Adsorption of anionic azo dye Congo Red from aqueous solution by Cationic Modified Orange Peel Powder

Adsorption of anionic azo dye Congo Red from aqueous solution by Cationic Modified Orange Peel Powder

Journal of Molecular Liquids 220 (2016) 540–548 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevie...

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Journal of Molecular Liquids 220 (2016) 540–548

Contents lists available at ScienceDirect

Journal of Molecular Liquids journal homepage:

Adsorption of anionic azo dye Congo Red from aqueous solution by Cationic Modified Orange Peel Powder Venkata Subbaiah Munagapati, Dong-Su Kim ⁎ Department of Environmental Science and Engineering, Ewha Womans University, 11-1 Daehyun-Dong, Seodaemun-Gu, Seoul 120-750, Korea

a r t i c l e

i n f o

Article history: Received 29 March 2016 Received in revised form 28 April 2016 Accepted 29 April 2016 Available online xxxx Keywords: Adsorption Congo Red Kinetics Isotherms Thermodynamics Temperature

a b s t r a c t This study investigated the adsorption of Congo Red (CR), an anionic azo dye, from aqueous solution by using Cationic Modified Orange Peel Powder (CMOPP). The optimum conditions were determined by investigating the effects of pH, contact time, initial dye concentration and temperature. The adsorbent was characterized by FTIR analysis. The equilibrium data were analyzed using Langmuir and Freundlich isotherm models. The maximum adsorption capacity of CR on CMOPP was estimated as 107, 144, and 163 mg/g, respectively, at different temperatures (298, 308 and 318 K). Langmuir model fitted the equilibrium data better than the Freundlich isotherm. The kinetic results demonstrated that the adsorption of CR onto CMOPP was well described by pseudo-second-order kinetic model. The activation energy of adsorption (Ea) was found to be 30 kJ/mol by using the Arrhenius equation. The calculated thermodynamic parameters (ΔGo, ΔHo and ΔSo) showed that the adsorption of CR onto CMOPP was feasible, spontaneous and endothermic. Desorption experiments were carried out to explore the feasibility of regenerating the adsorbent and the adsorbed CR from CMOPP was desorbed using 0.1 M NaOH. The results indicated that CMOPP can be considered as a potential adsorbent for the removal of CR from aqueous media. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Dyes are water soluble and intensely colored substances, and are used for the coloration of various substrates, including paper, leather, cosmetics, food, hair and textiles. The deterioration of water quality by the presence of hazardous azo dyes has been of great concern and industrial revolution in pigment manufacturing, painting, photographic, cosmetic, tanning, dyeing and textile industries accelerated it [1]. Recent studies show that the aquatic environmental contamination by azo dyes is rated as the most polluting among all the industries. The widespread presence of synthetic dyes in water bodies, their bioaccumulation, potential toxicity, and adverse health effects have made the study of their environmental fate as highly important [2]. Around 60% of the dyes used in the textile plants are azo dyes which are characterized by azo groups (\\N_N\\) bound to the sp2 hybrid carbon atoms. They exist as mono, di, tri, and tetra azo dyes, with the azo groups mainly bound to the benzene and naphthalene rings [3]. Congo red [1-naphthalene sulfonic acid, 3.30-(4.40-biphenylenebis (azo)) bis (4-amino-) disodium salt] is a typical and the first synthetic dye of anionic azo dyes, which is synthesized by coupling tetrazotised benzidine with two molecules of napthionic acid. CR is used in the cellulose industries such as paper, pulp and cotton textile [4]. CR is

⁎ Corresponding author. E-mail address: [email protected] (D.-S. Kim). 0167-7322/© 2016 Elsevier B.V. All rights reserved.

a serious hazard to aquatic living organisms and can cause carcinogen in humans [5]. Considering the large discharged volume and effluent combustion, the wastewaters from the textile industry is rated as the most polluting among all industrial discharges. Their presence in water, even at very low concentrations is distinctly visible; it is undesirable and may dramatically affect the photosynthetic activity in the aquatic life due to the reduced light penetration. In view of these reasons, in the present studies the focus is on the removal of CR. Structural stability of CR is a major challenge for its removal from wastewater treatment. Dyes are toxic to micro-organisms and stable to both light and heat, they cannot be easily removed by conventional treatment due to their complex structure and synthetic origin. At present, a number of techniques have been employed for the removal of dye contaminants from waste water, which include ultrasound irradiation [6], photocatalysis [7], coagulation-flocculation [8], oxidation [9], ozonation [10], membrane separation [11] and biological treatments [12]. All these methods have some economic and/or environmental drawbacks, such as high capital and operating cost, sludge production and complexity of the treatment processes. Adsorption is a widely used technique for the removal of dyes as economical and environmentally friendly [13,14]. The process cost for the dye removal by adsorption depends mainly on the cost of the adsorbent and its regeneration. Orange peels are one of the valuable waste materials discarded from juice industry. Since world's orange production is estimated to be N60 million tons per year [15]. Orange peel principally consists

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of cellulose, pectin, hemicellulose, lignin, chlorophyll pigments and other low molecular weight hydro-carbons [16]. These components contain various functional groups, such as carboxyl and hydroxyl, which make orange peel a potential adsorbent material for removing dyes from aqueous solutions. In recent years, research interest has been focused on increasing the sorption capacity of biomass through physical and chemical modifications. These modifications are aimed at increasing the fraction of the effective functional groups such as carboxylate, hydroxyl, sulfate, phosphate, amide and amino groups on the biomass surface. Some researchers developed surface modified low-cost agricultural waste by-products that have been successfully applied in the removal of dyes from aqueous solution via physical and chemical modification, some of which are peanut husk [17], coir pith [18], wheat straw [19], barley straw [20], palm ash [21], straw [22], sugarcane bagasse [23], banana peel [24], sawdust [25] and aminated pumpkin seed powder [26]. In this study CMOPP was chosen as an adsorbent for the removal of CR from aqueous solution. Adsorption was evaluated as a function of various parameters such as pH, contact time, initial dye concentration and temperature. Results are fitted to pseudo-first-order and pseudo-second-order kinetic models to quantify the rate of adsorption of CR on the adsorbent's surface. Langmuir and Freundlich adsorption isotherm models were also studied. Thermodynamic parameters of the adsorption process have been studied and the changes in Gibbs free energy, entropy and enthalpy of adsorption were determined which are important for the design of a separation system. 2. Materials and methods 2.1. Adsorbate and reagents Congo Red dye (≥35% dye content, M.W. = 991.82, λmax = 497 nm) was procured from Sigma Aldrich, Korea. CR was used without further purification. Stock solution of CR was prepared in distilled water. The structure of the dye is shown in Fig. 1. In the adsorption experiments and modification process, the chemical with analytical reagent grade were used. 2.2. Preparation of Cationic Modified Orange Peel Powder (CMOPP) Orange peels were collected from a local market, washed with deionized water several times to remove ash and other contaminants. Then it was washed with double distilled water and was dried at 70 °C inside a convection oven for 24 h. The dried orange peel was crushed and sieved to be a smaller particle by a 150 mesh sieve. The obtained product was named OPP. The cationic modified adsorbent was prepared by employing a reported method [27]. For the preparation of modified adsorbent, 5 g of the washed OPP was treated with 20% (w/v) sodium hydroxide solution for 5 min then filtered and treated with 16% (v/v) dichloroethane in water at 70 °C for 90 min in a flask. The dichloroethane


treated adsorbent was then rinsed thoroughly with hot and cold water to remove excess of alkali and chemicals. The washed adsorbent was then aminated with the mixture of 10% (v/v) methyl amine and water at 70 °C for 90 min and filtered. The adsorbent was finally treated with dilute acetic acid for protonation. The resultant dried CMOPP was stored in a desiccator, and subsequently used as the adsorbent in the sorption experiments. The relevant partial reaction is given below: NaOH

Cl−CH 2 −CH2 −Cl

Adsorbent−OH → Adsorbent−ONa → CH 3 NH2 Adsorbent−O−CH2 −CH2 −Cl → Adsorbent−O−CH2 −CH2 −NH−CH 3 Hþ

→ Adsorbent−O−CH2 −CH2 −NHþ 2 −CH 3 Protonation

2.3. Characterization of the CMOPP Infrared spectra of the CMOPP and CR loaded CMOPP samples were obtained using a Fourier Transform Infrared Spectrophotometer (BIO-RAD, FTS-135, USA). For IR spectral studies, 10 mg of sample was mixed and ground with 100 mg of KBr and made into a pellet. The background absorbance was measured by using pure KBr pellet. The spectra of the samples were recorded in the wavenumber range of 4000–400 cm− 1. 2.4. Batch adsorption procedure Adsorption experiments were performed to investigate the removal of CR as a function of pH, contact time, initial dye concentration and temperature. For sorption experiments, 0.05 g of adsorbent was brought into contact with 30 mL of CR solution in a 50 mL polypropylene centrifuge tube. CR stock solution of 1000 mg/L was prepared. The pH values of the solutions were adjusted in the range of 3–9 using 0.1 M HCl and 0.1 M NaOH. All the sample tubes were agitated in an electrical thermostatic reciprocating shaker at 160 rpm. Adsorption kinetic study was conducted with an initial dye concentration of 300 mg/L at three temperatures of 298, 308 and 318 K. Samples were collected at various time intervals until the concentration of CR in the dilute phase became constant. Adsorption isotherm experiments were also carried out with different initial dye concentrations varying from 50 to 300 mg/L at different temperatures (298, 308 and 318 K). At the end of adsorption, 1 mL sample was collected and centrifuged at 3000 rpm for 10 min. The filtrate was then collected in polythene tubes and diluted before measurement. Then the concentration of the residual dye was determined spectrophotometrically by monitoring the absorbance at 497 nm for CR using UV–Vis Spectrophotometer (Optizen Pop, Korea). The dye uptake (q) was calculated from the difference between the concentrations of CR before and after sorption using the following the Eq. (1):

C i V i −C f V f M


where q is the uptake of CR (mg/g), Ci and Cf are the initial and final CR concentrations in the solution (mg/L), Vi and Vf are the initial and final (initial plus added HCl or NaOH solutions) solution volumes, respectively, and M is the mass of sorbent (g). 2.5. Batch mode desorption and reusability studies

Fig. 1. Molecular structure of CR.

Batch desorption experiments were carried out using different eluents such as 0.1 M HCl, deionized water, 0.1 M CH3COOH and 0.1 M NaOH. The adsorbed CR (300 mg/L) onto CMOPP was washed with deionized water several times and transferred into polypropylene


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centrifuge tubes. To this 30 mL of the desorbing eluent was added, and then the tubes were shaken at room temperature (298 K) using a mechanical shaker. The concentration of CR released from the CMOPP into aqueous phase was measured by UV–Vis spectrophotometer. The CMOPP after desorption was reused in adsorption experiment, and the process was repeated for five times. Desorption efficiency was calculated by using Eq. (2):

2.6. χ2 analysis To identify the suitable isotherm for adsorption of CR onto CMOPP, the chi-square (χ2) test was carried out using the experimental data, to find the best adsorption isotherm model. The χ2 value was calculated using Eq. (3) for evaluating the best fit model. 

Amountof CR desorbed X 100 Desorption efficiency ¼ Amountof CR adsorbed



χ ¼∑

qe −qe;m qe;m

2 !

Fig. 2. FTIR spectra of (A) CMOPP and (B) CR-loaded CMOPP.


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3. Results and discussions

the negatively charged surface and the anionic dye molecules. Therefore there are no more exchangeable anions on the outer surface of the adsorbent at higher pH and consequently the observed decreased adsorption. A similar type of behavior is also reported for the adsorption of the dye with different adsorbents [29,30].

3.1. FTIR analysis

3.3. Effect of contact time and temperature

The functional groups responsible for the adsorption of CR on the cell surfaces of adsorbent are confirmed by FTIR spectra. The FTIR spectra of CMOPP and CR-loaded CMOPP are shown in Fig. 2. The broad and strong band at 3429 cm− 1 was due to bounded hydroxyl or amine groups. The peak observed at 2921 cm− 1 was due to the \\CH asymmetric stretching. The peak at 1732 cm−1 was attributed to stretching vibration of carboxyl group. The bands at 1633 and 1434 cm−1 were attributed to asymmetric and symmetric stretching vibration of C_O groups. The C\\N stretching band was observed at 1326 cm− 1. The bands at 1165 and 1061 cm−1 were assigned to C\\O stretching of alcohols and carboxylic acids. After CR adsorption, the symmetrical stretching vibration bands of hydroxyl or amine groups were shifted to 3429 to 3408 cm−1 for CR-loaded CMOPP. The stretching band of carboxyl groups was shifted from 1732 to 1711 cm−1. The stretching bands at 1633, 1434 and 1373 cm− 1 were also shifted to 1615, 1432 and 1374 cm− 1 respectively. The C\\O peaks at 1165 and 1061 cm− 1 were shifted to 1163 and 1060 cm−1, respectively. The analysis of the FTIR spectral results in terms of frequency shifts indicate that these functional groups above mentioned are responsible for the binding of the CR to the surface of the CMOPP.

In order to determine the adsorption equilibrium time for CR, the contact time was varied from 0 to 300 min at different temperatures, and the results are shown in Fig. 4. From this figure it was observed that the adsorbed amount of CR increased with contact time up to 60 min, after that a maximum removal was attained. Therefore, 60 min was selected as the optimum contact time for all further experiments. Temperature has a vital effect on adsorption process as it can increase or decrease the amount of adsorption. Fig. 4 shows the effect of the temperature on the adsorption of CR onto CMOPP. The results revealed that the adsorption capacity increased with increasing temperature from 298 to 318 K. This suggests that the adsorption of CR onto CMOPP is endothermic and activated process indicating that the higher temperature is more favorable for the dye adsorption. The increase in CR adsorption with rise in temperature may be a result of increase in the mobility of CR molecules with temperature. Increasing temperature is known to increase the rate of diffusion of the adsorbed molecules across the external boundary layer and the internal pores of the adsorbent particles, owing to the decrease in the viscosity of the solution [31].

3.2. Effect of pH

For practical applications, the process design and operations control, the sorption kinetics is very important. Sorption kinetics in wastewater treatment is significant, as it provides valuable insights into the reaction pathways and the mechanism of the sorption reactions [32]. Also, the kinetics describes the solute uptake, which in turn controls the residence time of sorbate uptake at the solid-solution interface [33]. The experimental adsorption kinetics data were modeled using pseudo-firstorder [34] and pseudo-second-order [35] kinetics, which can be represented in their non-linear forms, as follows: Pseudo-first-order model

where qe,m equilibrium capacity which was obtained by calculating from a model (mg/g) and qe experimental data of equilibrium capacity (mg/g).

The pH of the aqueous solution is an important controlling parameter in the adsorption process [28]. The effect of initial solution pH on the adsorption of CR from aqueous solution was investigated in the pH range between 3.0 and 9.0 (which was adjusted with 0.1 M HCl or 0.1 M NaOH at the beginning of the experiment and not measured afterwards) at a constant temperature of 298 K and 300 mg/L initial CR concentration. As shown in Fig. 3, the amount of adsorption of the solute increases as the pH is decreasing. When the pH is changed from 3 to 9, the adsorption will decrease from 85 to 8 mg/g. The maximum adsorption of CR was observed at pH 3.0. Hence, all the succeeding investigations were performed at pH 3.0. At pH 3.0 a significantly highelectrostatic attraction exists between the positively charged surface of the adsorbent and anionic dye. As the pH of the system increases, the number of negatively charged sites increases and the number of positively charged sites decrease. A negatively charged surface site on the adsorbent does not favor the adsorption of the dye anions due to the electrostatic repulsion. Also, lower adsorption at higher pH may be due to the abundance of OH– ions and causing ionic repulsion between

Fig. 3. Effect of pH on the adsorption of CR onto CMOPP.

3.4. Adsorption kinetics

qt ¼ q1 ð1− expð−k1 t ÞÞ


Pseudo-second-order model qt ¼

q22 k2 t 1 þ q2 k2 t


Fig. 4. Effect of contact time and temperature on the adsorption of CR onto CMOPP.


V.S. Munagapati, D.-S. Kim / Journal of Molecular Liquids 220 (2016) 540–548

where qt is the amount of dye sorbed at time t (mg/g), q1 and q2 are the amount of dye sorbed at equilibrium (mg/g), k1 the pseudo-first-order rate constant (1/min) and k2 is the pseudo-second-order rate constant (g/mg. min). The kinetic parameters and coefficient of determination (R2) for each model are presented in Table 1. The low R2 values and the difference between experimental qe and theoretical q1 values indicate that the pseudo-first-order model was not well suited to describe the adsorption of CR by CMOPP. On the other hand, the R2 values for the pseudo-second-order model were relatively higher than that of the pseudo-second-order model. Moreover, the theoretical q2 values calculated by the pseudo-second-order model were close to the experimental qe values. Based on these results, it can be concluded that the pseudosecond-order kinetic model provided a good correlation for the adsorption of CR onto CMOPP in contrast to the pseudo-first-order model. The non-linear kinetic model curves for the adsorption of CR onto CMOPP at 298 to 318 K are shown in Fig. 5.

where Ce is the equilibrium solute concentration of dyes in solution (mg/L), qe is the adsorbed value of dye at equilibrium concentration (mg/g), qm is the maximum monolayer adsorption capacity (mg/g) and KL is the Langmuir adsorption constant (L/mg) relating the free energy of adsorption. The values of qmax increased from 107 to 163 mg/g (Table 2) with an increase in the temperature from 298 to 318 K. The rise in sorption capacity with temperature was because of rise in the kinetic energy of sorbent particles. Thus, the collision frequency between sorbent and sorbate increased, which resulted in the enhanced sorption on to the surface of the sorbent. The type of the Langmuir isotherm could be predicted based on whether the adsorption was favorable or unfavorable in term of equilibrium parameter or dimensionless constant separation factor RL [39], which is given by Eq. (8):

RL ¼

1 1 þ K L C0


3.5. Activation energy The relationship between the pseudo-second-order rate constant and temperature may be described by Eq. (6): lnk2 ¼ lnAo −



where k2 is the rate constant at different temperatures obtained from the pseudo-second-order kinetics, Ea is the Arrhenius activation energy (kJ/mol), A is the Arrhenius factor (g/mg min), R is the gas constant (8.314 J/mol K), and T is the absolute temperature (K). When ln k2 is plotted against 1/T, a straight line with a slope equal to –Ea/R is obtained (Fig. 6). The magnitude of Ea is commonly used to distinguish between physical and chemical adsorption. Physical adsorption reactions are readily reversible, equilibrium attained rapidly and thus the energy requirements are small (ranging from 5 to 40 kJ/mol). Chemical adsorption reactions involve strong forces and thus require large activation energies (ranging from 40 to 800 kJ/mol) [36]. The activation energy for CR onto CMOP was found as 30 kJ/mol. This result indicated that the adsorption process was a physical sorption. 3.6. Adsorption isotherms Equilibrium data permits evaluation of composite adsorption properties as it described how the adsorbate molecules when the adsorption process reaches equilibrium. This provides comprehensive understanding of the nature of the interaction [37]. In this study, the adsorption data was modeled by using two parameters non-linear form of Langmuir and Freundlich isotherms. Langmuir adsorption isotherm [38] is applicable in many adsorption processes. The basic assumption is the formation of a monolayer of adsorbate on the outer surface of the adsorbent and no further adsorption thereafter. Langmuir isotherm is expressed by Eq. (7): qe ¼

qm K L C e 1 þ K LCe


Table 1 Kinetic parameters obtained from pseudo-first-order and pseudo-second-order at different temperatures. Dye


Temp. (K)

qe (mg/g)

298 308 318

93 121 144


where KL is the Langmuir constant and C0 is the initial concentration of adsorbate. The values of RL indicate whether the isotherm is unfavorable (RL N 1), linear (RL = 1), favorable (RL b 1) or irreversible (RL = 0) [40]. The values of RL 0.26, 0.25 and 0.24 at the three varied and different temperatures were all in the range 0–1, which indicates that the isotherm was favorable. Freundlich isotherm [41] is an empirical equation describing that the adsorption surface becomes heterogeneous during the adsorption process. The Freundlich isotherm is commonly presented by the Eq. (9):

qe ¼ K f C 1=n e


where, Ce is the equilibrium concentration of dye in solution (mg/L), qe is the adsorbed value of dye at equilibrium concentration (mg/g), Kf and n are Freundlich constants characteristics of the system, indicating the adsorption capacity and the adsorption intensity. The values of Kf increased from 19 to 22 mg/g (Table 2) with increase in the temperature of the solution from 298 to 318 K. As the Kf is a measure of adsorption capacity, the increase in the value again confirms that the adsorption process of CR onto CMOPP is an endothermic process. The value of 1/n, one of Freundlich constants, indicates the type of isotherm. When 0 b 1/n b 1, the adsorption is favorable; when 1/n = 1, the adsorption is irreversible; and when 1/n N 1, the adsorption is unfavorable [42]. The 1/n values are found in the range of 0.35–0.26, when the temperature was altered from 298 to 318 K. The 1/n values between 0 and 1 indicated that the adsorption of CR onto CMOPP was favorable. Fig. 7 shows Langmuir and Freundlich adsorption isotherms of CR onto CMOPP at different temperatures. It is clear from Fig. 7 that data fit better to Langmuir model in comparison with the Freundlich model. This indicates the monolayer adsorption on the homogeneous surface of the adsorbent with identical binding sites of the adsorbent. The comparison among the isotherm models was based on the higher R2 and low χ2 values. The isotherm parameters for the two models along with chi-square (χ2) values are listed in Table 2. 3.7. Comparison with other adsorbents


q1 (mg/g)

k1 (L/min)


q2 (mg/g)

k2 (g/mg min)


89 114 136

0.0838 0.1942 0.3355

0.9033 0.8514 0.8622

97 120 140

0.0018 0.0026 0.0037

0.9772 0.9744 0.9791

A comparison between the results obtained in this work with the similar data from literature [43–59] is presented in Table 3. The value of CR uptake found in this work is significantly higher than reported for other adsorbents. Thus, the comparison of adsorption capacities shows that the CMOPP is an efficient adsorbent for the uptake of CR.

V.S. Munagapati, D.-S. Kim / Journal of Molecular Liquids 220 (2016) 540–548


Fig. 5. Adsorption kinetics of CR onto CMOPP at different temperatures (A) 298 K, (B) 308 K and (C) 318 K.

3.8. Thermodynamic evaluation The sorption thermodynamics is useful to investigate whether the process is spontaneous or not and also to obtain an insight into the sorption behavior. Parameters including the Gibbs free energy change of adsorption (ΔGo), enthalpy (ΔHo), and entropy (ΔSo) for the adsorption of CR onto CMOPP were calculated using the following equations: ΔG ¼ −RT lnK


where R is the universal gas constant (8.314 J/mol K), T is absolute temperature (K) and K (L/mg) is the equilibrium constant obtained by multiplying the Langmuir constant qm and KL [60]. ΔGo ¼ ΔHo −TΔSo ln K ¼




According to Eq. (12), the values of ΔSo and ΔHo can be determined from the slope and intercept of the ln K plot versus 1/T, respectively (Fig. 8). The Δ Go value indicates the degree of spontaneity of the adsorption process, where more negative values reflect a more energetically favorable adsorption process. We can find in Table 4 that the magnitude of ΔGo increased with the rise in temperature and the values of ΔGo were negative at all temperatures. In general, if the values of energy Δ Go are from 0 and −20 kJ/mol, then its indicate that the adsorption process is physisorption, while the values from − 80 to − 400 kJ/mol correspond to chemisorption [61,62]. The values of energy ΔGo suggest the adsorption is mainly a physisorption process. The ΔSo value was found to be positive which suggests an increase in the randomness at Table 2 Langmuir and Freundlich isotherm models for adsorption of CR on CMOPP. Dye

CR Fig. 6. The linearized Arrhenius plots for the adsorption of CR onto CMOPP.

Temp. (K)

298 308 318


Freundlich 2


qmax (mg/g)

KL (L/mg)



Kf (mg/g)




107 144 163

0.0185 0.0201 0.0207

0.9998 0.9908 0.9851

0.5 4.8 6.9

19 20 22

2.89 3.02 3.81

0.9650 0.9629 0.9620

7.2 9.8 11.3


V.S. Munagapati, D.-S. Kim / Journal of Molecular Liquids 220 (2016) 540–548

Fig. 7. Adsorption isotherms of CR onto CMOPP at different temperatures (A) 298 K, (B) 308 K and (C) 318 K.

the solid/solution interface during the adsorption. This positive value may be explained as follows: during the adsorption of the dye, the adsorbate species displace the adsorbed solvent molecules to gain more translational entropy than was lost by the adsorbate, thus allowing randomness in the system [63]. The positive ΔHo value obtained indicated that the adsorption process was endothermic in nature. The magnitude

of ΔHo is used to indicate where the adsorption is physical or chemical. In general, the heat evolved during physical adsorption generally falls into the range of 8–25 kJ/mol, while the heat of chemical adsorption generally falls into the range of 80–200 kJ/mol [64]. The values of Δ Ho suggest that the adsorption is mainly is a physical adsorption process. The calculated values of thermodynamic parameters Δ Go, Δ So and Δ Ho for the adsorption of CR on CMOPP are given in Table 4.

Table 3 Comparison of CR sorption capacity of CMOPP with that of different sorbents. Adsorbent

qmax (mg/g)



Spent mushroom Neem leaf powder Eucalyptus wood saw dust Jute stick powder Sugarcane bagasse Jujuba seed Ackee apple Bentonite Groundnut shell carbon Bamboo dust carbon Treated sunflower stalks Activated red mud Phoenix dactylifera seeds Crosslinked cellulose dialdehyde Fe-Zn bimetallic nanoparticles Rubber seeds Modified wheat straw Cone biomass of Pinus brutia CMOPP

147.1 41.2 66.6 35.7 38.2 55.56 161.89 158.7 110.8 101.9 155.2 7.08 61.72 42.03 28.56 9.82 118 102.8 163

2.0 – 7.0 7.0 5.0 2.0 3.0 6.8 – – – 7.0 2.0 4.0 4.0 6.0 5.0 4.0 3.0

[43] [44] [45] [46] [47] [48] [49] [50] [51] [51] [52] [53] [54] [55] [56] [57] [58] [59] This study

Fig. 8. Plot of ln K versus 1/T for the estimation of thermodynamic parameters for adsorption of CR onto CMOPP.

V.S. Munagapati, D.-S. Kim / Journal of Molecular Liquids 220 (2016) 540–548 Table 4 Thermodynamic parameters for the adsorption of CR on CMOPP. Temp. (K)

ΔGo (kJ/mol)

ΔSo (kJ/mol K)

ΔHo (kJ/mol)

298 308 318

−1.8879 −2.5121 −3.2330




There is a gradual decrease in CR adsorption with the increase in the number of cycles. After a sequence of five cycles, the CR adsorption capacity of the adsorbent wad reduced from 98% to 90%. The loss in the adsorption capacity of the adsorbent for dye ions was found to be b8%. This might be due to the ignorable amount of adsorbent loss during the adsorption-desorption process. Therefore, CMOPP could be used for five cycles in CR adsorption studies with a small loss in the total adsorption capacity.

3.9. Desorption and regeneration studies

4. Conclusions

Desorption studies are important in order to regenerate the adsorbent and recover the dye ions. A successful desorption process requires the proper selection of the eluents, which strongly depends on the type of adsorbent and the adsorption mechanism. For this purpose, 0.1 M HCl, deionized water, 0.1 M CH3COOH and 0.1 M NaOH was examined as eluents. Results obtained in the desorption studies are shown in Fig. 9(A). The maximum percentage recovery of CR was 98% with 0.1 M NaOH. It was found that 0.1 M NaOH as a good eluent for desorption of CR compared to other three eluents namely 0.1 M HCl, deionized water, and 0.1 M CH3COOH. In adsorption process, to keep the processing cost down and to open the possibility of recovering the dyes extracted from the liquid phase, it is desirable to regenerate the adsorbent material. In this study 0.1 M NaOH was used as an eluent agent. The regenerated adsorbent was reused for adsorption-desorption cycles as shown in Fig. 9(B). An efficiency of 98% recovery of CR was obtained with 0.1 M NaOH in the first cycle and is therefore suitable for regeneration of adsorbent.

This study identified CMOPP as a suitable adsorbent for CR removal in batch experiments. The batch study parameters; pH of solution, contact time, initial dye concentration and temperature was found to be effective on the adsorption process. Acidic pH was found to enhance the CR dye removal the most. The equilibrium data fitted well to the Langmuir isotherm. The maximum monolayer adsorption capacity of CR was 163 mg/g at an optimum pH 3.0. The kinetic studies revealed that the adsorption process followed well the pseudo-second-order kinetic model well. Activation energy was calculated as 30 kJ/mol and this value suggests that the adsorption of CR onto CMOPP was a physical adsorption. Thermodynamic studies confirmed that the process was spontaneous and endothermic in nature. The reusability of the adsorbent was good after five consecutive adsorption-desorption cycles without any considerable loss in adsorption capacity. The interactions between CR and functional groups on the cell wall surface of the adsorbent were confirmed by FTIR analysis. Based on the results, it can be concluded that CMOPP is an effective and efficient adsorbent for the removal of CR from aqueous solutions. Acknowledgements This research was supported by the R&D Program for Society of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (Grant No: NRF-2013M3C8A3078596). This study was supported by the R&D Center for Valuable Recycling (Global-Top Environmental Technology Development Program) funded by the Ministry of Environment (Project No: GT-11-C-01-070-0). References

Fig. 9. Desorption studies of CR: (A) desorption time, and (B) number of adsorptiondesorption cycles.

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