, Bedford Park, Adelaide, SA 5042, Australia b Division of Information Technology, Engineering and Environment, Mawson Institute, University of South Australia, Mawson Lakes, SA 5095, Australia

h i g h l i g h t s • Natural zeolites were modified with graphene oxide (GO) via a condensation reaction. • The GO was then modified using a diazonium salt to produce carboxy-GO/zeolite powder. • The kinetics of rhodamine B adsorption onto the carboxy-GO/zeolite was very fast.

a r t i c l e

i n f o

Article history: Received 27 February 2013 Received in revised form 20 May 2013 Accepted 21 May 2013 Available online 29 May 2013 Keywords: Natural zeolites Graphene oxide Adsorption kinetics Rhodamine B Diazonium salts

a b s t r a c t Graphene oxide (GO) nanosheets were grafted to acid-treated natural clinoptilolite-rich zeolite powders followed by a coupling reaction with a diazonium salt (4-carboxybenzenediazoniumtetrafluoroborate) to the GO surface. Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA) revealed successful grafting of GO nanosheets onto the zeolite surface. The application of the adsorbents for the adsorption of rhodamine B from aqueous solutions was then demonstrated. After reaching adsorption equilibrium the maximum adsorption capacities were shown to be 50.25, 55.56 and 67.56 mg g−1 for pristine natural zeolite, GO grafted zeolite (GO-zeolite) and benzene carboxylic acid derivatized GO-zeolite powders, respectively. The adsorption behavior was fitted to a Langmuir isotherm and shown to follow a pseudo-second-order reaction model. Further, a relationship between surface functional groups, pH and adsorption efficiency was established. Results indicate that benzene carboxylic acid derivatized GO-zeolite powders are environmentally favorable adsorbents for the removal of cationic dyes from aqueous solutions. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Dye effluents generated from textile printing and dye manufacturing often exhibit toxic effects on aquatic environments [1]. In order to remove these dyes, and other contaminants, physical, chemical and biological methods such as adsorption [2], coagulation [3], chemical oxidation [4], photodegradation [5], and aerobic or anaerobic treatment have been developed [6]. By far the most simple, efficient, cost-effective and versatile method amongst adsorption technologies is the use of solid porous materials, for example, activated carbon [7], aerogels [8], zeolites [2], clays [9] and kaolinite [10]. In particular, activated carbon is widely used for the removal of effluent dyes due to its high surface area, porous structure, and high adsorption efficiency and capacity

∗ Corresponding author. E-mail address: Amanda.Ellis@flinders.edu.au (A.V. Ellis). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.05.041

[11,12]. However, there is a high energy cost to production and issues associated with regeneration. Therefore, there has been an increasing focus on utilizing low-cost environmentally friendly materials for environmental remediation. One material in particular, natural zeolites, affords a cheap option to effluent clean-up. Natural zeolites are microporous aluminosilicate crystalline minerals with 3-dimensional frameworks of tetrahedrally coordinated SiO4 or AlO4 [13]. Various cations (e.g., Na+ , K+ , Ca2+ , and Mg2+ ) are loosely held in the zeolite pores, channels or cavities which can be exchanged with other organic compounds (e.g., dyes, humic acids or phenolic compounds) and inorganic cations (e.g., Hg2+ , Pb2+ , Ag+ , Cu2+ , Cd2+ , Cr2+ , Co2+ or Mn2+ ) [14–19]. The use of natural/modified zeolites for environmental remediation and wastewater treatment applications has been gaining significant interest due to their microporosity, moderate/high surface area, ion-exchange properties, low cost and worldwide occurrence [20,21]. Recent studies have shown that natural zeolites have a good affinity for basic dyes in aqueous solution

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Scheme 1. Three possible interactions between carboxy-GO/zeolite sample and rhodamine B at (a) pH = 3 and (b) pH = 5.

(e.g., methylene blue [2,22], reactive red 195 [23], reactive blue 21 [23] and rhodamine B [22,24]). Potentially, the dye adsorption performance of natural zeolites could be enhanced by surface modification and/or functionalization. A good candidate for this is graphene oxide (GO), a single layer nanosheet of oxygenated graphene sheets [25]. The oxygen functionalities in GO include epoxy and hydroxyl groups, which lie on the basal planes of the GO sheets while carboxyl groups are mostly found at the GO edges [25]. These oxygen containing functional groups make the GO nanosheets negatively charged and hydrophilic. Moreover, GO has a strong propensity to interact with positively charged species such as metal ions [26], dyes [27–29] and biomolecules [30]. Further modification of GO can be achieved using diazonium salts which are covalently functionalized on the electron-rich sp2 carbon domains of GO nanosheets [25,31,32]. Additional functional group density can be imparted to the surface (e.g., SH, NH2 and COOH groups) and potentially enhance GO’s performance in the adsorption of dyes and other organic compounds. Here, we functionalize the surface of a clinoptilolite-rich natural zeolite with GO nanosheets followed by the covalent attachment of a diazonium salt, 4-carboxybenzenediazoniumtetrafluoroborate, to the GO surface. Thus producing a so-called carboxy-GO/zeolite adsorbent. The adsorption affinity, kinetics and equilibrium loading capacity of both the pristine natural zeolites and the carboxy-GO/zeolite adsorbents toward aqueous solutions of rhodamine B were investigated. We also show the impact of surface functionality, initial dye concentration, pH and dye exposure time on the adsorption of rhodamine B onto the adsorbents.

2. Experimental 2.1. Materials Australian natural zeolite (diameter: 2 mm, chemical composition: 68.26% SiO2 , 12.99% Al2 O3 , 1.37% Fe2 O3 , 0.83% MgO, and 0.23% TiO2 ) was purchased from Zeolite Australia Limited (New South Wales, Australia) and ground in a mortar and pestle before use.

GO was purchased from Graphene Supermarket (BET surface area: ∼400 m2 g−1 ), USA, and used as received. Rhodamine B (molecular weight 479.01 g mol-1 , CAS Number 81-88-9 and molecular structure shown in Scheme 1), 4-aminobenzoic acid, fluoroboric acid and sodium nitrite were purchased from Sigma-Aldrich (Australia). Hydrochloric acid, sodium hydroxide, acetone and ethanol were supplied from Chem Supply (Australia). All aqueous solutions were prepared using Milli-Q water (18.2 M cm at 25 ◦ C). 2.2. Acid treatment of natural zeolite powder Zeolite powder (0.5 g) was suspended in Milli-Q water (20 cm3 ). 32% (v/v) aqueous hydrochloric acid (0.1 cm3 ) was added and the suspension stirred for 30 min. The resulting acid-treated zeolite powder was then separated via centrifugation at 3000 rpm for 5 min, followed by successive washings and centrifugation with Milli-Q water. 2.3. Functionalization of acid-treated zeolite powder with GO nanosheets GO powder (5 mg) was dispersed in Milli-Q water (5 cm3 ) and sonicated (Unisonics sonicator, FXP10MH) for 1 min. The GO dispersion (1 cm3 , 40 mg dm−3 ) was added to the acid-treated zeolite powder (∼0.5 g) in Milli-Q water (10 cm3 ). The suspension was sonicated for 10 s, stirred for 30 min and then placed in an oven at 110 ◦ C for 12 h. The GO/zeolite powder was cooled to room temperature for further use. 2.4. Synthesis of 4-carboxybenzenediazoniumtetrafluoroborate (carboxy-BDTFB) According to Blanch et al.’s procedure [33], 4carboxybenzenediazoniumtetrafluoroborate (carboxy-BDTFB) was synthesized by dissolving 13.72 g 4-aminobenzoic acid in a mixture of 34 cm3 48% fluoroboric acid and 40 cm3 Milli-Q water and the solution was cooled down to 0 ◦ C. 6.8 g sodium nitrite was dissolved in 15 cm3 Milli-Q water and was added drop-wise to the solution, stirred for 30 min. The precipitate was collected via vacuum filtration and purified by dissolving in a minimum amount

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of acetone, followed by re-precipitation by the addition of diethyl ether. The product carboxy-BDTFB was obtained as a pale orange powder and stored in a sealed brown vial at 0 ◦ C. 2.5. Surface modification of GO-zeolites with carboxy-BDTFB Carboxy-BDTFB (0.08 g) was dissolved in Milli-Q water (16 cm3 ) via brief sonication (<30 s). GO-zeolite powder (0.6 g) was then added into the solution and the suspension was sonicated for 5 s and stirred for 16 h in a sealed brown vial. The diazonium functionalized GO-zeolite powder was washed 3 times with acetone, ethanol and Milli-Q water using centrifugation at 3000 rpm for 5 min each time. The resulting carboxy-GO/zeolite powder was stored in a sealed vial for further use. 2.6. Characterization The morphology of natural zeolite and GO-zeolite samples was determined by focused ion beam scanning electron microscopy (FIB/SEM) using a Helios NanoLab dual-beam FIB/SEM operating at 10 kV. The particle size distributions of aqueous solutions (1% (w/v)) of natural zeolite, GO-zeolite and carboxy-GO/zeolite samples were determined using a Malvern Mastersizer X, UK. Zeta potentials of dilute suspensions of natural zeolite, GOzeolite and carboxy-GO/zeolite samples were determined using a Nano-ZS Zetasizer (Malvern, UK) in electrophoretic light scattering mode. Suspensions (0.01% (w/w)) were made by adding dry natural/functionalized zeolite (0.25 g) powders to KNO3 (50 cm3 , 10−3 M) and stirred for 20 min. After stirring each suspension was allowed to rest for 5 min and the supernatant was collected via transfer pipetting. The pH of the supernatant was adjusted between 1.5 and 9.0, using sodium hydroxide (0.1 M) or hydrochloric acid (0.1 M), for zeta potential measurements. Fourier transform infrared (FTIR) spectroscopy was used for functional group analysis of the natural zeolite, GO-zeolite and carboxy-GO/zeolite samples. A FTIR Nicolet iN10 MX FT-IR Microscope spectrometer was used to record FTIR spectra. Each sample was made into a KBr pellet sample using to dry KBr at a 1:100 ratio (sample:KBr). FTIR spectra were then recorded on the KBr pellets using 128 scans between 4000 and 650 cm−1 with a resolution of 4 cm−1 in absorbance mode, with background subtraction. Raman measurements of the natural zeolite, GO-zeolite and carboxy-GO/zeolite samples were carried out on an Alpha300RS microscopy/spectroscopy instrument (Witec, Germany) using a 532 nm laser with a maximum power of <60 mW. Single spectra were recorded at an acquisition time of 10 s. Thermogravimetric analysis (TGA) was carried out using a TA Instruments TGA 2950, USA. The natural zeolite, GO-zeolite and carboxy-GO/zeolite samples (∼20 mg powder) were heated at 10 ◦ C min−1 from 30 to 800 ◦ C in a N2 atmosphere. Specific surface area (Brunauer–Emmett–Teller, BET) analysis of the natural zeolite, GO-zeolite and carboxy-GO/zeolite samples (∼0.5 g powder) were measured using a BEL Sorp Max (Japan) with nitrogen adsorption gas at 77 K and a pressure p/p0 = 1 × 10−8 . 2.7. Adsorption experiments In order to investigate the adsorption behavior of the natural zeolite, GO-zeolite and carboxy-GO/zeolite adsorbents adsorption isotherm experiments were carried out by varying the initial rhodamine B concentration from 1 to 500 mg dm−3 . Standard solutions of rhodamine B (1–500 mg dm−3 ) were prepared by dilution of a 500 mg dm−3 rhodamine B stock solution with Milli-Q water. The adsorbents (0.01 g) were dispersed in each of the standard aqueous rhodamine B solution (10 cm3 ) and the suspension magnetically

Fig. 1. Raman spectra of (a) pristine natural zeolite, (b) GO-zeolite and (c) carboxyGO/zeolite samples.

stirred for 60 min to ensure equilibrium. The suspensions were then filtered through a 0.45 ␮m membrane filter (Millipore, nylon membrane) and the filtrate analyzed using a UV–visible spectrophotometer (Varian Cary 50 scan spectrophotometer) at a wavelength of 554 nm. The rhodamine B concentration was then calculated from a calibration curve. The effect of pH (3.0–9.0), contact time (0–60 min) and initial rhodamine B concentrations (1–500 mg dm−3 ) on the adsorption kinetics and isotherms were studied, 3 replicates of each were performed. Sodium hydroxide (0.1 M) and hydrochloric acid (0.1 M) were used to control the pH of each suspension. The rhodamine B adsorption capacity q (mg g−1 ) and adsorption efficiency were calculated from Eqs. (1) and (2), respectively [34]: q=

(C0 − C)V W

Adsorption efficiency =

(1) C0 − C × 100% C0

(2)

where C0 and C is the initial and equilibrium concentration of rhodamine B (mg dm−3 ), respectively; V is the volume of the solution (dm3 ) and W is the dry mass of the adsorbent (g). 3. Results and discussion 3.1. Structural and chemical characterization of natural zeolite, GO-zeolite and carboxy-GO/zeolite samples Fig. 1(a)–(c) shows the Raman spectra of natural zeolite, GOzeolite and carboxy-GO/zeolite samples, respectively. A prominent band at 463 cm−1 for the natural zeolite is attributed to the characteristic Si O and of tetrahedral silica in the zeolite structure (Fig. 1(a)) [35]. Additional bands in the low frequency region at 380 cm−1 and 315 cm−1 are assigned to the spectroscopic signatures of cyclo-aluminosilicates containing six or more Si atoms [36]. In Fig. 1(b), two characteristic peaks at 1353 cm−1 and 1609 cm−1 were observed for the GO-zeolite sample, which are attributed to the D-band (structural defects or partially disordered structures of sp2 domains) and G-band (in-plane vibration mode of sp2 domains) of GO, respectively [37]. After surface functionalization of the GO with the carboxy-diazonium salt the D-band and G-band were red-shifted to 1349 cm−1 (4 cm−1 ) and 1604 cm−1 (5 cm−1 ), respectively (Fig. 1(c)). The red-shift can be ascribed to the disruption of the GO sp2 domains by the additional chemical bonds between carbon atoms of the GO and the newly attached carboxylated benzene ring [37,38]. Furthermore, the D/G intensity ratio was observed to increase slightly for the carboxyGO/zeolite (ID /IG = 0.993) than for the GO-zeolite (ID /IG = 0.987)

Y. Yu et al. / Journal of Hazardous Materials 260 (2013) 330–338

Fig. 2. FTIR spectra of (a) pristine natural zeolite, (b) GO-zeolite and (c) carboxyGO/zeolite samples.

sample, indicating a slight increase in disordered carbon in the carboxy-GO/zeolite sample [37,39]. Fig. 2(a)–(c) shows the FTIR spectra of the natural zeolite, GO-zeolite and carboxy-GO/zeolite samples, respectively. For the natural zeolite, a broad peak was observed at 1160 cm−1 due to Si O Si asymmetric stretching, while the peak at 814 cm−1 is ascribed to Al O Si stretching vibrations (Fig. 2(a)) [40,41]. The broad peak at 3664 cm−1 and a peak at 1637 cm−1 are attributed to the O H stretching and bending vibrations of water molecules, respectively, indicating entrapped water in the natural zeolite structure [41,42]. In the case of the GO-zeolite (Fig. 2(b)) and carboxy-GO/zeolite (Fig. 2(c)) samples, additional bands were observed at 1757 cm−1 and 1622 cm−1 , attributed to the carbonyl stretching vibrations and the skeletal vibrations of the graphitic domains from the basal plane of GO, respectively [43]. In addition, the spectrum of the carboxy-GO/zeolite sample (Fig. 2(c)), shows two new bands at 1521 cm−1 and 1445 cm−1 associated with the asymmetric and symmetric stretching modes of carboxylate groups [44]. The FTIR results indicate the successful functionalization of the natural zeolite powders with GO nanosheets, followed by modification of the GO surface with a benzene carboxylic acid. In Fig. S3, the new band at 1345 cm−1 associated with the stretching mode vibrations of the C N species of the rhodamine B molecules, was found in the rhodamine B loaded natural zeolite, GO/zeolite and carboxy-GO/zeolite samples, which indicates the incorporation of rhodamine B molecules onto the natural/functionalized zeolite surface [68].

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Fig. 3(a) shows the FIB/SEM image of the natural zeolite samples showing a highly microporous drusy texture with small crystalline laminar crystals on the surface [19]. Similar images were obtained for the acid-treated zeolites (data not shown). After modification with GO, Fig. 3(b) indicates that the GO nanosheets partially cover the zeolite surface. These GO nanosheets remained on the zeolite surface even after multiple washings and sonication in water. This observation suggests that the GO is strongly bound to the zeolite surface. This is most likely through a condensation reaction occurring under the conditions used (110 ◦ C for 12 h) between carboxyl groups on the GO surface and hydroxyl groups on the acid-treated zeolite surface forming a covalent ester linkage. Fig. 3(c) shows a high resolution image of the GO coated zeolites revealing the randomly aggregated and crumpled nanosheets on the zeolite surface. The TGA of the natural zeolite, GO-zeolite and carboxyGO/zeolite samples are shown in Fig. S2a–c. The weight loss at 120–200 ◦ C is attributed to the desorption of entrapped water in the zeolite matrix [45]. The weight loss of 5.5 ± 0.2 wt%, 5.8 ± 0.1 wt% and 6.4 ± 0.3 wt% was observed from 200 to 500 ◦ C for the natural zeolite, GO-zeolite and carboxy-GO/zeolite samples, respectively. The extra weight loss of 0.3 wt% and 0.9 wt% compare to the natural zeolite is attributed to the decomposition of organic moieties such as epoxy, carbonyl and carboxyl groups from the GO-zeolite/carboxy-GO/zeolite surface [46]. The 0.6 wt% weight loss of the carboxy-GO/zeolite sample compared to that of the GOzeolite sample indicates the presence of additional COOH groups on the GO surface. The weight loss at 500–800 ◦ C is associated to the decomposition of carbon from the GO basal planes [47] and also to the removal of silanol groups [48] in the zeolite structure. The BET surface area of the pristine natural zeolite powders was determined to be 11.97 ± 0.80 m2 g−1 , which is a low surface area compared to other synthesized adsorbents [7,49]. Specific surface areas of the GO-zeolite and carboxy-GO/zeolite particles were determined using BET analysis to be 17.81 ± 1.50 and 17.86 ± 1.30 m2 g−1 , respectively, which is ∼49% higher than that of the natural zeolite material. The increase in surface area is due to the coating of GO nanosheets (surface area ∼400 m2 g−1 ) on the surface of the zeolite [50]. In order to determine the surface charge on the natural zeolite, GO-zeolite and carboxy-GO/zeolite samples zeta potential analysis was performed, data is shown in Fig. 4(a)–(c), respectively. There is a clear monotonic increase in the magnitude of the zeta potential with increasing pH from 1.5 to 9.0 (Fig. 4(a)–(c)) with all three samples exhibiting a negative zeta potential over the pH range studied. The negative charge could result from the substitution of Si4+ ions by the Al3+ ions in the natural zeolite lattice

Fig. 3. SEM images of (a) pristine natural zeolite, (b) GO-zeolite and (c) GO-zeolite samples at high magnification.

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Fig. 4. Zeta potentials of (a) pristine natural zeolite, (b) GO-zeolite and (c) carboxyGO/zeolite powders as a function of pH in 10−3 M KNO3 solution (pH was decreased from 9.0 to 1.5).

and also from the broken Si O Si bonds at the particle surface [51,52]. Importantly, the zeta potentials become more negative for the GO-zeolite (Fig. 4(b)) and carboxy-GO/zeolite (Fig. 4(c)) samples [53]. This is expected due to the increase in oxygenated species on the zeolite surface. Notably, at pH > 4.2, the zeta potential of the carboxy-GO/zeolite samples (ranging from −41.9 to −52.9 mV) is more negative than that of the pristine natural zeolite (ranging from −20.7 to −44.3 mV) and the GO-zeolite particles (ranging from −35.4 to −42.8 mV). This increased negative charge on the carboxyGO/zeolite samples is due to the deprotonated COOH groups on the surface at pH > pKa (c.f. benzoic acid (∼4.2)) [54]. When the pH < pKa, the COOH groups are protonated and become neutral. In this case the zeta potential of the carboxy-GO/zeolite particles did not change significantly from that of the GO-zeolite particles (see Fig. 4(a)–(c)). 3.2. Adsorption studies of rhodamine B The adsorption capacities of GO-zeolite and carboxy-GO/zeolite powders were investigated for their effectiveness in the extraction of rhodamine B from aqueous solution and compared to pristine natural zeolites at pH 3–9 (Fig. 5(a)–(c), Fig. S4). Pristine natural zeolite powders showed a maximum adsorption capacity of 3.08 mg g−1 at pH = 3.0 (Fig. 5(a)). This is the pH at which rhodamine B is fully protonated (pKa for the aromatic carboxylic acid group present on the rhodamine B is ∼4.2) [55,56]. At this pH the adsorption is due to the interaction of the positively charged (cationic) rhodamine B molecules with the negatively charged surface of the

Fig. 5. Effect of pH on the adsorption of aqueous rhodamine B by (a) pristine natural zeolite, (b) GO-zeolite and (c) carboxy-GO/zeolite powders. Initial concentration of rhodamine B was 5.0 mg dm−3 (contact time: 60 min).

natural zeolite via electrostatic interactions (Fig. 5(a)). After surface functionalization of the natural zeolite the adsorption capacities at pH = 3.0 of the GO-zeolite and carboxy-GO/zeolite powders were determined to be 3.34 mg g−1 and 4.13 mg g−1 , Fig. 5(b) and (c), respectively. This increase in adsorption capacity after functionalization of the natural zeolite is again due to the enhanced electrostatic interactions between the adsorbents and adsorbate as the surface of these functionalized powders are more negatively charged than that of the pristine zeolite powders (see Scheme 1(a)) and zeta potential data (Fig. 4(a) and (c)). At pH = 5.0 rhodamine B transforms from a cationic form to a zwitterionic form due to the COOH groups on the rhodamine B becoming deprotonated [55,57]. This leads to an electrostatic repulsion between the rhodamine B and all the three negatively charged adsorbents (Scheme 1(b)), thereby decreasing the adsorption capacities of all samples at this pH (Fig. 5(a)–(c)), and most notably the natural zeolite. Aside from electrostatic interactions between the adsorbent and adsorbate there are also potential hydrophobic interactions (␲–␲ stacking) and hydrogen bonding interactions that can occur between the GO or the carboxylic acid groups on the GO surface with rhodamine B (Scheme 1(a) and (b)) [55]. Hydrophobic interactions can occur between the hydrophobic basal planes of the GO nanosheets with the aromatic rings of the rhodamine B leading to strong ␲–␲ stacking interactions at lower pH values (Scheme 1(a)) [58]. This in turn leads to a higher adsorption capacity (3.34 mg g−1 in Fig. 5(b), 4.13 mg g−1 in Fig. 5(c)) at pH = 3.0 than at higher pH value (e.g., pH = 5.0) (2.25 mg g−1 in Fig. 5(b), 3.01 mg g−1 in

Fig. 6. Effect of initial concentration (a) 2.5 mg dm−3 and (b) 5.0 mg dm−3 on the adsorption capacities of (i) pristine natural zeolite, (ii) GO-zeolite and (iii) carboxy-GO/zeolite powders, pH = 3.0.

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Table 1 Pseudo-first order and pseudo-second order kinetic parameters of rhodamine B (RhB) adsorption onto pristine natural zeolite, GO/zeolite and carboxy-GO/zeolite powders. Sample

Zeolite GO-zeolite Carboxy-GO/zeolite

RhB (mg dm−3 )

2.5 5.0 2.5 5.0 2.5 5.0

qe (exp) (mg g−1 )

2.15 3.02 2.27 3.34 2.50 4.13

Pseudo-first order model qe (cal) (mg g 1 )

R2

k2 (g mg−1 min−1 )

qe (cal) (mg g−1 )

R2

0.139 0.107 0.179 0.207 0.207 0.117

0.89 1.15 0.97 1.49 1.03 1.58

0.896 0.792 0.942 0.946 0.905 0.851

0.458 0.332 0.448 0.284 0.418 0.246

2.18 3.04 2.30 3.38 2.54 4.18

0.999 0.999 1.000 0.999 1.000 1.000

Fig. 5(c)) [58]. Importantly, at low pH (<4.2), the COOH groups on the GO-zeolite and carboxy-GO/zeolite samples are protonated and they can interact with the protonated —COOH groups on the rhodamine B through hydrogen bonding (Scheme 1(a)). Therefore at low pH the carboxy-GO/zeolite has a higher adsorption capacity of rhodamine B than that of GO-zeolite sample, which may be due to more hydrogen bonds forming between rhodamine B and carboxy-GO/zeolite sample which has more COOH groups on the surface after benzene carboxylic acid derivatization (Fig. 5(c)). As the pH was increased to greater than 5.0, the COOH groups become deprotonated and the hydrogen bonding between the COOH groups becomes weaker (Scheme 1(b)) [55], leading to lower adsorption capacities of rhodamine B due to electrostatic repulsions (2.25 mg g−1 in Fig. 5(b), 3.01 mg g−1 in Fig. 5(c)). 3.3. Adsorption kinetics Fig. 6(a) and (b) shows the adsorption kinetics of rhodamine B on natural zeolite, GO-zeolite and carboxy-GO/zeolite powders at initial concentrations of 2.5 and 5.0 mg dm−3 , respectively (for initial concentrations of 100 mg dm−3 : Fig. S5). At both initial rhodamine B concentrations (2.5 and 5.0 mg dm−3 ) the adsorption occurred rapidly within the first few minutes of exposure, reaching an equilibrium within 60 min. There was also a significant increase in the equilibrium adsorption capacity (qe ) for the three adsorbents studied (from 2.15, 2.27 and 2.50 mg g−1 to 3.01, 3.34 and 4.13 mg g−1 for the natural zeolite, GO-zeolite and carboxy-GO/zeolite powders, respectively) when the rhodamine B initial concentration was increased from 2.5 to 5 mg dm−3 . This is due to a stronger driving force for mass transfer and subsequent surface adsorption onto the zeolite powders at higher concentrations [59]. In order to evaluate the adsorption capacity of rhodamine B onto the surface of zeolite, GO-zeolite and carboxy-GO/zeolite powders both pseudo-first and second order empirical power law models were used. Empirical first and second order kinetic models are expressed by Eq. (4), for reaction exponents n = 1 and 2, respectively [60]: dqt = kn (qe − qt )n dt

(4)

where qt (mg g−1 ) and qe (mg g−1 ) represent adsorption capacity at contact time t and at equilibrium, respectively. k1 and k2 are the first and second order kinetic rate constants, respectively. Integrating Eq. (4) with the boundary conditions t = 0, qt = 0 and t = t, qt = qt to the linear form gives Eqs. (5) and (6), respectively: log(qe − qt ) = log qe − t 1 t = + qt qe k2 q2e

k1 t 2.303

Pseudo-second order model

k1 (min−1 )

(5)

(6)

The rate constants k1 and k2 may be determined from the slope of linear plots of log(qe − qt ) versus t and the intercept of the linear plot between t/qt versus t, respectively.

Table 1 indicates that the experimental data is better fitted to a pseudo-second order model (R2 > 0.999) than to a pseudofirst order model (R2 > 0.792), for the adsorption of rhodamine B onto all natural and functionalized zeolite powders. Furthermore, when using the pseudo-second order model the estimated qe values (Table 1) are in good agreement with the experimental values qe (exp), respectively. Thus, an empirical second order model provides a more suitable description of the adsorption kinetics of rhodamine B onto pristine zeolite, GO-zeolite and carboxyGO/zeolite powders. The k2 data (Table 1) shows that the rate of initial adsorption for the pristine/functionalized powders was much higher at a low rhodamine B concentrations (2.5 mg dm−3 ). In fact the adsorption process reaches equilibrium faster than at higher rhodamine B concentrations (5.0 mg dm−3 ). This may be due to the fact that the adsorption of rhodamine B onto saturated active sites of pristine natural and functionalized zeolite powders is the rate-limiting step at a higher rhodamine B concentration [61]. For the GO nanosheets coated zeolite adsorbents, its rate constant k2 (0.246–0.448 g mg−1 min−1 ) for rhodamine B adsorption is much lower than those of the pristine zeolite powders (0.332–0.458 g mg−1 min−1 ). This may be due to the GO nanosheets on the surface of GO-zeolite and carboxy-GO/zeolite samples hindering the transport of rhodamine B into the inner channels of zeolite, thus slowing down the adsorption kinetics for these samples. In contrast the pristine zeolite surface is uniform and porous, which enables the adsorption reaction to proceed faster. 3.4. Adsorption isotherms Fig. 7(a) and (b) shows the adsorption isotherms of rhodamine B onto different adsorbents at pH = 3.0 and 5.0. The adsorption capacity for rhodamine B increases with the increasing equilibrium rhodamine B concentration. In particular, the carboxyGO/zeolite sample has the highest adsorption capacity, estimated as 52.68 mg g−1 at an initial concentration of 500 mg dm−3 at pH = 3.0 (Fig. 6(a)), compared to the other two adsorbents. Upon increasing the pH to 5.0, the carboxy-GO/zeolite sample showed an adsorption capacity of 32.03 mg g−1 at an initial concentration of 500 mg dm−3 , which is approximately twice as high as that of the pristine natural zeolite powders (14.42 mg g−1 , Fig. 6(b)). Langmuir and Freundlich isotherms are commonly used to predict equilibrium parameters for the molecular adsorption at interfaces [60]. The Langmuir model assumes monolayer adsorption onto homogeneous active sites on adsorbents. This is expressed using Eq. (7) [62]. Ceq Ceq 1 = + q qmax qmax b

(7)

where qe and Ceq are the adsorption capacity (mg g−1 ) and rhodamine B concentration (mg dm−3 ) at equilibrium, respectively. b and qmax are the Langmuir constant (dm3 mg−1 ) and maximum adsorption capacity (mg g−1 ) determined by the intercept and slope of the linear plot of Ceq /q versus Ceq .

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Table 2 Langmuir and Freundlich isotherm parameters at (a) pH = 3.0 and (b) pH = 5.0. Sample

Langmuir model

Freundlich model

b (dm3 mg−1 )

qmax (mg g−1 )

R2

Kf (dm3 mg−1 )

n

R2

pH = 3.0 Zeolite GO-zeolite Carboxy-GO/zeolite

0.007169 0.007751 0.007851

50.25 55.56 67.56

0.999 0.998 0.995

2.086 2.543 4.152

2.044 2.096 2.385

0.989 0.987 0.992

pH = 5.0 Zeolite GO-zeolite Carboxy-GO/zeolite

0.00700 0.01098 0.02156

18.18 23.42 34.13

0.988 0.989 0.991

0.290 0.678 2.467

1.516 1.725 2.270

0.985 0.977 0.983

Fig. 7. Adsorption isotherms of rhodamine B onto (i) pristine natural zeolite, (ii) GO-zeolite and (iii) carboxy-GO/zeolite powders at (a) pH = 3.0 and (b) pH = 5.0. The dash curves are the Langmuir prediction based on the experimental data.

The Freundlich isotherm, which is based on the adsorption onto heterogeneous surface with uniform energy with no restriction to the formation of a monolayer, may be expressed as [63]: log qe = log Kf +

1 log Ceq n

(8)

where Kf and 1/n is the Freundlich adsorption constant (dm3 mg−1 ) and adsorption intensity (dimensionless). The parameters may be respectively calculated from the intercept and slope of the plot of log qe versus log Ceq . From Table 2, the data at pH = 3.0 and 5.0 shows that the regression coefficients (R2 ) of both the Langmuir and Freundlich models are all greater than 0.95, indicating that both of the models fit well. However, for the Langmuir model (R2 = 0.988–0.999) for the three adsorbents (pristine natural zeolite, GO-zeolite and carboxy-GO/zeolite) appeared to fit better than the Freundlich model (R2 = 0.977–0.992). This suggests that the best description of Table 3 Comparison of BET surface areas and maximum adsorption capacities for rhodamine B of different adsorbents. Adsorbent type

BET surface area (m2 g−1 )

Maximum adsorption capacity (mg g−1 )

Reference

Zeolite Zeolite MCM-22 Red mud Fly ash Kaolinite Reduced GO/ZnO GO Activated carbon Activated carbon GO-zeolite Carboxy-GO/zeolite

16 40 108 8.9 – – 23.3 522.7 476.8 17.8 17.9

13.22 52.69 5.56 10 46.08 32.6 154.8 263.85 28.49 55.56 67.56

[24] [49] [65] [66] [10] [67] [50] [7] [57] This work This work

the observed adsorption equilibrium behavior of rhodamine B onto the three adsorbents (Fig. 7(a) and (b)) is a Langmuir adsorption isotherm. Thus, adsorption may be assumed to occur at homogeneous binding sites on the surface of the adsorbents up to a monolayer coverage [64]. Compared to the pristine natural zeolite and GO-zeolite powders, carboxy-GO/zeolite powders have the highest maximum adsorption capacity at 67.56 mg g−1 , predicted by the Langmuir model at pH = 3.0 (Table 2). The kinetics of rhodamine B adsorption onto carboxy-GO/zeolite powders is considerably faster (98% recovery in 30 min) and the maximum adsorption capacity (67.56 mg g−1 ) is 1.3–5.1 times greater than those reported for natural or synthesized zeolite powders in similar rhodamine B adsorption studies [24,49] (Table 3). The functionalized zeolite powders in our work have superior adsorption efficiency for rhodamine B, which may be ascribed to the GO layer grafting and increased active sites after benzene carboxylic acid functionalization. 4. Conclusions Zeolite particles were successfully surface functionalized with GO nanosheets and a carboxylated diazonium salt. Subsequently, these new materials were shown to be effective adsorbents of rhodamine B from aqueous solution. Carboxy-GO/zeolite powders showed higher adsorption capacities compared to pristine natural zeolite and GO coated zeolite powders at acidic or moderate pH values primarily due to electrostatic interactions, ␲–␲ stacking and hydrogen bonding. The adsorption capacity for rhodamine B was greatest at pH = 3, reaching an equilibrium within 60 min, resulting in maximum adsorption capacities of 50.25, 55.56 and 67.56 mg g−1 for pristine zeolite, GO-zeolite and carboxy-GO/zeolite, respectively.

Y. Yu et al. / Journal of Hazardous Materials 260 (2013) 330–338

Acknowledgements The authors acknowledge the financial support by the Australian Research Council Linkage Grant (LP100100616). Thanks to Dr Mahaveer Kurkuri from University of Adelaide for his help with the BET surface area tests. This work was supported by the Australian Microscopy and Microanalysis Research Facility (AMMRF).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2013.05.041.

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