adsorption studies of remazol brilliant blue r dye on...

Columbia International Publishing American Journal of Modern Chemical Engineering (2014) Vol. 1 No. 1 pp. 1-12

Research Article

______________________________________________________________________________________________________________________________ *Corresponding e-mail: [email protected] 1* School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal,

Penang, Malaysia 2 Department of Pure and Applied Chemistry, Ladoke Akintola University of Technology, P.M.B 4000,

Ogbomoso, Oyo State, Nigeria 3 Faculty of Mechanical Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya,

76100 Durian Tunggal, Melaka, Malaysia 1

Adsorption Studies of Remazol Brilliant Blue R Dye on Activated Carbon Prepared from Corncob

Mohd Azmier Ahmad1*, Olugbenga Solomon Bello2, and Mohd Azhar Ahmad3

Received 20 October, 2013; Published online 14 June 2014 © The author(s) 2014. Published with open access at www.uscip.us

Abstract The removal of Remazol Brilliant Blue R (RBBR) reactive dye from aqueous solution using corncob based activated carbon (CCAC) was investigated in this study. Adsorption studies were carried out at different initial dye concentration, contact time and solution temperature. The RBBR dye adsorption was found to increase with increase in initial concentration and contact time. RBBR adsorption equilibrium fitted the Freundlich isotherm model most. The maximum adsorption capacity for the adsorption process is 333.3 mg/g. The rate of adsorption was found to follow the pseudo second order kinetic model. Intraparticle diffusion was not the only rate limiting mechanism governing the adsorption process, external mass transfer was also involved. The result demonstrated CCAC as a feasible adsorbent for RBBR dye removal.

Keywords: Activated carbon; Adsorption; Corncob; Remazol Brilliant Blue R

1. Introduction Textile industry is one of the largest industrial producers of wastewater with high colour (Hameed and El-Khaiary, 2008). Dyes are considered an objectionable type of pollutant because they consist of toxic properties which are known to cause respiratory toxicity, carcinogenesis, mutagenesis and teratogenesis (Ahmad and Alrozi, 2011). Several methods are available for colour removal from wastewater such as membrane separation, aerobic and anaerobic degradation using various microorganisms, chemical oxidation, coagulation and flocculation, adsorption using different kind of adsorbents and reverse osmosis. Among them, adsorption is a promising removal technique that produces effluents containing very low levels of dissolved organic compounds (Chandra and Mirna, 2007). The most widely used adsorbent in adsorption process is activated carbon (AC). The

Mohd Azmier Ahmad, Olugbenga Solomon Bello, and Mohd Azhar Ahmad / American Journal of Modern Chemical Engineering (2014) Vol. 1 No. 1 pp. 1-12

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commercially coal based AC is quite expensive due to the high price of the precursor. Recently, various low-cost AC adsorbents derived from the agricultural wastes were investigated such as rambutan peel (Ahmad and Alrozi, 2011), orange peel (Khaled et al., 2009), ginger waste (Ahmad and Kumar, 2010), oil palm empty fruit bunch (Hameed et al., 2009), and banana stem fibre (Haris and Sathasivam, 2006). Corn is a significant crop which can be found all around the world. The annual production worldwide is about 520×109 kg. Corncob is the by-product generated during corn processing. Since the ratio between corn grain and corncob may reach 100:18, a large quantity of corncobs were generated as waste (Cao et al., 2006). It is proposed to convert corncob into AC which can be used as an adsorbent to adsorb pollutants in water as well as to reduce solid waste generation (Khan and Wahab, 2007). In this study, corncob based activated carbon (CCAC) was used to remove RBBR from aqueous solution. The effects of different parameters including initial RBBR concentration, contact time, solution temperature and initial pH were studied. The adsorption kinetics, isotherms and thermodynamic were then analyzed.

2. Materials and Methods 2.1. Adsorbate Remazol Brilliant Blue R (RBBR) supplied by Sigma Aldrich (M) Ltd. was used as an adsorbate. RBBR has a molecular formula of C22H16N2Na2O11S3 with molecular weight of 626.54g/mol. Fig. 1 displays the chemical structure of RBBR.

Fig. 1. Chemical structure of RBBR

2.2. Preparation of activated carbon Corncob was obtained from the local market in Parit Buntar, Perak, Malaysia. Corncob was firstly washed thoroughly and subsequently dried in an oven at 378 K for 24 h. The dried precursor was ground and sieved to the size of 1-2 mm and loaded in a stainless steel vertical tubular reactor placed in a tube furnace. Carbonization step was carried out at 973 and held for 2 h under nitrogen flow. The char produced was impregnated with Na2CO3 pellets at ratio of 1:2.5. Deionized water was then added to dissolve the Na2CO3 pellets. The mixture was then dehydrated in an oven at 373 K for 24 h to remove moisture. The activation step was done using similar reactor as in carbonization step but at final temperature of 1030 K. Once the final activation temperature was reached, the gas flow was switched from nitrogen to carbon dioxide at flow rate of 150 mL/min for 3 h. The sample was then cooled to room temperature under nitrogen flow and washed with hot deionized water to recover the remaining Na2CO3. The washing process was continued using 0.1 M HCl until the pH of the washed solution reached 6.5-7. Then, the samples were dried at 378 K for 24 h and stores in desiccators for further studies.


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2.3. Batch equilibrium studies Batch equilibrium tests were carried out for adsorption of RBBR on the CCAC at different initial RBBR concentration, contact time, solution temperature and solution. The dye concentration was measured by using UV-visible spectrophotometer (Shimadzu UV-1800) at the maximum wavelength of 590 nm. The amount of adsorbate adsorbed at equilibrium, qe (mg/g) was calculated according to Eq. (1).

( )

( )

where Co and Ce(mg/L) are the liquid-phase concentrations of adsorbate at initial and at equilibrium, respectively. V is the volume of the solution and W is the mass of adsorbent used. In order to study the effects of initial adsorbate concentration (25-300 mg/L) and contact time on the adsorption uptake, 200 mL of adsorbate solution with known initial RBBR concentration were prepared in a series of 250 mL conical flasks. 0.2 g of CCAC was added into each flask containing RBBR solution. The flasks were covered with aluminium foil and placed in an isothermal water bath shaker at constant temperature of 303 K with rotation speed of 135 rpm for 24 h. For study on effect of adsorption temperature, the experiment was carried out at 303, 318 and 333 K. 2.4. Batch kinetic studies The procedure of kinetic adsorption test was identical to that of batch equilibrium test, however the aqueous sample was taken at preset time intervals. The RBBR concentration was similarly measured and the uptake RBBR uptake at any time, qt (mg/g), was calculated by Eq. (2).

( )

( )

where Ct is the liquid-phase concentration of RBBR at any time, t.

3. Results and Discussion 3.1. Effect of contact time and dye initial concentration The effect of contact time and initial concentration on the RBBR dye uptake using CCAC at 303 K is shown in Fig. 2. It was observed that the dye adsorption was rapidly increased at initial stage of 2 h and thereafter it became slower until it attained equilibrium where no more dye can be removed from the solution. Initially a large number of surface sites of the CCAC were available for adsorption. After a lapse of time, the remaining surface sites were difficult to be occupied because of the repulsion between the solute molecules of the solid and bulk phases (Hameed and El-Khaiary, 2008a). As the initial dye concentration increase from range 25 to 300 mg/L, the RBBR adsorption uptake at equilibrium was found to increase from1.33 to 58.48 mg/g. The initial dye concentration provided a powerful driving force to overcome the mass transfer resistance between the aqueous and solid phases (Tan et al., 2009). Higher initial concentration has high number of ions competing for the available sites of CCAC resulting in higher RBBR adsorption capacity.


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Fig. 2. Effect of initial dye concentration and contact time on RBBR uptake onto CCAC

A longer time was required by the RBBR dye of higher initial concentration to reach equilibrium. The contact times needed for low RBBR dye concentrations (25-100 mg/L) to reach equilibrium were around 3-4 h. However, for high concentrations (200-300 mg/L), 22-24 h was required. Initially, RBBR dye molecules have to encounter the boundary layer effect to diffuse from boundary layer film onto the adsorbent surface. Then, the dye molecules have to diffuse into the porous structure of the adsorbent (Hameed et al., 2009). Thus, RBBR dye solution with higher initial concentration would take relatively longer contact time to attain equilibrium due to the higher amount of adsorbate molecules. 3.2. Effect of solution temperature Fig. 3 shows the adsorption capacity, qe of RBBR on the prepared CCAC at various initial dye concentrations (25-300 mg/L) at temperature of 303, 318 and 333 K. From the figure, the adsorption capacity increased from 5.68 to 67.61 mg/g with increase in temperature from 303 to 333 K, indicating the endothermic nature of the adsorption reaction. It was explained that at higher temperature, dye molecules have more sufficient energy to undergo an interaction with active sites at the surface of adsorbent. In addition, the rate of diffusion of the adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent particle was also increased (Hameed and El-Khaiary, 2008b). Similar trend was reported by Idris et al. (2011) for the removal of malachite green by using rubber seed coat based AC.

Fig. 3. Effect of solution temperature on RBBR dye uptake at various initial concentrations


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3.3. Adsorption isotherms Equilibrium isotherm equations are used to describe the experimental sorption data and the parameters obtained from Langmuir, Freundlich and Temkin isotherm models. The applicability of these isotherm equations were compared by judging the correlation coefficients, R2. The Langmuir adsorption model predicted the existence of monolayer of adsorbate at the outer surface of the adsorbent. Once an adsorbate molecule occupies a site, no further adsorption can take place. The linear form of Langmuir isotherm equation is given as (Langmuir, 1918):

e

oLoe

e CQKQq

C 11

(3)

where Ce (mg/L) is the equilibrium concentration of the RBBR dye, qe (mg/g) is the amount of RBBR adsorbed per unit mass of adsorbent. Qo (mg/g) and KL (L/mg) are Langmuir constants related to adsorption capacity and rate of adsorption, respectively. Freundlich model is based on sorption on a heterogeneous surface of varied affinities (Freundlich, 1906). The logarithmic form of Freundlich model is given as:

eFe Cn

Kq log1

loglog

(4)

where KF and n are Freundlich constants with n as a measure of the deviation of the model from linearity of the adsorption and KF (mg/g (L/mg)1/n) indicates the adsorption capacity of the adsorbent. In general, n>1 suggests that adsorbate is favourably adsorbed on the adsorbent. The higher the n value the stronger the adsorption intensity. Temkin isotherm assumes that the heat of adsorption of all the molecules in the layer would decrease linearly with coverage due to adsorbent-adsorbate interaction. The adsorption is characterized by a uniform distribution of binding energies (Temkin and Pyzhev, 1940). Temkin model is expressed as:

eT

T

e CAb

RTq ln

(5)

where RT/bT = B (J/mol), which is the Temkin constant related to heat of sorption whereas AT (L- /g) is the equilibrium binding constant corresponding to the maximum binding energy. R (8.314 J/mol K) is the universal gas constant and T (K) is the absolute solution temperature. Table 1 summarizes all the constants and correlation coefficient, R2 values obtained from the three isotherm models applied for adsorption of RBBR dye on the CCAC. On the basis of the R2, Freundlich isotherm best represents the equilibrium adsorption data with better fit as compared to the other isotherms. This proves the heterogeneous nature of the CCAC surface. The value of n is greater than unity, suggesting that RBBR dye was favourably adsorbed by CCAC.


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Table 1 Isotherm parameters for the adsorption of RBBR dye onto CCAC at different temperatures

Temperature 303 K 318 K 333 K Langmuir isotherm

Qo(mg/g) 333.3 208.3 149.27 KL(L/mg) 0.0009 0.0019 0.0036 R2 0.9435 0.9463 0.9755 Freundlich isotherm KF (mg/g) (L/mg)1/n 0.9749 0.7747 0.6474 n 1.2544 1.1396 1.0771 R2 0.9886 0.9946 0.9993 Temkin isotherm AT (L/g) 0.051 0.058 0.067 B 20.99 22.48 23.24 R2 0.9366 0.9591 0.9787

The essential characteristics of Langmuir equation can be expressed in terms of dimensionless separation factor, RL, defined as:

oL

LCK

R

1

1 (6)

where Co (mg/L) is the highest initial solute concentration whereas RL value implies that the adsorption is unfavourable (RL>1), linear (RL=1), favourable (0<RL<1), or irreversible (RL=0). Fig. 4 represents the calculated RL values versus the initial concentration of RBBR dye at 303, 318 and 333 K. It can be seen that all the RL values obtained were between 0 and 1, showing that the adsorption of RBBR dye on the CCAC was favourable at the conditions being studied. As the RBBR dye initial concentration increased from 25 to 300 mg/L, the RL values were found to decrease, indicating that the adsorption was more favourable at higher RBBR dye concentration.

Fig. 4. Effect of RBBR dye initial concentration on separation factor


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3.4. Adsorption kinetics The applicability of the pseudo-first-order and pseudo-second-order models were tested for the adsorption of RBBR onto CCAC. The pseudo-first order kinetic model found by Langergren and Svenska (1898) was widely used to predict sorption kinetic and was defined as:

( ) (7)

where qe and qt (mg/g) are the amounts of adsorbate adsorbed at equilibrium and at any time, t (h), respectively and k1 (1/h) is the adsorption rate constant. The linear plot of ln (qe-qt) versus t gives a slope of k1 and intercept of ln qe as shown in Fig. 5.

Fig. 5. Pseudo-first order kinetic plot for the adsorption of RBBR onto CCAC at 303 K

The values of k1 and R2 obtained from the plots for adsorption of RBBR on the adsorbent at 303 K are reported in Table 2. It was observed that the R2 values obtained for the pseudo-first order model did not show a consistent trend. Besides, the experimental qe values are too low compared to the calculated values obtained from the linear plots. This indicates that the adsorption of RBBR dye on the CCAC is not a first-order reaction. Table 2 Pseudo-first order and pseudo-second order kinetic model parameters for the adsorption

of RBBR dye onto CCAC at 303 K. Co (mg/L)

qe,exp

(mg/g) Pseudo-first-order kinetic model Pseudo-second-order kinetic model

qe1 (mg/g) k1 (1/h) R2 qe2 (mg/g) k2 (g/mg h) R2 25 5.68 4.35 0.49 0.945 5.84 0.21 0.987 50 10.93 7.40 0.56 0.978 10.95 0.17 0.994 100 21.93 11.53 0.59 0.973 21.56 0.15 0.999 200 40.36 20.37 0.63 0.940 40.16 0.09 0.998 250 50.80 31.95 0.52 0.911 49.75 0.04 0.980 300 58.48 38.22 0.40 0.929 52.63 0.04 0.988


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The pseudo-second-order equation by Ho and Mckay (1998) predicts the behaviour over the whole range of adsorption and is expressed as:

tqqkq

t

eet

112

2

(8)

where k2 (g/mg h) is the rate constant of second-order adsorption. The linear plot of t/qt versus t (Fig. 6) gave 1/qe as the slope and 1/k2qe

2 as the intercept. From Table 2, all the R2 values obtained from the pseudo-second order model were close to unity, indicating that the adsorption of RBBR dye on CCAC fitted well into this model.

Fig. 6. Pseudo-second order kinetic plot for the adsorption of RBBR onto CCAC at 303 K

3.5. Adsorption mechanism The kinetic results were further analyzed for the diffusion mechanism by using the intraparticle diffusion model. The intraparticle diffusion equation is expressed as:

ipit Ctkq 21

(9)

where kpi (mg/g h1/2), the rate parameter of stage i, is obtained from the slope of the straight line of qt versus t1/2. Ci represent the boundary layer effect which means that the larger the intercept, the greater the contribution of the surface sorption in the rate-controlling step (Hameed et al., 2009). The qt versus t1/2 will be linear if the intraparticle diffusion occurs. The rate limiting process is only due to the intraparticle diffusion if the plot passes through the origin. Otherwise, some other mechanisms along with intraparticle diffusion are also involved (Tan et al., 2009). Fig. 7 demonstrates the intraparticle diffusion plots for the adsorption of RBBR dye on CCAC at 303 K for various RBBR dye initial concentrations. As can be seen from the plot, the first sharper region completed within the first 15 minutes was the instantaneous adsorption and is probably due to a strong electrostatic attraction between dye and the external surface of adsorbent. The second region is a gradual adsorption stage, which can be


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attributed to intraparticle diffusion of dye molecules through the pores of adsorbent (Hameed and El-Khaiary, 2008a). In some cases, the third region existed especially when the RBBR dye initial concentration were high, which was referred to as the final equilibrium stage when intraparticle diffusion started to slow down due to the extremely low adsorbate concentration in solution (Wang et al., 2010). It was observed that the linear lines of the second and third stages did not pass through the origin. Therefore, intraparticle diffusion was not the only rate limiting-step, boundary layer effect may also be involved in the process (Tan et al., 2009).

Fig. 7. Plot of intraparticle diffusion model for RBBR dye adsorption onto CCAC at 303 K

The values of kpi, Ci and R2 obtained for the three regions from the plots are reported in Table 3. It showed that the kp values for the three regions increase as the initial dye concentration increase. The result revealed that an increase in adsorbate concentration results in an increase in the driving force, which leads to an increase in the RBBR dye diffusion rate (Weng et al., 2009). It was observed that the values of intercept increase with increase in initial dye concentration from 25-300 mg/L which is an indication of the increase in the thickness of the boundary layer (Khaled et al., 2009).

Table 3 Intraparticle diffusion model parameters for the RBBR adsorption onto CCAC at 303 K. Co (mg/L)

kp1 (mg/g h1/2)

kp2 (mg/g h1/2)

kp3

(mg/g h1/2)

C1 C2 C3 (R1)2 (R2)2 (R3)2

25 2.66 2.62 0.23 0 0.34 4.40 1 0.942 0.801 50 8.05 4.49 0.17 0 2.12 9.98 1 0.973 0.842

100 21.38 7.38 0.24 0 7.86 20.57 1 0.949 0.837 200 41.14 13.71 0.31 0 15.06 38.27 1 0.935 0.968 250 43.84 19.17 1.36 0 12.14 43.70 1 0.922 0.966 300 42.02 20.82 2.33 0 11.92 46.99 1 0.945 0.999


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3.6. Adsorption thermodynamics Thermodynamic parameters were considered to characterize the adsorption process; the standard enthalpy (ΔH0, kJ/mol), standard free energy (ΔG0, kJ/mol) and standard entropy (ΔS0, kJ/mol). The values of ΔH0 and ΔS0 were obtained from Eq. 10:

RT

H

R

SK

oo

L

ln

(10)

where R is the universal gas constant (8.314 J/mol K), T is the absolute solution temperature (K) and KL is the Langmuir isotherm constant obtained earlier (L/mg). The ΔH0 and ΔS0values were calculated from the slope and intercept of the Van’t Hoff plot of ln KL versus 1/T respectively. ΔG0

can then be calculated using the relation below:

L

o KRTG ln (11) In order to determine the activation energy of adsorption, Arrhenius equation was applied using Eq. 12:

RT

EAk a lnln 2

(12)

where k2 is the rate constant obtained from the pseudo second-order kinetic model (g/mg h), Ea is the Arrhenius activation energy of adsorption (kJ/mol) and A is the Arrhenius factor. When lnk2 was plotted against 1/T, a straight line with slope of −Ea/R was obtained. The calculated values of ∆Ho, ∆So, ∆Go and Ea for adsorption of RBBR dye on CCAC are given in Table 4. The positive value of ∆Ho further confirmed that the adsorption process was endothermic in nature and this is consistent with the results obtained earlier where the RBBR dye uptakes increase with increase in solution temperature. Table 4 Thermodynamic parameters for the adsorption of RBBR on CCAC

∆Ho(kJ/mol) ∆So (kJ/mol) Ea (kJ/mol) ∆Go(kJ/mol) 303 K 318 K 333 K

39.20 0.071 8.56 17.70 16.59 15.58 The positive values of ∆Go reflect the non-spontaneous nature of the adsorption processes at the range of temperature studied. The positive ∆Go values also revealed that the reaction rate is decreasing with an increase in temperature (Deniz and Saygideger, 2010). However, the positive value of ΔSo obtained shows the affinity of the adsorbent for adsorbate and the increased randomness at the solid-solution interface with some structural changes in the adsorbates and adsorbents during the adsorption process. This phenomenon had also been observed in the adsorption of malachite green dye from aqueous solution by Polygonumorientale Linn based AC (Wang et al., 2010). The Ea value of 8.56 kJ mol-1 indicates the feasibility of the physical adsorption process.


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4. Conclusions

In this study, CCAC was found to be suitable for RRBR dye removal. The adsorption of RBBR dye was found to increase with increase in contact time, RBBR initial concentration and solution temperature. The Freundlich isotherm model and the pseudo-second order kinetic model were proved to fit the adsorption equilibrium and kinetic data, respectively. From the thermodynamic studies, the adsorption process was endothermic and non-spontaneous in nature.

Acknowledgements The authors gratefully acknowledge the financial support received in the form of research grants from Universiti Sains Malaysia (Short term grant) and Ministry of Education Malaysia (Knowledge Transfer Programme Grant) for this project.

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http://dx.doi.org/10.1016/S0923-0467%2898%2900076-1



http://dx.doi.org/10.1021/ja02242a004


http://dx.doi.org/10.1016/j.desal.2009.12.012


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