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Abstract

This study investigates the potential use of activated carbon prepared from coffee waste (CW) as an adsorbent for the removal of congo red dye from aqueous solution. The oxygen-containing groups of activated carbon prepared from CW play an important role in dyes ions adsorption onto activated carbon prepared from CW. The activated carbon is characterized by scanning electron microscopy and Fourier transform infrared (FTIR) spectroscopy. Adsorption experiments were carried out as batch studies at different contact time, pH, and initial dye concentration. The dye adsorption equilibrium was attained after 120 min of contact time. Removal of dye in acidic solutions was better than in basic solutions. The adsorption of dye increased with increasing initial dye concentration. The equilibrium data were revealed that Langmuir model was more suitable to describe the congo red adsorption and demonstrated excellent reusability potential with desorption greater than 90% throughout six consecutive adsorption–desorption cycles. Experimental data founded that kinetics followed a pseudo-second-order equation. Thermodynamic study showed that the adsorption was a spontaneous and exothermic process. According to the FTIR analyses, hydrogen bonding and electrostatic interactions between dyes and oxygen-containing functional groups on activated carbon prepared from CW are dominant mechanisms for dye adsorption.

Keywords

Activated carbon oxygen-containing functional groups adsorption mechanism desorption

Introduction

The discharge of dye effluents from textile, leather, paper, and plastics industries into the environment poses severe problems to many forms of life. Removal of toxic dyes from the environment is an important challenge. Ideally, a removal processes must be simple, effective and inexpensive. Several processes have been suggested to remove dyes from wastewaters. These methods include biological and physico-chemical processes (Bhatnagar and Sillanpää, 2010; Gupta and Suhas, 2009; Gupta et al., 2012).

However, the adsorption process has been proven as a most effective and reliable method for dye removal. The major advantages of an adsorption treatment for the control of water pollution are less investment in terms of initial development cost, simple design, easy operations, free from generation of toxic substances, and easy and safe recovery of the adsorbent as well as adsorbate materials (Djilani et al., 2015; Mittal et al., 2010; Saleh and Gupta, 2014; Zhou et al., 2015). Activated carbons are widely used as adsorbents in wastewater treatment which enable the adsorption of both cationic and anionic pollutants in effluent (Chen et al., 2012; Hiremath et al., 2012; Saleh and Gupta, 2011). Recently, the use of agricultural waste as activated carbon precursors has been found to be renewable and relatively less expensive (Hirunpraditkoon et al., 2011; Mittal et al., 2009a, 2009b). Therefore, in recent years, people have been focusing on the activated carbon preparation based on agricultural waste and lignocelluloses materials which are effective and very inexpensive (Orkun et al., 2012; Sugumaran et al., 2012) such as coffee waste (CW) (Lafi and Hafiane, 2015), date pits (Theydan and Ahmed, 2012), coconut shell (Olafadehan et al., 2012), apicot stones (Djilani et al., 2015). Physical and chemical activations are the common methods for production of activated carbons. Physical activation involves carbonization or pyrolysis of the carbonaceous materials at a high temperature in the presence of oxidizing gas such as steam, air, and carbon dioxide (Gonzalez et al., 2009; Guo et al., 2009). By chemical activation, it is possible to prepare activated carbon in the presence of activating agents such as K₂CO₃, ZnCl₂, H₃PO₄ and KOH (Abbas and Trari, 2015; Bouchemal et al., 2011; Cruz et al., 2012; Demiral et al., 2008; Ketcha et al., 2012; Olowoyo and Orere, 2012).

In this study, the adsorptive removal of congo red (CR) dye from aqueous solution onto activated carbon coffee waste (ACCW) was investigated. The effect of different parameters such as pH, adsorbent dosage, temperature, contact time and initial dye concentration were investigated. The adsorption kinetics and isotherms for CR adsorption onto ACCW were also studied, the reusability potential of ACCW is also established and the mechanism of CR adsorption was discussed.

Materials and methods

Chemicals

All the used reagents (CH₃CO₂K, Na₂CO₃, NaHCO₃, NaOH and HCl) were analytical-grade reagents.

Congo red, a diazo dye, was used as a surrogate indicator to simulate industrial wastewater in order to evaluate the adsorption capacity of MACCW in the present work. The molecular structure of CR is illustrated in Figure 1 and the chemical formula is C₃₂H₂₂N₆Na₂O₆S₂ with Color Index 22120. The molecular weight of CR is 696.65 g mol⁻¹ and the IUPAC name is (1-napthalene sulfonic acid, 3,3-(4,4-biphenylenebis (azo)) bis(4-amino-) disodium salt). Congo red contains –NH₂ and –SO₃ functional groups. The color of CR changes from red to blue in the presence of inorganic acids. The change of color is due to the resonance between charged canonical structures (Finar, 1986). The red color is stable in the pH range of 5–10 (Özcan and Özcan, 2004).

Figure 1.

Molecular structure of CR.

Stock solution (500 mg/L) was prepared by dissolving CR with distilled water. Working solutions were obtained by diluting the stock solution with distilled water to the desired concentration.

Preparation of activated carbon coffee waste

The 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, sieved and the portion with a size of 250–800 µm was used for activated carbon production.

Fifty gram CW and 50 g CH₃CO₂K were mixed to obtain a ratio of 1:1 by weight. Distilled water enough to wet this mixture (200 mL) was added, mixed well, and allowed to stand for 24 h at room temperature. Then, it was dried at 105°C for 24 h and carbonized for 1 h at 450°C (heating rate: 10°C min⁻¹) in muffle furnace. The carbonized sample was washed with distilled water to obtain a pH in the range of 6.8–7; it was oven dried at 100°C and finally, the ACCW was packaged in an airtight container for further use.

The yield of activated carbon which is defined as the ratio of final weight of the obtained product after washing and drying to the weight of dried precursor initially used was calculated based on the following equation:

Y i e l d (%) = \frac{W_{f}}{W_{o}} \times 100

(1)where W_f and W₀ are the weight of final activated carbon product (g) and the weight of dried activated carbon (g), respectively.

Characterization

The techniques used to characterize the ACCW, include Brunauer Emmett Teller (BET), Fourier transform infrared (FTIR) spectroscopy, Boehm titration and pH point of zero charge (pH_PZC).

The surface area, total pore volume and average pore diameter of the samples were determined from the adsorption isotherms of nitrogen at 77 K by using micrometrics model, ASAP 2020, 700VA, made in U.S.A. volumetric analyzer. Mesopore volume was calculated by subtracting the total volume obtained at a relative pressure of 0.99 from the micropore volume obtained from t-plot.

The FTIR analysis was done using a Fourier transform spectrophotometer model IRAffinty − 1 SHIMADZU. The spectra of the ACCW before and after adsorption were in a range of 4000–400 cm^–1.

Boehm titration was used to determine lactonic, phenolic and carboxylic groups on the surface of ACCW. For measurement, 0.15 g of ACCW was added to 50 mL of 0.01 M of these solutions (NaOH, or Na₂CO₃, or NaHCO₃, or HCl). The mixtures were stirred for 48 h at constant speed: 220 r/min, at room temperature. Then, the suspensions were filtrated. To determine the acidic group content, back-titrations of the filtrate (10 mL) were achieved with standard HCl (0.01 M). Basic group’s contents were also determined by titration of the filtrate with NaOH (0.01M) (Boehm, 1966).

For pH_PZC determination, after 0.15 g ACCW is mixed with a series of 50 mL NaCl 0.01 M solutions with an initial pH value in the range of 2–12, resultant suspension was agitated for 48 h at 220 r/min. Solution pH was adjusted using 0.1 M NaOH and HCl. After the end of agitation, the final pH was measured and plotted versus the initial pH. The pH_PZC is determined at the value for which pH_final = pH_initial (Lopez-Ramon et al., 1999).

Adsorption experiments

An adsorption study of CR was performed using ACCW adsorbent. The batch experiments were performed using a set of 100-mL Erlenmeyer flasks containing a known quantity of adsorbent (50 mg) and a pre-defined volume (25 mL in each flask) of dye solution at a fixed initial concentration (50 mg/L) with a constant solution pH. The flasks were then placed in a shaker at 220 r/min and a temperature of 25°C. The samples were examined at specific time intervals, and the solutions were filtered at equilibrium to determine the equilibrium concentrations.

Kinetic studies were carried out by varying the initial CR concentration (from 20 to 120 mg/L), pH = 3 at 25°C and the amount of adsorbent equal to 50 mg. The contact time was varied between 0 and 420 min. After a predetermined contact time, the sample was removed from the shaker and filtrated and the residual CR concentration was determined using a UV-Vis spectrophotometer (Shimadzu, Japan) at 497 nm. The amount of adsorption at equilibrium, q_e (mg/g), was calculated by:

q_{e} = (C_{i} - C_{e}) \frac{V}{W}

(2)where C_i and C_e 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:

% D y e r e m o v a l = \frac{C_{i} - C_{e}}{C_{i}} \times 100

(3)

The effect of pH on the sorption process of CR dye by ACCW 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). Twenty milliliters of CR dye (50 mg/L) was mixed with 100 mg of ACCW 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 497 nm.

The effect of contact time and initial dye concentration on the adsorption process was accomplished by using 25 mL of 50 mg/L or 100 mg/L of CR dye at pH = 3. There solutions were added to 100 mg of ACCW adsorbent at 25 ± 1°C and the mixture was shaken at constant speed of 220 r/min. Samples were withdrawn at different time intervals (0–420 min), filtrated and analyzed for remaining dye concentration.

The effect of adsorbent dosage was performed by varying the adsorbent dosage of ACCW from 0.1 to 20 g/L. 25 mL of the dye solution (50 mg/L) was adjusted to a pH = 3, and the batch experiment was performed at 25 ± 1°C by using a 4 h shaking time.

Finally, the thermodynamics parameters were determined by using a 25 mL dye solution (50 mg/L) and 100 mg of adsorbent dosage at pH = 3 for 4 h and the procedure was completed as described above.

Desorption experiments

In this study, to determine the desorption behavior of the CR-loaded ACCW; three solutions were used as a desorbing agent: hydrochloric acid solution, sodium hydroxide solution, sodium chloride solution; the concentration of solutes was set as 0.01 M. Typically, 0.1 g of adsorbent was added to 25 mL of 50 mg/L CR solution at pH 3 and shaken for 90 min. Then, the CR adsorbed by ACCW was separated and the residual CR 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 25 mL of distilled water at different pH values (3.2–12). After the mixture was shaken for a predetermined time, the desorbed CR concentration was determined spectrophotometrically. The desorption efficiency of CR was calculated as the ratio of the desorbed amount (q_de) to the adsorbed amount (q_ad).

D e s o r p t i o n e f f i c i e n c y (%) = \frac{q_{d e}}{q_{a d}} \times 100

(4)

In order to check the reusability of the regenerated adsorbents, seven cycles of consecutive adsorption–desorption studies were performed. All the experiments (adsorption and desorption) were performed for the three solutions.

Results and discussion

Characterization of ACCW

The characterization results of ACCW are shown in Table 1. The surface acidity and basicity are important criteria used to describe the surface chemistry of carbon adsorbents. The surface properties of activated carbon are significantly affected by the type and quantity of the surface functional groups. From Table 1, it can be observed that the number of acidic sites on ACCW increased which can be explained by the fact that oxygenated groups were introduced onto the surface of the activated carbon prepared from CW during chemical activation (Bouchemal et al., 2011; Djilani et al., 2015; Hirunpraditkoon et al., 2011).

Table 1.

Physical and chemical properties of ACCW.

Precursor	ACCW
Carbonization temperature (°C)	450
Carbonization time (h)	1
CH₃CO₂K impregnation (%)	100
Yield (%)	18.7
Carboxylic group (mmol/g)	1.341
Phenolic groups (mmol/g)	1.210
Lactonic groups (mmol/g)	0.022
Total acidity (mmol/g)	2.573
Total basicity (mmol/g)	2.015
pH_PZC	7.05
Specific surface area (m²/g)	219.69
Total volume pore (cm³/g)	0.125
Volume of micropore (cm³/g)	0.055
Average pore diameter (nm)	4.04

ACCW: activated carbon coffee waste.

The BET analysis of ACCW gave BET surface area of 219.69 m²/g, average pore diameter of 40.04 nm and a cumulative pore volume of 0.125 cm³/g. The relatively high surface area was attributed to the intercalation of potassium metal from intermediate reaction of KOH (formed through hydrolysis of potassium metal) with carbon (Stavropoulos and Zabaniotou, 2005). The micropores are clearly visible, facilitating the easy diffusion of a large number of dye molecules in to the pore structure and the adsorption of dye molecules onto the surface of the adsorbent.

Figure 2 presents the morphology of CW and ACCW according to SEM micrographs taken. It is obvious that its surface was not smooth, but full of cavities. These cavities can be characterized as channels onto the surface of CW instead of pores, given the small surface area from BET analysis (∼2.9 m²/g) and the prepared ACCW presents a microporous structure with different pore diameters. The ACCW morphology has changed and the surface became smoother with less visible pores, indicating an adsorption on both the surface and within pores.

Figure 2.

SEM micrographs of the CW (A) and the ACCW (B).

To examine the surface charges of the ACCW (Table 1), the point of zero charge (pH_pzc) occurred at pH 7.05. The ACCW was positively charged below this pH and negatively charged above it.

Effect of pH

The pH of an aqueous solution is one of the most important factors in the adsorption of anionic dyes because of its impact on both the surface binding sites of the adsorbent and the ionization process of the dye molecule. The influence of the initial pH of the CR solution on its adsorption on ACCW is shown in Figure 3. The adsorption capacity decreased from 25.73 mg/g (96.8% removal efficiency) to 8.87 mg/g (33.7% removal efficiency) when the pH was changed from 2.9 to 10.2 and maximum adsorption was observed at pH = 3. This pH effect can be explained by the different interaction between CR and ACCW in terms of surface charge, degree of ionization and speciation. CR is a pH sensitive dye and exposure to HCl causes color change from red to blue, due to π–π* transition of azo group shift to higher wavelength because of protonation (Onida et al., 2001). CR is an acidic dye (the isoelectric point is near 3), and its sulfonate moiety contains negative sulfonic groups (–SO₃^–) (Yaneva and Georgieva, 2012). At an acidic solution, the CR was dissociated to polar groups (R–SO₃^–) and the acidic medium is favorable for the adsorptions of dye onto ACCW because the surface of ACCW seems to be positive at its surface, which induced electrostatic interactions between its surface and R–SO₃^–; however at high pH, the presence of excess OH^– ions competes with the dye anions for the adsorption sites. Similar results have been reported for the adsorption of CR on activated carbon (Liu et al., 2014; Purkait et al., 2007).

Figure 3.

Effect of initial pH on the adsorption of CR on ACCW (adsorbent dosage = 100 mg, initial CR concentration = 50 mg/L, solution volume = 25 mL, temperature = 25°C, contact time = 4 h).

Effect of contact time and initial dye concentration

The effects of contact time and initial concentration of dye on the removal of CR are presented in Figure 4. The amount of dye adsorbed at equilibrium increased from 9.35 to 51.87 mg/g with an increase in the initial dye concentrations from 20 to 120 mg/L. The initial dye concentration provides the necessary driving force to overcome the resistance to the mass transfer of dye molecules between the aqueous phase and the solid phase. The increase in initial dye concentration also enhances the interaction between CR and adsorbent. Therefore, an increase in initial concentration of CR enhances the adsorption uptake of CR. This is due to the increase in the driving force of the concentration gradient, as an increase in the initial dye concentration. As seen in Figure 4, it is evident that time has significant influence on the adsorption of dye at the initial concentration of 120 mg/L. It can be seen that the adsorption of anionic dye was quite rapid in the first 120 min, then gradually increased with the prolongation of contact time. After 180 min of contact time, no obvious variation in adsorbed dye was observed. Based on these results, 180 min was taken as the equilibrium time in batch adsorption experiments. A similar type of behavior is also reported for the adsorption of the methyl orange at mesoporous carbon CMK-3 (Mohammadi et al., 2011).

Figure 4.

Effect of the contact time on the removal of CR (T: 25°C, adsorbent dose: 100 mg, pH = 3).

Effect of the adsorbent dosage

A dosage study is an important experiment in adsorption studies because it determines the capacity of an adsorbent for a given initial concentration of dye in solution. Experiments were conducted to examine the effect of the ACCW (g) on CR uptake using an initial dye concentration of 50 mg/L. The effect of adsorbent dosage on CR adsorption is shown in Figure 5. An increase in adsorbent dosage from 0.1 to 20 g/L resulted in an increase in CR adsorption from 41.1% to 99.2%, which suggests that there is an increase in the number of sorption sites on the adsorbent surface of ACCW (Dawood and Sen, 2012; Djilani et al., 2015; Namasivayam and Kavitha, 2002; Sumanjit et al., 2015). Therefore, 4 g/L was chosen as the optimum adsorbent dosage for the following experiments, because higher amounts of adsorbent did not increase appreciably the CR adsorption.

Figure 5.

Effect of adsorbent dosage on the removal efficiency of CR and the adsorption capacity of ACCW (initial CR concentration = 50 mg/L, solution volume = 25 mL, pH = 3.0, temperature = 25°C, contact time = 4 h).

Adsorption isotherm

In this work, four models were used to describe the relationship between the amount of CR 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 4 g/L, temperature 25 ± 1°C, contact time 4 h and stirring speed 220 r/min) was judged by the correlation coefficient R² value of each plot. The higher R² values indicate the fitness.

Langmuir isotherm

The non-linear form of the Langmuir isotherm model (Langmuir, 1916) is given as:

q_{e} = \frac{q_{m} b C_{e}}{(1 + b C_{e})}

(5)where C_e is the equilibrium concentration of dyes adsorbed in mg/L, q_e is the amount of dyes adsorbed in mg/g, q_m and b (Langmuir constants) are the monolayer adsorption capacity and affinity of adsorbent towards adsorbate, respectively.

A plot of q_e against C_e gave a fitted curve, and the Langmuir constants were generated from the plot of sorption data in Table 2. The essential factor R_L, calculated for all concentrations for dyes revealed that the entire adsorption process was favorable for two dyes since their values were in the range of 0 < R_L < 1. The R_L equation (Hall et al., 1966) is given by:

R_{L} = \frac{1}{(1 + b C_{0})}

(6)where b is the Langmuir constant and C₀ the initial concentration dye (mg/L).

Table 2.

Parameters of adsorption models for CR adsorption onto ACCW.

Isotherm model	Parameter	ACCW
Langmuir	q_m (mg/g)	90.90
	b (L/mg)	0.150
	R²	0.992
Freundlich	n	1.46
	K_F	12.03
	R²	0.989
Temkin	A	5.37
	B	10.68
	R²	0.976
D–R	q_mD-R (mg/g)	42.64
	β_D-R (mol²/j²)	4 × 10^–7
	E (j/mol)	1582
	R²	0.87

CR: congo red; ACCW: activated carbon coffee waste.

Freundlich isotherm

The non-linear form of the Freundlich model is given (Freundlich, 1906) as:

q_{e} = K_{F} C_{e} \frac{1}{n}

(7)where q_e is the amount adsorbed at equilibrium, mg/g; C_e is the equilibrium concentration of the adsorbate, mg/L; K_F and n are the Freundlich equilibrium coefficients. The value of n gives information on favorability of adsorption process and K_F is the adsorption capacity of the adsorbate.

A plot of q_e against C_e gave poor curves indicating that the adsorption process did not follow the model. The values of the Freundlich equilibrium coefficients K_F and n were generated from the plot of sorption data in Table 2. The parameter 1/n ranging between 0 and 1 measures the adsorption intensity or surface heterogeneity.

Temkin isotherm

The Temkin model is expressed (Temkin and Pyzhev, 1940) as:

q_{e} = B l n (A C_{e})

(8)where k_T is the equilibrium binding constant corresponding to the maximum binding energy, B is related to the head of adsorption, q_e is the experimental adsorption capacity (mg/g), C_e is the concentration of dyes adsorbed at equilibrium (mg/L)

B = \frac{R T}{b_{T}}

(9)where 1/b_T indicates the adsorption potential of the adsorbent; R is the universal gas constant (8.314 J/kmol) and T is the temperature in Kelvin (K).

A plot of q_e against C_e indicates a linear decrease in adsorption energy as the adsorption sites are 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.

Dubinin–Radushkevich (D–R) isotherm

The Dubinin–Radushkevich model (Dubinin and Radushkevich, 1947) 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 follows:

q_{e} = q_{m} \exp (- β ε^{2})

(10)where q_e is the amount of dye molecules (mol/L), q_m is the maximum adsorption capacity (mol/g), β is the activity coefficient related to adsorption mean free energy mol²/j² and ε is the Polanyi potential given by

ε = R T l n (1 + \frac{1}{C_{e}})

(11)

A plot of q_e against ε² gave nonlinear graphs (figure not shown). The adsorption mean free energy, E (kJ/mol) is given as:

E = \frac{1}{{(2 β)}^{0.5}}

(12)

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 obtained of the four models were calculated and are represented 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 on to surface containing finite number of identical sorption sites. The maximum adsorption capacity of ACCW for CR is 99.90 mg/g at 25°C.

The values of the parameter R_L indicate that the adsorption is favorable (0 < R_L < 1). These were found to be 0.252–0.052 for ACCW in the concentration range studied, and the value of the free energy estimated from the D–R model E < 8 kJ/mol indicates that adsorption process is of physical nature.

The isotherm profile of ACCW for CR is shown in Figure 6. It is obvious that the experimental results are well represented by the Langmuir isotherm.

Figure 6.

Isotherm modeling of CR on the adsorption onto ACCW at 25°C.

Adsorption kinetics

In order to elucidate the CR adsorption process on ACCW, particularly the potentially rate-controlling step, the dye adsorption data were analyzed using the pseudo first-order, pseudo-second-order and intra-particle diffusion models, which are described below.

The pseudo-first-order kinetic model was suggested by Lagergren and Sven (1898) for the adsorption of solid/liquid systems and its formula is given as:

\log (q_{e} - q_{t}) = \log (q_{e}) - (\frac{k_{1}}{2.303}) t

(13)where q_t (mg/g) and q_e (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 (q_e – q_t) against t. However, the calculated and experimental q_e values do not agree (Table 4), and the pseudo-first-order kinetic model does not describe the adsorption of CR on ACCW.

The linear form of the pseudo-second-order kinetic model can be expressed as (Ho and McKay, 1999):

\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{t}{q_{e}}

(14)where k₂ (g/(mg min)) is the rate constant of the pseudo-second-order adsorption, obtained from linear plots of t/q_t against t (Table 3). 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.

Table 3.

Comparison of adsorption capacities of various adsorbent for CR.

Dye	C_o (mg/L)	Pseudo-second-order kinetics
Dye	C_o (mg/L)	q_e,exp (mg/g)	q_e,cal (mg/g)	k₂ (g mg min^–1)	R ²	Δq (%)
CR	20	9.41	9.43	0.052	0.993	0.34
	50	25.59	25.64	0.041	0.999	1.26
	100	42.88	43.47	0.001	0.994	2.05
	120	54.54	55.55	0.002	0.999	0.97

CR: congo red.

Prediction of the rate-limiting step is also an important factor to be considered in the adsorption process. Therefore, the kinetic results were analyzed by the intra- particle model, which is expressed as (Weber and Morris, 1963):

q_{t} = k_{i d} t^{\frac{1}{2}} + C

(15)where q_t is the amount adsorbed at time t, t^0.5 is the square root of the time and k_id (mg/(g min^1/2)) is the rate constant for intra-particle diffusion. The intercept, C, in a plot of q_t versus t^0.5 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 plot of q_t (mg/g) versus t^0.5 is shown in Figure 7 for various initial CR concentrations. The plots for different pH and adsorbent dosages showed similar trends and are not presented here. 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, which indicates that adsorption of CR on ACCW 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.

Figure 7.

Intra-particle diffusion model for different initial CR concentrations.

The suitability of the kinetic model to describe the adsorption process was validated by the normalized standard deviation, Δq (%) is defined as:

Δ q (%) = 100 \times \sqrt{\frac{\sum^{​} [(q_{t, \exp} - q_{t, c a l}) / q_{t, \exp}]^{2}}{n - 1}}

(16)where n is the number of data points, q_t,exp and q_t,cal are the experimental and calculated adsorption capacities, respectively. Pseudo-second-order kinetic model yielded the best fit, with highest correlation coefficients and lowest normalized standard deviation, Δq values which ranged between 0.34% and 2.05% for CR concentration dye ranging from 20 to 120 mg/L.

Thermodynamics parameters

A study of temperature dependence for adsorption process gives information on whether the reaction is spontaneous or not, with aid of thermodynamic parameters such as change in Gibbs free energy (ΔG⁰), enthalpy (ΔH⁰), and entropy (ΔS⁰), the thermodynamic of adsorption reaction towards spontaneity can be evaluated. These parameters were calculated using the following equations:

Δ G^{0} = - R T \ln (K_{0})

(17)

\ln K_{0} = - \frac{Δ G^{0}}{R T} = - \frac{Δ H^{0}}{R T} + \frac{Δ S^{0}}{R}

(18)where R is the universal gas constant (8.314 J/mol), T is the absolute temperature (K) and K₀ is the distribution coefficient equals to q_e/C_e. The values of ΔH⁰ and ΔS⁰ calculated from the slope and intercept of the plot of ln (K₀) versus 1/T are listed in Table 4. The negative values of ΔG⁰ mean that the process is feasible and adsorption is spontaneous thermodynamically. Generally, ΔG⁰ values for physisorption are in the range –20 to 0 kJ/mol, and those for chemisorption range between –80 and –400 kJ/mol (Liu et al., 2010, Mohammadi et al., 2011).

Table 4.

Kinetic parameters for adsorption of CR onto ACCW.

	ΔH⁰ (kJ/mol)	ΔS⁰ (kJ/mol.K)	ΔG⁰ (kJ/mol)
	ΔH⁰ (kJ/mol)	ΔS⁰ (kJ/mol.K)	298 K	308 K	318 K	328 K
ACCW	–25.464	–0.743	–3.296	–2.296	–1.911	–0.0988

CR: congo red; ACCW: activated carbon coffee waste.

In this study, the ΔG⁰ values are in the range of –3.29 to –0.98 kJ/mol indicating that adsorption is than usual physisorption. The exothermic nature of the process is confirmed by the negative value of ΔH⁰ (–25.46 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 value of ΔH⁰ indicates that for van der Waals interaction, hydrophobic interaction could also be to the adsorption process in addition to electrostatic attraction. The negative ΔS⁰ value suggests a decrease in the randomness at adsorbate–solution interface during the adsorption process (Djilani et al., 2015).

Proposed adsorption mechanism

Adsorbent surface characteristics play a significant part in the adsorption processes (Ahmad and Kumar, 2010). ACCW contains functional groups on their surfaces having acidic and/or alkaline characteristics. These groups would have an influence on the dye’s adsorption processes, though the pH value of the dye solution, temperature, and the concentration will affect the overall adsorption behavior and mechanism. However, the surface charge density of the ACCW would decrease as the pH increases (Ahmad and Kumar, 2010). The hydroxyl groups existing on the ACCW surface could gain or lose a proton causing a change in the surface charge that fluctuates with changing pH values.

At low pH (pH = 3), the surface functional groups are protonated; thus a positive charge would be formed on the surface. In acidic medium, the protonation of –OH and –COOH groups present at ACCW surface (i.e. C_x–OH + H⁺↔C_x–OH₂⁺, C_x = carbon) occurs (since pH_PZC of the adsorbent is 7.05, below which the adsorbent surface is positive). Generally, the carboxyl groups presented a pK_a value between 3.0 and 5.0 (Yu et al., 2007). At pH lower than pK_a, carboxylate groups carried positive charge resulting in electrostatic interaction between negatively charged SO₃^– groups and positively charged adsorbent surface.

Its surface principally comprises oxygen, nitrogen, hydrogen. Among these heteroatoms, the oxygen-containing functional groups such as carbonyl, carboxyl, and phenol H-bonding between oxygen and nitrogen contain functional groups of CR and ACCW surface (Newconbe and Drikas, 1997).

In basic medium, the carboxylic groups of ACCW are expected to completely ionize; therefore, electrostatic repulsion between anionic CR and anionic ACCW surface lowers the adsorption capacity.

Additional support for the explanation mentioned above is obtained from FTIR spectra of ACCW and CR. The spectra of ACCW are shown in Figure 8. The FTIR spectroscopic analysis showed broad band at 3460 cm⁻¹ is a characteristic of the stretching vibration of hydrogen-bonded hydroxyl groups (from carboxyls, phenols or alcohols) and water adsorbed in the activated carbon. The peaks at 2919 and 2853 cm⁻¹ are caused by C–H vibration. The small band at about 1732 cm⁻¹ is usually assigned to C=O stretching vibrations of ketones, aldehydes, lactones or carboxyl groups. The band at 1593 cm⁻¹ was due to C=C vibrations in aromatic rings. The absorption bands at 1646 and 1465 cm⁻¹ indicate respectively the presence of COO of carboxyl and C–O groups on the adsorbent surface. Bands in the range of 1382 cm⁻¹ are attributed to COO^– symmetric stretching vibration. Another absorption band appearing around 1120 and 1020 cm⁻¹ could be attributed to the stretching; C–O stretching of ether group and O–C–O stretching of COOH (Reffas et al., 2010).

Figure 8.

FTIR spectra of ACCW.

For the spectra of CR dye (Figure 9), the region between 1800 and 3700 cm⁻¹ presents one major peak around 3460 cm⁻¹ (the H-bonded –OH group and NH₂ group). Peaks at 1586 cm⁻¹, 1453 cm⁻¹ and 1376 cm⁻¹ correspond to amine group, –N–H bending, and –S=O stretching vibration, respectively.

Figure 9.

FTIR spectra of CR.

The color of CR in aqueous solution is solid red at pH around 7. The color of CR changes to dark blue at acid pH and to red at alkaline pH (10–12), but this red color is slightly different from original red at the neutral pH. CR exists as an anionic form at basic pH (sulfonate groups) and as a cationic form at acid pH. At an acidic solution, the CR was dissociated to polar groups (R–SO₃^–). The acidic medium is favorable for the adsorptions of CR onto ACCW because the surface of ACCW seems to be positive at its surface, which induced electrostatic interactions between its surface and R–SO₃^–. Figure 10 shows the proposed mechanism of CR adsorption onto ACCW. Ahmad and Kumar (2010) also reported that the NH₂, –N=N–, –HN–N and –SO₃ groups of CR were involved in the adsorption.

Figure 10.

Mechanism for CR adsorption onto ACCW surface at pH = 3.

Desorption and reuse

Desorption study is usually applied to elucidate the adsorption mechanism and to recover the depleted adsorbent (Dawood and Sen, 2012). Desorption experiments were carried out to find the optimum pH-desorption conditions. Figure 11 shows CR desorption efficiency at the first desorption cycle from the CR-adsorbed ACCW with the increasing of pH of the desorbing agent. Low desorption efficiency of 9.06% for CR-adsorbed ACCW was obtained with neutral distilled water. The best desorption was obtained at alkaline pH. The desorption increased slowly from 0.66 to 42.4% with pH changing from 3.2 to 12.

Figure 11.

Congo red desorption efficiency at different pH values.

To compare the efficiency of different solutions in desorption study, the concentration of solutes were set as 0.01 M (except the deionized water). If the attached dye molecule desorbed by deionized water, the desorption is weak; otherwise, if it is desorbed by alkaline solution, the adsorption is dominated by electrostatic interaction and hydrogen bonding in adsorption process (Ahmad and Kumar, 2010). According to Figure 12, hydroxide solution is the most efficient one (95.22% for ACCW), the hydrochloric acid solution, sodium chloride solution and deionized water were inefficient to desorb CR molecules in the present study.

Figure 12.

Reusability of the ACCW.

Performance of the prepared ACCW

In order to have an idea about the efficiency of the prepared ACCW, a comparison of the anionic dye adsorption of this work and other relevant studies is reported in Table 5. The adsorption capacity (q_max) is the parameter used for the comparison. One can conclude that the value of q_max is in good agreement with those of most previous works, suggesting that CR could be easily adsorbed on ACCW prepared in this work. This indicates that the CW, abundant in Tunisia, is a cheap and effective adsorbent for the CR. ACCW is promising adsorbent for anionic dyes owing to pH_pzc and our perspective is to achieve the adsorption tests in column mode using industrial effluents. Such results are currently under way and will be reported in a next future.

Table 5.

Thermodynamic parameters for adsorption of CR onto ACCW.

Adsorbent	q_max (mg/g)	References
Date stone activated carbon AC (Zn CO₂) 800	35.21	Bouchemal et al. (2011)
Date stone activated carbon AC (Zn 600, CO₂ 800)	30.22	Bouchemal et al. (2011)
Date stone activated carbon AC (Zn 600)	18.32	Bouchemal et al. (2011)
Date stone activated carbon AC (CO₂ 800)	15.23	Bouchemal et al. (2011)
Apricot stone activated carbon	32.85	Abbas and Trari (2015)
Surfactant-modified zeolites	69.94	Liu et al. (2014)
Commercial activated carbon	300	Purkait et al. (2007)
CTAB-Eichhornia crassipes	103.1	Sumanjit et al. (2015)
Activated carbon from coir pith	6.72	Namasivayam and Kavitha (2002)
Acid treated pine cone	40.19	Dawood and Sen (2012)
Luffa cylindrica cellulosic fiber	17.39	Gupta et al. (2014)
Eucalyptus wood (Eucalyptus globulus) Sawdust	31.25	Mane and Babu (2013)
Eichhonia charcoal	56.8	Kaur et al. (2012)
Bael shell carbon	98.02	Ahmad and Kumar (2010)
Cationic surfactant modified wheat straw	71.2	Cheng et al. (2014)
Bottom ash	142.81	Mittal et al. (2009b)
Deoiled soya	1083.33	Mittal et al. (2009b)
Activated carbon coffee waste	90.90	This study

CR: congo red; ACCW: activated carbon coffee waste.

Conclusions

The study gives the CR adsorption capacity and adsorption process of ACCW. Chemical activation by potassium acetate introduces oxygen-containing functional groups on carbon surfaces, and the functional groups play an important role in dye adsorption. Batch experiment studies indicated that the adsorption process was fast enough, as maximum removal took place within 180 min of contact time and the removal efficiency of dye was improved in acidic solutions. The adsorption of dye increased with increasing initial dye concentration. Fitting equilibrium data isotherms showed that Langmuir model was more suitable to describe the CR adsorption with maximum monolayer adsorption capacity of 90.90 mg/g at 25°C. The adsorption kinetics was found to follow closely the pseudo-second-order kinetic model. The negative values of ΔG₀ and ΔH₀ showed that the adsorption was a spontaneous and exothermic process. Based on the study on initial pH, adsorption thermodynamics, adsorption kinetics and FTIR analyses, the ACCW adsorption is mainly attributed to hydrogen bonding and electrostatic interaction between dyes species and oxygen-containing functional groups on the carbon surfaces.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

References

Abbas

Trari

(2015) Kinetic equilibrium and thermodynamic study on the removal of Congo red from aqueous solutions by adsorption onto apricot stone. Process Safety and Environmental Protection 98: 424–436.

Ahmad

Kumar

(2010) Adsorptive removal of congo red dye from aqueous solution using bael shell carbon. Applied Surface Science 257: 1628–1633.

Bhatnagar

Sillanpää

(2010) Utilization of agro-industrial and municipal waste materials as potential adsorbents for water treatment: A review. Chemical Engineering Journal 157: 277–296.

Boehm

(1966) Chemical identiﬁcation of surface groups. Advances in Catalysis 16: 179–274.

Bouchemal

Merzougui

Addoun

(2011) Adsorption en milieux aqueux de deux colorants sur charbons actifs à base de noyaux de datte. Journal de la Société Algérienne de Chimie 21: 1–14.

Chen

Huang

et al . (2012) Preparation of phosphoric acid activated carbon from sugarcane bagasse by mechanochemical processing. BioRe-sources 51: 9–16.

Cheng

Zhang

Guo

et al . (2014) Adsorption of Congo red from aqueous solutions using cationic surfactant modified wheat straw in batch mode: Kinetic and equilibrium study. Journal of the Taiwan Institute of Chemical Engineers 45: 2578–2583.

Cruz

Pirila

Huuuhtanen

et al . (2012) Production of activated carbon from Cocoa (Theobroma cacao) pod husk. Civil Engineering and Environmental Systems 2: 1–6.

Dawood

Sen

(2012) Removal of anionic dye Congo red from aqueous solution by raw pine and acid-treated pine cone powder as adsorbent: Equilibrium, thermodynamic, kinetics, mechanism and process design. Water Research 46: 1933–1946.

10.

Demiral

Tumsek

et al . (2008) Pore structure of activated carbon prepared from hazelnut bagasse by chemical activation. Surface and Interface Analysis 40: 616–619.

11.

Djilani

Zaghdoudi

Djazi

et al . (2015) Adsorption of dyes on activated carbon prepared from apricot stones and commercial activated carbon. Journal of the Taiwan Institute of Chemical Engineers 53: 112–121.

12.

Dubinin

Radushkevich

(1947) Proceedings of the academy of sciences of the USSR. Physical Chemistry Section 55: 327–329.

13.

Finar

(1986) Organic Chemistry: The Fundamental Principles. England: Addison Wesley Longman Ltd.

14.

Freundlich

HMF

(1906) Über dies adsorption in Lösungen. Zeitschrift Für Physikalische Chemie 57: 385–470.

15.

Gonzalez

Roman

Encinar

et al . (2009) Pyrolysis of various biomass residues and char utilization for the production of activated carbons. Journal of Analytical and Applied Pyrolysis 85: 134–141.

16.

Guo

Peng

et al . (2009) Effects of CO₂ activation on porous structures of coconut shell-based activated carbons. Applied Surface Science 255: 8443–8449.

17.

Gupta

Suhas (2009) Application of low-cost adsorbents for dye removal: A review. Journal of Environmental Management 90: 2313–2342.

18.

Gupta

Ali

Saleh

et al . (2012) Chemical treatment technologies for waste-water recycling – An overview. RSC Advances 2: 6380–6388.

19.

Gupta

Pathania

Agarwal

et al . (2014) Amputation of Congo Red dye from wastewater using microwave induced grafted Luffa cylindrica cellulosic fiber. Carbohydrate Polymers 111: 556–566.

20.

Hall

Eagleton

Acrivos

et al . (1966) Pore and solid diffusion kinetics in ﬁxed adsorption under constant pattern conditions. Industrial & Engineering Chemistry Fundamentals 5: 212–223.

21.

Hiremath

Shivayogimath

Shivalingappa

(2012) Preparation and characterization of granular activated carbon from corncob by KOH activation. International Journal of Research in Chemistry Environment 2: 84–87.

22.

Hirunpraditkoon

Tunthong

Ruangchai

et al . (2011) Adsorption capacities of activated carbons prepared from bamboo by KOH activation. World Academy of Science, Engineering and Technology 78: 711–715.

23.

McKay

(1999) Pseudo-second order model for sorption processes. Process Biochemistry 344: 51–65.

24.

Kaur

Rani

Mahajan

(2012) Adsorption kinetics for the removal of hazardous dye Congo Red biowaste materials as adsorbents. Journal of Chemistry 2013: 1–12.

25.

Ketcha

Dina

DJD

Ngomo

et al . (2012) Preparation and characterization of activated carbons obtained from maize cobs by zinc chloride activation. American Chemical Science Journal 2: 136–160.

26.

Lafi

Hafiane

(2015) Removal of methyl orange (MO) from aqueous solution using cationic surfactants modified coffee waste (MCWs). Journal of the Taiwan Institute of Chemical Engineers 58: 424–433.

27.

Lagergren

Sven

(1898) About the theory of so-called adsorption of soluble substances. Kungliga Svenska Vetenskapsak Ademien Handlingar 24: 1–39.

28.

Langmuir

(1916) The constitution and fundamental properties of solids and liquids. Journal of the American Chemical Society 38: 2221–2295.

29.

Liu

Zheng

Wang

et al . (2010) Adsorption isotherm, kinetic and mechanism studies of some substituted phenols on activated carbon fibers. Chemical Engineering Journal 157: 348–356.

30.

Liu

Ding

et al . (2014) Adsorption of the anionic dye Congo red from aqueous solution onto natural zeolites modified with N, N-dimethyl dehydroabietylamine oxide. Chemical Engineering Journal 248: 135–144.

31.

Lopez-Ramon

Stoeckli

Moreno-Castilla

et al . (1999) On the characterization of acidic and basic surface sites on carbons by various techniques. Carbon 37: 1215–1221.

32.

Mane

Babu

PVV

(2013) Kinetic and equilibrium studies on the removal of Congo red from aqueous solution using Eucalyptus wood (Eucalyptus globulus) saw dust. Journal of the Taiwan Institute of Chemical Engineers 44: 81–88.

33.

Mittal

Kaur

Malviya

et al . (2009) Adsorption studies on the removal of coloring agent phenol red from wastewater using waste materials as adsorbents. Journal of Colloid and Interface Science 337: 345–354.

34.

Mittal

Malviya

et al . (2009) Adsorptive removal of hazardous anionic dye “Congo red” from wastewater using waste materials and recovery by desorption. Journal of Colloid and Interface Science 340: 16–26.

35.

Mittal

Malviya

et al . (2010) Decoloration treatment of a hazardous triarylmethane dye, Light Green SF (Yellowish) by waste material adsorbents. Journal of Colloid and Interface Science 342: 518–527.

36.

Mohammadi

Khani Gupta

(2011) Adsorption process of methyl orange dye onto mesoporous carbon material – Kinetic and thermodynamic studies. Journal of Colloid and Interface Science 362: 457–462.

37.

Namasivayam

Kavitha

(2002) Removal of congo red from water by adsorption onto activated carbon prepared from coir pith, an agricultural solid waste. Dyes and Pigments 54: 47–58.

38.

Newconbe

Drikas

(1997) Adsorption of NOM activated carbon: Electrostatic and NON-electrostatic effects. Carbon 35: 1239–1250.

39.

Olafadehan

Jinadu

Salami

et al . (2012) Treatment of brewery wastewater effluent using activated carbon prepared from coconut shell. International Journal of Applied Science and Technology 2: 165–178.

40.

Olowoyo

Orere

(2012) Preparation and characterization of activated carbon made from palm-kernel shell, coconut shell, groundnut shell and obeche wood (investigation of apparent density, total ash content, moisture content, particle size distribution parameters). International Journal of Research in Chemistry and Environment 2: 32–35.

41.

Onida

Flora

Arean

et al . (2001) Permeability of micelles of surfactant containing MCM-41 silica as monitored by embedded dye molecule. Chemical Communications 21: 2216–2217.

42.

Orkun

Karatepe

Yavuz

(2012) Influence of temperature and impregnation ratio of H₃PO₄ on the production of activated carbon from hazelnut shell. Acta Physica Polonica A 121: 277–280.

43.

Özcan

(2004) Adsorption of acid dyes from aqueous solutions onto acid activated bentonite. Journal of Colloid and Interface Science 276: 39–46.

44.

Purkait

Maiti

DasGupta

et al . (2007) Removal of congo red using activated carbon and its regeneration. Journal of Hazardous Materials 145: 287–295.

45.

Reffas

Bernardet

David

et al . (2010) Carbons prepared from coffee grounds by H₃PO₄ activation: Characterization and adsorption of methylene blue and Nylosan Red N-2RBL. Journal of Hazardous Materials 175: 779–788.

46.

Saleh

Gupta

(2011) Functionalization of tungsten oxide into MWCNT and its application for sunlight-induced degradation of rhodamine B. Journal of Colloid and Interface Science 362: 337–344.

47.

Saleh

Gupta

(2014) Processing methods, characteristics and adsorption behavior of tire derived carbons: A review. Advances in Colloid and Interface Science 211: 93–101.

48.

Stavropoulos

Zabaniotou

(2005) Production and characterization of activated carbons from olive-seed waste residue. Microporous and Mesoporous Materials 82: 79–85.

49.

Sugumaran

Susan

Ravichandran et al . (2012) Production and characterization of activated carbon from banana empty fruit bunchand Delonix regia fruit pod. International Journal of Sustainable Energy 3: 125–132.

50.

Sumanjit

Mahajan

Gupta

(2015) Modification of surface behaviour of Eichhornia crassipes using surface active agent: An adsorption study. Journal of Industrial and Engineering Chemistry 21: 189–197.

51.

Temkin

Pyzhev

(1940) Recent modiﬁcations to Langmuir isotherms. Acta Physiochimica USSR 12: 217–225.

52.

Theydan

Ahmed

(2012) Adsorption of methylene blue onto biomass-based activated carbon by FeCl₃ activation: Equilibrium, kinetics, and thermodynamic studies. Journal of Analytical and Applied Pyrolysis 97: 116–122.

53.

Weber

Morris

(1963) Kinetics of adsorption on carbon from solutions. Journal of the Sanitary Engineering Division 89: 31–59.

54.

Yaneva

Georgieva

(2012) Insights into Congo red adsorption on agroindustrial materials-spectral, equilibrium, kinetic, thermodynamic, dynamic and desorption studies. A review. International Review of Chemical Engineering 4: 127–146.

55.

Tong

Sun

et al . (2007) Biomass grafted with polyamic acid for enhancement of cadmium (II) and lead (II) biosorption. Reactive and Functional Polymers 67: 564–572.

56.

Zhou

Zhang

Cheng

(2015) Removal of organic pollutants from aqueous solution using agricultural wastes: A review. Journal of Molecular Liquids 212: 739–762.

Adsorption of congo red dye from aqueous solutions by prepared activated carbon with oxygen-containing functional groups and its regeneration

Abstract

Keywords

Introduction

Materials and methods

Chemicals

Preparation of activated carbon coffee waste

Characterization

Adsorption experiments

Desorption experiments

Results and discussion

Characterization of ACCW

Effect of pH

Effect of contact time and initial dye concentration

Effect of the adsorbent dosage

Adsorption isotherm

Langmuir isotherm

Freundlich isotherm

Temkin isotherm

Dubinin–Radushkevich (D–R) isotherm

Adsorption kinetics

Thermodynamics parameters

Proposed adsorption mechanism

Desorption and reuse

Performance of the prepared ACCW

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

References