Physico-chemical and surface characterization of adsorbent prepared from groundnut shell by ZnCl 2 activation and its ability to adsorb colour

Indian Journal of Chemical Technology Vol. 13, July 2006, pp. 319-328 Physico-chemical and surface characterization of adsorbent prepared from groundnut shell by ZnCl 2 activation and its ability to adsorb colour R Malik, D S Ramteke* & S R Wate Environmental Impact & Risk Assessment Division National Environmental Engineering Research Institute, Nagpur 440 020, India Email: dsramteke_neeri@rediffmail.com, ds_ramteke@neeri.res.in Received 23 November 2005; revised received 17 May 2006; accepted 20 May 2006 Physico-chemical and surface characterization of activated carbon prepared from groundnut shell, an abundantly available carbonaceous solid waste from oil processing mills in India, by chemical activation were carried out. The effect of various activation conditions viz. time, temperature and ZnCl 2 / char ratio on the % yield of product and adsorption efficiency in terms of iodine number was studied. Changes in surface morphology of char before and after activation were examined by scanning electron microscopy which showed well developed pore structure in activated carbon demonstrating corrosive effect of ZnCl 2. Surface modifications through chemical changes were studied by FTIR spectroscopy which showed considerable development and increase of surface functional groups in activated carbon as compared to char. The developed adsorbent was utilized for the removal of acid dye from aqueous solution and its adsorption capacity was found to be 55.5 mg/g of the adsorbent for 100 ppm initial concentration of dye solution. Adsorption parameters for Freundlich and Langmuir isotherm models were examined for dye adsorption and for the validity of models to analyze the equilibrium data, it was found that Freundlich model fitted better than Langmuir. The results indicate that Groundnut Shell based Powdered Activated Carbon (GSPAC) could be employed as low-cost alternative adsorbent to commercial activated carbon in the wastewater treatment for removal of acid dyes. Keywords: Activated carbon, Groundnut shell, Porosity, FTIR, Fuchsin acid, Adsorption isotherms IPC Code: B01D15/00, C01B31/00 The high cost of commercial activated carbon restricts its application, so, there is need to undertake studies to substitute the costlier commercial activated carbon with the unconventional, low cost and locally available agricultural waste based adsorbents 1. India is an agricultural country and generates considerable amount of agricultural wastes. Residues from agriculture and agro-industries are the non-product outputs from the growing and processing of raw agricultural products such as rice, corn, bean and peanut etc. 2. Disposal of agricultural by-products is a major economic and ecological issue these days. The conversion of by-products to adsorbents, such as activated carbon, represents a possible outlet. Agricultural by-products 3 are renewable sources of raw materials for activated carbon production because the development of methods to utilize the waste materials is greatly desired and production of activated carbons from waste is an interesting possibility 4. Successful studies on these materials could be beneficial to the developing countries and could be easily incorporated in development of appropriate technologies. India is the second largest producer of groundnut throughout the world, which has 8-milion hectares area of cultivation producing 8004,000 metric tons/y. Groundnut shell is a carbonaceous, fibrous solid waste which encounters disposal problem and is generally used for its fuel value. Therefore, it was of interest to prepare a higher value product such as activated carbon from groundnut shell. These byproducts 5 have the advantage of greater percentage of non-carbon constituents in their composition compared to coal or peat and therefore offer a greater chance of retaining functional groups, especially oxygenated groups in the carbonized product. However, the lower carbon content of by-products translates to lower activated carbon yields, but the low cost of waste cancel out the lower yields. Basically, there are two different processes for the preparation of activated carbon: physical and chemical activation 6. Physical activation involves carbonization of carbonaceous precursor followed by activation of resulting char in the presence of activating agents such as carbon dioxide or steam.

320 INDIAN J. CHEM. TECHNOL., JULY 2006 Chemical activation on the other hand, involves the carbonization of precursor in the presence of chemical agents. In physical activation, elimination of large amount of internal carbon mass is necessary to obtain a well developed porous structure, whereas in chemical activation process, chemical agents used are dehydrating agents that influence pyrolytic decomposition and inhibit the formation of tar, thus enhancing yield of carbon 7 and after washing, large amount of porosity is formed after elimination of chemical from product. Chemical activation has more advantages 8 over physical activation with respect to higher yield, more surface area and better development of porous structure oxygenated surface complexes in carbon. The dye adsorption tests help to determine the capacity of carbon to remove organic contaminants and colored bodies from aqueous solution. Due to the large degree of organics present in these molecules and the stability of modern dyes; conventional, physico-chemical and biological treatment methods are ineffective for their removal 1,9. This leads to the study of other effective methods. The adsorption process is one of the efficient methods to remove dyes from effluent but commercially available activated carbons are very expensive and adsorption capacities of non-conventional adsorbents are not very large, new adsorbents are still under development. As basic dyes are not commonly used in textile industries 10, so, there is a need to search for materials that could bind with anionic dyes, which are commonly used in textile industries. The aim of this work was to develop an adsorbent with appropriate porosity from groundnut shell by ZnCl 2 activation and to investigate its adsorption capacity by removal of anionic dye from aqueous solution. The influence of activation variables on the adsorption characteristics and % yield has been investigated. To observe the applicability of product in treatment system, its characterization has been done for physical, chemical and adsorption properties. SEM and FTIR studies were carried out to see the porosity and surface functional groups development in the product upon activation. Freundlich and Langmuir equations were used to fit the equilibrium isotherm and influence of adsorbent dose and contact time was studied on dye adsorption. The results will be useful for further application of GSPAC in colour removal from wastewater. Experimental Procedure Adsorbent preparation Groundnut shell received from local oil mill was washed thoroughly, dried, crushed and sieved to 2 mm size. The biomass waste was kept in covered stainless steel vessels in an electric furnace and heated at 400 o C for 30 min. The char so produced was used for chemical activation by impregnating it with ZnCl 2 for 24 h in different ratios and dried at 100±5 o C. The impregnated char was carbonized at 650 o C for 15 min. After that, the activated product was treated with (1:1) HCl for the removal of ZnCl 2. The recovered ZnCl 2 (80%) was recycled back into system for impregnation. Further, the product was washed with hot distilled water for the removal of chlorides and acidity. It was finally dried and sieved to get particular particle size. Various conditions like ZnCl 2 to char ratio, activation temperature and time were optimized for the preparation of adsorbent based on percent yield and adsorption efficiency in terms of iodine number. Percentage yield of product during variation in charring and activation conditions are given in Table 1. Percentage yield was calculated based on the wt. difference of initial raw material and char during initial carbonization and wt. difference of char and chemical free activated product after activation. The chemical recovery (C.R.) was estimated 6 as follows: Wt. of product prior to washing Wt. of product after washing C.R. = 100 Wt. of chemical agent added The GSPAC was characterised for it s quality control parameters and compared with commercial powdered activated carbon (CPAC) obtained from Merck. The results of characterization of adsorbents are an average of three replicates and the precision of the experiments was in the range of 1-5%. Analysis of activated carbon The activated carbon was analyzed for its moisture, ash, volatile matter and fixed carbon content (by difference) using the Method 11 IS 1350, 1984. Ultimate analysis was done by using CHNS analyzer (Carlos Erva, Model No. 1108, Italy).

MALIK et al.: ADSORBENT FROM GROUNDNUT SHELL BY ZnCl 2 ACTIVATION 321 Characterization of adsorbent Apparent density and particle size determination Particle size was determined using particle size Analyzer (CILAS 1180 Liquid) and apparent density was determined by a procedure described by Hassler 12. Determination of surface area The activated carbon and char samples were also characterized by measuring the surface area. Nitrogen gas was used for the adsorption and surface areas were calculated by BET N 2 method using the instrument Micromeritics, ASAP-2000. Determination of iodine number Iodine number indicates the extent of micropore distribution in the carbon. Iodine number (mg iodine/g carbon) is determined by using a 0.1 N standardized iodine solution and extrapolating to 0.02 N by an assumed isotherm slope using the standard method 13. The titrant used was 0.1 N sodium thiosulphate. Determination of methylene blue number Methylene blue number is correlated with ability of activated carbon to adsorb colour and high molecular weight substances. It was measured 13 by extent of adsorption in milligrams of methylene blue adsorbed by one gram of activated carbon in equilibrium with a solution of methylene blue having a concentration of 1.0 mg L -1. SEM studies To observe the surface pore structure of activated carbons, SEM studies were carried out using Electron Probe Micro Analyzer (Model Jeol-JXA 840 A, Japan). For char and activated carbon samples, the foremost requirement is that they must be moisture free. The pulverized sample was mounted onto a substrate with a conductive adhesive. Coating with a thin film of conducting material is the primary requisite for all non-conducting specimens to be examined in SEM. In the present study, conducting material coating on specimen was done with gold metal by vacuum evaporation to get uniform thickness of specimen during analysis. FTIR determination The surface functional groups on char and activated carbon were studied by diffuse reflectance infrared fourier transform (DRIFT) spectrometer (Perkin-Elmer, Model Spectrum One, USA). For FTIR analysis, aliquots of the samples were diluted or mixed in KBr and KBr pellets were then vacuum pressed. Absorbance spectra were recorded from 4000 to 400 cm -1. By comparison to the standard frequency patterns 14, various characteristic chemical bonds (or stretching) were determined from which certain functional groups could be derived. Determination of acidic and basic oxides Quantitative determination of surface functional groups of activated carbon was done according to the Boehm titration method 15. The surface acidity was determined by putting 1 g of dried PAC in 50 ml of basic solutions of 0.1 N: NaHCO 3, Na 2 CO 3 and NaOH and stirred for 72 h. The suspension was then filtered and the excess basic solution was titrated with 0.1 N HCl solution. Titration of the basic surface functional groups, chromene and pyrone was also done by placing 1 g of PAC into 50 ml of 0.1 N HCl solution. After 72 h of stirring and then a filtration, the excess of acid was titrated. Estimated errors of the quantitative determination of acidic or basic surface functional groups were about 5%. Preparation of synthetic dye solution Fuchsin acid 16 {2-amino-5-[(4-amino-3-sulphophenyl) (4-imino-3-sulpho-2,5-cyclohexadien-1-ylidene) methyl]3-methylbenzene sulphonic acid disodium salt (I)}, also commonly known as Acid magenta, Acid rubin, Acid roseine etc. was obtained from BDH, U.K. Double distilled water was used for the preparation of purplish red dye solution and dye calibration curve was prepared for a concentration range of 2.5-25 mg L -1 using UV-Visible spectrophotometer (Spectronic Genesys 2, USA) at 546 nm on absorbance mode. Instrument had spectral slit width of 2 nm with split beam and dual detectors with accuracy of ± 1 nm and precision ± 0.5 nm. Na + - O 3 S H 2 N H 3 C (I) NH 2 SO 3 - SO 3 - Na + NH 2 +

322 INDIAN J. CHEM. TECHNOL., JULY 2006 Batch adsorption experiment Adsorption experiments with GSPAC were carried out by agitating 0.06 g of adsorbents with 100 ml of dye solution having conc. 100 mg L -1 in a series of BOD bottles of 300 ml capacity at temperature of 30±1 o C at 150 rpm on a rotary orbital shaker. The samples were withdrawn from shaker at predetermined time intervals (5-120 min) and adsorbent was separated from the solution by centrifugation at 5000 rpm for 15 min. Residual dye concentration was determined using absorbance values of dye before and after the treatment at 546 nm by spectrophotometeric method. Isotherm studies were conducted by shaking different quantities of adsorbent varying from 0.02 to 0.2 g in 100 ml (100 mg L -1 ) of dye solution for a time period equal to equilibrium time. All experiments were replicated and the average results were used in data analysis. Fig. 1 Effect of variation of ZnCl 2 /char ratio in terms of iodine number Results and Discussion Effect of variation in ZnCl 2 /Char ratio on iodine value To observe the effect of ZnCl 2 concentration during activation, carbonized char prepared at 400 o C temperature for 30 min of heating time was impregnated with ZnCl 2 in ratios from 0.5 to 2.0 and carbonized at 700 o C for 30 min. It was observed that adsorptive power of carbon in terms of the value of iodine number increases with the increase of ZnCl 2 /char ratio (Fig. 1), which proves that adsorption efficiency is dependent upon concentration of ZnCl 2. In the course of chemical activation with ZnCl 2 at temperatures around 550-560 o C, the gases released on thermal decomposition create voids and porosity in the carbonized material. Moreover, impregnation with ZnCl 2 followed by carbonization dehydrates the cellulose material resulting in aromatization of the carbon skeleton and development of pore structure. The porosity developed in the carbon structure is also the result of the removal of ZnCl 2 from the particle due to intense washing 7,17. During activation, ZnCl 2 is responsible for degradation of organic material to release volatile matter and development of micro porous structure that results in the increase of the iodine value. The same pattern have been obtained in other investigations also 7,17,18 where increase in ZnCl 2 ratio causes decrease in % yield but enhanced values of surface area, pore volume, adsorption parameters like iodine and methylene blue number etc. Fig. 2 Effect of temperature on iodine number Effect of activation temperature on iodine value At the optimized ratio of 1.75 (ZnCl 2 /Char), activation of char was done at various temperatures (600-800 o C) for 30 min. The product was evaluated for its iodine number and it was observed that the adsorptive capacity of the product was found to be comparative at temperatures between 650 and 750 o C but decreases at 800 o C (Fig. 2). Comparative values of iodine number from 650 to 750 o C clearly indicate that 650 o C is the optimum temperature for the activation of groundnut shell for the production of carbon. Effect of activation time on iodine value Keeping the optimized ratio of ZnCl 2 /char at 1.75 and temperature at 650 o C, impregnated char was carbonized for different time intervals (5-60 min). It was observed that with increase of activation time up to 15 min there was sharp increase in the iodine number which decreases further on the increase of activation time (Fig. 3). Decrease in iodine number on increase in time after 15 min indicates that due to prolonged activation time, there may be possibility of clogging of pores by condensation of liquid volatile matter released (tar etc.) during early duration of activation.

MALIK et al.: ADSORBENT FROM GROUNDNUT SHELL BY ZnCl 2 ACTIVATION 323 Fig. 3 Effect of time on iodine number Effect of activation on % yield of product Percent yield is an important parameter to know the rate of carbonization process. Results for percent yield of char and activated carbon during various activation conditions are shown in Table 1. It was observed that percent yield decreases with increase in temperature during I st step of carbonization, it indicates that release of volatile matter present in biomass increases with increase of temperature. But during second step of carbonization, % yield remains almost constant during variations in activation temperature and time except for variation in ZnCl 2 /char ratio. Any nutshell 19 commonly consists of cellulose, hemi-cellulose and lignin, cellulose and lignin decompose in the temperature range of 300-430 and 250-550 o C, respectively. Hemi-Cellulose decomposes at much lower temperature i.e. 250-350 o C. The results of % yield for variation in activation temperature and time are in good agreement with the above statement because up to 555 o C, maximum volatile matter is released due to decomposition of cellulose, hemi-cellulose and lignin and that s why % yield remains almost constant on increase of temperature and time because there is no volatile matter left for release. Percent yield decreases due to increase in ZnCl 2 /char ratio, this may be due to release of more volatile matter w.r.t. more cracking in char by increase in ZnCl 2 concentration. As it is known 20 that inorganic salts always catalyze the char condensation and at the same time these salts catalyze the char-gasification reactions also. Characterization of product for various quality control parameters The adsorbent, prepared from groundnut shell at the optimized conditions of 650 o C, 15 min of activation time and ZnCl 2 /Char ratio of 1.75, was further characterized for its quality control parameters Table 1 % yield of product biomass for various conditions during pyrolysis Pyrolysis stage Optimization condition Variation in parameter 1 Char preparation Temperature ( o C) 300 350 400 2 Activation ZnCl 2 /Char ratio 0.5 0.75 1.0 1.25 1.5 1.75 2.0 Temperature ( o C) 600 650 700 750 800 Time (min) 5 15 30 45 60 % Yield 65 45 40 76 76 74 73 72 69 57 66 66 65 65 65 67 67 66 66 65 Table 2 Proximate and ultimate analysis of powdered activated carbon Analysis Value (%) Proximate Moisture 9.0 Ash 8.2 Volatile matter 11.4 Fixed carbon 71.4 Ultimate Carbon 75.8 Hydrogen 2.95 Nitrogen 1.53 Sulphur 0.32 as in relation to the knowledge of adsorptive behaviour of activated carbon, it is often desirable to have information about the basic properties of activated carbon that determines the utility of the carbon. Adsorbent was characterized for its proximate and ultimate analysis (Table 2). From the proximate analysis, it was observed that moisture, ash and volatile matter % was high which may be due to its plant origin. However, aromatized carbon was found

324 INDIAN J. CHEM. TECHNOL., JULY 2006 Table 3 Physical and adsorptive characteristics of powdered activated carbon obtained from groundnut shell Parameter Value (%) Apparent density (g/cm 3 ) 0.23 Particle size (mm) 0.075 Surface area (m 2 /g) Char 550 Activated carbon 1200 Loss on Ignition (%) 91.8 Iodine number 1114 Methylene blue number 238 Fig. 4 SEM micrograph of groundnut shell based char before activation to be significant which is evident from the carbon percentage in ultimate analysis. Physical and adsorptive characteristics of adsorbent are shown in Table 3. Bulk density and particle size of adsorbent are essential parameters to know before its application in treatment system. The density of the adsorbent depends not only on the nature of the starting material but also the preparation process. Surface area is very high for groundnut shell based adsorbent which shows it s high adsorption capacity. Adsorbent was characterized for its adsorption properties and it s having high iodine and methylene blue number. Adsorptive properties are directly linked with the porosity of activated carbon as the highly porous carbon can adsorb relatively large quantities of adsorbate. Iodine number can be correlated with ability to adsorb low molecular weight substances as the iodine molecule is relatively small and so provides a measure of surface area or capacity available to small adsorbates. Iodine numbers for commercial adsorbents range from 300 to 1200 mg/g. On the other hand, methylene blue number indicates ability of adsorbing high molecular weight substances and colour. Any carbon showing methylene blue number above 200, indicates the carbon with good activity for adsorption. So results of adsorptive properties indicate that groundnut shell is a very good source of raw material for the production of activated carbon of high efficiency. Porosity development upon activation To study the effect of activation on porosity development, SEM studies were carried out for groundnut shell based char and activated carbon (Figs 4 and 5). SEM micrograph of char shows the Fig. 5 SEM micrograph of groundnut shell based powdered activated carbon after activation presence of some flaky structure and rudimentary pores while SEM micrograph of activated carbon shows the presence of wide pores which result due to chemical activation with ZnCl 2 which is highly corrosive and dehydrating in nature. This activation process is dependant on raw material, which is composed of mainly cellulose, and due to the electrolytic action of activation agent, cellulose undergoes swelling, during which the arrangement of the molecules in the direction of longitudinal axis remains unchanged, but the lateral bonds are broken down with the result that the inter- and intra-micelle voids increase 21 under the effect of chemical reactions. Due to dehydration, decomposition of organic matter takes place that causes high porosity of activated carbon under the effect of chemical activation. Micrographs of char and carbon show that activation plays key role in porosity development which is largely responsible for the extent of surface area and adsorptive capacity of carbon.

MALIK et al.: ADSORBENT FROM GROUNDNUT SHELL BY ZnCl 2 ACTIVATION 325 Table 4 Some fundamental IR absorption frequencies of GSPAC and char Band position GSPAC (cm -1 ) Possible assignment Band position char (cm -1 ) Possible assignment 3746 O-H stretching of hydroxyl group 3442 O-H stretching of hydroxyl group 3226, 2339 N-H stretching 2963 C-H alkane asymmetric stretching 2957 C-H stretching 2344 N-H stretching 2116 -C C-H alkyne stretching 1588 1436 CO 2 - asymmetric stretching In plane C-H rocking 1518 N-H deforming 1255 COOR, ester stretching 1238 CH 3 COOR, ester stretching 634 C-S stretching 1038 C=S stretching 887 O-O stretching 707, 623 > C = O stretching Table 5 Amount of surface functional groups present on GSPAC analyzed by selective acid-base neutralization Surface functional group (mmol g -1 ) Carbon GSPAC Carboxyl 0.590 Lactonic 1.296 Phenolic 1.864 Acidic 3.750 Total basic 4.440 Fig. 6 FTIR spectra of groundnut shell based char Surface chemistry The drift spectra of char and activated carbon are shown in Figs 6 and 7, respectively. There is formation of more surface functional groups on carbon as compared to char due to activation (Table 4). New surface functional groups exhibited by absorption bands at 3746, 3226, 2116, 1518, 1038 and 707 cm -1 were developed in carbon sample. The drift spectra of groundnut shell based activated carbon show that both types i.e. acidic and basic surface functional group are present on its surface. The presence of sulphur and amino groups on the surface of groundnut shell based carbon can be attributed to its leguminous plant origin as evident from high content of N and S in elemental analysis. Presence of more nitrogen groups on surface of activated carbon helps in removal of dyes and organic pollutants. However, selective acid base neutralization method gives more information on the oxygen surface functionality of the activated carbon studied. The results of the acid-base neutralization from the respective aqueous solutions are given in Table 5. Fig. 7 FTIR spectra of groundnut shell based activated carbon GSPAC shows more no. of total basic groups compared to acidic groups which will also account for efficient uptake of acidic dye. Effect of contact time on fuchsin acid adsorption The percentage of dye removal as a function of time is shown in Fig. 8 with initial 100 mg L -1 concentration of dye and adsorbent dosage of 0.06 g for 100 ml of dye solution. The uptake of dye by adsorbents occurs at a faster rate corresponding to 81% removal in equilibrium time of 45 min and after

326 INDIAN J. CHEM. TECHNOL., JULY 2006 Fig. 8 Effect of time on % removal of fuchsin acid dye this desorption takes place leading to again adsorption. In batch type adsorption, monolayer of adsorbate is normally formed on the surface of adsorbent and the rate of removal of adsorbate species from aqueous solution is controlled primarily by the rate of transport of the adsorbate species from the exterior/outer sites to the interior site of the adsorbent particles 22,23. Effect of adsorbent dose on dye adsorption The percent adsorption increased from 67.5 to 99% as the adsorbent dose was increased from 0.02 g-0.2 g 100 ml -1 at 100 mg L -1 dye concentration on equilibrium time of 45 min (Fig. 9). It is apparent that by increasing the adsorbent dose, the amount adsorbed per unit mass decreases. It is readily understood that number of available adsorption sites as well as surface area increases by increasing the adsorbent dose and it, therefore, results in the increase of amount of adsorbed dye. Although percent adsorption increases with increase in adsorbent dose but amount adsorbed per unit mass decreases. This statement can be supported from the results that the trend in % adsorption shows very sharp increase from 67.5 to 92% at the dose increment from 0.02-0.12 g 100 ml -1 but on further increasing the dose, the maximum % adsorption i.e. 99% is attained at the dose of 0.18 g 100 ml -1 as clearly shown in Fig. 9. So increase in adsorption is there w.r.t. increase in dose but amount adsorbed per unit mass decreasing with increasing dose. The decrease in adsorption density with increase in adsorbent dose is mainly because of unsaturation of adsorption sites through the adsorption process 24,25. Adsorption isotherms The results obtained for adsorption of dye by GSPAC were analysed by the well-known models given by Freundlich and Langmuir. These isotherms 26 are represented by equations: Fig. 9 Effect of dose on % removal of fuchsin acid dye The linearized Freundlich isotherm is shown in Eq. (1): X l log = log K + logc M n (1) The Freundlich isotherm equation considers heterogeneous surfaces and is based on the idea that the adsorption depends on the energy of the adsorption sites. The linear form of Langmuir isotherm is represented in Eq. (2): 1 1 1 1 = + X / M Q bq C (2) The Langmuir equation describes adsorption on strongly homogenous surfaces. The model assumes uniform energies of adsorption onto the surface and no transmigration of adsorbate in the plane of the surface. Where X/M = amount of solute adsorbed per unit weight of adsorbent (mg/g), C = concentration of solute remaining in solution at equilibrium (mg/l), Q = amount of solute adsorbed per unit weight of adsorbent in forming a complete monolayer on the surface (mg/l) (or related to maximum adsorption capacity), b = a constant related to the energy or net enthalpy, K and n = Freundlich constants related to adsorption capacity and adsorption intensity, respectively. Isotherm studies The data obtained from isotherm studies were tested for their applicability to the two isotherm models. Table 6 summarizes the value of the coefficients for both isotherms models, which were calculated from the best-fit lines. Freundlich isotherm fits the data (Fig. 10) very well as the values of 1< n <10 show favourable adsorption 27 of fuchsin acid

MALIK et al.: ADSORBENT FROM GROUNDNUT SHELL BY ZnCl 2 ACTIVATION 327 Table 6 Freundlich and Langmuir adsorption isotherm constants for GSPAC for dye removal Adsorbent Freundlich Langmuir K n R 2 Q b R 2 GSPAC 47.206 2.889 0.936 125 0.65 0.68 on GSPAC. The values of Langmuir constants indicate favourable conditions for adsorption but low values of correlation coefficients show that langmuir model (Fig. 11) is less fitted to present adsorption study compared to Freundlich model. In general, according to R 2 criterion, Freundlich model fits better for dye adsorption for present adsorption study. Adsorption mechanism As maximum dyes are ph dependent, so adsorption experiments were done at its natural ph. The complexity of aqueous solution adsorption arises from the large number of variables involved 28. These include the ph of the solution, the ionic strength, solute-solute interaction and solute-solvent interaction. Activated carbon is one of the synthetic adsorbents which have a high degree of surface complexity. Undoubtedly, the ph value of the dye solution plays an important role in the whole adsorption process and particularly on the adsorption capacity. It is necessary to consider the basic models for carbon basicity because of the potential relevance of activated carbon materials to liquid phase applications, such as metal sorption, organic species sorption etc. The basic surface can be represented as 29 : S B + H + S B And commonly in the form 29 : (3) COH + H + + COH 2 (4) In Eqs (3) and (4), the basic properties are restricted to hydroxyl type groups 31 or, at most, extend to include other oxygen functional groups 32. More detailed models to account for carbon surface basicity are available in literature 33. The presence of chromene (benzpyran) type structures was postulated to account for acid uptake (i.e. carbon basicity). It has been reported 1 that the basic surface properties arise from two types of interactions (i) electron-donor acceptor (EDA) complex formation that predominates in carbon of low oxygen content and (ii) pyrone type Fig. 10 Freundlich adsorption isotherm for fuchsin acid dye by GSPAC Fig. 11 Langmuir adsorption isotherm for fuchsin acid dye by GSPAC group contribution, which prevails in carbons of high oxygen content. Two possible mechanisms of adsorption of acid dye on GSPAC may be considered: (a) electrostatic interaction between the protonated groups of carbon and acidic dye and (b) the chemical reaction between the adsorbate and the adsorbent. At low ph, a significantly high electrostatic attraction exists between the positively charged surface of the adsorbent and anionic dye. Also, the surface of GSPAC contains significant amount of basic groups as shown by FTIR spectra and results of Boehm titration, chemisorption might be operative. Conclusions Activation of char with ZnCl 2 results in considerable modifications of its textural and adsorption properties. ZnCl 2 causes hydrolytic reactions, weakens the precursor structure and release of volatile matter takes place producing microporous structure in carbon which results in high surface area and adsorptive capacity of carbon as compared to char. Activation process also modifies the surface chemistry of adsorbent by formation of more functional groups as compared to char, which surely

328 INDIAN J. CHEM. TECHNOL., JULY 2006 enhances the performance of adsorbent. The results of dye adsorption experiments show that activated carbon prepared from low cost material i.e. groundnut shell has suitable adsorption capacity for removal of Fuchsin acid dye and adsorption is highly dependent on adsorbent dose and contact time. Adsorption pattern is valid for both Freundlich and Langmuir Isotherms but Freundlich model fits better and shows high correlation coefficient values indicating heterogenous adsorption. The physico-chemical and adsorptive properties of groundnut shell based adsorbent indicate its potential use as an adsorbent for applicability in water and wastewater treatment system. Acknowledgement The authors are thankful to the Director, NEERI, Nagpur for providing facilities to carry out this research work and permission for this publication. References 1 Bailey S E, Olin J J, Bricka R M & Adrian D D, Water Res, 33 (1999) 2469. 2 Tsai W T, Chang C Y, Wang S Y, Chang C F, Chien S F & Sun H F, Resour Conserv Recycl, 32 (2001) 43. 3 Anukar K, Collin G J, Faujan B H, Zulkarnian A Z, Hussain Z M & Abdullah H A, Res J Chem Environ, 5(3) (2001) 21. 4 Hayashi J, Kazehaya A, Muroyama K & Watkinson A P, Carbon, 38 (2000) 1873. 5 Christopher A T, Wayne E M & Mitchell M J, Energeia, 9(3) (1998) 1. 6 Ahmadpour A & Do D D, Carbon, 34(4) (1996) 471. 7 Rodriguez-Reinoso F & Molina-Sabio M, Carbon, 30(7) (1992) 1111. 8 Lillo-Rodenas M A, Carzrola-Ameros D & Linares-Solano A, Carbon, 41 (2003) 265. 9 Malik P K, Dyes Pigms, 56 (2003) 239. 10 Mckay G, J Chem Technol Biotechnol, 32 (1982) 759. 11 IS 1350 (Part 1): Methods of Test for Coal and Coke: Part 1, Proximate Analysis (Second Revision, Amendment (1), 1984, 28. 12 Hassler J W, Purification with Activated Carbon (Chemical Publishing Company, New York), 1974, 350. 13 U S Environmental Protection Agency, Process design manual for carbon adsorption, Technology Transfer EPA, 625, 1-71-0029, 1973, B2-B16. 14 Socrates G, Infrared Characteristic Group Frequencies (A Wiley-Interscience Publ, John Wiley & Sons, Chichester- Toronto), 1980. 15 Boehm H P, Adv Catal, 16 (1966) 179. 16 The Merck Index, 12 th edn, edited by Budavari S (Published by Merck Research Laboratories, Division of Merck & Co, Inc, White House Station, NJ), 1996, 972. 17 Caturala F, Molina-Sabio M & Rodriguez-Reinoso F, Carbon, 29(7) (1991) 999. 18 Hu Z, Srinivasan M P & Yaming N, Carbon, 39 (2001) 877. 19 Raveendran K, Ganesh A & Kartic C K, Fuel, 75(8) (1996) 987. 20 Raveendran K & Ganesh A, Fuel, 77(7) (1998) 769. 21 Smisek M & Cerny S, Active Carbon Manufacture, Properties and Application (Elsevier Publishing Company, Amsterdem), 1970. 22 Hall K R, Eagleton L C, Acrivos A & Vemenlen T, Ind Eng Chem Fund, 5 (1996) 212. 23 Kannan N & Sundaran M M, Dyes Pigms, 1 (2001) 25. 24 Ozacar M & Sengil I A, Bioresourc Technol, 96 (2005) 791. 25 Yu L J, Shukla S S, Dorris K L, Shukla A & Margrove J L, J Hazard Mater, B100 (2003) 53. 26 Ramakrishna K R & Viraraghvan T, Waste Manage, 17(8) (1997) 483. 27 Mckay G, Blair H S & Garden J R, J Appl Polym Sci, 27 (1982) 3043. 28 Al-degs Y, Khraisheh M A M, Allen S J & Ahmad M N, Water Res, 34(2) (2000) 927. 29 Muller C, Radke C J & Prausnitz J M, J Phys Chem, 84 (1980) 369. 30 Noh J S & Schwartz J A, J Colloid Interface Sci, 130(1) (1989) 157. 31 Corapcioglu M O & Huang C P, Carbon, 25 (1987) 269. 32 Abosti G M K & Scarroni A W, Carbon, 28 (1990) 79. 33 Leon y Leon C A, Solar J M, Calemma V & Radovic L R, Carbon, 30 (1992) 797.