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Removal of chromium (VI) from dilute aqueous solutions by activated carbon developed from Terminalia arjuna nuts activated with zinc chloride
Chemical Engineering Science 60 (2005) 3049 – 3059 www.elsevier.com/locate/ces
Removal of chromium (VI) from dilute aqueous solutions by activated carbon developed from Terminalia arjuna nuts activated with zinc chloride Kaustubha Mohanty, Mousam Jha, B.C. Meikap∗ , M.N. Biswas Department of Chemical Engineering, Indian Institute of Technology (IIT), Kharagpur, P.O. Kharagpur Technology, Dist: Midnapur (West), West Bengal, Pin - 721302, India Received 17 August 2004; received in revised form 29 October 2004; accepted 4 December 2004 Available online 16 March 2005
Abstract Different structured activated carbons were prepared from Terminalia arjuna nuts, an agricultural waste, by chemical activation with zinc chloride for the adsorption of Cr(VI) from dilute aqueous solutions. The most important parameter in chemical activation was found to be the chemical ratio (activating agent/precursor, g/g). Carbonization temperature and time are the other two important variables, which had significant effect on the pore structure of carbon. A high surface area of 1260 m2 /g was obtained at a chemical ratio of 300%, carbonization time and temperature of 1 h and 500 ◦ C, respectively. The activated carbon developed shows substantial capability to adsorb Cr(VI) from dilute aqueous solutions. The parameters studied include pH, adsorbent dosage, contact time, and initial concentrations. The kinetic data were best fitted to the Lagergren pseudo-first-order model. The isotherm equilibrium data were well fitted by the Langmuir and Freundlich models. The maximum removal of chromium was obtained at pH 1.0 (about 99% for adsorbent dose of 2 g/l and 10 mg/l initial concentration). 䉷 2005 Elsevier Ltd. All rights reserved. Keywords: Pollution; Terminalia arjuna nuts; Activated carbon; Chemical activation; Surface area; Cr(VI) removal; Waste water treatment; Water pollution
1. Introduction Water pollution due to toxic heavy metals has been a major cause of concern for chemists and environmental engineers. The industrial (Dwari et al., 2004) and domestic wastewater, if properly not managed, is responsible for causing severe damage to the environment and adversely affecting the health of the people. Chromium is one of the contaminants, which exist in hexavalent and trivalent forms. Hexavalent form is more toxic (Smith and Lec, 1981) than trivalent and requires more concern. Strong exposure of Cr(VI) causes cancer in the digestive tract and lungs (Kaufaman, 1970) and may cause epigastric pain, nausea, vomitting, severe diarrhoea, and hemorrhage (Browning, 1969). It is therefore, essential to remove Cr(VI) from wastewater ∗ Corresponding author. Tel.: +91 3222 283958; fax: +91 3222 282250. E-mail addresses:[email protected], [email protected] (B.C. Meikap).
0009-2509/$ - see front matter 䉷 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.12.049
before disposal. The main sources of chromium (VI) are tannery, paint, ink, dye, and from aluminium manufacturing industries etc. The treatment of chromium bearing effluents has been reported by several methods, such as reduction precipitation, ion exchange, electrochemical reduction, evaporation, reverse osmosis and direct precipitation (Marshal, 1980; Pat terson, 1985). Most of these methods need high capital cost and recurring expenses such as chemicals, which are not suitable for small-scale industries. The process of adsorption is by far the most versatile and widely used technique for the removal of metal ions. Activated carbon has been the water industry’s standard adsorbent for the reclamation of municipal and industrial wastewater for potable use for almost three decades (Fornwalt and Hutchins, 1966). Despite its prolific use in the water and waste industries, activated carbon remains an expensive material. In recent years, research interest in the production of low-cost alternatives to activated carbon has grown.
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Activated carbons (ACs) exhibit a great adsorption capacity, owing to their highly developed pore structures characterized by large surface areas. International growing demand of this adsorbent, mainly because of their use in applications related to their environmental mitigation, has led to search for new, available low-cost feedstocks of renewable character. In practice, coal and agricultural by-products of lignocellulosic materials are two main sources for the production of commercial activated carbons. Agricultural wastes have emerged as a better choice. There are a quite large number of studies regarding the preparation of activated carbons from agricultural wastes (Kadirvelu et al., 2003; Nag et al., 1999) which include nutshells (Ahmadpour and Do, 1997), fruit stones (Lussier et al., 1994), bagasse (Mohan et al., 2002), coirpith (Kadrivelu et al., 2001), oil palm waste (Lua and Guo, 1998) and agricultural residues from sugarcane (Blanco Castro et al., 2000), rice (Srinivasan et al., 1998) and peanut (Periasamy and Namasivayam, 1996; Ri cordel et al., 2001), sawdust (Marquez-Montesinos et al., 2001) and canes from some easy-growing wood species (Basso et al., 2002). Basically, there are two different processes for the preparation of activated carbon: physical activation and chemical activation. In comparison with physical activation, there are two important advantages of chemical activation. One is the lower temperature in which the process is accomplished. The other is that the global yield of the chemical activation tends to be greater since burn-off char is not required. Among the numerous dehydrating agents, zinc chloride in particular is the widely used chemical agent in the preparation of activated carbon. Knowledge of different variables during the activation process is very important in developing the porosity of carbon sought for a given application. Chemical activation by ZnCl2 improves the pore development in the carbon structure and because of the effect of chemicals; the yields of carbon are usually high (Ahmadpour and Do, 1997). In this work it has been reported the results obtained on the preparation of activated carbons from Terminalia arjuna nuts with zinc chloride activation and their ability to remove hexavalent chromium from dilute aqueous solutions. Different preparation variables on the characteristics of activated products were studied to find the optimum conditions for making activated carbons with well-developed porosity. The influence of several operating parameters, such as pH, adsorbent dosage, contact time, and initial concentrations on the adsorption capacity, were investigated. Kinetic models were used to identify the possible mechanisms of such adsorption process. The Langmuir and Freundlich models were used to analyse the adsorption equilibrium. 2. Experimental technique 2.1. Preparation of activated carbon Terminalia arjuna nuts collected from nearby locality was first washed with distilled water to remove the water-soluble
Table 1 Proximate analysis of Terminalia arjuna nut used as raw material Component
Quantities
Moisture 3% Volatile matter 20% Fixed carbon 76% Ash 1%
76 3 20 <1
impurities and surface adhered particles and then dried at 60 ◦ C to remove the moisture and other volatile impurities. Then the precursor was ground in the ball mill and sieved to a particle size range of 150–200 m. The proximate analysis of the precursor is presented in Table 1. Chemical activation of the powdered precursor was done with ZnCl2 . 10 g of dried precursor was well mixed with distilled water so that 100 ml concentrated solution contained 10 g of ZnCl2 . The chemical ratio is defined as the ratio of chemical activating agent (ZnCl2 ) to the precursor. The chemical ratio (activating agent/precursor) was 100% in this case. The mixing was performed at 50 ◦ C for 1 h. After mixing, the slurry was subjected to vacuum drying at 100 ◦ C for 24 h. The resulting chemical-loaded samples were placed in a stainless-steel tubular reactor and heated (5 ◦ C/min) to the final carbonization temperature under a nitrogen flow rate of 150 ml/min STP. Samples were held at the final temperature (carbonization temperature) for different carbonization times of 1, 2, 3 h before cooling down under nitrogen. Nitrogen entering in the reactor was first preheated to 250–300 ◦ C in a pre-heater. The products were washed sequentially with 0.5 N HCl, hot water and finally cold distilled water to remove residual organic and mineral matters, then dried at 110 ◦ C. In all experiments, heating rate and nitrogen flow was kept constant. The experiments were carried out for different chemical ratio (100–300%) and carbonization temperature (300–600 ◦ C). Weight loss of the carbon samples was calculated on a chemical free basis and chemical recovery (CR) was estimated according to CR =
WP i − WPf × 100, WC
(1)
where WP i and WPf are the weight of products before and after washing and Wc is weight of chemical used. The BET surface areas of these activated carbons were measured in Flowsorb-2300. The Flowsorb-2300 was used to find out the surface area of activated carbon on the basis of mono-layer adsorption at or near the boiling point of gases. The monolayer formation was achieved with a nitrogen–helium of 30% by volume. The adsorbed monolayer of nitrogen were established at atmospheric pressure and at the temperature of liquid nitrogen. In this method first sample quantity is optimized after several trial runs so that surface area falls with in 0.5–2.5 m2 . The sample is dried at 105–110 ◦ C and is free from any gases or vapours for which 200–250 ◦ C for 15 min is adequate. Relatively adsorbing and
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desorbing nitrogen at test position can be employed as a degassing means. Surface area in Flowsorb-2300 is displayed in terms of quantity of sample contained in sample tube. Then the displayed number is converted to specific surface area by dividing the weight of the sample.
2.2. Sorption procedure The adsorption capability of the prepared activated carbon toward Cr(VI) was investigated using aqueous solutions of the metal. A stock solution of 1000 mg/l was prepared by dissolving the necessary amount of potassium dichromate in distilled water. Analytical-grade reagents were used in all cases. The stock solution was diluted as required to obtain standard solutions of concentrations ranging between 10 and 100 mg/l. All the adsorption experiments were carried out at a constant temperature of 25 ◦ C in a incubator shaker using fractions of 200 m average particle size for all of the carbons. Concentrations of Cr(VI) in solution were analysed by UV-Spectrophotometer. Adsorption of Cr(VI) on developed activated carbon was conducted containing different weighted amounts of each sample with 100 ml solution of 10, 20 and 30 mg/l of initial concentration. The solutions were kept in a thermal shaker at controlled temperature (25 ◦ C) for a period of 24 h. Significant pH drift during the adsorption test was studied by measuring pH at start and end of each experiment. Once the equilibrium was attained, the slurries were filtered through 0.45 m membranes and equilibrium concentration was determined. The batch process was used so there is no need for volume correction. Batch adsorption experiments were performed by contacting 0.4 g of the selected carbon samples with 200 ml of the aqueous solution of different initial concentration (10, 20, 30 mg/l) at natural solution pH. Continuous mixing was provided during the experimental period with a constant agitation speed of 120 rpm for better mass transfer with high interfacial area of contact (Meikap et al., 2001). The remaining concentration of Cr(VI) in each sample after adsorption at different time intervals, was determined spectrophotometrically. Adsorption equilibrium isotherms on the selected carbon were determined using sample dosages of 0.4 g/200 ml aqueous solutions of initial concentrations in the range of 10–100 mg/l, at pH 1.0. For these experiments, the flasks were shaken, keeping the temperature (25 ◦ C) constant and agitation speed (120 rpm) for the minimum contact time required to attain equilibrium, as determined from the kinetic measurements detailed above. The effect of pH on the equilibrium adsorption of Cr(VI) on selected carbon was further investigated by employing different initial concentrations (10, 20 and 30 mg/l) and varying adsorbent dosages. The pH values were adjusted with dilute sulfuric acid and sodium hydroxide solutions. For these experiments, the flasks were shaken, keeping constant the
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temperature (25 ◦ C) and agitation speed (120 rpm) for the minimum contact time required to attain equilibrium, as determined from the kinetic measurements detailed above. The effect of adsorbent dosages on the equilibrium adsorption of Cr(VI) on the selected carbon was further investigated by employing different initial concentrations (10, 20 and 30 mg/l) at pH 1.0. The adsorbent dosages were varied from 0.5–3.5 g/l. The chromium concentration retained in the adsorbent phase was calculated according to qe =
(Co − Ce )V , W
(2)
where Co and Ce are the initial and equilibrium concentrations (mg/l), respectively, of Cr(VI) in solution, V is the volume (l), and W is the weight (g) of the adsorbent.
3. Results and discussion The results for the characterization of the prepared activated carbon are discussed in this paper. The tests for characterization include weight loss, chemical recovery, BET surface area and adsorption capacities of the activated carbon towards Cr(VI) removal. 3.1. Weight loss 3.1.1. Effect of carbonization time The effect of carbonization time on weight loss is shown in Fig. 1. It can be seen that carbonization time does not have much effect on the weight loss. The experimental data also showed that the weight loss increased with increasing carbonization time for a fixed carbonization temperature, as more volatiles were released then. The values of weight loss are 51.9, 52.3, 53.8 and 53.9 for 30, 60, 120, 150 and 150 min time, respectively.
3.1.2. Effect of carbonization temperature The effect of carbonization temperature on weight loss is shown in Fig. 2. Carbonization temperature also does not have much effect on the weight loss. Overall weight loss was found to increase with increasing temperature, resulting in decreasing yield of char as temperature increased. This weight loss was essentially due to the devolatilization of the Terminalia arjuna fibres upon heating and as expected, the quantity of volatiles evolved increased with increasing temperature. The values of weight loss are 50.9, 51.4, 52.3 and 53.8 with increase in time. The final yields of the chars resulting from carbonization at different temperature were fond to be about 48–52% of the original weight of the predried extracted fibre.
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Fig. 1. Effect of carbonization time on weight loss carbonization temperature = 500 ◦ C, chemical ratio = 100%.
Fig. 2. Effect of carbonization temperature on weight loss carbonization time = 60 min, chemical ratio = 100%.
3.1.3. Effect of chemical ratio The effect of chemical ratio on weight loss is shown in Fig. 3. It can be seen that adding chemical agents to the precursor decreases the weight loss of the carbon products. The reduction of weight loss is most likely due to the effect of the chemical agent in which it promotes the condensation (polymerization) reactions. These reactions, which occur among the aromatic hydrocarbons and tar-forming compounds, result in the formation of large molecules (polycyclic aromatics) in the structure of activated products and increase the carbon yield. 3.2. Chemical recovery 3.2.1. Effect of carbonization time The effect of carbonization time on chemical recovery is shown in Fig. 4. It can be seen that chemical recovery decreases with the carbonization time. This might be due to the evaporation of ZnCl2 from the precursor at longer carbonization time. 3.2.2. Effect of carbonization temperature The effect of carbonization temperature on chemical recovery is shown in Fig. 5. Decrease in chemical recovery with temperature can be explained as a consequence of ZnCl2 evaporation from the precursor at higher temperature.
Fig. 3. Effect of chemical ratio on weight loss carbonization time=60 min, carbonization temperature = 500 ◦ C.
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Fig. 4. Effect of carbonization time on chemical recovery carbonization temperature = 500 ◦ C, chemical ratio = 100%.
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Fig. 6. Effect of chemical ratio on chemical recovery carbonization time = 60 min, carbonization temperature = 500 ◦ C.
3.2.3. Effect of chemical ratio The effect of chemical ratio on chemical recovery is shown in Fig. 6. A portion of chemicals added to the carbon precursor can be recovered during the washing stage of the activated products after carbonization. The change in chemical reagent recovery with the chemical ratio is shown in Fig. 6. It is obvious that with increasing the chemical ratio, the recovery should be increased as well. However beyond 200% the chemical recovery remains constant. This can be explained as the increase in recovery reaches a saturated value beyond which it again bind with activated carbon and dilution effects nullify the increasing effect. 3.3. BET surface area
Fig. 5. Effect of carbonization temperature on chemical recovery carbonization time = 60 min, chemical ratio = 100%.
3.3.1. Effect of carbonization time The effect of carbonization time on BET surface area is shown in Fig. 7. The surface area first increase with carbonization time and reaches its maximum at 1 h and thereafter it decreases. The surface area decrease (after 1 h) was possibly due to some of the pores being sealed off as a result of sintering at excessive time duration. This phenomenon is confirmed by measuring the pore volume of activated carbon. Pore volume measured at 60 min was found more (0.606 ml/g) than that at 120 min (0.578 ml/g). Generally, a longer carbonization time is needed to enhance porosity as well as to clear blocked pore entrances before detrimental effects set in at prolonged times. From
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Fig. 7. Effect of carbonization time on BET surface area carbonization temp. = 500 ◦ C, chemical ratio = 100%.
Fig. 9. Effect of chemical ratio on BET surface area temperature=500 ◦ C, carbonization time = 60 min.
an initial high surface area, it deteriorated with increasing carbonization time. 3.3.2. Effect of carbonization temperature The effect of carbonization temperature on BET surface area is shown in Fig. 8. When the carbonization temperature was 300 ◦ C, pyrolysis reactions had just commenced, thereby producing very small BET surface area. This phenomenon was due to the inadequacy of heat energy produced at this low carbonization temperature for any substantial evolution of volatiles necessary for pore development. As the temperature was increased to 400 ◦ C and then to 500 ◦ C, more volatile matters were released progressively during carbonization, thereby resulting in the development of some new pores, and hence the BET surface area increased progressively. The decrease in surface area with further increase in temperature to 600 ◦ C might be due to a sintering effect at high temperature, followed by shrinkage of the char, and realignment of the carbon structure which resulted in reduced pore areas.
Fig. 8. Effect of carbonization temperature on BET surface area carbonization time = 60 min, chemical ratio = 100%.
3.3.3. Effect of chemical ratio The effect of chemical ratio on BET surface area is shown in Fig. 9. It shows that surface area increases with increase
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Fig. 10. (a) Effect of contact time on Cr(VI) removal. (b) Comparison of effect of contact time on % removal of Cr(VI) at different activated carbon doses and initial concentrations at constant Ph.
in chemical ratio. The two distinct regions of pore evolution can be observed. In the first region (Chemical ratio < 150%), the surface area increase at a high rate but in the second region (chemical ratio > 150%), the rate of porosity increase becomes slower. These phenomena may be due to the formation of multiplayer of multi-layers of activating agent, which is more prominent at higher chemical ratio. As a result there is a slow change in surface area.
carbon is more at initial stages, which gradually decreases and becomes almost constant after an optimum period of 600 min. A comparison at different activated carbon dose (1.0 and 2.0 g/l) with a constant pH=5.6 has been made and presented in Fig. 10 for the adsorption efficiency. It has been found that under identical conditions higher doses of activated carbon enhances the removal efficiency of Cr(VI). This is quite obvious that at higher dose activated carbon adsorb more quantity of Cr(VI), which resulted in increased efficiency.
3.4. Chromium removal 3.4.1. Contact time study The effect of time of contact for three different concentrations of Cr(VI) (10,20,30 mg/l) with activated carbon dose (2 g/l) at natural pH of solution (pH 5.85) is shown in Fig. 10. The chromium (VI) percent adsorption increases with time until the equilibrium is attained between the amounts of chromium adsorbed on the activated carbon and the remaining in solution. The figure shows that the adsorption of Cr(VI) increases with time from 0 to 300 min and more and then becomes almost constant up to the end of experiment. It can be concluded that the rate of Cr(VI) binding with activated
3.5. Adsorption kinetics Numerous kinetic models have been proposed to elucidate the mechanism by which pollutants may be adsorbed. The mechanism of adsorption depends on the physical and/or chemical characteristics of the adsorbent as well as on the mass transport process (Meikap et al., 2002). In order to investigate the mechanism of Cr(VI) adsorption, Lagergren proposed a pseudo-first-order kinetic model. The integral form of the model is log(qe − q) = log qe −
Kad t, 2.303
(3)
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Fig. 11. (a) Cr(VI) uptake by Terminalia arjuna nuts according to the Lagergren model. (b) Lagergren plot for Cr(VI) at constant pH.
Table 2 Lagergren constants Initial Cr(VI) Conc. (mg/l)
Kad (min−1 )
R2
10 20 30
0.01414 0.01087 0.0125
0.9922 0.9928 0.9835
where q is the amount of Cr(VI) sorbed (mg/g) at time t (min), qe is the amount of Cr(VI) sorbed at equilibrium (mg/g) and Kad is the equilibrium rate constant of pseudofirst-order adsorption (min−1 ). For calculating the rate constant of the adsorption at room temperature, Lagergren equation is plotted for three different initial Cr(VI) concentrations (10, 20 and 30 mg/l) with a fixed adsorbent dose shown in Fig. 11. The values of Kad for three different initial concentrations along with the correlation coefficients are given in Table 2. The resulting straight lines are showing the applicability of the pseudo-first-order kinetics. Detailed experiments were conducted to find out adsorption kinetics and results of Lagergen model for two different adsorbent doses shown in Fig. 11. The results show that the adsorption of Cr(VI) on activated carbon follow the first order kinetics.
3.5.1. Effect of pH Earlier studies have indicated that solution pH is an important parameter affecting adsorption of heavy metals. Cr(VI) removal was studied as a function of pH for three different initial concentrations for a fixed adsorbent dose (2 g/l) and the results are shown in Fig. 12. It is clear from this figure (Fig. 12) that the percent adsorption of Cr(VI) decreases with increase in pH from pH 1.0 to 5.0 and after pH 5.85 (natural pH) no adsorption takes place at all. It is important that the maximum adsorption at all the concentrations takes place at pH 1.0. This behaviour can be explained considering the nature of the adsorbent at different pH in metal adsorption. The cell wall of activated carbon contains a large number of surface functional groups. The pH dependence of metal adsorption can largely be related to the type and ionic state of these functional groups and also on the metal chemistry in solution. Adsorption of Cr(VI) below pH 3.0 (maximum at pH 1.0) suggests that the negatively charged species (chromate/dichromate in the sample solution) bind through electrostatic attraction to positively charged functional groups on the surface of activated carbon because at this pH more functional groups carrying positive charge would be exposed. But at above pH 3.0, it seems that activated carbon possesses more functional groups carrying a net negative
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Fig. 13. Effect of sorbent dose on Cr(VI) removal at solution pH = 1.0.
Fig. 12. Effect of pH on percent removal of Cr(VI).
charge, which tends to repulse the anions. However, there is also the removal above pH 3.0, as indicated by Fig. 12, but the rate of removal is considerably reduced. Hence, it could be said that above pH 3.0, other mechanism like physical adsorption on the surface of adsorbent could have taken an important role in the adsorption of Cr(VI) and exchange mechanism might have reduced. In addition adjustment of pH is very important. The possibilities of other subsidiary substances and hydrogen bonding may affect the adsorption process and need careful analysis. 3.6. Sorbent dose study The effect of adsorbent dosage on the percentage removal Cr(VI) has been shown in Fig. 13. It can be seen from the figure that initially the percentage removal increases very sharply with the increase in adsorbent dosage but beyond a certain value 0.25–0.3 g, the percentage removal reaches almost a constant value. This trend is expected because as the adsorbent dose increases the number adsorbent particles increases and thus more Cr(VI) is attached to their surfaces. A maximum removal of 99.52% was observed at adsorbent dosage of 2 g/l at pH 1.0 for an initial Cr(VI) concentration of 10 mg/l. Therefore, the use of 2 g/l adsorbent dose is justified for economical purposes.
3.6.1. Adsorption isotherms Several models have been used in the literature to describe the experimental data of adsorption isotherms. The Langmuir and Freundlich models are the most frequently employed models. In the present work both models were used. The chromium sorption isotherm followed the linearized Freundlich model as shown in Fig. 14. The relation between the metal uptake capacity ‘qe ’ (mg/g) of adsorbent and the residual metal ion concentration ‘Ce ’ (mg/l) at equilibrium is given by ln qe = ln k +
1 ln Ce , n
(4)
where the intercept ln k is a measure of adsorbent capacity, and the slope 1/n is the sorption intensity. The isotherm data fit the Freundlich model well (R 2 =0.9612). The values of the constants k and 1/n were calculated to be 3.915 and 0.551. Since the value of 1/n is less than 1, it indicates a favourable adsorption. The Langmuir equation relates solid phase adsorbate concentration (qe ), the uptake, to the equilibrium liquid concentration (Ce ) as follows: KL bC e , (5) qe = 1 + bC e where KL and b are the Langmuir constants, representing the maximum adsorption capacity for the solid phase loading and the energy constant related to the heat of adsorption respectively. It can be seen from Fig. 15 that the
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K. Mohanty et al. / Chemical Engineering Science 60 (2005) 3049 – 3059 Table 3 Adsorption isotherms parameters at pH = 1.0 for Cr(VI) removal Parameters
KL (mg/g)
b (l/g)
R2
k (l/g)
1/n
Values
28.43
0.1183
0.9913 Langmuir 0.9612 Lagergren
3.915
0.551
to 1.0 for maximum removal of Cr(VI) are presented in Table 3. 4. Conclusions
Fig. 14. Freundlich adsorption isotherm.
Fig. 15. Langmuir adsorption isotherm.
isotherm data fits the Langmuir equation well (R 2 =0.9913). The values of KL & b were determined from the figure and were found to be 28.43 mg/g and 0.1183 l/mg, respectively. The outcome values of parameters k, n, KL , b, R 2 for all the experiments with pH of solution equal
The Terminalia arjuna nut precursor is found to be a good raw material for developing activated carbons. This study demonstrated that ZnCl2 is a suitable activating agent for the preparation of high-porosity carbons from Terminalia arjuna nut. For the carbonization of the ZnCl2 treated sample, ZnCl2 plays an important role in retarding tar escape during carbonization. The washing process following carbonization with ZnCl2 has a significant influence on the surface properties of resulting char. It was found that acid washing is a necessary step for the preparation of high-porosity carbons. High surface area of activated carbons was prepared from the chemical activation of Terminalia arjuna nuts with ZnCl2 as chemical agent. Results shows that the porosity of resulting activated carbon increases with carbonization temperature and carbonization time to a maximum up to a temperature of 500 ◦ C and 1 h, respectively and then began to decrease thereafter. Also the porosity increases with chemical ratio up to 300% and then becomes almost constant. The optimal conditions for the production of high surface area activated carbon from Terminalia arjuna nut by chemical activation are: chemical ratio (activating agent/precursor) of 300%, carbonization time of 1 h and carbonization temperature of 500 ◦ C. At this optimal condition, the BET surface area obtained was 1260 m2 /g. The aqueous adsorption tests indicate that the Terminalia arjuna nut-derived activated carbon has a notable adsorption capacity for Cr(VI). The kinetics of Cr(VI) adsorption followed nicely the pseudo-first-order rate expression. The Langmuir and Freundlich models fit the isotherm data well. It is interesting to mention that the Lagergen model is compatible with other two models. The uptake of the Cr(VI) was greatly affected by the solution pH. The data thus obtained may be helpful for designing and establishing a continuous treatment plant for water and wastewaters enriched in Cr(VI). The cost of removal is expected to be quite low, as the adsorbent is cheap and easily available in large quantities as compared and reported in the literature.
Notation 1/n b
sorption intensity, dimensionless Langmuir constant, l/g
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Ce equilibrium chromium concentration, mg/l Co initial chromium concentration , mg/l Chemical ratio activating agent (ZnCl2 )/precursor, g/g CR chemical recovery, g/g k measure of adsorbent capacity, l/g Kad equilibrium rate constant of pseudo-first-order adsorption, min−1 KL Langmuir constant, mg/g qe amount of chromium adsorbed at equilibrium, mg/g T temperature, ◦ C t time, min V volume of the solution, l W weight of the adsorbent, g Wc weight of chemical used, g weight of product after washing, g Wpf Wpi weight of product before washing, g Acknowledgements The authors thankfully acknowledge the reviewers for suggesting valuable technical comments of this paper. References Ahmadpour, A., Do, D.D., 1997. The preparation of activated carbon from Macademia Nutshell by chemical activation. Carbon 35 (12), 1723–1732. Basso, M.C., Cerrella, E.G., Cukierman, A.L., 2002. Activated carbons developed from a rapidly renewable biosource for removal of cadmium(II) and nickel(II) ions from dilute aqueous solutions. Industrial and Engineering Chemistry Research 41, 180–189. Blanco Castro, J., Bonelli, P.R., Cerrella, E.G., Cukierman, A.L., 2000. Phosphoric acid activation of agricultural residues and baggasse from sugar cane: influence of the experimental conditions on adsorption characteristics of activated carbons. Industrial and Engineering Chemistry Research 39, 4166–4170. Browning, E., 1969. Chromium in Toxicity of Industrial Metals, second ed. Butterworths and Co., London, pp. 76–96. Dwari, R.K., Biswas, M.N., Meikap, B.C., 2004. Performance characteristics for particles of Sand-FCC and Fly-Ash in a novel hydrocyclone. Chemical Engineering Science 59, 671–684.
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Fornwalt, H.J., Hutchins, R.A., 1966. Purifying liquids with activated carbon. Chemical Engineering 73, 179–184. Kadrivelu, K., Thamaraiselvi, K., Namasivayam, C., 2001. Adsorption of nickel(II) from aqueous solution onto activated carbon prepared from coirpith. Separation and Purification Technology 24, 497–505. Kadirvelu, K., Kavipriya, M., Karthika, C., Radhika, M., Vennilamani, N., Pattabhi, S., 2003. Utilization of various agricultural wastes for activated carbon preparation and application for the removal of dyes and metal ions from aqueous solutions. Bioresource Technology 87, 129–132. Kaufaman, D.B., 1970. Acute potassium dichromate poisoning in man. American Journal of Diseases of Children 119, 374–381. Lua, C.A., Guo, J., 1998. Preparation and characterization of chars from oil palm waste. Carbon 36 (11), 1663–1670. Lussier, M.G., Shull, J.C., Miller, D.J., 1994. Activated carbons from cherry stones. Carbon 32 (8), 1493–1498. Marquez-Montesinos, F., Cordero, T., Rodriguez-Mirasol, J., Rodriguez, J.J., 2001. Powdered activated carbons from sawdust. Separation Science and Technology 36 (14), 3191–3206. Marshal, S., 1980. Metal and Inorganic Waste Reclaiming Encyclopedia. Wayers Beta Corp., Park-Ridge, New Jersey, pp. 56–78. Meikap, B.C., Kundu, G., Biswas, M.N., 2001. Prediction of the interfacial area of contact in a variable area multi-stage bubble column. Industrial and Engineering Chemistry Research 40 (26), 6194–6200. Meikap, B.C., Kundu, G., Biswas, M.N., 2002. Scrubbing of Fly-Ash laden SO2 in a modified multi-stage bubble column scrubber. American Institute of Chemical Engineers Journal 48 (8), 2074–2083. Mohan, D., Singh, K.P., 2002. Single- and multi-component adsorption of cadmium and zinc using activated carbon derived from bagasse—an agricultural waste. Water Research 36, 2304–2318. Nag, A., Gupta, N., Biswas, M.N., 1999. Removal of chromium (VI) and arsenic (III) by chemically treated saw dust. Indian Journal of Environmental Protection 19 (1), 25–29. Patterson, J.W., 1985. Industrial Waste Water Treatment Technology. Ann Arbor Science Publication, Inc., Ann Arbor, Michigan, pp. 53–62. Periasamy, K., Namasivayam, C., 1996. Removal of copper(II) by adsorption onto peanut hull from water and copper plating industry waste water. Chemosphere 32, 769–789. Ricordel, S., Taha, S., Cisse, I., Dorange, G., 2001. Heavy metals removal by adsorption onto peanut husks carbon: characterization kinetic study and modeling. Separation and Purification Technology 24, 389–401. Smith, R.G., Lec, D.H.K., 1981. Chromium in Metallic Contaminants and Human Health. Academic Press, New York. Srinivasan, K., Balasubramaniam, N., Ramakrishnan, T.V., 1998. Studies on chromium removal by rice husk carbon. Indian Journal of Environmental Health 30, 376–387.
Removal of chromium (VI) from dilute aqueous solutions by activated carbon developed from Terminalia arjuna nuts activated with zinc chloride Kaustubha Mohanty, Mousam Jha, B.C. Meikap∗ , M.N. Biswas Department of Chemical Engineering, Indian Institute of Technology (IIT), Kharagpur, P.O. Kharagpur Technology, Dist: Midnapur (West), West Bengal, Pin - 721302, India Received 17 August 2004; received in revised form 29 October 2004; accepted 4 December 2004 Available online 16 March 2005
Abstract Different structured activated carbons were prepared from Terminalia arjuna nuts, an agricultural waste, by chemical activation with zinc chloride for the adsorption of Cr(VI) from dilute aqueous solutions. The most important parameter in chemical activation was found to be the chemical ratio (activating agent/precursor, g/g). Carbonization temperature and time are the other two important variables, which had significant effect on the pore structure of carbon. A high surface area of 1260 m2 /g was obtained at a chemical ratio of 300%, carbonization time and temperature of 1 h and 500 ◦ C, respectively. The activated carbon developed shows substantial capability to adsorb Cr(VI) from dilute aqueous solutions. The parameters studied include pH, adsorbent dosage, contact time, and initial concentrations. The kinetic data were best fitted to the Lagergren pseudo-first-order model. The isotherm equilibrium data were well fitted by the Langmuir and Freundlich models. The maximum removal of chromium was obtained at pH 1.0 (about 99% for adsorbent dose of 2 g/l and 10 mg/l initial concentration). 䉷 2005 Elsevier Ltd. All rights reserved. Keywords: Pollution; Terminalia arjuna nuts; Activated carbon; Chemical activation; Surface area; Cr(VI) removal; Waste water treatment; Water pollution
1. Introduction Water pollution due to toxic heavy metals has been a major cause of concern for chemists and environmental engineers. The industrial (Dwari et al., 2004) and domestic wastewater, if properly not managed, is responsible for causing severe damage to the environment and adversely affecting the health of the people. Chromium is one of the contaminants, which exist in hexavalent and trivalent forms. Hexavalent form is more toxic (Smith and Lec, 1981) than trivalent and requires more concern. Strong exposure of Cr(VI) causes cancer in the digestive tract and lungs (Kaufaman, 1970) and may cause epigastric pain, nausea, vomitting, severe diarrhoea, and hemorrhage (Browning, 1969). It is therefore, essential to remove Cr(VI) from wastewater ∗ Corresponding author. Tel.: +91 3222 283958; fax: +91 3222 282250. E-mail addresses:[email protected], [email protected] (B.C. Meikap).
0009-2509/$ - see front matter 䉷 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.12.049
before disposal. The main sources of chromium (VI) are tannery, paint, ink, dye, and from aluminium manufacturing industries etc. The treatment of chromium bearing effluents has been reported by several methods, such as reduction precipitation, ion exchange, electrochemical reduction, evaporation, reverse osmosis and direct precipitation (Marshal, 1980; Pat terson, 1985). Most of these methods need high capital cost and recurring expenses such as chemicals, which are not suitable for small-scale industries. The process of adsorption is by far the most versatile and widely used technique for the removal of metal ions. Activated carbon has been the water industry’s standard adsorbent for the reclamation of municipal and industrial wastewater for potable use for almost three decades (Fornwalt and Hutchins, 1966). Despite its prolific use in the water and waste industries, activated carbon remains an expensive material. In recent years, research interest in the production of low-cost alternatives to activated carbon has grown.
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Activated carbons (ACs) exhibit a great adsorption capacity, owing to their highly developed pore structures characterized by large surface areas. International growing demand of this adsorbent, mainly because of their use in applications related to their environmental mitigation, has led to search for new, available low-cost feedstocks of renewable character. In practice, coal and agricultural by-products of lignocellulosic materials are two main sources for the production of commercial activated carbons. Agricultural wastes have emerged as a better choice. There are a quite large number of studies regarding the preparation of activated carbons from agricultural wastes (Kadirvelu et al., 2003; Nag et al., 1999) which include nutshells (Ahmadpour and Do, 1997), fruit stones (Lussier et al., 1994), bagasse (Mohan et al., 2002), coirpith (Kadrivelu et al., 2001), oil palm waste (Lua and Guo, 1998) and agricultural residues from sugarcane (Blanco Castro et al., 2000), rice (Srinivasan et al., 1998) and peanut (Periasamy and Namasivayam, 1996; Ri cordel et al., 2001), sawdust (Marquez-Montesinos et al., 2001) and canes from some easy-growing wood species (Basso et al., 2002). Basically, there are two different processes for the preparation of activated carbon: physical activation and chemical activation. In comparison with physical activation, there are two important advantages of chemical activation. One is the lower temperature in which the process is accomplished. The other is that the global yield of the chemical activation tends to be greater since burn-off char is not required. Among the numerous dehydrating agents, zinc chloride in particular is the widely used chemical agent in the preparation of activated carbon. Knowledge of different variables during the activation process is very important in developing the porosity of carbon sought for a given application. Chemical activation by ZnCl2 improves the pore development in the carbon structure and because of the effect of chemicals; the yields of carbon are usually high (Ahmadpour and Do, 1997). In this work it has been reported the results obtained on the preparation of activated carbons from Terminalia arjuna nuts with zinc chloride activation and their ability to remove hexavalent chromium from dilute aqueous solutions. Different preparation variables on the characteristics of activated products were studied to find the optimum conditions for making activated carbons with well-developed porosity. The influence of several operating parameters, such as pH, adsorbent dosage, contact time, and initial concentrations on the adsorption capacity, were investigated. Kinetic models were used to identify the possible mechanisms of such adsorption process. The Langmuir and Freundlich models were used to analyse the adsorption equilibrium. 2. Experimental technique 2.1. Preparation of activated carbon Terminalia arjuna nuts collected from nearby locality was first washed with distilled water to remove the water-soluble
Table 1 Proximate analysis of Terminalia arjuna nut used as raw material Component
Quantities
Moisture 3% Volatile matter 20% Fixed carbon 76% Ash 1%
76 3 20 <1
impurities and surface adhered particles and then dried at 60 ◦ C to remove the moisture and other volatile impurities. Then the precursor was ground in the ball mill and sieved to a particle size range of 150–200 m. The proximate analysis of the precursor is presented in Table 1. Chemical activation of the powdered precursor was done with ZnCl2 . 10 g of dried precursor was well mixed with distilled water so that 100 ml concentrated solution contained 10 g of ZnCl2 . The chemical ratio is defined as the ratio of chemical activating agent (ZnCl2 ) to the precursor. The chemical ratio (activating agent/precursor) was 100% in this case. The mixing was performed at 50 ◦ C for 1 h. After mixing, the slurry was subjected to vacuum drying at 100 ◦ C for 24 h. The resulting chemical-loaded samples were placed in a stainless-steel tubular reactor and heated (5 ◦ C/min) to the final carbonization temperature under a nitrogen flow rate of 150 ml/min STP. Samples were held at the final temperature (carbonization temperature) for different carbonization times of 1, 2, 3 h before cooling down under nitrogen. Nitrogen entering in the reactor was first preheated to 250–300 ◦ C in a pre-heater. The products were washed sequentially with 0.5 N HCl, hot water and finally cold distilled water to remove residual organic and mineral matters, then dried at 110 ◦ C. In all experiments, heating rate and nitrogen flow was kept constant. The experiments were carried out for different chemical ratio (100–300%) and carbonization temperature (300–600 ◦ C). Weight loss of the carbon samples was calculated on a chemical free basis and chemical recovery (CR) was estimated according to CR =
WP i − WPf × 100, WC
(1)
where WP i and WPf are the weight of products before and after washing and Wc is weight of chemical used. The BET surface areas of these activated carbons were measured in Flowsorb-2300. The Flowsorb-2300 was used to find out the surface area of activated carbon on the basis of mono-layer adsorption at or near the boiling point of gases. The monolayer formation was achieved with a nitrogen–helium of 30% by volume. The adsorbed monolayer of nitrogen were established at atmospheric pressure and at the temperature of liquid nitrogen. In this method first sample quantity is optimized after several trial runs so that surface area falls with in 0.5–2.5 m2 . The sample is dried at 105–110 ◦ C and is free from any gases or vapours for which 200–250 ◦ C for 15 min is adequate. Relatively adsorbing and
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desorbing nitrogen at test position can be employed as a degassing means. Surface area in Flowsorb-2300 is displayed in terms of quantity of sample contained in sample tube. Then the displayed number is converted to specific surface area by dividing the weight of the sample.
2.2. Sorption procedure The adsorption capability of the prepared activated carbon toward Cr(VI) was investigated using aqueous solutions of the metal. A stock solution of 1000 mg/l was prepared by dissolving the necessary amount of potassium dichromate in distilled water. Analytical-grade reagents were used in all cases. The stock solution was diluted as required to obtain standard solutions of concentrations ranging between 10 and 100 mg/l. All the adsorption experiments were carried out at a constant temperature of 25 ◦ C in a incubator shaker using fractions of 200 m average particle size for all of the carbons. Concentrations of Cr(VI) in solution were analysed by UV-Spectrophotometer. Adsorption of Cr(VI) on developed activated carbon was conducted containing different weighted amounts of each sample with 100 ml solution of 10, 20 and 30 mg/l of initial concentration. The solutions were kept in a thermal shaker at controlled temperature (25 ◦ C) for a period of 24 h. Significant pH drift during the adsorption test was studied by measuring pH at start and end of each experiment. Once the equilibrium was attained, the slurries were filtered through 0.45 m membranes and equilibrium concentration was determined. The batch process was used so there is no need for volume correction. Batch adsorption experiments were performed by contacting 0.4 g of the selected carbon samples with 200 ml of the aqueous solution of different initial concentration (10, 20, 30 mg/l) at natural solution pH. Continuous mixing was provided during the experimental period with a constant agitation speed of 120 rpm for better mass transfer with high interfacial area of contact (Meikap et al., 2001). The remaining concentration of Cr(VI) in each sample after adsorption at different time intervals, was determined spectrophotometrically. Adsorption equilibrium isotherms on the selected carbon were determined using sample dosages of 0.4 g/200 ml aqueous solutions of initial concentrations in the range of 10–100 mg/l, at pH 1.0. For these experiments, the flasks were shaken, keeping the temperature (25 ◦ C) constant and agitation speed (120 rpm) for the minimum contact time required to attain equilibrium, as determined from the kinetic measurements detailed above. The effect of pH on the equilibrium adsorption of Cr(VI) on selected carbon was further investigated by employing different initial concentrations (10, 20 and 30 mg/l) and varying adsorbent dosages. The pH values were adjusted with dilute sulfuric acid and sodium hydroxide solutions. For these experiments, the flasks were shaken, keeping constant the
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temperature (25 ◦ C) and agitation speed (120 rpm) for the minimum contact time required to attain equilibrium, as determined from the kinetic measurements detailed above. The effect of adsorbent dosages on the equilibrium adsorption of Cr(VI) on the selected carbon was further investigated by employing different initial concentrations (10, 20 and 30 mg/l) at pH 1.0. The adsorbent dosages were varied from 0.5–3.5 g/l. The chromium concentration retained in the adsorbent phase was calculated according to qe =
(Co − Ce )V , W
(2)
where Co and Ce are the initial and equilibrium concentrations (mg/l), respectively, of Cr(VI) in solution, V is the volume (l), and W is the weight (g) of the adsorbent.
3. Results and discussion The results for the characterization of the prepared activated carbon are discussed in this paper. The tests for characterization include weight loss, chemical recovery, BET surface area and adsorption capacities of the activated carbon towards Cr(VI) removal. 3.1. Weight loss 3.1.1. Effect of carbonization time The effect of carbonization time on weight loss is shown in Fig. 1. It can be seen that carbonization time does not have much effect on the weight loss. The experimental data also showed that the weight loss increased with increasing carbonization time for a fixed carbonization temperature, as more volatiles were released then. The values of weight loss are 51.9, 52.3, 53.8 and 53.9 for 30, 60, 120, 150 and 150 min time, respectively.
3.1.2. Effect of carbonization temperature The effect of carbonization temperature on weight loss is shown in Fig. 2. Carbonization temperature also does not have much effect on the weight loss. Overall weight loss was found to increase with increasing temperature, resulting in decreasing yield of char as temperature increased. This weight loss was essentially due to the devolatilization of the Terminalia arjuna fibres upon heating and as expected, the quantity of volatiles evolved increased with increasing temperature. The values of weight loss are 50.9, 51.4, 52.3 and 53.8 with increase in time. The final yields of the chars resulting from carbonization at different temperature were fond to be about 48–52% of the original weight of the predried extracted fibre.
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Fig. 1. Effect of carbonization time on weight loss carbonization temperature = 500 ◦ C, chemical ratio = 100%.
Fig. 2. Effect of carbonization temperature on weight loss carbonization time = 60 min, chemical ratio = 100%.
3.1.3. Effect of chemical ratio The effect of chemical ratio on weight loss is shown in Fig. 3. It can be seen that adding chemical agents to the precursor decreases the weight loss of the carbon products. The reduction of weight loss is most likely due to the effect of the chemical agent in which it promotes the condensation (polymerization) reactions. These reactions, which occur among the aromatic hydrocarbons and tar-forming compounds, result in the formation of large molecules (polycyclic aromatics) in the structure of activated products and increase the carbon yield. 3.2. Chemical recovery 3.2.1. Effect of carbonization time The effect of carbonization time on chemical recovery is shown in Fig. 4. It can be seen that chemical recovery decreases with the carbonization time. This might be due to the evaporation of ZnCl2 from the precursor at longer carbonization time. 3.2.2. Effect of carbonization temperature The effect of carbonization temperature on chemical recovery is shown in Fig. 5. Decrease in chemical recovery with temperature can be explained as a consequence of ZnCl2 evaporation from the precursor at higher temperature.
Fig. 3. Effect of chemical ratio on weight loss carbonization time=60 min, carbonization temperature = 500 ◦ C.
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Fig. 4. Effect of carbonization time on chemical recovery carbonization temperature = 500 ◦ C, chemical ratio = 100%.
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Fig. 6. Effect of chemical ratio on chemical recovery carbonization time = 60 min, carbonization temperature = 500 ◦ C.
3.2.3. Effect of chemical ratio The effect of chemical ratio on chemical recovery is shown in Fig. 6. A portion of chemicals added to the carbon precursor can be recovered during the washing stage of the activated products after carbonization. The change in chemical reagent recovery with the chemical ratio is shown in Fig. 6. It is obvious that with increasing the chemical ratio, the recovery should be increased as well. However beyond 200% the chemical recovery remains constant. This can be explained as the increase in recovery reaches a saturated value beyond which it again bind with activated carbon and dilution effects nullify the increasing effect. 3.3. BET surface area
Fig. 5. Effect of carbonization temperature on chemical recovery carbonization time = 60 min, chemical ratio = 100%.
3.3.1. Effect of carbonization time The effect of carbonization time on BET surface area is shown in Fig. 7. The surface area first increase with carbonization time and reaches its maximum at 1 h and thereafter it decreases. The surface area decrease (after 1 h) was possibly due to some of the pores being sealed off as a result of sintering at excessive time duration. This phenomenon is confirmed by measuring the pore volume of activated carbon. Pore volume measured at 60 min was found more (0.606 ml/g) than that at 120 min (0.578 ml/g). Generally, a longer carbonization time is needed to enhance porosity as well as to clear blocked pore entrances before detrimental effects set in at prolonged times. From
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Fig. 7. Effect of carbonization time on BET surface area carbonization temp. = 500 ◦ C, chemical ratio = 100%.
Fig. 9. Effect of chemical ratio on BET surface area temperature=500 ◦ C, carbonization time = 60 min.
an initial high surface area, it deteriorated with increasing carbonization time. 3.3.2. Effect of carbonization temperature The effect of carbonization temperature on BET surface area is shown in Fig. 8. When the carbonization temperature was 300 ◦ C, pyrolysis reactions had just commenced, thereby producing very small BET surface area. This phenomenon was due to the inadequacy of heat energy produced at this low carbonization temperature for any substantial evolution of volatiles necessary for pore development. As the temperature was increased to 400 ◦ C and then to 500 ◦ C, more volatile matters were released progressively during carbonization, thereby resulting in the development of some new pores, and hence the BET surface area increased progressively. The decrease in surface area with further increase in temperature to 600 ◦ C might be due to a sintering effect at high temperature, followed by shrinkage of the char, and realignment of the carbon structure which resulted in reduced pore areas.
Fig. 8. Effect of carbonization temperature on BET surface area carbonization time = 60 min, chemical ratio = 100%.
3.3.3. Effect of chemical ratio The effect of chemical ratio on BET surface area is shown in Fig. 9. It shows that surface area increases with increase
K. Mohanty et al. / Chemical Engineering Science 60 (2005) 3049 – 3059
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Fig. 10. (a) Effect of contact time on Cr(VI) removal. (b) Comparison of effect of contact time on % removal of Cr(VI) at different activated carbon doses and initial concentrations at constant Ph.
in chemical ratio. The two distinct regions of pore evolution can be observed. In the first region (Chemical ratio < 150%), the surface area increase at a high rate but in the second region (chemical ratio > 150%), the rate of porosity increase becomes slower. These phenomena may be due to the formation of multiplayer of multi-layers of activating agent, which is more prominent at higher chemical ratio. As a result there is a slow change in surface area.
carbon is more at initial stages, which gradually decreases and becomes almost constant after an optimum period of 600 min. A comparison at different activated carbon dose (1.0 and 2.0 g/l) with a constant pH=5.6 has been made and presented in Fig. 10 for the adsorption efficiency. It has been found that under identical conditions higher doses of activated carbon enhances the removal efficiency of Cr(VI). This is quite obvious that at higher dose activated carbon adsorb more quantity of Cr(VI), which resulted in increased efficiency.
3.4. Chromium removal 3.4.1. Contact time study The effect of time of contact for three different concentrations of Cr(VI) (10,20,30 mg/l) with activated carbon dose (2 g/l) at natural pH of solution (pH 5.85) is shown in Fig. 10. The chromium (VI) percent adsorption increases with time until the equilibrium is attained between the amounts of chromium adsorbed on the activated carbon and the remaining in solution. The figure shows that the adsorption of Cr(VI) increases with time from 0 to 300 min and more and then becomes almost constant up to the end of experiment. It can be concluded that the rate of Cr(VI) binding with activated
3.5. Adsorption kinetics Numerous kinetic models have been proposed to elucidate the mechanism by which pollutants may be adsorbed. The mechanism of adsorption depends on the physical and/or chemical characteristics of the adsorbent as well as on the mass transport process (Meikap et al., 2002). In order to investigate the mechanism of Cr(VI) adsorption, Lagergren proposed a pseudo-first-order kinetic model. The integral form of the model is log(qe − q) = log qe −
Kad t, 2.303
(3)
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Fig. 11. (a) Cr(VI) uptake by Terminalia arjuna nuts according to the Lagergren model. (b) Lagergren plot for Cr(VI) at constant pH.
Table 2 Lagergren constants Initial Cr(VI) Conc. (mg/l)
Kad (min−1 )
R2
10 20 30
0.01414 0.01087 0.0125
0.9922 0.9928 0.9835
where q is the amount of Cr(VI) sorbed (mg/g) at time t (min), qe is the amount of Cr(VI) sorbed at equilibrium (mg/g) and Kad is the equilibrium rate constant of pseudofirst-order adsorption (min−1 ). For calculating the rate constant of the adsorption at room temperature, Lagergren equation is plotted for three different initial Cr(VI) concentrations (10, 20 and 30 mg/l) with a fixed adsorbent dose shown in Fig. 11. The values of Kad for three different initial concentrations along with the correlation coefficients are given in Table 2. The resulting straight lines are showing the applicability of the pseudo-first-order kinetics. Detailed experiments were conducted to find out adsorption kinetics and results of Lagergen model for two different adsorbent doses shown in Fig. 11. The results show that the adsorption of Cr(VI) on activated carbon follow the first order kinetics.
3.5.1. Effect of pH Earlier studies have indicated that solution pH is an important parameter affecting adsorption of heavy metals. Cr(VI) removal was studied as a function of pH for three different initial concentrations for a fixed adsorbent dose (2 g/l) and the results are shown in Fig. 12. It is clear from this figure (Fig. 12) that the percent adsorption of Cr(VI) decreases with increase in pH from pH 1.0 to 5.0 and after pH 5.85 (natural pH) no adsorption takes place at all. It is important that the maximum adsorption at all the concentrations takes place at pH 1.0. This behaviour can be explained considering the nature of the adsorbent at different pH in metal adsorption. The cell wall of activated carbon contains a large number of surface functional groups. The pH dependence of metal adsorption can largely be related to the type and ionic state of these functional groups and also on the metal chemistry in solution. Adsorption of Cr(VI) below pH 3.0 (maximum at pH 1.0) suggests that the negatively charged species (chromate/dichromate in the sample solution) bind through electrostatic attraction to positively charged functional groups on the surface of activated carbon because at this pH more functional groups carrying positive charge would be exposed. But at above pH 3.0, it seems that activated carbon possesses more functional groups carrying a net negative
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Fig. 13. Effect of sorbent dose on Cr(VI) removal at solution pH = 1.0.
Fig. 12. Effect of pH on percent removal of Cr(VI).
charge, which tends to repulse the anions. However, there is also the removal above pH 3.0, as indicated by Fig. 12, but the rate of removal is considerably reduced. Hence, it could be said that above pH 3.0, other mechanism like physical adsorption on the surface of adsorbent could have taken an important role in the adsorption of Cr(VI) and exchange mechanism might have reduced. In addition adjustment of pH is very important. The possibilities of other subsidiary substances and hydrogen bonding may affect the adsorption process and need careful analysis. 3.6. Sorbent dose study The effect of adsorbent dosage on the percentage removal Cr(VI) has been shown in Fig. 13. It can be seen from the figure that initially the percentage removal increases very sharply with the increase in adsorbent dosage but beyond a certain value 0.25–0.3 g, the percentage removal reaches almost a constant value. This trend is expected because as the adsorbent dose increases the number adsorbent particles increases and thus more Cr(VI) is attached to their surfaces. A maximum removal of 99.52% was observed at adsorbent dosage of 2 g/l at pH 1.0 for an initial Cr(VI) concentration of 10 mg/l. Therefore, the use of 2 g/l adsorbent dose is justified for economical purposes.
3.6.1. Adsorption isotherms Several models have been used in the literature to describe the experimental data of adsorption isotherms. The Langmuir and Freundlich models are the most frequently employed models. In the present work both models were used. The chromium sorption isotherm followed the linearized Freundlich model as shown in Fig. 14. The relation between the metal uptake capacity ‘qe ’ (mg/g) of adsorbent and the residual metal ion concentration ‘Ce ’ (mg/l) at equilibrium is given by ln qe = ln k +
1 ln Ce , n
(4)
where the intercept ln k is a measure of adsorbent capacity, and the slope 1/n is the sorption intensity. The isotherm data fit the Freundlich model well (R 2 =0.9612). The values of the constants k and 1/n were calculated to be 3.915 and 0.551. Since the value of 1/n is less than 1, it indicates a favourable adsorption. The Langmuir equation relates solid phase adsorbate concentration (qe ), the uptake, to the equilibrium liquid concentration (Ce ) as follows: KL bC e , (5) qe = 1 + bC e where KL and b are the Langmuir constants, representing the maximum adsorption capacity for the solid phase loading and the energy constant related to the heat of adsorption respectively. It can be seen from Fig. 15 that the
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K. Mohanty et al. / Chemical Engineering Science 60 (2005) 3049 – 3059 Table 3 Adsorption isotherms parameters at pH = 1.0 for Cr(VI) removal Parameters
KL (mg/g)
b (l/g)
R2
k (l/g)
1/n
Values
28.43
0.1183
0.9913 Langmuir 0.9612 Lagergren
3.915
0.551
to 1.0 for maximum removal of Cr(VI) are presented in Table 3. 4. Conclusions
Fig. 14. Freundlich adsorption isotherm.
Fig. 15. Langmuir adsorption isotherm.
isotherm data fits the Langmuir equation well (R 2 =0.9913). The values of KL & b were determined from the figure and were found to be 28.43 mg/g and 0.1183 l/mg, respectively. The outcome values of parameters k, n, KL , b, R 2 for all the experiments with pH of solution equal
The Terminalia arjuna nut precursor is found to be a good raw material for developing activated carbons. This study demonstrated that ZnCl2 is a suitable activating agent for the preparation of high-porosity carbons from Terminalia arjuna nut. For the carbonization of the ZnCl2 treated sample, ZnCl2 plays an important role in retarding tar escape during carbonization. The washing process following carbonization with ZnCl2 has a significant influence on the surface properties of resulting char. It was found that acid washing is a necessary step for the preparation of high-porosity carbons. High surface area of activated carbons was prepared from the chemical activation of Terminalia arjuna nuts with ZnCl2 as chemical agent. Results shows that the porosity of resulting activated carbon increases with carbonization temperature and carbonization time to a maximum up to a temperature of 500 ◦ C and 1 h, respectively and then began to decrease thereafter. Also the porosity increases with chemical ratio up to 300% and then becomes almost constant. The optimal conditions for the production of high surface area activated carbon from Terminalia arjuna nut by chemical activation are: chemical ratio (activating agent/precursor) of 300%, carbonization time of 1 h and carbonization temperature of 500 ◦ C. At this optimal condition, the BET surface area obtained was 1260 m2 /g. The aqueous adsorption tests indicate that the Terminalia arjuna nut-derived activated carbon has a notable adsorption capacity for Cr(VI). The kinetics of Cr(VI) adsorption followed nicely the pseudo-first-order rate expression. The Langmuir and Freundlich models fit the isotherm data well. It is interesting to mention that the Lagergen model is compatible with other two models. The uptake of the Cr(VI) was greatly affected by the solution pH. The data thus obtained may be helpful for designing and establishing a continuous treatment plant for water and wastewaters enriched in Cr(VI). The cost of removal is expected to be quite low, as the adsorbent is cheap and easily available in large quantities as compared and reported in the literature.
Notation 1/n b
sorption intensity, dimensionless Langmuir constant, l/g
K. Mohanty et al. / Chemical Engineering Science 60 (2005) 3049 – 3059
Ce equilibrium chromium concentration, mg/l Co initial chromium concentration , mg/l Chemical ratio activating agent (ZnCl2 )/precursor, g/g CR chemical recovery, g/g k measure of adsorbent capacity, l/g Kad equilibrium rate constant of pseudo-first-order adsorption, min−1 KL Langmuir constant, mg/g qe amount of chromium adsorbed at equilibrium, mg/g T temperature, ◦ C t time, min V volume of the solution, l W weight of the adsorbent, g Wc weight of chemical used, g weight of product after washing, g Wpf Wpi weight of product before washing, g Acknowledgements The authors thankfully acknowledge the reviewers for suggesting valuable technical comments of this paper. References Ahmadpour, A., Do, D.D., 1997. The preparation of activated carbon from Macademia Nutshell by chemical activation. Carbon 35 (12), 1723–1732. Basso, M.C., Cerrella, E.G., Cukierman, A.L., 2002. Activated carbons developed from a rapidly renewable biosource for removal of cadmium(II) and nickel(II) ions from dilute aqueous solutions. Industrial and Engineering Chemistry Research 41, 180–189. Blanco Castro, J., Bonelli, P.R., Cerrella, E.G., Cukierman, A.L., 2000. Phosphoric acid activation of agricultural residues and baggasse from sugar cane: influence of the experimental conditions on adsorption characteristics of activated carbons. Industrial and Engineering Chemistry Research 39, 4166–4170. Browning, E., 1969. Chromium in Toxicity of Industrial Metals, second ed. Butterworths and Co., London, pp. 76–96. Dwari, R.K., Biswas, M.N., Meikap, B.C., 2004. Performance characteristics for particles of Sand-FCC and Fly-Ash in a novel hydrocyclone. Chemical Engineering Science 59, 671–684.
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