Removal of Resorcinol from Aqueous Solution by

American Chemical Science Journal 16(1): 1-13, 2016, Article no.ACSJ.27637 ISSN: 2249-0205

SCIENCEDOMAIN international www.sciencedomain.org

Removal of Resorcinol from Aqueous Solution by Activated Carbon: Isotherms, Thermodynamics and Kinetics Omer El-Amin Ahmed Adam1* 1

Department of Chemistry, Faculty of Science and Arts, Al Baha University, Baljurashi 65635, P.O.Box 1988, Albaha, Saudi Arabia. Author’s contribution The sole author designed, analyzed and interpreted and prepared the manuscript. Article Information DOI: 10.9734/ACSJ/2016/27637 Editor(s): (1) Ling Bing Kong, School of Materials Science and Engineering Nanyang Technological University, Singapore. Reviewers: (1) Sameer Amereih, Palestine Technical University-Kadoori, Tullkarm, Palestine. (2) Jeriffa De Clercq, Ghent University, Belgium. (3) Farid I El-Dossoki, Port Said University, Egypt. Complete Peer review History: http://www.sciencedomain.org/review-history/15699

Original Research Article

Received 11th June 2016 th Accepted 29 July 2016 th Published 6 August 2016

ABSTRACT The adsorption of resorcinol from aqueous solutions onto activated carbon was studied. The specific surface area and point of zero charge of activated carbon were determined. Adsorption experiments were carried out in a batch process and various experimental parameters such as agitation time, initial resorcinol concentration, adsorbent dose, pH and temperature were studied. The equilibrium sorption data were fitted into Langmuir, Freundlich, Tempkin and Dubinin– Radushkevich (DR) isotherms. The Langmuir isotherm model appears to have better regression coefficients, with a maximum adsorption capacity range (208.33 - 223.21 mg/g). The resorcinol adsorption was strongly dependent on solution pH, and the solute removal become significant at pH ≅ pHpzc. Thermodynamic parameters: ∆H°, ∆S° and ∆G° were computed from the experimental data. These values show that the adsorption is endothermic and spontaneous process. Moreover, the relative value of ∆Hads (34.506 kJ/mol) confirms a physical adsorption. Kinetics of adsorption was investigated and the data were treated according to pseudo-first-order, pseudo-second-order and intra-particle diffusion models. It was found that the adsorption process of resorcinol onto activated carbon follows pseudo-second-order model. _____________________________________________________________________________________________________ *Corresponding author: E-mail: [email protected]; Permanent address: Chemistry Department, University of Kassala, P.O. Box 266, Kassala 31111, Sudan.

Adam; ACSJ, 16(1): 1-13, 2016; Article no.ACSJ.27637

Keywords: Adsorption equilibrium; activated carbon; resorcinol; kinetics; thermodynamics. ml/0.1 g). The carbon had been soaked in distilled water for 24 h to remove fines and soluble ash and dried to constant weight at 110°C, after which the carbon was sieved to 325 mesh and stored in a calcium chloride desiccator until use.

1. INTRODUCTION In recent years, there has been an increasing interest in finding solutions for the efficient removal of contaminants from water, soil, and air. Adsorption processes have been widely studied and applied as an effective, efficient, and economic approach for water purification. The adsorption of solutes from aqueous solution by any adsorbent is affected by several parameters such as the initial concentration of adsorbate (C0), temperature (T), adsorbent dosage (m), solution pH and contact time (t) [1].

2.2 Characterization of Activated Carbon The specific surface area of activated carbon was determined by the method of methylene blue (MB) described by Hang and Brindly [9]. The adsorption capacity of MB was obtained from batch adsorption experiments. A series of 25 mL bottles were employed. Each bottle was filled with 25 mL of MB (Acros Organics, pure) solution of varying concentrations (1.0 to 10.0 mg/L, 10 standards) and 0.02 g of activated carbon. The stoppered bottles were shaken at 25 °C and 200 rpm for 24 hrs. A 3.0 mL portion of each solution was withdrawn after 24 hours and centrifuged. The MB concentrations were determined spectrophotometrically using UV–Visible spectrophotometer (Apel PD-303 UV, Japan) at λ = 665 nm.

Hydroxy aromatic compounds such as catechol (C), resorcinol (R), and hydroquinone (HQ), were used widely as industrial solvents [2]. These compounds were found in the effluents of industries such as textile, paper and pulp, steel, petrochemical, petroleum refinery, rubber, dye, plastic, pharmaceutical, cosmetic etc., and in the wastewater of synthetic coal fuel conversion processes [3-4]. Resorcinol (1,3-dihydroxybenzene) is an important organic compound, used externally as an antiseptic and disinfectant. It is usually employed to produce dyes, plastics, and synthetic fibers [5-7]. It is released into the environment from a number of anthropogenic sources, including production, processing, and consumer uses, especially from hair dyes and pharmaceuticals. In addition, localized high concentrations can appear in coal conversion wastewater or wastewater in regions with oil shale mining. It is irritating to eyes and skin and mucous membranes [8].

Surface functional groups were determined by standard neutralization titration with NaOH, Na2CO3, NaHCO3, and HCl (0.05N in water) according to the Boehm procedure [10]. The number and type of acid groups were calculated by considering that the difference between NaOH and Na2CO3 consumption corresponds to the weakly acidic phenolic groups, while difference between Na2CO3and NaHCO3 consumption corresponds to the lactonic groups. Carboxylic groups were therefore quantified by direct titration with NaHCO3. On the other hand, total basic sites were evaluated by titration with HCl.

In the present work, we have studied the potential of resorcinol adsorption on activated carbon. The effects of temperature, pH, contact time, initial resorcinol concentration and adsorbent dose were examined. The adsorption isotherms, kinetic and thermodynamic parameters were deduced from the adsorption measurements.

2.3 pHpzc The point of zero charge (pHpzc) was determined from acid–base titration. The method proposed by Rivera-Utrilla et al. [11] was followed. Aliquots with 50 mL of 0.01 M NaCl solution were prepared in different flasks. Their pH was adjusted from 2 to 12 by addition of 0.01 M solutions of NaOH or HCl. When the pH value was constant, 0.15 g of carbon sample was added to each flask and it was sealed and placed in a shaker for three days. Blank tests were also made without sample to eliminate the influence of CO2 on pH. The pHpzc value is the point where

2. MATERIALS AND METHODS 2.1 Activated Carbon Commercial activated carbon was used in the batch experiments (Powder, extra pure, minimum methylene blue adsorption (0.15 % solution) ≥ 12

2


the curve pHfinal vs pHinitial crosses the line pHinitial = pHfinal.

2.7 Effect of Solution pH on Resorcinol Adsorption

2.4 Analytical Procedure

The effect of solution pH on resorcinol adsorption was investigated according to the following procedure. A mass of 0.20 g of dried activated carbon was added to a number of 250 ml glass bottles containing 100.0 ml of 100 mg/L resorcinol solution. The pH of the solutions was adjusted over the range pH 2 – pH 12 using 0.1 M HCl or 0.1 M NaOH solutions (prior to the addition of the adsorbent). The bottles were sealed and placed in the shaker at a speed 200 rpm and 25°C for 24 hours, after which the solution concentration was determined spectrophotometrically.

Both initial and final equilibrium concentrations of resorcinol was determined with a single beam UV/Visible spectrophotometer (Apel PD-303UV, Japan) using a (1.0 cm) light-path cell, at a wavelength of 273 nm. In accordance with the Beer-Lambert law, the absorbance was found to vary linearly with concentration in the range of concentrations used. Each equilibrium concentration value was an average of three measurements.

2.5 Adsorption Experiments 3. RESULTS AND DISCUSSION Batch adsorption experiments were conducted by mixing AC with R solutions with desired concentration in 25 mL glass flask. The glass flasks were stoppered during the equilibration period and placed on a temperature-controlled shaker at a speed 200 rpm, after which, the samples were removed and the supernatant solution was separated from the adsorbent by filtration using Whatman No. 41 filter paper. The amount of adsorption at equilibrium (qe mg/g) and sorption efficiency (%) were calculated according to the expressions:

qe =

(C 0 − C e ) V m

Sorption efficiency % =

3.1 Adsorbent Characterization The adsorption data of MB onto activated carbon (Fig. 1) was analyzed according to the linear form of Langmuir equation [13]:

C q

1 1 e = + C e K .q q e L max max

(3)

where qe is the amount of MB adsorbed (mg/g carbon), qmax is the maximum adsorption capacity (mg/g), and KL is the equilibrium adsorption constant related to the free energy of the adsorption (L/mg). Ce is the equilibrium solution concentration of MB (mg/L). Values of Langmuir parameters were calculated from the slope and intercept of the linear plot of Ce/qe versus Ce. The value of qmax obtained was 30.96 mg MB/g carbon. Depending on this value the specific surface area (S) of activated carbon can be calculated from Eq. (4) [14].

(1)

(C0 − C e ) × 100 (2) C0

Where Co and Ce are the initial and equilibrium concentrations (mg/L), respectively, V the volume of solution (L), m is the mass of the carbon material (g) [12]. All experiments were performed in duplicate.

S = q max × CSA× N A

2.6 Adsorption Kinetics

(4)

where CSA is the cross-sectional area occupied by MB molecule (130 °A2) and NA is the −1 Avogadro's number (mol ). The specific surface area of activated carbon was found to be 75.78 2 m /g.

A 100 mL of R aqueous solutions with concentration ranging from 60 to 120 mg/L was placed in four glass flasks containing 0.1 g of activated carbon (AC). The glass flasks were stoppered and placed in a temperature-controlled shaker at a speed 200 rpm. The pH was adjusted to 7.8 and the temperature to 25°C. The concentration of R in the solution was determined at known time intervals.

The surface chemistry of activated carbons is basically determined by functional groups and pHpzc of their surfaces. The results of Boehm titration method are summarized in Table 1. The data indicate that the activated carbon has both

3


acidic and basic properties. Some examples of oxygen containing acidic functional groups are carboxylic, lactonic, and phenolic [15]. On the other hand, it was proposed that certain oxygen containing surface groups such as ketonic, pyronic, chromenic, and p-electron system of carbon basal planes can contribute to the carbon basicity [16]. Activated carbon used in this study has more basic properties and has a density of surface functional groups of (10.33 group/nm2). A 2 value of 14.9 group/nm has been reported for an activated carbon [17]. From the total surface acidic functional groups the phenolic groups are more dominating, followed by carboxylic groups and then come the lactonic groups.

3.2 Equilibrium Isotherms Adsorption isotherm is a graphical representation between the equilibrium concentration of the adsorbate and the adsorbed amount at constant temperature. It characterize the distribution of adsorbed species between liquid and adsorbent at various equilibrium concentrations, based on a set of assumptions that are mainly the heterogeneity of the adsorbent, the type of coverage and the type of interaction between the adsorbate species. The adsorption isotherm of resorcinol on AC was studied at 25°C, 30°C, 35°C and 40°C an initial solution pH of 7.8. As seen from (Fig. 3), and according to the classification of Brunauer, Emett, and Teller [19] the adsorption isotherms of resorcinol on AC are of type IV. This indicates that the interactions between the molecules of resorcinol and the surface of adsorbent are stronger than the interactions between the adsorbed molecules.

To determine the pHpzc, the pH drift method was used. The experimental results are shown in (Fig. 2). The pHpzc is the point where the curve of pHfinal vs. pHinitial intersects the line pHinitial = pHfinal, with the value for the activated carbon being 7.8. At pH < pHpzc, the carbon surface has a net positive charge, while at pH > pHpzc the surface has a net negative charge [18].

250

30 200

qe ( mg/g)

qe (mg/g)

25 20 15

150

100

10 50

5 0

0

0

0.00

0.10

0.20

0.30

0.40

10

20

30

40

50

60

Ce (mg/L)

Ce (mg/L)

Fig. 1. Adsorption isotherm of methylene blue on activated carbon at 25°°C

Fig. 3. Adsorption isotherms of resorcinol on activated carbon at: ()), 25°C; (+), 30°C; ('), 35°C; ((), 40°C

12

3.2.1 Langmuir isotherm

10

pHfinal

8

The Langmuir isotherm is represented by Eq. (3). This model assumes that the sites are homogeneously distributed on the surface of the adsorbent and that there is no interaction among the adsorbed molecules. The correlation of Resorcinol adsorption data with the Langmuir 2 isotherm model was high, with R values between (0.991 - 0.998) (Table 2). The essential characteristics of Langmuir model can be expressed by dimensionless constant called separation factor, RL, which is given by this equation:

6 4 2 0 0

2

4

6

8

10

12

14

pHinitial Fig. 2. Point of zero charge (pHpzc) of the activated carbon, determined by the pH drift method 4


RL =

1 1 + K L C0

q max = K F C1/n 0

(5)

where C0 is the initial concentration of the solute in the bulk solution (mg L−1).

where C0 is the highest initial adsorbate concentration (mg/L) [20]. RL value indicates the adsorption nature to be unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) and irreversible (RL = 0) [21]. The RL for resorcinol adsorption by AC was found to be (0.0352 – 0.1231) for concentration of 0 – 60 mg/L of resorcinol, indicating favorable adsorption.

3.2.3 Tempkin isotherm Tempkin isotherm equation assumes that the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbent–adsorbate interactions and that the adsorption is characterized by a uniform distribution of the binding energies up to some maximum binding energy. The Tempkin isotherm has generally been applied in the linear form as follows [31]:

3.2.2 Freundlich isotherm The Freundlich isotherm model is the well-known earliest relationship describing the adsorption process. This model applies to adsorption on heterogeneous surfaces with the interaction between adsorbed molecules and the application of the Freundlich equation also suggests that sorption energy exponentially decreases on completion of the sorption centers of an adsorbent. For adsorption from solution the Freundlich isotherm is expressed by Eq. (6) [27]:

lnq

e

= lnK

F

+ 1 lnC e n

(7)

qe =

RT lnK BT

T

+

RT lnC BT

(8)

e

where BT is the Tempkin constant related to the −1 heat of adsorption (J mol ), R is the gas −1 −1 constant (8.314 J mol K ), T is the Temperature (K), and KT is the empirical Tempkin constant related to the equilibrium binding constant related to the maximum binding −1 −1 energy (L mg ), (L mol ), respectively. The plot of qe vs. lnCe enables the determination of the isotherm constants BT and KT from the slope and the intercept (Table 2). The Tempkin isotherm takes into account the effects of the interaction of the adsorbate and the adsorbing species. The model assumes that the heat of adsorption (a function of temperature) of all of the molecules in the layer would decrease linearly rather than logarithmically with coverage due to adsorbate– adsorbent interactions [32].

(6)

where, KF is the Freundlich constant, which indicates the relative adsorption capacity of the adsorbent related to the bonding energy, and n is the heterogeneity factor representing the deviation from linearity of adsorption and is also known as Freundlich coefficient. Freundlich equilibrium constants were determined from the plot of lnqe versus lnCe. The n value indicates the degree of non-linearity between solution concentration and adsorption as follows: if n = 1, then adsorption is linear; if n < 1, then adsorption is a chemical process; if n > 1, then adsorption is a favorable physical process. The values of (n) (Table 2), show that surface is heterogeneous and possess great affinity for resorcinol [28,29].

3.2.4 Dubinin–Radushkevich isotherm The DR isotherm is commonly used to describe the sorption isotherms of a single solute system. This isotherm, apart from being analogous to the Langmuir isotherm, is more general than the latter as it does not assume the homogeneity of the surface or constant adsorption potential [33].

According to Freundlich model the maximum adsorption capacity can be calculated by [30]:

Table 1. Boehm titration results and surface density of functional groups and pHpzc of activated carbon Basic groups µ.mol/g

Lactonic groups µ.mol/g

Carboxylic groups µ.mol/g

Phenolic groups µ.mol/g

675.00

31.25

93.75

500.00

Total acidic groups µ.mol/g 625.00 5

Total functional groups µ.mol/g 1300.00

Density of group on the surface (group/nm2) 10.33

pHpzc

7.8

Adam; ACSJ, 16(1): 1-13, 2016; Article no.ACSJ.27637 -1

The DR equation is represented as follows [34]:

lnqe = lnq

max

− βε 2

amount of resorcinol adsorbed, qt (mg.g ), increased with increasing initial concentration and remained nearly constant after about 150 min. It is clear that the adsorption at different initial concentrations was rapid in the initial stages and gradually decreased with the progress of adsorption until the equilibrium reached. The percentage sorption is given in Fig. 5, which showed that the percentage of R sorption increases with decreasing sorbate concentration. This is because at lower concentration there are sufficient active sites that the sorbate can easily occupy. However, at higher concentrations, active sorption sites are not sufficiently available for the sorbate to occupy.

(9)

In this equation, qmax is the DR monolayer capacity (mg.g-1), β is a constant related to the adsorption energy (mol2.kJ-2), and ε is the Polanyi potential, calculated from Eq. (9) [35]:

ε = RTln (1 + 1/C e )

(10)

The mean free energy of adsorption (E), defined as the free energy change when one mole of ion is transferred from infinity in solution to the surface of the solid, was calculated from the β value using the following relation [36]:

1 2β

(11)

qe / (mg.g-1)

E =

The calculated values of DR parameters are given in Table 2. It is reported that when the value of (E) is below 8 kJ/mol, the adsorption process can be considered as the physical adsorption [26]. From Table 2, it can be observed that the obtained values of mean free energy, E, are limited within the range of (1.396 – 1.954). Based on these data, it can thus be concluded that the effect of physical adsorption will play a dominating role in the adsorption process of resorcinol onto activated carbon.

160 140 120 100 80 60 40 20 0

0

100

200

300

t / min Fig. 4. Effect of initial concentration on the adsorption of resorcinol on activated carbon at 20°C: ()), 60 mg/L; (+), 80 mg/L; ('), 100 mg/L; ((), 120 mg/L

2

A perusal of the regression coefficients (R ) for the four isotherms (Table 2) reveals that the adsorption data of resorcinol onto activated carbon is best described by the Langmuir isotherm, followed by the Tempkin isotherm, DR isotherm, and Freundlich isotherm. The removal of R by adsorption onto various adsorbents can be found in the literature [22-26]. Table 3 compare Langmuir and Freundlich parameters and the maximum adsorption capacity for the adsorption of R onto various adsorbents. The values of maximum adsorption capacity obtained using Langmuir equation are higher than those calculated using Freundlich equation.

70

Sorption %

60 50 40 30 20 10 0

0

100 t / min 200

300

Fig. 5. The percent of removal for resorcinol adsorption on activated carbon at 25°C: ()), 60 mg/L; (+), 80 mg/L; ('), 100 mg/L; ((), 120 mg/L

3.3 Effect of Contact Time and Initial Concentration

3.4 Effect of the Adsorbent Dosage

The effect of contact time (5 - 250 min) on qt values for Co = 120, 100, 80, and 60 mg/L at T = 20°C is shown in (Fig. 4). The equilibrium time and the uptake of resorcinol are dependent of initial concentration of the adsorbate. The

The percentage removal resorcinol by adsorption onto activated carbon adsorbent was studied by varying the adsorbent dose in the range from 0.4 to 3.2 g/l, with a solution of 100 mg/l of resorcinol 6


concentration, at agitation time of 200 rpm, at a fixed pH = 7.8 and at a temperature of 25°C. The effect of adsorbent dose on percent uptake of resorcinol is shown in (Fig. 6), which indicated rapid increase in adsorption with increasing doses. Adsorption was ≅ 97% at 1.6 g/L and on further increase of the adsorbent dose adsorption percentage did not increase.

temperatures 298, 303, 308 and 313 K and at optimum conditions, at a fixed pH = 7.8 and contact time of 200 min. (Fig. 3) shows that the -1 uptake capacity of resorcinol (mg.g ) increases with increasing temperature, indicating that the adsorption is an endothermic process. The spontaneity and feasibility of resorcinol adsorption on AC was studied by determination of thermodynamic parameters, by the following equations [4]:

3.5 Effect of Initial pH Change of pH value in adsorption system could lead to the transformation of chemical characteristics on the surface of activated carbon and the form of the adsorbate, thus it plays an important role in the adsorption. As shown in (Fig. 7), the adsorption capacity of resorcinol onto activated carbon increased from pH 2 to 8 and then decreased from pH 8 to 12. In fact, it can be said that the optimum condition of pH for resorcinol adsorption by activated carbon was 7.8 .When pH of the solution is lower than pHpzc the carbon surface has a net positive charge, while at higher than pHpzc the surface has a net negative charge. The pKa value of resorcinol is 9.4 (at 25°C) [37] indicate that resorcinol is present almost entirely in the protonated form under environmental conditions (pH 5–8). Low adsorption would be expected at pH > pKa and pHpzc because both adsorbent and adsorbate are negatively charged and electrostatic repulsion may be one of the dominant mechanisms [38]. Similar results were observed for the adsorption of catechol and resorcinol on granular activated carbon [4].

∆G o = − RTlnK d ∆G o = ∆H o − T ∆S o

(13)

where T is temperature (K); Kd the distribution -1 o coefficient (qe/Ce) (mg g ); ∆G the standard -1 o Gibbs free energy (kJ mol ); ∆H the standard -1 o enthalpy (kJ mol ), and ∆S the standard entropy -1 -1 ° (J mol K ). The Gibbs energy change ∆G , was ° obtained by Eq. (12). The magnitude of ∆H and ° ∆S was calculated from the slope and yintercept from the plot of ln Kd vs 1/T. The thermodynamic parameters were summarized in Table 4. o

The free energy change, ∆G , is negative, indicating that the adsorption process is feasible and spontaneous. The more negative the ∆Go, the more spontaneous the adsorption was and, hence, it is expected a higher adsorption o capacity. The positive value of ∆H indicates that the adsorption is endothermic in nature, and the o positive value of ∆S , indicates increased randomness at the solid/solution interface with some structural changes in the adsorbate and the adsorbent [2].

3.6 Effect of Temperature The effect of solution temperature was studied by conducting the experiment at different

120

300

100 80

qe (mg/g)

200

60 150

Sorption %

40

100

20

50 0

Sorption %

350

250

qe (mg/g)

(12)

0 0

1

2

3

4

adsorbent dose (g) Fig. 6. Effect of adsorbent dose on the adsorption of resorcinol on activated carbon at 25°C

7


Table 2. Isotherm constants for Resorcinol adsorption by activated carbon T/ °C

25 30 35 40

KL (L/mg) 0.059 0.108 0.166 0.227

Langmuir qmax RL (mg/g) 208.33 0.1231 217.39 0.0712 220.75 0.0475 223.21 0.0352

2

R

0.998 0.997 0.991 0.994

KF (mg/g) 22.45 26.52 35.30 36.43

Freundlich 2 n R 1.74 1.80 1.99 1.93

0.911 0.895 0.875 0.849

BT (J/mol) 51.334 50.534 55.184 53.786

Tempkin KT (L/g) 0.805 1.000 1.581 1.695

2

R

0.989 0.993 0.987 0.974

qmax (mg/g) 760 677 442 435

D-R E (kJ/mol) 1.396 1.510 1.869 1.954

2

R

0.929 0.964 0.970 0.960

Table 3. Langmuir and Freundlich constants for adsorption of resorcinol on various adsorbents Adsorbent AC

GAC GAC GAC ACC HJ-1

T/ °C 25 30 35 30 30 30 25 30

Langmuir KL (L/mg) qmax (mg/g) 0.059 208.33 0.108 217.39 0.166 220.75 0.046 140.72 0.047 142.82 0.038 209.00 0.143 232.33 0.003 126.58

Freundlich n 1.74 1.80 1.99 4.24 4.35 2.45 4.00 1.69

KF (mg/g) 22.45 26.52 35.30 34.52 34.83 24.25 11.87 2.01

Reference qmax (mg/g 86.76 97.98 115.12 120.44 145.35 147.72 37.54 46.21

This work This work This work Mondal and Balomajumder [22] Kumar et al. [23] Bayram et al. [24] Kumar et al. [25] Huang et al. [26]

AC, Activated Carbon; GAC, Granular Activated Carbon; ACC, Activated Carbon Cloth; HJ-1, Hypercrosslinked resin

8

Adam; ACSJ, 16(1): 1-13, 2016; Article no.ACSJ.27637 2

∆H° value as calculated from equations (12 & 13) is 34.506 kJ/mol. Physical adsorption and chemisorption can be classified, to a certain extent, by the magnitude of the enthalpy change. It is accepted that bonding strengths of < 84 kJ/mol are typically those of physical adsorption type bonds. Chemisorption bond strengths can range from 84 to 420 kJ/mol [17].

(min). The rate constant, R , and qe are listed in Table 5. 3.7.2 Pseudo-second-order kinetics The pseudo-second-order kinetic equation is described as [39]:

t = q t

140

qe (mg/g)

120

1 t + k q 2 e

(15)

100

where k2 (mg.g-1.min-1) is the rate constant and qe and qt (mg/g) are the amount of resorcinol adsorbed on AC at equilibrium and at time t (min), respectively. Values of k2 and qe were calculated from the slopes and intercepts of the linear plots of t/qt against t (Fig. 8).

80 60 40 20 0 0

2

4

6

pH

8

10

12

14

3.00

Fig. 7. Effect of pH on the adsorption of resorcinol on activated carbon at 25°C

t/qt (min.g/mg)

2.50

3.7 Adsorption Kinetics To evaluate the resorcinol adsorption rate onto active carbon, pseudo first-order, pseudosecond-order, and intra-particle diffusion models were used. The kinetic experiments were performed with 60, 80, 100, and 120 mg/L solutions of the adsorbate at 298.15 K, with active carbon dosage of 0.4 g/L in all experiments.

1.50 1.00 0.50 0.00 0

50

100

150

200

t / min

250

Fig. 8. Pseudo-second-order kinetics for resorcinol adsorption on activated carbon at 25°C: ()), 60 mg/L; (+), 80 mg/L; ('), 100 mg/L; ((), 120 mg/L

3.7.1 Pseudo-first-order kinetics

As depicted in Tables 5, according to the 2 correlation coefficients (R ), the pseudosecond-order equation fits the experimental kinetic data better than the pseudo- first-order equation. These results indicated that the pseudo-second order model can likely predict resorcinol adsorption onto AC. Previous studies also found that the adsorption of catechol and resorcinol from aqueous solutions on GAC follows the pseudo second-order kinetics [4].

The pseudo-first-order kinetic equation [16] may be written as:

ln(qe − q t ) = lnqe − k1t

2.00

(14)

where qt is the amount of resorcinol removed at time t (mg/g), qe is the adsorption capacity at equilibrium (mg/g), k1 is the pseudo-first-order -1 rate constant (min ), and t is the contact time

Table 4. Thermodynamic parameters for resorcinol adsorption by active carbon at different temperatures T / °C 25 30 35 40

o

-1

-1

o

∆S (J.mol .K ) 133 133 133 133

-1

∆H (kJ. mol ) 34.506 34.506 34.506 34.506

9

o

-1

∆G (kJ. mol ) -5.124 -5.789 -6.454 -7.119


Table 5. Kinetic parameters for resorcinol adsorption by activated carbon Co/mg/L 60 80 100 120

Pseudo -1 k1 (min ) -3 1.52×10 -3 1.35×10 -3 1.27×10 -2 1.38×10

first-order model 2 qe (mg/g) R 65.11 0.974 89.26 0.973 84.72 0.962 76.91 0.930

Pseudo second-order model -1 -1 k2(mg.g .min ) qe (mg/g) 3.62 90.91 4.13 114.94 7.41 123.46 13.40 138.89

10

2

R 0.993 0.994 0.994 0.998

Intra-particle diffusion model -1 -1/2 Ki (mg.g .min ) C (mg/g) Ri 5.38 11.74 0.868 6.82 12.74 0.884 6.90 24.41 0.805 7.13 43.89 0.686

2

R 0.949 0.948 0.903 0.800


3.7.3 Intra-particle diffusion model

adsorption amount, C, to the final adsorption amount, qref. According to those authors, Ri value can be divided into: (i) Ri = 1, no initial adsorption; (ii) 1 > Ri > 0.9, weak initial adsorption; (iii) 0.9 > Ri > 0.5, intermediately initial adsorption; (iv) 0.5 > Ri > 0.1, strong initial adsorption and (v) Ri < 0.1, approaching complete initial adsorption. Ri values between 0.686 and 0.884 (Table 5) were obtained suggesting that there is a considerably intermediate initial adsorption.

qe (mg/g)

To study the diffusion mechanism of adsorption, the kinetic results were evaluated using the intra-particle diffusion model. During the intra-particle diffusion process, the adsorbate species are most probably transferred from the bulk of the solution into the solid phase [40]. 180 160 140 120 100 80 60 40 20 0

4. CONCLUSION

0

5

10

t0.5

/

15

The present study shows that the AC is an effective adsorbent for the removal of resorcinol from aqueous solutions. Adsorption isotherms of R on AC were studied and modeled using Langmuir, Freundlich, Tempkin and Dubinin– Radushkevich models. Equilibrium adsorption data was best represented by the Langmuir and Tempkin isotherms.

20

min0.5

Fig. 9. Intra-particle diffusion plots for resorcinol adsorption on activated carbon at 25°C: ()), 60 mg/L; (+), 80 mg/L; ('), 100 mg/L; ((), 120 mg/L

The adsorption capacity of resorcinol onto activated carbon increased from pH 2 to 8 and then decreased from pH 8 to 12. Adsorption of resorcinol is favorably influenced by the increase in temperature indicating endothermic nature of the adsorption process. The sorption kinetics followed pseudo second-order model. It may be concluded that the AC can be used for the removal of resorcinol from aqueous solutions.

The Weber and Morris intra-particle diffusion model [41] can be expressed as shown in Eq. (16):

q t = k i t 0.5 + C

(16)

where ki is the intra-particle diffusion rate constant (mg.g-1.min-1/2) and C is a constant related to the thickness of the boundary layer (mg.g-1) and is determined from the plot of qt 1/2 versus t (above Fig. 9). The adsorption plots of resorcinol at all initial concentrations did not pass through the origin and therefore the intra-particle diffusion was not only the rate-limiting step, but the external mass transfer also played an important role in the adsorption process [42]. The intercept C value increases with increasing initial resorcinol concentration, indicating that the initial adsorption increases with increasing initial concentration of the adsorbate [4,43]. According to Wu et al. [44] this intercept, C, indicates the occurrence of a rapid adsorption within a short period of time. They had investigated the initial adsorption using the intercept C to define an initial adsorption factor (Ri), expressed as:

C R i = 1− ( ) q ref

COMPETING INTERESTS Author has declared that no competing interests exist.

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2.

3.

(17)

Equation (17) indicates that Ri can be represented in terms of the ratio of the initial 11

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