Removal of cationic methylene blue and malachite green dyes from aqueous solution by waste materials of Daucus carota

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1 Journal of Saudi Chemical Society (2014) 18, King Saud University Journal of Saudi Chemical Society ORIGINAL ARTICLE Removal of cationic methylene blue and malachite green dyes from aqueous solution by waste materials of Daucus carota Atul Kumar Kushwaha, Neha Gupta, M.C. Chattopadhyaya * Environmental Chemistry Research Laboratory, Department of Chemistry, University of Allahabad, Allahabad (UP) , India Received 4 May 2011; accepted 19 June 2011 Available online 25 June 2011 KEYWORDS Adsorption; Daucus carota; Malachite green; Methylene blue; Isotherm Abstract In present study adsorption capacity of waste materials of Daucus carota plant (carrot stem powder: CSP and carrot leaves powder: CLP) was explored for the removal of methylene blue (MB) malachite green (MG) dye from water. The morphology and functional groups present were investigated by scanning electron microscope (SEM) and Fourier transform infrared (FTIR) spectroscopy. The operating variables studied were ph, adsorbent dose, ionic strength, initial dye concentration, contact time and temperature. Equilibrium data were analysed using Langmuir and Freundlich isotherm models and monolayer adsorption capacity of adsorbents were calculated. Kinetic data were studied using pseudo-first and pseudo-second order kinetic models and the mechanism of adsorption was described by intraparticle diffusion model. Various thermodynamic parameters such as enthalpy of adsorption DH, free energy change DG and entropy DS were estimated. Negative value of DH and negative values of DG showed that the adsorption process was exothermic and spontaneous. Negative value of entropy DS showed the decreased randomness at the solid liquid interface during the adsorption of MB and MG onto CSP and CLP. ª 2011 King Saud University. Production and hosting by Elsevier B.V. All rights reserved. * Corresponding author. Tel.: addresses: atulkk2008@gmail.com (A.K. Kushwaha), neha. evs07@gmail.com (N. Gupta), mcc46@rediffmail.com (M.C. Chattopadhyaya) ª 2011 King Saud University. Production and hosting by Elsevier B.V. All rights reserved. Peer review under responsibility of King Saud University. doi: /j.jscs Production and hosting by Elsevier 1. Introduction Many manufacturing industries such as paper, plastics, cosmetics, textile and food use dyes for colouring their products. The discharge of effluents from these industries contain large amount of dyes, not only damage the aesthetic nature of receiving water bodies, but also may be toxic to aquatic life. Methylene blue (MB) and malachite green (MG) are cationic dyes, have wide applications which includes colouring paper, dyeing cottons, wools, silk, leather and coating for paper stock. Although methylene blue is not strongly hazardous, it can cause some harmful effects, such as heartbeat increase, vomiting, shock, cyanosis, jaundice, quadriplegia, and tissue

2 Removal of cationic methylene blue and malachite green dyes from aqueous solution 201 necrosis in humans (Yi and Zhang, 2008). Though the external use of MG as an antiseptic, antibacterial and antiprotozoan agent is well known but its oral consumption is toxic, hazardous and carcinogenic due to presence of nitrogen (Bulut et al., 2008). Contact to malachite green with skin and eye causes irritation with redness and pain. Therefore, the treatment of effluents containing such dyes is of great interest due to its harmful impacts on receiving waters. As dyes are designed to resist breakdown with time, exposure to sunlight, water, soap and oxidizing agent, cannot be easily removed by conventional wastewater treatment processes due to their complex structure and synthetic origins (Wang et al., 2008). Adsorption techniques to remove dyes from water have been widely used. It has been found to be an economical and effective treatment method for removal of dyes due to its sludge free clean operation. Several materials such as silica gel (Kushwaha et al., 2010; Samiey and Toosi, 2010), clay (Almeida et al., 2009; Tahir and Rauf, 2006), zeolite (Han et al., 2009), sawdust (Garg et al., 2004), activated carbon (Altenor et al., 2009; Yener et al., 2008), algae (Bekci et al., 2009), agricultural wastes (Aksu and Isoglu, 2006; Chowdhury et al., 2011), plant wastes (Gupta et al., 2011; Weng et al., 2009) have been tried as an adsorbent for colour removal from the aquatic medium. Daucus carota (carrot) is a herb of Umbelliferae family. The stems are erect, branched, tough and furrowed. The leaves are very finely divided and all the leaves embrace the stem with the sheathing base. Main-crop carrots are generally taken up in winter and spring season. Thereafter, leaves and stem remain as agricultural wastes. The present paper is an attempt to explore the possibility to utilize the waste materials of carrot plant (stems and leaves) as a new adsorbent for the removal of methylene blue and malachite green dyes from aqueous solution. 2. Materials and methods 2.1. Preparation of adsorbents The waste carrot leaves and stems were washed thoroughly by double distilled water to remove the dust and other impurities. Leaves were firstly dried at room temperature and then in hot air oven at 60 C. The washed stems were boiled in double distilled water for 45 min and dried in a hot air oven at 60 C The dried leaves and stem were ground in a domestic grinder and screened to separate the particles of lm. Resulting carrot leaves powder (CLP) and carrot stem powder (CSP) were kept in a glass bottle for use in adsorption studies. Some physical properties of adsorbents were analysed using proximate analysis (Ponnusami et al., 2009) and are given in Table Adsorption experiments A stock solution of 1000 mg/l of MB and MG was prepared by dissolving an appropriate amount of each dye which was diluted to required concentration. Batch adsorption experiments were carried out to investigate the effect of ph, ionic strength, adsorbent dose, initial dye concentration, contact time and temperature on the adsorption of MB and MG on CLP and CSP. The experiments were carried out in 150 ml conical flasks by mixing a pre-weighed amount of adsorbent with 50 ml of dye solution and the solution was agitated at 200 rpm on a stirrer at constant temperature and centrifuged (Remi Research centrifuge). The dye concentration in supernatant solution was determined at characteristic wavelength (MB k max = 665 nm; MG k max = 617 nm) by double beam UV visible spectrophotometer (Systronics 2202). The percentage removal of dye and amount of dye adsorbed on adsorbents (q e ) was calculated by Eq. (1) and (2), respectively: % removal ¼ 100ðC 0 C e Þ=C 0 ð1þ q e ¼ðC 0 C e ÞV=M ð2þ where q e is the amount of dye adsorbed on adsorbent at equilibrium (mg/g), C 0 and C e are the initial and equilibrium concentration (mg/l) of dye in solution, respectively, V is the volume of solution (L) and M is the mass of adsorbent (g) Characterization of CLP and CSP FT-IR studies were carried out on Fourier transform infrared spectrophotometer (FTLA 2000, ABB Bomem, Quebec, Canada) using potassium bromide (KBr) disc method, within the range cm 1 to identify the functional groups responsible for the adsorption. The morphology of CLP and CSP were investigated by FEI Quanta 200 scanning electron microscope operated at 20 kv accelerated voltage. 3. Results and discussion 3.1. Characterization FT-IR spectra of CSP and CLP (Fig. 1) revealed the presence of similar functional groups in both the adsorbents. Broad Table 1 Some physical properties of adsorbents. Physical properties (%) CSP CLP Moisture content Volatile matter Ash Fixed carbon Figure 1 FTIR spectra of CSP and CLP.

3 202 A.K. Kushwaha et al. band around 3400 cm 1 was due to stretching vibration of O H bond in hydroxyl groups. The peaks observed at 2934 and 1386 cm 1 were assigned to the stretching and bending vibration of C H bond in methyl groups, respectively. The peak located at 1736 cm 1 was characteristic of carbonyl group stretching. The peak at 1603 cm 1 was due to bending vibration of O H groups. The strong C O band at 1052 cm 1 also confirms the lignin structure of the PSP and PLP (Han et al., 2010). The peak at 1250 cm 1 may be from the stretching vibration of C O in phenols. The SEM micrograph of a CLP and CSP sample at 1000 magnification depicted in Fig. 2(a) and (b) showed the fibrous and rough surface in CSP and CLP and pores can be seen in CLP which provides suitable binding sites for MB and MG dye molecule Determination of ph pzc of CSP and CLP The effect of ph on adsorption can be described on the basis of point zero charge (ph pzc ), which is the point at which the net charge of the adsorbent is zero (El-Qada et al., 2006). The ph at point zero charge (ph pzc ) of the CSP and CLP was determined by the solid addition method (de Oliveira Brito et al., 2010). Initial ph of 0.1 N KNO 3 solutions (ph i ) was adjusted from ph 3 to 11 by adding either 0.1 N HCl or 0.1 N NaOH. Adsorbent dose (2 g/l) was added to 50 ml of 0.1 N KNO 3 solution in 150 ml conical flasks and stirred for 30 min of contact time and final ph (ph f ) of solution was measured. The difference between the initial and final ph (ph f ph i ) was plotted against the initial ph (ph i ) and the point where ph f ph i = 0 was taken as the ph pzc (Fig. 3). The ph pzc of CSP and CLP was found to be 6.12 and 6.1, respectively. The adsorption amount of increased with increasing ph from 3 to 7. Thereafter increase in ph did not cause significant increase in adsorbed amount of dye. At ph < ph PZC, the surface becomes positively charged, concentration of H + were high and they compete with MB and MG cations for vacant adsorption sites causing a decrease in dye uptake. At the solution ph > ph PZC, the adsorbent surface negatively charged and favours uptake of cationic dyes due to increased electrostatic force of attraction. At ph 7, surface of adsorbent was negatively charged to its maximum extent. Further increase in ph did not increase surface charge intensity as well as adsorption capability. Therefore, ph 7 was used for adsorption studies. Figure 3 The plots of ph f ph i vs. ph (adsorbent dose: 2 g/l; temperature: 303 K; contact time: 30 min). Figure 4 Effect of adsorbent dose on adsorption of MB/MG onto CSP/CLP (initial dye concentration: 10 mg/l; ph: 7; temperature: 303 K; contact time: 30 min). Figure 2 SEM images of CSP (a) and CLP (b).

4 Removal of cationic methylene blue and malachite green dyes from aqueous solution 203 Figure 5 The effect of NaCl concentration on adsorption of MB/MG on CSP/CLP (adsorbent dose: 2 g/l; initial dye concentration: 10 mg/l; ph: 7; temperature: 303 K; contact time: 30 min) Effect of adsorbent dose Adsorbent dose is an important parameter influencing adsorption processes since it determines the adsorption capacity of an adsorbent for a given initial concentration of the adsorbate at the operating conditions. The effect of CSP/CLP dose on removal of MB and MG were studied in range of g/l. Fig. 4 showed that the % removal of dye increased from 40% to 82%, 45% to 87%, 32% to 67% and 35% to 75% for CSP-MB, CLP-MB, CSP-MG and CLP-MG systems, respectively, as adsorbent dose increased from 0.5 to 2 g/l. Such a trend is attributed to an increase in the adsorptive surface area and the availability of more binding sites. Further increase in adsorbent dose, did not show significant increase in % removal of dye, therefore, 2 g/l adsorbent dose was chosen for the successive experiments Effect of ionic strength Various salts and metal ions exist in the dye containing waste water. The presence of ions leads to high ionic strength, which may significantly affect the performance of the adsorption process. Fig. 5 showed that the salt (NaCl) existing in the solution affected the MB/MG adsorption onto CSP/CLP. It was seen that the adsorption amount of dye decreased from 4.1 to 3.4 mg/g, 4.35 to 3.9 mg/g, 3.35 to 2.75 mg/g and 3.75 to 3.3 mg/g for CSP-MB, CLP-MB, CSP-MG and CLP-MG systems, respectively, as the salt concentrations increased from 0.0 to 0.1 mol/l. This could be attributed to the competitive effect between dye cations and Na + ions for the available adsorption sites. As the ionic strength increased, the activity of MB and MG and the active sites for adsorption decreased, therefore, the adsorption amount of MB and MG decreased (Han et al., 2007) Adsorption kinetic studies A study of kinetics of adsorption is desirable as it provides information about the mechanism of adsorption, which is Figure 6 Adsorption kinetics of the adsorption of MB/MG on CSP/CLP (adsorbent dose: 2 g/l; initial dye concentration: 10 mg/ L; ph: 7; temperature: 303 K). important for efficiency of the process. The effect of contact time on adsorption of MB and MG onto CSP and CLP adsorbents was studied at 10 mg/l. From Fig. 6, it can be seen that the rate of adsorption was very rapid at initial period of contact time, thereafter, it decreased gradually with time until adsorption was reached at equilibrium point. This trend of adsorption kinetics was due to the adsorption of dye on the exterior surface of adsorbent at the initial period of contact time. When the adsorption on the exterior surface reached saturation point, the dye diffused into the pores of the adsorbent and was adsorbed by the interior surface of the adsorbent. The equilibrium time for CSP-MB, CLP-MB, CSP-MG and CLP- MG systems was 30 min, 30 min, 25 min and 25 min, respectively, thereafter, no further adsorption occurred with prolonged time. In order to understand the process of adsorption, three kinetic models (i) Lagergren s pseudo-first order model (Eq. (3)), (ii) Ho s pseudo-second order model (Eq. (4)) and (iii) Weber and Morris intraparticle diffusion model (Eq. (5)) were applied to analyse the experimental data. The linearized form of pseudo-first order kinetic model (Lagergren, 1898) can be written as: lnðq e q t Þ¼ln q e k 1 t ð3þ where q e and q t are the amount of dye adsorbed at equilibrium and at time t (mg/g), respectively and k 1 (min 1 ) is rate constant of adsorption. The values of k 1 and q cal e were calculated from the slopes ( k 1 ) and intercepts (ln q e ) of the plots of ln (q e q t ) vs. t (Fig. 7), respectively, and are presented in Table 2. Plots of Eq. (3) showed high correlation coefficient values but the calculated q e values of pseudo-first order equation were lower than the experimental one, indicated that pseudofirst order model was not fit to describe kinetic data. The pseudo-second order adsorption kinetics (Ho and McKay, 1999) can be written as follows: t=q t ¼ 1=k 2 q 2 e þ t=q e ð4þ where k 2 is the rate constant of adsorption (g/mg min), q e and q t are the amount of dye adsorbed at equilibrium and at time t

5 204 A.K. Kushwaha et al. Table 2 Adsorption system Kinetic parameters for adsorption of MB/MG on CSP/CLP. q exp e Pseudo-first order Pseudo-second order Intraparticle diffusion q cal e k 1 R 2 q cal e k 2 R 2 k i C R 2 CSP-MB CLP-MB CSP-MG CLP-MG Figure 7 Pseudo-first order kinetics plots for the adsorption of MB/MG on CSP/CLP (adsorbent dose: 2 g/l; initial dye concentration: 10 mg/l; ph: 7; temperature: 303 K). (mg/g), respectively. The values of k 2 and q cal e were calculated from the intercepts (1/k 2 q 2 e ) and slopes (1/q e) of the plots of t/q t vs. t (Fig. 8), respectively and are presented in Table 2. The correlation coefficients of pseudo-second order were closer to unity and calculated q e values computed from pseudo-second order equation showed good agreement with experimental values, indicating the applicability of pseudo-second order kinetic model for CSP-MB, CLP-MB, CSP-MG and CLP-MG systems. To identify the importance of diffusion in the adsorption process, mathematical expression of intraparticle diffusion model (Weber and Morris, 1963) was used: q t ¼ k i t 0:5 þ C ð5þ where k i is the intraparticle diffusion constant (mg/g min 0.5 ) and the intercept (C) reflects the boundary layer effect. The values of k i were calculated from slopes (k i ) of the plots of q t vs. t 0.5 (Fig. 9) and are presented in Table 2. Fig. 9 showed the involvement of two steps in adsorption process, first one representing adsorption of dyes on the surface of adsorbent and second one described diffusion of dyes to adsorption site. Surface adsorption mechanism was dominant in first 5 min of contact time, thereafter, diffusion became a rate-limiting process. The values of C were higher for MB dye revealed more surface adsorption of MB than MG. Rates of diffusion were higher in CLP than CSP, results in high adsorption capacity of CLP. From results it can be concluded that both surface adsorption and intraparticle diffusion mechanism was followed by all the four adsorption systems. Figure 8 Pseudo-second order kinetics plots for the adsorption of MB/MG on CSP/CLP (adsorbent dose: 2 g/l; initial dye concentration: 10 mg/l; ph: 7; temperature: 303 K) Equilibrium studies For the equilibrium study of adsorption of MB and MG onto CSP and CLP the experiments were conducted at different initial dye concentrations (10 50 mg/l). The uptake of dye was increased from 4.1 to 19 mg/g, 4.35 to mg/g, 3.35 to 14.7 mg/g and 3.75 to mg/g for CSP-MB, CLP-MB, CSP-MG and CLP-MG systems, respectively, as initial dye concentration increased from 10 to 50 mg/l. This may be attributed to an increase in the initial dye concentration, increase in the concentration gradient which provides a driving force to overcome all mass transfer resistances of dyes between the aqueous and solid phase. However the percentage removal of dye decreased from 82% to 76%, 87% to 83%, 67% to 58.8% and 75% to 68.2% for CSP-MB, CLP-MB, CSP-MG and CLP-MG systems, respectively, as initial dye concentration increased from 10 to 50 mg/l. Adsorptions isotherms are important for the description of how molecules of adsorbate interact with adsorbent surface. The most widely used isotherm models are Langmuir and Freundlich models. The Langmuir isotherms are based upon an assumption of monolayer adsorption onto surface of adsorbent containing a finite number of adsorption sites of uniform energies of adsorption. It can be written as follows (Ghoul et al., 2003): q e ¼ Q m bc e =ð1 þ bc e Þ ð6þ

6 Removal of cationic methylene blue and malachite green dyes from aqueous solution 205 Figure 9 Intraparticle diffusion plots for the adsorption of MB/ MG on CSP/CLP (adsorbent dose: 2 g/l; initial dye concentration: 10 mg/l; ph: 7; temperature: 303 K). Figure 11 The Frendlich plots for the adsorption of MB/MG on CSP/CLP (adsorbent dose: 2 g/l; ph: 7; temperature: 303 K). MB and MG in solution, Q m is the monolayer adsorption capacity (mg/g) and b is the Langmuir constant (L/mg) related to the free energy of adsorption. The values of Q m and b were calculated from the slopes (1/Q m ) and intercepts (1/bQ m ) of the linear plots of C e /q e vs. C e (Fig. 10) and are given in Table 3. The linearity of plots revealed that the adsorption followed Langmuir isotherm model. The maximum monolayer adsorption capacity of CSP-MB, CLP-MB, CSP-MG and CLP-MG systems was found to be 55.5 mg/g, 66.6 mg/g, 43.4 mg/g and 52.6 mg/g, respectively. The Freundlich isotherm assumes that adsorption process occurs on heterogeneous surfaces and the capacity of adsorption is related to the concentration of MB and MG at the equilibrium. The Freundlich equation is expressed as follows (Kumar et al., 2006): Figure 10 The Langmuir plots for the adsorption of MB/MG on CSP/CLP (adsorbent dose: 2 g/l; initial dye concentration: 10 mg/ L; ph: 7; temperature: 303 K). The linearized form of Eq. (6) can be written as Eq. (7) C e =q e ¼ 1=bQ m þ C e =Q m ð7þ where q e is the adsorption density (mg/g) of MB and MG at equilibrium, C e is the equilibrium concentration (mg/l) of q e ¼ K f C 1=n e ð8þ Logarithmic form of Eq. (8) can be written as Eq. (9) ln q e ¼ ln K f þð1=nþln C e ð9þ where K f and n are Freundlich constants related to adsorption capacity [mg g 1 (mg L 1 ) 1/n ] and adsorption intensity of adsorbents, respectively. The values of the K f and n were calculated from the intercepts (ln K f ) and slopes (1/n) of the plots ln q e vs. ln C e (Fig. 11) and are presented in Table 3. The values of regression correlation coefficients for all the four adsorption systems are closer to unity indicated the data were also fitted well in Freundlich isotherm model. The values of Table 3 Isotherm parameters for adsorption of MB/MG on CSP/CLP. Adsorption system Langmuir isotherm parameters Freundlich isotherm parameters Q max b R 2 K F 1/n R 2 CSP-MB CLP-MB CSP-MG CLP-MG

7 206 A.K. Kushwaha et al. Figure 12 Effect of temperature on adsorption of MB/MG on CSP/CLP (adsorbent dose: 2 g/l; initial dye concentration: 10 mg/ L; ph: 7). adsorption capacity was found to be in the order CLP-MB > CSP-MB > CLP-MG > CSP-MG. The values of 1/n were less than one revealed favourable adsorption conditions Thermodynamic studies The study of the adsorption process with respect to temperature can give valuable information about the free energy change and enthalpy change during adsorption. The effect of temperature on the adsorption capacities was studied by carrying out a series of experiments at 303, 313 and 323 K for all the four systems (Fig. 12). The adsorption decreased from 4.1 to 3.95 mg/g, 4.35 to 4.15 mg/g, 3.35 to 3.2 mg/g and 3.75 to 3.55 mg/g for CSP-MB, CLP-MB, CSP-MG and CLP-MG systems, respectively, as temperature increased from 303 to 323 K indicating that the adsorption process was exothermic in nature. The decrease in the equilibrium adsorption of MB and MG with increase in temperature indicated that the adsorption of dye on CSP and CLP was favourable at low temperature. This may be due to tendency of the dye molecules to escape from the solid phase to the bulk phase with an increase in temperature of the solution. Thermodynamic parameters such as enthalpy (DH ), entropy (DS ) and Gibb s free energy (DG ) were determined by Eq. (10) (Nandi et al., 2009) and (11). lnðq e m=c e Þ¼DS =R DH =RT ð10þ DG ¼ DH TDS ð11þ where m is the adsorbent dose (g/l), C e is the equilibrium concentration (mg/l) of the MB in solution, q e is the amount of MB adsorbed at equilibrium (mg/g) and q e m is the solid-phase concentration (mg/l) at equilibrium. R is the gas constant (8.314 J/mol/K) and T is the temperature (K). DH, DS and DG are changes in enthalpy (kj/mol), entropy (J/mol/K) and Gibb s free energy (kj/mol), respectively. The values of DH and DS were determined from the slopes ( DH /R) and the intercept (DS /R) of the plots of ln (q e m/c e ) vs. 1/T. The DG values were calculated using Eq. (11). The values of thermodynamic parameters are presented in Table 4. Negative values of DG indicated that the adsorption process was feasible and spontaneous in nature. Spontaneity of adsorption was found to be in the order CLP- MB > CSP-MB > CLP-MG > CSP-MG. Negative values of DH suggested that the exothermic nature of adsorption and negative values of DS described the randomness at the adsorbent-solution interface decreased during the adsorption. The values of DS were more negative for CLP based systems, indicated that the dye was more stable on CLP than CSP. The DH values obtained were in the range of 8.2 to 12.6 kj/ mol, indicated the presence of physical adsorption mechanism. 4. Conclusion In this study, CSP and CLP were successfully used for the removal of MB and MG dye from aqueous solution. ph 7 and temperature 303 K was found to be optimum for removal of MB and MG by CSP and CLP. The equilibrium of adsorption of MB/MG onto CSP/CLP was suitably described by the Langmuir and Freundlich isotherm models. Monolayer adsorption capacities of CLP were higher suggested that CLP is better adsorbent than CSP. The process of adsorption was best described by the pseudo-second order kinetics and intraparticle diffusion model. The dye uptake process was found to be controlled by both surface and pore diffusion, with surface diffusion at the earlier stages followed by pore diffusion at later stages. The thermodynamics parameters indicated that the adsorption was spontaneous, exothermic and physical in nature. Finding indicated that the waste materials of D. carota could be used as a potential adsorbent for the removal of MB and MG from aqueous solution and is inexpensive material for treating industrial wastewater. Acknowledgements Authors are thankful to Prof. Avinash C. Pandey and Mr. Prashant K. Sharma of Nanotechnology Application Centre, University of Allahabad, India, for recording SEM and the Table 4 Thermodynamic parameters for adsorption of MB/MG on CSP/CLP. Adsorption system DH (kj/mol) DS (J/mol/K) DG (kj/mol) 303 K 313 K 323 K CSP-MB CLP-MB CSP-MG CLP-MG

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