Adsorption of Reactive Dyes by Palm Kernel Shell Activated Carbon: Application of Film Surface and Film Pore Diffusion Models

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1 ISSN: ; CODEN ECJHAO E- Chemistry htt:// 2009, 6(4), Adsortion of Reactive Dyes by Palm Kernel Shell Activated Carbon: Alication of Film Surface and Film Pore Diffusion Models M. M. NOUROUZI, T.G. CHUAH * and THOMAS S.Y.CHOONG Deartment of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra, Malaysia, UPM Serdang, Selangor, Malaysia. chuah@eng.um.edu.my Received 1 October 2008; Revised 17 February 2009; Acceted 15 March 2009 Abstract: The rate of adsortion of two reactive dyes, Reactive Black 5 and Reactive Red E onto alm kernel shell-based activated carbon was studied. The exeriment was carried out to investigate three models: film diffusion model, filmsurface and film-ore diffusion models. The results showed that the external coefficients of mass transfer decreased with increasing of initial adsorbate concentration. In addition, it was found that the adsortion rocess was better described by using the two resistance models, i.e. film-surface diffusion. Keywords: Adsortion, Reactive dye, Palm kernel shell, Film-surface-ore diffusion. Introduction Industries, such as textile, ceramic, aer, rinting and lastic, use dye as their raw material, thus generating a large amount of colored wastewater. Adsortion methods are romising in decolorizing textile effluents, but this alication is limited by the high cost of adsorbents. The activated carbon derived from agricultural wastes is imortant due to the fact that it is inexensive, adequate to remove organic and inorganic contaminants from wastewater and locally available 1. Malaysia is a major roducer of alm oil. In 2002 alone, the amount of alm kernel shell generated in Malaysia was aroximately 4.3 million tons which can cause serious disosal roblem 2. Considering the volume of this waste, several studies were initiated to utilize Palm Kernel Shell as the crude material for activated carbon and it is reorted that a good quality roduct can be obtained, such as its granular structure, insolubility in water, chemical stability and high mechanical strength 2,3. It is imortant to be able to redict the rate at which ollutant is removed from aqueous solutions in order to design aroriate sortion treatment lants. Therefore, the kinetics that describes the dye utake rate needs to be determined. The adsortion mechanism can be described by three essential stes 4 :

2 950 T.G. CHUAH et al. 1. External mass transfer from bulk solution to adsorbent surface across the boundary layer surrounding the adsorbent surface article. 2. Intraarticle diffusion within the internal structure of article. Internal diffusion is diffusion of molecule inside the ores and surface diffusion is diffusion of the molecules on surface hase. 3. Adsortion at an interior site. Generally, the total rate of the kinetic rocess is controlled by the rate of the slowest rocess. The transort of the adsorbate from the bulk of fluid hase to the external surface of adsorbent forms an imortant ste in the utake rocess. Single resistance models involve only a liquid film resistance, ore diffusion resistance or surface diffusion resistance 5. In most cases, single resistance models are not adequate to exlain the adsortion rocess for orous adsorbents. Therefore, two resistance mass transfer models which incororate both external and intraarticle diffusion effects is alied 6. Two resistance models can be divided into: 1. Film-ore diffusion model. 2. Film-surface diffusion model. A model which incororated film, ore diffusion and concentration deendent surface diffusion was introduced 7 and later on a three-resistance model based on external mass transfer, ore and concentration deendent surface diffusion for removal of methylene blue by PKS based-ac was resented 8. They exressed that the film-ore-concentrationdeendent surface diffusion (FPCDSD) model was able to fit the exerimental data using a single set of mass transfer arameters for a wide range of initial dye concentrations. However, a two resistance model may be easier for imlementation. Exerimental PKS-based activated carbon sulied from KD Technology Sdn. Bhd was used without any chemical or hysical treatment. Two reactive dyes were used for this investigation, namely, Reactive Black 5 (RB 5) and Reactive Red E (RR E) sulied from Texchem-Pack Bhd. The chemical structure of these dyes is given in Table 1. According to data obtained from H meter, for single solution H of the solution of RB 5 and RRE was 6.5 and 6.4, resectively. For binary solution, treating both dyes RB 5 and RR E in one solution, H was 6.4. To reare the stock solution, g ( ) of each dye was dissolved in 1 L distilled water. In order to maintain homogenous condition, the solution was shaken for 5 hours using orbital incubator shaker (Seilau Saintifik, Malaysia) at 28 o C. Then the solution was ket in dark lace for avoiding any off-color due to sunlight. The stock solution would then be diluted into desired concentration. Batch kinetic study Batch kinetic studies were erformed to investigate the dynamic behavior of PKS - based activated carbon for removal of reactive dyes. The exeriments were accomlished in shaking conical flasks with 1000 ml dye solution at a constant temerature of 28 C (+ 2 C), using an incubator. The H of the solutions was without any modification. Samle of 1 ml was carefully withdrawn at every 3 minutes for the first 30 minutes and at every 5 minutes for the next 30 minutes of adsortion rocess. For the next 60 minutes, samling was done every 10 minutes and eventually every 60 minutes for the next 20 hours and every 6 hours until equilibrium oint. The same rocedure was alied for kinetic study for binary mixture at concentration of 20 mg/l (for each dye) and activated carbon mass of 2 g/l.

3 Adsortion of Reactive Dyes by Palm Kernel Shell Activated Carbon 951 Table 1. Proerties of RB 5 and RR E 9. C.I. Reactive Black 5 C.I. Reactive Red E Tye Reactive dye Reactive dye Chemical structure Molecular length, nm Molecular width, nm Max wave length of adsortion (λ max,) nm Theoretical model External mass transfer The mass transfer rate at the external surface layer of the adsorbent article 10 is: N t = k f S A (c b -c s ) (1) Where N t is adsortion rate at time t, k f is the interhase mass transfer coefficient, S A is surface area, c b and c s are adsorbate concentration in the bulk of fluid and that at the fluidarticle interface, resectively. The differential mass balance is given by: Vdcb N = dt 3M S A = a ρ t = Mdq dt where q is average adsorbed-hased concentration, M is mass of adsorbent, V is volume of solution, a is article radius and ρ is ellet density. The concentration of adsorbate, q, with resect to time is related to the mass transfer coefficient k f by: dq 3k f (4) = ( cb cs ) dt a ρ Equation 4 can be solved with aroriate initial condition: (5) cb = cbo and qs = 0, t < 0 Intraarticle mass transfer Film-surface diffusion Fick s second law of diffusion suggested that the adsorbate molecule is transferred through the adsorbent articles by creeing from one adsortion site to another on the solid surface. The surface diffusivity D s of adsorbed molecules is assumed here to be concentration indeendent. The mass transort in the sherical article is described by following equation. 1 2 D 1 =, 0, 0 2 r r a t t r r r Equation 6 was solved with aroriate initial and boundary conditions: S (2) (3) (6)

4 952 T.G. CHUAH et al. = 0, r = 0 The second-boundary condition for external mass transfer is 10 : 3D a s q = 0, 0 r a, t < 0 (7) 3k f = ( c a ρ b cr= ), t The external mass transfer coefficient, k f, is estimated from the single-resistance model. To calculate the value of the solid hase diffusivity, D s, equation 6 was solved numerically, considering the value of k f obtained from single-resistance model. Film-ore diffusion Pore diffusion equations are described as follow: a 0 c D c (10) ε + ρ = ( r 2 ), 0 r a 2 t t r c = 0, q = 0, 0 r a t < 0 (11) c = 0, r = 0, t 0 (12) c (13) k f ( cb c) = D ( ), r = a, t 0 where D is ore diffusivity. Both the two resistance models were solved by using finite-difference method, as aroximating the satial derivation by central difference exression. Results and Discussion To evaluate the external mass diffusion, the values of external mass transfer coefficient k f were calculated for different initial concentrations. Table 2 resents the values of k f for adsortion of RR5 and RR E onto PKS based-ac. It can be noted that these values, ranging between to cm/s, decrease with an increase in initial dye concentration. The value of k f in a single resistance mass transort model is normally exected to be constant, variation of external mass coefficient shows that intraarticle diffusion is laying a significant role in the mass transort rocess 6. It is exressed that the values of k f decreased with increasing of initial adsorbate concentration 11. The same results were shown for adsortion of dye onto bagasse ith 12. It can be noted from the Table 2 that it agrees with finding of this work. It is ossible that increasing the concentration of dyes, considering the large molecule of dye, causes the reduction of mobility of transferring adsorbates into the boundary layer 4. Table 2. External mass transfer coefficient k f for adsortion of RB 5 and RR E onto PKSbased AC (single system). Isotherm model Interhase Mass Transfer Coefficient, cm/s 100, mg/l 20, mg/l RB 5 / PKS Pellet RR E / PKS Pellet By changing the values of k f and D s it is ossible to obtain the best fit to the exerimental curves for batch adsortion. Table 3 resents the values of D s for adsortion of RB 5 and RR E with dye concentration 20 mg/l onto PKS-based AC with dosage 2 g/l. (8) (9)

5 Adsortion of Reactive Dyes by Palm Kernel Shell Activated Carbon 953 Some values of the surface diffusivity, reorted in other literature are listed in Table 3, as well. It can be seen that the values of D s ranges between to cm 2 /s. From Table 3, it can be noted that the values of D s for RB 5 and RR E are different from values of other literatures. The difference shows secificity of adsortion. In other word interaction between adsorbate and adsorbent is characteristic for each system and it is not common for all systems 13. Table 3. Values of D s, reorted in the literature on adsortion of different dyes using different adsorbents and this work. Material Dye Surface Diffusivity, cm 2 /s Reference Filtrasorb-400 AC Cibacron reactive red Filtrasorb-400 AC Tectilon Red 2B PKS-AC RB This work. PKS-AC RR E This work. Figures 1(a) and 1(b) show the comarisons of results of both adsortions of RB 5 and RR E onto PKS-based activated carbon with redictions based on the film-surface diffusion and film-ore diffusion models. It is obvious from these figures that rediction based on the film-surface diffusion model fits better with exeriment data than the film-ore diffusion model, and it agrees with both dyes. The values of SSE for film-surface diffusion are significantly less than values of SSE for film-ore diffusion, as shown in Table 5. As a result, it indicated that the adsortion rocess was governed by the resistance models, i.e. film-surface diffusion mechanism. The values of D are as listed in Table 4. Time, h Figure 1(a). Comarison between rediction of film-surface diffusion (S.D) and film-ore diffusion (P.D) models with exeriments data from adsortion of RB 5 onto PKS-based activated carbon at dye concentration of 20 mg/l and dosage of adsorbent 2 g/l (single system). Cb/Cbo Cb/Cbo mg/l S.D 20 mg/l Ex data 20 mg/l P.D mg/l S.D 20 mg/l Ex data 20 mg/l P.D Time, h Figure 1(b). Comarison between rediction of film-surface diffusion (S.D) and film-ore diffusion (P.D) models with exeriments data from adsortion of RR E onto PKS-based activated carbon at dye concentration of 20 mg/l and dosage of adsorbent 2 g/l (single system).

6 954 T.G. CHUAH et al. Table 4. Values of D for adsortion of RB 5 and RR E onto PKS based-ac. Kinetic Model D, cm 2 /s RB 5/PKS RR E/PKS Table 5. Comarison of SSE of film-surface diffusion and film-ore diffusion models for adsortion of RB 5and RR E onto PKS based-ac. Kinetic model SSE Film-surface diffusion Film-ore diffusion RB 5/PKS RR E/PKS Conclusion The rate of the kinetic rocess for both dyes was better described by two resistance models. Based on SSE, the film-surface diffusion model was able to fit exerimental better than the film-ore diffusion model for both RB 5 and RR E on alm kernel based activated carbon. Acknowledgement The authors are grateful to KD Technology Sdn. Bhd. for sulying granular PKS. References 1. Mohan D and Pittman Jr C U Hazardous Materials., 2007, 142, Ma A N, Palm Oil Engineering Bulletin, 2002, Chuah T G, Jumasiah A, Gimbon J, Choong T S Y and Azni I, Desalination, 2005, 186, McKay G, Bino M J and Altememi A, Water Res., 1986, 20, Al-Qoudah Z, Water Res., 2000, 34, Choy K K H, Ko D C K, Cheung C W C, Porter J F and McKay G, J Colloid Interface Sci., 2004, 271, Choy K K H, Porter J F and McKay G, Adsortion, 2001, 7, Choong T S Y, Wong T N, Chuah T G and Idris A, J Colloid Interface Sci., 2006, 301, Kim S I, Yamamoto T, Endo A, Ohmori T and Nakaiwa M, Microorous and Mesoorous Materials, 2006, 96, Tien C, Adsortion calculation and modeling; Butterworth-Heinemann, 1994, Tsai W T, Chang CY, Ing C H and Chang C F, J Colloid Interface Sci., 2004, 275, Mckay G, El Geundi M and Nassar M M, Water Res., 1988, 22, Meshko V, Markovska L, Mincheva M and Rodrigues A E, Water Res., 2001, 14, Yang X Y and Al-Duri, Chemical Engineering Journal, 2001, 83, Walker G M and Weatherley L R, Water Res., 1999, 33,

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