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Removal of Cr(VI) from aqueous solution by polysulfone microcapsules containing Cyanex 923 as extraction reagent
Desalination 259 (2010) 179–186
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Desalination j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d e s a l
Removal of Cr(VI) from aqueous solution by polysulfone microcapsules containing Cyanex 923 as extraction reagent Senar Ozcan ⁎, Ali Tor, Mehmet Emin Aydin Selcuk University, Department of Environmental Engineering, Campus, 42031 Konya, Turkey
a r t i c l e
i n f o
Article history: Received 15 January 2010 Received in revised form 31 March 2010 Accepted 2 April 2010 Available online 7 May 2010 Keywords: Microcapsules Cyanex 923 Chromium(VI) Sorption
a b s t r a c t This paper describes the removal of Cr(VI) from aqueous solution in batch sorption technique by using polysulfone microcapsules containing Cyanex 923 as extraction reagent. The microcapsules containing Cyanex 923 (MCs-Cyanex 923) were prepared by solvent evaporation method and characterized by using optical microscope, Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FT-IR) and Thermal Gravimetric Analysis (TGA). The results showed that maximum removal of Cr(VI) was obtained when ratio of Cyanex 923 to polysulfone was 1.0. The complex formation between Cr(VI) and Cyanex 923 favoured at initial pH of 1.0. The equilibrium time was 30 min and it obeyed the pseudo second-order kinetic model. The Redlich Peterson and Langmuir isotherm models better represented the sorption data in comparison to Freundlich isotherm. The Langmuir sorption capacity of MCs-Cyanex 923 for Cr(VI) was 0.430 mmol g−1. The MCs-Cyanex 923 preferably sorbed Cr(VI) against various metal ions, including Cr(III), Ni(II), Pb(II), Cu(II), Zn(II), Cd(II), Co(II). The regeneration studies also showed that MCs-Cyanex 923 could be re-used for the adsorption of Cr(VI) from aqueous solutions over 3 cycles. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The extensive use of chromium in various industries, such as leather tanning, metallurgy, electroplating, etc. has caused the release of chromium to the environment. Cr(III) and Cr(VI) are the chromium oxidation states usually encountered in the environment. Oxidizing 2− 2− potential of the Cr(VI) species, such as HCrO− 4 , Cr2O7 and CrO4 , make them highly toxic to bacteria, plants and animals [1]. Human toxicity of Cr(VI) includes lung cancer, liver, kidney and gastric damage and epidermal irritation [2]. The limit for the discharge of Cr(VI) into inland surface waters is 0.1 mg/L. The maximum contaminant level of Cr(VI) for the drinking water is 0.05 mg/L [3]. Therefore, the removal of Cr(VI) from waters is important for environmental health. For the treatment of aqueous solution by adsorption, activated carbon is the most widely used adsorbent. However, it is very expensive and has high operating costs [4]. Therefore, in recent years, considerable attention has been devoted to the study of different types of low-cost materials in order to remove the pollutants from aqueous solution. Many workers have employed the brown coal [5], humic acid extracted from brown coals [6], waste tyre [7] and zeolites [8], etc., for removing metals from the aqueous solution. Liquid–liquid extraction is one of the major techniques for the removal and recovery of metals from the aqueous solution [9]. However, this method can suffer from significant drawbacks, such as
⁎ Corresponding author. Tel.: +90 332 223 1983; fax: +90 332 241 0635. E-mail address: [email protected] (S. Ozcan). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.04.009
the formation of stable emulsions and disposal of solvent waste, etc. In order to avoid these shortcomings, alternative methods have been developed by employing the solid phase instead of organic solvent, such as supported liquid membrane [10], solvent impregnated resin [11], activated composite membrane [12], chelating fiber [13], polysulfone hollow fiber membrane [14–16] and extractant microcapsule [17]. The main disadvantage of supported liquid membrane and solvent impregnation resin is their short lifetime due to the progressive leakage of both extractant and solvent from the membrane and resin [10,11]. The activated composite membrane has slow transport kinetics [12]. When the chelating fiber was treated with strong acid and eluted with large volumes of water, the fiber was found to swell, as resulted in the problems for re-use [13]. The synthesis of the polysulfone hollow fiber membrane containing chelating reagent is complex and needs a long time [14–16]. In recent years, the polymeric microcapsules (MCs) have been extensively used for the removal and recovery of heavy metals from the aqueous solutions [18–21]. The MCs correspond to a porous polymeric matrix that contains an immobilized suitable extraction reagent, which is chosen to selectively extract the desired metals. The potential advantages of MCs are easy phase separation, large specific interfacial area, minimal use of organic solvent, high selectivity and more stability. Applications of MCs have been studied in many fields, but their potential use in hydrometallurgy is remarkable [19]. For example, Yang et al. [22] investigated Cu(II) sorption using polysulfone MCs containing Di-2-ethylhexyl phosphoric acid. The extraction experiments showed that the MCs can be used for recovery metal
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ions in dilute solutions and have a sufficient stability for repeated processes. In other work, Ochoa et al. [21] reported the preparation of polysulfone/polyvinylpyrrolidone MCs containing Aliquat 336 for the extraction of Cr(VI) from aqueous solution. They reported that MCs with Aliquat 336 prepared from 2:1 polysulfone/polyvinylpyrrolidone ratio achieved the highest Cr(VI) extractive performance (92% of Cr extraction for contact time of 60 min) and the best breakthrough point in column tests (up to 10 h in the first cycle). Cyanex 923 has gained prominence as an extractant during the last decade because of better selectivity, hydrolytic stability and easier phase separations. Its active component is the mixture of tertiary octyl and hexyl phosphine oxides. This extractant is an effective compound to remove Cr(VI) from industrial effluents and is widely used in liquid– liquid extraction process [12,23,24]. Because it has not been studied, this work attempts both obtaining of polysulfone MCs containing Cyanex 923 as extractant and investigation of their potential use for the removal of Cr(VI) from aqueous solution. Equilibrium and kinetics information derived from the experimental results is also analyzed and discussed. It is fundamental to know this information to understand the basic principles that govern the sorption process and for scaling it up to a practical system based on continuous columns packed with MCs. 2. Experimental 2.1. Reagents and solvent
tions. Ionic strength of all solutions was 0.01 M maintained with NaNO3. Initial pH of the solutions determined by using ionmeter (Orion EA940, USA) was adjusted to the desired level with 0.1 M NaOH or 0.1 M HCl solutions. The experiments were performed by shaking a known amount of MCs-Cyanex 923 in 20 mL of Cr(VI) solutions on a temperature controlled orbital shaker (Gallenkamp, UK) at 200 rpm and 25 ± 1 °C for an equilibrium time. Then, the solution were filtered through a Whatman No. 42 filter paper. The studied parameters were initial pH of the solution (1.0–6.6), contact time (10–240 min), initial Cr(VI) concentration (0.096– 6.280 mmol/L), the ratio of Cyanex 923 to polysulfone in dispersed phase (0.25–1.25 g/g) and MCs-Cyanex 923 dosage (0.5–5 g/L). Moreover, the effects of Cr(III), Ni(II), Pb(II), Cu(II), Zn(II), Cd(II), Co (II) on the sorption of Cr(VI) were investigated. The amount of Cr(VI) sorbed was calculated from Eq. (1). q = ðC0 −Ce Þ:V = m
ð1Þ
Where q, Cr(VI) sorbed (mmol/g); C0, initial concentration of Cr (VI) (mmol/L); Ce, concentration of Cr(VI) in solution at equilibrium (mmol/L); V, solution volume (L); m, MCs-Cyanex 923 dosage (g). The concentration of Cr(VI) and other metal ions were determined by using Atomic Absorption Spectrometer (ContrAA 300, Analytik Jena) in air-acetylene flame. Cr(VI) concentration in the presence of Cr (III) was determined by UV spectrophotometry [25]. Hence, Cr(III) concentration was determined by the subtracting of Cr(VI) from the
The chemical reagents used for experiments and preparation of MCsCyanex 923 were of analytical grade. CH2Cl2, HCl, K2CrO4, CrCl3·6H2O, CuCl2·2H2O, NiCl2·6H2O, ZnCl2, CoCl2·6H2O, 3CdSO4·8H2O, Pb(NO3)2, NaOH, NaCl, were from Merck Co. (Darmstadt, Germany). Cyanex 923 was obtained from CYTEC (Canada). Composition of the Cyanex 923 was as follows: dioctyl-monohexyl phosphine oxide (40–44%), monooctyldihexyl phosphine oxide (28–32%) and tri-n-octylphosphine oxide (12–16%). Polysulfone and gum arabic were obtained from Aldrich (Steinheim, Germany). 2.2. Preparation of MCs-Cyanex 923 The MCs-Cyanex 923 were prepared by the following procedure described by Bari et al. [18]. A known amount of polysulfone and desired amount of Cyanex 923 were dissolved in CH2Cl2 as a dispersed phase. The continuous phase was prepared by dissolving 1.0 wt.% gum arabic in 500 mL distilled water. The dispersed phase was introduced into the continuous phase under agitation speed of 600 rpm at room temperature for 2 h. After, CH2Cl2 was permitted to evaporate completely under the same conditions, the polysulfone MCs-Cyanex 923 were obtained. The resulting MCs-Cyanex 923 were washed for three times with distilled water, then desiccated at room temperature. 2.3. Characterization of MCs TGA of the MCs was performed with SETARAM thermal gravimetric analyzer at the temperature range of 25–800 °C with 10 °C/min heating ramp in argon atmosphere (gas flow rate: 20 mL/min). The surface morphology of samples was examined by SEM (Jeol, JSM 5310, Japan). The optical miscroscope images of MCs were obtained with an optical microscope equipped with a digital camera (Nikon Eclipse E-400 equiped with Nikon DS camera). FT-IR analysis was carried out by a single channel Fourier transform spectrophotometer (Perkin-Elmer 1605) at wave number between 4000 and 650 cm−1 with a resolution of 0.01 cm−1. 2.4. Batch sorption experiments The Cr(VI) solutions were prepared by diluting the stock solution (1000 mg/L Cr(VI) prepared from K2CrO4) to the desired concentra-
Fig. 1. (a) and (b) Optical microscope images of MCs-Cyanex 923 (composition of dispersed phase: polysulfone: Cyanex 923: dichloromethane = 2 g: 2 g; 50 mL, continuous phase: 1 wt.% gum arabic solution; agitation speed: 600 rpm).
S. Ozcan et al. / Desalination 259 (2010) 179–186
total–chromium concentration. All experiments were carried out in duplicate or triplicate and variations between replicate samples within an experiment range less than 3%. 3. Results and discussion 3.1. Preparation and characterization of MCs-Cyanex 923 In the preparation of MCs, gum arabic is used as dispersant to prevent the aggregation of MCs. Bari et al. [18] reported that obtaining MCs is hard without dispersant due to the adhesive aggregation caused by the high viscosity of the polystyrene solution. They also stated ideal MCs containing Cyanex 272 was obtained when 1 wt.% gum arabic solution was used as a continuous phase. Thus, for the preparation of MCs-Cyanex 923 in this study, the concentration of gum arabic solution was selected as 1 wt.%. MCs-Cyanex 923 were prepared at various agitation speeds by using continuous phase (1 wt.% gum arabic solution) and dispersed phase (polysulfone: 2 g; Cyanex 923: 2 g; CH2Cl2, 50 mL). The optimum agitation speed was determined as 600 rpm. At an agitation speed below or above 600 rpm, the dispersed phase droplets had a trend for aggregation. Therefore, in further experiments, 600 rpm was chosen as agitation speed for the preparation of MCs-Cyanex 923. The optical microscope images of the MCs-Cyanex 923 show that MCs containing Cyanex 923 are spherical (Fig. 1a,b). The diameter of the MCs-Cyanex 923 was determined as 273(±48) µm by taking into account fifteen MCs-Cyanex 923. As seen in Fig. 2, FT-IR assignments of blank MCs can be summarized as follows. Band at 2967.05 cm−1 was observed due to vibrations of C–H strecthing involving methyl group. Bands at 1583.47 and 1485.82 cm−1 were attributed to the streching vibrations of aromatic C=C groups. Bands at 1408.52 and 1363.97 cm−1 corresponded to asymetric and symetric C–H bending deformation of methyl group, respectively. Doublet resulting from
181
asymetric O=S=O strecthing of sulfone group was seen at 1322.57 and 1293.72 cm−1. Band at 1234.32 cm−1 indicated asymmetric C–O–C strecthing of aryl ether group. Band at 1147.28 cm−1 was attributed to symmetric O=S=O stretching of sulfone group. Bands at 1103.13 and 1078.80 cm−1 corresponded to aromatic ring vibrations. In Fig. 2a,b, the peaks at 1205.22 cm−1 and 1206.63 cm−1, which correspond to the P=O group, demonstrated Cyanex 923 was succesfully coated on the MCs. The SEM images also indicated that coating of Cyanex 923 was confirmed by the morphological appearance of the MCs prepared with and without Cyanex 923 (Fig. 3a,b). TGA and its first derivative (dTGA) curves for blank MCs and MCsCyanex 923 are presented in Fig. 4a,b. The blank MCs showed a weight loss of about 68% at temperatures ranging from 500 to 590 °C, mainly due to the loss of volatiles of polysulfone. Similar result was reported by Ball and Boettner [26] for the termal analysis of polysulfone. The weight losses (1.01, 7.25 and 16.19%) at temperature ranges of 120–160, 175–275 and 280–370 °C, respectively, were due to the coated Cyanex 923 on the MCs. The total weight loss (28.96%) of Cyanex 923 in this temperature range (120–370) was in good agreement with the encapsulated amount of Cyanex 923 (26%) by MCs, prepared with the composition polysulfone: Cyanex 923: dichloromethane =2 g: 0.25 g: 50 mL. 3.2. Sorption experiments 3.2.1. Effect of Cyanex 923 and polysulfone ratio on the sorption of Cr(VI) The effects of Cyanex 923 and polysulfone ratio on both sorption of Cr(VI) at pH 1 and encapsulation efficiency are presented in Fig. 5. The experiments of this section were performed at various amounts of Cyanex 923 by keeping a constant amount of polysulfone (2 g) in 50 mL of CH2Cl2. The encapsulation efficiency can be defined as the weight ratio of encapsulated extractant in the MCs. It was found that encapsulation efficiency increased with an increase in the ratio of
Fig. 2. The infrared spectrum of (a) blank MCs (containing no Cyanex 923), (b) MCs-Cyanex 923 and (c) Cyanex 923.
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S. Ozcan et al. / Desalination 259 (2010) 179–186 −
2−
+ H2 O
−
2−
+H
2HCrO4 ⇔Cr2 O7 HCr2 O7 ⇔Cr2 O7
þ
K3 = 35:5 K4 = 0:85
ð4Þ ð5Þ
According to both Eqs. (2–5) and their K values, when Cr(VI) concentration is equal or less than 1x10−3 M at pH 6, approximately 75% 2− of Cr(VI) exists as HCrO− 4 and 25% of Cr(VI) exists as CrO4 [29]. In an acidic solution, when Cr(VI) concentration is less than 0.02 M, HCrO− 4 is the dominant species and when Cr(VI) concentration is greater than 2− 0.02 M then Cr2O2− 7 is the dominant species [24]. Furthermore, Cr2O7 − converts into HCrO4 in acidic aqueous solution at a total Cr(VI) concentration less than (1.26–1.74) × 10−2 M [30]. Based on this explanation and Refs. [31–33], in the present study, Cr(VI) ions exist as HCrO− 4 in the aqueous solution because its initial concentration ranged from 9.6 × 10−5 to 6.28 × 10−3 M. Moreover, Bayrakci et al. [34] stated that the monoanion have a smaller free energy of hydration than does the dianionic form. It is expected that there is a smaller loss in the hydration energy when HCrO− 4 is transferred from the aqueous solution to the extractant phase. In order to investigate the effect of initial pH of the feed phase on the sorption of Cr(VI) by MCs-Cyanex 923, the experiments were performed at the initial pH of 1.0, 2.9, 3.9, 5.1 and 6.6. A pH deviation of ±0.1 was observed for each pH measurement. The results in Fig. 6 showed that the sorption of Cr(VI) was maximum at an initial pH of 1.0. The obtained result can be attributed to that formation of the complex between Cr(VI) and Cyanex 923 mostly favours at solution pH 1.0. Similar result was reported by Agrawal et al. [24], who studied the extractive removal of Cr(VI) from industrial waste solution by using Cyanex 923. Agrawal et al. [24] also pointed out that Cr(VI) was extracted from the hydrochloric acid media by Cyanex 923 according to the following reactions: −
þ
−
−
HCrO4 + H + Cl ⇔CrO3 Cl
−
Fig. 3. SEM images of (a) blank MCs and (b) MCs-Cyanex 923.
CrO3 Cl
Cyanex 923 to polysulfone. Additionally, the sorption of Cr(VI) increases with increasing the ratio of polysulfone to Cyanex 923 up to 1.0. This is expected because increasing the amount of Cyanex 923 increases the formation of complex between the Cr(VI) and Cyanex 923. Whilst, the sorption of Cr(VI) decreased when Cyanex 923 to polysulfone ratio was beyond 1.0. This result can be explained as follows: at high loading of Cyanex 923 (Cyanex 923 to polysulfone ratio N1.0), crystallization of the extractant in the pore of polymeric matrix may occur and limit accessibility to reactive sites for the interaction between the Cr(VI) and the extractant retained in the porous structure of the MCs. Similar result was reported by Navarro et al. [27], who studied cadmium extraction from the hydrochloric acid solutions using Amberlite XAD-7 impregnated with Cyanex 921. In addition, Cyanex 923 was coagulated when its amount was more than 2.5 g (Cyanex 923 to polysulfone ratio N1.25) in 50 mL of dichloromethane. Therefore, the optimum ratio of Cyanex 923 to polysulfone was chosen as 1.0 for further experiments. 3.2.2. Effect of initial pH of solution and sorption mechanism The chromate ions may exist in the aqueous solution in different 2− 2− − ionic forms (HCrO− 4 , CrO4 , Cr2O7 , HCr2O7 ), which depend on the Cr (VI) concentration and pH of the solution. The equilibrium for the species of Cr(VI) and related equilibrium constants (K) are presented in Eqs. (2–5) [28,29]. −
H2 CrO4 ⇔HCrO4 + H −
2−
HCrO4 ⇔CrO4
+H
þ
þ
ð2Þ
K1 = 1:21 −7
K2 = 3 × 10
ð3Þ
+ H2 O
ð6Þ
+ H + 2·Cyanex−923⇔HCrO3 Cl·2·Cyanex−923
ð7Þ
þ
Overall reaction can be written as follows: −
þ
−
HCrO4 +2H +Cl +2·Cyanex−923⇔HCrO3 Cl·2·Cyanex−923+H2 O ð8Þ
Based on this reaction, it can be stated that rate of the complex formation depends on the solution pH. A decrease in pH increases the rate of the complex formation (HCrO3Cl·2·Cyanex-923) between HCrO− 4 and Cyanex 923. Therefore, the initial pH of the solution should be equal to 1.0 for an efficient sorption of Cr(VI) by MCsCyanex 923 (see Fig. 6). After 1 h of contact time, the equilibrium pH values were 0.5, 2.5, 3.6, 4.8 and 6.5 for the initial pH of 1.0, 2.9, 3.9, 5.1 and 6.6, respectively. Apart from the HCl, Agrawal et al. [24] examined the other mineral acids (i.e., H2SO4 and HNO3) in order to adjust the pH of the solution, hence, efficiency of Cr(VI) extraction. They reported that the maximum extraction efficiency was obtained when HCl was involved in the extraction. Therefore, in further experiments of this study, the initial pH of the feed phase was adjusted to 1.0 ± 0.1 by using HCl. 3.2.3. Effect of contact time and kinetic evaluation It is seen in Fig. 7 that the sorption of Cr(VI) attained equilibrium after 30 min without dependence of initial Cr(VI) concentration. According to Barassi et al. [19], the concentration of the extractant on the surface of MCs permits the evaluation of the rate of the chemisorption by means of pseudo second-order kinetic model.
S. Ozcan et al. / Desalination 259 (2010) 179–186
183
Fig. 4. TGA and its first derivatives of (a) blank MCs (polysulfone: dichloromethane =2 g: 50 mL, continuous phase: 1 wt.% gum arabic solution) and (b) MCs-Cyanex 923 (polysulfone: Cyanex 923: dichloromethane =2 g: 0.25 g: 50 mL, continuous phase: 1 wt.% gum arabic solution).
Therefore, the experimental data was applied to the pseudo secondorder kinetic model given as Eq. (9) [35]. 2 t = qt = 1 = k2:qe +t = qe
ð9Þ
Where qe and qt are the amounts of Cr(VI) sorbed (mmol/g) at equilibrium and time t (min), respectively, k2 is the rate constant of
pseudo second-order chemisorption (g/(mmol. min)). For the studied concentrations, the rate constant (k2) and theoretical equilibrium sorption capacities, qe (calculated), were calculated from the slope and the intercept of the linear plots of the pseudo second-order kinetic model. The coefficients of determination (R2) and theoretical and experimental qe values suggested that the sorption of Cr(VI) by MCs-Cyanex 923 followed the second-order type reaction kinetics (Table 1).
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S. Ozcan et al. / Desalination 259 (2010) 179–186 Table 1 Values of sorption rate constant for pseudo second-order kinetic model. Cr(VI) concentration, mmol/L 0.096 0.192 0.962
Fig. 5. Effect of Cyanex 923 to polysulfone ratio in dispersed phase on the sorption of Cr(VI) and encapsulation efficiency (concentration of Cr(VI): 0.385 mmol/L, initial pH of solution: 1.0(±0.1), contact time: 1 h, shaking speed: 200 rpm, temperature: 25 °C, ionic strength: 0.01 M, amount of MCs-Cyanex 923: 1.5 g/L; amount of polysulfone in dispersed phase: 2 g, and volume of dichloromethane: 50 mL).
qe (experimental) (mmol/g)
k2 [g/(mmol. min)]
qe (calculated) (mmol/g)
R2
0.043 0.090 0.320
15.923 8.128 2.805
0.053 0.095 0.321
0.999 0.999 0.999
3.2.4. Effect of initial concentration of Cr(VI) and sorption isotherm models The influence of initial concentration of Cr(VI) on its removal was examined by varying the initial Cr(VI) concentration at pH 1.0(±0.1) and 1 h of equilibrium time. The results demonstrated that increasing the initial concentration improved the amount of the Cr(VI) sorbed. For instance, the amounts of Cr(VI) sorbed were 0.043, 0.090 and 0.305 mmol/g, for the initial concentrations of 0.096, 0.192 and 0.961 mmol/L, respectively. This may be due to the increasing driving force for mass transfer with the concentration of Cr(VI) [36]. The experimental data was analyzed with two parameters isotherm models including Langmuir and Freundlich models and three parameters Redlich–Peterson isotherm model. The Langmuir isotherm models the monolayer coverage of the sorption surfaces and assumes that sorption take places on a structurally homogeneous surface of the sorbent. This isotherm is given as Eq. (10) [37]. qe = Q o :b:Ce = ð1 + b:Ce Þ
ð10Þ
The linear form of the Langmuir isotherm model can be presented as Eq. (11); Ce = qe = ð1 = Q o :bÞ + ðCe = qe Þ
ð11Þ
Where Ce is the concentration of Cr(VI) (mmol/L) at equilibrium, Q 0 is the monolayer capacity of the adsorbent (mmol/g) and b is the Langmuir sorption constant (L/mmol). The Freundlich equation is derived to model the multilayer sorption and for the sorption on heterogeneous surfaces. The Freundlich model is formulated as Eq. (12) [38]. 1=n
qe = k:Ce Fig. 6. Effect of initial pH of the solution on the sorption of Cr(VI) (concentration of Cr(VI): 0.385 mmol/L, contact time: 1 h, amount of MCs-Cyanex 923: 1.5 g/L).
ð12Þ
Linearized form of the Freundlich equation is given by the following equation: log qe = log k + ð1 = nÞ log Ce
ð13Þ
Where Ce is the equilibrium concentration (mmol/L), k is the sorption capacity (mmol/ g) and n is an empirical parameter. The Redlich–Peterson isotherm [39] incorporates three parameters into an empirical isotherm, and therefore, can be applied either in homogeneous or heterogeneous systems due to its high versatility. The Redlich–Peterson equation is: β qe = KR :Ce = 1 + aR :Ce
Fig. 7. Effect of contact time on the sorption of Cr(VI) (concentrations of Cr(VI): 0.096, 0.192 and 0.962 mmol/L, initial pH of solution: 1.0(± 0.1), amount of MCs-Cyanex 923: 1.5 g/L).
ð14Þ
Where, KR is Redlich–Peterson isotherm constant (L/mmol), aR is also a constant (L/mmol)β and β is the exponent which lies between zero and one. This isotherm model involves varying the isotherm parameter, KR to obtain the maximum (R2) value for the linear regression of ln Ce vs. ln[(KR·Ce/qe) − 1]. For β = 1, Eq. (14) reduces to Langmuir equation and for β = 0, it reduces to Henry's equation. Eq. (14) can be converted to a linear form by taking logarithms: ln ½ðKR :Ce = qe Þ−1 = ln aR + β: ln Ce
ð15Þ
S. Ozcan et al. / Desalination 259 (2010) 179–186
Fig. 8. Non-linear isotherm plots for the sorption of Cr(VI) by MCs-Cyanex 923 (initial pH of solution: 1.0(± 0.1), contact time: 1 h, amount of MCs-Cyanex 923: 1.5 g/L).
Fig. 8 shows the Langmuir, Freundlich and Redlich–Peterson isotherm models and the experimental data for the sorption of Cr(VI) by MCs-Cyanex 923. The values of the isotherm constants were obtained from the slope and intercept of the plots of the linear form of each isotherm. The Langmuir sorption capacity, Q0, and the sorption equilibrium constant, b, were 0.430 mmol/g and 4.024 L/mmol, respectively. The Freundlich isotherm constant, k, was 0.262 mmol/g and n was 2.786; the Redlich–Peterson isotherm constant KR was 3.000 L/mmol, aR was 8.566 (L/mmol)β and β was 0.858. The R2 values for the Langmuir, Freundlich and Redlich–Peterson isotherm models were 0.992, 0.895 and 0.994, respectively, which means that sorption of Cr(VI) by MCsCyanex 923 could be better described by the Redlich–Peterson and the Langmuir isotherm models. The Chi-square (χ2) test, defined by Eq. (16), was also employed to find out the best-fit isotherm model for the sorption data [40]. h i 2 2 χ = Σ ðqe –qm Þ = qm
used for the synthesis of MCs may also affect both encapsulation efficiency and pyhsical property, hence the sorption capacity of the MCs. For example, Barassi et al. [19] used the in situ radical polymerization method for the preparation of MCs-Aliquat 336 with 87% of encapsulation and 20 µm of mean diameter. In the present study, the solvent evaporation method was used in order to obtain the MCs-Cyanex 923 because it was easier than radical polymerization method. The encapsulation and mean diameter of MCs-Cyanex 923 were determined as 75% and 273 µm, respectively. Therefore, it is expected that the sorption capacity of MCs-Aliquat 336 is higher than that of MCs-Cyanex 923 due to higher surface area and encapsulation efficiency. Nevertheless, it should be emphasized that the potential use of Cyanex 923 for the removal or recovery of solutes from water is more extensive than that of Aliquat 336 because it has affinity towards for both inorganic and organic solutes, i.e. phenol, carboxylic acid, ect. In addition, the major advantage of Cyanex 923 over similar extraction reagents, e.g. trioctylphosphine oxide, is that it is completely miscible with all common hydrocarbon diluents even at low ambient temperatures. The major benefit of high solubility lies in the ability to prepare concentrated, stable solvents which can recover solutes (e.g. acetic acid) that are normally only weakly extracted by this type of reagent [42]. MCs-Cyanex 923 showed comparable Cr(VI) sorption capacity with Lewatit MP64 (0.400 mmol/g) and Lewatit MP500 (0.410 mmol/g) anion-exchange resins [43]. The sorption capacity of MCs-Cyanex 923 was lower than that of commercial activated carbon (0.580 mmol/g) [6]. However, the commercial activated carbon is very expensive and has high operating costs. 3.2.5. Effect of MCs-Cyanex 923 dosage An increase in the MCs-Cyanex 923 dosage increased the removal of Cr(VI) (Fig. 9). This is consistent with the expectation that higher sorbent dosage results in lower q values. The concentration of Cyanex 923 is related to MCs-Cyanex 923 concentration through surface site density [44]. Therefore, the removal increased with increasing the microcapsule dosage, whereas q decreased.
ð16Þ
Where qm is the equilibrium capacity obtained by calculating from the model (mmol/ g), and qe is experimental data of the equilibrium capacity (mmol/g). Small number of χ2 indicates that data from the model is close to the experimental data whereas, a large number of χ2 indicates that data from the model is different from the experimental data. The χ2 values for the Langmuir, Freundlich and Redlich– Peterson isotherm models were 0.022, 0.110 and 0.025, respectively. This means that the best-fit isotherm models were the Langmuir and Redlich–Peterson isotherm models. In fact, for our experimental data, the Redlich–Peterson model supported the Langmuir isotherm model, meaning that the sorption process would occur in a monolayer over a homogeneous surface. This result can be also inferred from the value (0.858) of β, which is close to unity [41]. The maximum Cr(VI) sorption capacity of the MCs-Cyanex 923 was estimated as 0.430 mmol/g. The maximum Cr(VI) sorption capacity of the MCs containing Aliquat 336 was in the range of 0.857–0.967 mmol/g [19]. The higher result may be attributed to the difference between the sorption mechanisms of MCs containing Aliquat 336 and Cyanex 923. Aliquat 336, a commercial quaternary ammonium salt [(C8–10H17–21)3·CH3]N+·Cl−, reacts with the anionic chromium species (HCrO− 4 ) according to the following anion exchange mechanism [19]. þ
185
−
−
þ
−
3.2.6. Effect of other metal ions on the removal of Cr(VI) The effects of other metal ions, including Cr(III), Cu(II), Pb(II), Zn(II), Ni(II), Co(II) and Cd(II), on the removal of Cr(VI) by MCs-Cyanex 923 were investigated. The results showed that Cr(III), Cu(II), Pb(II), Ni(II),
−
½ðC8–10 H17–21 Þ3 :CH3 N :Cl þHCrO4 ⇔½ðC8–10 H17–21 Þ3 :CH3 N :HCrO4 þ Cl
However, the sorption of Cr(VI) by Cyanex 923 is based on the complexation mechanism as described in Section 3.2.2. The procedure
Fig. 9. The variation of sorption of Cr(VI) with MCs-Cyanex 923 dosage (concentration of Cr(VI): 0.962 mmol/L, initial pH of solution: 1.0(± 0.1), contact time: 1 h, amount of MCs-Cyanex 923: 1.5 g/L).
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Table 2 Effect of other metal ions on the removal of Cr(VI) by MCs-Cyanex 923. (Experimental conditions: concentration of each metal ion: 10 mg/L, dosage of MCs-Cyanex 923: 5 g/L, initial pH: 1.0(± 0.1) and contact time: 1 h). Metal ion in aqueous solution
Removal, %
Cr(III) Ni(II) Cu(II) Co(II) Pb(II) Zn(II) Cd(II) Cr(VI)
No No No No No 10 5 89
removal removal removal removal removal
and Co(II) were not removed from the solution (Table 2). Contrary, 10% of Zn(II) and 5% of Cd(II) were removed from aqueous solution by MCsCyanex 923. This can be attributable to that Cr(III), Cu(II), Pb(II), Ni(II), and Co(II) are present as their cationic forms in the aqueous solution used in the presented study, and thus they were not removed by MCsCyanex 923. This was not the case for Zn(II) and Cd(II) which are present in the solution as the corresponding chloro-complexes, and were sorbed by MCs-Cyanex 923. Similar results were reported by Alguacil et al. [45], who studied the facilitated transport of Cr(VI) through supported liquid membrane containing Cyanex 921 and Cyanex 923. In addition, Agrawal et al. [24] investigated the effect of various metal ions (i.e., Cr(III), Ni(II), Ca(II), Mg(II), Fe(II), Zn(II), Cu(II), etc.) on the extraction of Cr(VI) by Cyanex 923 and they reported that only Zn(II) was found to be interfering species. The results also indicated that Cr(VI) was preferably removed by MCs-Cyanex 923 against other metal ions. However, sorption percentage of Cr(VI) in the presence of these ions was 89%. This value was lower than that (95%) of the prensence of only-Cr(VI) in the aqueous solution. The result may be because of a decrease in nominal Cyanex 923 concentration of the MCs due to the competitive sorption of Zn(II), Cd(II) and Cr(VI) [45,46]. Similar findings were reported in the related literatures [12,24]. 3.2.7. Regeneration and reusability of MCs-Cyanex 923 The regeneration experiments were carried out to see whether the MCs-Cyanex 923 loaded with Cr(VI) could be chemically regenerated. According to the literature, the most efficient solution was NaOH for stripping of Cr(VI) from Cr(VI)–Cyanex 923 complex [24]. Therefore, in the presented study, 0.5 M NaOH (10 mL) was used for the desorption of Cr(VI) from the loaded MCs-Cyanex 923. After desorption of Cr(VI), the MCs-Cyanex 923 were washed with distilled water until neutralization and then desiccated at room temperature. The adsorption process was repeated by using the regenerated MCs. It was found that after 3 cycles there is no change in the Cr(VI) sorption capacity of the MCs-Cyanex 923. In desorption experiments, it was observed that almost the total recovery of Cr(VI) occurs, suggesting that the MCs-Cyanex 923 are regenerable and can be used several times. 4. Conclusion The following conclusions can be made from the presented study. The results from the FT-IR, TGA, SEM of the MCs clearly indicated that immobilization of Cyanex 923 onto the MCs was successfully performed. The MCs-Cyanex 923 exhibited significant sorption capability for Cr(VI). The maximum removal of Cr(VI) was obtained when the ratio of Cyanex 923 to polysulfone was 1.0 at an initial pH of 1.0. The sorption of Cr(VI) by MCs-Cyanex 923 was presented by means of the complexation mechanism. The sufficient time for the
sorption equilibrium was 30 min and it obeyed the pseudo secondorder kinetic model. The Langmuir sorption capacity of MCs-Cyanex 923 for Cr(VI) was estimated as 0.430 mmol/g. The Cr(VI) was preferably sorbed by prepared microcapsules against the other metal ions [i.e., Cr(III), Ni(II), Cu(II), Zn(II), Cd(II), Co(II) and Pb(II) etc.]. The regeneration studies also showed that MCs-Cyanex 923 can be used several times for the sorption of Cr(VI) from the aqueous solutions.
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Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d e s a l
Removal of Cr(VI) from aqueous solution by polysulfone microcapsules containing Cyanex 923 as extraction reagent Senar Ozcan ⁎, Ali Tor, Mehmet Emin Aydin Selcuk University, Department of Environmental Engineering, Campus, 42031 Konya, Turkey
a r t i c l e
i n f o
Article history: Received 15 January 2010 Received in revised form 31 March 2010 Accepted 2 April 2010 Available online 7 May 2010 Keywords: Microcapsules Cyanex 923 Chromium(VI) Sorption
a b s t r a c t This paper describes the removal of Cr(VI) from aqueous solution in batch sorption technique by using polysulfone microcapsules containing Cyanex 923 as extraction reagent. The microcapsules containing Cyanex 923 (MCs-Cyanex 923) were prepared by solvent evaporation method and characterized by using optical microscope, Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FT-IR) and Thermal Gravimetric Analysis (TGA). The results showed that maximum removal of Cr(VI) was obtained when ratio of Cyanex 923 to polysulfone was 1.0. The complex formation between Cr(VI) and Cyanex 923 favoured at initial pH of 1.0. The equilibrium time was 30 min and it obeyed the pseudo second-order kinetic model. The Redlich Peterson and Langmuir isotherm models better represented the sorption data in comparison to Freundlich isotherm. The Langmuir sorption capacity of MCs-Cyanex 923 for Cr(VI) was 0.430 mmol g−1. The MCs-Cyanex 923 preferably sorbed Cr(VI) against various metal ions, including Cr(III), Ni(II), Pb(II), Cu(II), Zn(II), Cd(II), Co(II). The regeneration studies also showed that MCs-Cyanex 923 could be re-used for the adsorption of Cr(VI) from aqueous solutions over 3 cycles. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The extensive use of chromium in various industries, such as leather tanning, metallurgy, electroplating, etc. has caused the release of chromium to the environment. Cr(III) and Cr(VI) are the chromium oxidation states usually encountered in the environment. Oxidizing 2− 2− potential of the Cr(VI) species, such as HCrO− 4 , Cr2O7 and CrO4 , make them highly toxic to bacteria, plants and animals [1]. Human toxicity of Cr(VI) includes lung cancer, liver, kidney and gastric damage and epidermal irritation [2]. The limit for the discharge of Cr(VI) into inland surface waters is 0.1 mg/L. The maximum contaminant level of Cr(VI) for the drinking water is 0.05 mg/L [3]. Therefore, the removal of Cr(VI) from waters is important for environmental health. For the treatment of aqueous solution by adsorption, activated carbon is the most widely used adsorbent. However, it is very expensive and has high operating costs [4]. Therefore, in recent years, considerable attention has been devoted to the study of different types of low-cost materials in order to remove the pollutants from aqueous solution. Many workers have employed the brown coal [5], humic acid extracted from brown coals [6], waste tyre [7] and zeolites [8], etc., for removing metals from the aqueous solution. Liquid–liquid extraction is one of the major techniques for the removal and recovery of metals from the aqueous solution [9]. However, this method can suffer from significant drawbacks, such as
⁎ Corresponding author. Tel.: +90 332 223 1983; fax: +90 332 241 0635. E-mail address: [email protected] (S. Ozcan). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.04.009
the formation of stable emulsions and disposal of solvent waste, etc. In order to avoid these shortcomings, alternative methods have been developed by employing the solid phase instead of organic solvent, such as supported liquid membrane [10], solvent impregnated resin [11], activated composite membrane [12], chelating fiber [13], polysulfone hollow fiber membrane [14–16] and extractant microcapsule [17]. The main disadvantage of supported liquid membrane and solvent impregnation resin is their short lifetime due to the progressive leakage of both extractant and solvent from the membrane and resin [10,11]. The activated composite membrane has slow transport kinetics [12]. When the chelating fiber was treated with strong acid and eluted with large volumes of water, the fiber was found to swell, as resulted in the problems for re-use [13]. The synthesis of the polysulfone hollow fiber membrane containing chelating reagent is complex and needs a long time [14–16]. In recent years, the polymeric microcapsules (MCs) have been extensively used for the removal and recovery of heavy metals from the aqueous solutions [18–21]. The MCs correspond to a porous polymeric matrix that contains an immobilized suitable extraction reagent, which is chosen to selectively extract the desired metals. The potential advantages of MCs are easy phase separation, large specific interfacial area, minimal use of organic solvent, high selectivity and more stability. Applications of MCs have been studied in many fields, but their potential use in hydrometallurgy is remarkable [19]. For example, Yang et al. [22] investigated Cu(II) sorption using polysulfone MCs containing Di-2-ethylhexyl phosphoric acid. The extraction experiments showed that the MCs can be used for recovery metal
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ions in dilute solutions and have a sufficient stability for repeated processes. In other work, Ochoa et al. [21] reported the preparation of polysulfone/polyvinylpyrrolidone MCs containing Aliquat 336 for the extraction of Cr(VI) from aqueous solution. They reported that MCs with Aliquat 336 prepared from 2:1 polysulfone/polyvinylpyrrolidone ratio achieved the highest Cr(VI) extractive performance (92% of Cr extraction for contact time of 60 min) and the best breakthrough point in column tests (up to 10 h in the first cycle). Cyanex 923 has gained prominence as an extractant during the last decade because of better selectivity, hydrolytic stability and easier phase separations. Its active component is the mixture of tertiary octyl and hexyl phosphine oxides. This extractant is an effective compound to remove Cr(VI) from industrial effluents and is widely used in liquid– liquid extraction process [12,23,24]. Because it has not been studied, this work attempts both obtaining of polysulfone MCs containing Cyanex 923 as extractant and investigation of their potential use for the removal of Cr(VI) from aqueous solution. Equilibrium and kinetics information derived from the experimental results is also analyzed and discussed. It is fundamental to know this information to understand the basic principles that govern the sorption process and for scaling it up to a practical system based on continuous columns packed with MCs. 2. Experimental 2.1. Reagents and solvent
tions. Ionic strength of all solutions was 0.01 M maintained with NaNO3. Initial pH of the solutions determined by using ionmeter (Orion EA940, USA) was adjusted to the desired level with 0.1 M NaOH or 0.1 M HCl solutions. The experiments were performed by shaking a known amount of MCs-Cyanex 923 in 20 mL of Cr(VI) solutions on a temperature controlled orbital shaker (Gallenkamp, UK) at 200 rpm and 25 ± 1 °C for an equilibrium time. Then, the solution were filtered through a Whatman No. 42 filter paper. The studied parameters were initial pH of the solution (1.0–6.6), contact time (10–240 min), initial Cr(VI) concentration (0.096– 6.280 mmol/L), the ratio of Cyanex 923 to polysulfone in dispersed phase (0.25–1.25 g/g) and MCs-Cyanex 923 dosage (0.5–5 g/L). Moreover, the effects of Cr(III), Ni(II), Pb(II), Cu(II), Zn(II), Cd(II), Co (II) on the sorption of Cr(VI) were investigated. The amount of Cr(VI) sorbed was calculated from Eq. (1). q = ðC0 −Ce Þ:V = m
ð1Þ
Where q, Cr(VI) sorbed (mmol/g); C0, initial concentration of Cr (VI) (mmol/L); Ce, concentration of Cr(VI) in solution at equilibrium (mmol/L); V, solution volume (L); m, MCs-Cyanex 923 dosage (g). The concentration of Cr(VI) and other metal ions were determined by using Atomic Absorption Spectrometer (ContrAA 300, Analytik Jena) in air-acetylene flame. Cr(VI) concentration in the presence of Cr (III) was determined by UV spectrophotometry [25]. Hence, Cr(III) concentration was determined by the subtracting of Cr(VI) from the
The chemical reagents used for experiments and preparation of MCsCyanex 923 were of analytical grade. CH2Cl2, HCl, K2CrO4, CrCl3·6H2O, CuCl2·2H2O, NiCl2·6H2O, ZnCl2, CoCl2·6H2O, 3CdSO4·8H2O, Pb(NO3)2, NaOH, NaCl, were from Merck Co. (Darmstadt, Germany). Cyanex 923 was obtained from CYTEC (Canada). Composition of the Cyanex 923 was as follows: dioctyl-monohexyl phosphine oxide (40–44%), monooctyldihexyl phosphine oxide (28–32%) and tri-n-octylphosphine oxide (12–16%). Polysulfone and gum arabic were obtained from Aldrich (Steinheim, Germany). 2.2. Preparation of MCs-Cyanex 923 The MCs-Cyanex 923 were prepared by the following procedure described by Bari et al. [18]. A known amount of polysulfone and desired amount of Cyanex 923 were dissolved in CH2Cl2 as a dispersed phase. The continuous phase was prepared by dissolving 1.0 wt.% gum arabic in 500 mL distilled water. The dispersed phase was introduced into the continuous phase under agitation speed of 600 rpm at room temperature for 2 h. After, CH2Cl2 was permitted to evaporate completely under the same conditions, the polysulfone MCs-Cyanex 923 were obtained. The resulting MCs-Cyanex 923 were washed for three times with distilled water, then desiccated at room temperature. 2.3. Characterization of MCs TGA of the MCs was performed with SETARAM thermal gravimetric analyzer at the temperature range of 25–800 °C with 10 °C/min heating ramp in argon atmosphere (gas flow rate: 20 mL/min). The surface morphology of samples was examined by SEM (Jeol, JSM 5310, Japan). The optical miscroscope images of MCs were obtained with an optical microscope equipped with a digital camera (Nikon Eclipse E-400 equiped with Nikon DS camera). FT-IR analysis was carried out by a single channel Fourier transform spectrophotometer (Perkin-Elmer 1605) at wave number between 4000 and 650 cm−1 with a resolution of 0.01 cm−1. 2.4. Batch sorption experiments The Cr(VI) solutions were prepared by diluting the stock solution (1000 mg/L Cr(VI) prepared from K2CrO4) to the desired concentra-
Fig. 1. (a) and (b) Optical microscope images of MCs-Cyanex 923 (composition of dispersed phase: polysulfone: Cyanex 923: dichloromethane = 2 g: 2 g; 50 mL, continuous phase: 1 wt.% gum arabic solution; agitation speed: 600 rpm).
S. Ozcan et al. / Desalination 259 (2010) 179–186
total–chromium concentration. All experiments were carried out in duplicate or triplicate and variations between replicate samples within an experiment range less than 3%. 3. Results and discussion 3.1. Preparation and characterization of MCs-Cyanex 923 In the preparation of MCs, gum arabic is used as dispersant to prevent the aggregation of MCs. Bari et al. [18] reported that obtaining MCs is hard without dispersant due to the adhesive aggregation caused by the high viscosity of the polystyrene solution. They also stated ideal MCs containing Cyanex 272 was obtained when 1 wt.% gum arabic solution was used as a continuous phase. Thus, for the preparation of MCs-Cyanex 923 in this study, the concentration of gum arabic solution was selected as 1 wt.%. MCs-Cyanex 923 were prepared at various agitation speeds by using continuous phase (1 wt.% gum arabic solution) and dispersed phase (polysulfone: 2 g; Cyanex 923: 2 g; CH2Cl2, 50 mL). The optimum agitation speed was determined as 600 rpm. At an agitation speed below or above 600 rpm, the dispersed phase droplets had a trend for aggregation. Therefore, in further experiments, 600 rpm was chosen as agitation speed for the preparation of MCs-Cyanex 923. The optical microscope images of the MCs-Cyanex 923 show that MCs containing Cyanex 923 are spherical (Fig. 1a,b). The diameter of the MCs-Cyanex 923 was determined as 273(±48) µm by taking into account fifteen MCs-Cyanex 923. As seen in Fig. 2, FT-IR assignments of blank MCs can be summarized as follows. Band at 2967.05 cm−1 was observed due to vibrations of C–H strecthing involving methyl group. Bands at 1583.47 and 1485.82 cm−1 were attributed to the streching vibrations of aromatic C=C groups. Bands at 1408.52 and 1363.97 cm−1 corresponded to asymetric and symetric C–H bending deformation of methyl group, respectively. Doublet resulting from
181
asymetric O=S=O strecthing of sulfone group was seen at 1322.57 and 1293.72 cm−1. Band at 1234.32 cm−1 indicated asymmetric C–O–C strecthing of aryl ether group. Band at 1147.28 cm−1 was attributed to symmetric O=S=O stretching of sulfone group. Bands at 1103.13 and 1078.80 cm−1 corresponded to aromatic ring vibrations. In Fig. 2a,b, the peaks at 1205.22 cm−1 and 1206.63 cm−1, which correspond to the P=O group, demonstrated Cyanex 923 was succesfully coated on the MCs. The SEM images also indicated that coating of Cyanex 923 was confirmed by the morphological appearance of the MCs prepared with and without Cyanex 923 (Fig. 3a,b). TGA and its first derivative (dTGA) curves for blank MCs and MCsCyanex 923 are presented in Fig. 4a,b. The blank MCs showed a weight loss of about 68% at temperatures ranging from 500 to 590 °C, mainly due to the loss of volatiles of polysulfone. Similar result was reported by Ball and Boettner [26] for the termal analysis of polysulfone. The weight losses (1.01, 7.25 and 16.19%) at temperature ranges of 120–160, 175–275 and 280–370 °C, respectively, were due to the coated Cyanex 923 on the MCs. The total weight loss (28.96%) of Cyanex 923 in this temperature range (120–370) was in good agreement with the encapsulated amount of Cyanex 923 (26%) by MCs, prepared with the composition polysulfone: Cyanex 923: dichloromethane =2 g: 0.25 g: 50 mL. 3.2. Sorption experiments 3.2.1. Effect of Cyanex 923 and polysulfone ratio on the sorption of Cr(VI) The effects of Cyanex 923 and polysulfone ratio on both sorption of Cr(VI) at pH 1 and encapsulation efficiency are presented in Fig. 5. The experiments of this section were performed at various amounts of Cyanex 923 by keeping a constant amount of polysulfone (2 g) in 50 mL of CH2Cl2. The encapsulation efficiency can be defined as the weight ratio of encapsulated extractant in the MCs. It was found that encapsulation efficiency increased with an increase in the ratio of
Fig. 2. The infrared spectrum of (a) blank MCs (containing no Cyanex 923), (b) MCs-Cyanex 923 and (c) Cyanex 923.
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S. Ozcan et al. / Desalination 259 (2010) 179–186 −
2−
+ H2 O
−
2−
+H
2HCrO4 ⇔Cr2 O7 HCr2 O7 ⇔Cr2 O7
þ
K3 = 35:5 K4 = 0:85
ð4Þ ð5Þ
According to both Eqs. (2–5) and their K values, when Cr(VI) concentration is equal or less than 1x10−3 M at pH 6, approximately 75% 2− of Cr(VI) exists as HCrO− 4 and 25% of Cr(VI) exists as CrO4 [29]. In an acidic solution, when Cr(VI) concentration is less than 0.02 M, HCrO− 4 is the dominant species and when Cr(VI) concentration is greater than 2− 0.02 M then Cr2O2− 7 is the dominant species [24]. Furthermore, Cr2O7 − converts into HCrO4 in acidic aqueous solution at a total Cr(VI) concentration less than (1.26–1.74) × 10−2 M [30]. Based on this explanation and Refs. [31–33], in the present study, Cr(VI) ions exist as HCrO− 4 in the aqueous solution because its initial concentration ranged from 9.6 × 10−5 to 6.28 × 10−3 M. Moreover, Bayrakci et al. [34] stated that the monoanion have a smaller free energy of hydration than does the dianionic form. It is expected that there is a smaller loss in the hydration energy when HCrO− 4 is transferred from the aqueous solution to the extractant phase. In order to investigate the effect of initial pH of the feed phase on the sorption of Cr(VI) by MCs-Cyanex 923, the experiments were performed at the initial pH of 1.0, 2.9, 3.9, 5.1 and 6.6. A pH deviation of ±0.1 was observed for each pH measurement. The results in Fig. 6 showed that the sorption of Cr(VI) was maximum at an initial pH of 1.0. The obtained result can be attributed to that formation of the complex between Cr(VI) and Cyanex 923 mostly favours at solution pH 1.0. Similar result was reported by Agrawal et al. [24], who studied the extractive removal of Cr(VI) from industrial waste solution by using Cyanex 923. Agrawal et al. [24] also pointed out that Cr(VI) was extracted from the hydrochloric acid media by Cyanex 923 according to the following reactions: −
þ
−
−
HCrO4 + H + Cl ⇔CrO3 Cl
−
Fig. 3. SEM images of (a) blank MCs and (b) MCs-Cyanex 923.
CrO3 Cl
Cyanex 923 to polysulfone. Additionally, the sorption of Cr(VI) increases with increasing the ratio of polysulfone to Cyanex 923 up to 1.0. This is expected because increasing the amount of Cyanex 923 increases the formation of complex between the Cr(VI) and Cyanex 923. Whilst, the sorption of Cr(VI) decreased when Cyanex 923 to polysulfone ratio was beyond 1.0. This result can be explained as follows: at high loading of Cyanex 923 (Cyanex 923 to polysulfone ratio N1.0), crystallization of the extractant in the pore of polymeric matrix may occur and limit accessibility to reactive sites for the interaction between the Cr(VI) and the extractant retained in the porous structure of the MCs. Similar result was reported by Navarro et al. [27], who studied cadmium extraction from the hydrochloric acid solutions using Amberlite XAD-7 impregnated with Cyanex 921. In addition, Cyanex 923 was coagulated when its amount was more than 2.5 g (Cyanex 923 to polysulfone ratio N1.25) in 50 mL of dichloromethane. Therefore, the optimum ratio of Cyanex 923 to polysulfone was chosen as 1.0 for further experiments. 3.2.2. Effect of initial pH of solution and sorption mechanism The chromate ions may exist in the aqueous solution in different 2− 2− − ionic forms (HCrO− 4 , CrO4 , Cr2O7 , HCr2O7 ), which depend on the Cr (VI) concentration and pH of the solution. The equilibrium for the species of Cr(VI) and related equilibrium constants (K) are presented in Eqs. (2–5) [28,29]. −
H2 CrO4 ⇔HCrO4 + H −
2−
HCrO4 ⇔CrO4
+H
þ
þ
ð2Þ
K1 = 1:21 −7
K2 = 3 × 10
ð3Þ
+ H2 O
ð6Þ
+ H + 2·Cyanex−923⇔HCrO3 Cl·2·Cyanex−923
ð7Þ
þ
Overall reaction can be written as follows: −
þ
−
HCrO4 +2H +Cl +2·Cyanex−923⇔HCrO3 Cl·2·Cyanex−923+H2 O ð8Þ
Based on this reaction, it can be stated that rate of the complex formation depends on the solution pH. A decrease in pH increases the rate of the complex formation (HCrO3Cl·2·Cyanex-923) between HCrO− 4 and Cyanex 923. Therefore, the initial pH of the solution should be equal to 1.0 for an efficient sorption of Cr(VI) by MCsCyanex 923 (see Fig. 6). After 1 h of contact time, the equilibrium pH values were 0.5, 2.5, 3.6, 4.8 and 6.5 for the initial pH of 1.0, 2.9, 3.9, 5.1 and 6.6, respectively. Apart from the HCl, Agrawal et al. [24] examined the other mineral acids (i.e., H2SO4 and HNO3) in order to adjust the pH of the solution, hence, efficiency of Cr(VI) extraction. They reported that the maximum extraction efficiency was obtained when HCl was involved in the extraction. Therefore, in further experiments of this study, the initial pH of the feed phase was adjusted to 1.0 ± 0.1 by using HCl. 3.2.3. Effect of contact time and kinetic evaluation It is seen in Fig. 7 that the sorption of Cr(VI) attained equilibrium after 30 min without dependence of initial Cr(VI) concentration. According to Barassi et al. [19], the concentration of the extractant on the surface of MCs permits the evaluation of the rate of the chemisorption by means of pseudo second-order kinetic model.
S. Ozcan et al. / Desalination 259 (2010) 179–186
183
Fig. 4. TGA and its first derivatives of (a) blank MCs (polysulfone: dichloromethane =2 g: 50 mL, continuous phase: 1 wt.% gum arabic solution) and (b) MCs-Cyanex 923 (polysulfone: Cyanex 923: dichloromethane =2 g: 0.25 g: 50 mL, continuous phase: 1 wt.% gum arabic solution).
Therefore, the experimental data was applied to the pseudo secondorder kinetic model given as Eq. (9) [35]. 2 t = qt = 1 = k2:qe +t = qe
ð9Þ
Where qe and qt are the amounts of Cr(VI) sorbed (mmol/g) at equilibrium and time t (min), respectively, k2 is the rate constant of
pseudo second-order chemisorption (g/(mmol. min)). For the studied concentrations, the rate constant (k2) and theoretical equilibrium sorption capacities, qe (calculated), were calculated from the slope and the intercept of the linear plots of the pseudo second-order kinetic model. The coefficients of determination (R2) and theoretical and experimental qe values suggested that the sorption of Cr(VI) by MCs-Cyanex 923 followed the second-order type reaction kinetics (Table 1).
184
S. Ozcan et al. / Desalination 259 (2010) 179–186 Table 1 Values of sorption rate constant for pseudo second-order kinetic model. Cr(VI) concentration, mmol/L 0.096 0.192 0.962
Fig. 5. Effect of Cyanex 923 to polysulfone ratio in dispersed phase on the sorption of Cr(VI) and encapsulation efficiency (concentration of Cr(VI): 0.385 mmol/L, initial pH of solution: 1.0(±0.1), contact time: 1 h, shaking speed: 200 rpm, temperature: 25 °C, ionic strength: 0.01 M, amount of MCs-Cyanex 923: 1.5 g/L; amount of polysulfone in dispersed phase: 2 g, and volume of dichloromethane: 50 mL).
qe (experimental) (mmol/g)
k2 [g/(mmol. min)]
qe (calculated) (mmol/g)
R2
0.043 0.090 0.320
15.923 8.128 2.805
0.053 0.095 0.321
0.999 0.999 0.999
3.2.4. Effect of initial concentration of Cr(VI) and sorption isotherm models The influence of initial concentration of Cr(VI) on its removal was examined by varying the initial Cr(VI) concentration at pH 1.0(±0.1) and 1 h of equilibrium time. The results demonstrated that increasing the initial concentration improved the amount of the Cr(VI) sorbed. For instance, the amounts of Cr(VI) sorbed were 0.043, 0.090 and 0.305 mmol/g, for the initial concentrations of 0.096, 0.192 and 0.961 mmol/L, respectively. This may be due to the increasing driving force for mass transfer with the concentration of Cr(VI) [36]. The experimental data was analyzed with two parameters isotherm models including Langmuir and Freundlich models and three parameters Redlich–Peterson isotherm model. The Langmuir isotherm models the monolayer coverage of the sorption surfaces and assumes that sorption take places on a structurally homogeneous surface of the sorbent. This isotherm is given as Eq. (10) [37]. qe = Q o :b:Ce = ð1 + b:Ce Þ
ð10Þ
The linear form of the Langmuir isotherm model can be presented as Eq. (11); Ce = qe = ð1 = Q o :bÞ + ðCe = qe Þ
ð11Þ
Where Ce is the concentration of Cr(VI) (mmol/L) at equilibrium, Q 0 is the monolayer capacity of the adsorbent (mmol/g) and b is the Langmuir sorption constant (L/mmol). The Freundlich equation is derived to model the multilayer sorption and for the sorption on heterogeneous surfaces. The Freundlich model is formulated as Eq. (12) [38]. 1=n
qe = k:Ce Fig. 6. Effect of initial pH of the solution on the sorption of Cr(VI) (concentration of Cr(VI): 0.385 mmol/L, contact time: 1 h, amount of MCs-Cyanex 923: 1.5 g/L).
ð12Þ
Linearized form of the Freundlich equation is given by the following equation: log qe = log k + ð1 = nÞ log Ce
ð13Þ
Where Ce is the equilibrium concentration (mmol/L), k is the sorption capacity (mmol/ g) and n is an empirical parameter. The Redlich–Peterson isotherm [39] incorporates three parameters into an empirical isotherm, and therefore, can be applied either in homogeneous or heterogeneous systems due to its high versatility. The Redlich–Peterson equation is: β qe = KR :Ce = 1 + aR :Ce
Fig. 7. Effect of contact time on the sorption of Cr(VI) (concentrations of Cr(VI): 0.096, 0.192 and 0.962 mmol/L, initial pH of solution: 1.0(± 0.1), amount of MCs-Cyanex 923: 1.5 g/L).
ð14Þ
Where, KR is Redlich–Peterson isotherm constant (L/mmol), aR is also a constant (L/mmol)β and β is the exponent which lies between zero and one. This isotherm model involves varying the isotherm parameter, KR to obtain the maximum (R2) value for the linear regression of ln Ce vs. ln[(KR·Ce/qe) − 1]. For β = 1, Eq. (14) reduces to Langmuir equation and for β = 0, it reduces to Henry's equation. Eq. (14) can be converted to a linear form by taking logarithms: ln ½ðKR :Ce = qe Þ−1 = ln aR + β: ln Ce
ð15Þ
S. Ozcan et al. / Desalination 259 (2010) 179–186
Fig. 8. Non-linear isotherm plots for the sorption of Cr(VI) by MCs-Cyanex 923 (initial pH of solution: 1.0(± 0.1), contact time: 1 h, amount of MCs-Cyanex 923: 1.5 g/L).
Fig. 8 shows the Langmuir, Freundlich and Redlich–Peterson isotherm models and the experimental data for the sorption of Cr(VI) by MCs-Cyanex 923. The values of the isotherm constants were obtained from the slope and intercept of the plots of the linear form of each isotherm. The Langmuir sorption capacity, Q0, and the sorption equilibrium constant, b, were 0.430 mmol/g and 4.024 L/mmol, respectively. The Freundlich isotherm constant, k, was 0.262 mmol/g and n was 2.786; the Redlich–Peterson isotherm constant KR was 3.000 L/mmol, aR was 8.566 (L/mmol)β and β was 0.858. The R2 values for the Langmuir, Freundlich and Redlich–Peterson isotherm models were 0.992, 0.895 and 0.994, respectively, which means that sorption of Cr(VI) by MCsCyanex 923 could be better described by the Redlich–Peterson and the Langmuir isotherm models. The Chi-square (χ2) test, defined by Eq. (16), was also employed to find out the best-fit isotherm model for the sorption data [40]. h i 2 2 χ = Σ ðqe –qm Þ = qm
used for the synthesis of MCs may also affect both encapsulation efficiency and pyhsical property, hence the sorption capacity of the MCs. For example, Barassi et al. [19] used the in situ radical polymerization method for the preparation of MCs-Aliquat 336 with 87% of encapsulation and 20 µm of mean diameter. In the present study, the solvent evaporation method was used in order to obtain the MCs-Cyanex 923 because it was easier than radical polymerization method. The encapsulation and mean diameter of MCs-Cyanex 923 were determined as 75% and 273 µm, respectively. Therefore, it is expected that the sorption capacity of MCs-Aliquat 336 is higher than that of MCs-Cyanex 923 due to higher surface area and encapsulation efficiency. Nevertheless, it should be emphasized that the potential use of Cyanex 923 for the removal or recovery of solutes from water is more extensive than that of Aliquat 336 because it has affinity towards for both inorganic and organic solutes, i.e. phenol, carboxylic acid, ect. In addition, the major advantage of Cyanex 923 over similar extraction reagents, e.g. trioctylphosphine oxide, is that it is completely miscible with all common hydrocarbon diluents even at low ambient temperatures. The major benefit of high solubility lies in the ability to prepare concentrated, stable solvents which can recover solutes (e.g. acetic acid) that are normally only weakly extracted by this type of reagent [42]. MCs-Cyanex 923 showed comparable Cr(VI) sorption capacity with Lewatit MP64 (0.400 mmol/g) and Lewatit MP500 (0.410 mmol/g) anion-exchange resins [43]. The sorption capacity of MCs-Cyanex 923 was lower than that of commercial activated carbon (0.580 mmol/g) [6]. However, the commercial activated carbon is very expensive and has high operating costs. 3.2.5. Effect of MCs-Cyanex 923 dosage An increase in the MCs-Cyanex 923 dosage increased the removal of Cr(VI) (Fig. 9). This is consistent with the expectation that higher sorbent dosage results in lower q values. The concentration of Cyanex 923 is related to MCs-Cyanex 923 concentration through surface site density [44]. Therefore, the removal increased with increasing the microcapsule dosage, whereas q decreased.
ð16Þ
Where qm is the equilibrium capacity obtained by calculating from the model (mmol/ g), and qe is experimental data of the equilibrium capacity (mmol/g). Small number of χ2 indicates that data from the model is close to the experimental data whereas, a large number of χ2 indicates that data from the model is different from the experimental data. The χ2 values for the Langmuir, Freundlich and Redlich– Peterson isotherm models were 0.022, 0.110 and 0.025, respectively. This means that the best-fit isotherm models were the Langmuir and Redlich–Peterson isotherm models. In fact, for our experimental data, the Redlich–Peterson model supported the Langmuir isotherm model, meaning that the sorption process would occur in a monolayer over a homogeneous surface. This result can be also inferred from the value (0.858) of β, which is close to unity [41]. The maximum Cr(VI) sorption capacity of the MCs-Cyanex 923 was estimated as 0.430 mmol/g. The maximum Cr(VI) sorption capacity of the MCs containing Aliquat 336 was in the range of 0.857–0.967 mmol/g [19]. The higher result may be attributed to the difference between the sorption mechanisms of MCs containing Aliquat 336 and Cyanex 923. Aliquat 336, a commercial quaternary ammonium salt [(C8–10H17–21)3·CH3]N+·Cl−, reacts with the anionic chromium species (HCrO− 4 ) according to the following anion exchange mechanism [19]. þ
185
−
−
þ
−
3.2.6. Effect of other metal ions on the removal of Cr(VI) The effects of other metal ions, including Cr(III), Cu(II), Pb(II), Zn(II), Ni(II), Co(II) and Cd(II), on the removal of Cr(VI) by MCs-Cyanex 923 were investigated. The results showed that Cr(III), Cu(II), Pb(II), Ni(II),
−
½ðC8–10 H17–21 Þ3 :CH3 N :Cl þHCrO4 ⇔½ðC8–10 H17–21 Þ3 :CH3 N :HCrO4 þ Cl
However, the sorption of Cr(VI) by Cyanex 923 is based on the complexation mechanism as described in Section 3.2.2. The procedure
Fig. 9. The variation of sorption of Cr(VI) with MCs-Cyanex 923 dosage (concentration of Cr(VI): 0.962 mmol/L, initial pH of solution: 1.0(± 0.1), contact time: 1 h, amount of MCs-Cyanex 923: 1.5 g/L).
186
S. Ozcan et al. / Desalination 259 (2010) 179–186
Table 2 Effect of other metal ions on the removal of Cr(VI) by MCs-Cyanex 923. (Experimental conditions: concentration of each metal ion: 10 mg/L, dosage of MCs-Cyanex 923: 5 g/L, initial pH: 1.0(± 0.1) and contact time: 1 h). Metal ion in aqueous solution
Removal, %
Cr(III) Ni(II) Cu(II) Co(II) Pb(II) Zn(II) Cd(II) Cr(VI)
No No No No No 10 5 89
removal removal removal removal removal
and Co(II) were not removed from the solution (Table 2). Contrary, 10% of Zn(II) and 5% of Cd(II) were removed from aqueous solution by MCsCyanex 923. This can be attributable to that Cr(III), Cu(II), Pb(II), Ni(II), and Co(II) are present as their cationic forms in the aqueous solution used in the presented study, and thus they were not removed by MCsCyanex 923. This was not the case for Zn(II) and Cd(II) which are present in the solution as the corresponding chloro-complexes, and were sorbed by MCs-Cyanex 923. Similar results were reported by Alguacil et al. [45], who studied the facilitated transport of Cr(VI) through supported liquid membrane containing Cyanex 921 and Cyanex 923. In addition, Agrawal et al. [24] investigated the effect of various metal ions (i.e., Cr(III), Ni(II), Ca(II), Mg(II), Fe(II), Zn(II), Cu(II), etc.) on the extraction of Cr(VI) by Cyanex 923 and they reported that only Zn(II) was found to be interfering species. The results also indicated that Cr(VI) was preferably removed by MCs-Cyanex 923 against other metal ions. However, sorption percentage of Cr(VI) in the presence of these ions was 89%. This value was lower than that (95%) of the prensence of only-Cr(VI) in the aqueous solution. The result may be because of a decrease in nominal Cyanex 923 concentration of the MCs due to the competitive sorption of Zn(II), Cd(II) and Cr(VI) [45,46]. Similar findings were reported in the related literatures [12,24]. 3.2.7. Regeneration and reusability of MCs-Cyanex 923 The regeneration experiments were carried out to see whether the MCs-Cyanex 923 loaded with Cr(VI) could be chemically regenerated. According to the literature, the most efficient solution was NaOH for stripping of Cr(VI) from Cr(VI)–Cyanex 923 complex [24]. Therefore, in the presented study, 0.5 M NaOH (10 mL) was used for the desorption of Cr(VI) from the loaded MCs-Cyanex 923. After desorption of Cr(VI), the MCs-Cyanex 923 were washed with distilled water until neutralization and then desiccated at room temperature. The adsorption process was repeated by using the regenerated MCs. It was found that after 3 cycles there is no change in the Cr(VI) sorption capacity of the MCs-Cyanex 923. In desorption experiments, it was observed that almost the total recovery of Cr(VI) occurs, suggesting that the MCs-Cyanex 923 are regenerable and can be used several times. 4. Conclusion The following conclusions can be made from the presented study. The results from the FT-IR, TGA, SEM of the MCs clearly indicated that immobilization of Cyanex 923 onto the MCs was successfully performed. The MCs-Cyanex 923 exhibited significant sorption capability for Cr(VI). The maximum removal of Cr(VI) was obtained when the ratio of Cyanex 923 to polysulfone was 1.0 at an initial pH of 1.0. The sorption of Cr(VI) by MCs-Cyanex 923 was presented by means of the complexation mechanism. The sufficient time for the
sorption equilibrium was 30 min and it obeyed the pseudo secondorder kinetic model. The Langmuir sorption capacity of MCs-Cyanex 923 for Cr(VI) was estimated as 0.430 mmol/g. The Cr(VI) was preferably sorbed by prepared microcapsules against the other metal ions [i.e., Cr(III), Ni(II), Cu(II), Zn(II), Cd(II), Co(II) and Pb(II) etc.]. The regeneration studies also showed that MCs-Cyanex 923 can be used several times for the sorption of Cr(VI) from the aqueous solutions.
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