Preparation and Characterization of Activated Carbons from

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Ind. Eng. Chem. Res. 2005, 44, 4128-4138

Preparation and Characterization of Activated Carbons from Terminalia Arjuna Nut with Zinc Chloride Activation for the Removal of Phenol from Wastewater Kaustubha Mohanty,† Mousam Jha, B. C. Meikap,* and M. N. Biswas‡ Department of Chemical Engineering, Indian Institute of Technology (IIT), Kharagpur, Dist: Midnapur(W), West Bengal, Pin-721 302, India

Nuts of Terminalia Arjuna, an agricultural waste, were used to prepare activated carbons by zinc chloride activation under four different activation atmospheres, to develop carbons with substantial capability, and to adsorb phenol from dilute aqueous solutions. Experiments were carried out at different chemical ratios (activating agent/precursor). Effect of carbonization temperature and time are the important variables, which had significant effect on the pore structure of carbon. Developed activated carbon was characterized by SEM analysis. Pore volume and surface area were estimated by Hg porosimetry and BET surface area analyses. The carbons showed surface area and micropore volumes of around 1260 m2/g and 0.522 cm3/g, respectively. The activated carbon developed shows substantial capability to adsorb phenol from dilute aqueous solutions. The kinetic data were fitted to the models of intraparticle diffusion, pseudo-secondorder, and Lagergren model which followed more closely the pseudo-second-order chemisorption model. The isotherm equilibrium data were well-fitted by the Langmuir and Freundlich models. The maximum removal of phenol was obtained at pH 3.5 (about 93% for adsorbent dose of 10 g/L and 100 g/L initial concentration). Introduction The presence of phenols and phenolic compounds in wastewater is a major problem for adverse effects on aquatic life and stringent environmental regulations attracts the attention of chemists and environmental engineers for its control. Phenol includes a variety of hydroxybenzenes and substituted hydroxybenzenes. These are common water pollutants. Phenolic compounds are toxic in nature. Phenol is colorless solid at room temperature and soluble in water. The major sources containing phenols are the wastewaters from processing manufacturing industries engaged in oil refining, coal tar processing, petrochemical production, coke oven byproducts, plastic industry, textile processing, leather processing, insecticides production, manufacture of dyes and dyeing, glass production, etc.1-5 The discharge of phenolic waste into waterways may adversely affect human health as well as that of flora and fauna. Ingestion of a small amount of phenol by human beings may cause nausea, vomiting, paralysis, coma, greenish or smoky colored urine, and even death from respiratory failure or cardiac arrests. Phenols also impart undesirable taste to water even at extremely low concentration. Fatal poisoning may also occur by adsorption of phenol by skin, if a large area of it is exposed. Various physicochemical methods have been proposed for the treatment of wastewaters containing phenolic wastes.1,4 The choice of treatment depends on effluent characteristics such as concentration of phenol, pH, temperature, flow volume, biological oxygen demand, * To whom correspondence should be addressed. Tel: +913222-283958(O)/283959(R). Fax: +91-3222-282250. E-mail: [email protected]. † E-mail: [email protected]. ‡ E-mail: [email protected].

the economics involved, and the social factor like standard set by government agencies. However, adsorption on to the surface of activated carbons is the most widely used method.6 Activated carbons have the advantage of exhibiting a high adsorption capacity for organic pollutants due to their high specific surface area, adequate pore size distribution, and relatively high mechanical strength.2,7 In view of the high cost and tedious procedures for the preparation and regeneration of activated carbons, there is a continuing search for low-cost potential adsorbents. In practice, coal and agricultural byproducts of lignocellulosic materials are two main sources for the production of commercial activated carbons. Agricultural wastes have emerged as a better choice. Though raw agricultural wastes can be used as adsorbents without further treatment, activation could enhance their adsorption capacity. The production of activated carbons from agricultural wastes converts unwanted, surplus agricultural waste, of which billions of kilograms are produced annually, to useful valuable adsorbents, such as nutshells,8,9 fruit stones,10 bagasse,11 coirpith,12 oil palm waste,13 and agricultural residues from sugarcane,14 rice,15 peanuts,16,17 sawdust,18 and canes from some easy-growing wood species.19,20 Basically, there are two different processes for the preparation of activated carbon: physical activation and chemical activation. The effect of different chemical reagents on the production and quality of activated carbon was studied extensively by different researchers.8,9,13,20,21 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 burnoff char is not required. Among the numerous dehydrating agents, zinc chloride in particular is the widely used chemical agent

10.1021/ie050162+ CCC: $30.25 © 2005 American Chemical Society Published on Web 04/26/2005

Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 4129 Table 1. Proximate Analysis of Terminalia Arjuna Nut Used as Raw Material component

quantities

moisture (%) volatile matter (%) fixed carbon (%) ash (%)

3 20 76 150%), where in addition to pore opening some portion of the chemicals are responsible for the widening of the micropores, the rate of porosity increase becomes slower. The destructive effect of high ZnCl2 ratio on the micropore structure of active carbon can be observed in this region. SEM Analysis of the Activated Carbons Scanning electron microscopy (SEM) technique was used to observe the surface physical morphology of the nuts derived from activated carbon. Figures 4 and 5 are SEM photographs of the activated carbon at 1000 and 5000 times magnification. Pores of different size and different shapes could be observed. It can be seen from

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Figure 4. SEM micrographs of the activated carbons prepared at 500 °C with 100% chemical ratio at 30, 60, and 120 min and at 500 °C with 200% chemical ratio for 30 min.

Micropore Size Distribution of Activated Carbons

Figure 5. SEM micrographs of the activated carbons prepared at at 500 °C, chemical ratio of 300%.

the micrographs that the external surface of the chemically activated carbon is full of cavities. The reason for the formation of the cavities on the ZnCl2-activated carbon is not clear. According to the micrographs, it seems that the cavities resulted from the evaporation of ZnCl2 during carbonization, leaving the space previously occupied by the ZnCl2. The carbonization temperature for chemical activation was too low to cause the agglomeration of the char structure. Since the carbonization temperature for physical activation was high (900 °C), caking and agglomeration occurred on the char structure and thus resulted in the formation of chars with an intact external surface. It merits mentioning that there was a noticeable scattering of salt particles, probably attributed to the remaining zinc chloride, present on the activated carbon. Some particles were even trapped into the pores and could possibly block the entry of pores to some extent. The above findings support the results obtained by Hg porosimetry of activated carbon. Therefore, it seems that the adsorptive capacity of the products could be further increased if the washing procedure was improved.

The micropore size distribution of ZnCl2 activated carbons obtained at 500 °C and 300% chemical ratio at three different carbonization times were measured and are shown in Figure 6. The micropore size distribution is important for explaining the possible highest adsorption capacity. From the figure it has been found that at 60 min the micropore volumes obtained are more than those obtained at 120 and 30 min. It is interesting to note that the maximum peak is obtained at around 2.0 nm and the peak sweeps right for 60 min duration. This is possibly due to some of the pores being sealed off as a result of sintering at excessive time duration, thereby resulting in a fewer number of micropores. It has also been found that at a carbonization time of 30 min the variation in pore size distribution is not enough. This may be explained as a relatively longer time of carbonization facilitates the escape of gases and vapors from activated carbon pores, leaving a better pore size distribution than a shorter time. Kinetics of Phenol Adsorption Process The relationship between contact time and phenol removal by activated carbon obtained by ZnCl2 activation of Terminalia Arjuna nuts for four different initial concentrations is presented in Figure 7. The equilibrium time kept 5.0 h for phenol adsorption. It can be seen from the figure that an increase in initial phenol concentration results increase in the phenol removal. These experiments were performed with an adsorbents dose of 5 g/L at natural pH of the solution. It can be concluded that the rate of phenol binding with activated carbon increases more rapidly in the initial stages. And after some point the variation is not considerably enough and becomes almost constant after a period of 300 min. Numerous kinetic models have been proposed to elucidate the mechanism by which pollutants may be

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Figure 6. Micropore size distribution of the activated carbon obtained at 500 °C, for carbonization time 30, 60, and 120 min and chemical ratio of 300%.

adsorbed. The mechanism of adsorption depends on the physical and/or chemical characteristics of the adsorbent as well as on the mass transport process. To investigate the mechanism of phenol adsorption, three kinetic models were considered as follows. (1) Lagergren Model. Lagergren proposed a pseudofirst-order kinetic model. The integral form of the model is

log(qe - q) ) log qe -

Kad t 2.303

(3)

where q is the amount of phenol sorbed (mg g-1) at time t (min), qe is the amount of phenol sorbed at equilibrium (mg g-1), and Kad is the equilibrium rate constant of pseudo-first-order adsorption (min-1). This model was successfully applied to describe the kinetics of many adsorption systems. (2) Pseudo-Second-Order Model. The adsorption kinetics may also be described by a pseudo-second-order reaction. The linearized-integral form of the model is

1 1 t ) + t q K q 2 qe

(4)

2 e

where K2 is the pseudo-second-order rate constant of adsorption. (3) Diffusion Model. The intraparticle diffusion model is based on the theory proposed by Weber and Morris.24 According to this theory,

q ) kdxt

(5)

Figure 7. Effect of contact time on percent removal of phenol at initial concentration of 10, 25, 50, and 100 ppm, adsorbent dose ) 5 g/L.

where kd is the rate constant of intraparticle diffusion (mg g-1 min-1/2). The applicability of the above three models can be examined by each linear plot of log(qe - q) vs t, (t/q) vs t, and q vs t1/2, respectively, and are presented in Figures 8, 9, and 10. To quantify the applicability of each model, the correlation coefficient, R2, was calculated from these plots. The linearity of these plots indicates the applicability of the three models. However, the correlation coefficients, R2, showed that the pseudo-second-order model, an indication of a chemisorption mechanism, fits better the experimental data (R2 > 0.998) than the pseudo-first-order model (R2 is in the range of 0.9530.971). The intraparticle diffusion was also involved in the adsorption of phenol by activated carbons (Figure 10). The linear portion of the plot for a wide range of contact time between adsorbent and adsorbate does not pass through the origin. This deviation from the origin or near saturation may be perhaps due to the difference in the rate of mass transfer in the initial and final stages of adsorption.25,26 Further, such deviation from origin indicates that the pore diffusion is not the only ratecontrolling step.26,27 From Figure 10 it may be seen that there are two distinct regionssthe initial pore diffusion due to external mass-transfer effects followed by the intraparticle diffusion.26,28 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.


Figure 8. Kinetics of phenol removal according to the Lagergren model at initial phenol concentration of 10, 25, 50, and 100 ppm.

The phenol sorption isotherm was carried out with activated carbon obtained by ZnCl2 activation of Terminalia Arjuna nuts, following the linearized Langmuir model as shown in Figure 11. The Langmuir equation relates solid-phase adsorbate concentration (qe), the uptake, to the equilibrium liquid concentration (Ce) as follows,

qe )

(

)

KLbCe 1 + bCe

(6)

where KL and b are the Langmuir constants, representing the maximum adsorption capacity for the solidphase loading and the energy constant related to the heat of adsorption, respectively. It can be seen from the figure that the isotherm data fits the Langmuir equation well (R2 ) 0.9997). The values of KL and b were determined from the figure and were found to be 11.175 mg/g and 0.0148 L/mg, respectively. The phenol adsorption isotherm followed the linearized Freundlich model as shown in Figure 12. The relation between the phenol uptake capacity “qe”(mg/g) of adsorbent and the residual phenol concentration “Ce” (mg/L) at equilibrium is given by

1 ln qe ) ln k + ln Ce n

(7)

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 (R2 ) 0.9995). The values of the constants k and 1/n were calculated to be 0.018 and 0.954. Since the value of 1/n is less than 1, it indicates a favorable adsorption.

Figure 9. Kinetics of phenol removal according to pseudo-secondorder model at initial phenol concentration of 10, 25, 50, and 100 ppm.

The adsorption of phenol by activated carbons obtained by ZnCl2 activation was studied at various pH values. The experiments were performed for an initial concentration of 100 ppm for different adsorbent doses (1, 5, and 10 g/L) and the results are shown in Figure 13. It is clear from this figure that the percent adsorption of phenol increases with increase in pH from pH 2.0 to pH 3.5 and decreases thereafter. It is important that the maximum adsorption at all the concentrations takes place at pH 3.5. This behavior can be explained considering the nature of the adsorbent at different pH in phenol adsorption. The cell wall of activated carbon contains a large number of surface functional groups and facilitates for adsorption. The pH dependence of phenol adsorption can largely be related to type and ionic state of these functional groups and also to the ionic chemistry in solution. Adsorption of phenol up to pH 3.5 suggests that the negatively charged phenolate ions 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 pH above 3.5, it seems that activated carbon possesses more functional groups carrying a net negative charge, which tends to repulse the anions. However, there is removal above pH 3.5 also, as indicated by figure, but the rate of removal is considerably reduced. Hence, it could be said that above pH 3.5, other mechanisms like physical adsorption on the surface of adsorbent could have taken an important role in the adsorption of phenol and the exchange mechanism might have reduced.3

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Figure 10. Kinetics of phenol removal according to the intraparticle diffusion model at initial phenol concentration of 10, 25, 50, and 100 ppm.

Conclusions This study has demonstrated that high surface area activated carbons can be prepared from the chemical activation of Terminalia Arjuna nuts with ZnCl2 as activating agent. For the carbonization of the ZnCl2treated sample, the release of moisture and ZnCl2 represents most of the evolution, indicating that 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 highporosity carbons. Study of various parameters during chemical activation revealed that the most important variable is the ratio of chemical agent to the nut precursor. The other important operation variables with a direct effect on the porosity development are temperature and carbonization time. Under the experimental conditions investigated, the best conditions for the production of high surface area activated carbon from Terminalia Arjuna nut by chemical activation are chemical ratio (activating agent/precursor) of 200%, carbonization time of 1 h, and carbonization temperature of 500 °C. At these optimal conditions, the BET surface area and micropore volume obtained were 1260 m2/g and 0.560 cm3/g, respectively. Developed activated carbon was characterized by SEM analysis. The batch adsorption tests indicate that the Terminalia Arjuna nut derived activated carbon had a notable adsorption capacity for phenol from wastewater. The kinetics of phenol adsorption followed nicely the pseudosecond-order rate expression. The Langmuir and Fre-

Figure 11. Langmuir isotherm model. Adsorbent dose ) 5 g/L, temperature ) 25 °C.

Figure 12. Freundlich isotherm model. Adsorbent dose ) 5 g/L, temperature ) 25 °C.

undlich models fit the isotherm data well. Solution pH has a great effect on the uptake of phenol. The data thus


Literature Cited

Figure 13. Effect of pH on phenol removal for adsorbent dose of 1, 5, and 10 g/L.

obtained may be helpful for designing and establishing a continuous treatment plant for water and wastewaters enriched in phenol and phenolic compounds. Acknowledgment The authors gratefully acknowledge the help received from Dr. B. V. R. Murthy, Mr. J. N. Mohanty, and Mr. R. K. Dwari of Regional Research Laboratory (CSIR), Bhubaneswar, Orissa, India, for porosity analysis of samples. Nomenclature CR ) chemical recovery WPi ) weight of product before washing (g) WPf ) weight of product after washing (g) WC ) weight of chemical used (g) Q ) amount of phenol adsorbed at time “t” (mg/g) qe ) amount of phenol adsorbed at equilibrium (mg/g) C0 ) initial phenol concentration (mg/L) Ce ) equilibrium phenol concentration (mg/L) V ) volume of the solution (L) W ) weight of the adsorbent (g) Kad ) equilibrium rate constant of pseudo-first-order adsorption (min-1) K2 ) pseudo-second-order rate constant of adsorption (mg-1 g min-1) kd ) rate constant of intraparticle diffusion (mg g-1 min-1/2) k ) measure of adsorbent capacity 1/n ) sorption intensity KL ) Langmuir constant (mg/g) b ) Langmuir constant (L/mg)

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Received for review February 9, 2005 Revised manuscript received March 29, 2005 Accepted March 30, 2005 IE050162+

Preparation and Characterization of Activated Carbons from

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