LALLAN SINGH YADAV REMOVAL OF PHENOL FROM AQUEOUS … No3_p263-273_Jul … · BIJAY KUMAR MISHRA1 ARVIND KUMAR2 1Centre of Studies in Surface Science and Technology, School of Chemistry,

Chemical Industry & Chemical Engineering Quarterly

Available on line at Association of the Chemical Engineers of Serbia AChE www.ache.org.rs/CICEQ

Chem. Ind. Chem. Eng. Q. 22 (3) 263−273 (2016) CI&CEQ

263

LALLAN SINGH YADAV1

BIJAY KUMAR MISHRA1

ARVIND KUMAR2 1Centre of Studies in Surface

Science and Technology, School of Chemistry, Sambalpur University,

Jyoti Vihar, Odisha, India 2Department of Chemical

Engineering, National Institute of Technology, Rourkela, Odisha,

India

SCIENTIFIC PAPER

UDC 547.562.1:628.3:544:661.183.2

DOI 10.2298/CICEQ150327040S

REMOVAL OF PHENOL FROM AQUEOUS SOLUTION BY MAHUA SEED ACTIVATED CARBON: KINETIC, ISOTHERM, MASS TRANSFER AND ISOSTERIC HEAT OF ADSORPTION STUDIES

Article Highlights • Pseudo-second order kinetic model primarily governed the phenol uptake process • The average effective diffusivity (6.4×10-13 m2/s) of phenol was estimated using the

Boyd model • The maximum uptake of phenol (54 mg/g) was obtained at 333 K • Overcome the difficulty of recovering the photocatalyst after use • The negative values of ΔG indicate that the sorption of phenol was spontaneous Abstract

Mahua (Madhuca longifolia) seed activated carbon (MSAC) has been dev-eloped as an effective adsorbent for the removal of phenol from contaminated wastewaters. The prepared MSAC was characterized for various physico-chemical properties, by Fourier transform infra-red (FTIR) and scanning elec-tron microscopy (SEM) analysis. Laboratory batch experiments were per-formed to investigate the effect of MSAC dosage (w), pH, contact time (t), and initial phenol concentration (co) on sorption efficiency at optimal conditions. The maximum adsorption capacity of phenol was obtained at pH 6, t = 5 h and MSAC dosage = 1.2 g/l. The kinetics data of phenol adsorption was very well described by the pseudo-second-order kinetic model. The equilibrium adsorp-tion data were best fitted to the Langmuir isotherm. The average effective diffusion coefficient 6.4×10-13 m2/s was calculated from the experimental data. Thermodynamic studies confirmed the sorption process to be spontaneous and exothermic. The isosteric heat of adsorption of phenol was found to inc-rease with an increase in the surface loading indicating that MSAC have more homogeneous surface.

Keywords: phenol, adsorption isotherm, mass transfer, thermodynamic parameters, isosteric heat.

Phenol and its derivatives are considered to be very toxic and harmful to living organisms even at low concentrations. There are various sources of phenols that pollute the surface and ground water [1]. Several environmental regulatory agencies of the world have set the maximum permissible discharge concentration of phenol in the effluent for their discharge into receiving water bodies [2]. Therefore, in order to pro- Correspondence: A. Kumar, Department of Chemical Engineer-ing, National Institute of Technology, Rourkela - 769 008, Odisha, India. E-mail: [email protected] Paper received: 27 March, 2015 Paper revised: 30 July, 2015 Paper accepted: 29 September, 2015

tect public health and the environment, the treatment of phenolic wastewater is necessary before their safe disposal. Several traditional methods can be used for the removal of phenolic compounds from aqueous solutions [3]. Adsorption is considered to be the most powerful separation and purification method because of various reasons [4]. During the past few decades, several low cost adsorbents have been used for removing organic pollutants from wastewater [5–7]. Though the adsorption of pollutants onto activated carbon is a well-known process, there are several factors that limit its use as an adsorbent [8]. There-fore, new adsorbents that are economically viable,

L.S. YADAV et al.: REMOVAL OF PHENOL FROM AQUEOUS SOLUTION… Chem. Ind. Chem. Eng. Q. 22 (3) 263−273 (2016)

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easily available, having strong affinity and high load-ing capacity need to be explored.

In the present work, Mahua seeds activated carbon (MSAC) was developed in the laboratory and used as low cost adsorbent for the removal of phenol from aqueous solution. Effects of experimental para-meters such as initial pH of the solution, contact time, initial phenol concentration, MSAC concentration, etc. were studied. The mechanism of phenol adsorption onto MSAC was evaluated in terms of thermo-dynamics and kinetics. The adsorption isotherms were described by using Langmuir and Freundlich isotherm models.

MATERIALS AND METHODS

All chemicals used in this work were of anal-ytical reagent grade. Deionized water was used for all dilutions. A stock solution of 1000 mg/l phenol was prepared by dissolving appropriate amount of phenol (Merck, Darmstadt, Germany) in 1 L of deionized water. The required concentration of phenol solution was prepared by diluting appropriate volume of the stock solution. The pH of the solution was adjusted by addition of 0.1 M HCl or 0.1 M NaOH solutions. Adsorption of phenol onto MSAC was investigated through a batch process.

Details of MSAC preparation from mahua seed are presented in this paragraph. A tubular reactor was used for the production of activated carbon. The pre-cursor sample was pyrolyzed at 750 K for 5 h under N2 atmosphere. Sufficient amount of carbonized mat-erial was treated with saturated solution of NaOH for activation to increase the adsorption capacity of MSAC [9]. The sample was then dried in an oven at a temperature of 115 °C for 8 h. These mixtures were heated to 450–650 °C for 2–3 h in a nitrogen atmo-sphere in the horizontal furnace with a heating rate of 5 °C/min. The activated carbon was taken out from the furnace and allowed to cool at ambient conditions. After chemical activation, the final product was washed with double distillated water (DDW) until the pH of the residual solution became 6-7. All the water soluble impurities were removed by washing with DDW [10]. Immediately after washing, the sample was dried in the oven at 110 °C for 120 min and the activated carbon prepared was preserved for end use. Pre-pared MSAC was characterized for textural character-istics and point of zero charge. Moreover, Fourier transform infra-red (FTIR) and scanning electron mic-roscopy (SEM) analysis was also done.

For adsorption experiments, 50 ml of phenol sol-ution in the concentrations range of 50–150 mg/l was

transferred into a series of conical flasks. Known amount of MSAC was added to the solution, and the mixture was agitated on a mechanical water bath shaker at 250 rpm. After reaching equilibrium, the suspension was filtered through 0.45 µm of syringe driven filter membrane (Millex, Millipore), and the supernatant was analyzed for residual phenol con-centration using a double beam UV–vis spectrophoto-meter. The amount of the phenol adsorbed by the MSAC was calculated using the following equation:

( )= −e o e /q c c w (1)

where co (mg/l) is the initial concentration of phenol solution, ce (mg/l) is the equilibrium concentration of phenol in aqueous solution, w (g/l) is the MSAC con-centration, and qe (mg/g) is the amount of calculated phenol uptake onto MSAC.

Marquardt’s percent standard deviation (MPSD) error function [11] was also employed in this study to find out the most suitable kinetic and isotherm model to represent the experimental data. MPSD error function is given as:

=

−=

− 2

e,exp e,cal

e,exp1

1100

n

i i

q qMPSD

n p q (2)

where qe,exp and qe,cal are the experimental and cal-culated equilibrium adsorbate uptake, respectively, and n is the number of data points.

RESULTS AND DISCUSSION

Characterization of MSAC

The detailed characterization of MSAC is pre-sented in Table 1. The bulk density of MSAC was found to be 0.77 kg/m3. The proximate analysis shows that the amount of carbon content present in MSAC was more than 55% and that the amount of ash was only 3.6%. Chemical analysis of ash showed presence of silicon, iron, magnesium and calcium oxides in MSAC. Ultimate analysis of MSAC showed 58.33% carbon; 3.98% hydrogen; 2.37% nitrogen, and oxygen by balance was found to be 34.28 %; and trace amounts of sulfur were also observed.

The Brunauer-Emmett-Teller (BET) surface area of MSAC was found to be 45 m2/g using N2 adsorp-tion isotherm. The Barrett-Joyner-Halenda (BJH) ads-orption/desorption surface area of pores was found to be ∼22/16 m2/g. The single point total pore volume of pores (< 2621.3 Å) was 0.134 cm3/g, whereas the cumulative pore volume of pores (20 < d < 2500 Å) was 0.053 cm3/g. The average pore diameter by BET method was found to be 22.5 Å, whereas the BJH


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adsorption/desorption average pore diameter was found to be 33.78 Å/56.12 Å. Micropores (d < 20 Å) accounted for 25.6% (190 m2/g) of the pore surface area and 20% (0.02 cm3/g) of the pore volume and the mesopores (20 Å < d < 500 Å) accounted for 34.2% (76 m2/g) of the pore surface area. The mic-ropore areas and external surface areas were deter-mined using t-plot method. The surface acidity and alkalinity were determined to be 5.46 and 2.73 mmol/g, respectively.

Table 1. Physicochemical characteristics of MSAC

Characteristic Value

Proximate analysis

Moisture 3.37%

Volatile matter 38%

Ash 3.6%

Fixed Carbon 55.03%

Bulk density 0.7702 kg/m3

Dry density 0.7449

Porosity 0.4784

Specific gravity 0.7994

Void ratio 0.0720

Heating value 7454 cal/g

Average particle size 574.28 μm

Chemical analysis of ash

Insoluble 78.35%

SiO2 2.46%

Fe2O3 and Al2O3 2.92%

CaO 14%

MgO 1.09%

Others 1.18%

Ultimate analysis of adsorbents

Carbon 58.33%

Hydrogen 3.98%

Nitrogen 2.37%

Sulfur 1.04%

EDX Analysis

Carbon 49.11

Nitrogen 23.39

Oxygen 24.10

Magnesium 0.32

Phosphorus 1.96

Sulfur 0.31

Potassium 0.81

Surface area of pores, m2/g

BET 45

BJH (adsorption/desorption) 22.25/16.62

BJH cumulative pore volume, cm3/g

Single Point Total 0.134

BJH adsorption 0.053

Table 1. Continued

Characteristic Value

BJH cumulative pore volume, cm3/g

BJH desorption 0.051

Average pore diameter, Ǻ

BET 22.5

BJH adsorption 33.78

BJH desorption 56.12

Iodine no. 190

Methylene blue no. 76

Point of zero charge 6.12

Boehm titration

Surface acidity 5.46 mmol/g

Surface alkalinity 2.73 mmol/g

The morphologies of blank and phenol-loaded MSAC were examined by scanning electron micro-scopy (SEM) analysis. The SEMs of the blank and phenol-loaded MSAC are shown in Figure 1. These figures indicate the surface texture and porosity of the blank and phenol loaded adsorbent. SEM micro-graphs of MSAC at 600× magnification (Figure 1) show the fibrous structure and strands of the fibre. It can be inferred from these figures that the surface texture of the blank MSAC changes (rough surface morphology to smoother morphology) after the ads-orption of phenol. These changes in morphology are in good agreement with the literature for various mat-erials [12].

(a)

(b)

Figure 1. SEM of: a) virgin MSAC; b) phenol loaded MSAC.


266

The changes in the FTIR spectra of MSAC due to adsorption of phenol are shown in Figure 2. The chemical structure of the adsorbent is of vital impor-tance in understanding the sorption process. The sorption capacity of adsorbents is strongly influenced by the chemical structure of their surface. The car-bon-oxygen functional groups are by far the most important structures in influencing the surface charac-teristics and surface behavior of MSAC. The OH stretching vibrations band of surface hydroxyl groups and chemisorbed water was observed between 3000 and 3700 cm-1. The shifting of this band at lower wave numbers indicates the presence of strong hydrogen bonds between surface functional groups and ads-orbed water molecules [13]. This stretching is due to both the silanol groups (Si-OH) and adsorbed water (peak at 3400 cm-1) on the surface [14]. The FTIR spectra of the samples below 1800 cm-1 show change in chemical structure of the surface and structural oxygen groups. The peak referred to the band of 175- -1550 cm-1 can also be attributed to C=O moieties stretching vibration in carboxylic, ester, lactonic or anhydride groups, and conjugated systems like diket-one, ketoesters and keto-enol structures [15]. The FTIR spectra peak at 1056 cm-1 may be due to C-O stretching vibrations [16]. The FTIR spectra band around ∼1400 cm-1 may be due to the carboxyl-carbonate structures. The functional groups sug-gested most often are: i) carboxyl groups, ii) phenolic hydroxyl groups, iii) carbonyl groups and iv) lactone groups [17]. The bands referred to the peak at 900– -1100 cm–1 are due to Si–O and Si–O–Si stretching and bending vibrations. [18]. Phenol is adsorbed on car-bon surface in four different states: physically ads-orbed phenol, hydrogen-bonded phenol, phenol in interaction with a Lewis acid site, and Bronsted acid

site [19]. The FTIR spectra of the virgin and phenol loaded MSAC are shown in Figure 2. Two clear peaks around 1450–1500 cm-1 and 1620–1650 cm-1 can be identified that seem to be affected by phenol ads-orption. The peak around 1400 cm-1 in the range from 1400 to 1500 cm-1 is the characteristic band of phenol bonded to Lewis site. The peaks around 1630 cm-1 in the range of 1600–1650 cm-1 are due to the bond of phenol to the Bronsted site [20]. These peaks are normally attributed to adsorbed bound water onto the sorbent surface. The peak around 1400 cm-1 indicates that water was lost and the phenol interacted with the cationic surface of the adsorbents within the interlayer space [21-22]. The peak around 1630 cm-1 is attri-buted to hydrogen bending vibrations reflecting the presence of bound water. Peak around 1600 cm-1 may also be due to conjugated hydrocarbon bonded carboxyl groups. Upon phenol adsorption, these peaks get partially shifted.

Effect of MSAC concentration

The effects of MSAC concentration (keeping unchanged pH, adsorbate concentration, contact time and temperature) on the uptake of phenol from aque-ous solutions were investigated by using different MSAC concentrations in the range of 0.5–6 g/l and initial phenol concentration of 100 mg/l at pH 6. As the MSAC concentration was increased from 0.5 to 6 g/l, the equilibrium adsorption capacity (qc) of MSAC decreased from 76.33 to 14.36 mg/g, whereas the phenol removal efficiency increased from 38.17 to 86.16%. The increase in adsorption percentage of phenol was due to the increased availability of active adsorption sites with an increase in MSAC dosage. The adsorption capacity, the amount adsorbed per unit mass of adsorbent, decreases mainly due to insufficient of adsorption sites [23] and due to particle

Figure 2. The comparision of FTIR spectra of virgin MSAC and phenol loaded MSAC.


267

interaction, such as aggregation, resulting from high adsorbent concentration. Such aggregation cause decrease in total surface area of the sorbent and an increase in diffusion path length [24-26].

Effect of pH

The solution pH is the one of the most important factor that controls the sorption of phenol. Figure 3 shows the effect of solution pH on the adsorption capacity of MSAC for phenol. The results presented in the figure reveal that the adsorption increased with an increase in pH of the phenol solution initially but thereafter it declined. As shown in Figure 3, the amount of phenol sorbed by MSAC slowly increased with an increase in pH of phenol solution from 2 to 6 and the optimum pH observed at 6. Maximum uptake of phenol was about 31.99 mg/g at pH 6.

Figure 3. The effect of pH on the removal of phenol from

aqueous solution using the MSAC (w = 1.2 g/l; pH 6; co = 100 mg/l; t = 5 h; T = 303 K).

The effect of pH may be explained in terms of pHzpc (zero point of charge) of the adsorbent, at which the adsorbent is neutral. The surface charge of the adsorbent is positive when suspension pH is below pHzpc (pHzpc 9 determined by solid addition method [27] for MSAC) [25]. The surface charge of the ads-orbent became positive, thus increasing the ads-orption significantly.

Before a phenolic compound attaches to the surface of the adsorbent (M+=MSAC), it undergoes a dissociation process as illustrated by the following equation:

− +PhOH PhO +H (3)

The phenolic compound ionizes to phenolate anion and a proton. The proposed equilibrium model for the adsorption can be presented as:

− + + − + +PhO H +M PhO M +H (4)

Effect of contact time and initial phenol concentration on sorption efficiency

The effect of contact time and initial phenol con-centration (50–150 mg/l) was investigated at optimum

pH of 6 in the presence of 1.2 g/l of MSAC. It was observed that an increase in the phenol concentra-tions increases the phenol sorption capacity from 12.46 to 37.52 mg/g in 5 h equilibrium time. The increase in adsorption capacity may be due to the higher adsorption rate of phenol and the utilization of all available active sites for adsorption at higher phe-nol concentration. A further increase in the contact time has a negligible effect on the rate of phenol ads-orption. The rate of phenol uptake is higher in the beginning due to larger surface area of the MSAC being available for phenol adsorption. Later as the adsorbed material forms a monolayer, the capacity of the adsorbent gets exhausted and no further uptake of phenol was observed. It was observed that after 5 h of mixing the adsorption capacity of the adsorbent was exhausted.

Adsorption kinetic studies

Adsorption kinetics describes the relationship of solute uptake rate of the adsorption and the adsorp-tion time. The results obtained for rate of adsorption of phenol on to the MSAC at different concentrations and contact times were analyzed by using the pseudo-first-order, pseudo-second-order and Boyd models. The pseudo-first-order and second-order kin-etic models were used to test the experimental kinetic data by nonlinear fitting. The pseudo first-order equa-tion is given as follows:

= −f ed

( )d

tt

q k q qt

(5)

Eqation (5) can be linearized into the following form:

− = −e e fln( ) lntq q q k t (6)

Eqation (6) can further be written as:

= − −e e fexp(ln )tq q q k t (7)

where kf (min-1) is the rate constant of pseudo-first order adsorption, qe and qt (µg/g) are the adsorbed amount at equilibrium and at time t.

The pseudo-second order model can be repre-sented as given below:

= − 2s e

d( )

dt

tq k q qt

(8)

Integrating Eq. (8) and using conditions that qt = 0 at t = 0, the following equation is obtained:

=+

2s e

s e1tk q tq

k q t (9)


268

The initial sorption rate, h (mg/g min) at t = 0 is defined as:

= 2s eh k q (10)

where ks is the pseudo-second order rate constant (g/mg.min). h, qe and ks can be determined from the non-linear regression of Eq. (9).

The best-fit values of h, qe and ks along with the correlation coefficient values of phenol under different concentration range for the pseudo-first order and pseudo-second order models are shown in Table 2.

The kinetic data were analyzed for the phenol uptake by plotting qt versus t to the pseudo-first-order, pseudo-second-order and Boyd kinetic models. It is clear from the plot of qt versus t that an increase in initial phenol concentration leads to the increase in adsorption capacity, qe, this indicates that the initial phenol concentration is an important parameter in determining the adsorption capacity of phenol uptake onto MSAC. Figure 4 shows that the pseudo-second order model fits the experimental data better than pseudo-first-order model for the entire adsorption period of phenol concentrations. Moreover, it was further noticed from the regression data in Table 2 that the values of qe,cal, obtained from the pseudo-second-order model are closer to the experimental results than qe,exp obtained from pseudo-first-order model. Furthermore, for various concentrations of phenol, the constants calculated from the plots are given in Table 2. The R2 values are excellent and calculated qe values match well with experimental ones. Therefore, the sorption of phenol onto MSAC follows the second-order reaction kinetics. Thus sup-porting the basics assumption in the model that chemisorption plays a major role in this adsorption

system. Similar results have been reported for phenol adsorption from aqueous solutions onto Luffa cylin-drical fibers [28].

Figure 4. The Effect of contact time on the adsorption of phenol

by MSAC (w = 1.2 g/l; pH 6.0; t = 5 h; T = 303 K).

Effective diffusivity

To identify whether surface diffusion controlled the adsorption process, the kinetic data were anal-yzed using Boyd kinetic expression, which is given as follows:

π= − −6

1 exp( )t tF B (11)

= − − −0.4977 ln(1 )t tB F (12)

where = et tF q q is the fractional attainment of equilibrium at time t and Bt is the mathematical func-

Table 2. Kinetic parameters for the removal of phenol by MSAC (t = 5 h; co, 50-150 mg/l; w = 1.2 g/l; pH 6); co: initial phenol concentra-tion; qe,exp: experimental equilibrium uptake capacity; qe,cal: calculated equilibrium uptake capacity (mg/g); kf: 1st order reaction rate cons-tant; R2: regression coefficient; ks: 2nd order reaction rate constant; MPSD: Marquardt's percent standard deviation; h: initial sorption rate

co / mg l-1 Parameter

qe,exp / mg g-1 qe,cal / mg g-1 kf / min-1 R2 MPSD / %

Pseudo-first order kinetic model

50 12.16 10.91 0.0185 0.9751 15.3793

75 21.34 21.83 0.0196 0.9965 17.7663

100 28.87 25.82 0.0186 0.9719 16.0179

150 32.96 32.64 0.0170 0.9679 27.8690

Pseudo-second order kinetic model

qe,cal / mg g-1 ks / g mg-1 min-1 h / mg g-1 min-1 R2 MPSD / %

50 14.20 0.0012 0.2357 0.9888 11.4595

75 25.51 0.0007 0.4769 0.9813 14.5877

100 33.90 0.0005 0.5574 0.9902 12.1747

150 35.64 0.0008 0.9968 0.9676 20.0298


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tion of F. The particles are assumed to be spherical. Effective diffusivity was calculated by the kinetic

model given by Boyd et al. [29]:

π = −

2e

2 2a

1ln

1 t

D tF R

(13)

where De is the effective diffusion coefficient of ads-orbents in the adsorbent phase (m2/s) and Ra is the radius of the adsorbent particles. De is obtained from the slope of a plot of ln[1/(1-Ft

2)] versus t. The aver-age value of De as calculated was 6.4×10-13 m2/s for the adsorption of phenol onto MSAC. Previous inves-tigators have reported effective diffusivity values of 0.388×10-10 and 3.19×10-10 m2/s for phenol and hydro-quinone adsorption onto GAC, respectively [27,30]. Aravindhan et al. [23] obtained the average effective diffusivity value of 1.44×10-9 cm2/s for phenol removal by modified green macroalgae, where they suggested that the intra-particle diffusion is negligible in com-parison to external mass transfer.

Equilibrium isotherms

The results of phenol removal by MSAC at equi-librium were studied by the various isotherm models available in the literature for adsorption such as the Langmuir, Freundlich, Temkin and Redlich-Peterson, each of them being able to provide useful information on the sorption mechanism. The complete details about these isotherms are presented elsewhere [31]. Figure 5 shows the isotherm at 303 K and the fitting of various isotherms. The parametric values for different isotherm equations along with the values of the coef-ficient of determination (R2) and MPSD values for the fit of adsorption data at various temperatures are given in Table 3. The qmax value of the Langmuir iso-therm represents the uptake of MSAC. It is observed that qmax increases with an increase in temperature. The 1/n and KF values of the Freundlich isotherm equation model are the measures of the surface heterogeneity and adsorption intensity of the adsorb-ate, respectively. The values of 1/n < 1 indicate favor-able adsorption. The Freundlich and Langmuir iso-therms constants values obtained in some of studies are compared with the values obtained in the present work. The qmax = 54.75 mg/g (at 338 K and pH 6) in the present work was comparable with the previous works [30,32,33].

The values of the Temkin parameters presented in Table 2 indicate uniform distribution of binding energy up to maximum value and that phenol bio-sorption by MSAC (BT = 13.15 kJ/ mol) is exothermic in nature.

Figure 5. Equilibrium adsorption isotherms at different tem-

peratures for phenol-MSAC system, (w = 1.2 g/l; pH 6.0; t = 5 h; T = 303 K) experimental data points given by the sym-

bols and the lines predicted by the adsorption isotherms equation.

The R2 values (given in Table 3) alone are not enough to determine the best isotherm model to rep-resent the experimental data because they are mostly greater than 0.99 for all four models. The MPSD values are smaller for the Langmuir model compared to the other models. Therefore, the Langmuir model may be taken to represent the adsorption isotherm data of the phenol-MSAC system adequately and satisfactorily.

Table 4 lists some of the low cost materials used as adsorbents for the removal of phenol and their adsorption capacity values. From Table 4, it is obs-erved that the sorption capacity of MSAC determined from the present study is higher than the non-con-ventional low cost adsorbents previously reported for the uptake of phenol from its aqueous solutions [32].

Effect of temperature on thermodynamics parameter on adsorption of phenol

To study the thermodynamics of adsorption of phenol on MSAC, thermodynamic constants such as enthalpy change, ΔH, free energy change, ΔG, and entropy change, ΔS, were calculated through Eqs. (12)–(14). The values of these parameters are given in Table 5. Thermodynamic parameters ΔH, ΔS and ΔG for phenolic ions-MSAC system was calculated using the following equations:

Δ = − dlnG RT K (14)

Δ Δ= − +d1

lnH SK

R T R (15)

ρρ−= =o e ed

e e

c c qVKc w c

(16)

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270

Table 3. Isotherm Parameters for the removal of phenol onto MSAC at different temperatures (t = 5 h; w = 1.2 g/l; pH 6); qe: equilibrium uptake capacity; ce: equilibrium concentration; KF: Freundlich sorption capacity; KT: Langmuir constant of surface energy (l/mg); β: degree of heterogeneity - R-P model; KR and aR: constants in R-P model; BT: constant related to heat of sorption; KT: equilibrium binding constant; MPSD: Marquardt's percent standard deviation

Temperature, K Parameter

Freundlich, = 1e F e

nq K c

KF / mg g-1 (mg l)-1/n 1/n R2 MPSD

308 8.6364 0.3514 0.9737 6.3609

318 6.9544 0.3954 0.9849 3.2927

328 4.6494 0.4823 0.9956 2.2021

338 4.0344 0.4934 0.9962 2.0052

Langmuir, =+m L e

eL e1

q K cq

K c

qm / mg g-1 KL / l mg-1 R2 MPSD

308 48.8584 0.0516 0.9993 1.8200

318 50.6921 0.0398 0.9997 1.2113

328 57.2834 0.0247 0.9997 0.6624

338 54.7573 0.0220 0.9986 1.4357

Temkin, = +e T T T eln lnq B K B c

KT / l mg-1 BT R2 MPSD

308 0.5127 10.6962 0.9890 4.3286

318 0.3463 11.6853 0.9956 2.8578

328 0.1987 13.7420 0.9994 0.7179

338 0.1766 13.1581 0.9977 1.6090

Redlich-Peterson, β=+

R ee

R e1

K cq

a c

KR / l g-1 aR / l mg-1 β R2 MPSD

308 1.9833 0.0206 1.1396 0.9992 1.7783

318 1.2924 0.0011 1.6865 0.9916 6.6568

328 14.1841 2.6369 0.5432 0.9966 2.0683

338 121.4294 29.6700 0.5090 0.9965 1.9961

Table 4. Recent reported adsorption capacities q (mg/g) of non-conventional low cost adsorbents for phenol [32]

Adsorbent Adsorption capacity, q / mg g–1 Optimum conditions, w / g l-1, pH

Sawdust based Activated carbon 2.82 2, 3.5

Rice husk carbon 22 15, 2.7

Baggase fly ash 12-13 10, 6.5

Dried sewage sludge 16 5, 6.5

Neutralized red mud 5.13 1, 6

Modified attapulgite 1.4 10, 10

Olive pomace 4-5 10, 9

Macro alga 20 6, 6

Luffa cylindrical 10.37 3, 7

Chitin 12.7 3, 2

Chitin 21.5 1, 1

Mahua seed activated carbon (MSAC) 48-55 1.2, 6


271

Table 5. Thermodynamic parameter for phenol adsorption onto MSAC (t = 5 h; w = 1.2 g/l; pH 6; co, 50-150 mg/l)

co / mg l-1 -ΔH / kJ mol-1 ΔS / kJ mol-1 K-1

-ΔG / kJ mol-1

T / K

308 318 328 338

50 10.31 25.99 18.35 18.62 18.68 19.42

75 10.76 22.57 17.77 17.94 18.05 18.68

100 6.73 33.00 16.86 17.28 17.51 18.22

125 29.39 -43.72 16.21 15.29 14.77 14.40

150 4.27 -38.92 16.23 16.64 17.07 17.79

where ρ = 1000 kg/m3; the density of the solution mixture, and Kd is the distribution coefficient.

Generally, the absolute magnitude of the change in free energy for physisorption is between -20 and 0 kJ/mol and for chemisorption in the range of -80 to -400 kJ/mol. Table 5 indicates that the change in free energy for biosorption of phenol onto MSAC varies from -18.35 to -14.77 kJ/mol from temperature range 303 to 333 K, with an initial concentrations of 50-150 mg/l. Hence, this process can be considered as physisorption. The negative values of ΔG shows that the biosorption of phenol onto MSAC is spon-taneous. It can also be noted that the change in free energy decreased with an increase in temperature. The negative value of change in enthalpy (ΔH) shows that the biosorption is exothermic in nature. The negative values of ΔS suggest that the adsorbed species are stable. Immobilization of the phenol mole-cules as a result of adsorption will decrease the deg-ree of freedom of the phenol molecules and that is why the ΔS values are negative [23,34,35].

Mass transfer study

A mass transfer phenomenon was studied with the help of the following equations:

β + − = − + +

1 s0

1 1ln ln

1 1tc wK wK S t

c wK wK wK (17)

ρ ε=

−sp p p

6(1 )wS

d (18)

where co and ct are the initial phenol concentration (mg/l) and concentration at time t of the adsorbate species, respectively, w (g/l) is mass of the adsorbent. K (l/g) is the Langmuir constant, ρp (cm/s) is the mass transfer coefficient and Ss (l/cm) is the outer surface of the adsorbent per unit volume. According to above equation, surface mass transfer is controlling when t tends to zero. A plot of

− + 0ln 1 1tc c wK and t result in a straight line (Figure 6). The values of ρ1 have been calculated

from slope and intercept of the line. At co 100 mg/l, ρ1 values 5.25×10-5, 2.49×10-5 and 1.61×10-5 cm/s were obtained at temperature 308, 318 and 328 K, res-pectively, indicating that the transfer of phenol from bulk of the liquid to adsorbent surface is very rapid and the adsorbent is suitable for removal of phenol from aqueous solution. Similar findings related to determination of mass transfer coefficient for various adsorbate-adsorbent systems have been reported previously [36–41].

Figure 6. Mass transfer plot for adsorption of phenol- MSAC

system (co = 150 mg/l; t = 5 h; w = 1.2 g/l; pH 6).

Isosteric heat of adsorption

Clausius-Clapeyron equation is used to cal-culate apparent isosteric heat of adsorption ( Δ st,aH ) assuming constant surface coverage (qe, 15, 20, 30, 40 and 50 mg/g). The Clausius-Clapeyron equation is given as follows [27]:

−Δ= st,ae

2

dlnd

HcT RT

(19)

or:


272

Δ =e

est, a

d lnd(1/ ) q

cH RT

(20)

For this purpose, the equilibrium concentration (ce) at a constant equilibrium amount of adsorbed solute, qe, is obtained from the adsorption isotherm data at different temperatures. Δ st,aH is calculated from the slope of the ln ce versus 1/T plot for different qe of phenol onto MSAC [27]. The isosters corres-ponding to different equilibrium adsorption uptake of phenol by all the adsorbents is shown in Figure 7. The variation of Δ st,aH for the adsorption of phenol with the surface loading is presented in Figure 8. The Δ st,aH is increasing with increasing surface loading for MSAC indicating that MSAC have more homo-geneous surface. The dependence of heat of adsorp-

tion with surface coverage is usually observed to dis-play the adsorbent-adsorbate interaction. The nega-tive values of isosteric heat of adsorption for all the adsorbate-adsorbent systems shows that the sorption of phenol is an exothermic process [27].

CONCLUSION

This study shows that MSAC could be used as an effective adsorbent for the removal of phenol from aqueous solutions. Maximum adsorption of phenol occurred at pH 6. Equilibrium between the phenol in the solution and on the MSAC surface was practically achieved in 300 min. The sorption kinetics followed a pseudo-second-order model. The Langmuir isotherm model best represented the equilibrium sorption data adequately and satisfactorily at all temperatures. The negative values of changes in the Gibbs free energy and heat of adsorption at all the temperature indi-cated the adsorption of phenol onto MSAC is spon-taneous and exothermic in nature. The present study showed that the MSAC can be used effectively as an adsorbent for the removal of phenol from aqueous solutions.

Acknowledgements

The authors highly thank the Director of NIT Rourkela for providing financial assistance to the first author during the course of the work.

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LALLAN SINGH YADAV1

BIJAY KUMAR MISHRA1

ARVIND KUMAR2 1Centre of Studies in Surface Science and Technology, School of Chemistry,

Sambalpur University, Jyoti Vihar, Odisha, India

2Department of Chemical Engineering, National Institute of Technology,

Rourkela, Odisha, India

NAUČNI RAD

UKLANJANJE FENOLA IZ VODENOG RASTVORA POMOĆU AKTIVNOG UGLJENIKA SEMENA MAHUE: ISTRAŽIVANJE KINETIKE, RAVNOTEŽE, PRENOSA MASE I IZOSTERNE TOPLOTE ADSORPCIJE

Aktivni ugalј semena Mahue (Madhuca longifolia, MSAC) je razvijen kao efikasan adsorbent za uklanjanje fenola iz kontaminiranih otpadnih voda. Pripremlјenom MSAC su određena različita fizičko-hemijska svojstva infracrvenom spektroskopijom sa Furijeovom transformacijom (FTIR) i skenirajućom elektronskom mikroskopijom (SEM). Laboratorij-skim šaržnim eksperimentima ispitan je uticaj MSAC doze, pH, vremena kontakta i početne koncentracije fenola na efikasnost sorpcije pod optimalnim uslovima. Maksimalni kapacitet adsorpcije fenola je dobijen na pH 6, sa vremenom kontkata 5 h i MSAC dozom 1,2 g/l. Kinetički podaci o adsorpciji fenola su veoma dobro opisani kinetičkim modelom pseudo-drugog reda, a podaci o ravnoteži Langmirovom izotermom. Iz eksperimentalnih podataka izračunat je prosečni efektivni koeficijent difuzije od 6,4×10-13 m2/s. Termodina-mička proučavanja potvrdila su da je proces sorpcije spontan i egzoterman. Isosterna toplote adsorpcije fenola se povećava sa povećanjem površine adsorbenta, što ukazuje da MSAC ima homogeniju površinu.

Ključne reči: fenol, adsorpciona izoterma, prenos mase, trmodinamički parametri, izosterna toplota.

Documents

LALLAN SINGH YADAV REMOVAL OF PHENOL FROM AQUEOUS … No3_p263-273_Jul … · BIJAY KUMAR MISHRA1 ARVIND KUMAR2 1Centre of Studies in Surface Science and Technology, School of Chemistry,