Properties of activated carbon prepared from rice husk with chemical activation. Samah Babiker Daffalla* and Hilmi Mukhtar

Int. J. Global Environmental Issues, Vol. 12, Nos. 2/3/4, 2012 107 Properties of activated carbon prepared from rice husk with chemical activation Samah Babiker Daffalla* and Hilmi Mukhtar Department of Chemical Engineering, Faculty of Engineering, Universiti Teknologi PETRONAS, 31750, Tronoh, Malaysia E-mail: Samahb.daffalla@gmail.com E-mail: hilmi_mukhtar@petronas.com.my *Corresponding author Maizatul Shima Shaharun Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, 31750, Tronoh, Malaysia E-mail: maizats@petronas.com.my Abstract: The present work involves an investigation of the possible use of activated carbon developed from rice husk by chemical activation with zinc chloride (ZnCl 2 ) and phosphoric acid (H 3 PO 4 ) under different activation conditions for the removal of phenol from artificial wastewater. The physical and surface properties of the developed adsorbents were characterised using FTIR and SEM. A comparison between ZnCl 2 and H 3 PO 4 shows that the efficiency of phenol removal by H 3 PO 4 activated carbon is generally lower than that of ZnCl 2 activated carbon for both activation temperatures. After 24 hrs, removal efficiency of up to 90% could be achieved with 0.5 g ZnCl 2 activated carbon, either prepared at 500 C or 600 C activation temperature. However, for 0.1g ZnCl 2 activated carbon, an efficiency of 80% to 85% and 69% to 74% could be achieved at 500 C and 600 C activation temperatures, respectively. For H 3 PO 4 activated carbon prepared at 500 C, the efficiency was 45% to 48% and 48% to 56% for 0.1 g and 0.5 g of adsorbent respectively. While for H 3 PO 4 activated carbon prepared at 600 C, an efficiency of 41% to 45% and 43% to 51% could be achieved with 0.1 g and 0.5 g, respectively. The kinetics of phenol adsorption on both ZnCl 2 and H 3 PO 4 activated carbons were found to follow the pseudo-second-order kinetic model. Keywords: activated carbon; phenol adsorption; rice husk; adsorbent functional group; treatment of wastewater. Reference to this paper should be made as follows: Daffalla, S.B., Mukhtar, H. and Shaharun, M.S. (2012) Properties of activated carbon prepared from rice husk with chemical activation, Int. J. Global Environmental Issues, Vol. 12, Nos. 2/3/4, pp.107 129. Copyright 2012 Inderscience Enterprises Ltd.

108 S.B. Daffalla et al. Biographical notes: Samah Babiker Daffalla obtained her MSc in Chemical Engineering from University of Khartoum (UofK), Sudan in 2008. Currently she is a PhD student at Universiti Teknologi PETRONAS (UTP), Malaysia. She received her BSc (Hons.) in Chemical Engineering from University of Khartoum, Sudan in the year 2004. Her research interests are environmental engineering and wastewater treatment. Hilmi Mukhtar received his PhD from University of Wales Swansea, UK in 1995. Currently, he is an Associate Professor at the Department of Chemical Engineering, Universiti Teknologi PETRONAS, Malaysia. His research interest is the separation processes, focusing on the removal of carbon dioxide from natural gas and treatment of wastewater using membrane separation techniques and adsorbents. Currently, he also serves as a reviewer for the Canadian Journal of Chemical Engineering (CJChE). He holds the Fellow of Institution of Chemical Engineers (IChemE), UK. Maizatul Shima Shaharun obtained her PhD in Chemical Engineering from Universiti Teknologi PETRONAS (UTP), Malaysia. Currently she is a Senior Lecturer at UTP. She received her BSc (Hons.) in Chemistry from University of Liverpool and MSc in Chemical Process Technology from University of Durham, UK in the year 1998 and 2000, respectively. Her research interests are catalysis, reaction kinetics, molecular modelling and water treatment. This paper is a revised and expanded version of a paper entitled Properties of activated carbon prepared from rice husk with chemical activation presented at 2010 International Conference on Environment, Penang, Malaysia, 13 15 December 2010. 1 Introduction The presence of hazardous chemicals in the environment continues to be an important concern. The demand for more stringent control and protection of our water resources from pollution has mounted steadily in recent decades (Jern, 2006). Phenol, as a class of organic compounds, has been known as a common and hazardous contaminant in water environment. Phenol constitutes the 11th of the 126 chemicals, which has been designated as priority pollutants by the US Environmental Protection Agency (USEPA) (Nayak and Singh, 2007). The content of phenols in industrial wastewater is usually in the range of 0.1 6,800 mg/l (Busca et al., 2008). The allowable discharge limit for phenol is 0.1 mg/l and 0.001 mg/l (Standard A) set by the USEPA and the Malaysia Environmental Quality Act (MEQA), 1974 respectively (Ahmaruzzaman and Sharma, 2005). Human consumption of water contaminated with phenol can cause severe pain leading to damage of the capillaries and ultimately causing death. Phenol is released to the environments from various industry sectors particularly, iron-steel, coke, petroleum, pesticide, paint, solvent, pharmaceutics, wood preserving chemicals, and paper and pulp industries (Busca et al., 2008). Various methods such as microbial degradation, adsorption, chemical oxidation, incineration and solvent extraction have previously been used for removal of phenol from wastewater (Aksu, 2005; Jain et al., 2002; Radetski et al., 2009). Among those methods, adsorption process has emerged as the best for removing phenolic compounds from

Properties of activated carbon prepared from rice husk 109 aqueous streams. The efficiency of the adsorption process is mainly due to the characteristic of the adsorbent such as high surface area, high adsorption capacity, microporous structure and special surface reactivity. To the present, activated carbon is the most widely used adsorbent for removal of phenol from waste water due to its good adsorption capacity for phenol (Sandro et al., 2009; Somnath et al., 2007; Vimal et al., 2006). In spite of this, its use particularly for wastewater treatment is limited due to its high price. Activated carbon is quite expensive and the higher the quality the greater the cost (Cherifi et al., 2009). Consequently, there has been a growing interest in developing and implementing various potential adsorbents for removal of phenol from water, and researchers are always in a hunt for developing more suitable, efficient, cheap and easily accessible types of adsorbents, particularly from the waste materials. Considerable efforts have been made by many researchers to prepare activated carbons from agricultural wastes (Dimitrios et al., 2008; Jun ichi et al., 2000; Tzong-Horng, 2009). The abundance and availability of agricultural by-products make them good sources of cheap raw materials for natural adsorbents. Rice husk, an agricultural waste, mostly used as fuel in boiler furnaces of various industries to produce steam, has been reported as a good adsorbent for many metals and basic dyes (Kumar and Bandyopadhyay, 2006; Wan Ngah, and Hanafiah, 2008). According to the statistics compiled by the Malaysian Ministry of Agriculture, there are more than 408,000 ton of rice husk produced in Malaysia annually (Chuah et al., 2005). Activating the rice husk is an inert atmosphere produces a highly porous carbon with a very high surface area. Thus, converting rice husk into effective adsorbent can be an attractive option to eliminate waste materials (Liou and Wu, 2009). The present work focuses on the development and characterisation of activated carbon from rice husk by chemical activation using zinc chloride and phosphoric acid. Different preparation variables including chemical ratios of activating agent and precursor, and carbonisation temperature were studied. The produced activated carbons were used to remove phenol from artificial wastewater. Kinetic models were used at different adsorbent doses to identify the possible mechanisms of such adsorption process. The effects of the presence of surface functional groups on phenol adsorption were also investigated. 2 Material and methods 2.1 Preparation of activated carbon The raw material, rice husk (RRH) was obtained from a nearby rice mill. The proximate and ultimate analysis of rice husk is shown in Table 1. The rice husk was washed thoroughly with distilled water to remove adhering soil and clay, and then dried in air at 105 C in an oven for 24 hrs. The rice husk was milled and then passed through different sieves. The milled rice husk particles of sizes between 500 250 µm were selected for further pretreatment. The rice husk was then refluxed with 1M NaOH solution for 1 h to reduce ash content in the sample. Subsequently, the rice husk was impregnated with H 3 PO 4 or ZnCl 2 and water at various impregnation ratios (mass ratio of activating agent to dried rice husk) from 1/1 to 2/1. The mixture was heated in a horizontal tubular furnace (model, TSH17/75/450-2416-2116) under pure nitrogen gas at a heating rate of 10 C/min to the final carbonisation temperature and was held for one hour. Two

110 S.B. Daffalla et al. carbonisation temperatures were selected, i.e., 500 C and 600 C. Then the activated carbon was washed with 3M HCl at room temperature for 1 h. Table 1 Proximate and Ultimate analysis of rice husk Proximate analysis (wt %) Moisture content 10.5 Ash content 12 Fixed carbon 77.5 Ultimate analysis (wt %) Carbon 41.16 Sulphur 0.061 Nitrogen 1.075 Hydrogen 6.06 Oxygen 39.64 2.2 Characterisation of adsorbents The adsorbents were characterised in terms of morphological characteristics using Scanning Electron Microscope (SEM-EDX, model LE01430VP). In addition, the functional groups present on the adsorbent s surface were determined using Fourier Transform Infrared spectrophotometer (FTIR, model 8400S). The spectra range chosen was from 4,000 to 400 cm 1. 2.3 Batch studies The performance of the prepared adsorbents was evaluated through batch process with varying adsorbent doses. A mixture of 1,000 mg/l phenol and activated rice husks was agitated at 190 rpm in a mechanical shaker. The ph of the solution was 6.1 ± 0.1. Several experiments were carried out to observe the effects of different adsorbent doses. The sorption study was performed using adsorbent dose of 0.1 g and 0.5 g, and shaking time from 0 to 24 hrs. From an earlier analysis, the equilibrium time was found to be 4 hrs and 2 hrs for 0.1 g and 0.5 g adsorbent dose respectively; the same equilibrium time was applied. At each adsorbent dose, the effects of different parameters such as impregnation ratio and activation temperature were investigated. The suspensions in all sorption assays were filtered to remove any suspended adsorbent. Initial and final concentration of phenol was determined by finding out the absorbance of the solution at 460 nm wavelength using UV/Vis-Spectrophotometer (DR500). The percentage of phenol removal and equilibrium adsorption uptake, q e (mg/g), were calculated using the following relationships: ( Co Ct) % Removal Efficiency = 100 (1) C o

Properties of activated carbon prepared from rice husk 111 ( e ) Amount Adsorbed q ( o e) C C V = ( mg of adsorbate / g of adsorbant) (2) W where C o and C e (mg/l) are the initial and equilibrium liquid-phase concentrations of phenol, respectively, C t (mg/l) is the concentration of phenol at time t, V is the volume of the solution (l) and W is the mass of dry adsorbent (g). 3 Results and discussions 3.1 Yield of activated carbon The yield of carbon as a function of activation temperature and impregnation ratio for both H 3 PO 4 and ZnCl 2 activated samples is determined using the relationship of equation (3). mass of activated carbon Carbon yield(%) = 100 (3) mass of dried rice husk It was found that, the yield of ZnCl 2 and H 3 PO 4 activated carbon prepared at 500 C increased from 48% to 52% and 52% to 53%, respectively, as the impregnation ratio was increased from 1/1 to 2/1. This may be due to the presence of the activating agents which influence the pyrolytic decomposition and inhibit the formation of tar or ash, thus enhancing carbon yield. On the other hand, for the adsorbents prepared at 600 C a lower yield was obtained for both ZnCl 2 (47% 50%) and H 3 PO 4 (50% 52%), for both impregnation ratios. This is expected, as increasing the temperature will release volatile compounds, thereby reducing the yield. The same result was reported by Lua and Ting (2004). 3.2 Characterisation of activated carbon using SEM Figure 1 shows the morphological characteristics of raw rice husk, ZnCl 2 and H 3 PO 4 activated carbons. It is obvious that, as impregnation ratio increased, large pores of different shapes could be observed for ZnCl 2 and H 3 PO 4 activated carbons. This may be due to the activation which occurs only at the exterior of the rice husk, and decreases the formation of pores. An increase in the activating agent promotes the contact area between rice husk and activating agent, and therefore, increases the surface area and porosity of carbon. The mechanism for phosphoric acid activation tends to produce a well developed porosity besides high carbon yield, since H 3 PO 4 degrades cellulose, hemicellulose and lignin. According to the micrograph, it seems that the cavities on the surfaces resulted from the evaporation of the activating agent during carbonisation, leaving the space previously occupied by the activating agent (Prahas et al., 2008).

112 S.B. Daffalla et al. Figure 1 SEM for (a) RRH (b) 1:1 ZnCl 2 (c) 2:1 ZnCl 2 (d) 1:1 H 3 PO 4 (e) 2:1 H 3 PO 4 activated carbons prepared at 500 C, magnified 1,000 times (see online version for colours) (a) Porosity (b) (c)

Properties of activated carbon prepared from rice husk 113 Figure 1 SEM for (a) RRH (b) 1:1 ZnCl 2 (c) 2:1 ZnCl 2 (d) 1:1 H 3 PO 4 (e) 2:1 H 3 PO 4 activated carbons prepared at 500 C, magnified 1,000 times (continued) (see online version for colours) (d) (e) 3.3 Characterisation of adsorbent using FTIR Figure 2 shows the ZnCl 2 and H 3 PO 4 activated carbons with various impregnation ratios at 500 C. The bands at 2,000 2,400 cm 1, 1,662 cm 1, 1,479 cm 1 and 1,200 cm 1 can be attributed to C C, C = C, CH 2 and CO, respectively. A peak around 1,714 cm 1 shows the presence of stretching vibration of C = O in ketones, aldehyde, lactone, and carboxyl. The presence of the broad band at 3,400 2,000 cm 1 and the peak around 1,714 cm 1 indicates the existence of carboxylic groups (Prahas et al., 2008). For H 3 PO 4 activated carbon, the band at 2,364 2,374 cm 1 is ascribed to C C vibration in alkyne groups. The band is more intense than that of the raw rice husk at 2,370 cm 1 due to the release of light volatile matter such as H, resulting from the heat treatment process. Puziy et al. (2002), reported that the peak at 1,220 1,180 cm 1 may be attributed to the phosphorous-containing group P = O, C O stretching vibrations in P O C linkage or P = OOH bond. When rice husks are activated with H 3 PO 4 and ZnCl 2, the bands at

114 S.B. Daffalla et al. 1,080 cm 1 and 862 476 cm 1 decreased, which correspond to Si-O-Si and Si-H functional group respectively, indicating the removal of ash in carbons. Figure 3 shows that for both activation procedures, the intensity of C C, C = C, CH 2 and CO bands decreases as the activation temperature increases to 600 C, indicating that the proportion of carbon content increases at high temperatures. The same result was reported by Liou and Wu (2009). Figure 2 FTIR spectra of (a) 1:1 RRH/ZnCl 2 (b) 2:1 ZnCl 2 (c) 1:1 H 3 PO 4 (d) 2:1 H 3 PO 4 activated carbons prepared at 500 C (see online version for colours) RRH (a) (b)

Properties of activated carbon prepared from rice husk 115 Figure 2 FTIR spectra of (a) 1:1 RRH/ZnCl 2 (b) 2:1 ZnCl 2 (c) 1:1 H 3 PO 4 (d) 2:1 H 3 PO 4 activated carbons prepared at 500 C (continued) (see online version for colours) (c) (d)

116 S.B. Daffalla et al. Figure 3 FTIR spectra of (a) 1:1 RRH/ZnCl 2 (b) 2:1 ZnCl 2, (c) 1:1 H 3 PO 4 (d) 2:1 H 3 PO 4 activated carbons prepared at 600 C (see online version for colours) AC(1:1) RRH (a) (b)

Properties of activated carbon prepared from rice husk 117 Figure 3 FTIR spectra of (a) 1:1 RRH/ZnCl 2 (b) 2:1 ZnCl 2, (c) 1:1 H 3 PO 4 (d) 2:1 H 3 PO 4 activated carbons prepared at 600 C (continued) (see online version for colours) (c) (d)

118 S.B. Daffalla et al. Figure 4 Pseudo-second-order kinetics for adsorption of phenol on ZnCl 2 (a) and (b), H 3 PO 4 (c) and (d) activated carbons prepared at 500 C for different impregnation ratios (adsorbent dose = 0.1 g and 0.5 g) (see online version for colours) (a) (b)

Properties of activated carbon prepared from rice husk 119 Figure 4 Pseudo-second-order kinetics for adsorption of phenol on ZnCl 2 (a) and (b), H 3 PO 4 (c) and (d) activated carbons prepared at 500 C for different impregnation ratios (adsorbent dose = 0.1 g and 0.5 g) (continued) (see online version for colours) (c) (d)

120 S.B. Daffalla et al. Table 2 Pseudo-first-order, Pseudo-second-order and Elovich model, constant and correlation coefficient for adsorption of phenol by ZnCl 2 and H 3 PO 4 activated carbons prepared at 500 C for different impregnation ratios (adsorbent dose = 0.1 g and 0.5 g) ZnCL 2 AC (0.1g) ZnCL 2 AC (0.5g) H 3 PO 4 AC (0.1g) H 3 PO 4 AC (0.5g) Model Parameters 1:1 2:1 1:1 2:1 1:1 2:1 1:1 2:1 Pseudo first order Pseudo second order Elovich q e,exp (mg/g) 795.34 791.78 123.02 164.47 645.08 582.90 133.16 137.31 q e,cal (mg/g) 81.82 84.79 91.56 41.98 190.23 135.84 63.84 58.59 K 1 (min 1 ) 0.0417 0.0417 0.0254 0.0307 0.0065 0.0062 0.0150 0.0253 R 2 0.7570 0.7570 0.9561 0.8070 0.9716 0.9693 0.9675 0.9354 q e (%) 36.63 36.45 10.44 30.40 26.65 28.99 28.78 23.40 q e,cal (mg/g) 792.56 788.95 119.14 159.48 561.49 520.57 110.78 128.39 K 2 (g/mg.min) 0.0023 0.0022 0.0007 0.0039 0.0007 0.001 0.0023 0.0023 R 2 0.9999 0.9999 0.9940 0.9989 0.9975 0.9982 0.9849 0.9950 q e (%) 0.143 0.146 1.288 1.24 4.898 4.042 6.863 2.652 q e,cal (mg/g) 828.75 826.41 116.8 168.2 532.81 499.68 105.95 129.60 a 1.22 10 9 5.51 10 8 25.02 2.58 10 4 1.75 10 9 1.37 10 13 5.07 10 3 1.34 10 3 b 0.0267 0.0257 0.0413 0.0733 0.0430 0.0646 0.1044 0.0722 R 2 0.9198 0.9198 0.9938 0.9544 0.789 0.877 0.803 0.8998 q e (%) 1.715 1.785 2.065 0.925 6.578 5.369 8.341 2.292

Properties of activated carbon prepared from rice husk 121 3.4 Kinetic study for the removal of phenol Kinetic studies were conducted to understand the uptake rates of phenol on the surfaces of adsorbents and to determine the equilibrium time required for phenol uptake by ZnCl 2 and H 3 PO 4 activated carbons. The results showed that the equilibrium time required for the adsorption of phenol on ZnCl 2 and H 3 PO 4 activated carbons at 500 C was 4 hrs and 2 hrs for 0.1 g and 0.5 g of adsorbent respectively. The kinetic of the adsorption data were analysed using three different kinetic models. 1 A simple pseudo first-order equation, which was an early proposal by Lagergren, is described by equation (4) (Hameed et al., 2008): ln ( e t) ln e 1 q q = q k t (4) where q e and q t (mg/g) are amount of phenol adsorbed at equilibrium and at time, t (min), respectively and k 1 (min 1 ) is the adsorption rate constant. The plot of ln(q e q t ) versus t (figure not shown) gave the slope of k 1 and intercept of lnq e. It can be seen in Table 2 that the coefficient of determination (R 2 values) for the pseudo-first order kinetic model is in the range of 0.757 0.9716. Besides, the experimental q e values did not agree with the calculated ones. So, the adsorption of phenol onto ZnCL 2 and H 3 PO 4 activated carbon was not a first-order reaction. 2 The Pseudo-second order kinetic model has also been applied widely and is described by equation (5) (Ho and Mckay, 1999): t 1 1 = + t (5) q k q q 2 t 2 e e where k 2 (g/mg min) is the rate constant of second-order adsorption. The linear plot of t/q t versus t gave 1/q e as the slope and 1/k 2 q 2 e as the intercept. The linear plot of t/q t versus t, as shown in Figure 4 and Table 2, shows a good agreement between the experimental and calculated q e values. In addition, the coefficient of determination R 2 values for the second-order kinetic model were almost equal to unity, which indicates that the kinetics of phenol adsorption on both ZnCl 2 and H 3 PO 4 activated carbons followed a second order kinetics. 3 The Elovich kinetic model [equation (6)], which is normally used in cases of chemisorptions was also applied. 1 1 qt = ln( ab) ln( t) b + b (6) where a and b are the constants for this model obtained from the slope and intercept of the linear plot of qt versus ln t (figure not shown). The coefficient of determination (R 2 values) for the Elovich kinetic model is in the range of 0.789 0.994, and the calculated q e values (see Table 2) are quite close to the experimental values. Even though these results are good, but a comparison with the pseudo second order kinetic model show that the latter gives a better fit between experimental and calculated data. In order to evaluate the fitting of the experimental data and the prediction accuracy of the models utilised in the present work, the normalised standard deviation is employed, q e (%), which is defined as (Hameed et al., 2008):

122 S.B. Daffalla et al. Δ q (%) = 100 e ( ) ) 2 qe,exp qe, cal qe,exp (7) ( N 1) where N is the number of data points, and q e,exp and q e,cal (mg/g) are the experimental and calculated equilibrium adsorption capacity value, respectively. Figure 5 Effect of adsorbent amount on percent removal of phenol for (a) 1:1 ZnCl 2 (b) 2:1 ZnCl 2 activated carbon and (c)1:1 H 3 PO 4 and (d) 2:1 H 3 PO 4 activated carbons (adsorbent activation temperature = 500 C, adsorbent dose = 0.1 g and 0.5 g, equilibrium time = 240 min) (see online version for colours) (a) (b)

Properties of activated carbon prepared from rice husk 123 Figure 5 Effect of adsorbent amount on percent removal of phenol for (a) 1:1 ZnCl 2 (b) 2:1 ZnCl 2 activated carbon and (c)1:1 H 3 PO 4 and (d) 2:1 H 3 PO 4 activated carbons (adsorbent activation temperature = 500 C, adsorbent dose = 0.1 g and 0.5 g, equilibrium time = 240 min) (continued) (see online version for colours) (c) (d)

124 S.B. Daffalla et al. Figure 6 Effect of adsorbent amount on percent removal of phenol for (a) 1:1 ZnCl 2 (b) 2:1 ZnCl 2 activated carbon and (c) 1:1 H 3 PO 4 and (d) 2:1 H 3 PO 4 activated carbons (adsorbent activation temperature = 600 C, adsorbent dose = 0.1 g and 0.5 g equilibrium time = 240 min) (see online version for colours) (a) (b)

Properties of activated carbon prepared from rice husk 125 Figure 6 Effect of adsorbent amount on percent removal of phenol for (a) 1:1 ZnCl 2 (b) 2:1 ZnCl 2 activated carbon and (c) 1:1 H 3 PO 4 and (d) 2:1 H 3 PO 4 activated carbons (adsorbent activation temperature = 600 C, adsorbent dose = 0.1 g and 0.5 g equilibrium time = 240 min) (continued) (see online version for colours) (c) (d)

126 S.B. Daffalla et al. 3.5 Effect of adsorbent amount Figure 5 and Figure 6 present the effect of adsorbent dose (0.1 g and 0.5 g) on removal efficiency of phenol at equilibrium time (240 min). It is apparent that the adsorption efficiency increases by increasing the adsorbent dose. This is due to the increasing number of available adsorption sites as more adsorbent is added. Generally after 24 hrs, removal efficiency of up to 90% could be achieved with 0.5 g ZnCl 2 activated carbon, either prepared at 500 C or 600 C activation temperature. However, by using 0.1 g adsorbent under the same experimental conditions, the removal efficiency of the adsorbent prepared at 500 C was between 80% to 85%, while that for the adsorbent prepared at 600 C was 69% to 74%. A comparison between ZnCl 2 and H 3 PO 4 shows that the efficiency of phenol removal by H 3 PO 4 activated carbon is generally lower than that of ZnCl 2 activated carbon. For H 3 PO 4 activated carbon prepared at 500 C, the efficiency was 45% to 48% and 48% to 56% for 0.1 g and 0.5 g of adsorbent respectively. While for H 3 PO 4 activated carbon prepared at 600 C, an efficiency of 41% to 45% and 43% to 51% could be achieved with 0.1 g and 0.5 g, respectively. This behaviour might be attributed to the difference in their chemical structure and porosity of activated carbon using different activating agent. It also observed that activation temperature has a significant effect on removal efficiency. In this study, increasing the activation temperature from 500 C to 600 C reduced the phenol removal efficiency. Liou and Wu (2009) reported that, when the activation temperature is higher than 500 C, violent gasification reactions may cause a part of the micropore structure to be destroyed by collapsing or combining together, resulting in reducing the removal efficiency. Figure 7 FTIR spectra of (a) ZnCl 2 and (b) H 3 PO 4 activated carbons prepared at 500 C and 1:1 impregnation ratio before and after phenol sorption (see online version for colours) ZnCl 2 AC+ phenol ZnCl 2 AC (a)

Properties of activated carbon prepared from rice husk 127 Figure 7 FTIR spectra of (a) ZnCl 2 and (b) H 3 PO 4 activated carbons prepared at 500 C and 1:1 impregnation ratio before and after phenol sorption (continued) (see online version for colours) H3PO4 AC H3PO4 AC +phenol (b) Table 3 FTIR spectra of ZnCl 2 activated carbon prepared at 500 C before and after phenol sorption (1:1 impregnation ratio) Adsorbent OH C H C C C = O C = C C C, C O ZnCl 2 AC 3,442.70 2,854.45 2,364.57 1,635.59 1,548.35 1,205.43 ZnCl 2 AC after phenol sorption 3,446.56 2,850.59 2,476.43 1,633.59 1,549.50 1,149.50 Table 4 FTIR spectra of H 3 PO 4 activated carbon prepared at 500 C before and after phenol sorption (1:1 impregnation ratio) Adsorbent OH C H C C C = O C = C C C, C O H 3 PO 4 AC 3,442.70 2,856.38 2,125.41 1,714.6 1,596.95 1,201.57 H 3 PO 4 AC after phenol sorption 3,448.49 2,887.24 2,102.26 1,766.67 1,585.38 1,249.79 3.6 Characterisation of adsorbent using FTIR after adsorption experiments The effects of the presence of the surface functional group on adsorption of phenol were analysed by observing the shifting of the FTIR peaks after the adsorption experiment. Figure 7 and Table 3 and Table 4, show that shifting occurs at lower and higher wave numbers for the ZnCl 2 and H 3 PO 4 activated carbons, indicating that these groups may contribute to adsorption of phenol onto the surface of adsorbent.

128 S.B. Daffalla et al. 4 Conclusions The present investigation has shown that ZnCl 2 and H 3 PO 4 activated carbons are a promising low-cost adsorbent for the removal of phenol from aqueous solutions. Adsorption of phenol was found to increase with increasing adsorbent dose. After 24 hrs, removal efficiency of up to 90% could be achieved with 0.5 g ZnCl 2 activated carbon, either prepared at 500 C or 600 C activation temperature. However, for 0.1 g ZnCl 2 activated carbon, an efficiency of 80% to 85% and 69% to 74% could be achieved at 500 C and 600 C activation temperatures respectively. On the other hand, the efficiency of 0.1 g and 0.5 g H 3 PO 4 activated carbon was less than ZnCl 2 activated carbon, which was 45% to 48% and 48% to 56%, respectively for H 3 PO 4 activated carbon prepared at 500 C. While for H 3 PO 4 activated carbon prepared at 600 C, an efficiency of 41% to 45% and 43% to 51% could be achieved with 0.1 g and 0.5 g, respectively. The kinetics of the adsorption process was found to follow the pseudo-second-order kinetic model. FTIR analysis was conducted on the prepared activated carbon before and after phenol adsorption to study the surface chemistry of the activated carbon. The FTIR results show that the OH, C H, C C, C = O, C = C, C C and C O groups contribute to the adsorption of phenol onto the surface of the adsorbent. References Ahmaruzzaman, M. and Sharma, D.K. (2005) Adsorption of phenols from wastewater, J. Colloid Interface Sci., Vol. 287, No. 1, pp.14 24. Aksu, Z. (2005) Application of biosorption for the removal of organic pollutants: a review, Process Biochemistry, Vol. 40, Nos. 3 4, pp.997 1026. Busca, G., Berardinelli, S., Resini, C. and Arrighib, L. (2008) Technologies for the removal of phenol from fluid streams: a short review of recent developments, J. Hazard. Mater., Vol. 160, pp.265 288. Cherifi, H., Haninia, S. and Bentahar, F. (2009) Adsorption of phenol from wastewater using vegetal cords as a new adsorbent, Desalination, Vol. 244, Nos. 1 3, pp.177 187. Chuah, T.G., Jumasiah, A., Azni, I., Katayon, S. and Thomas Choong, S.Y. (2005), Rice husk as a potentially low-cost biosorbent for heavy metal and dye removal: an overview, Desalination, Vol. 175, No. 3, pp.305 316. Dimitrios, K., Bethanis, S., Paraskeva, P. and Diamadopoulos, E. (2008) Production of activated carbon from bagasse and rice husk by a single-stage chemical activation method at low retention times, Bioresource Technology, Vol. 99, No. 15, pp.6809 6816. Hameed, B.H., Tan, I.A.W. and Ahmad, A.L. (2008) Adsorption isotherm, kinetic modeling and mechanism of 2, 4, 6-trichlorophenol on coconut husk-based activated carbon, Chem. Eng. Journal, Vol. 144, No. 2, pp.235 244. Ho, Y.S. and Mckay, G. (1999) Comparative sorption kinetic studies of dye and aromatic compounds onto fly ash, J. Environ. SCI. Health, Vol. A34, No. 5, pp.1179 1204. Jain, A.K., Suhas and Bhatnagar, A. (2002), Methylphenols removal from water by low-cost adsorbents, J. Colloid Interface Sci., Vol. 251, No. 1, pp.39 45. Jern, NG.W. (2006) Industrial Wastewater Treatments, Imperial College Press, Covent Garden, London. Jun ichi H, Kazehaya, A., Muroyama, K. and Watkinson, A.P. (2000) Preparation of activated carbon from lignin by chemical activation, Carbon, Vol. 38, No. 13, pp.1873 1878. Kumar, U. and Bandyopadhyay, M. (2006) Sorption of cadmium from aqueous solution using pretreated rice husk, Bioresource Technology, Vol. 97, No. 1, pp.104 109.

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