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Evaluation of chitosan-carbon based biocomposite for efficient removal of phenols from aqueous solutions
Journal of Water Process Engineering 16 (2017) 56–63
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Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe
Evaluation of chitosan-carbon based biocomposite for efficient removal of phenols from aqueous solutions Usha Soni a , Jaya Bajpai a , Sunil Kumar Singh b , A.K. Bajpai a,∗ a b
Bose Memorial Research Laboratory, Department of Chemistry, Government Autonomous Science College, Jabalpur, MP, India Department of Chemistry, Guru Ghasidas Central University, Bilaspur, CG, India
a r t i c l e
i n f o
Article history: Received 19 September 2016 Received in revised form 27 November 2016 Accepted 15 December 2016 Keywords: Adsorption Biopolymer Nanocomposites Phenol Isotherm
a b s t r a c t Nanocomposite particles of chitosan and activated carbon were prepared for removal of phenols from aqueous solutions. The nanoparticles were characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) spectroscopy, and particle size and charge analysis. Removal of phenol from aqueous solution was optimized by varying experimental conditions like initial concentration of phenol, pH, adsorbent doses, temperature and contact time. Equilibrium adsorption studies and kinetics of adsorption process showed that adsorption process followed Freundlich isotherm and pseudo-second order kinetic model, respectively. The maximum adsorption capacity of phenol was found to be 409 mg/g. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Contamination of surface and ground waters with aromatic compounds are one of the most serious environmental global problems that have endangered the whole civilization including human beings and living organisms [1]. Owning to its inherent toxicity and good solubility phenols are considered to be one of the potential organic pollutants being discharged into the environment causing severe physiological disorders [2,3]. Major sources of phenol pollutants in the aquatic environments are waste water from the outlets of fine chemical plants such as paints, pesticides, coal conversion, polymeric resins, petroleum and petrochemical industries [4]. Degradation of these substances produces phenols and its derivatives in the environment and its further chlorination as a part of disinfection produces chlorinated phenols which are more toxic and cause other severe hazards. The tolerance limit of the phenol contents in the drinking water is not very high and the concentration should not exceed 0.002 mg/L as per the Indian standard [5]. Phenol is highly toxic and mutagenic substance at large concentration and may be absorbed by the skin. Treatments of this organic compound containing water have drawn significant concern because of the high toxicity of the phenolic compounds.
∗ Corresponding author. E-mail address: [email protected] (A.K. Bajpai). http://dx.doi.org/10.1016/j.jwpe.2016.12.004 2214-7144/© 2016 Elsevier Ltd. All rights reserved.
In recent years, however, many techniques have been applied for purification of phenols contaminated water like ozonolysis, photolysis and photocatalytic decomposition but the success rate is quite low [6]. Traditionally biological treatments, activated carbon adsorption, reverse osmosis, ion exchange and solvent extraction are most widely used techniques for removing phenols and related organic substances [7–10]. However, the adsorption appears to be the most prominent technique for the water reuse in terms of the initial cost, flexibility and simplicity of the design, ease of operation and insensitivity to toxic pollutants [11–16]. It also does not lead to the formation of the harmful substances. Activated carbons are another vital adsorbents and exhibit good adsorption capacity for many organic pollutants [17], but its high cost due to its difficult regeneration and high disposal cost as well as poor adsorption ability to phenols push to explore other adsorbents which could be equally effective but biodegradable so as to reduce the cost of disposal and regeneration. In recent years polymeric adsorbents have found extensive use as an alternative to activated carbons due to their economic viability, adsorbent regeneration properties, presence of large number of functional groups, ease of derivatization into more useful forms, and fairly good mechanical strength. Chitosan is a kind of positively charged polysaccharide prepared by the N-de acetylation of chitin which makes up the shell and shrimps. Due to the primary, secondary hydroxyl groups and highly reactive amino groups of chitosan as well as the property of the nontoxicity and biodegradability, it has been regarded as one of the preferred materials for separation studies. However, the adsorp-
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tion capacity of phenol on chitosan is reported to be low [18]. and some studies on chemical modification of chitosan have also been carried out to enhance its activity through addition of novel ligand into its structure [19]. Many industrial applications of chitosan are known due to the secondary amino groups of chitosan which show poly-cationic, chelating and film forming properties along with high solubility in the dilute acids.
using Zetasizer Nano (Backman Coulter Delso Nano C.) Dynamic light scattering technique was used to determine particle size. The zeta potential distribution was determined from the zeta potential (Mv) versus intensity (kcps) curve and the measurements were performed at 25 ◦ C with the count rate of 2272.3 kcps.
2. Experimental
The concentration of phenol in aqueous solution was determined by measuring absorbance at wavelength of 269 nm, using a UV spectrometer (model Shimadzu UV-1500). In order to reduce measurement errors in all the experiments, the UV adsorption intensity of each solution sample was measured in triplicate and the average value was used to calculate the equilibrium concentration based on standard calibration curve, whose correlation coefficient (R2 ) was found to be 0.9872.
2.1. Materials Chitosan, activated carbon, acetic acid, toluene, tripolyphosphate (TPP) and phenol used were of analytical reagent grade and supplied by Merck India. Water used for preparation of solutions was generated in the laboratory by double distilling the deionized water in a distillation unit. The stock solution of phenol was prepared by dissolving 100 mg phenol in one litter water. All other required reagents were of analytical grade quality. 2.2. Preparation of the adsorbent Chitosan based biosorbent was prepared by emulsion crosslinking method. In brief, for preparation of adsorbent a known amount of chitosan was dissolved in 10 mL of 1% acetic acid solution and after its homogenous mixing a requisite amount of activated carbon was added in it. The whole mixture was stirred on a magnetic stirrer for about 1 h at room temperature while for preparing w/o emulsion, 10 mL paraffin-oil was added in to homogenous mixture of chitosan-charcoal suspension. The above solutions were mixed on a magnetic stirrer for 2 h to form a stable emulsion. Now to this stable emulsion 10 mL of TPP solution of 0.1 M concentration was dropwise added and stirred for 4 h at room temperature. The nanocomposite particles so prepared were cleaned by washing frequently with toluene and acetone. The final particles were dried at room temperature and stored in an air tight polyethylene bag. 2.3. Characterization Prior to using the prepared adsorbent for adsorption experiments, the adsorbent was characterized by the following techniques.
2.4. Quantification of phenol solution
2.5. Adsorption studies (Batch process) Adsorption experiments were carried out using the batch contact method. In brief, 50 mg of chitosan based nanocomposite was added into a 10 mL of phenol solution at constant pH and temperature. The suspension was shaken on a thermostat shaker (Rivotech India) for 1 h to attain equilibrium at the room temperature (30 ◦ C). After shaking was over the mixture was filtered through the Whatman filter paper (2.5 size particle retention). The adsorption capacity of the adsorbent was determined by material balance of the initial and equilibrium concentrations of the solution. The adsorbed amount and the percentage removal of phenol were calculated by using the following equations, respectively.
Absorbed amount mg/g = % removal =
Ci − Cf ×V m
Ci − Cf Ci
(1) (2)
Where Ci is the initial and the Cf the final concentrations of the metal ion solutions (mg/L), V is the volume of the adsorbate solution, and m being is the weight of adsorbent (chitosan composite). The pH of the solution was adjusted using required volumes of 0.1 M HCL and/or 0.1 N NaOH before adding the adsorbent. 2.6. Statistical analysis
2.3.1. Fourier transforms infrared (FTIR) spectroscopy The FTIR spectra of prepared chitosan based biosorbent were recorded on a FTIR-8400, Shimadzu spectrophotometer. Samples for the spectral analysis were prepared by mixing adsorbent and KBr in1:10 proportion and the spectra were obtained in the range of 4000–400 cm−1 with a resolution of 2 cm−1 .
All measurements were done at least 3 times and graphs have been plotted along with the respective error bars. 3. Results and discussion 3.1. Fourier transform infrared (FTIR) studies
2.3.2. SEM/EDX analysis A scanning electron microscope inter-phased with an electron dispersive X-ray spectrometer (SEM/EDX, JEO, JSM-5800LV) was used to study the surface morphologies and elemental analysis of chitosan based nanocomposite and activated charcoal. 2.3.3. X-Ray diffraction (XRD) analysis In order to ascertain crystalline nature of the chitosan based nanocomposite. XRD analysis were performed on a rotating X-ray diffractometer in the 2 range of 10–70 ◦ C. 2.3.4. Particle size and particle charge analysis The sample was prepared by dispersing a definite amount of nanoparticles in ethylene glycol, having a viscosity 0.0100 cP as the dispersant. The dispersant was placed in the disposable zeta cell and the surface charge of the nanocomposite was determined
FTIR spectra of chitosan, chitosan-carbon nanocomposite particles, before and after adsorption are shown in Fig. 1(b–c), respectively. The FTIR spectrum of chitosan(1-a) shows a broad and strong band at 3431cm−1 is due to overlapping of the OH and N H stretching vibration of functional groups engaged in hydrogen bond [20]. The band at 2910 cm−1 and 2874 cm−1 is due to symmetric and asymmetric − CH2 vibration attributed to pyranose ring. A peak at 1653cm.−1 is C O stretching in amide group band,1572cm−1 is N H bending in non acetylated 2-amino glucose primary amine, The sharp peak at 1381cm−1 is C N stretching. The peaks at 1068cm−1 ,1035cm−1 ,are skeletal vibration involving the CO stretching which are characteristic of chitosan saccharide structure and 663cm−1 is due to −NH2 wagging vibration peak. The following changes were observed in the FTIR spectra of chitosan based nanoparticles after adsorption (Fig. 1b), the absorption
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Fig. 2. XRD spectra of chitosan − carbon nanocomposites.
Fig. 1. FTIR spectra of (a) chitosan (b) chitosan-carbon nanocomposite (c) phenol adsorbed chitosan based nanocomposite.
the nanocomposite and the peak at 20.6◦ which corresponds to a d (110) spacing of 0.45 nm X-ray diffraction (b) become weakened and the intensity of the peak became low. These evidences suggested that strong interaction occurred between activated carbon and chitosan molecule in the formation of nanocomposite. Many new sharp peaks were observed in XRD spectra of the nanocomposite which illustrated the presence of different alumino-silicate minerals in activated carbon. Major peaks were observed at 2 = 28, 29.2, 34, 36.9 and 40.1.The peak at 29.2 is for zeolites, and at 2 = 26 peaks corresponds to graphite. Spectra also depicts so many small and medium diffraction peaks at various 2 values which suggest for the presence of a number of other alumino silicates such as quartz, sodium silicates etc. The mean grain size of composite particles was calculated using Debye-Scherrer formula as shown in equation K ˇCOS
peak 3431 cm−1 has a shift to 3504 cm-1 and the peak for antisymmetric deformation of NH vibration in NH3 + ion is 1696 cm−1 [21,22]. Some new bands appears, a band at 1149 cm-1 that corresponds to P = O stretching vibration in phosphate ion [23]. and peak at 3745 cm−1 is due to surface functional group OH stretching vibration that present on activated charcoal [24]. A significant difference can be seen in the FTIR spectra of composite nanoparticles before and after adsorption, peaks are shifted and/or broadened indicating that the functional groups present on the composite nanoparticles is involved in interaction with the phenol.
Where is mean size, k is the shape factor (0.94),  is broadening of the diffaction angle and is diffraction wavelenth (1.54 A0 ).The estimated average crystal size of particles was calculated to be 28.1 nm with lattice strain 0.0074. The amorphous and crystalline nature of particles can also be quantified in terms of degree of crystallinity. The numerical formula to calculate the percent crystallinity of biocomposite is given in the following equation
3.2. X-Ray diffraction (XRD) analysis
xc%
XRD analysis is based on constructive interference of monochromatic X-rays. It is a non-destructive technique widely used to investigate the interlayer changes and the crystalline properties of the synthesized material. The inter-planer distances may be calculated by the following Braggs’ equation.
Where Ac and Aa are the area of crystalline and amorphous phases respectively [26]. The percent crystallinity of nanocomposite was found to be more than 80%. Chitosan is semi crystalline in nature, Impregnation of activated charcoal in the matrix increases its overall crystallinity and many sharp peaks with different intensities were observed in XRD spectra of chitosan based nanocomposites.
n = 2d(hkl)Sin
(3)
where is the wavelength of the X-ray, is the scattering angle, n is an integer representing the order of the diffraction peak, d is the inter planer distance of the lattices, and (hkl) are the Miller indices. The XRD spectra of chitosan based carbon composite particles were recorded and results are depicted in Fig. 2 which clearly indicates a crystalline nature of particles due to sharpness of peaks in the spectra. For the pure chitosan, there were two peaks around 2 value 10◦ and 200 [25] Chitosan retained semi crystalline characteristic in the nanocomposites. The peak of chitosan at 2 = 100 is disappearing in
=
Ac Aa + Ac
(4)
(5)
3.3. Particle size and charge analysis Particles size distribution of chitosan-carbon nanocomposite particles is shown in Fig. 3 .It was observed that aggregate size of chitosan based composite particles were found to lie in the range of 1200 to about 3000 nm. The zeta potential of composite particles is commonly used to characterize the surface charge properties of surface active materials. High electric surface charge shows high value of zeta potential of the composite particles due to repulsive forces between particles which leads to stop it from aggregation
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Fig. 3. Image showing particle size distribution curve of prepared chitosan-carbon nanocomposite particles. Fig. 5. Effect of initial concentration of phenol solution on its percent removal at [Temp] = 28 ± 2 ◦ C. [nanocomposites] = 0.05 g, [Shaking time] = 60 min.
[26,27]. The zeta potential of composite particles was measured before and after adsorption. Before adsorption it was found to be 54.75 mV, which indicates cationic nature of particles. After adsorption of phenol it decreases up to 1.02 mV which clearly indicates that phenol were adsorbed in the form of phenolate ions.
3.4. Scanning electron microscope with electron dispersive X-ray Emission electron microscopy is the most widely used technique for the investigation of shape, size morphology and porosity of the chitosan based nanoparticles matrices. SEM microphotographs of the activated native carbon and chitosan-carbon nanocomposite nanoparticles are shown in Fig. 4(a and b), respectively. The sem image of the native carbon particles show that the particles are well aggregated and the size of nanoparticles lie in the range up to 50 nm. On the other hand, the image (b) is entirely different from the image (a) and it reveals that during preparation of chitosan nanoparticles by microemulsion crosslinking method and in the immediate presence of native carbon nanoparticles, well defined nanocomposites are prepared which are up to 300 nm in width and several microns in length. This is an unusual finding and suggests that during crosslinking of chitosan nanoparticles, they act as micro reactors inside which nanocompsoite are fabricated.
3.5. Effect of phenol concentration The effect of initial concentration of phenol on its percentage removal was studied by varying the concentration of phenol from 2 to 8 mg per 10 mL of solution at fixed dose of adsorbent (0.05 g). The results are shown in Fig. 5 which clearly show that the percent removal of phenol increases from 12.5% to 69% as the initial concentration of phenol is increased from 2 to 8.0 mg per 10 mL of phenol solution. The observed results may be explained by the fact that the increased phenol concentrations provide the maximum driving force to overcome all the mass transfer resistances of phenol from the aqueous phase to solid phase resulting in higher probability of collision between phenol and the active sites. This obviously results in an increase in percent removal of phenol. 3.6. Effect of pH pH of the phenol solution is an important factor that may influences the uptake of the adsorbate. Experiments were conducted in the pH range 2–8 using 50 mg of chitosan based composite particles with 10 mL of phenol solution of 100 mg/L adsorbate solution at room temperature. In the alkaline range, the pH was varied using NaOH, whereas in the acidic range pH was adjusted using HCl. Results are shown in Fig. 6 which clearly reveal that the percent
Fig. 4. SEM images of (a) activated carbon, and (b) chitosan-carbon nanocomposite.
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6. Effect of pH on% Fig. [Temp] = 28 ± 2 ◦ C.[nanocomposites] = 0.05 g, [Shaking time] = 60 min.
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removal [Phenol
of phenol at solution] = 10 mg L−1 ,
removal of phenol increased with increase in pH up to 4 pH. Thus 78% removal of phenol at 4 pH was observed and then the percent removal of phenol decreases with further increase in pH. The observed results may be explained as below: It is well known that the zero point charge (ZPC) of carbon is about 8.0, ie. below this pH, the carbon surfaces shall be in positively charged state. It is also known that the pKa of phenol is about 10 and below this pH, the phenol molecules will also be present in unionized state. Now at pH 2.0, the adsorbent will have net positive charge over its surfaces and, therefore, the phenol molecules will be bound to the adsorbent surface through electrostatic attraction between the pai electron clouds of phenol and positive charged adsorbent surfaces. When the pH is raised, the phenol molecules will be slightly ionized and phenoxide ion will bound to the positively charged surfaces of the adsorbent. At pH 4.0, the adsorption of phenol reaches at optimum and, therefore, maximum removal of phenol is observed. However, beyond pH 4.0, the phenol molecules produce phenoxide ions which due to electrostatic repulsion between these ions and negatively charges of the surfaces result in a decreasing adsorption of phenol. This obviously results in a lower removal of phenol from the pH 4.0–8.0. Similar type of results has been reported by others [20]. The results can be explained as being the result of the increased electrostatic repulsion between the sorbate and sorbent since both are negatively charged over this pH range. This shows the phenol adsorption takes place mainly in its dissociated form adhering to the fact that the phenol has a pKa of 9.9 below which it remains undissociated. When pH of the solution goes beyond the pKa, phenol exists as negatively charged phenolate ions. Due to the electron rich nature of the oxygen atom in phenolate ions the hydrogen bonding efficiency decreases. Therefore, phenols are effectively adsorbed on to the adsorbent as the molecules but not phenolate ions. It means that molecular interactions are involved in the adsorption process. 3.7. Effect of adsorbent dose The effect of adsorbent doses on the removal of phenol was studied in the range 10–100 mg//10 mL of phenol solutions. It is clear from Fig. 7 that% removal of phenol increases with increasing adsorbent dose up to 70 mg and then it becomes almost constant thus showing no further effect of adsorbent dose. The increase in% removal of phenol could be explained by the fact that increase in
Fig. 7. Effect of adsorbent doses on% removal of phenol [Temp] = 28 ± 2 ◦ C, [Phenol solution] = 10 mg L−1 , [Shaking time] = 60 min.
Fig. 8. Effect of temperature on% removal of phenol [nanocomposites]=0.05 g, [Phenol solution] = 10 mg L−1 , [Time]=60 min.
adsorbent dose results in increase in number of active sites at which the phenol molecules get adsorbed. However, after the adsorbent dose of 70 mg, the percent removal of phenol assumes an asymptotic value which can be explained by the fact that there is an overlapping of active sites at high doses which decreases the surface area as was observed by others [20]. 3.8. Effect of temperature The effect of temperature on the percent removal of phenol has been studied by increasing the temperature of adsorption system from 20 ◦ C to 50 ◦ C. The results are shown in Fig. 8 which shows that on increasing the temperature of the adsorption system, the percent removal of phenol constantly decreases. The results clearly reveal that an optimum removal of 70% is obtained at 20 ◦ C. The decrease in percentage removal of phenol with increasing temperature may be due to damage of active binding sites and weakening of binding forces between the phenol molecules and the adsorbent. Similar type of results has also been reported elsewhere [28]. Another possible reason for the observed decrease in removal of phenol could be that on increasing the temperature from 20 to
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Table 1 Pseudo first order and Lagergren kinetic constant for the adsorption of phenol on the composite. qe experimental
mg g
33.605
K1
qe calculated
3.9 × 10−2
75
mg g
R2 0.9956
cability of the pseudo first order model to the adsorption process of the phenol onto chitosan based particles is unfeasible. The linearized form of pseudo-second order kinetics is given by the following equation: t 1 t = 2 2 + qt qe K qe
Fig. 9. Effect of shaking time on% removal of phenol [Temp] = 28 ± 2 ◦ C [nanocomposites] = 0.05 g, [Phenol solution] = 10 mg L−1 .
50 ◦ C, the solubility of phenol in water also increases that obviously results in less removal of phenol. The observed results also reveal that the adsorption of phenol onto the chitosan nanocomposites is an exothermic process.
(6)
From the equation, the plot of qtt against t should give a linear relationship. The qe andK2 can be determined from the slope and intercept of the plot. The pseudo second-order kinetic parameters are presented in Table 1. From the above data we see that the values qe (experimented) values are quite closer to those of qe (calculated) values for pseudo second order kinetics. Hence, it can be concluded that the adsorption of phenol on chitosan based nano particles is described by pseudo second order kinetic mode. 3.11. Adsorption isotherms
3.9. Effect of contact time Contact time is an inevitably a fundamental parameter for an economical waste water treatment process. The effect of the contact time on the percent removal of phenol is shown in Fig. 9. The extent of phenol removal increases with time and after a certain time of contact, it starts decreasing. The observed increase in the initial contact time period may be explained by the reason that greater the time of contact, larger would be the interactions between the phenol molecules and adsorbent. This obviously brings out an increase in percent removal of phenol. However, after a certain time period, 60 min in the present study, the percent removal of phenol starts decreasing which may be attributed to beginning of desorption of phenol molecules from the adsorbent. The observed desorption clearly results in a falling percent removal of phenol. The observed desorption of phenol is quite expected since the phenol molecules adsorb onto the adsorbent through physical forces which after large agitation time period acquire enough kinetic energy that breaks up the binding forces between the phenol molecules and agitating adsorbent particles. 3.10. Adsorption kinetics Kinetic study is helpful in prediction of adsorption rate constants, adsorption capacity and adsorption mechanism. The applicability of pseudo-first order and pseudo second order kinetic models were examined in this study at constant temperature of 25 ◦ C. The linear form of pseudo- first order model of Lagergreen [29] is generally expressed as follows: Log (qe − qt ) = log (qe ) − K1 t (4). Where qe and qt the adsorption capacities of the phenol per unit weight of nanocomposite particles at equilibrium and at time t (min), respectively (mg/g) and K 1 is the pseudo first order rate constant (min−1 ), K1 andqe values are determined from the slope and the intercepts of the graph which is plotted between Log (qe − qt ) vs t. The results obtained are summarized in Table 1. From the data shown in Table 1 it can be seen that experimental qe values do not agree with the calculated ones that shows appli-
Several models have been published in the literature to describe the equilibrium adsorption systems. Freundlich and Langmuir isotherms are commonly used to describe the adsorption characteristics of adsorbent utilized in water and wastewater treatment. Therefore, adsorption data of phenol on chitosan based nano composite were employed to test Freundlich and Langmuir models in this study. Which have been tested to describe the relationship between the amounts of phenol adsorbed (qe) and its equilibrium concentration (Ce) in the solution. 3.11.1. Freundlich isotherm Freundlich isotherm assumes that the uptake of phenol occurs on a heterogeneous surface by multilayer adsorption and that the amount of metal ions adsorbed increases infinitely with an increase in concentration. It is a most popular model for a single solute system, based on the distribution of solute between the solid phase and aqueous phase at equilibrium. Freundlich isotherm is an empirical equation and expressed as, qe = Kf Ce1/n
(7)
where qe is the amount of phenol adsorbed per unit mass of adsorbent (mg/g), Ce is the equilibrium concentration of metal ions (mg/L), Kf is the measure of adsorption capacity(empirical constants depending on several environmental factors), 1/n being the adsorption intensity (an empirical parameter representing the energetic heterogeneity of the adsorption sites). The equation is conveniently used in the linear form by taking the logarithmic of both sides as shown below: logqe = logKf + logCe
(8)
According to the above equation, a plot of (log qe) against (logCe) yields a straight line (Fig. 10) which indicates the confirmation of the Freundlich isotherm for adsorption. The constants can be obtained from the slope and the intercept of the linear plot of the experimental data and summarized in Table 2. The value of n indicates a favourable physical adsorption. Vander-Waal’s forces involved in adsorption.
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reflects monolayer adsorption. Moreover, the Langmuir constants b and Q0 can be evaluated from the slope and intercept of linear equation, respectively. The values of Q0 and b obtained in this work are presented in Table 2 respectively. 3.11.3. Temkin isotherm Temkins isotherm model was also applied to the experimental data. The Temkin isotherm equation assumes that the heat of adsorption of all the molecules in layer decreases linearly with coverage due to adsorbent–adsorbate interactions, and that the adsorption is characterized by a uniform distribution of the bonding energies, up to some maximum binding energy was carried out by plotting the quantity sorbed qe against lnCe and the constants were determined from the slope and intercept. The model is represented by the following equation qe = RT Fig. 10. Freundlich isotherm for adsorption of phenol onto chitosan − carbon nanocomposites, Shaking time = 60 min, Temp. = 28 ±2 ◦ C, nanocomposite = 0.05 g. Table 2 Data showing the values of adsorption isotherms obtained for adsorption of phenol on chitosan based nanocomposites. Parameters Isotherm 2
R
n Freundlich kf(mg/g) Langmuir 2
0.9721 0.09 1.7×10−7 0.9178 0.568 −1.103
Tempkin R2 B AT(L/g)
09863 115118 138
D-R model R2 qm(mol/g)  (mol2 /j2 )
0.9301 9862 −6.88
Q 0bCe 1 + bCe
Eq. (11) can be expressed in its linear form as: qe = BlnAT + BlnCe
lnqe = lnqm −  ∈ 2
(9)
where a is the amount of phenol adsorbed per unit mass of adsorbent (mg/g), Ce = is the equilibrium concentration of phenol ions (mg/L), Q0 is a measure of adsorption capacity of adsorbent (mg/g), b is the Langmuir constant which is measure of energy of adsorption (L/mg). The above equation can be rearranged to give the following linear Ce Ce 1 + = a Q0 Q 0b
(12)
(13)
3.11.4. Dubinin–Radushkevich isotherm Dubinin–Radushkevich isotherm is generally applied to express the adsorption mechanism with a Gaussian energy distribution onto a heterogeneous surface. The model has often successfully fitted high solute activities and the intermediate range of concentrations data well. The linear form of D–R isotherm is presented as the following equation:
3.11.2. Langmuir isotherm The Langmuir adsorption isotherm is often used to describe adsorption of solutes from a liquid solution. A basic assumption of the Langmuir theory is that sorption takes place at specific homogeneous sites within the adsorbent. It is then assumed that once a phenol occupies a site, no further sorption can take place at that site. The rate of sorption to the surface should be proportional to a driving force multiplied by area. The driving force for adsorption is the concentration in the solution, and the area is the amount of bare surface available on the adsorbent. The Langmuir adsorption isotherm is perhaps the best known of all the isotherms and is often expressed as: a=
RT RT RT lnAT + lnCe B = bT bT bT
(11)
where AT = Temkin isotherm equilibrium binding constant(L/g), bT = Temkin isotherm constant, R = universal gas constant (8.314 J/mol/K), T = Temperature at 298 K, B = Constant related to heat of sorption (J/mol). The values of AT and B are shown in Table 2.
Value
R Q0(mg/g) b (L/g)
qe =
RT ln(AT Ce) b
(10)
The above equation can be used for the linearization of experimental data by plotting Ce/a against Ce, and a good fit of this equation
(14)
where qe is the amount of chromium ions adsorbed per unit mass of adsorbent (mg/g), qm is theoretical isotherm saturation capacity (mg/g),  is the constant of the sorption energy (mol2 /J2 ), and ∈ = Polanyi potential, which is described as ∈ = RTIn
1+
1 Ce
(15)
where T is the solution temperature (K) and R is the gas constant and is equal to 8.314 J/mol K. Values of qm and  , are calculated from the intercept and slope of the plot by plotting ln qe versus ∈ 2 and the values are summarized in Table 2, As seen in Table 2, that Langmuir isotherm is unsuitable to describe the sorption of phenol and Freundlich equation fits to adsorption system well. (Correlation coefficient R2 > 0.97). The parameters of the fitted curve are summarized in the Table 2. Langmuir isotherm is based on a monolayer homogenous surface, while the Freundlich isotherm is applied to adsorption process on heterogeneous surface. 4. Conclusions The adsorption of phenol from water using chitosan based nano bio composites was investigated; results show outstanding adsorption ability for phenol. Various experimental parameters such as contact time, adsorbent dose, and temperature and solution pH were optimized. The removal of phenol was found to be highly influenced by initial phenol concentration and phenol speciation which is directly related to solution pH. Equilibrium adsorption
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studies show that the data fit very well in the Freundlich equation of multilayer adsorption, this result indicate the heterogeneity of the adsorbent surface. The values of R2 obtained from pseudo secondorder model were higher than 0.999, indicating that the adsorption process obeyed the pseudo second-order model. Finally the adsorption experiments indicated that chitosan based nano composite was efficient adsorbent for the removal of phenol from water. References [1] Z.P. Zhang, S.S. Feng, The drug encapsulation efficiency in vitro drug release, cellular uptake and cytotoxicity of paclitaxel-loaded poly (lactide)-tocopheryl polyethylene glycol succinate nanoparticles, Biomaterials 27 (2006) 4025–4033. [2] N. Calace, Nardi E. Petronio, B.M. Pietroletti, Adsorption of phenols by papermill sludges, Environ. Pollut. 118 (2002) 315. [3] F. Akbal, A.N. Onar, Particle size and shape effects on adsorption rate parameters, Environ. Monit. Assess. 83 (2003) 301. [4] Guido Buscaa, Silvia Berardinelli, Carlo Resini, Laura Arrighi, Technologies for the removal of phenol from fluid streams: a short review of recent developments, J. Hazard. Mater. 160 (2008) 265–288. [5] Md. Ahmaruzzman, Adsorption of phenolic compounds on low cost adsorbents: a review, Adv. Colloid Interface Sci. 143 (2008) 48. [6] M.Q. Hu, Y.M. Xu, J.C. Zhao, Efficient photosensitized degradation of 4-chlorophenol over immobilized aluminium tetrasulphophthalocyanine in the presence of hydrogen peroxide, Langmuir 20 (2004) 6302. [7] O. Tepe, A.Y. Dursun, Combined effects of external mass transfer and biodegradation rates on removal of phenol by immobilized Ralstoniaeutropha in a packed bed reactor, J. Hazard. Mater. 151 (2008) 9. [8] A. Dabrowski, P. Podkoscielny, M. Hubicki, M. Barczak, Adsorption of phenolic compounds by activated carbon −a critic review, Chemoshpere 58 (2005) 1985. [9] G. Dursun, H. Cecek, A.Y. Dursum, Adsorption of phenol from aqueous solution by using carbonised beet pulp, J. Hazard. Mater. 125 (2005) 175. [10] J.F. Patterson, Industries Waste Water Treatment Technology, 2nd ed., Butterworths, London, 1985. [11] H.L. Wang, J.L. Chen, Z.C. Zai, Y.L. Chen, Q.X. Zhang, J. Chin, Studies on removal of phenol using ionic immobilised polymeric micro capsule, Environ. Chem. 23 (2004) 188. [12] Z.H. Fei, J.L. Chen, J.G. Cai, Y.P. Qiu, A.M. Li, Q.X. Zhang, Chin. J. Polym. Sci. 22 (6) (2004) 529. [13] Z.C. Zhai, J.L. Chen, Z.H. Fei, H.L. Wang, A.M. Li, Q.X. Zhang, React. Funct. Polym. 57 (2003) 93. [14] J.G. Cai, A.M. Li, H.Y. Shi, Z.H. Fei, C. Long, Q.X. Zhang, J. Hazard. Mater. 124 (2005) 173.
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[15] J.G. Cai, A.M. Li, H.Y. Shi, Z.H. Fei, C. Long, Q.X. Zhang, Adsorption behaviour of phenolic compounds onto polymeric adsorbents modified with benzoyl groups, Chemosphere 61 (2005) 502. [16] L.S. Wang, Organic Pollutant Chemistry, 1st ed., China Science Press, Beijing, 2004 (p.40). [17] F. Derbyshire, M. Jagtoyen, R. Andrews, A. Rao, I. Martin-Gullon, E. Grulke, Carbon materials in environmental applications, in: L.R. Radovic (Ed.), Chemistry and Physics of Carbon, vol. 27, Marcel Dekker, New York, 2001, p. 1–66. [18] Jian-Mei Li, Xiang-Guang Meng, Chang-Wei Hu, Juan Du, Adsorption of phenol and p-nitrophenol onto functional chitosan, Bioresour. Technol. 100 (3) (2009) 1168–1173. [19] R.S. Juang, C.Y. Ju, Kinetics of sorption of Cu (II)- ethylene-diaminetetraacetic acid chelated anions on cross-linked polyaminated chitosan beads, Ind. Eng. Chem. Res. 32 (1998) 386. [20] N.S. Kumar, M. Suguna, M.V. Subbaiah, A.S. Reddy, N.P. Kumar, A. Krishnaiah, Adsorption of phenolic compounds from aqueous solutions onto chitosan-coated perlite beads as biosorbent, Ind. Eng. Chem. Res. 49 (2010) 9238–9247. [21] C.N.R. Rao, Chemical Application of Infrared Spectroscopy, Academic Press, New-York London, 1963. [22] M. Pierog, M. Gierszewska-Druzynska, J. Ostrowska-Czubenko, Effect of Crosslinking agents on swelling behaviour of Chitosan hydrogel membranes, in: M.M. Jaworska (Ed.), Progress on Chemistry and Application of Chitin and Its Derivatives, Polish Chitin Society, Lodz, 2009. [23] D.E.C. Corbridge, E.J. Lowe, The Infrared Spectra of some inorganic phosphorous compounds, J. Chem. Soc. 493 (1954) 502. [24] P. Sugumaran, V. Priya Susan, P. RaviChandran, S. Sheshadri, Production and characterisation of Activated Carbon from Banana empty fruit bunch and Detonix region Fruit Pod, J. Sustain. Energy Environ. 3 (2012) 125–132. [25] M. Hasegawa, A. Isogai, F. Onabe, M. Usuda, R.H. Atalla, Characterization of cellulose–chitosan blends films, J. Appl. Polym. Sci. 45 (1992) 1873–1879. [26] G. Behl, M. Sharma, S. Dahiya, A. Chhikara, M. Chopra, Synthesis, characterization, and evaluation of radical scavenging ability of ellagic acid-loaded nano-gels, J. Nanomater. (2011) 1–9, http://dx.doi.org/10.1155/ 2011/695138. [27] Zhang W. Ming, Chen J. Long, Pan B. Cai, Zhang Q. Xing, B. Zhang, Synergistic adsorption of phenol from aqueous solution onto polymeric adsorbents, J. Hazard. Mater. 28 (2006) 123–129. [28] B. Agrawal, P. Sengupta, C. Balomajumder, Equilibrium, kinetic and thermodynamic studies of simultaneous co-adsorptive removal of phenol and cyanide using chitosan, Int. J. Chem. Nuclear Metall. Mater. Eng. 7 (11) (2013) 863–870. [29] S. Lagergren, Zur theorie der sogenannten adsorption gelo¨ ster stoffe, Kungliga Svenska Vetenskapsakademiens. Handlingar 24 (1898) 1–39.
Contents lists available at ScienceDirect
Journal of Water Process Engineering journal homepage: www.elsevier.com/locate/jwpe
Evaluation of chitosan-carbon based biocomposite for efficient removal of phenols from aqueous solutions Usha Soni a , Jaya Bajpai a , Sunil Kumar Singh b , A.K. Bajpai a,∗ a b
Bose Memorial Research Laboratory, Department of Chemistry, Government Autonomous Science College, Jabalpur, MP, India Department of Chemistry, Guru Ghasidas Central University, Bilaspur, CG, India
a r t i c l e
i n f o
Article history: Received 19 September 2016 Received in revised form 27 November 2016 Accepted 15 December 2016 Keywords: Adsorption Biopolymer Nanocomposites Phenol Isotherm
a b s t r a c t Nanocomposite particles of chitosan and activated carbon were prepared for removal of phenols from aqueous solutions. The nanoparticles were characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) spectroscopy, and particle size and charge analysis. Removal of phenol from aqueous solution was optimized by varying experimental conditions like initial concentration of phenol, pH, adsorbent doses, temperature and contact time. Equilibrium adsorption studies and kinetics of adsorption process showed that adsorption process followed Freundlich isotherm and pseudo-second order kinetic model, respectively. The maximum adsorption capacity of phenol was found to be 409 mg/g. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Contamination of surface and ground waters with aromatic compounds are one of the most serious environmental global problems that have endangered the whole civilization including human beings and living organisms [1]. Owning to its inherent toxicity and good solubility phenols are considered to be one of the potential organic pollutants being discharged into the environment causing severe physiological disorders [2,3]. Major sources of phenol pollutants in the aquatic environments are waste water from the outlets of fine chemical plants such as paints, pesticides, coal conversion, polymeric resins, petroleum and petrochemical industries [4]. Degradation of these substances produces phenols and its derivatives in the environment and its further chlorination as a part of disinfection produces chlorinated phenols which are more toxic and cause other severe hazards. The tolerance limit of the phenol contents in the drinking water is not very high and the concentration should not exceed 0.002 mg/L as per the Indian standard [5]. Phenol is highly toxic and mutagenic substance at large concentration and may be absorbed by the skin. Treatments of this organic compound containing water have drawn significant concern because of the high toxicity of the phenolic compounds.
∗ Corresponding author. E-mail address: [email protected] (A.K. Bajpai). http://dx.doi.org/10.1016/j.jwpe.2016.12.004 2214-7144/© 2016 Elsevier Ltd. All rights reserved.
In recent years, however, many techniques have been applied for purification of phenols contaminated water like ozonolysis, photolysis and photocatalytic decomposition but the success rate is quite low [6]. Traditionally biological treatments, activated carbon adsorption, reverse osmosis, ion exchange and solvent extraction are most widely used techniques for removing phenols and related organic substances [7–10]. However, the adsorption appears to be the most prominent technique for the water reuse in terms of the initial cost, flexibility and simplicity of the design, ease of operation and insensitivity to toxic pollutants [11–16]. It also does not lead to the formation of the harmful substances. Activated carbons are another vital adsorbents and exhibit good adsorption capacity for many organic pollutants [17], but its high cost due to its difficult regeneration and high disposal cost as well as poor adsorption ability to phenols push to explore other adsorbents which could be equally effective but biodegradable so as to reduce the cost of disposal and regeneration. In recent years polymeric adsorbents have found extensive use as an alternative to activated carbons due to their economic viability, adsorbent regeneration properties, presence of large number of functional groups, ease of derivatization into more useful forms, and fairly good mechanical strength. Chitosan is a kind of positively charged polysaccharide prepared by the N-de acetylation of chitin which makes up the shell and shrimps. Due to the primary, secondary hydroxyl groups and highly reactive amino groups of chitosan as well as the property of the nontoxicity and biodegradability, it has been regarded as one of the preferred materials for separation studies. However, the adsorp-
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tion capacity of phenol on chitosan is reported to be low [18]. and some studies on chemical modification of chitosan have also been carried out to enhance its activity through addition of novel ligand into its structure [19]. Many industrial applications of chitosan are known due to the secondary amino groups of chitosan which show poly-cationic, chelating and film forming properties along with high solubility in the dilute acids.
using Zetasizer Nano (Backman Coulter Delso Nano C.) Dynamic light scattering technique was used to determine particle size. The zeta potential distribution was determined from the zeta potential (Mv) versus intensity (kcps) curve and the measurements were performed at 25 ◦ C with the count rate of 2272.3 kcps.
2. Experimental
The concentration of phenol in aqueous solution was determined by measuring absorbance at wavelength of 269 nm, using a UV spectrometer (model Shimadzu UV-1500). In order to reduce measurement errors in all the experiments, the UV adsorption intensity of each solution sample was measured in triplicate and the average value was used to calculate the equilibrium concentration based on standard calibration curve, whose correlation coefficient (R2 ) was found to be 0.9872.
2.1. Materials Chitosan, activated carbon, acetic acid, toluene, tripolyphosphate (TPP) and phenol used were of analytical reagent grade and supplied by Merck India. Water used for preparation of solutions was generated in the laboratory by double distilling the deionized water in a distillation unit. The stock solution of phenol was prepared by dissolving 100 mg phenol in one litter water. All other required reagents were of analytical grade quality. 2.2. Preparation of the adsorbent Chitosan based biosorbent was prepared by emulsion crosslinking method. In brief, for preparation of adsorbent a known amount of chitosan was dissolved in 10 mL of 1% acetic acid solution and after its homogenous mixing a requisite amount of activated carbon was added in it. The whole mixture was stirred on a magnetic stirrer for about 1 h at room temperature while for preparing w/o emulsion, 10 mL paraffin-oil was added in to homogenous mixture of chitosan-charcoal suspension. The above solutions were mixed on a magnetic stirrer for 2 h to form a stable emulsion. Now to this stable emulsion 10 mL of TPP solution of 0.1 M concentration was dropwise added and stirred for 4 h at room temperature. The nanocomposite particles so prepared were cleaned by washing frequently with toluene and acetone. The final particles were dried at room temperature and stored in an air tight polyethylene bag. 2.3. Characterization Prior to using the prepared adsorbent for adsorption experiments, the adsorbent was characterized by the following techniques.
2.4. Quantification of phenol solution
2.5. Adsorption studies (Batch process) Adsorption experiments were carried out using the batch contact method. In brief, 50 mg of chitosan based nanocomposite was added into a 10 mL of phenol solution at constant pH and temperature. The suspension was shaken on a thermostat shaker (Rivotech India) for 1 h to attain equilibrium at the room temperature (30 ◦ C). After shaking was over the mixture was filtered through the Whatman filter paper (2.5 size particle retention). The adsorption capacity of the adsorbent was determined by material balance of the initial and equilibrium concentrations of the solution. The adsorbed amount and the percentage removal of phenol were calculated by using the following equations, respectively.
Absorbed amount mg/g = % removal =
Ci − Cf ×V m
Ci − Cf Ci
(1) (2)
Where Ci is the initial and the Cf the final concentrations of the metal ion solutions (mg/L), V is the volume of the adsorbate solution, and m being is the weight of adsorbent (chitosan composite). The pH of the solution was adjusted using required volumes of 0.1 M HCL and/or 0.1 N NaOH before adding the adsorbent. 2.6. Statistical analysis
2.3.1. Fourier transforms infrared (FTIR) spectroscopy The FTIR spectra of prepared chitosan based biosorbent were recorded on a FTIR-8400, Shimadzu spectrophotometer. Samples for the spectral analysis were prepared by mixing adsorbent and KBr in1:10 proportion and the spectra were obtained in the range of 4000–400 cm−1 with a resolution of 2 cm−1 .
All measurements were done at least 3 times and graphs have been plotted along with the respective error bars. 3. Results and discussion 3.1. Fourier transform infrared (FTIR) studies
2.3.2. SEM/EDX analysis A scanning electron microscope inter-phased with an electron dispersive X-ray spectrometer (SEM/EDX, JEO, JSM-5800LV) was used to study the surface morphologies and elemental analysis of chitosan based nanocomposite and activated charcoal. 2.3.3. X-Ray diffraction (XRD) analysis In order to ascertain crystalline nature of the chitosan based nanocomposite. XRD analysis were performed on a rotating X-ray diffractometer in the 2 range of 10–70 ◦ C. 2.3.4. Particle size and particle charge analysis The sample was prepared by dispersing a definite amount of nanoparticles in ethylene glycol, having a viscosity 0.0100 cP as the dispersant. The dispersant was placed in the disposable zeta cell and the surface charge of the nanocomposite was determined
FTIR spectra of chitosan, chitosan-carbon nanocomposite particles, before and after adsorption are shown in Fig. 1(b–c), respectively. The FTIR spectrum of chitosan(1-a) shows a broad and strong band at 3431cm−1 is due to overlapping of the OH and N H stretching vibration of functional groups engaged in hydrogen bond [20]. The band at 2910 cm−1 and 2874 cm−1 is due to symmetric and asymmetric − CH2 vibration attributed to pyranose ring. A peak at 1653cm.−1 is C O stretching in amide group band,1572cm−1 is N H bending in non acetylated 2-amino glucose primary amine, The sharp peak at 1381cm−1 is C N stretching. The peaks at 1068cm−1 ,1035cm−1 ,are skeletal vibration involving the CO stretching which are characteristic of chitosan saccharide structure and 663cm−1 is due to −NH2 wagging vibration peak. The following changes were observed in the FTIR spectra of chitosan based nanoparticles after adsorption (Fig. 1b), the absorption
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Fig. 2. XRD spectra of chitosan − carbon nanocomposites.
Fig. 1. FTIR spectra of (a) chitosan (b) chitosan-carbon nanocomposite (c) phenol adsorbed chitosan based nanocomposite.
the nanocomposite and the peak at 20.6◦ which corresponds to a d (110) spacing of 0.45 nm X-ray diffraction (b) become weakened and the intensity of the peak became low. These evidences suggested that strong interaction occurred between activated carbon and chitosan molecule in the formation of nanocomposite. Many new sharp peaks were observed in XRD spectra of the nanocomposite which illustrated the presence of different alumino-silicate minerals in activated carbon. Major peaks were observed at 2 = 28, 29.2, 34, 36.9 and 40.1.The peak at 29.2 is for zeolites, and at 2 = 26 peaks corresponds to graphite. Spectra also depicts so many small and medium diffraction peaks at various 2 values which suggest for the presence of a number of other alumino silicates such as quartz, sodium silicates etc. The mean grain size of composite particles was calculated using Debye-Scherrer formula as shown in equation K ˇCOS
peak 3431 cm−1 has a shift to 3504 cm-1 and the peak for antisymmetric deformation of NH vibration in NH3 + ion is 1696 cm−1 [21,22]. Some new bands appears, a band at 1149 cm-1 that corresponds to P = O stretching vibration in phosphate ion [23]. and peak at 3745 cm−1 is due to surface functional group OH stretching vibration that present on activated charcoal [24]. A significant difference can be seen in the FTIR spectra of composite nanoparticles before and after adsorption, peaks are shifted and/or broadened indicating that the functional groups present on the composite nanoparticles is involved in interaction with the phenol.
Where is mean size, k is the shape factor (0.94),  is broadening of the diffaction angle and is diffraction wavelenth (1.54 A0 ).The estimated average crystal size of particles was calculated to be 28.1 nm with lattice strain 0.0074. The amorphous and crystalline nature of particles can also be quantified in terms of degree of crystallinity. The numerical formula to calculate the percent crystallinity of biocomposite is given in the following equation
3.2. X-Ray diffraction (XRD) analysis
xc%
XRD analysis is based on constructive interference of monochromatic X-rays. It is a non-destructive technique widely used to investigate the interlayer changes and the crystalline properties of the synthesized material. The inter-planer distances may be calculated by the following Braggs’ equation.
Where Ac and Aa are the area of crystalline and amorphous phases respectively [26]. The percent crystallinity of nanocomposite was found to be more than 80%. Chitosan is semi crystalline in nature, Impregnation of activated charcoal in the matrix increases its overall crystallinity and many sharp peaks with different intensities were observed in XRD spectra of chitosan based nanocomposites.
n = 2d(hkl)Sin
(3)
where is the wavelength of the X-ray, is the scattering angle, n is an integer representing the order of the diffraction peak, d is the inter planer distance of the lattices, and (hkl) are the Miller indices. The XRD spectra of chitosan based carbon composite particles were recorded and results are depicted in Fig. 2 which clearly indicates a crystalline nature of particles due to sharpness of peaks in the spectra. For the pure chitosan, there were two peaks around 2 value 10◦ and 200 [25] Chitosan retained semi crystalline characteristic in the nanocomposites. The peak of chitosan at 2 = 100 is disappearing in
=
Ac Aa + Ac
(4)
(5)
3.3. Particle size and charge analysis Particles size distribution of chitosan-carbon nanocomposite particles is shown in Fig. 3 .It was observed that aggregate size of chitosan based composite particles were found to lie in the range of 1200 to about 3000 nm. The zeta potential of composite particles is commonly used to characterize the surface charge properties of surface active materials. High electric surface charge shows high value of zeta potential of the composite particles due to repulsive forces between particles which leads to stop it from aggregation
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Fig. 3. Image showing particle size distribution curve of prepared chitosan-carbon nanocomposite particles. Fig. 5. Effect of initial concentration of phenol solution on its percent removal at [Temp] = 28 ± 2 ◦ C. [nanocomposites] = 0.05 g, [Shaking time] = 60 min.
[26,27]. The zeta potential of composite particles was measured before and after adsorption. Before adsorption it was found to be 54.75 mV, which indicates cationic nature of particles. After adsorption of phenol it decreases up to 1.02 mV which clearly indicates that phenol were adsorbed in the form of phenolate ions.
3.4. Scanning electron microscope with electron dispersive X-ray Emission electron microscopy is the most widely used technique for the investigation of shape, size morphology and porosity of the chitosan based nanoparticles matrices. SEM microphotographs of the activated native carbon and chitosan-carbon nanocomposite nanoparticles are shown in Fig. 4(a and b), respectively. The sem image of the native carbon particles show that the particles are well aggregated and the size of nanoparticles lie in the range up to 50 nm. On the other hand, the image (b) is entirely different from the image (a) and it reveals that during preparation of chitosan nanoparticles by microemulsion crosslinking method and in the immediate presence of native carbon nanoparticles, well defined nanocomposites are prepared which are up to 300 nm in width and several microns in length. This is an unusual finding and suggests that during crosslinking of chitosan nanoparticles, they act as micro reactors inside which nanocompsoite are fabricated.
3.5. Effect of phenol concentration The effect of initial concentration of phenol on its percentage removal was studied by varying the concentration of phenol from 2 to 8 mg per 10 mL of solution at fixed dose of adsorbent (0.05 g). The results are shown in Fig. 5 which clearly show that the percent removal of phenol increases from 12.5% to 69% as the initial concentration of phenol is increased from 2 to 8.0 mg per 10 mL of phenol solution. The observed results may be explained by the fact that the increased phenol concentrations provide the maximum driving force to overcome all the mass transfer resistances of phenol from the aqueous phase to solid phase resulting in higher probability of collision between phenol and the active sites. This obviously results in an increase in percent removal of phenol. 3.6. Effect of pH pH of the phenol solution is an important factor that may influences the uptake of the adsorbate. Experiments were conducted in the pH range 2–8 using 50 mg of chitosan based composite particles with 10 mL of phenol solution of 100 mg/L adsorbate solution at room temperature. In the alkaline range, the pH was varied using NaOH, whereas in the acidic range pH was adjusted using HCl. Results are shown in Fig. 6 which clearly reveal that the percent
Fig. 4. SEM images of (a) activated carbon, and (b) chitosan-carbon nanocomposite.
60
6. Effect of pH on% Fig. [Temp] = 28 ± 2 ◦ C.[nanocomposites] = 0.05 g, [Shaking time] = 60 min.
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removal [Phenol
of phenol at solution] = 10 mg L−1 ,
removal of phenol increased with increase in pH up to 4 pH. Thus 78% removal of phenol at 4 pH was observed and then the percent removal of phenol decreases with further increase in pH. The observed results may be explained as below: It is well known that the zero point charge (ZPC) of carbon is about 8.0, ie. below this pH, the carbon surfaces shall be in positively charged state. It is also known that the pKa of phenol is about 10 and below this pH, the phenol molecules will also be present in unionized state. Now at pH 2.0, the adsorbent will have net positive charge over its surfaces and, therefore, the phenol molecules will be bound to the adsorbent surface through electrostatic attraction between the pai electron clouds of phenol and positive charged adsorbent surfaces. When the pH is raised, the phenol molecules will be slightly ionized and phenoxide ion will bound to the positively charged surfaces of the adsorbent. At pH 4.0, the adsorption of phenol reaches at optimum and, therefore, maximum removal of phenol is observed. However, beyond pH 4.0, the phenol molecules produce phenoxide ions which due to electrostatic repulsion between these ions and negatively charges of the surfaces result in a decreasing adsorption of phenol. This obviously results in a lower removal of phenol from the pH 4.0–8.0. Similar type of results has been reported by others [20]. The results can be explained as being the result of the increased electrostatic repulsion between the sorbate and sorbent since both are negatively charged over this pH range. This shows the phenol adsorption takes place mainly in its dissociated form adhering to the fact that the phenol has a pKa of 9.9 below which it remains undissociated. When pH of the solution goes beyond the pKa, phenol exists as negatively charged phenolate ions. Due to the electron rich nature of the oxygen atom in phenolate ions the hydrogen bonding efficiency decreases. Therefore, phenols are effectively adsorbed on to the adsorbent as the molecules but not phenolate ions. It means that molecular interactions are involved in the adsorption process. 3.7. Effect of adsorbent dose The effect of adsorbent doses on the removal of phenol was studied in the range 10–100 mg//10 mL of phenol solutions. It is clear from Fig. 7 that% removal of phenol increases with increasing adsorbent dose up to 70 mg and then it becomes almost constant thus showing no further effect of adsorbent dose. The increase in% removal of phenol could be explained by the fact that increase in
Fig. 7. Effect of adsorbent doses on% removal of phenol [Temp] = 28 ± 2 ◦ C, [Phenol solution] = 10 mg L−1 , [Shaking time] = 60 min.
Fig. 8. Effect of temperature on% removal of phenol [nanocomposites]=0.05 g, [Phenol solution] = 10 mg L−1 , [Time]=60 min.
adsorbent dose results in increase in number of active sites at which the phenol molecules get adsorbed. However, after the adsorbent dose of 70 mg, the percent removal of phenol assumes an asymptotic value which can be explained by the fact that there is an overlapping of active sites at high doses which decreases the surface area as was observed by others [20]. 3.8. Effect of temperature The effect of temperature on the percent removal of phenol has been studied by increasing the temperature of adsorption system from 20 ◦ C to 50 ◦ C. The results are shown in Fig. 8 which shows that on increasing the temperature of the adsorption system, the percent removal of phenol constantly decreases. The results clearly reveal that an optimum removal of 70% is obtained at 20 ◦ C. The decrease in percentage removal of phenol with increasing temperature may be due to damage of active binding sites and weakening of binding forces between the phenol molecules and the adsorbent. Similar type of results has also been reported elsewhere [28]. Another possible reason for the observed decrease in removal of phenol could be that on increasing the temperature from 20 to
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Table 1 Pseudo first order and Lagergren kinetic constant for the adsorption of phenol on the composite. qe experimental
mg g
33.605
K1
qe calculated
3.9 × 10−2
75
mg g
R2 0.9956
cability of the pseudo first order model to the adsorption process of the phenol onto chitosan based particles is unfeasible. The linearized form of pseudo-second order kinetics is given by the following equation: t 1 t = 2 2 + qt qe K qe
Fig. 9. Effect of shaking time on% removal of phenol [Temp] = 28 ± 2 ◦ C [nanocomposites] = 0.05 g, [Phenol solution] = 10 mg L−1 .
50 ◦ C, the solubility of phenol in water also increases that obviously results in less removal of phenol. The observed results also reveal that the adsorption of phenol onto the chitosan nanocomposites is an exothermic process.
(6)
From the equation, the plot of qtt against t should give a linear relationship. The qe andK2 can be determined from the slope and intercept of the plot. The pseudo second-order kinetic parameters are presented in Table 1. From the above data we see that the values qe (experimented) values are quite closer to those of qe (calculated) values for pseudo second order kinetics. Hence, it can be concluded that the adsorption of phenol on chitosan based nano particles is described by pseudo second order kinetic mode. 3.11. Adsorption isotherms
3.9. Effect of contact time Contact time is an inevitably a fundamental parameter for an economical waste water treatment process. The effect of the contact time on the percent removal of phenol is shown in Fig. 9. The extent of phenol removal increases with time and after a certain time of contact, it starts decreasing. The observed increase in the initial contact time period may be explained by the reason that greater the time of contact, larger would be the interactions between the phenol molecules and adsorbent. This obviously brings out an increase in percent removal of phenol. However, after a certain time period, 60 min in the present study, the percent removal of phenol starts decreasing which may be attributed to beginning of desorption of phenol molecules from the adsorbent. The observed desorption clearly results in a falling percent removal of phenol. The observed desorption of phenol is quite expected since the phenol molecules adsorb onto the adsorbent through physical forces which after large agitation time period acquire enough kinetic energy that breaks up the binding forces between the phenol molecules and agitating adsorbent particles. 3.10. Adsorption kinetics Kinetic study is helpful in prediction of adsorption rate constants, adsorption capacity and adsorption mechanism. The applicability of pseudo-first order and pseudo second order kinetic models were examined in this study at constant temperature of 25 ◦ C. The linear form of pseudo- first order model of Lagergreen [29] is generally expressed as follows: Log (qe − qt ) = log (qe ) − K1 t (4). Where qe and qt the adsorption capacities of the phenol per unit weight of nanocomposite particles at equilibrium and at time t (min), respectively (mg/g) and K 1 is the pseudo first order rate constant (min−1 ), K1 andqe values are determined from the slope and the intercepts of the graph which is plotted between Log (qe − qt ) vs t. The results obtained are summarized in Table 1. From the data shown in Table 1 it can be seen that experimental qe values do not agree with the calculated ones that shows appli-
Several models have been published in the literature to describe the equilibrium adsorption systems. Freundlich and Langmuir isotherms are commonly used to describe the adsorption characteristics of adsorbent utilized in water and wastewater treatment. Therefore, adsorption data of phenol on chitosan based nano composite were employed to test Freundlich and Langmuir models in this study. Which have been tested to describe the relationship between the amounts of phenol adsorbed (qe) and its equilibrium concentration (Ce) in the solution. 3.11.1. Freundlich isotherm Freundlich isotherm assumes that the uptake of phenol occurs on a heterogeneous surface by multilayer adsorption and that the amount of metal ions adsorbed increases infinitely with an increase in concentration. It is a most popular model for a single solute system, based on the distribution of solute between the solid phase and aqueous phase at equilibrium. Freundlich isotherm is an empirical equation and expressed as, qe = Kf Ce1/n
(7)
where qe is the amount of phenol adsorbed per unit mass of adsorbent (mg/g), Ce is the equilibrium concentration of metal ions (mg/L), Kf is the measure of adsorption capacity(empirical constants depending on several environmental factors), 1/n being the adsorption intensity (an empirical parameter representing the energetic heterogeneity of the adsorption sites). The equation is conveniently used in the linear form by taking the logarithmic of both sides as shown below: logqe = logKf + logCe
(8)
According to the above equation, a plot of (log qe) against (logCe) yields a straight line (Fig. 10) which indicates the confirmation of the Freundlich isotherm for adsorption. The constants can be obtained from the slope and the intercept of the linear plot of the experimental data and summarized in Table 2. The value of n indicates a favourable physical adsorption. Vander-Waal’s forces involved in adsorption.
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reflects monolayer adsorption. Moreover, the Langmuir constants b and Q0 can be evaluated from the slope and intercept of linear equation, respectively. The values of Q0 and b obtained in this work are presented in Table 2 respectively. 3.11.3. Temkin isotherm Temkins isotherm model was also applied to the experimental data. The Temkin isotherm equation assumes that the heat of adsorption of all the molecules in layer decreases linearly with coverage due to adsorbent–adsorbate interactions, and that the adsorption is characterized by a uniform distribution of the bonding energies, up to some maximum binding energy was carried out by plotting the quantity sorbed qe against lnCe and the constants were determined from the slope and intercept. The model is represented by the following equation qe = RT Fig. 10. Freundlich isotherm for adsorption of phenol onto chitosan − carbon nanocomposites, Shaking time = 60 min, Temp. = 28 ±2 ◦ C, nanocomposite = 0.05 g. Table 2 Data showing the values of adsorption isotherms obtained for adsorption of phenol on chitosan based nanocomposites. Parameters Isotherm 2
R
n Freundlich kf(mg/g) Langmuir 2
0.9721 0.09 1.7×10−7 0.9178 0.568 −1.103
Tempkin R2 B AT(L/g)
09863 115118 138
D-R model R2 qm(mol/g)  (mol2 /j2 )
0.9301 9862 −6.88
Q 0bCe 1 + bCe
Eq. (11) can be expressed in its linear form as: qe = BlnAT + BlnCe
lnqe = lnqm −  ∈ 2
(9)
where a is the amount of phenol adsorbed per unit mass of adsorbent (mg/g), Ce = is the equilibrium concentration of phenol ions (mg/L), Q0 is a measure of adsorption capacity of adsorbent (mg/g), b is the Langmuir constant which is measure of energy of adsorption (L/mg). The above equation can be rearranged to give the following linear Ce Ce 1 + = a Q0 Q 0b
(12)
(13)
3.11.4. Dubinin–Radushkevich isotherm Dubinin–Radushkevich isotherm is generally applied to express the adsorption mechanism with a Gaussian energy distribution onto a heterogeneous surface. The model has often successfully fitted high solute activities and the intermediate range of concentrations data well. The linear form of D–R isotherm is presented as the following equation:
3.11.2. Langmuir isotherm The Langmuir adsorption isotherm is often used to describe adsorption of solutes from a liquid solution. A basic assumption of the Langmuir theory is that sorption takes place at specific homogeneous sites within the adsorbent. It is then assumed that once a phenol occupies a site, no further sorption can take place at that site. The rate of sorption to the surface should be proportional to a driving force multiplied by area. The driving force for adsorption is the concentration in the solution, and the area is the amount of bare surface available on the adsorbent. The Langmuir adsorption isotherm is perhaps the best known of all the isotherms and is often expressed as: a=
RT RT RT lnAT + lnCe B = bT bT bT
(11)
where AT = Temkin isotherm equilibrium binding constant(L/g), bT = Temkin isotherm constant, R = universal gas constant (8.314 J/mol/K), T = Temperature at 298 K, B = Constant related to heat of sorption (J/mol). The values of AT and B are shown in Table 2.
Value
R Q0(mg/g) b (L/g)
qe =
RT ln(AT Ce) b
(10)
The above equation can be used for the linearization of experimental data by plotting Ce/a against Ce, and a good fit of this equation
(14)
where qe is the amount of chromium ions adsorbed per unit mass of adsorbent (mg/g), qm is theoretical isotherm saturation capacity (mg/g),  is the constant of the sorption energy (mol2 /J2 ), and ∈ = Polanyi potential, which is described as ∈ = RTIn
1+
1 Ce
(15)
where T is the solution temperature (K) and R is the gas constant and is equal to 8.314 J/mol K. Values of qm and  , are calculated from the intercept and slope of the plot by plotting ln qe versus ∈ 2 and the values are summarized in Table 2, As seen in Table 2, that Langmuir isotherm is unsuitable to describe the sorption of phenol and Freundlich equation fits to adsorption system well. (Correlation coefficient R2 > 0.97). The parameters of the fitted curve are summarized in the Table 2. Langmuir isotherm is based on a monolayer homogenous surface, while the Freundlich isotherm is applied to adsorption process on heterogeneous surface. 4. Conclusions The adsorption of phenol from water using chitosan based nano bio composites was investigated; results show outstanding adsorption ability for phenol. Various experimental parameters such as contact time, adsorbent dose, and temperature and solution pH were optimized. The removal of phenol was found to be highly influenced by initial phenol concentration and phenol speciation which is directly related to solution pH. Equilibrium adsorption
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studies show that the data fit very well in the Freundlich equation of multilayer adsorption, this result indicate the heterogeneity of the adsorbent surface. The values of R2 obtained from pseudo secondorder model were higher than 0.999, indicating that the adsorption process obeyed the pseudo second-order model. Finally the adsorption experiments indicated that chitosan based nano composite was efficient adsorbent for the removal of phenol from water. References [1] Z.P. Zhang, S.S. Feng, The drug encapsulation efficiency in vitro drug release, cellular uptake and cytotoxicity of paclitaxel-loaded poly (lactide)-tocopheryl polyethylene glycol succinate nanoparticles, Biomaterials 27 (2006) 4025–4033. [2] N. Calace, Nardi E. Petronio, B.M. Pietroletti, Adsorption of phenols by papermill sludges, Environ. Pollut. 118 (2002) 315. [3] F. Akbal, A.N. Onar, Particle size and shape effects on adsorption rate parameters, Environ. Monit. Assess. 83 (2003) 301. [4] Guido Buscaa, Silvia Berardinelli, Carlo Resini, Laura Arrighi, Technologies for the removal of phenol from fluid streams: a short review of recent developments, J. Hazard. Mater. 160 (2008) 265–288. [5] Md. Ahmaruzzman, Adsorption of phenolic compounds on low cost adsorbents: a review, Adv. Colloid Interface Sci. 143 (2008) 48. [6] M.Q. Hu, Y.M. Xu, J.C. Zhao, Efficient photosensitized degradation of 4-chlorophenol over immobilized aluminium tetrasulphophthalocyanine in the presence of hydrogen peroxide, Langmuir 20 (2004) 6302. [7] O. Tepe, A.Y. Dursun, Combined effects of external mass transfer and biodegradation rates on removal of phenol by immobilized Ralstoniaeutropha in a packed bed reactor, J. Hazard. Mater. 151 (2008) 9. [8] A. Dabrowski, P. Podkoscielny, M. Hubicki, M. Barczak, Adsorption of phenolic compounds by activated carbon −a critic review, Chemoshpere 58 (2005) 1985. [9] G. Dursun, H. Cecek, A.Y. Dursum, Adsorption of phenol from aqueous solution by using carbonised beet pulp, J. Hazard. Mater. 125 (2005) 175. [10] J.F. Patterson, Industries Waste Water Treatment Technology, 2nd ed., Butterworths, London, 1985. [11] H.L. Wang, J.L. Chen, Z.C. Zai, Y.L. Chen, Q.X. Zhang, J. Chin, Studies on removal of phenol using ionic immobilised polymeric micro capsule, Environ. Chem. 23 (2004) 188. [12] Z.H. Fei, J.L. Chen, J.G. Cai, Y.P. Qiu, A.M. Li, Q.X. Zhang, Chin. J. Polym. Sci. 22 (6) (2004) 529. [13] Z.C. Zhai, J.L. Chen, Z.H. Fei, H.L. Wang, A.M. Li, Q.X. Zhang, React. Funct. Polym. 57 (2003) 93. [14] J.G. Cai, A.M. Li, H.Y. Shi, Z.H. Fei, C. Long, Q.X. Zhang, J. Hazard. Mater. 124 (2005) 173.
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