Equilibrium and kinetic studies on biosorption of 2,4,6-trichlorophenol from aqueous solutions by Acacia leucocephala bark

Equilibrium and kinetic studies on biosorption of 2,4,6-trichlorophenol from aqueous solutions by Acacia leucocephala bark

Colloids and Surfaces B: Biointerfaces 94 (2012) 125–132 Contents lists available at SciVerse ScienceDirect Colloids and Surfaces B: Biointerfaces j...

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Colloids and Surfaces B: Biointerfaces 94 (2012) 125–132

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb

Equilibrium and kinetic studies on biosorption of 2,4,6-trichlorophenol from aqueous solutions by Acacia leucocephala bark Nadavala Siva Kumar ∗ , Heung-Sik Woo, Kim Min Department of Safety Environmental System Engineering, Dongguk University, Gyeongju 780-714, Republic of Korea

a r t i c l e

i n f o

Article history: Received 23 September 2011 Received in revised form 14 December 2011 Accepted 21 January 2012 Available online 7 February 2012 Keywords: Biosorption Acacia leucocephala bark Kinetics and isotherms modeling

a b s t r a c t Biosorption of 2,4,6-trichlorophenol (2,4,6-TCP) from aqueous solution by biomass prepared from Acacia leucocephala bark, an agricultural solid waste has been investigated in the present study. All the experiments are carried out by batch mode technique. The resulting biosorbent was characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) techniques. The effect of experimental parameters such as contact time, effect of pH (2–10), initial concentration of adsorbate (50–200 mg L−1 ) and amount of biosorbent dosage was evaluated. The removal was found to be pH dependent, and maximum removal was found to be at pH 5.0. The equilibrium time was found to be 3 h. The biosorbent dose was increased, and the percentage removal of 2,4,6-TCP was increased, while the adsorption capacity at equilibrium qe (mg g−1 ) (amount of 2,4,6-TCP loaded per unit weight of adsorbent) decreased. Biosorption kinetic and isotherm studies showed the pseudo-second-order kinetics with a good correlation coefficient (R2 = 0.999), and both Langmuir and Freundlich isotherms were the best choices to describe the adsorption behaviors. The maximum monolayer biosorption capacity of A. leucocephala bark for 2,4,6-TCP was found to be 256.4 mg g−1 , at 30 ± 1 ◦ C according to Langmuir model. This study demonstrated for the first time that the A. leucocephala bark could be an alternative for more costly adsorbents used for removal of 2,4,6-TCP from aqueous media. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Highly chlorinated phenol derivatives, such as 2,4,6trichlorophenol (2,4,6-TCP) have been commonly used as pesticides, herbicides, wood preservatives, and defoliants since the early 1930s [1]. Chlorophenols are a group of chemicals in which chlorine’s (between one and five) have been added to phenol. The main pollution sources containing chlorophenols are the wastewaters from pesticide, paint, solvent, pharmaceutics, wood, paper and pulp industries as well as drinking water treatment are the most important chlorophenol pollution sources [2]. Even low concentrations can be an obstacle to the use or reuse of water. Phenols give an unpleasant taste and odor to drinking water and can exert negative effects on different biological processes. Most of these compounds are known or suspected to be human carcinogens [3]. The chemical known as 2,4,6-trichlorophenol (TCP) is a toxic, mutagenic and carcinogenic pollutant. It is found in the emissions from fossil fuel combustion, municipal waste incineration and chlorination of water containing phenol or certain aromatic acids with hypochlorite or during disinfection of water

∗ Corresponding author. Tel.: +82 54 770 2253; fax: +82 54 770 2280. E-mail addresses: [email protected], [email protected] (N. Siva Kumar). 0927-7765/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2012.01.048

[4]. Adverse effects on the human nervous system and many health disorders have been reported to be caused by 2,4,6-TCP, such as respiratory effects, cardiovascular effects, gastrointestinal effects as well as cancer [5]. The stable C Cl bond and the position of chlorine atoms relative to the hydroxyl group are responsible for their toxicity and persistence in the biological environment [6]. In view of the high toxicity, carcinogenic properties, structural stabilization and persistence in the environment, it is necessary to removal of 2,4,6-TCP from wastewaters before discharge into water bodies. From the literature, various processes have been employed for the removal of chlorophenols from aqueous media, such as biological treatment using anaerobic granular sludge [2] and dead fungi [7], catalytic wet oxidation [5], adsorption technology using activated clay [4], fuel oil fly ash [8], membrane filtration [9], biological degradation [10], electrochemical oxidation [11] and photocatalytic degradation [12]. Other treatment technologies include air stripping, incineration, precipitation, ion exchange and solvent extraction to remove phenolic materials from aqueous solution. Traditionally, in water treatment the most popular and widely used method is adsorption onto the surface of activated carbon. Adsorption on activated carbon is one of the most effective and widely used techniques in treating high strength and low volume of phenolic wastewaters [13]. However, the activated carbon has a number of disadvantages, such as relatively high cost and tedious procedures

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for its preparation and regeneration. Polymer based adsorbents are widely employed for the removal of phenols [14–16], but the high cost of polymers has also stimulated interest in examining the feasibility of using cheaper adsorbents. This has led many researchers to search for more cost-effective and efficient adsorbents to remove organic contaminants from water and wastewater. The search for new and innovative treatment techniques has focused attention on the adsorption capacities of other adsorbents, such as agricultural by-products and lignocellulosic residues [17–19], which are readily available and do not need to be regenerated due to their low cost. The application of lignocellulosic materials in the wastewater treatment was the object of interest for several authors owing to their natural abundance in the environment or because they are wastes from industrial activities. Although pine bark fulfils these requirements its potential has been underestimated. A few studies were published suggesting good efficiencies in heavy metals removal from wastewater by pine bark [20] owing to its exchangeable surface cations. Regarding the sorption of organic pollutants by pine bark, Bras et al. and Ratola et al. [21,22] reported interesting results for organochlorine pesticides, while Haussard et al. and Chiu et al. attained acceptable sorption for hydrocarbons [23,24]. Numerous studies on the sorption of metals and organic pollutants by these alternative adsorbents in batch systems have been reported [25]. Adsorption of phenol on formaldehyde-pretreated Pinus pinaster bark via equilibrium and kinetics was studied by Vazquez et al. [26]. Recently, agricultural solid wastes have been considered as one of the most promising adsorbents. However, to the best of our knowledge, the biosorption and removal of 2,4,6-TCP from aqueous solutions by A. leucocephala bark has not been reported in literature. A. leucocephala bark, agricultural and easily available waste, could be an alternative for more costly wastewater treatment processes. Further advantages are that they are readily available and do not need regeneration. In particular, A. leucocephala’s native range through South and Southeast Asia is non-contiguous. Its largest continuous distribution is in arid India through Sri Lanka, Bangladesh, Burma and much of Thailand. A. leucocephala is an important dry-season fodder and pasture tree. Leaves, tender shoots and pods are eagerly eaten by goats, sheep and cattle. All mature A. leucocephala put on an annual layer of bark, which contributes to the increasing diameter of the stems. In some species the outermost layer dies and is annually discarded either in long strips or in variably sized flakes. Due to the high number of A. leucocephala trees in India, a massive amount of bark is produced, which is being disposed of as waste. Lignocellulosic waste materials are available in abundance, renewable and low in cost. The A. leucocephala bark fulfils these requests, and its potential has been under estimated. A few studies were published suggesting good efficiencies in heavy metals and phenolic compounds removal from aqueous solution by A. leucocephala bark [27–29]. In the interest of the environment, according to authors’ survey, there is no extensive study on the biosorptive removal of 2,4,6-trichlorophenol from aqueous solutions using A. leucocephala bark in literature. In addition, this material was chosen as novel biosorbent in this study due to being of its naturally abundant, renewable and thus costeffective biomass. The objective of the present work is to introduce a new low cost adsorbent such as A. leucocephala bark and also to examine its effectiveness in removing 2,4,6-TCP from aqueous solutions. This adsorption study has not been reported in literature. The A. leucocephala bark was characterized by scanning electron microscopy (SEM). Further this A. leucocephala bark was characterized before and after adsorption of 2,4,6-TCP by Fourier transform infrared (FTIR) spectroscopy studies. The influence of experimental parameters such as pH, contact time, biosorbent dosage and initial 2,4,6-TCP concentrations were studied. The sorption of 2,4,6-TCP

at solid–liquid interfaces has been studied extensively under equilibrium conditions. Further the kinetic and the equilibrium models involved in the sorption process were evaluated at different initial 2,4,6-TCP concentrations. 2. Materials and methods 2.1. Materials A. leucocephala bark was collected from a local A. leucocephala tree near Pullampet, Kadapa District, Andhra Pradesh, India, for the preparation of biosorbent. Materials including, 2,4,6trichlorophenol were purchased from Aldrich (St. Louis, USA) and were not purified prior to use. The chemical 2,4,6-TCP has a formula of C6 H3 Cl3 O, with molecular weight of 197.46 g mol−1 . Stock solution was prepared by dissolving 1.0 g of TCP in 1 L of double distilled water. This stock solution was used to prepare 50, 100, 150, and 200 mg L−1 solutions of 2,4,6-TCP. The solution pH was adjusted by adding 0.1 M HCl and 0.1 M NaOH solutions. Water used for preparation of solutions and cleaning adsorbents was generated in the laboratory by double distilling the deionized water in a quartz distillation unit. 2.2. Preparation of biosorbent A. leucocephala bark was thoroughly washed with distilled water to remove mud and dirt. Then A. leucocephala bark was soaked in 0.1 M NaOH to remove lignin based color materials followed by 0.1 M H2 SO4 . Finally it was washed with distilled water several times and dried in an oven at 80 ◦ C for 6 h and cooled at room temperature in desiccators. The physical, chemical and surface characterization of A. leucocephala bark was carried out using standard methods, and the results are given in Table 1. The dried A. leucocephala bark was stored in desiccators until used. The dried bark was ground to fine powder sieved to 55–75 ␮m mesh with standard testing sieve and used as biosorbent without any pretreatment for 2,4,6-TCP adsorption. 2.3. Batch studies Batch equilibrium tests were carried out for adsorption of 2,4,6TCP on the A. leucocephala bark. In order to explore the effect of influencing factors, such as solution pH, contact time, quantity of adsorbent dosage, and the adsorbate initial concentration of 2,4,6-TCP, a series of batch experiments were conducted. For adsorption equilibrium, experiments were conducted in a set of 125 mL Erlenmeyer flasks, where 100 mL of 2,4,6-TCP solutions with different initial concentrations (50–200 mg L−1 ) were placed. An amount of 0.1 g of A. leucocephala bark was transferred to the flask before the addition of the 2,4,6-TCP solution. The pH of the experimental solutions was adjusted from 2.0 to 10.0 by using 0.1 M NaOH or HCl. The solutions were stirred on a shaking Water Bath Table 1 Physical, chemical and surface characterization of Acacia leucocephala bark biosorbent. Parameter

Value

Color Odor Weight loss (%) Apparent (bulk) density (g cm3 ) Moisture content (%) Ash content (%) BET surface area (m2 g−1 ) Carbon (%) Hydrogen (%)

Light yellowish None 54.8 0.251 7 4.88 0.27 44.8 5.65

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Temperature controller of 220 rpm at 30 ± 1 ◦ C. For equilibrium studies, the experiment was carried out for 3 h to ensure equilibrium was reached. Samples were taken from the tubes, and the solutions were separated from the adsorbent by filtering through Whatman No. 50 filter paper (2.7 ␮m size particle retention) to eliminate any fine particles. Then the concentrations of 2,4,6-TCP in the supernatant solutions before and after adsorption were determined by measuring absorbance using UV–vis spectrophotometer (Shimadzu UV-1601 Spectrophotometer, Japan) at its maximum wavelength of 296 nm. The 2,4,6-TCP uptake at equilibrium, qe (mg g−1 ), was calculated from the following equation: qe =

(C0 − Ce )V W

(1)

where C0 and Ce (mg L−1 ) are the liquid-phase concentrations of 2,4,6-TCP at initial and equilibrium, respectively. V (L) is the volume of the solution, and W (g) is the mass of the dry bark used. For batch kinetic studies, the same procedure was followed, but the aqueous samples were taken at preset time intervals. The concentrations of 2,4,6-TCP were similarly measured. The 2,4,6-TCP uptake at a time t, qt (mg g−1 ), was calculated by qt =

(C0 − Ct )V W

(2)

where Ct (mg L−1 ) is the liquid-phase concentration of 2,4,6-TCP at any time, t (min). 3. Results and discussion 3.1. Characterization of A. leucocephala bark 3.1.1. Fourier transform infrared (FTIR) studies The FTIR spectral analysis is important in identifying the different functional groups of the biosorbent surface which are responsible for biosorption of 2,4,6-TCP. FTIR spectra in the range of 4000–500 cm−1 for the A. leucocephala bark before biosorption and after biosorption of 2,4,6-TCP are shown in Fig. 1(a) and (b), respectively. The spectra display a number of absorption peaks, indicating the complex nature of the biosorbent material examined. Fig. 1(a) and (b) shows the FTIR spectra had a broad and strong absorption band peaks at 3426–3442 cm−1 suggests the presence of OH and NH2 groups. On the other hand, the two peaks appearing at 2920–2921 cm−1 represent the asymmetrical

0.7

% Transmittance

(b)

1510 1423 1652 1060

2921

655

3442

0.6

1511 1319

2920

537

1644

(a) 0.5 3426

4000

3500

3000

2500

2000

1500

1000

500

0

Wavenumbers cm-1 Fig. 1. FTIR spectra of 2,4,6-trichlorophenol: (a) before biosorption of Acacia leucocephala bark and (b) after 2,4,6-TCP biosorption.

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and symmetrical stretching vibration of methylene ( CH2 ) group, respectively. The complicated nature of the adsorption bands in the 1652–1500 cm−1 range suggests that aromatic ring bands and double bond (C C) vibrations overlap the aforesaid C O stretching vibration bands and OH binding vibration bands. The peak appearing at 1652 cm−1 arises from C O stretching in amide groups. The peak at 1060 cm−1 corresponds to C O stretching vibration of alcohols and carboxylic acids. As we observe in Fig. 1(b) a significant difference can be seen in the FTIR spectra of biosorbent before and after biosorption. The band at 3426 cm−1 corresponding to OH and NH groups shifts to the higher frequency (3442 cm−1 ). The FTIR band of C O stretching shifted to higher frequency due to involvement of carboxyl ( C O) group in the adsorption process of 2,4,6-TCP with pure bark. This shift and/or broadening of some of the FTIR spectral peaks of the biosorbent in the presence of the 2,4,6-TCP studied provides a clear indication that the functional groups like NH2 , OH and –C O present on the A. leucocephala bark surface are involved in 2,4,6-TCP adsorption. 3.1.2. Scanning electron microscopic (SEM) Studies For morphological characteristics SEM of biosorbent A. leucocephala bark was carried out. Scanning electron microscopy (SEM; Model Evo15, Carl Zeiss, England) is widely used to study the morphological features and surface characteristics of the biosorbent material. In the present study, scanning electro-micrographs (SEM) show the surface texture and morphology of the biosorbent (Fig. 2(a) and (b)). It is evident from the micrograph that the biosorbent has a well-defined rod clusters in net/mat format (500×). At 2.50× magnification, the biosorbent has an irregular structure, thus making it possible for the biosorption of 2,4,6-TCP on different parts of the biosorbent. 3.2. Effect of pH Solution pH usually influences the adsorption to a large extent, as it affects the properties of both adsorbent and adsorbate. Optimization of pH for adsorption medium plays a vital role in biosorption studies. The pH of the adsorption medium is the most significant parameter in the treatment of chlorophenols by the adsorbent [30]. During the current investigations removal was studied in both the acidic and alkaline ranges of pH. Fig. 3 shows the effect of solution pH on the 2,4,6-TCP removal on the A. leucocephala bark. In order to optimize the pH for maximum removal efficiency, experiments were conducted in the pH range from 2.0 to 10.0 using 0.1 g of A. leucocephala bark with 100 mL of 100 mg L−1 initial 2,4,6-TCP concentrations at 30 ± 1 ◦ C. In acidic range the pH was varied using 0.1 M HCl while in alkaline range the pH was varied using 0.1 M NaOH. As can be seen from Fig. 3, the 2,4,6TCP removal was found to decrease significantly with increase in initial pH of the solution from pH 2.0 to 10.0. In this study, the highest 2,4,6-TCP removal was achieved at pH 5.0, with 2,4,6-TCP adsorption capacity as high as 75.68 mg g−1 , at 2,4,6-TCP initial concentration of 100 mg L−1 . But, adsorption capacity was considerably decreased when the pH of the initial solution was above 5.0. The 2,4,6-TCP uptake was the highest where the pH was below the pKa (pKa value 5.99, at ambient temperature) of 2,4,6-TCP. The lower pKa value is associated with the electron withdrawal effect of the chlorine substitution on the aromatic ring, thus reducing the overall electron density of the aromatic ring of the adsorbate [31]. This was because at acidic pH, the 2,4,6-TCP was undissociated and the dispersion interactions predominated. However, at basic pH, the 2,4,6-TCP dissociated, forming phenolate anions, while the surface functional groups are either neutral or negatively charged. The electrostatic repulsion between the identical charges lowers the adsorption capacities. Besides, the phenolate anions have higher solubilities in aqueous solution and form stronger adsorbate–water

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100 80

% removal

70 90

% removal

60

85

50 40

80

Adsorption capacity (mg g -1 )

30

75

20

Adsorption capacity (mg g-1)

90 95

10 70 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Amount of adsorbent dose (g) Fig. 4. Effect of biosorbent dosage level on the biosorption of 2,4,6-trichlorophenol onto Acacia leucocephala bark [% removal of 2,4,6-TCP and biosorption capacity (mg g−1 )]. Experimental conditions: for 2,4,6-TCP: initial concentrations = 100 mg L−1 , biosorbent dosage = 0.1–0.8 g, contact time 4 h, Temp = 30 ± 1 ◦ C, pH 5.0. The error bars represent ±SD with n = 3.

bonds. So the adsorption is more difficult at higher pH. The decrease in adsorption capacity may also be due to the competing hydroxide ions. On the whole, the protonated phenolic compounds dominating at low pH were more adsorbable than the ionized forms. A similar trend was reported in the adsorption of TCP on coconut shell-based activated carbon [13], oil palm empty fruit bunch-based activated carbon [32] and activated clay [5] as well as adsorption of 2,4,6-trichlorophenol on coconut husk-based activated carbon [33]. From the experimental results, pH 5.0 was selected as an optimum pH value. 3.3. Effect of biosorbent dosage

Fig. 2. Scanning electron micrographs of the Acacia leucocephala bark at (a) 500× and (b) 2.50× magnification.

Adsorption capacity (mg g-1)

80 70

The effect of different doses of the A. leucocephala bark on removal of 2,4,6-TCP was carried out, and the results have been presented in Fig. 4. The amount of adsorbent was varied from 0.1 to 0.8 g while all the variables such as pH, rpm, contact time, and temperature were kept constant. It can be seen from Fig. 4, the percentage removal of 2,4,6-TCP increased with the increase in adsorbent dose while loading capacity, qe (mg g−1 ) (amount of 2,4,6-TCP loaded per unit weight of adsorbent) gradually decreased. It is clear from Fig. 4 that the percent removal of 2,4,6-TCP increases by increasing adsorbent dose and it reached 95% when the dose was 0.5 g. This increase in loading capacity is due to the availability of higher number of solutes (2,4,6-TCP) per unit mass of adsorbent, i.e., higher solute/adsorbent ratio. 3.4. Effect of contact time and initial concentration

60 50 TCP (100 mg L-1 )

40 30 20 1

2

3

4

5

6

7

8

9

10

11

Solution pH Fig. 3. Effect of pH on the biosorption of 2,4,6-trichlorophenol onto Acacia leucocephala bark. Experimental conditions: for 2,4,6-TCP: initial concentrations = 100 mg L−1 , contact time 4 h, biosorbent dosage = 0.1 g, Temp = 30 ± 1 ◦ C, agitation rate 220 rpm. The error bars represent ±SD with n = 3.

Two parameters, namely, contact time and initial concentration, have a pronounced effect on the removal of adsorbate species from aqueous solutions. In the present studies, the effect of initial concentration on the removal of 2,4,6-TCP from aqueous solutions was carried out. All the other parameters like temperature (30 ± 1 ◦ C), mass of adsorbents, initial concentration and contact time were kept constant. Adsorptive uptake of 2,4,6-TCP (mg g−1 ) increased with increase in contact time and reached equilibrium. The removal is rapid in the initial stages, decreases slowly, and acquires a maximum at the time of equilibrium, viz., 3 h. This result is interesting because equilibrium time is one of the important parameters for economical wastewater treatment applications. The same result was found by Hameed in adsorption of the 2,4,6trichlorophenol onto activated clay [5]. However, for adsorbate solutions with higher initial concentrations, lower equilibrium times were required. Based on this result, the contact time was

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-1

Adsorption capacity (mg g )

160

3.5.2. Pseudo-second-order kinetic model The pseudo-second-order kinetic model can be given in the following form [35]:

-1

50 mg L -1 100 mg L -1 150 mg L -1 200 mg L

140 120

1 1 t = + t qt qe k2 q2e

100 80 60 40 20 0 0

30

60

90

120

150

180

210

240

270

Time (min) Fig. 5. Effect of contact time on 2,4,6-trichlorophenol biosorption [different initial 2,4,6-TCP concentrations. () C0 = 50 mg L−1 , (䊉) C0 = 100 mg L−1 , () C0 = 150 mg L−1 , () C0 = 200 mg L−1 ; pH = 5.0; biosorbent dosage = 0.1 g; contact time 4 h, agitation rate: 220 rpm, Temp = 30 ± 1 ◦ C]. The error bars represent ±SD with n = 3.

fixed at 3 h for the rest of the batch experiments to make sure that equilibrium was reached in all cases. It is clear from Fig. 5 that the graphs are single and smooth, indicating monolayer coverage of the adsorbent surface by 2,4,6-TCP.

3.5. Biosorption kinetics study The kinetics of biosorption describes the rate of adsorbate uptake onto A. leucocephala bark and it controls the equilibrium time. The kinetic adsorption data were investigated to understand the dynamics of the adsorption process in terms of the order of the rate constant. The pseudo-first-order, pseudo-second-order kinetic models and Elovich equation were applied to study the kinetics of the adsorption process, as the intraparticle diffusion model was further tested to determine the diffusion mechanism of the adsorption system. The kinetics is fitted with the pseudo-first-order and pseudosecond-order models, which are extensively used in kinetic studies. The comparison was done between the experimental data, calculated data and the regression coefficient (R2 ).

3.5.1. Pseudo-first-order kinetic model The pseudo-first-order equation given by Langergren and Svenska [34] can be expressed as: ln(qe − qt ) = ln qe − k1 t

129

(3)

where qe and qt are the amounts of 2,4,6-TCP adsorbed at equilibrium and at time t in mg g−1 , respectively, and k1 is the pseudo-first-order rate constant (min−1 ). The values of ln(qe − qt ) are calculated from the experimental data and plotted against t; k1 is calculated from the slope of the plot. From Table 2, for the first-order kinetic model, the obtained R2 values were surprisingly low. The calculated qe (cal) were much lower than the experimental qe (exp), which might be attributed to the adsorption rate, as a result the values of ln(qe − qt ) varied considerably. Therefore, the adsorption of 2,4,6-TCP on the A. leucocephala bark is not a pseudo-first-order reaction, especially compared to the pseudosecond-order kinetic model which agrees well with the reaction process.

(4)

where k2 is the rate constant of pseudo-second-order adsorption (g mg−1 min−1 ). The values of t/qt are plotted against t, qe and k2 and are calculated from the slope and intercept of the plot. The linear plot of t/qt versus t determined the slope 1/qe and the intercept 1/k2 q2e . From Table 2, the values of the correlation coefficient for the second-order kinetic model was found to be in the range 0.997–0.999, indicating excellent applicability of the pseudosecond-order kinetic model to describe the adsorption process of 2,4,6-TCP on A. leucocephala bark. Based on higher correlation coefficients (R2 ) and agreement between experimental and calculated qe , values are closer to unity for pseudo-second-order kinetic model; therefore, the biosorption kinetics could well be approximated more favorably by pseudo-second-order kinetics model rather than pseudo-first-order kinetics for 2,4,6-TCP. 3.5.3. Elovich equation The Elovich equation is commonly used to determine the kinetics of chemisorption of gases onto heterogeneous solids, and in recent years this equation has been found to be valid to describe the sorption of pollutants from aqueous solution [36]. The equation has been applied satisfactorily to some chemisorption processes and has been found to cover a wide range of slow adsorption rates. The Elovich equation is one of the most useful models for describing chemisorption. The Elovich equation could be written in the following form [37]: qt =

1 b

ln(ab) +

1 ln t b

(5)

where a (mg g−1 min−1 ) is the initial sorption rate and b (g mg−1 ) is the desorption constant related to the extent of surface coverage and activation energy for chemisorption. The parameters (1/b) and (1/b)ln(ab) can be obtained from the slope and intercept of the linear plot of qt versus ln t. The R2 values obtained from Elovich equation was in the range of 0.885–0.962 for 2,4,6-TCP initial concentrations of 50–200 mg L−1 (Table 2). The calculated qe values from Elovich model agreed quite well with the experimental equilibrium concentration values. This suggests that the sorption system studied belongs to the pseudo-second-order kinetic model based on the assumption that the rate determining step may be chemisorption, involving valence forces through sharing or exchange of electrons between adsorbent and adsorbate. 3.5.4. Validity of kinetic models The applicability of the three kinetic models above to describe the adsorption process was further validated by the normalized standard deviation. In order to quantitatively compare the applicability of each model, a normalized standard deviation qt (%) was calculated, which is defined as:



qt (%) = 100 ×

 [(qt,exp − qt,cal )/qt exp ]2 N−1

(6)

where the subscripts ‘exp’ and ‘cal’ refer to the experimental and calculated values, respectively and N is the number of data points. The lower the value of qt (%) in the model is a better fit for the data. The calculated rate constants for the models, their corresponding regression (R2 ) and normalized standard deviation values are listed in Table 2. The qt (%) obtained for the pseudo-first-order kinetic model ranged from 10.27% to 12.78% for 2,4,6-TCP. The initial concentration ranged from 50 to 200 mg L−1 , which was relatively high as compared to the qt (%) values of 0.002–0.034% and 0.05–0.049%

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Table 2 Biosorption rate constants of 2,4,6-TCP on Acacia leucocephala bark. The error bars represent ±SD with n = 3. Parameter

Initial concentration of 2,4,6-trichlorophenol (mg L−1 ) 50

100

150

200

Pseudo-first-order-kinetic model q(e, exp) (mg g−1 ) q(e, cal) (mg g−1 ) k1 (min−1 ) R2 qt (%)

42.21 ± 3.13 12.03 ± 1.07 0.008 ± 0.002 0.993 12.78

74.56 ± 2.45 26.77 ± 3.60 0.014 ± 0.001 0.963 10.27

103.13 ± 3.33 33.41 ± 9.68 0.013 ± 0.003 0.913 11.42

143.54 ± 2.27 40.97 ± 6.45 0.014 ± 0.001 0.942 12.76

Pseudo-second-order-kinetic model q(e, cal) (mg g−1 ) k2 (g mg−1 min−1 ) R2 qt (%)

40.65 ± 2.29 0.002 ± 0.0004 0.998 0.034

76.33 ± 2.61 0.001 ± 0.0006 0.997 0.014

104.16 ± 3.97 9.72 × 10−4 ± 0.0007 0.997 0.002

144.92 ± 2.42 8.28 × 10−4 ± 0.0003 0.999 0.002

Elovich equation q(e, cal) (mg g−1 ) (1/b)ln(ab) (mg g−1 ) 1/b (mg g−1 ) R2 qt (%)

40.34 ± 2.36 16.45 ± 0.52 4.59 ± 0.36 0.885 0.049

73.14 ± 2.12 28.42 ± 1.96 8.61 ± 0.78 0.962 0.009

100.86 ± 3.06 46.16 ± 2.09 10.53 ± 0.95 0.936 0.012

141.44 ± 2.04 74.25 ± 10.22 12.94 ± 1.61 0.955 0.005

Weber–Morris kid C R2

1.065 ± 0.10 26.60 ± 1.14 0.938

1.971 ± 0.19 47.66 ± 0.41 0.995

2.437 ± 0.24 69.47 ± 0.65 0.981

2.914 ± 0.36 103.55 ± 6.53 0.988

obtained for the pseudo-second-order kinetic model and Elovich equation, respectively. The results suggest that the higher R2 values obtained with the pseudo-second-order kinetic model, the lower qt (%) values and the calculated qe (cal) values are closer to the experimental data than the calculated values of pseudo-first-order kinetic model. The result suggested that the pseudo-second-order adsorption is predominant and that chemisorption mainly controls the rate of the TCP adsorption process [38,39]. The similar phenomena have also been observed in the adsorption of TCP on activated clay [5], and oil palm empty fruit bunch-based activated carbon [32] were also found to be best represented by the pseudosecond-order kinetic model. 3.5.5. Intraparticle diffusion study The above kinetic models were not able to identify the diffusion mechanisms and rate controlling steps in the adsorption process, the intraparticle diffusion model based on the theory proposed by Weber and Morris [40] was tested. It is an empirical functional relationship, common to most adsorption processes, where uptake varies almost proportionally with t1/2 rather than with the contact time t. According to this theory, the intraparticle diffusion equation can be expressed as: qt = kid t 1/2 + C

(7)

where qt (mg g−1 ) is the amount adsorbed at time t (min), kid (mg g−1 min1/2 ) is the intraparticle diffusion rate constant and C is the value of intercept which gives an idea about the boundary layer thickness, i.e., the larger intercept; the greater is the boundary effect. Thus, the intraparticle diffusion constant kid can be obtained from the slope of the plot of qt versus the square root of time. The kid values obtained from the slope of the linear portions of the curve of different initial concentrations are shown in Table 2. If the adsorption process follows the intraparticle diffusion model, then qt versus t1/2 will be linear, and if the plot passes through the origin, then intraparticle diffusion is the sole rate limiting step. Otherwise, some other mechanism along with intraparticle diffusion is also involved. The plots do not present a linear relationship over a period of time, but the lines did not pass through the origin, which suggested that intraparticle diffusion was present [41]. However, this was not the only rate controlling step, some other rate

controlling steps might be involved and may affect the adsorption of 2,4,6-TCP. 3.6. Biosorption isotherm models Several mathematical models have been applied for describing equilibrium studies for the removal of pollutants by adsorption on solid surfaces. Equilibrium modeling for the process of removal of 2,4,6-TCP was carried out by using the Langmuir, Freundlich, Temkin and Dubinin–Radushkevich adsorption isotherm models. Adsorption isotherm is basically important to describe how adsorbate interacts with adsorbents and is critical in optimizing the use of adsorbents. Selection of an isotherm equation depends on the nature and type of the system. The Langmuir model assumes that adsorption takes place at specific homogeneous sites on the surface of the adsorbent and also, when a site is occupied by an adsorbate molecule, no further adsorption can take place at this site. The Langmuir equation is expressed by the following expression [42]: 1 1 1 = + qe qm KL Ce qm

(8)

where qe (mg g−1 ) is the amount of 2,4,6-TCP adsorbed per unit mass of adsorbent, Ce (mg L−1 ) is the equilibrium concentration of the 2,4,6-TCP, qm is the monolayer biosorption capacity of the adsorbent (mg g−1 ) and KL (mg L−1 ) is the Langmuir equilibrium constant, respectively. The graph of 1/qe versus 1/Ce was plotted to determine qm and KL . Table 3 Isotherm parameters of Langmuir, Freundlich, Temkin and D–R isotherms for biosorption of 2,4,6-TCP on Acacia leucocephala bark. R2

Isotherms

Constants

Langmuir

qm (mg g−1 ) 256.4

b (L mg−1 ) 0.002

0.999

Freundlich

KF ((mg g−1 ) (L mg−1 )1/n ) 1.754

n 1.032

0.995

bT 196.07

0.9043

E (kJ mol−1 ) 13.36

0.8107

−1

Temkin

A (L g 6.98

Dubinin–Radushkevich

qs (mg g−1 ) 3.512

)

N. Siva Kumar et al. / Colloids and Surfaces B: Biointerfaces 94 (2012) 125–132

The essential characteristics of Langmuir isotherm can be expressed by a dimensionless constant called separation factor (or equilibrium parameter), RL , which is defined by Weber and Chakkravorti as [43], RL =

1 1 + KL C0

1 log Ce n

(10)

where KF and n are Freundlich constants with n giving an indication of how favourable the adsorption process and KF ((mg g−1 )(L mg−1 )1/n ) is the biosorption capacity of the adsorbent. The plot of log qe versus log Ce gave a straight line with slope of 1/n and intercept of log KF . The slope of 1/n ranging between 0 and 1 is a measure of adsorption intensity or surface heterogeneity, becoming more heterogeneous as its value gets closer to zero [44]. The value of 1/n below one indicates a normal Langmuir isotherm while 1/n above one is indicative of cooperative adsorption. Temkin isotherm [45] contains a factor that explicitly takes into account the adsorbent–adsorbate interactions. The heat of adsorption of all the molecules in the layer would decrease linearly with coverage due to adsorbent–adsorbate interactions. The adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy. The linear form of the Temkin isotherm is expressed as: qe =

 RT  bT

(8.314 J mol−1 K) is the universal gas constant and T (K) is the absolute solution temperature. Another model for the analysis of isotherms of a high degree of rectangularity is Dubinin–Radushkevich isotherm which has the following form [46],

(9)

where C0 (mg L−1 ) is the initial amount of adsorbate and KL is the Langmuir sorption constant (L mg−1 ) described above. RL indicates the nature of adsorption. Adsorption is favorable when the value of RL is between 0 and 1. The RL parameter is considered as a more reliable indicator of the adsorption. The value of RL indicates the type of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0). The value of RL also lies between 0.701 and 0.903 which shows that the adsorption is favorable. The value of RL is less than 1 and great than 0, suggesting the favourable uptake of 2,4,6-TCP by A. leucocephala bark. Freundlich isotherm is suitable for a highly heterogeneous surface. The application of the Freundlich equation suggests that sorption energy exponentially decreases on completion of the sorption centers of an adsorbent. The well-known logarithmic form of Freundlich isotherm is given by the following form: log qe = log KF +

131

ln A +

 RT  bT

ln Ce

(11)

where RT/bT = B (J mol−1 ), which is the Temkin constant related to heat of sorption whereas A (L g−1 ) is the equilibrium binding constant corresponding to the maximum binding energy. R

qe = qs exp(−BD–R ε2 )

(12)

where qs is the Dubinin–Radushkevich monolayer capacity (mg g−1 ), B a constant related to sorption energy, and ε is the Polanyi potential which is related to the equilibrium concentration as follows



ε = RT ln 1 +

1 Ce

 (13)

where R is the gas constant (8.314 J mol−1 K−1 ) and T is the absolute temperature(K). The constant BD–R gives the mean free energy E of adsorption per molecule of the adsorbate when it is transferred to the surface of the solid from infinity in the solution and can be computed by using the relationship: E=



1

(14)

2BD–R

A plot of ln(qe ) versus ε2 enables the constants qs and E to be determined from the slope and intercept, respectively. The purpose of applying equilibrium data to the D–R model is mainly to clarify the adsorption type and evaluate the nature of interaction between sorbate and solid. The value of E (13.36 kJ mol−1 ) is between 8 and 16 kJ mol−1 which shows that the adsorption process is associated with chemical ion-exchange mechanism [47]. All the correlation coefficient, R2 values and the constants obtained from the four isotherm models applied for adsorption of 2,4,6TCP on the A. leucocephala bark are summarized in Table 3. Since the value of R2 nearer to 1 indicates that the respective equation better fits the experimental data. The Langmuir and Freundlich gave the highest R2 values which were greater than 0.995, showing that the adsorption of 2,4,6-TCP on the A. leucocephala bark was best described by these two models. The experimental data yielded excellent fits within the following isotherms order: Langmuir > Freundlich > Temkin > Dubinin–Radushkevich, based on R2 values. Table 4 lists the comparison of the maximum monolayer adsorption capacity of various types of chlorophenols on various adsorbents. It is clear from this table that the adsorption capacity of the A. leucocephala bark used in the present studies is significant.

Table 4 Comparison of monolayer adsorption capacities, qm (mg g−1 ), of various chlorophenols on various adsorbents. Adsorbent

Adsorbate

Sorption capacity (qm , mg g−1 )

Reference

Acacia leucocephala bark Anaerobic granular sludge Activated clay Coconut shell-based activated carbon Acacia leucocephala bark powder EFB-based activated carbon Coconut husk-based activated carbon Coconut husk Coir pith carbon (ZnCl2 activation) MMIPs (magnetic molecularly imprinted polymers) Loosestrife-based activated carbon Rice straw-based carbon Palm pith carbon Coir pith carbon Cattail fiber activated carbon (CFAC) CTS/␥-Fe2 O3 /FACs magnetic composite Granular activated carbon

2,4,6-Trichlorophenol 4-Chlorophenol 2,4,6-Trichlorophenol 2,4,6-Trichlorophenol 2-Chlorophenol and 4-chlorophenol 2,4,6-Trichlorophenol 2,4,6-Trichlorophenol 2,4,6-Trichlorophenol 2,4,6-Trichlorophenol 2,4,6-Trichlorophenol 2,4,6-Trichlorophenol 3-Chlorophenol 2,4-Dichlorophenol 2,4-Dichlorophenol 2,4,6-Trichlorophenol and 2,4-dichlorophenol 2,4,6-Trichlorophenol 2,4,6-Trichlorophenol

256.4 6.32 123.46 122.33 147.05 and 181.81 500.00 716.10 191.73 172 246.73 367.65 14.20 19.16 19.12 192.31 and 142.86 106.4 847.11

Present study [2] [5] [13] [29] [32] [33] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57]

132

N. Siva Kumar et al. / Colloids and Surfaces B: Biointerfaces 94 (2012) 125–132

4. Conclusions The present investigation showed that A. leucocephala bark was a promising low-cost biosorbent to be used in the removal of 2,4,6TCP from aqueous medium over a wide range of concentrations. The following conclusions are made based on the results of the present study: • The biosorbent was characterized by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) techniques. • Biosorption of 2,4,6-TCP was found to increase with increase in contact time and initial concentration while acidic solution pH 5 was more favourable for the adsorption of 2,4,6-TCP on the A. leucocephala bark. • When the A. leucocephala bark biosorbent dosage was increased, the equilibrium adsorption capacity (mg g−1 ) of A. leucocephala bark gradually decreased, whereas the percent removal efficiency increased. • The results showed good correlation coefficients and agreement between experimental and calculated values of qe , and lower qt (%) values showed that the 2,4,6-TCP adsorption followed pseudo-second-order kinetic model and Elovich equation rather than the pseudo-first-order kinetics. The results of the intraparticle diffusion model suggest that intraparticle diffusion was not the only rate controlling step. • Equilibrium data were fitted to linear models of Langmuir, Freundlich, Temkin and Dubinin–Radushkevich, and the equilibrium data were best described by the Langmuir and Freundlich isotherm models, with maximum monolayer biosorption capacity of 256.4 mg g−1 at 30 ± 1 ◦ C. Based on all the above results, it can be concluded that the A. leucocephala bark is an effective and alternative biosorbent, for the removal of 2,4,6-TCP from aqueous medium in terms of high biosorption capacity, abundantly available in nature at low cost. References [1] [2] [3] [4] [5] [6]

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