Removal of methyl orange (MO) from aqueous solution using cationic surfactants modified coffee waste (MCWs)

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Removal of methyl orange (MO) from aqueous solution using cationic surfactants modified coffee waste (MCWs) Ridha Lafi∗, Amor Hafiane Laboratory of Wastewater Treatment, CERTE, BP 273, Soliman 8020, Tunisia

a r t i c l e

i n f o

Article history: Received 6 January 2015 Revised 18 May 2015 Accepted 28 June 2015 Available online xxx Keywods: Methyl orange Modified coffee waste Surfactant Adsorption Kinetics Thermodynamics

a b s t r a c t In this present work, a commercial coffee waste was treated using cationic surfactants cetyltrimethyl ammonium bromide (CTAB) or cetylpyridinium chloride (CPC) to enhance its adsorption capacity for the removal of methyl orange dye (MO, anionic dye) from aqueous solutions. A series of experiments were undertaken in an agitated batch to assess the effect of key parameters such as pH, contact time, adsorbent dose and ionic strength. Maximum methyl orange dye adsorption onto modified commercial coffee waste (MCWs) was observed at pH 3.5 with 0.1 g/50 ml of adsorbent dose. Modeling study shows that pseudo-second-order kinetic model and Langmuir adsorption isotherm model provide better fitness to the experimental data. The maximum adsorption capacity (62.5 mg/g at 25 °C) was obtained with CPC modified commercial coffee waste. Calculated thermodynamic parameters ࢞G0 , ࢞H0 and ࢞S0 showed that adsorption is spontaneous and exothermic. The FT-IR analysis showed that possible mechanisms controlling MO adsorption on the MCWs included electrostatic interactions and hydrophobic interaction. © 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Removal of toxic dyes from the environment is an important challenge. Ideally, a removal processes must be simple, effective and inexpensive. Conventional methods of dyes removal from wastewater have been used. These methods include biological and physico– chemical processes [1,2]. The activated carbon with large surface area is effective and widely used as adsorbent [3], but its cost is quite high. Therefore, many researchers pay attention to the use of different types of low-cost materials from biomass products such as peanut husk [4,5], sugarcane bagasse [6,7], peanut hulls [8], wheat straw [9,10] as adsorbent. Commercial coffee waste can be used as an available adsorbent to remove cationic dyes from wastewater [11,12]. The chemical composition of the insoluble coffee waste cell’s wall is largely made up of lignin and some structural proteins that contain hydroxyl and carboxylic groups. However the capacity about anionic dyes was very low. To enhance the remove of the anionic pollutants, several researchers used cationic surfactant to modify agricultural products [13–16]. But investigations using commercial coffee waste modified by surfactants for dyes adsorption were not, in our knowledge, reported in the literature. Therefore in the present study, two cationic surfactants cetyltrimethyl ammonium bromide (CTAB) and cetylpyridinium chloride (CPC) were used to modify the surface of CW to enhance its capacity toward anionic pollutants. Methyl orange ∗

Corresponding author. Tel.: +216 79 325 750; fax: +216 79 325 802. E-mail address: ridha.lafi@yahoo.fr (R. Lafi).

(MO, C14 H14 N3 NaO3 S, molecule weight 327.36 g mol/l, CAS 547-58-0) Fig. S1 (Supplementary information) is selected as anionic dye model. It is one of the well-known acidic/anionic dyes, and it has been widely used in textile, printing, paper, food, and pharmaceutical industries. Azo dyes are well known as carcinogenic organic substances. Like many other dyes of its class, MO do not inadvertently enter the body through ingestion, it is metabolized into aromatic amines by intestinal microorganisms [17]. The aim of this study is to improve the adsorption property of commercial coffee waste towards MO dye by using two cationic surfactants. Kinetic, thermodynamic and isotherms of MO removal on this novel material were investigated. 2. Materials and methods 2.1. Chemicals All the used reagents (Na2 CO3 , NaHCO3 , NaOH, HCl) were analytical-grade reagents and the chemical properties of cationic surfactants used in this study are represented in Table S1 (Supplementary information). Double distilled water was used for preparing all of the solutions and reagents. The initial pH is adjusted with 0.1 M HCl and NaOH solutions. All the adsorption experiments were carried out at room temperature (25 ± 1 °C). A sock solution of 1.0 g/l of methyl orange dye (MO, C14 H14 N3 NaO3 S, molecule weight 327.34 g mol/l, CAS 547-58-0) was prepared by dissolving the appropriate amount of dye with distilled water.

http://dx.doi.org/10.1016/j.jtice.2015.06.035 1876-1070/© 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: R. Lafi, A. Hafiane, Removal of methyl orange (MO) from aqueous solution using cationic surfactants modified coffee waste (MCWs), Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.035

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2.2. Preparation of modified coffee waste (MCWs) The coffee waste (CW) was collected from a coffee shop. The collected materials were washed several times with boiled distilled water to remove any adhering dirt and color until pH was reached 6.1 in the residual liquid. The CW was then dried in the oven at 60 °C for 24 h, ground and sieved to obtain a particle sizes in the range 250– 800 μm. The coffee waste was modified by impregnating it with the cationic surfactants as follows: 12.5 g of CW were mixed with 0.5 l of 0.027 mol/l surfactant solutions, and then the mixtures were agitated in shaker machine with 220 rpm at room temperature 25 ± 1 °C for 48 h. The modified coffee waste was then separated from the liquid and washed several times with distilled water to remove superficially retained surfactants. Finally, the treated coffee waste was dried in an oven at 60 °C overnight and stored in an airtight glass. The resulting MCWs were designated as CTAB-CW and CPC-CW, respectively. 2.3. Characterization The techniques used to characterize the samples include Fourier Transform infrared spectroscopy (FTIR), Boehm titration and pH point of zero charge (PZC).The FTIR analysis was done using a Fourier Transform Spectrophotometer model IRAffinty−1 SHIMADZU. The determination of surface functional groups was based on the Boehm titration method [18]. The number of the basic sites was calculated from the amount of HCl that reacted with the coffee adsorbents. The various free acidic groups were derived using the assumption that NaOH neutralizes carboxyl, lactone and phenolic groups, Na2 CO3 neutralizes carboxyl and lactone. The NaHCO3 neutralizes only carboxyl groups. The excess of base or acid was then determined by back titration using NaOH (0.10 mol/l) and HCl (0.10 mol/l) solutions. The point of zero charge (pHPZC ) was evaluated according to titration procedure described in literature [19]. Three aqueous solutions of different pH values (3, 6 and 11) were prepared. Several amounts of CW and MCWs (0.05, 0.1, 0.5, 1.0, 3.0, 7.0 and 10.0% w/w) were added to 20 ml of each solution. The aqueous suspensions containing different amounts of the adsorbent were kept in equilibrium for 24 h under agitation (220 rpm) at 25 °C. The pH of each solution was then measured. The pH was determined as the converging pH value from the pH versus adsorbent mass curve. 2.4. Equilibrium studies Equilibrium studies were carried out by contacting fixed amount of MCWs (0.10 g) with 50 ml of dye solution of different initial concentrations in 100 ml stopper conical flasks at room temperature (25 ± 1 °C) and pH of 3.5. The initial and equilibrium concentrations of dye were analyzed using thermospectronic UV 1 Spectrophotometer at a wavelength of maximum absorbance 463 nm. The amount of adsorption at equilibrium, qe (mg/g), was calculated by:

V qe = (Ci − Ce ) W

(1)

where Ci and Ce are the initial and final (equilibrium) concentrations of dye (mg/l), respectively. V is the volume of dye solution (l), and W is the weight of the adsorbent used (g). The percentage removal of dye was calculated as follows:

% Dye removal =

[m5G;July 19, 2015;14:8]

Ci − Ce × 100 Ci

(2)

The sorption characteristics of methyl orange dye by MCWs adsorbent were studied by using the batch equilibrium method under various experimental controlling parameters such as the reaction pH, contact time, adsorbent dosage and temperature. The effect of contact time on the sorption process was accomplished by using 50 ml of 50 mg/l of MO dye. This solution was added

to 0.1 g of MCWs adsorbent at 25 ± 1 °C and the mixture was shaken at constant speed of 120 rpm. Samples were withdrawn at different time intervals (0–420 min), filtrated and analyzed for remaining dye concentration. The effect of pH on the sorption process of MO dye by MCWs was accomplished by adjusting the pH value of dye solution in the pH range of 2.7–10.5 by addition of dilute aqueous solutions of HCl or NaOH (0.1 M). 50 ml of MO dye (50 mg/l) was mixed with 0.1 g of MCWs and the mixture was shaken for 4 h at 25 ± 1 °C. This was then followed by filtration and the absorbance of remaining dye solution was measured at 463 nm. The effect of adsorbent dosage was performed by varying the adsorbent dosage of MCWs from 0.2 to 20 g/l. 50 ml of the dye solution (50 mg/l) was adjusted to a pH 3.5, and the batch experiment was performed at 25 ± 1 °C by using a 4 h shaking time. For the study of salt effect, the experiments were carried out with the NaCl at different concentrations ranging from 0 to 10 g/l using a 50 ml dye solution (50 mg/l) and 0.1 g of adsorbent dosage at pH 3.5 for 4 h and the procedure was completed as described above. Finally, the effect of temperature (25, 35, 45 and 55 °C) was studied by using a 50 ml dye solution (50 mg/l) and 0.1 g of adsorbent dosage at pH 3.5 for 4 h. 2.5. Desorption experiments To determine the desorption behavior of the MO-loaded MCWs; 0.01 M NaOH solution was used as a desorbing agent. Typically, 0.1 g of fresh adsorbent was added to 50 ml of 50 mg/l MO solution at pH 3.5 and shaken for 240 min. Then, the MO adsorbed by MCWs was separated and the residual MO concentration was measured using spectrophotometer. The dye-loaded adsorbent was washed gently with water to remove any not adsorbed dye and dried. The desorption process was performed by mixing the dried adsorbent with 50 ml of distilled water at different pH values (3.2–12). After the mixture was shaken for a predetermined time, the desorbed MO concentration was determined spectrophotometrically. The desorption efficiency of MO was calculated as the ratio of the desorbed amount (qde ) to the adsorbed amount (qad ).

Desorption efficiency (%) =

qde × 100 qad

(3)

In order to check the reusability of the regenerated adsorbents, three cycles of consecutive adsorption desorption studies were performed. All the experiments (adsorption and desorption) were performed in a batch setup taking two replicates and average values were obtained. 3. Results and discussion 3.1. Characterization of CW and MCWs The point of zero charge (pHpzc ) of CW occurred at pH 5.3. The CW was positively charged below this pH and negatively charged above it. Many researchers reported that the CW particles have negative charge [12]. In the case of MCWs, the pHpzc are 4.9 and 4.2 for CPC-CW and CTAB-CW respectively (Table 1). The surface charges of the CW impregnated with cationic surfactants (i.e. linear alkyl quaternary ammonium) were probably positive at all pHs because the hydrophilic positively charged head groups of the surfactants are arranged toward the aqueous phase. For this reason, the capacity of coffee waste to adsorb positive ions increases in proportion to the quantity of cationic surfactant attached to its surface. 3.2. Surfactant adsorption on CW In order to determine the Effect of pH on the adsorption of surfactant onto MCWs, 0.027 M CTAB or CPC solutions was applied and at

Please cite this article as: R. Lafi, A. Hafiane, Removal of methyl orange (MO) from aqueous solution using cationic surfactants modified coffee waste (MCWs), Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.035

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R. Lafi, A. Hafiane / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10 Table 1 Surface chemical characteristics of the CW and MCWs.

Carboxylic groups (mmol/g) Lactonic groups (mmol/g) Phenolic groups (mmol/g) Total acid groups (mmol/g) Total basic groups (mmol/g) pHPZC

CW

CPC-CW

CTAB-CW

0.225 0.015 0.27 0.51 0.1 5.3

0.245 0.014 0.28 0.539 0.113 4.9

0.264 0.014 0.29 0.568 0.121 4.2

3

ified adsorbent has become positive and accordingly, more appropriate for the adsorption of the anionic adsorbates like the MO molecules. Similar results were also obtained by cationic surfactantmodified barley straw, and wheat straw for removal of anionic dyes [13,14,24]. In the present research, it can be assured that the bilayer is formed. Fig. 2 best schematizes the modification procedure of CW using cationic surfactant. Owing to the different configurations of the CTAB and CPC surfactants on CW, various interactions may be involved in the adsorption of the MO from aqueous solution.

8

3.3. Effect of adsorbent dosage CW CW-CTAB CW-CPC

7

The effect of adsorbent dosage on MO adsorption is shown in Fig. 3, an increase in adsorbent dosage from 0.2 to 20 g/l conduct to an increase in MO adsorption from 41.1 to 99.3 and from 37.9 to 98.5 for CTAB-CW and CPC-CW respectively, which suggests that there is an increase in the number of sorption sites on the adsorbent surface with increasing CTAB and CPC [13,25]. The constant removal efficiency of MO observed after the initial increase is probably due to the aggregation of adsorbent particles by increasing adsorbent dose. Such aggregation decreased the effective surface area of adsorbent [13]. Therefore, 2 g/l was chosen as the optimum adsorbent dosage for the following experiments, because higher amounts of MCWs did not increase appreciably the MO adsorption.

pHf

6

5

4

3 2

4

6

8

10

12

3.4. Effect of solution pH

pHi Fig. 1. Effect of initial pH on the adsorption of surfactant on CW (Adsorbent dosage = 1.25 g/50 ml, initial CTAB or CPC concentration = 0.027 M, temperature =25 °C, contact time = 48 h).

pHs ranging from 2 to 12 by adding either 0.1 M HCl or NaOH. Next, 50 ml of each was transferred into a series of conical flasks and then 1.25 g of the MCWs adsorbent was taken to each solution. The solutions were agitated for 48 h and the final pH values of the solution were measured. The results are illustrated in Fig. 1. It can be seen that the final pH in presence of surfactant modified adsorbents have significantly decreased compared to that in presence of distilled water by CW. As a matter of fact, the surface charges of the CW impregnated with cationic surfactants (i.e. linear alkyl quaternary ammonium) were probably positive at all pHs because the hydrophilic positively charged head groups of the surfactants are arranged toward the aqueous phase. The polar nature of coffee waste can remove large quantities of metal ions and dyes from aqueous solutions since the chemical composition of the insoluble coffee waste cell’s wall is largely made up of lignin and some structural proteins [20]. The mechanism was due to adsorption of cationic surfactants onto adsorbent surface may follow two approach. (i): The non-polar portion (alkyl) of CTAB and CPC may interact with CW surface through hydrophobic bonding and the polar (positively charged) head group directed toward the bulk of the solution. (ii): The adsorption of the cationic surfactant onto the negatively charged surface of the adsorbent can be considered to be controlled by two steps; (1) the formation of surfactant monolayer through the ion exchange and electrostatic attraction and (2) the formation of surfactant bilayer via hydrophobic interactions [21–23]. As a matter of fact, firstly, the positive head of the surfactants are exchanged with the interlayer exchangeable cations within the CW, there by forming a surfactant monolayer with outward pointing head groups. Secondly, the bilayer is organized by the attachment of the surfactant alkyl chains to the outer surface of the monolayer by means of the hydrophobic– hydrophobic interactions. Therefore, the external surface of the mod-

The initial pH has distinguished role on the chemistry of dye and the surface binding sites of the adsorbents. The influence of the initial pH on the adsorption of MO onto MCWs is shown in Fig. 4. The adsorption capacity decreased from 23.48 mg/g to 7.06 mg/g and from 23.64 mg/g to 8.11 mg/g for CTAB-CW and CPC-CW respectively, when the pH was changed from 2.9 to 10.3. The maximum adsorption was observed at pH 3.5. These results could be explained by the different interaction between MO and MCWs in terms of surface charge and degree of ionization of dye. In acidic medium, the lower pH leads to an increase in H+ ion concentration and the MCWs surface becomes more positively charged. The strong electrostatic attraction between the positively charged adsorption site and the anionic MO molecule (pKa MO = 3.42 [26]) results in high adsorption of MO dye. Furthermore, the higher adsorption capacity may be attributed to more hydrogen bonding formed from –SO3 − to –SO3 H at lower pH. Apparently, under acidic conditions MCWs can act as hydrogen bonding donor and receptor. At higher pH the MO decreased because MO is in non-protonated form SO3 − and the negative groups offer only weak electrostatic attraction (point zero charge of MCWs (pHPZC = 4.9 for CW-CPC and pHPZC = 4.2 for CW-CTAB), but no strong hydrogen bonding. Also with the increase of pH, there was competition between OH− and MO for positively charged adsorption sites, which caused the decline of adsorption rate. However, there is still existence of higher adsorption capacity at higher pH value and this showed that the intermolecular interaction such as π –π dispersive interactions between aromatic rings, –N=N– groups of the dye molecule also contribute to the adsorption. A similar behavior has been reported in the literature [15,27,28]. 3.5. Effect of contact time From Fig. 5, it was seen that the whole process was evidently divided by three sections: (1) an initial rapid stage (to 60 min) where adsorption rate was fast, (2) a slower second stage where adsorption rate became lower (60–120 min), and (3) a slowest equilibrium adsorption stage. At the beginning of adsorption, the driving force of MO concentration between MCWs and MO solution was larger, and adsorption

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Fig. 2. (a) Surfactant monolayer formation on the adsorbent surface, (b) surfactant bilayer formation on the adsorbent surface and (c) schematic representation of the interactions of MO molecules with MCWs.

26 24 22 20

qe (mg/g)

% MO removal

100 90 80

CTAB CPC

70

CTAB CPC

18 16 14 12

60

10

50

8

40

6 2

30 0

2

4

6

8

10

12

14

16

18

20

22

3

4

5

6

7

8

9

10

11

pH

Adsorbent dosage (g/L)

Fig. 3. Effect of adsorbent dosage on the removal of MO (initial MO concentration = 50 mg/l, solution volume = 50 ml, pH = 3.5, temperature = 25 °C, contact time = 4 h).

Fig. 4. Effect of initial pH on the adsorption of MO on MCWs (Adsorbent dosage = 2 g/l, initial MO concentration = 50 mg/l, solution volume = 50 ml, temperature =25 °C, contact time = 4 h).

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R. Lafi, A. Hafiane / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10 Table 2 Parameters of adsorption models for MO adsorption onto MCWs.

25

Isotherm model

qe (mg/g)

5

CTAB CPC

20

Langmuir

15 Freundlich

10

Temkin

5

D-R

0 0

30

60

90

120

150

180

210

240

270

Parameter

qm (mg/g) b (mg/l) R2 n KL (mg/g) R2 A B R2 qmD-R (mg/g) β D-R (mol2 /J2 ) E (J/mol) R2

MCWs CTAB-CW

CPC-CW

58.82 0.395 0.998 2.29 16.10 0.889 1.98 8.66 0.985 40.04 2 × 10−7 2236 0.89

62.5 0.290 0.985 2.03 14.23 0.939 1.75 9.26 0.954 39.56 3 × 10−7 1825 0.871

Time (min) Fig. 5. Effect of contact time on the adsorption of MO on MCWs (Adsorbent dosage = 2 g/l, initial MO concentration = 50 mg/l, solution volume = 50 ml, temperature = 25 °C, contact time = 0–4 h).

qe (mg/g)

24 22 20 CTAB CPC

18

3.7. Adsorption isotherm In this work, four models were used to describe the relationship between the amount of MO adsorbed and its equilibrium concentration. The applicability of the isotherm models to the adsorption study within the experimental conditions (concentration range 20– 120 mg/l, adsorbent dose 2 g/l, temperature 25 ± 1 °C, contact time 4 h and stirring speed 110 rpm) was judged by the correlation coefficient R2 value of each plot. The higher R2 value indicates the fit. 3.7.1. Langmuir isotherm The non-linear form of the Langmuir isotherm model [31] is given as:

16

qe =

14 12 10 0

2

4 6 NaCl (g/L)

8

10

Fig. 6. Effect of ionic strength on the adsorption of MO on MCWs (Adsorbent dosage = 2 g/l, initial MO concentration = 50 mg/l, solution volume = 50 ml, temperature = 25 °C, contact time = 4 h).

speed was relative fast. The value of qt increased significantly with time. After this stage, the driving force became smaller, and value of qt increase with small range. This behavior may be an indication that external mass transfer (first) and intraparticle diffusion mass transfer (second) may be controlled MO adsorption onto MCWs. 3.6. Effect ionic strength The effect of ionic strength on MO adsorption onto MCWs was studied at fixed pH=3.5 and at different NaCl concentrations. Fig. 6 shows that the effect of added salt is weak as the adsorption capacity decreased only slightly (from 22.57 to 21.78 mg/g and from 22.17 to 21.13 mg/g for CTAB-CW and CPC-CW) when NaCl concentration increased from 0 to 10 g/l. This slight decrease could be attributed to the competition between MO and chloride anions for the sorption sites. Previous studies have shown that if electrostatic attraction is the main adsorption mechanism, ionic strength has a significant negative effect on the adsorption process [29,30]. Thus these results mean that the electrostatic interaction between the surface of MCWs and MO is not the domination interaction in the adsorption of MO onto MCWs and provides support for the hypothesis that hydrophobic interaction between hydrophobic parts of MO and MCWs plays also a role in the adsorption of MO onto MCWs.

qm bCe

(1 + bCe )

(4)

where Ce (mg/l) is the equilibrium concentration of dyes adsorbed, qe (mg/g) is the amount of dyes adsorbed, qm and b (Langmuir constants) are the monolayer adsorption capacity and affinity of adsorbent towards adsorbate, respectively. A plot of qe against Ce gave a fitted curve, and the Langmuir constants were generated from the plot of sorption data (Table 2). The essential factor RL , calculated for all dye concentrations revealed that the entire adsorption process were favorable since its values were in the range of 0 < RL < 1. The RL equation [32] is given by:

RL =

1

(1 + bC0 )

(5)

where b is the Langmuir constant and C0 (mg/l) is the initial concentration dye. 3.7.2. Freundlich isotherm The non-linear form of Freundlich model [33] is given as:

qe = KF Ce 1/n

(6)

Where qe (mg/g) is the amount of dye adsorbed at equilibrium, Ce (mg/l) is the equilibrium concentration of the adsorbate, KF and n are the Freundlich equilibrium coefficients. The value of n gives information on favorability of adsorption process and KF is the adsorption capacity of the adsorbate. A plot of qe against Ce gave poor curves indicating that the adsorption process did not follow this model. The values of the Freundlich equilibrium coefficients KF and n were generated from the plot of sorption data (Table 2). The parameter 1/n ranging between 0 and 1 measure the adsorption intensity or surface heterogeneity. 3.7.3. Temkin isotherm The Temkin model [34] is expressed as:

qe = Bln(A Ce )

(7)

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where kT is the equilibrium binding constant corresponding to the maximum binding energy, B is related to the heat of adsorption, qe (mg/g) is the experimental adsorption capacity and Ce (mg/l) is the concentration of dyes adsorbed at equilibrium.

B=

RT bT

(8)

where 1/bT indicates the adsorption potential of the adsorbent, R is the universal gas constant (8.314 J/mol K) and T is the temperature in Kelvin (K). A plot of qe against Ce indicates a linear decrease in adsorption energy as the adsorption sites is filled. The heat of adsorption of all the molecules in the layer decreases linearly with surface coverage due to adsorbent–adsorbate interactions. It assumes also that adsorption is characterized by uniform distribution of binding energies up to a maximum value. 3.7.4. Dubinin–Radushkevich (D–R) isotherm The Dubinin–Radushkevich model [35] is used to estimate the characteristic porosity and the apparent free energy of adsorption. It helps to determine the nature of adsorption processes whether physical or chemical. The D–R sorption is more general than the Langmuir isotherm because it does not assume a homogenous surface or constant sorption potential. The non-linear presentation of the D–R isotherm equation is:



qe = qm exp −βε 2



ε = RT ln 1 +

 1

Ce

(10)

A plot of qe against ε 2 gave non linear graphs (figure not shown). The adsorption mean free energy E (kJ/mol) is given as:

E=

1

(2β)0.5

Adsorbent

Dye

qmax (mg/g)

References

Modified wheat straw Ethylenediamine modified wheat straw Modified wheat straw Surfactant modified zeolite Modified wheat straw Modified peanut husk Surfactant modified barley straw Surfactant modified barley straw HCl-treated Sugarcane bagasse polyethyleneimine treated Peanut husk Acetic acid treated Peanut husk CTAB-TW CTAB-CW CPC-CW

CR CR MO CR LG LG AB (40) RB5 DV (51) RSN BG CR MO MO

71.2 (30 °C) 73.4 (30 °C) 50.4 (30 °C) 69.94 (30 °C) 70.01 (30 °C) 146.2 (30 °C) 47.6 (25 °C) 25.4 (25 °C) 39.6 (30 °C) 79.5 (30 °C) 79.7 (30 °C) 106.4 (30 °C) 58.82 (25 °C) 62.5 (25 °C)

[13] [9] [11] [23] [10] [14] [22] [22] [7] [5] [4] [28] This study This study

60 50 40 30

(9)

where qe (mg/l) is the amount of dye molecules, qm (mg/g), is the maximum adsorption capacity β is the activity coefficient related to adsorption mean free energy mol2 /J2 and ε is the Polanyi potential given by



Table 3 Adsorption capacity of anionic dyes by other materials.

qe (mg/g)

6

[m5G;July 19, 2015;14:8]

Experimental CTAB-CW Langmuir CTAB-CW Experimental CPC-CW Langmuir CPC-CW

20 10 0 0

5

10

15

20

25

Equilibrium MO Concentration (mg/L) Fig. 7. Isotherm modeling of MO on the adsorption onto MCWs at 25 °C.

(11)

The mean free energy gives information about chemical or physical adsorption. With the value of 8 < E< 16 kJ/mol, the sorption process follows chemical ion-exchange, while the values of E < 8 kJ/mol, the sorption is of a physical nature. The parameters of the four models were calculated and were listed in Table 2. By comparing the correlation coefficients, it can be concluded that Langmuir isotherm provides a good model for the sorption system, which is based on monolayer sorption onto surface containing finite number of identical sorption sites. The maximum adsorption capacities of MCWs for MO, at 25 °C, are 58.82 and 62.5 mg/g for CTAB-CW and CPC-CW, respectively. The comparison of adsorption capacity of MCWs with that of various adsorbents was given in Table 3, shows that modified coffee waste, available and cheap by-product, can be used as potential material to minimize the concentration of MO from aqueous solution. The values of the parameter RL indicate that the adsorption is favorable (0 < RL < 1). These were found to be 0.116–0.019 for CTABCW and 0.152–0.026 for CPC-CW in the concentration range studied, and the value of the free energy estimated from the D-R model E < 8 kJ/mol indicating that adsorption process is of physical nature. The isotherm profiles of MCWs for MO are shown in Fig. 7. It is obvious that the experimental results are well represented by the Langmuir isotherm. 3.8. Adsorption kinetics study Several steps were used to examine the adsorption dynamics controlling of sorption process such as chemical reaction, diffusion con-

trol and mass transfer. Kinetic models are used to test experimental data from the adsorption of MO onto MCWs. The kinetics of MO adsorption onto MCWs is required for selecting optimum operating conditions for the full-scale batch process. The kinetic parameters, which are helpful for the prediction of adsorption rate, give important information for designing and modeling the adsorption processes. Thus, pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion kinetic models were used for the adsorption of MO onto MCWs. The conformity between experimental data and the model-predicted values was expressed by the correlation coefficients R2 . The pseudo-first-order kinetic model was suggested by Lagergren [36] for the adsorption of solid/liquid systems and its formula is given as:



log (qe − qt ) = log (qe ) −



k1 t 2.303

(12)

where qt (mg/g) and qe (mg/g) denote the amount of dye adsorbed at any time t (min) and at the equilibrium time, respectively, and k1 (1/min) is the rate constant of the pseudo-first-order adsorption. k1 is calculated from a plot of log (qe – qt ) against t. However, the calculated and experimental qe values do not agree (Table 4), and the pseudo-first-order kinetic model does not describe the adsorption of MO on MCWs. The linear form of the pseudo-second-order kinetic model can be expressed as [37]:

1 t t = + qt qe k2 q2e

(13)

Please cite this article as: R. Lafi, A. Hafiane, Removal of methyl orange (MO) from aqueous solution using cationic surfactants modified coffee waste (MCWs), Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.035

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R. Lafi, A. Hafiane / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10 Table 4 Kinetic parameters for adsorption of methyl orange (50 mg/l) onto MCWs. MCWs

Pseudo-first-order kinetics

CTAB-CW CPC-CW

2

qe (mg/g)

k1 (1/min)

R

14.92 17.74

0.023 0.023

0.931 0.976

Table 6 Thermodynamic parameters for the adsorption of MO onto MCWs.

Pseudo-second-order kinetics qe (mg/g)

k2 (g/(mg min))

R

26.31 26.31

0.0036 0.0029

0.998 0.997

CTAB-CW CPC-CW

CTAB-CW CPC-CW

C

kid (mg/g min1/2 )

R2

α

β

R2

9.699 8.582

1.454 1.442

0.991 0.986

22.54 13.51

0.283 0.267

0.971 0.972

where k2 (g/(mg min)) is the rate constant of the pseudo-secondorder adsorption, obtained from linear plots of t/qt against t (Table 4). The experimental and calculated values for qe were similar; also by comparing the correlation coefficients (R2 ) values obtained for pseudo-first-order and pseudo-second-order kinetics, it can be concluded that the adsorption mechanism is a pseudo-second-order reaction. The simple Elovich model equation is generally expressed by the following equation [38]:

qt =

1

β

ln (αβ) +

1

β

ln (t )

࢞S0 (kJ/mol)

−9.387 –11.399

−0.005 – 0.009

࢞G0 (kJ/mol) 298K

308K

318K

328K

−7.807 −8.625

−7.754 −8.532

−7.702 −8.439

−7.649 −8.346

50

Elovich equation

(14)

Plot of qt versus ln (t) should yield a linear relationship, if the Elovich is applicable, with a slope of (1/β ) and an intercept of (1/β ) ln (αβ ). The Elovich constants obtained from the slope and the intercept of the straight line reported in Table 5. The correlation coefficients R2 are very wavy and ranged from low value to high value without definite role. The diffusion into pores could play a role in MO adsorption onto MCWs. Therefore, the kinetic results were analyzed by the intra- particle model, which is expressed as [39]:

MO desorption efficiency (%)

Intraparticle diffusion model

࢞H0 (kJ/mol)

MCWs 2

Table 5 Kinetic parameters for adsorption of methyl orange (50 mg/l) onto MCWs. MCWs

7

40 CW-CTAB CW-CPC

30

20

10

0 2

4

6

8 pH

10

12

14

Fig. 8. Methyl orange desorption efficiency at different pH values.

is the square root of the where qt time and kid (mg/g min1/2 ) is the rate constant for intra-particle diffusion. The intercept, C, in a plot of qt versus t0.5 represents the thickness of the boundary layer. If C is zero then intra-particle diffusion is the sole rate-limiting step, and the larger the value of C the greater the contribution of surface adsorption. The plots were not linear over the whole time range, but could be separated into two-three linear regions, which suggests that there are multiple stages to the adsorption process. The R2 values are all > 0.9 (Table 5), which indicates that adsorption of MO on MCWs can be described by intra-particle diffusion, but since these lines do not pass through the origin, we can conclude that intra-particle diffusion is not the sole rate controlling step.

where R (8.314 J/molK) is the universal gas constant, T (K) is the absolute temperature and K0 is the equilibrium constant equals to qe /Ce . The plot of lnK0 against 1/T yields à straight line from which ࢞H0 and ࢞S0 were calculated from the slop and intercept, respectively. The thermodynamic parameters obtained at various temperatures investigated for 50 mg/l dyes concentrations are represented in Table 6. The negative values of G0 mean that the process is feasible and adsorption is spontaneous thermodynamically. Generally, G0 values for physisorption are in the range of -20 to 0 kJ/mol, and those for chemisorption range between −80 and −400 kJ/mol [40]. In this study, the G0 values are in the range of −7.80 to −7.64 kJ/mol for CW-CTAB and −8.62 to −8.34 kJ/mol for CW-CPC, indicating that adsorption is mainly physisorption. The exothermic nature of the process is confirmed by the negative value of H0 (−9.38 kJ/mol and −11.39 kJ/mol). Compared with the adsorption energy from different forces (i.e., Van der Waals forces 4–10 kJ/mol, hydrophobic bond forces about 5 kJ/mol, coordination exchange about 40 kJ/mol, and chemical bond forces > 60 kJ/ mol), the absolute value of H0 indicates that Van der Waals interaction, hydrophobic interaction may contribute to the adsorption process in addition to electrostatic attraction. The value of entropy is negative S0 due to decreased randomness at the solid/solution interface during adsorption of MO onto MCWs.

3.9. Thermodynamics parameters

3.10. Desorption and reuse

To further understand the adsorption process taking place between MO and MCWs, thermodynamic parameters were determined. The Gibbs free energy (࢞G0 ) was calculated using the equation

One of the important steps in the development of adsorption based technologies is desorption of loaded adsorbent which enables reuse of the biomass and recovery of adsorbate. Desorption experiments were carried out to find the optimum pHdesorption conditions. Fig. 8 shows MO desorption efficiency at the first desorption cycle from the MO-adsorbed MCWs with the increasing of pH of the desorbing agent. Low desorption efficiency of 8.15 and 7.80% for MO-adsorbed CW-CTAB and MO-adsorbed CW-CPC, respectively, was obtained with neutral distilled water. The best desorption was obtained at alkaline pH. The reason of desorption was that the

qt = kid t 1/2 + C

(15)

is the amount adsorbed at time t, t0.5

G0 = −RT ln(K0 )

(16)

The Van’t Hoff equation was used to calculate the values of enthalpy (࢞H0 ) and entropy (࢞S0 )

lnK0 = −

G 0 RT

= −

H 0 RT

+

S 0 R

(17)

Please cite this article as: R. Lafi, A. Hafiane, Removal of methyl orange (MO) from aqueous solution using cationic surfactants modified coffee waste (MCWs), Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.035

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R. Lafi, A. Hafiane / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10

100

102

90

100 98

CW-CTAB CW-CPC

70

96 Transmittance (%)

MO desorption efficiency (%)

80

60 50 40 30

1465

(b)CW-CTAB

1520

94 92 90

1607

(a) CW

2853

1540

1741 1662

1197

(c) CW-CTAB-MO

88

1032

2929

86 1107

84 82 80 4000

3500

3000

2500

2000

1032 2929 1107

3500

3000

2500

2000

1500

1000

500

1368

98 3414

1368

Fig. 11. FTIR spectra of CW/CW-CPC/CW-CPC-MO.

100

96

1197 (b) CW-CPC

-1 Wavenumber (cm )

Fig. 9. Reusability of the modified coffee waste.

102

1662 2853

86

80 4000

3

Cycle adsorption-desorption

Transmittance (%)

88

82

2

1741

90

84

1

(a) CW

92

10 0

1607 3414

94

20

1465

1540

(c) CW-CPC-MO

1500

1000

500

-1

Wavenumber (cm ) Fig. 10. FTIR spectra of CW/CW-CTAB/CW-CTAB-MO.

positive charge on the surface of adsorbent and OH− was competitive to bind these sites at stronger base condition. So the interaction became weak between amino group on adsorbent surface and the negative group (−SO3 − ) on the dye molecule. MO dye desorption efficiency after three cycles of regeneration were 82.78 and 50.44% for CTAB-CW and CPC-CW, respectively (Fig. 9), which is promising for potential recovery of the adsorbate and reuse of the adsorbent. 3.11. FTIR of CW, MCWs for adsorption and proposed mechanism The FTIR spectra of coffee waste before and after the adsorption of dyes are shown in Figs. 10 and 11. The FTIR spectroscopic analysis showed broad band at 3414/cm representing bonded −OH groups on the surface of CW. The broad band was attributed to inter-and intra-molecular hydrogen bonding of polymeric compounds (macromolecular associations), such as alcohols, phenols and carboxylic acids, as in pectin, cellulose and lignin [41,42]. The peaks at 2929 and 2853/cm are caused by C−H vibration whereas the adsorption peaks at 1741/cm was assigned to the resulting carboxyl linkage derived from xanthenes derivatives such as caffeine [43]. The absorption bands at 1662, 1540 and 1465/cm indicates respectively the presence of COO of carboxyl, secondary amine group N−H and C−O groups on the adsorbent surface. Bands in the range of 1368 /cm are attributed to COO− symmetric stretching vibration. Another ab-

sorption band appearing around 1107 and 1032/cm and could be attributed to the stretching, C−O stretching of ether group and C−O stretching of COOH [44]. The Comparison of FTIR of CW with that of CPC-CW shows that the absorption intensity of –CH3 (2929, 1368/cm) increased while the peak of –CH2 (2853/cm) was slightly resolved. This was due to the increase in the aliphatic carbon content (from CPC) in CPC-CW. These indicated the existence of CPC on the surface of CPC-CW. The Comparison between CW and CTAB-CW FTIRs shows that the vibration peak from –CH2 of CTAB (at 2853/cm and 1465/cm) becomes stronger while the band at 3414/cm (from – NH2 and –OH) is broadened. This showed that quaternary ammonium groups were successfully introduced to the chain backbone after modification. Two new peaks at 1607 and 1520/cm (assigned to aromatic skeletal vibrations and –N=N– stretching vibrations, respectively) are observed in the spectrum of MO-adsorbed MCWs. The bands at 1039 and 1195/cm attributed to S=O stretching after adsorption by MCWs indicates that the –SO3 − groups of MO are involved in the adsorption process. Based on above analysis and previous publications and the results described, we propose that the mechanisms controlling adsorption of MO on MCWs at solution pH 3.5 (Fig. 12a, b), includes (i) electrostatic interaction between negatively charged SO3 − groups of MO and the positively charged adsorbent that results from protonated amine R–N+ (CH3 )2 ) groups and (ii) hydrophobic–hydrophobic interactions between hydrophobic parts of MO and MCWs. To investigate the contribution of electrostatic interactions, we initially studied the effect of initial pH in the range of 2.7–10.5. Fig. 4 shows that the sorption capacities for MO on MCWs decrease rapidly from pH 3.5 to pH 10.5. This result demonstrates that the sorption could occur between sulfonic acid groups (SO3 − ) and protonated amine R–N+ (CH3 )2 ) groups. The large reduction in dye adsorption at highly basic conditions can be attributed to the electrostatic repulsion between the deprotonated –NH2 of MCWs and negatively charged dye molecules. Moreover, at lower pHs , more amine groups are protonated, with a high charge density along the starch chains, it adopts an extended conformation as the electrostatic repulsion effect that are good for sorption. The effect of ionic strength shows that sorption capacities of MO on MCWs decrease upon addition of small quantities of sodium chloride via screening the Coulombic potential between the dyes and charged adsorbents, indicating electrostatic mechanism of the sorption courses [45]. We can deduce that sulfonic acid groups of benzene rings were attached with R–N+ (CH3 )2 ) of the MCWs [46,47].

Please cite this article as: R. Lafi, A. Hafiane, Removal of methyl orange (MO) from aqueous solution using cationic surfactants modified coffee waste (MCWs), Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.035

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9

Fig. 12. Proposed mechanisms for MO adsorption on MCWs at pH 3.5: (a, b) electrostatic interaction; and (c) hydrophobic interaction.

4. Conclusions

Supplementary Materials

This study highlighted the feasibility of modified commercial coffee waste by cationic surfactants for the removal of methyl orange dye from aqueous solution in batch mode. Maximum removal of methyl orange dye adsorption onto modified commercial coffee waste (MCWs) was observed at pH 3.5 with 0.1 g/50 ml adsorbent dose. Experimental data showed better agreement with pseudo-secondorder kinetic model and Langmuir adsorption isotherm model and maximum dye removal (62.5 mg/g at 25 °C) was obtained with CPC modified commercial coffee waste. The thermodynamic study revealed that the adsorption process was spontaneous and exothermic in nature and exhausted MCWs can be regenerated by diluted NaOH solution and reused. The mechanism for the adsorption of MO on the MCWs may include hydrophobic interaction, and electrostatic interaction. The results revealed that MCWs could be used as a novel adsorbent for the removal of methyl orange dye from aqueous solutions.

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Please cite this article as: R. Lafi, A. Hafiane, Removal of methyl orange (MO) from aqueous solution using cationic surfactants modified coffee waste (MCWs), Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.06.035