Journal of Hazardous Materials 273 (2014) 174–182

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Copper removal using bio-inspired polydopamine coated natural zeolites Yang Yu a , Joseph G. Shapter a , Rachel Popelka-Filcoff b , John W. Bennett c , Amanda V. Ellis a,∗ a Flinders Centre for Nanoscale Science & Technology, School of Chemical and Physical Sciences, Flinders University, Sturt Road, Bedford Park, Adelaide 5042, SA, Australia b School of Chemical and Physical Sciences, Flinders University, Sturt Road, Bedford Park, Adelaide 5042, SA, Australia c Centre for Nuclear Applications, Australian Nuclear Science and Technology Organisation, Lucas Heights 2234, NSW, Australia

h i g h l i g h t s • • • • •

Natural zeolites were modified with bio-inspired polydopamine. A 91.4% increase in Cu(II) ion adsorption capacity was observed. Atomic absorption and neutron activation analysis gave corroborative results. Neutron activation analysis was used to provide accurate information on 30+ elements. Approximately 90% of the adsorbed copper could be recovered by 0.1 M HCl treatment.

a r t i c l e

i n f o

Article history: Received 19 November 2013 Received in revised form 16 March 2014 Accepted 23 March 2014 Available online 29 March 2014 Keywords: Natural zeolites Polydopamine Copper adsorption Neutron activation analysis

a b s t r a c t Herein, for the first time, natural clinoptilolite-rich zeolite powders modified with a bio-inspired adhesive, polydopamine (PDA), have been systematically studied as an adsorbent for copper cations (Cu(II)) from aqueous solution. Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) revealed successful grafting of PDA onto the zeolite surface. The effects of pH (2–5.5), PDA treatment time (3–24 h), contact time (0 to 24 h) and initial Cu(II) ion concentrations (1 to 500 mg dm−3 ) on the adsorption of Cu(II) ions were studied using atomic absorption spectroscopy (AAS) and neutron activation analysis (NAA). The adsorption behavior was fitted to a Langmuir isotherm and shown to follow a pseudo-second-order reaction model. The maximum adsorption capacities of Cu(II) were shown to be 14.93 mg g−1 for pristine natural zeolite and 28.58 mg g−1 for PDA treated zeolite powders. This impressive 91.4% increase in Cu(II) ion adsorption capacity is attributed to the chelating ability of the PDA on the zeolite surface. Furthermore studies of recyclability using NAA showed that over 50% of the adsorbed copper could be removed in mild concentrations (0.01 M or 0.1 M) of either acid or base. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Copper is recognized as one of the most harmful pollutants in the environment due to its high toxicity, non-degradability and bioaccumulation [1]. There are many sources of copper (Cu(II) ion) pollution which are mostly produced by industries such as petroleum refining, paints and pigments, paper and pulp, steel-works, chemical manufacturing, metallurgical mining and

∗ Corresponding author. Tel.: +61 8 8201 3104; fax: +61 8 8201 2905. E-mail address: Amanda.Ellis@flinders.edu.au (A.V. Ellis). http://dx.doi.org/10.1016/j.jhazmat.2014.03.048 0304-3894/© 2014 Elsevier B.V. All rights reserved.

electroplating [1,2]. Commonly accepted methods for the removal of copper from wastewater include precipitation, ion exchange, membrane filtration, adsorption (physi-, chemi- and bio-sorption), electrodeposition, and flotation [3]. Among them the use of solid porous materials as adsorbents is considered to be the most reliable, simple, efficient and cost-effective method. Activated carbon has been by far the most widely used material for the removal of Cu(II) ions from effluents but is costly to produce and difficult to regenerate [4]. Therefore, there has been increased focus on developing low-cost, non-toxic and environmentally friendly processes based on the use of natural materials for the removal of Cu(II) ions from wastewater streams.

Y. Yu et al. / Journal of Hazardous Materials 273 (2014) 174–182

Of these natural products, abundant and low cost naturally occurring zeolites have gained attention due to their porous structures, high surface areas and ion-exchange properties [5]. Natural zeolites are made up of a microporous aluminosilicate crystalline mineral with a 3-dimensional framework of tetrahedrally coordinated SiO4 or AlO4 [6]. Within the cavities, and channels of zeolites, various ion-exchangeable cations (e.g., Na+ , K+ , Ca2+ , and Mg2+ ) are loosely held and can be readily displaced by other inorganic cations (e.g., Hg2+ , Pb2+ , Ag+ , Cu2+ , Cd2+ , Cr3+ , Zn2+ , Ni2+ , Co2+ and Mn2+ ) or organic compounds (e.g., dyes, humic substances and phenolics) [7–12]. Recent studies have shown that natural or functionalized zeolites can be used as an efficient adsorbent for metal ion (e.g., Cu2+ ) removal in aqueous solution [12–14]. Interestingly, the performance of metal ion adsorption can be enhanced by surface functionalization/modification with non-toxic and biodegradable biopolymers (e.g., chitosan [15], algae [16], cellulose [17] and 3,4dihydroxy-l-phenylalanine (dopamine (DA)) [18]. DA is particularly interesting as it is a key compound in the formation of marine adhesive proteins [19]. In this work we demonstrate the use of polydopamine (PDA), well known as a universal surface functionalization agent, to modify natural zeolites [20–22]. In 2007, Lee et al. reported that multifunctional surface-adherent PDA films could be coated onto a wide range of organic and inorganic materials (e.g., metals, plastics, semiconductors and synthetic polymers) through a simple dip-coating process in an aerobic alkaline DA solution [23]. The actual structure of PDA is still ambiguous. However various structures have been proposed wherein dihydroxyindoline, indolinedione and dopamine units are covalently bonded or linked by hydrogen bonding in supramolecular architectures [20,23–25]. The existence of catechol groups in DA is crucial for the adhesion which forms strong hydrogen bonds within the PDA films and the substrates onto which it is placed [23,26,27]. Importantly for metal adsorption applications the catechol groups [23,27,28] and nitrogen heteroatoms [29] in PDA’s structure have a strong propensity to interact with positively charged metal ion species (e.g., Cu2+ and Fe3+ ). To date some of the more notable methods used to identify metal ion adsorption capacities and kinetics of zeolites have included spectrometries such as, atomic absorption (AA) [30], X-ray fluorescence (XRF) [31] and inductively coupled plasma atomic emission spectroscopy (ICP-AES) [32]. However, exact quantification of Cu in the zeolite matrix itself has until now yet to be determined. In the present work, we systematically investigated the adsorption of Cu(II) ions onto clinoptilolite-rich natural zeolite that had been functionalized with the bio-inspired adhesive PDA. Batch experiments were conducted to investigate the adsorption affinity, kinetics and equilibrium loading capacity of both pristine natural zeolites and PDA functionalized zeolite powders toward Cu(II) ions in aqueous solution. The impact of the surface functionality, initial metal ion concentration and pH were also examined. The Cu(II) ion adsorption capacity was determined by both AAS and neutron activation analysis (NAA). NAA provides the sensitivity, precision and accuracy necessary to analyze Cu as well as another 50+ elements in the zeolite substrate, simultaneously [33].

2. Experimental 2.1. Materials Australian natural zeolite (10 ␮m, 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). Dopamine hydrochloride (DA), Tris(hydroxymethyl)aminomethane (Tris) and CuSO4 ·5H2 O were purchased from Sigma-Aldrich (Australia). Hydrochloric acid,

175

potassium hydroxide, acetone and ethanol were supplied from Chem Supply (Australia). All aqueous solution were prepared using Milli-Q water (18.2 M cm at 25 ◦ C). 2.2. Surface modification of natural zeolite powder by polydopamine (PDA) Tris powder (182 mg) was dissolved in Milli-Q water (100 mL) after which DA powder (200 mg) was added. Natural zeolite powder (1 g) was then suspended in the Tris-DA solution for 3–24 h. The Tris-DA solution had a pH of ∼9.5 and the color changed from transparent to brown within 1 min and then black after 1 h as the dopamine oxidized to PDA on the surface of the zeolite [23]. The resulting PDA-coated zeolite powder was then separated via centrifugation at 3000 rpm for 10 min, followed by successive washing and centrifugation with Milli-Q water and dried in a vacuum desiccator for 24 h. The zeolite powders treated with PDA at different reaction times (3–24 h) were denoted by PDA3h-zeolite, PDA6hzeolite, PDA14h-zeolite, PDA18h-zeolite and PDA24h-zeolite. 2.3. Spectroscopic analysis Fourier transform infrared (FTIR) spectroscopy was used for functional group analysis of the natural zeolite and PDA-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. X-ray photoelectron spectroscopy (XPS) analysis was used to characterize surface functional groups of natural zeolite and PDA-zeolite using a Leybold LHS-10 spectrometer equipped with a monochromatic Al K␣ source (1486.6 eV). The X-ray source operated at 13 kV, 20 m and the vacuum was better than 10−9 Torr. Data were recorded in the VAMAS format and fitted to mixed Gaussian–Lorentzian components using the CasaXPS software package. All binding energies in the XPS results were calibrated using C 1s = 284.5 eV (adventitious carbon peak) [34]. Transmission electron microscopy (TEM) images were taken on a Tecnai G2 Spirit TEM an accelerating voltage of 120 kV. Pristine-zeolite, PDA6h-zeolite and PDA24h-zeolite samples were deposited out of solution onto carbon-coated copper grids for TEM analysis. 2.4. Morphology, particle size and zeta potential analysis The morphology of natural zeolite and PDA-zeolite samples was determined by scanning electron microscopy (SEM) using a CAMScan MX2500 operating at 10 kV. Zeta potentials of dilute suspensions of natural zeolite and PDA-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 and PDA modified zeolite powders (0.25 g) 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. For zeta potential measurements the pH of the supernatant was adjusted between 1.0 and 10.0, using potassium hydroxide (0.1 M) or hydrochloric acid (0.1 M). 2.5. Thermal analysis Thermogravimetric analysis (TGA) was carried out using a TA Instruments TGA 2950, USA. The natural zeolite and PDA-zeolite

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samples (∼20 mg powder) were heated at 10 ◦ C min−1 from 30 to 800 ◦ C in a N2 atmosphere (flow rate 60 cm3 min−1 ).

37 elements in mg kg−1 or % is shown in Table 1S. The residue concentration of Cu(II) ions in solution after adsorption was tested by AAS using the same procedure described above.

2.6. Surface area analysis 3. Results and discussion Specific surface area (Brunauer–Emmett–Teller, BET) analysis of the natural zeolite, and PDA-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 . The total pore volumes were calculated from the amount adsorbed of nitrogen at a pressure p/p0 = 0.99. The pore size distributions were evaluated by the BJH model following the adsorption branch of the nitrogen adsorption/desorption isotherm. 2.7. Adsorption experiments In order to investigate the adsorption behavior of the natural zeolite and PDA-zeolite adsorbents adsorption isotherm experiments were carried out by varying the initial Cu(II) ion concentration from 1 to 500 mg dm−3 . Standard solutions of Cu(II) (1 to 500 mg dm−3 ) were prepared by dilution of a 500 mg dm−3 CuSO4 ·5H2 O stock solution with Milli-Q water. The adsorbents (0.01 g) were dispersed in each of the standard aqueous Cu(II) ion solutions (10 cm3 ) and the suspension magnetically stirred for 24 h to ensure equilibrium. The suspensions were then filtered through a 0.45 ␮m membrane filter (Millipore, Millex-HN, Nylon, USA) and the filtrate analyzed using a flame atomic absorption spectrophotometer (FAAS, GBC933 plus) with a hollow cathode lamp (Photron lamps) at a wavelength of 327.4 nm and air-acetylene flame. The Cu(II) ion concentration was then calculated from a calibration curve. The effect of pH (2.0 to 5.5), contact time (0 to 24 h) and initial Cu(II) ion concentrations (1 to 500 mg dm−3 ) on the adsorption kinetics and isotherms were studied, three replicates of each were performed. Potassium hydroxide (0.1 M) and hydrochloric acid (0.1 M) were used to control the pH of each suspension. The Cu(II) adsorption capacity q (mg g−1 ) and adsorption efficiency were calculated from Eqs. (1) and (2), respectively [32]: q=

(C0 − C) V W

Adsorption efficiency =

3.1. Structural and chemical characterization of natural zeolite and polydopamine modified samples Fig. 1(a–c) shows the FTIR spectra of the DA, pristine natural zeolite and PDA-zeolite samples, respectively. For the DA monomer, absorption bands associated with the CH2 , aromatic phenol and NH2 in DA structure were observed at 1343 cm−1 , 1321 cm−1 , 1198 cm−1 and 1019 cm−1 , which can be attributed to CH2 bending vibration, C O H asymmetric bending vibration, C O asymmetric vibration, and C N stretching vibration, respectively [35]. For the natural zeolite, Si O Si asymmetric stretching and Al O Si stretching vibrations were observed at 1079 cm−1 and 796 cm−1 (Fig. 1(b)) [36,37]. The broad peak at 3378 cm−1 , and a peak at 1627 cm−1 , are attributed to the O H stretching and bending vibrations of water molecules entrapped in natural zeolite structure, respectively [37,38]. In the case of the PDA-zeolite samples (Fig. 1(c) and inset (PDA deposition with time)) additional bands were observed at 1505 cm−1 and 1442 cm−1 , attributed to N H stretching [39] and N H scissoring vibrations [35,40], arising from the indole or indoline structures of the PDA layer [20,35]. The XPS spectra of pristine natural zeolite and PDA-zeolites samples are shown in Fig. 2(a–d). Three major peaks of C 1s, O 1s and Si 2p and several minor peaks of Ca 2p and Al 2p were observed in the pristine natural zeolite sample (Fig. 2(a)), which is consistent with the chemical composition of natural zeolite. After PDA treatment, a new peak at ∼399 eV from N 1s appears in the XPS spectrum of the zeolites treated with PDA (3, 6 and 24 h) (Fig. 2(a)). Furthermore, the atomic composition of N ( N or N H from PDA) and C (aromatic carbon from PDA) increased as a function of PDA reaction time (Table 1), indicating the growth of PDA polymer on the surface of the zeolite. The XPS spectrum

(1) (C0 − C) × 100% C0

(2)

where C0 and C is the initial and equilibrium concentration of Cu(II) (mg dm−3 ), respectively; V is the volume of the solution (dm3 ) and W is the dry mass of the adsorbent (g). 2.8. Neutron activation analysis (NAA) experiments The solid zeolite samples for NAA analysis was prepared by the following procedure: The adsorbents (1 g) were dispersed in each of the standard aqueous Cu(II) ion solution (initial concentration: 100 mg dm−3 or 500 mg dm−3 , volume: 100 cm3 ) and the suspension magnetically stirred for 24 h to ensure equilibrium. The suspensions were filtered through a 5 ␮m filter paper (150 mm, Advantac) and the solid was gently washed with Milli-Q water three times to remove the surface adsorbed Cu(II) ions. Then the filter paper was placed in an oven at 80 ◦ C overnight and the solid samples were collected for NAA analysis. Sub-samples each (50 mg) were subjected to short and long irradiations in the 20 MW OPAL research reactor operated by the Australian Nuclear Science and Technology Organisation (ANSTO). NAA was carried out using the k0 -method of standardization in order to maximize the number of elements that could be quantified [33]. Sample preparation, irradiation, decay, counting and calculation procedures followed the standard procedures at the NAA facility [33]. The concentration of

Fig. 1. FTIR spectra of (a) DA monomer, (b) pristine natural zeolite and (c) PDA24hzeolite samples. Inset shows the FTIR spectra of zeolite treated with DA as a function of reaction time (0–24 h) in the range of 1300–1800 cm−1 .

Y. Yu et al. / Journal of Hazardous Materials 273 (2014) 174–182

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Fig. 2. (a) XPS survey of pristine natural zeolite and PDA-zeolite samples. High resolution XPS spectra of (b) C 1s, (c) N 1s and (d) O 1s of PDA24h-zeolite sample.

Table 1 Atomic composition of pristine and PDA-zeolite samples, determined from XPS analysis.

O

N

Si

31.85 43.39 61.35 73.82

39.16 32.53 23.90 17.81

0.00 4.67 4.78 5.49

20.09 15.87 7.79 2.88

Weight (%)

C

pristine zeolite

98

Atomic composition (%)

Sample

Pristine zeolite PDA3h-zeolite PDA6h-zeolite PDA24h-zeolite

100

PDA3h-zeolite

96

PDA6h-zeolite

94

PDA14h-zeolite

92

PDA24h-zeolite

90 weight loss 0 - 200 oC 6.60 % 0h 6.49 % 6.43 % 6.86 % 6.63 % 24h

88 86 84 82

of C 1s of PDA24h-zeolite sample can be curve-fitted with several peak components at 284.6 eV, 285.6 eV and 287.7 eV, which can be attributed to C H, C O/C N and C O, respectively (Fig. 2(b)) [41]. The high resolution XPS spectrum for N 1s (Fig. 2(c)) appears in the PDA treated zeolite sample at ∼399.0 eV. This was deconvoluted into two peak components: at 399.8 eV for the N H groups and at 398.6 eV for the N species, attributable to the indole or indoline structures in PDA [35,41]. The high resolution XPS spectrum for O 1s (Fig. 2(d)) shows two major peak components at 529.8 eV and 531.6 eV, which can be attributed to C O and O H, respectively [25]. The XPS results presented here may help to predict the possible structure of PDA. The N-H species may arise from the amine groups in the heterocycle of PDA and the N may arise from the pyridinelike structure of PDA, which is consistent with literature [35]. Fig. 3 shows the TGA data for the natural zeolite and PDA-zeolite samples. The weight loss between 120 ◦ C and 200 ◦ C is attributed to the desorption of entrapped water molecules from the zeolite matrix [42,43]. The weight loss between 200 ◦ C and 500 ◦ C and between 500 ◦ C and 800 ◦ C is associated with the decomposition of organic matter [44] and the removal of silanol groups [45] in the zeolite structure, respectively. The bifurcation of PDA-zeolites and pristine zeolite samples begins at 300 ◦ C. The extra weight loss of 1.9 wt%–6.6 wt% (Fig. 3) between 300 ◦ C and 800 ◦ C compared to the natural zeolite is attributed to the decomposition of PDA from the PDA-zeolite surface [35,46]. Fig. 1S shows the amount of PDA polymer grafted onto the zeolite surface increases rapidly after the first 3 h, then follows a linear relationship (R2 = 0.98) with the PDA treatment time (3–24 h), indicating

weight loss 500 - 800 oC 0.47 % 0h 1.61 % 1.97 % 2.71 % 3.93 % 24h

80 0

100

200

weight loss 200 - 500 oC 3.55 % 0h 4.39 % 4.59 % 5.26 % 7.21 % 24h

300

400

500

600

700

800

Temperature (°C) Fig. 3. TGA analyses of pristine natural zeolite and PDA treated zeolites.

the surface coverage of PDA can be simply controlled by reaction time. This has been previously observed in literature [47], which was also proved by TEM analysis (Fig. 2S) where the PDA coating was shown to be approximately 6.8 nm after 6 h deposition and approximately 12.6 nm after 24 h deposition. The BET surface area of the pristine natural zeolite powders was determined to be 14.65 ± 0.60 m2 g−1 (Fig. 4(a)), which is a low surface area compared to synthesized adsorbents [48,49]. Specific surface areas of the PDA-zeolite particles (Fig. 4(a)) decreased from 6.59 ± 0.50 to 4.57 ± 0.30 m2 g−1 with 3 h to 24 h PDA treatment time, a decrease in surface area of ∼55–69% compared to that of the natural zeolites. The corresponding total pore volumn (Fig. 4(b)) of the PDA-zeolite samples with 3 h to 24 h treatment time (0.044 ± 0.002 to 0.031 ± 0.001 cm3 g−1 ) shows a similar trend, which decrease ∼21–45% compared to that of the pristine zeolites (0.056 ± 0.002 cm3 g−1 ). The pore size distribution of the pristine and PDA-zeolite samples determined by using the BJH model was shown in Fig. 4(c). It indicates that the nanoporosity and mesoporosity of pristine zeolite gradually decrease due to the PDA treatment. These results indicate that increased PDA treatment times partially blocks the main zeolite pore channels [44].

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(a)

14 12

qe (mg·g-1)

Surface area (m 2·g-1)

16

10 8 6 4 2 0

3

6

12

9

15

18

21

PDA24h-zeolite

0.06

0.05

0.03

0.02 6

9

12

15

18

21

24

PDA treatment time (h) 0.05

(c) 0.04

pristine zeolite PDA3h-zeolite PDA6h-zeolite PDA14h-zeolite PDA24h-zeolite

0.03

4

5

6

of the zeta potential with increasing pH from 1.0 to 10.0 (Fig. 5) was observed indicating the pristine zeolite sample exhibits a negative zeta potential over the pH range studied, which is consistent with literature [43]. The negative charge is likely a result of the substitution of Si4+ ions by Al3+ ions in the natural zeolite lattice and also from cleaved Si O Si bonds at the particle surface [43,50]. An isoelectric point (IEP) close to 0.5 was observed for the pristine zeolite [50,51]. A charge reversal is observed for the PDA-zeolite samples at lower pH ranges and the IEP now shifts to a higher pH value (∼3.0 to 4.2) with increasing PDA treatment time (3–24 h, Fig. 5). This observed higher IEP of the PDA-coated zeolites may arise from the protonation of indole or indoline groups of the PDA which act to neutralize the initial negative charge of the zeolite surface [52]. After 24 h treatment, the IEP of the PDA24h-zeolite is pH = ∼4.3 which is similar to that of PDA films in literature (∼4.0) [52], indicating nearly full coverage of the PDA on the zeolite surface. At higher pH (pH > 5, Fig. 5) the PDA-zeolite samples surface becomes negatively charged and this may favor the adsorption of positively charged Cu(II) ions.

0.04

3

3

Fig. 6. Effect of pH on the adsorption of Cu(II) ions by pristine natural zeolite and PDA24h-zeolite powders. Initial concentration of Cu(II) ions was 25 mg dm−3 (contact time: 24 h).

(b)

0

2

pH

24

PDA treatment time (h) Total pore volume(cm3·g-1)

pristine zeolite

1

0

dV(logD) (cm3⋅g-1)

18 16 14 12 10 8 6 4 2 0

0.02 0.01

3.2. Adsorption studies of Cu(II) ions 0 10

100

1000

D (Å) Fig. 4. (a) BET surface area, (b) total pore volume and (c) pore size distribution of the pristine natural zeolite powders and PDA-zeolites samples with different PDA treatment time (3–24 h).

Surface charge plays a critical role in ion adsorption and therefore zeta potential analysis was performed in order to determine the surface charge on the natural zeolite and PDA-zeolite samples. The data are shown in Fig. 5. A monotonic increase in the magnitude

zeta potential (mV)

40 30

pristine zeolite

20

PDA3h-zeolite

10

PDA6h-zeolite

0

PDA14h-zeolite

-10

PDA18h-zeolite

-20

PDA24h-zeolite

-30 -40 -50 -60 0

1

2

3

4

5

6

7

8

9

10

11

pH Fig. 5. Zeta potentials of pristine natural zeolite and PDA-zeolite powders as a function of pH in 10−3 M KNO3 solution (pH was decreased from 10.0 to 1.0).

The pH not only influences the Cu(II) ion speciation in solution, but also affects the interfacial chemistry of the adsorbents. At higher pH (> 5.5), copper ions start to form Cu(OH)2 and precipitate from solution which makes these pH’s unsuitable for adsorption experiments [53]. Therefore, for this work a pH of between 2 and 5.5 was selected for the Cu(II) ion adsorption studies. The adsorption capacities of pristine natural zeolite and PDA-zeolite powders (24 h treated) were investigated for their effectiveness in the adsorption of Cu(II) ions from aqueous solution between pH = 2 and 5.5 (Fig. 6). Pristine natural zeolite powders showed a maximum adsorption capacity of 11.01 mg g−1 at pH = 5.5. At this pH the adsorption is due to the interaction of the positively charged Cu(II) ions with the negatively charged surface of the natural zeolite via electrostatic interactions (Fig. 5). At pH < 5.5, the observed decrease in Cu(II) ion adsorption is due to the competition between H+ ions and the adsorbate (Cu(II)) for the same negatively charged adsorption sites on the pristine zeolite surface [12]. After surface functionalization of the natural zeolite the adsorption capacity at pH = 5.5 of the PDA24h-zeolite powder was determined to be 15.65 mg g−1 (Fig. 6), which is 42.1% higher than that of the pristine zeolite. However, the magnitude of the zeta potential of the PDA24h-zeolite powder (−26.2 mV, Fig. 5) at pH = 5.5 is much lower than that of the pristine zeolite (−40.1 mV, Fig. 5), indicating potentially lower electrostatic interactions between the PDA24h-zeolite adsorbent and the positively charged Cu(II) ions. In light of this the increase in adsorption capacity after 24 h PDA functionalization may be strongly impacted by the presence of nitrogen heteroatoms (e.g., N H and N )

Y. Yu et al. / Journal of Hazardous Materials 273 (2014) 174–182

18

16

a

17

12

15

qt (mg·g-1)

qe (mg·g-1)

a

14

16 14 13 12 11

10 8 6 25 mg·dm-3 50 mg·dm-3 100 mg·dm-3

4

10 0

3

6

9

12

15

18

21

2

24

Time (h)

0 0

7

3

6

9

12

15

18

21

24

Time (h)

b

25

6

b 20

5

qt (mg·g -1)

qe (mg·g-1)

179

4 3 0

3

6

9

12

15

18

21

24

Time (h) 9

10 25 mg·dm-3 50 mg·dm -3 100 mg·dm -3

5 0

c

8

15

0

3

6

9

12

15

18

21

24

qe (mg·g-1)

Time (h) 7 Fig. 8. Adsorption kinetics of (a) pristine natural zeolite and (b) PDA24h-zeolite powders at different initial concentration of Cu(II) ions (25–100 mg dm−3 ) (contact time: 24 h, pH = 5.5).

6 5 4 3 0

3

6

9

12

15

18

21

24

Time (h) Fig. 7. Effect of PDA treatment time on the adsorption of Cu(II) ions by PDA-coated zeolite powders. (contact time: 24 h, pH = 5.5). (a) Initial concentration of Cu(II) ions was 25 mg dm−3 , adsorbent dosage was 1 g dm−3 , by AAS analysis. Initial concentration of Cu(II) ions was (b) 100 mg dm−3 and (c) 500 mg dm−3 , adsorbent dosage was 10 g dm−3 , determined by NAA.

or catechol groups of PDA which have a stronger propensity for complexation or coordination with Cu(II) ions in aqueous solution [23,27–29]. At a pH < 4 the Cu(II) ions adsorption decreased due to the increasing electrostatic repulsion between the positively charged adsorbent (pH < IEP of PDA24h-zeolite) and positively charged adsorbate (Cu(II) ions). Since both adsorbent types showed the maximum adsorption efficiency for Cu(II) ions at pH = 5.5, this pH was selected as the parameter for all further adsorption experiments and subsequent analyzes. Fig. 7(a) shows the effect of PDA treatment time (3–24 h) on the adsorption of Cu(II) ions by PDA functionalized zeolite powders, indicating the adsorption capacity increases as the reaction time increases. As previously described the surface coverage of PDA on the zeolite surface increases as a function of reaction time (see TGA data, Fig. 3 and zeta potential results, Fig. 5), therefore the amount of functional groups (nitrogen heteroatoms and catechol groups) that can coordinate with the Cu(II) ions on the PDA-zeolite surface also increases, which consequently enhances the adsorption capacity. Similar trends were observed by NAA in which a higher adsorbent dosage (10 g dm−3 ) and initial Cu(II) ion concentration (100 and 500 mg dm−3 ) were used (Fig. 7(b) and (c)).

Importantly, when NAA was used prior to the Cu(II) adsorption studies in order to determine the elemental composition of the samples (Table 1S) no detectable elemental Cu was observed in the pristine zeolite matrix. However, Cu (4.4 ± 0.2–7.7 ± 0.3 mg) was observed in 1 g of the PDA-coated zeolite samples after Cu(II) ion adsorption (Table 2S). The NAA data for the PDA24h-zeolite shows the highest adsorption capacity (15.65 mg g−1 ) and so the 24 h PDA treatment time was selected for subsequent experiments. The Cu concentration determined by NAA analysis in the zeolite and the PDA functionalized zeolite samples is in agreement with (albeit 1.62–2.73% higher, data not shown) than that determined by AAS analysis.

3.3. Adsorption kinetics The adsorption kinetics of Cu(II) ions on pristine natural zeolite and PDA24h-zeolite powders at initial concentrations of 25 to 100 mg dm−3 were shown in Fig. 8(a) and (b), respectively. At all initial Cu(II) ion concentrations (25 to 100 mg dm−3 ) the adsorption occurred rapidly within the first 3 h of exposure, reaching an equilibrium within 24 h. The adsorption capacity (qe ) for the two adsorbents studied (from 11.01 mg g−1 and 15.65 mg g−1 to 13.63 mg g−1 and 22.51 mg g−1 for the natural zeolite and PDA24h-zeolite powders, respectively) at equilibrium increased significantly when the Cu(II) ion initial concentration was increased from 25 to 100 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 [43,54]. The adsorption capacity of Cu(II) ions onto the surface of zeolite and PDA24h-zeolite powders were evaluated by pseudo-first and second order empirical power law models. Empirical first and

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second order kinetic models are expressed by Eq. (3), for reaction exponents n = 1 and 2, respectively [55]:

35

dqt = kn (qe − qt )n dt

25

30

qe (mg·g-1)

(3)

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. (3) with the boundary conditions t = 0, qt = 0 and t = t, qt = qt to the linear form gives Eqs. (4) and (5), respectively: log (qe − qt ) = log qe −

20 15 10 zeolite

5

PDA24h-zeolite

0 0

k1 t 2.303

100

(4)

1 t t = + qt qe k2 q2e

(5)

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. The regression coefficients (R2 ) data in Table 2 indicates that the experimental kinetic data fits better to a pseudo-second order model (R2 > 0.999) than to a pseudo-first order model (R2 > 0.873), for the adsorption of Cu(II) ions onto natural and PDA-coated zeolite powders. Furthermore, the estimated qe values (Table 2), using the pseudo-second order model, are in good agreement with the experimental values qe (exp). Thus, the empirical second order model is suitable for the description of the adsorption kinetics of Cu(II) ions onto pristine zeolite and PDA24h-zeolite powders. The k2 data (Table 2) shows that the rate of initial adsorption for the pristine and PDA-coated powders was higher at low Cu(II) ion concentrations (25 mg dm−3 ). This may be due to the fact that the adsorption of Cu(II) ions onto saturated active sites of pristine natural and PDA-coated zeolite powders is the rate-limiting step at a higher Cu(II) ion concentration [43,56]. For the PDA polymer coated zeolite adsorbents, the rate constant k2 (0.042–0.048 g mg−1 h−1 ) for Cu(II) ion adsorption is slightly lower than those of the pristine zeolite powders (0.039–0.045 g mg−1 h−1 ). This may be due to the PDA films coated on the zeolite surface blocks the porous pores and hinders the transport of Cu(II) ions into the inner channels of the zeolite matrix for ion exchange. In contrast the relatively uniform porous surface of pristine zeolite is available for Cu(II) ions adsorption, which enables the adsorption reaction to proceed faster. 3.4. Adsorption isotherms The adsorption isotherms of Cu(II) ions onto pristine natural zeolite and PDA24h-zeolite samples was shown in Fig. 9. The adsorption capacity for Cu(II) ions increases with the increasing equilibrium Cu(II) ion concentration. To predict the equilibrium parameters, Langmuir and Freundlich isotherms are commonly used for the molecular adsorption at interfaces [55]. The Langmuir model presumes a monolayer

200

300

400

500

Ce (mg·dm-3) Fig. 9. Adsorption isotherms of pristine natural zeolite and PDA24h-zeolite powders. (contact time: 24 h, pH = 5.5).

adsorption onto the homogeneous active sites on adsorbents. This is expressed using Eq. (6) [57]. Ceq Ceq 1 + = q qmax qmax b

(6)

where qe and Ceq are the adsorption capacity (mg g−1 ) and Cu(II) ion 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 . 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 [58] log qe = log Kf +

1 log Ceq n

(7)

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 . The regression coefficients (R2 ) data in Table 3 indicates that the Langmuir model (R2 = 0.998–0.999) for the two adsorbents (pristine natural zeolite, PDA24h-zeolite) fits much better than the Freundlich model (R2 = 0.781–0.905). A Langmuir adsorption isotherm can be used for the best description of the observed adsorption equilibrium behavior of Cu(II) ions onto the two adsorbents (Fig. 9). Thus, adsorption may be assumed to occur at homogeneous binding sites on the adsorbent surface up to monolayer coverage of adsorbate [59]. Compared to the pristine natural zeolite, PDA24h-zeolite powders have the maximum adsorption capacity at 28.58 mg g−1 predicted by the Langmuir model (Table 3), which is 91.9% higher than the pristine samples (14.93 mg g−1 ). This value is similar, or higher than, the Cu(II) ion adsorption capacity of other modified zeolites samples or low-cost materials (Table 4). Considering the low surface area (4.57 ± 0.30 m2 g−1 ) of the product after functionalization, the PDA-zeolite powders in our work still have superior adsorption efficiency for Cu(II) ions, which may be ascribed to the

Table 2 Pseudo-first order and pseudo-second order kinetic parameters of Cu(II) adsorption onto pristine natural zeolite and PDA24h-zeolite powders. Sample

Zeolite

PDA24h-zeolite

Cu(II) (mg dm−3 )

qe (exp) (mg g−1 )

25 50 100 25 50 100

11.1 12.2 13.6 15.6 18.2 22.5

Pseudo-first order model −1

k1 (h

0.399 0.395 0.556 0.577 0.472 0.443

)

Pseudo-second order model −1

qe (cal) (mg g 8.2 8.9 9.6 15.4 12.9 14.6

)

R

k2 (g mg−1 h−1 )

qe (cal) (mg g−1 )

R2

0.963 0.945 0.883 0.873 0.928 0.949

0.048 0.047 0.042 0.045 0.043 0.039

11.7 12.8 14.0 16.5 18.8 23.3

1.000 0.999 1.000 1.000 0.999 1.000

2

Y. Yu et al. / Journal of Hazardous Materials 273 (2014) 174–182

181

Table 3 Langmuir and Freundlich isotherm parameters of pristine natural zeolite and PDA24h-zeolite powders. Langmuir model

Sample

Zeolite PDA24h-zeolite

Freundlich model

B (dm3 mg−1 )

qmax (mg g−1 )

R2

Kf (dm3 mg−1 )

N

R2

0.177 0.085

14.93 28.58

0.999 0.998

7.139 8.301

7.605 4.646

0.781 0.905

Table 4 Comparison of BET surface areas and maximum adsorption capacities for Cu(II) ions of different adsorbents. Sample

BET surface area (m2 g−1 )

qmax (mg g−1 )

Reference

Zeolite-based geopolymers Zeolite/cellulose acetate blend fiber Humic acid-immobilized surfactant-modified zeolite Activated carbon Fly ash activated with bentonite Chitosan Polydopamine nanoparticles Pristine zeolite PDA24h-zeolite

– – – 886.72 6.14–21.33 1.75 – 14.65 4.57

5.97–20.1 16.2–21.7 19.8–21.5 17.83 7.29–24.30 85.21 34.4 14.93 28.58

[60] [14] [12] [61] [62] [63] [18] This work This work

increased active sites such as nitrogen heteroatoms (N H or N ) or catechol groups in PDA. This in turn means this material has a stronger propensity for complexation of Cu(II) ions [23,27–29]. Most importantly, this material is readily available and environmentally benign. Preliminary studies of recyclability show that 81.8%, 89.4%, 54.4% and 75.2% of Cu(II) ions can be recovered by 0.01 M HCl, 0.1 M HCl and 0.01 M KOH and 0.1 M KOH solutions from PDA functionalized zeolite, respectively, determined by NAA (Table 3S). The results indicate that 0.1 M HCl solution was the most suitable desorbing reagent for Cu(II) ions which is consistent with literature results [1].

4. Conclusions Zeolite particles were successfully surface functionalized with polydopamine (PDA) films and the functionalized PDA-zeolite samples showed significant higher adsorption capacities compared to the pristine natural zeolite at moderate pH values due to the presence of nitrogen heteroatoms (N H or N ) or catechol groups in PDA. The results of Cu(II) adsorption experiments investigated by AAS and NAA analysis were in a good agreement. The pristine zeolite and PDA-zeolite samples show maximum adsorption capacities for Cu(II) of 14.93 and 28.58 mg g−1 , respectively, at pH = 5.5, reaching an equilibrium within 24 h. 89.3% of adsorbed Cu(II) ions can be recovered from PDA-zeolite sample after extraction by 0.1 M HCl solution. These results confirm that the PDA functionalized zeolite has a potential to be used as a cost-effective and environmentally friendly adsorbent for copper ions extraction from water.

Acknowledgments We acknowledge the Australian Research Council Linkage grant no: LP100100616 and the Australian Institute of Nuclear Science and Engineering award ALNGRA13068 for funding. We also thank Dr Kristina Contantapolous for helpful discussions on polydopamine.

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. 2014.03.048.

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