Iranian Journal of Environmental Health Science & Engineering,Vol. 3, No. 4, 2006, pp. 239-246
COMPARATIVE EVALUATION OF COMMERCIAL AND SEWAGE SLUDGE BASED ACTIVATED CARBONS FOR THE REMOVAL OF TEXTILE DYES FROM AQUEOUS SOLUTIONS
*1S. Sreedhar Reddy, 2B. Kotaiah
*Corresponding author-Email: firstname.lastname@example.org Tel/Fax: +91 416 2243092
Received 19 June 2005; revised 5 July 2006; accepted 28 August 2006
Code Number: se06033
The sorption of dyes from aqueous solutions on to sludge-based activated carbon have been studied and compared with commercial activated carbon. Adsorption parameters for the Langmuir and Freundlich isotherms were determined and the effects of effluent pH, adsorbent dosage, contact time and initial dye concentration were studied. A pseudo-second order kinetic model has been proposed to correlate the experimental data.
Key words: Kinetics, textile dye, sorption, sludge based activated carbon, commercial activated carbon
Almost every industry uses coloring matter to color their products and the residual and unspent substances are discharged in to the environment, particularly aquatic environment. Discharge of such colored effluents imparts color to the receiving water and interferes with its intended beneficial use. In addition, recent reports suggest that several color-causing substances are micro toxic to the aquatic biota. Moreover if effort is not made to remove this visible pollution, public assumes that little effort is being expanded in removing other pollutants also. It is thus imperative that color must be removed from the effluent before disposal. Among different industrial wastewaters with different types of color causing substances, synthetic textile organic dye color wastes occupy a prominent position. Over 7 x 105 tons and approximately 10000 different types of dyes and pigments are produced worldwide annually. It is estimated that 10 to 15% of the dye is lost in the effluent during the dyeing process (Young et al., 1997; Danis et al., 1998). Various physical, chemical, physico-chemical and bio-chemical processes like sedimentation, equalization, neutralization, flotation, chemical precipitation, coagulation and flocculation, adsorption, ion-exchange, reverse osmosis, electro-chemical coagulation etc., have been investigated for the removal of dyes from aqueous solutions (McKay, 1982; Barton, 1987; Karithikeyan, 1990; karthikeyan, 1988; Venkatamohan, 1997). The stringent and rigid national effluent disposal standards call for an advanced treatment among which liquid-phase adsorption has been found highly efficient for removal of color in terms of initial cost, simplicity of design, ease of operation and insensitivity to toxic substances. Activated carbon is the most widely used adsorbent because it has excellent adsorption efficiency for organic compounds but its use is usually limited due to its high cost (Y. S. Ho et al., 1999; Rozada et al., 2003). In recent years this has prompted a growing research interest in the production of carbon based adsorbents from a range of residues mainly industrial or agricultural by products (Malik, 2003; Chiou and Li, 2003; Garg et al., 2003; Ozacar and Sengil, 2003; Shukla et al., 2002; Ramakrishna and Viraraghavan, 1997). The production of activated carbon from residues may allow waste producers to offset their increasing waste disposal cost against the cost of carbon production, while saving the non renewable natural resources and producing a valuable product with potential applications in pollution control.
The main focus of this research is to evaluate the adsorption potential of sewage sludge derived activated carbon (SAC) for the removal of dyes from aqueous solution. The effects of dose of SAC, aqueous solution pH and initial dye concentration in the aqueous solution on sorption of dye were investigated. The Langmuir and Freundlich isotherms were used to fit the equilibrium data. Similar experiments were carried out with commercial grade activated carbon (CAC) to compare the results.
MATERIALS AND METHODS
Sorbate and sorbents
The anaerobically digested and dewatered sewage sludge collected form Koyambedu metro sewage treatment plant, Chennai, TamilNadu, India, was used for making activated carbon as per the procedure suggested by Sreedharreddy, S. (2004). Characteristics of SAC in comparison with CAC are presented in Table 1. Surface area of activated carbon was measured as per guide lines of Poots et al., (1976). The textile dye laden aqueous solution was prepared as a mixture of three reactive dye formulations (Table 2), as per the procedure suggested by Sreedhar reddy et al., (2005). According to the information obtained from the local dyeing industries, usually 20% of the dyestuffs (hydrolyzed and / or unfixed form) and 100% of all assisting chemicals remain in the exhausted dye-bath. Dye-bath was suitably diluted to represent the typical strength of effluents from the washing and rising stages in the above-mentioned textile manufacturing plant. Composition of suitably diluted exhausted dye-bath is presented in Table 3.
Batch adsorption studies
Batch experiments were conducted to evaluate the adsorption potential of SAC and CAC for dyes from aqueous solutions.100 mL of dye solution of known concentration (Co) and initial pH was taken in a 300 mL capacity borosil made reagent bottle, with a required amount of adsorbent and was agitated at a speed of 100 rpm in a horizontal bench shaker (laboratory net scientific) at room temperature, for a specified period of contact time. Then the solution was filtered through a Whattman filter paper. The initial pH of the solution was adjusted by using either 0.1N NaOH or H2SO4. Spectornic made spectrophotometer was used to determine the concentration of dye at a wavelength corresponding to maximum absorbance. The filtrate was analyzed for the remaining dye concentration. The amount of dye adsorbed in mg/g at time, t was computed by using the following equation:
Where Co and Ct are the dye concentrations in mg/L initially and at a given time t, respectively. V is the volume of the dye solution in mL, and ms is the weight of activated carbon in grams.
Kinetic model theory
The reaction rate of a chemical reaction is defined as the change in concentration of a reactant or product per unit time. The concentrations of the products do not appear in the rate law because the reaction rate is being studied under conditions where the reverse reactions do not contribute to the overall rate. The reaction order and rate constant must be determined by experiment. In 1995, Ho presented a sorption pseudo-second order rate law expression showed how the rate depends on the sorption capacity on solid phase but not the concentration of the sorbate (Ho, Y.S., 1995; Ho, Y.S. and McKay, G., 2000).
Integrated pseudo-second order rate law can be obtained from Eq. (2) for the boundary conditions t=0-t and qt=0-qt, gives:
Eq. (3) can be rearranged to obtain a linear form:
Where qe is the amount of sorbate sorbed at equilibrium, (mg/g); t is the reaction time, (min); qt is the amount of sorbate sorbed at time t, (mg/g); k is the equilibrium rate constant of pseudo-second order sorption, (g/mg min). Ho's pseudo-second order rate law shows how the sorption capacity of sorbate depends on time. If the equilibrium sorption capacity of sorbate and the rate constant k are known, then the sorption capacity of sorbate at any time can be calculated. Using the linear form equation, Eq. (4), plotting t / qt , versus time,t will give a straight line with slope 1/ qe, and intercept 1/ kqe2.
The equilibrium studies conducted at a fixed initial concentration and varying adsorbent dose have been fitted in to the linearized Freundlich adsorption isotherm, which is of the form,
Ln (X/M) = ln Kf + 1/n ln Ce (5)
Where X is the amount of dye removed (mg), m is the weight of adsorbent used (g), Ce is the equilibrium concentration and Kf and n are constants incorporating all factors affecting the adsorption process such as adsorption capacity and intensity, respectively.
From Fig.1 Freundlich constants K¦ and n were found to be 9.2 mg/g and 2.45 for CAC, 4.6 mg/g and 2.116 for SAC respectively. The Langmuir isotherm, which is in the form of Eq. 6, was applied for adsorption equilibrium.
Ce/qe = 1/Qob + Ce/Qo (6)
Where Ce is the equilibrium concentration (mg/L), and qe in the amount of dye adsorbed per gram at equilibrium (mg/g). Qo and b are Langmuir constants. Qo and b were determined from the slope and intercept of the plot (Fig.2) and found to be 33.5 mg/g and 0.070 l/mg, for CAC and 25mg/g and 0.44 l/mg, for SAC, respectively.
Effect of initial concentration
The experimental results of the sorption of dye from aqueous solutions at various initial concentrations on to SAC and CAC are shown in Table 4. The equilibrium sorption capacity of SAC and CAC increased from 20 to 32.03 mg/g and 20.12 to 42 mg/g respectively with an increase in the concentration of reactive dye from 20 mg/L to 120 mg/L with an adsorbent dose of 1 g/L and at a solution pH of 3. Fig.3 shows a plot of t/qt against t, for the sorption on to SAC and CAC, for the pseudo second order model. The correlation coefficients (R2), the pseudo-second order rate parameter (ke), the initial sorption rate (h) and the equilibrium sorption capacity (qe) are shown in Table 4. The solid line curves in Fig.4 are the theoretical plots of equation (4), and the values of R2 are all extremely high (>0.997).
Effect of adsorbent dosage:
The sorption kinetics of reactive dye on to SAC and CAC are shown in Table 6. The Table shows that by that by increasing the adsorbent dose, the percentage dye removal increases, but adsorption density, i.e. the amount of dye adsorbed per unit mass, decreased.
Fig.4 shows a plot of t/qt against t for the sorption on to SAC and CAC, at various masses of adsorbent for the pseudo second order model. The correlation coefficients (R2), the pseudo-second order rate parameter (ke), the initial sorption rate (h) and the equilibrium sorption capacity (qe) as a function of adsorbent dosages are shown in Table 7.When increasing the adsorbent dosage from 1 g/L to 10 g/L, at pH=3, the equilibrium sorption capacity (qe) decreased form 38.46 to 14.22 mg/g for sludge derived activated carbon and from 40.81 to 14.20 mg/g for commercial activated carbon.
Effect of pH
To investigate the role of surface chemistry of SDCAC and CAC, on the adsorbent capacity of the reactive dyes, the effect of pH at equilibrium was studied. The experiments were carried out by varying the initial pH from 3 to 11, under a constant initial dye concentration of 120 mg/L, and a carbon dosage of 10 g/L. The dependence of the equilibrium adsorption capacity of SAC and CAC for reactive dye on equilibrium pH is presented in Table 8. Equilibrium sorption capacity of both the activated carbons decreased with increasing pH values of reactive dye solution. The maximum removal of reactive dye with sludge derived activated carbon and commercial activated carbon was hence observed at pH: 3 when the negative charge on the surface is very much reduced due to the excess of protons in solution. For SAC and CAC, the amount of dye adsorbed at equilibrium increased from 5.21 to 11.3 mg/g and 5.5 to 12.8 mg/g respectively with decrease in pH from 11 to 3. Results from a series of kinetic experiments at different pH conditions are shown in Table 9.
The data shows a good compliance with the pseudo-second-order equation and the regression coefficient (R2) for the linear plots of Fig.5. It is found that the equilibrium sorption capacity (qe), the rate constant, k and the initial sorption rate, h are a function of pH. The ultimate sorption capacity of sludge derived activated carbon and commercial activated carbon has decreased from 14.26 mg/g to 9.85 mg/g and 14.99 mg/g to 10 mg/g respectively with increase in pH from 3 to 11.
Adsorption studies performed on the adsorbents, SAC and CAC revealed the ability of both the carbons to remove reactive dye from cotton dyeing plant effluents. The equilibrium data have been analyzed using Langmuir and Freundlich isotherms. Linear plot of log (X/M) versus log Ce showed that the adsorption obeyed a Freundlich isotherm (Fig.1). McKay et al., (1982) have shown that n value between 2 and 10 indicate beneficial adsorption. The linear plot of Ce/qe versus Ce showed that the adsorption obeyed a Langmuir isotherm model (Fig.2). The adsorption capacity obtained in this experiment is in agreement with the results reported in the literature. The adsorption data obeyed both Freundlich and Langmuir models exhibiting heterogeneous surface conditions and monolayer adsorption (Lee et al., 1995). The favorable nature of adsorption can be expressed in terms of a dimensionless parameter RL that is given by the equation.
RL = 1/ 1+bCO (7)
Where b is the Langmuir constant (l/mg) and Co is the initial dye concentration (McKay et al., 1982). All RL values obtained using equation (7), for dye adsorption are greater than zero and less than unity, showing favorable adsorption of dye on to sewage sludge derived activated carbon and commercial activated carbon. The equilibrium sorption capacity of the SAC anc CAC has increased with an increase in the concentration of dye in the aqueous solution.This is due to the acceleration of dye diffusion from dyeing solutions at higher concentrations (Ozacar, 2003a; Sun and Xu, 1997). It is observed that by increasing the dose of SAC and CAC, the percentage of dye removal from aqueous solution increased but adsorption density, ie the amount of dye adsorbed per unit mass decreased. It is readily understood that, the number of available adsorption sites increases by increasing the adsorbent dose and it therefore, results in the increase of percentage of dye adsorbed. The decrease in adsorption density with increase in the adsorbent dosage is mainly because of unsaturation of adsorption sites through the adsorption process (Yu et al., 2003; Shukla et al., 2002). Another reason may be due the interparticle interaction, such as aggregation resulting from high adsorbent dosage. Such aggregation would lead to decrease in total surface area of the adsorbent and an increase in diffusional path length. The equilibrium sorption capacity of SAC and CAC has increased by varying the pH value of dye laden aqueous solution from 11 to 3. At acidic pH the positively charged species start dominating and the surface tends to acquire positive charge, while the adsorbate species are still negatively charged. As the adsorbent surface is positively charged, the increasing electrostatic attraction between negatively charged adsorbate species and positively charged adsorbent particles would lead to increased adsorption of reactive dye (Shukla et al., 2002). From Table 9 it is found that the equilibrium sorption capacity (qe), the rate constant (k) and the initial sorption rate (h) of SAC and CAC depends on pH of dye laden aqueous solution.
The writers are thankful to the management of Vellore Institute of Technology, Deemed University, Vellore 632014, India, for providing necessary facilities to carry out this research.
© 2006 Tehran University of Medical Sciences Publications
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