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Preparation of activated carbon from Alligator weed (Alternenthera philoxeroids) and its application for tartrazine removal: Isotherm, kinetics and spectroscopic analysis
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JECE 737 1–9 Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx
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Preparation of activated carbon from Alligator weed (Alternenthera philoxeroids) and its application for tartrazine removal: Isotherm, kinetics and spectroscopic analysis Pavan Kumar Gautama , Ravindra Kumar Gautama , Sushmita Banerjeea , Giusy Lofranob , Maria Angeles Sanromanc, Mahesh Chandra Chattopadhyayaa,* , J.D. Pandeya,* a
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Environmental Chemistry Research Laboratory, Department of Chemistry, University of Allahabad, Allahabad 211002, India Department of Chemistry and Biology, University of Salerno, via Giovanni Paolo II, 132 Fisciano, Salerno, Italy c Department of Chemical Engineering, Isaac Newton Building (F.F.T), University of Vigo, 36310 Vigo, Spain b
A R T I C L E I N F O
Keywords: Activated carbon Alligator weed Isotherms Kinetics SEM Tartrazine
A B S T R A C T
In this paper, the ability of activated carbon obtained from Alligator weed (AAW) to remove tartrazine from aqueous solutions was investigated. The as-prepared AAW was characterized by SEM, FTIR, BET surface area analyzer, and through the determination of pHzpc. The effect of different operational parameters like pH, initial concentration of tartrazine, contact time, and temperature on the sorption behavior was studied in batch mode. The adsorption of tartrazine was favored in acidic medium. The kinetic data were examined using various models viz. pseudo-first order, pseudo-second order, and intraparticle diffusion models. The fitness of the experimental data with pseudo-second order model was evident from the values of correlation coefficient (R2 > 0.99). The adsorption equilibrium was modeled with the Langmuir and Freundlich isotherm expressions. The isotherm data were better fitted to Freundlich model which indicates heterogeneous nature of the adsorbent. The Freundlich adsorption capacities (KF) were found to be 107.56, 131.67 and 183.4 mg g1 at 30, 40 and 50 C, respectively. The negative values of DG and positive value of DH demonstrated that the adsorption of tartrazine onto AAW were spontaneous and endothermic in nature, respectively. The regenerated AAW can be reused 4 times for the adsorption of tartrazine. ã 2015 Published by Elsevier Ltd.
8
Introduction
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Development and utilization of low-cost adsorbents for the treatment of dye bearing wastewater are driving scientific and academic interest because of the potential hazards associated with synthetic dyes and their byproducts [1]. Industries such as food, cosmetics, textiles, paper, rubber, and plastics are main consumers of dyestuffs [2]. Due to their complex aromatic structure and molecular size, dyes are resistant to photo-catalytic, thermal and biological degradation. Colored effluents are highly visible even in small concentration and seriously affect the aesthetic quality and transparency of water bodies, damaging the aquatic ecosystem [3]. Thin layer of discharged dyes, formed over the surfaces of recipient water bodies also reduces the amount of dissolved oxygen, thereby affecting the aquatic fauna [4,5]. Moreover, dye containing
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* Corresponding authors. Fax: +91 532 2541786. E-mail addresses: [email protected] (M.C. Chattopadhyaya), [email protected] (J.D. Pandey).
effluents increase biochemical oxygen demand. Additionally, they can have acute and chronic effects on living organisms depending upon their concentration and duration of exposure [6]. Therefore, elimination of dyes from water and wastewater has become a priority concern for researchers. A wide range of technologies including advanced oxidation processes [7], flocculation [8], electro-coagulation [9,10], membrane filtration [11], ultrasonic irradiation [12], photocatalysis [13], and microbial degradation [14], have been attempted for color removal from wastewater. The main drawbacks associated with these processes are high cost, low efficiency, limited versatility, and generation of sludge. Adsorption of dyes on efficient solid biomass support represents a simple and economically viable cleaning technology for color removal especially, where the sorbents are inexpensive and easily available [15]. More importantly, it imparts no side effect or toxicity to the water and this account for the superior removal of organic contaminants as compared to conventional treatments [16]. Therefore, it is preferred over other techniques in terms of cost, ease of operation, flexibility, negligible disturbances with diurnal variations, reusability and recyclability of the sorbent [17]. Several
http://dx.doi.org/10.1016/j.jece.2015.08.004 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: P.K. Gautam, et al., Preparation of activated carbon from Alligator weed (Alternenthera philoxeroids) and its application for tartrazine removal: Isotherm, kinetics and spectroscopic analysis, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j. jece.2015.08.004
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low-cost biosorbents such as mustard husk [1], lantana (Lantana camara) weed [18], chitosan [19], cactus [20], oil palm trunk fibres [21], almond shell [22], Borassus aethiopum shells [23], parthenium based activated carbon [24], and sphagnum moss peat [25] have been utilized for the removal of pollutants from aqueous media. Alligator weed (Alternenthera philoxeroids) is a native of South America and now it is widespread has around the world [26] due to its extremely vigorous growth and resistance to control measures. It also represents a serious threat to wetlands, rivers and irrigation systems [27]. This weed disrupts aquatic environment by covering the surface and restraining penetration of light [28]. This leads to impede gaseous exchange thereby causing anaerobic environment, which severely affects aquatic organisms [29]. In the past, the raw biomass of this weed has been successfully applied for the removal of metal ions from aqueous solutions [30,31] but its efficiency for Q2 the organic pollutants is yet to be explored. In the present study, a novel and low-cost adsorbent was prepared using biomass of abundantly available Alligator weed for the removal of an acidic dye tartrazine. Batch adsorption experiments were carried out using synthetic solutions of tartrazine, and the effects of operating parameters such as initial dye concentration, solution pH and contact time were investigated. The kinetics of adsorption system was studied, and various kinetic models, such as pseudo-first-order, pseudo-second-order and intra-particle diffusion models were tested with experimental data for their validity. The equilibrium sorption behavior of the adsorbents was studied using the adsorption isotherm techniques. Experimental data were fitted to the Langmuir and Freundlich isotherm models to determine the best isotherm to correlate the experimental data. Thermodynamic aspects of the adsorption process were studied by calculating the changes in Gibbs free energy, enthalpy and entropy. Desorption studies as for economic feasibility of adsorption process were also investigated.
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Materials and methods
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Tartrazine (Trisodium (4E)-5-oxo-1-(4-sulfonatophenyl)-4-[(4sulfonatophenyl)hydrazono]-3-pyrazolecarboxylate; C.I., 19,140; molecular formula, C16H9N4Na3O9S2; molar mass, 543.40 g mol1) was selected as an adsorbate and purchased from Sigma–Aldrich (USA). Chemical structure of the dye has been given in Fig. 1. It is light orange colored azo dye widely used in the food, cosmetics, pharmaceuticals and textile industries. Double distilled deionized water was used for preparing solutions. An Adventurer OhausAR2130 balance with a precision of 0.1 mg was used for weighing. Stock solutions of tartrazine were prepared by dissolving 1000 mg of dye in 1 L ultra pure water. This stock solution was diluted to prepare desired concentration range (25–200 mg L1) for adsorption experiment. A pH meter (pH meter 335, Systronics, India) was used for measuring pH of the solution. All the chemicals (NaOH, HCl, and H2SO4) used were of analytical grade with 98% purity and obtained from M/S Loba Chemie SD fine chemicals.
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Preparation of activated carbon
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Alligator weed (AW) was procured from a waterlogged area of Allahabad (India). The biomass was washed with distilled water and dried in sunlight for 4–5 days. The dried material was crushed down to powder and washed again 4–6 times with distilled water before to be dried in a hot air oven (Gupta scientific instruments, Ambala Cant, India) for 24 h. The powdered biomass of AW was treated with concentrated H2SO4 at 1:1 ratio followed by impregnation by drying the mixture at 120 C for 24 h. For carbonization, impregnated samples were heated at 600 C under nitrogen flow at a heating rate of 5 C min1. After this, the samples were allowed to cool down under N2 gas flow. The carbonized sample was washed 6–8 times with deionized water. The washed sample was dried in an oven at 11 C for 24 h. The dried material was then sieved with 44 BSS mesh to get fine particles and stored in air tight glass bottles, and kept in vacuum desiccators for the characterization and experimental studies.
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Characterization of AAW
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Fourier transform IR spectra of unloaded and dye loaded was obtained by using a Fourier transform infrared spectroscopy (FTLA 2000, ABB, Canada) to determine the surface functional groups. Morphological characteristics of sorbent were determined by performing scanning electron microscopy (JEOL-SEI) at an acceleration voltage of 15 kV. Before, scanning, sorbent was coated by a thin layer of gold using a sputter coater to make it conductive. BET (Brunauer, Emmett and Teller) surface area analyzer (Micromeritics ASAP 2020) was used to determine the surface area and pore volume. Elemental composition of the adsorbent was estimated using an elemental analyzer (CHN Autoanalyzer, Australia). Solid addition method was applied to calculate the pH of adsorbent at pHzpc which is the point at which the net charge on adsorbent is zero and 0.1 N HCl and 0.1 N NaOH were used to control the pH. For this, 0.5 g L1 was added to 50 mL of 0.1 N KNO3 solutions in a conical flask and agitated for 30 minand final pH of solution was measured. The difference between initial and final pH (pHf pHi) was plotted against the initial pH (pHi) and the point where pHf pHi = 0 was taken as pHzpc.
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Adsorption experiment
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Batch adsorption experiments were performed in order to investigate the removal efficiency of activated carbon of Alligator weed (AAW). Stock solution was diluted to prepare working solutions. The pH of the working solution was maintained with 0.1 M HCl or 0.1 M NaOH. The experiment was performed by adding 100 mL of adsorbate solution in 250 mL of Erlenmeyer conical flasks. An optimized dose of adsorbent (0.2 g) was selected (figure not shown) and used for all the subsequent adsorption experiments. The experiments were carried out at a stirring rate of 120 rpm on a shaking thermostat water bath (Macro scientific works Pvt. Ltd., MSW 275, Delhi). The uncertainty associated with temperature was estimated to within 0.5 C. After the equilibrium attainment, the adsorbent was separated by centrifugation (REMI R-8C BC, New Delhi, India) from its aqueous phase at 10,000 rpm for 15 min. A double beam UV-spectrophotometer (Systronics 2203, Ahmadabad, India) was used to determine the remaining concentration of tartrazine at 426 nm. The percentage removal of tartrazine and amount of tartrazine adsorbed were determined using the following relationships: 1 ð1Þ 100 Tartrazine removal ð%Þ ¼ ðC 0 C e Þ C0
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Fig. 1. Molecular structure of tartrazine.
Please cite this article in press as: P.K. Gautam, et al., Preparation of activated carbon from Alligator weed (Alternenthera philoxeroids) and its application for tartrazine removal: Isotherm, kinetics and spectroscopic analysis, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j. jece.2015.08.004
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JECE 737 1–9 P.K. Gautam et al. / Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx 1 Equilibrium adsorption capacityðmgg Þðqe Þ 1 V ¼ ðC 0 C e Þ M
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ð2Þ
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where C0 and Ce are the initial and equilibrium concentrations of tartrazine (mg L1), respectively, M is the mass of adsorbent (g) and V is the volume of solution (L).
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Desorption studies
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Desorption studies are very important as far economic feasibility of adsorption process is concerned. In the present work, 0.1 M NaOH aqueous solutions have been used for the regeneration of adsorbent due to its desorbing efficacy in our previously reported work [32]. The process was repeated several times until the tartrazine could not be detected in the filtrate. The regenerated AAW was washed thoroughly with distilled water to a neutral pH and the same adsorbent was reused 4 times for further adsorption experiments.
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Replication of batch experiment
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Each batch adsorption experiment was performed in triplicates and the average values have been reported to ensure reproducibility and accuracy with an error 5%. In the case of deviation larger than 5% more experiments were performed. The experimental data could be reproduced with accuracy greater than 95%. The error functions, sum of the squares of the errors (ERRSQ), were calculated across the studied range using the solver add-in with Microsoft’s Excel spreadsheet by the Eq. (3).
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ERRSQ ¼
P X
qe;cal qe;exp
i¼1
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2
ð3Þ
i
where qe,cal and qe,exp are the theoretical and experimental capacities (mg g1), respectively. Normalized standard deviations were also calculated for the selection of best adsorption model [23,24]. It was calculated from Eq. (4). Lower the value of normalized deviations, the better is the fit to experimental data.
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Normalized standard deviationDqe % vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u h i2 u tS qe;cal qe;exp =qe;exp ¼ 100 N 179 178
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Results and discussion
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Textural characteristics and elemental constituents
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Table 1 depicts estimated textural properties and elemental composition of AAW. The prepared activated carbon possesses specific surface area (303.09 m2 g1) and pore volume comparatively higher to inactivated or raw forms previously investigated [33]. However, the specific surface area obtained for AAW is lower than the reported for commercial activated carbon (Fluka, France) and higher than the activated carbon prepared from Lantana camara [18]. The mean pore diameter was estimated to be 41.38 Å indicating microporous morphology of the activated carbon. Elemental analysis confirmed the carbonaceous nature of the adsorbent which is present to the extent of 57.09%.
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Fourier transforms infrared spectroscopy (FTIR)
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In order to investigate the functional groups responsible for tartrazine removal, the FTIR spectra of prepared activated carbon was recorded (Fig. 2). These functional groups play significant role in understanding the chemistry of adsorption of adsorbate onto the surface of adsorbent [34]. From Fig. 2, it is evident that the AAW adsorbent displayed number of absorption peaks indicating the structural, elemental and constitutional complexity. The sharp peaks around 600 cm1 to 790 cm1 revealed the presence of sulphate group [35]. The bands in the region 800 cm1 may be assigned as mono-substituted aromatic rings or aromatic phosphate stretching (P–O–C). Gautam et al. [32] also reported similar observation in their work while studying the adsorption efficiency of raw Lantana camara biosorbents. The peaks observed in the 1000–1200 cm1 range may be explained by presence of C–O–C stretching of ether. Surprisingly, no any peaks was found in the region 2000–3500 cm1 as reported by [36] in case of raw AW biosorbents which corresponds to presence of OH of phenols and other primary and secondary amines. This may be attributed to vaporization of volatile functional groups during the process of activation of the sorbent. Shifting of peaks in dye loaded FTIR spectra confirmed the interaction of activated AW (AAW) sorbent with dye molecules.
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ð4Þ
where N is the number of measurements.
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Table 1 Physical characteristics of activated carbon. Typical properties BET surface area (m2 g1) Total pore volume (cm3 g1) Micropore surface area (m2 g1) Mesopore surface area (m2 g1) Meanpore diameter (Å)
303.09 0.215 255.79 47.30 41.38
Proximate analysis (wt%) Volatile matter Fixed carbon Ash
14.51 63.64 21.85
Elemental analysis (wt%) C H N S Oa
57.09 3.88 7.245 2.77 29.015
a
By difference.
Fig. 2. FTIR spectra of activated carbon and tartrazine loaded activated carbon of Alligator weed.
Please cite this article in press as: P.K. Gautam, et al., Preparation of activated carbon from Alligator weed (Alternenthera philoxeroids) and its application for tartrazine removal: Isotherm, kinetics and spectroscopic analysis, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j. jece.2015.08.004
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Fig. 3. SEM images of the pure activated carbon (a, b, c) and tartrazine loaded (d) activated carbon. 216
SEM micrographs
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Fig. 3 shows SEM micrographs of AAW adsorbents at different magnification before and after dye adsorption. Fig. 3(a–c) shows that AAW possesses uneven heterogeneous surface morphology with pores of variable sizes. This variability and heterogeneity facilitate the extent of adsorption due to the prevalence of active sites for the capture and adsorption of dye molecules. Kyzas et al. [37] equally reported similar morphological properties in untreated coffee residues which proved to be very effective for removing multicomponent dye solution. Fig 3(d) shows that the surface of adsorbent was fully covered by tartrazine molecules.
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Effect of pH
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The pH of the solution is an important factor as it governs both the surface chemistry of the adsorbent and solubility of dyes. The effect of pH on biosorption at constant temperature (30 C) of tartrazine after 60 min is shown in Fig. 4. It was observed that the removal efficiency of AAW increased with decreasing pH over the pH range 2–10 for all the concentration regime of tartrazine. Maximum removal was achieved (99.31%) at pH 2 for 25 mg L1dye concentration. Similarly, Habila et al. [38] determined that effective adsorption of tartrazine dye onto mixed-waste activated carbon occurred at pH 2. This results may be attributed to the presence of various functional groups such as OH, NH2, C¼C and C¼O, as indicated from the FTIR results, and also to the presence of partially charged groups in the tartrazine structure, such as COO, SO3 and OH. The presence of these groups allows the formation of covalent bonds, columbic forces, hydrogen bonds or weak van der Walls forces. The occurrence of the double bond serves to enhance the interaction between dye and the activated carbon [39]. This may also be explained by phenomenon of protonation of functional
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Fig. 4. Dependence of pH on tartrazine adsorption (adsorbent dose = 0.2 g, Shaking speed = 120 rpm, contact time = 60 min, temperature = 30 C.
groups present at the surface of adsorbent which create more positively charged sites on activated carbon [40]. These positively charged sites not only chemically bind with dye molecules but also stimulate their diffusion inside the surface wall of sorbent by electrostatic attraction, leading to an increment in percent removal of dye. With the increase in pH, the percent uptake was decreased due to the deprotonation of functional groups which causes electrostatic repulsion. The observation can further be understood in terms of zero point charge (pHzpc). This value was found to be 6.1 (Figure not shown). In general, pH < pHzpc favors the uptake of anionic dyes by electrostatic interaction via making the surface, positively charged. In case where pH > pHzpc, the rate of adsorption
Please cite this article in press as: P.K. Gautam, et al., Preparation of activated carbon from Alligator weed (Alternenthera philoxeroids) and its application for tartrazine removal: Isotherm, kinetics and spectroscopic analysis, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j. jece.2015.08.004
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is decreased due to the abundance of negatively charged sites on the surface of adsorbent which lower down the affinity of dyes for the surface. Similar findings have also been reported by Mittal et al. [41,42] on the removal behaviour of tartrazine using bottom ash, de-oiled soya and hen feather. Gautam et al. [18] reported similar conclusion on adsorption studies of tartrazine using activated Lantana camara biosorbents. These supporting informations gave us additional confidence to represent our results. Effect of initial adsorbate concentration and contact time The effect of initial dye concentration and contact time on adsorption of tartrazine for four initial concentrations (25, 50, 100 and 200 mg L1) of tartrazine is presented in Fig. 5, which shows that the percent removal of dye decreased with increase in initial dye concentration. This can be interpreted by tendency of dyes to make larger aggregates or micelles at higher initial concentration [43,44]. Aggregation inhibits their diffusion through the micropores of the activated carbon thereby reducing the percent removal of tartrazine. The adsorption was rapid at initial stage and then increases gradually up to the equilibrium attainment of 60 min for all the cases. No remarkable uptake was observed after this. These results are in accordance with the obtained by Goscianska and Pietrzak [45] who study the removal of tartrazine from aqueous solution by carbon nanotubes decorated with silver nanoparticles and determined that tartrazine adsorption equilibrium was rapidly attained after 60 min of contact time. In addition, the removal of the dye from acidic solutions was better than from basic ones. On the other hand the values of amount adsorbed (qe) (mg g1) increased with the increase in initial tartrazine concentration. At higher initial concentrations more dye molecules are available to interact with activated carbon. This could also be explained by the fact that the increase in initial dye concentration also accelerates driving forces that overcome the resistance to the mass transfer of tartrazine molecules from liquid phase to solid phase [46]. Habila et al. [38] reported that the amount of tartrazine adsorbed at the equilibrium time reveals the maximum adsorption capacity of the mixed-waste AC. The removal of tartrazine increased from 19 to 74.9 mg g1 as the initial concentration increased from 25 to 150 mg/L, indicating that the removal is affected by the initial concentration. Adsorption kinetics The most widely known models viz. Lagergren’s pseudo-firstorder and pseudo-second-order model were used to analyze
kinetics of the adsorptive removal of dye onto AAW. A linear form of pseudo-first-order model is expressed as: lnðqe qt Þ ¼ lnqe k1 t
t t 1 ¼ þ qt qe k2 q2e
300 301
ð5Þ
where qe and qt refer to the amount of dye adsorbed (mg g1) at equilibrium and at time t (min), respectively. The equilibrium rate constant k1and qe have been determined from the intercept and slope of the plot of log (qe qt) versus t. The larger differences between experimental and calculated qe values indicated that the pseudo-first order model did not fit well for the entire studied composition range of tartrazine (Table 2). Lower values of coefficient of determination (<0.959) also supported the observation. The values of first order rate constant k1 (min1), qe (mg g1) and R2 are given in Table 2. The linear form of pseudo-second-order model is expressed as [47]:
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ð6Þ
where k2 is the equilibrium rate constant of pseudo-second-order adsorption (g mg1 min1) estimated by calculating the slope and intercept of the plot of t/qt vs t. The recovered kinetic parameters along with fitness of good in terms of R2 values have been given in Table 2. The calculated qe values indicated good agreement with experimental values with pseudo-second-order kinetic model for all initial dyestuff concentration as shown in Table 2. The R2 values were greater than 0.999. This fact is also evident from the values of normalized standard deviations. This result indicated that the adsorption of tartrazine fitted well with the pseudo-second-order model, in agreement with the previous studies reporting the removal of tartrazine onto activated carbon of Lantana camara and papaya seeds have been reported by Gautam et al. [18] and Weber et al. [48].
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Intra-particle diffusion study
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The study is significant from the viewpoint of adsorption process as it indicates about the diffusivity of the adsorbate molecules, which plays important role in ascertaining the overall rate of sorption process. Although, upon investigation of kinetic parameters confirmed that pseudo-second-order equation agrees well with the experimental sorption data, the result obtained from this model failed to predict the diffusion process. Adsorption is a complex process which usually governed by more than one step such as boundary layer diffusion, intra-particle mass diffusion, or both [49]. Therefore, the slowest step which might be either film diffusion or particle diffusion can be designated as the overall rate determining step of the sorption process. The expression for intraparticle diffusion suggested by Weber and Morris [50] is given as:
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1
qt ¼ ki t2 þ C
Fig. 5. Effect of initial tartrazine concentration and contact time (adsorbent dose = 0.2 g, Shaking speed = 120 rpm, contact time = 60 min, temperature = 30 C.
5
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332 333 334 335 336 337 338 339 340 341 342 343
ð7Þ
where ki is the intra-particle diffusion rate constant (mg g1 min1/ 2 ) and C is the thickness of boundary layer (mg g1). The theory suggests that if the fitting of experimental data leads to linear plot and line passes through origin then pore diffusion controls the sorption process but any deviation from linearity indicates that film diffusion dominates the sorption process [51]. Table 2 reports the values of ki, C and R2 and Fig. 6 represents the intra-particle diffusion plots for various initial dye concentrations. The resultant plots obtained for all initial dye concentrations were linear over the whole time range but did not passes through the origin. Therefore the nonzero intercept of the plots advocate that intra-particle diffusion not involved in the adsorption process moreover film diffusion process dictate the overall rate of the adsorption process. This behavior is in accordance with the obtained by Dotto et al. [52]
Please cite this article in press as: P.K. Gautam, et al., Preparation of activated carbon from Alligator weed (Alternenthera philoxeroids) and its application for tartrazine removal: Isotherm, kinetics and spectroscopic analysis, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j. jece.2015.08.004
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P.K. Gautam et al. / Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx Table 2 Kinetic parameters for the adsorption process of tartrazine onto activated carbon of Alligator weed. Tartrazine concentration
Pseudo–first order
(mg L1)
k1 (min1)
qecal (mg g1)
qeexp (mg g1)
R2
ERRSQ
Normalized standard deviation (%)
25 50 100 200
0.044 0.043 0.035 0.041
34.04 60.81 94.14 169.01
119.5 229.1 438.35 835.85
0.933 0.911 0.931 0.959
7.30 103 2.83 104 1.18 105 4.44 105
41.29 42.41 45.34 46.06
Pseudo-second order Tartrazine concentration (mg L1)
k2 (g mg1 min1)
qecal (mg g1)
qeexp (mg g1)
R2
ERRSQ
Normalized standard deviation (%)
25 50 100 200
2.861 103 1.851 103 1.102 103 6.260 104
121.95 232.55 434.78 833.33
119.5 229.1 438.35 835.85
0.9977 0.9982 0.9989 0.9993
0.60 101 1.19 101 1.27 101 0.63 101
1.18 0.87 0.47 0.17
Intra-particle-diffusion
25 50 100 200
ki (mg g min1/2)
C (mg g1)
R2
4.055 7.087 11.689 20.809
87.38 172.03 339.31 628.1
0.9796 0.9819 0.9810 0.9957
capacity of adsorbent from the ratio between the quantity adsorbed at the interface and that remaining in the bulk at the fixed temperature at equilibrium [53]. The isotherm results were analyzed by fitting the equilibrium adsorption data to Langmuir and Freundlich model. Langmuir model is related to the assumption that maximum sorption corresponds to a saturated molecules of solute component with, no lateral interaction between adsorbed molecules [54]. The linear form of Langmuir isotherm model is expressed as: Ce 1 Ce þ ¼ qe ðK L Q m Þ Q m
360
and the tartrazine adsorption onto chitin occurred only by film diffusion.
361
Adsorption isotherms study
RL ¼
362
Study of adsorption isotherms is worthwhile to understand the complex mechanism of interaction between the adsorbate and adsorbent. Additionally, it also helps in predicting the adsorption
where C0 is the initial concentration of adsorbate (mg L1) and KL is Langmuir constant. The value of RL determines that the adsorption process is irreversible (RL = 0), linear (RL = 1), favorable (0 < RL < 1)
359
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1 ð1 þ K L C 0 Þ
30 40 50
Langmuir isotherm
367 368 369 370 371 372 373
375 374 376 377 378 379 380 381 382 383
ð9Þ
Table 3 Isotherm constants for the adsorption of tartrazine onto activated Alligator weed at different temperatures. Temperature ( C)
366
ð8Þ
where qe refers to the amount of dye adsorbed at equilibrium (mg g1), Ce is the equilibrium concentration of dye solution (mg L1), Qm is the maximum adsorption capacity (mg g1) and KL is the Langmuir constant (L mg1) related to the maximum adsorption capacity and energy of sorption. The values of Qm, KL, and R2 are presented in Table 3. The essential characteristic of Langmuir model can be described by a dimensionless separation factor RL which is calculated by following expression:
Fig. 6. Intra-particle diffusion plot for the adsorption of tartrazine on AAW at 30 C.
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Freundlich isotherm
Qm (mg g1)
KL (L mg1)
Normalized standard deviation (Dqe %)
R2
KF (mg g1)
n
Normalized standard deviation (Dqe %)
R2
77.118 98.765 172.428
15.440 10.850 6.006
22.01 17.71 13.91
0.8933 0.9150 0.9415
107.569 131.671 183.4
1.746 1.936 2.149
5.03 5.42 5.17
0.9968 0.9943 0.9950
Please cite this article in press as: P.K. Gautam, et al., Preparation of activated carbon from Alligator weed (Alternenthera philoxeroids) and its application for tartrazine removal: Isotherm, kinetics and spectroscopic analysis, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j. jece.2015.08.004
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or unfavorable (RL > 1). The values of RL was found to be in the range of 0–1, indicates the favorability of adsorption. In the present study RL values were calculated in the range of 0.0006–0.00032 for 30 C, 0.0036–0.00046 for 40 C and 0.0066–0.00083 for 50 C. All the RL values were found to be exist in the range of 0–1 which further indicates that adsorption of tartrazine on AAW is favorable in nature. The equilibrium data were further studied by Freundlich isotherm which is an empirical relationship proposed to describe heterogeneous systems [55]. The linear form of Freundlich isotherm is represented by following expression: 1 ð10Þ lnC e lnqe ¼ lnK F þ n
418
where KF (mg g1) and n are the constants representing the adsorption capacity and the adsorption intensity, respectively. The values KF, n and R2 were ascertained graphically from Freundlich plot at different temperatures and has been recorded in Table 3. It illustrates that the adsorption of tartrazine obeyed Freundlich model well. Adsorption capacity (KF) and adsorption intensity (n) increased with increasing temperature. The values of n were found greater than unity for all the studied temperature values, confirming that the tartrazine is favorably adsorbed onto AAW. The results clearly revealed that the multilayer coverage of adsorbate molecules on the surface of activated carbon facilitated by heterogeneous sites present at the surface of sorbent. Table 4 presents a comparison of the maximum adsorption capacity of tartrazine onto adsorbents from various sources. From the Table 4, it can be observed that AAW exhibits much higher sorption capacity for tartrazine as compared to other reported adsorbents. However, the difference observed for the sorption capacities mainly attributed to dye structure, functional groups, pHzpc, and surface area and porosity of each sorbent.
419
Thermodynamics
400 399 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417
420 421 422 423 424
Thermodynamic studies of the adsorption of tartrazine onto AAW were undertaken to observe the possible mechanism involved [56]. Thermodynamics parameters such as change in standard free energy (DG ), enthalpy (DH ) and entropy (DS ) were evaluated by following expression: logK c ¼
DS 2:303R
DH 2:303RT
ð11Þ
7
Table 5 Values of different thermodynamic parameters of adsorption for tartrazine onto AAW at various temperatures. Temperature ( C)
DG (kJ mol1)
DH (kJ mol1)
DS (J mol1 K1)
30 40 50
7.73 8.04 8.35
1.66
31.84
DG ¼ DH T DS
ð12Þ 1
where R (8.314 J mol K) is the gas constant, T is the absolute temperature (K) and Kc refers to the standard thermodynamic equilibrium defined by Cad/Ce, where Cad and Ce are equilibrium concentration of dye on the surface of AAW and equilibrium concentration of dye in the solution. The slopes and intercepts of the plot of log Kc versus 1/T were employed to calculate the values of DH and DS (Plot was not shown). The evaluated parameters at all the studied temperature values are listed in Table 5. The values of DG were negative for all the temperatures and decrease with the increase in temperature which exhibits the spontaneity of the sorption process. It also indicates that higher temperature accelerates the extent of adsorption by lowering the viscosity of solution [57]. This reduction in viscosity enhances the rate of mass transfer from the liquid to solid phase and makes adsorption easier [58]. Similar findings were reported by Rozada et al. [59] on the adsorption study of methylene blue and brilliant red from aqueous phase onto discarded tyres. Positive value of DH indicated the endothermic nature of the adsorption. Positive value of DS confirmed increase in randomness at the solid–liquid interface.
426 425 427
Recycling study
445
Reusability and recyclability of the sorbent are highly significant as far cost effectiveness of the adsorption process is concerned. Therefore, desorption study has also been undertaken to examine the possibility to reuse the adsorbent and regenerate the adsorbate. Tartrazine loaded AAW was recovered using an optimized concentration (0.1 M) of aqueous NaOH solutions of 100 mL. The experimental values of adsorption capacities and percent adsorbate recovery in each adsorption-desorption cycle are given in Table 6. The results indicated that the adsorption capacity of the regenerated adsorbent decreased sluggishly after every cycle of 15 min. The adsorbate regeneration efficiency was found to 90.4%
446
Table 4 Comparison of the maximum monolayer adsorption of tartrazine onto adsorbents from various sources. Adsorbents
Maximum adsorption capacity (mg g1)
Ref.
Activated carbon of Alligator weed Activated carbon of Lantana camara Saw dust Mixed-waste activated carbon Chitin Chitosan Cross-linked chitosan coated bentonite Polyaniline nano layer composite Hen feather Bottom ash Deoiled soya Commercial activated carbon Amberlite IRA-900 Amberlite IRA-910 Commercial activated carbon Alligator weed (Malachite green) Alligator weed (Methylene blue)
183.4 90.90 4.71 74.9 30 350 294.1 2.47 6.4 105 1.01 105 2.12 105 121.3 49.88 49.96 4.484 185.54 150.39
Present study [18] [46] [39] [52] [52] [61] [60] [42] [41] [41] [62] [63] [63] [64] [65] [66]
Please cite this article in press as: P.K. Gautam, et al., Preparation of activated carbon from Alligator weed (Alternenthera philoxeroids) and its application for tartrazine removal: Isotherm, kinetics and spectroscopic analysis, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j. jece.2015.08.004
428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444
447 448 449 450 451 452 453 454 455 456
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Table 6 Batch adsorption–desorption cycle of tartrazine. Cycle 1 2 3 4
457
Tartrazine qe (mg g1)
Recovery (%)
168.3 163.1 157.2 152.4
96.11 94.45 91.26 88.70
460
at the fourth cycle. The results showed that basic solution weakened the electrostatic interaction between the adsorbent and tartrazine, suggesting that AAW is a promising material for the treatment of dye laden wastewater.
461
Conclusions
462
481
Removal of tartrazine was investigated using AAW from aqueous solutions. The influence of initial adsorbate concentration, solution pH, temperature, and contact time on the adsorptive behavior was carefully examined. Acidic pH (2.0) favored the process of adsorption. The equilibrium data fitted well with Freundlich model, showing multilayer coverage of tartrazine molecules over the surface of AAW. This fact was visually supported by the SEM micrographs. The Freundlich maximum adsorption capacity was found to be 183.4 mg g1. Kinetics of the adsorption followed the pseudo-second order model. The rate of sorption process was controlled by the film diffusion mechanism. Thermodynamic parameters revealed that the adsorption process is endothermic, spontaneous with an increased randomness in nature. Reusability of the adsorbent was satisfactorily observed from desorption experiments. 0.1 M NaOH was considered as most suitable eluent and the regenerated adsorbent can be efficiently used up to four times. The results of the present work indicates that the AAW can be successfully employed as an efficient alternative adsorbent to commercial activated carbon for the removal of tartrazine from aqueous phase.
482
Acknowledgements
483
495
P.K. Gautam and R.K. Gautam are grateful to University Grant Commission, India for the financial aid in the form of RGNF Program and Senior Research Fellowship, respectively. S. Banerjee is thankful to Council for Scientific and Industrial Research, New Delhi, India, for providing Senior Research Fellowship. We are thankful to the Director, NCEMP, University of Allahabad, Allahabad for recording SEM images. The authors equally acknowledge the support and provision of the necessary facilities provided by the University of Allahabad, Allahabad, India. The support and encouragement of Prof. V.S. Tripathi from the Department of Chemistry, University of Allahabad is also appreciated. We are thankful to the anonymous reviewers for giving their kind criticisms and comments which fueled the zeal of the manuscript.
496
References
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463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480
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497 498 499 500 501 502 503 504 505
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Please cite this article in press as: P.K. Gautam, et al., Preparation of activated carbon from Alligator weed (Alternenthera philoxeroids) and its application for tartrazine removal: Isotherm, kinetics and spectroscopic analysis, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j. jece.2015.08.004
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Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
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Preparation of activated carbon from Alligator weed (Alternenthera philoxeroids) and its application for tartrazine removal: Isotherm, kinetics and spectroscopic analysis Pavan Kumar Gautama , Ravindra Kumar Gautama , Sushmita Banerjeea , Giusy Lofranob , Maria Angeles Sanromanc, Mahesh Chandra Chattopadhyayaa,* , J.D. Pandeya,* a
6 7
Environmental Chemistry Research Laboratory, Department of Chemistry, University of Allahabad, Allahabad 211002, India Department of Chemistry and Biology, University of Salerno, via Giovanni Paolo II, 132 Fisciano, Salerno, Italy c Department of Chemical Engineering, Isaac Newton Building (F.F.T), University of Vigo, 36310 Vigo, Spain b
A R T I C L E I N F O
Keywords: Activated carbon Alligator weed Isotherms Kinetics SEM Tartrazine
A B S T R A C T
In this paper, the ability of activated carbon obtained from Alligator weed (AAW) to remove tartrazine from aqueous solutions was investigated. The as-prepared AAW was characterized by SEM, FTIR, BET surface area analyzer, and through the determination of pHzpc. The effect of different operational parameters like pH, initial concentration of tartrazine, contact time, and temperature on the sorption behavior was studied in batch mode. The adsorption of tartrazine was favored in acidic medium. The kinetic data were examined using various models viz. pseudo-first order, pseudo-second order, and intraparticle diffusion models. The fitness of the experimental data with pseudo-second order model was evident from the values of correlation coefficient (R2 > 0.99). The adsorption equilibrium was modeled with the Langmuir and Freundlich isotherm expressions. The isotherm data were better fitted to Freundlich model which indicates heterogeneous nature of the adsorbent. The Freundlich adsorption capacities (KF) were found to be 107.56, 131.67 and 183.4 mg g1 at 30, 40 and 50 C, respectively. The negative values of DG and positive value of DH demonstrated that the adsorption of tartrazine onto AAW were spontaneous and endothermic in nature, respectively. The regenerated AAW can be reused 4 times for the adsorption of tartrazine. ã 2015 Published by Elsevier Ltd.
8
Introduction
9
Development and utilization of low-cost adsorbents for the treatment of dye bearing wastewater are driving scientific and academic interest because of the potential hazards associated with synthetic dyes and their byproducts [1]. Industries such as food, cosmetics, textiles, paper, rubber, and plastics are main consumers of dyestuffs [2]. Due to their complex aromatic structure and molecular size, dyes are resistant to photo-catalytic, thermal and biological degradation. Colored effluents are highly visible even in small concentration and seriously affect the aesthetic quality and transparency of water bodies, damaging the aquatic ecosystem [3]. Thin layer of discharged dyes, formed over the surfaces of recipient water bodies also reduces the amount of dissolved oxygen, thereby affecting the aquatic fauna [4,5]. Moreover, dye containing
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* Corresponding authors. Fax: +91 532 2541786. E-mail addresses: [email protected] (M.C. Chattopadhyaya), [email protected] (J.D. Pandey).
effluents increase biochemical oxygen demand. Additionally, they can have acute and chronic effects on living organisms depending upon their concentration and duration of exposure [6]. Therefore, elimination of dyes from water and wastewater has become a priority concern for researchers. A wide range of technologies including advanced oxidation processes [7], flocculation [8], electro-coagulation [9,10], membrane filtration [11], ultrasonic irradiation [12], photocatalysis [13], and microbial degradation [14], have been attempted for color removal from wastewater. The main drawbacks associated with these processes are high cost, low efficiency, limited versatility, and generation of sludge. Adsorption of dyes on efficient solid biomass support represents a simple and economically viable cleaning technology for color removal especially, where the sorbents are inexpensive and easily available [15]. More importantly, it imparts no side effect or toxicity to the water and this account for the superior removal of organic contaminants as compared to conventional treatments [16]. Therefore, it is preferred over other techniques in terms of cost, ease of operation, flexibility, negligible disturbances with diurnal variations, reusability and recyclability of the sorbent [17]. Several
http://dx.doi.org/10.1016/j.jece.2015.08.004 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: P.K. Gautam, et al., Preparation of activated carbon from Alligator weed (Alternenthera philoxeroids) and its application for tartrazine removal: Isotherm, kinetics and spectroscopic analysis, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j. jece.2015.08.004
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low-cost biosorbents such as mustard husk [1], lantana (Lantana camara) weed [18], chitosan [19], cactus [20], oil palm trunk fibres [21], almond shell [22], Borassus aethiopum shells [23], parthenium based activated carbon [24], and sphagnum moss peat [25] have been utilized for the removal of pollutants from aqueous media. Alligator weed (Alternenthera philoxeroids) is a native of South America and now it is widespread has around the world [26] due to its extremely vigorous growth and resistance to control measures. It also represents a serious threat to wetlands, rivers and irrigation systems [27]. This weed disrupts aquatic environment by covering the surface and restraining penetration of light [28]. This leads to impede gaseous exchange thereby causing anaerobic environment, which severely affects aquatic organisms [29]. In the past, the raw biomass of this weed has been successfully applied for the removal of metal ions from aqueous solutions [30,31] but its efficiency for Q2 the organic pollutants is yet to be explored. In the present study, a novel and low-cost adsorbent was prepared using biomass of abundantly available Alligator weed for the removal of an acidic dye tartrazine. Batch adsorption experiments were carried out using synthetic solutions of tartrazine, and the effects of operating parameters such as initial dye concentration, solution pH and contact time were investigated. The kinetics of adsorption system was studied, and various kinetic models, such as pseudo-first-order, pseudo-second-order and intra-particle diffusion models were tested with experimental data for their validity. The equilibrium sorption behavior of the adsorbents was studied using the adsorption isotherm techniques. Experimental data were fitted to the Langmuir and Freundlich isotherm models to determine the best isotherm to correlate the experimental data. Thermodynamic aspects of the adsorption process were studied by calculating the changes in Gibbs free energy, enthalpy and entropy. Desorption studies as for economic feasibility of adsorption process were also investigated.
75
Materials and methods
76
Tartrazine (Trisodium (4E)-5-oxo-1-(4-sulfonatophenyl)-4-[(4sulfonatophenyl)hydrazono]-3-pyrazolecarboxylate; C.I., 19,140; molecular formula, C16H9N4Na3O9S2; molar mass, 543.40 g mol1) was selected as an adsorbate and purchased from Sigma–Aldrich (USA). Chemical structure of the dye has been given in Fig. 1. It is light orange colored azo dye widely used in the food, cosmetics, pharmaceuticals and textile industries. Double distilled deionized water was used for preparing solutions. An Adventurer OhausAR2130 balance with a precision of 0.1 mg was used for weighing. Stock solutions of tartrazine were prepared by dissolving 1000 mg of dye in 1 L ultra pure water. This stock solution was diluted to prepare desired concentration range (25–200 mg L1) for adsorption experiment. A pH meter (pH meter 335, Systronics, India) was used for measuring pH of the solution. All the chemicals (NaOH, HCl, and H2SO4) used were of analytical grade with 98% purity and obtained from M/S Loba Chemie SD fine chemicals.
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Preparation of activated carbon
92
Alligator weed (AW) was procured from a waterlogged area of Allahabad (India). The biomass was washed with distilled water and dried in sunlight for 4–5 days. The dried material was crushed down to powder and washed again 4–6 times with distilled water before to be dried in a hot air oven (Gupta scientific instruments, Ambala Cant, India) for 24 h. The powdered biomass of AW was treated with concentrated H2SO4 at 1:1 ratio followed by impregnation by drying the mixture at 120 C for 24 h. For carbonization, impregnated samples were heated at 600 C under nitrogen flow at a heating rate of 5 C min1. After this, the samples were allowed to cool down under N2 gas flow. The carbonized sample was washed 6–8 times with deionized water. The washed sample was dried in an oven at 11 C for 24 h. The dried material was then sieved with 44 BSS mesh to get fine particles and stored in air tight glass bottles, and kept in vacuum desiccators for the characterization and experimental studies.
93
Characterization of AAW
109
Fourier transform IR spectra of unloaded and dye loaded was obtained by using a Fourier transform infrared spectroscopy (FTLA 2000, ABB, Canada) to determine the surface functional groups. Morphological characteristics of sorbent were determined by performing scanning electron microscopy (JEOL-SEI) at an acceleration voltage of 15 kV. Before, scanning, sorbent was coated by a thin layer of gold using a sputter coater to make it conductive. BET (Brunauer, Emmett and Teller) surface area analyzer (Micromeritics ASAP 2020) was used to determine the surface area and pore volume. Elemental composition of the adsorbent was estimated using an elemental analyzer (CHN Autoanalyzer, Australia). Solid addition method was applied to calculate the pH of adsorbent at pHzpc which is the point at which the net charge on adsorbent is zero and 0.1 N HCl and 0.1 N NaOH were used to control the pH. For this, 0.5 g L1 was added to 50 mL of 0.1 N KNO3 solutions in a conical flask and agitated for 30 minand final pH of solution was measured. The difference between initial and final pH (pHf pHi) was plotted against the initial pH (pHi) and the point where pHf pHi = 0 was taken as pHzpc.
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Adsorption experiment
129
Batch adsorption experiments were performed in order to investigate the removal efficiency of activated carbon of Alligator weed (AAW). Stock solution was diluted to prepare working solutions. The pH of the working solution was maintained with 0.1 M HCl or 0.1 M NaOH. The experiment was performed by adding 100 mL of adsorbate solution in 250 mL of Erlenmeyer conical flasks. An optimized dose of adsorbent (0.2 g) was selected (figure not shown) and used for all the subsequent adsorption experiments. The experiments were carried out at a stirring rate of 120 rpm on a shaking thermostat water bath (Macro scientific works Pvt. Ltd., MSW 275, Delhi). The uncertainty associated with temperature was estimated to within 0.5 C. After the equilibrium attainment, the adsorbent was separated by centrifugation (REMI R-8C BC, New Delhi, India) from its aqueous phase at 10,000 rpm for 15 min. A double beam UV-spectrophotometer (Systronics 2203, Ahmadabad, India) was used to determine the remaining concentration of tartrazine at 426 nm. The percentage removal of tartrazine and amount of tartrazine adsorbed were determined using the following relationships: 1 ð1Þ 100 Tartrazine removal ð%Þ ¼ ðC 0 C e Þ C0
130
Fig. 1. Molecular structure of tartrazine.
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JECE 737 1–9 P.K. Gautam et al. / Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx 1 Equilibrium adsorption capacityðmgg Þðqe Þ 1 V ¼ ðC 0 C e Þ M
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ð2Þ
152
where C0 and Ce are the initial and equilibrium concentrations of tartrazine (mg L1), respectively, M is the mass of adsorbent (g) and V is the volume of solution (L).
153
Desorption studies
154
Desorption studies are very important as far economic feasibility of adsorption process is concerned. In the present work, 0.1 M NaOH aqueous solutions have been used for the regeneration of adsorbent due to its desorbing efficacy in our previously reported work [32]. The process was repeated several times until the tartrazine could not be detected in the filtrate. The regenerated AAW was washed thoroughly with distilled water to a neutral pH and the same adsorbent was reused 4 times for further adsorption experiments.
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Replication of batch experiment
164
Each batch adsorption experiment was performed in triplicates and the average values have been reported to ensure reproducibility and accuracy with an error 5%. In the case of deviation larger than 5% more experiments were performed. The experimental data could be reproduced with accuracy greater than 95%. The error functions, sum of the squares of the errors (ERRSQ), were calculated across the studied range using the solver add-in with Microsoft’s Excel spreadsheet by the Eq. (3).
165 166 167 168 169 170 171
ERRSQ ¼
P X
qe;cal qe;exp
i¼1
173 172 174
2
ð3Þ
i
where qe,cal and qe,exp are the theoretical and experimental capacities (mg g1), respectively. Normalized standard deviations were also calculated for the selection of best adsorption model [23,24]. It was calculated from Eq. (4). Lower the value of normalized deviations, the better is the fit to experimental data.
175 176 177
Normalized standard deviationDqe % vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u h i2 u tS qe;cal qe;exp =qe;exp ¼ 100 N 179 178
3
Results and discussion
180
Textural characteristics and elemental constituents
181
Table 1 depicts estimated textural properties and elemental composition of AAW. The prepared activated carbon possesses specific surface area (303.09 m2 g1) and pore volume comparatively higher to inactivated or raw forms previously investigated [33]. However, the specific surface area obtained for AAW is lower than the reported for commercial activated carbon (Fluka, France) and higher than the activated carbon prepared from Lantana camara [18]. The mean pore diameter was estimated to be 41.38 Å indicating microporous morphology of the activated carbon. Elemental analysis confirmed the carbonaceous nature of the adsorbent which is present to the extent of 57.09%.
182
Fourier transforms infrared spectroscopy (FTIR)
193
In order to investigate the functional groups responsible for tartrazine removal, the FTIR spectra of prepared activated carbon was recorded (Fig. 2). These functional groups play significant role in understanding the chemistry of adsorption of adsorbate onto the surface of adsorbent [34]. From Fig. 2, it is evident that the AAW adsorbent displayed number of absorption peaks indicating the structural, elemental and constitutional complexity. The sharp peaks around 600 cm1 to 790 cm1 revealed the presence of sulphate group [35]. The bands in the region 800 cm1 may be assigned as mono-substituted aromatic rings or aromatic phosphate stretching (P–O–C). Gautam et al. [32] also reported similar observation in their work while studying the adsorption efficiency of raw Lantana camara biosorbents. The peaks observed in the 1000–1200 cm1 range may be explained by presence of C–O–C stretching of ether. Surprisingly, no any peaks was found in the region 2000–3500 cm1 as reported by [36] in case of raw AW biosorbents which corresponds to presence of OH of phenols and other primary and secondary amines. This may be attributed to vaporization of volatile functional groups during the process of activation of the sorbent. Shifting of peaks in dye loaded FTIR spectra confirmed the interaction of activated AW (AAW) sorbent with dye molecules.
194
ð4Þ
where N is the number of measurements.
Q4
Table 1 Physical characteristics of activated carbon. Typical properties BET surface area (m2 g1) Total pore volume (cm3 g1) Micropore surface area (m2 g1) Mesopore surface area (m2 g1) Meanpore diameter (Å)
303.09 0.215 255.79 47.30 41.38
Proximate analysis (wt%) Volatile matter Fixed carbon Ash
14.51 63.64 21.85
Elemental analysis (wt%) C H N S Oa
57.09 3.88 7.245 2.77 29.015
a
By difference.
Fig. 2. FTIR spectra of activated carbon and tartrazine loaded activated carbon of Alligator weed.
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Fig. 3. SEM images of the pure activated carbon (a, b, c) and tartrazine loaded (d) activated carbon. 216
SEM micrographs
217
226
Fig. 3 shows SEM micrographs of AAW adsorbents at different magnification before and after dye adsorption. Fig. 3(a–c) shows that AAW possesses uneven heterogeneous surface morphology with pores of variable sizes. This variability and heterogeneity facilitate the extent of adsorption due to the prevalence of active sites for the capture and adsorption of dye molecules. Kyzas et al. [37] equally reported similar morphological properties in untreated coffee residues which proved to be very effective for removing multicomponent dye solution. Fig 3(d) shows that the surface of adsorbent was fully covered by tartrazine molecules.
227
Effect of pH
228
The pH of the solution is an important factor as it governs both the surface chemistry of the adsorbent and solubility of dyes. The effect of pH on biosorption at constant temperature (30 C) of tartrazine after 60 min is shown in Fig. 4. It was observed that the removal efficiency of AAW increased with decreasing pH over the pH range 2–10 for all the concentration regime of tartrazine. Maximum removal was achieved (99.31%) at pH 2 for 25 mg L1dye concentration. Similarly, Habila et al. [38] determined that effective adsorption of tartrazine dye onto mixed-waste activated carbon occurred at pH 2. This results may be attributed to the presence of various functional groups such as OH, NH2, C¼C and C¼O, as indicated from the FTIR results, and also to the presence of partially charged groups in the tartrazine structure, such as COO, SO3 and OH. The presence of these groups allows the formation of covalent bonds, columbic forces, hydrogen bonds or weak van der Walls forces. The occurrence of the double bond serves to enhance the interaction between dye and the activated carbon [39]. This may also be explained by phenomenon of protonation of functional
218 219 220 221 222 223 224 225
229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245
Fig. 4. Dependence of pH on tartrazine adsorption (adsorbent dose = 0.2 g, Shaking speed = 120 rpm, contact time = 60 min, temperature = 30 C.
groups present at the surface of adsorbent which create more positively charged sites on activated carbon [40]. These positively charged sites not only chemically bind with dye molecules but also stimulate their diffusion inside the surface wall of sorbent by electrostatic attraction, leading to an increment in percent removal of dye. With the increase in pH, the percent uptake was decreased due to the deprotonation of functional groups which causes electrostatic repulsion. The observation can further be understood in terms of zero point charge (pHzpc). This value was found to be 6.1 (Figure not shown). In general, pH < pHzpc favors the uptake of anionic dyes by electrostatic interaction via making the surface, positively charged. In case where pH > pHzpc, the rate of adsorption
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is decreased due to the abundance of negatively charged sites on the surface of adsorbent which lower down the affinity of dyes for the surface. Similar findings have also been reported by Mittal et al. [41,42] on the removal behaviour of tartrazine using bottom ash, de-oiled soya and hen feather. Gautam et al. [18] reported similar conclusion on adsorption studies of tartrazine using activated Lantana camara biosorbents. These supporting informations gave us additional confidence to represent our results. Effect of initial adsorbate concentration and contact time The effect of initial dye concentration and contact time on adsorption of tartrazine for four initial concentrations (25, 50, 100 and 200 mg L1) of tartrazine is presented in Fig. 5, which shows that the percent removal of dye decreased with increase in initial dye concentration. This can be interpreted by tendency of dyes to make larger aggregates or micelles at higher initial concentration [43,44]. Aggregation inhibits their diffusion through the micropores of the activated carbon thereby reducing the percent removal of tartrazine. The adsorption was rapid at initial stage and then increases gradually up to the equilibrium attainment of 60 min for all the cases. No remarkable uptake was observed after this. These results are in accordance with the obtained by Goscianska and Pietrzak [45] who study the removal of tartrazine from aqueous solution by carbon nanotubes decorated with silver nanoparticles and determined that tartrazine adsorption equilibrium was rapidly attained after 60 min of contact time. In addition, the removal of the dye from acidic solutions was better than from basic ones. On the other hand the values of amount adsorbed (qe) (mg g1) increased with the increase in initial tartrazine concentration. At higher initial concentrations more dye molecules are available to interact with activated carbon. This could also be explained by the fact that the increase in initial dye concentration also accelerates driving forces that overcome the resistance to the mass transfer of tartrazine molecules from liquid phase to solid phase [46]. Habila et al. [38] reported that the amount of tartrazine adsorbed at the equilibrium time reveals the maximum adsorption capacity of the mixed-waste AC. The removal of tartrazine increased from 19 to 74.9 mg g1 as the initial concentration increased from 25 to 150 mg/L, indicating that the removal is affected by the initial concentration. Adsorption kinetics The most widely known models viz. Lagergren’s pseudo-firstorder and pseudo-second-order model were used to analyze
kinetics of the adsorptive removal of dye onto AAW. A linear form of pseudo-first-order model is expressed as: lnðqe qt Þ ¼ lnqe k1 t
t t 1 ¼ þ qt qe k2 q2e
300 301
ð5Þ
where qe and qt refer to the amount of dye adsorbed (mg g1) at equilibrium and at time t (min), respectively. The equilibrium rate constant k1and qe have been determined from the intercept and slope of the plot of log (qe qt) versus t. The larger differences between experimental and calculated qe values indicated that the pseudo-first order model did not fit well for the entire studied composition range of tartrazine (Table 2). Lower values of coefficient of determination (<0.959) also supported the observation. The values of first order rate constant k1 (min1), qe (mg g1) and R2 are given in Table 2. The linear form of pseudo-second-order model is expressed as [47]:
303 302 304 305 306 307 308 309 310 311 312 313 314
ð6Þ
where k2 is the equilibrium rate constant of pseudo-second-order adsorption (g mg1 min1) estimated by calculating the slope and intercept of the plot of t/qt vs t. The recovered kinetic parameters along with fitness of good in terms of R2 values have been given in Table 2. The calculated qe values indicated good agreement with experimental values with pseudo-second-order kinetic model for all initial dyestuff concentration as shown in Table 2. The R2 values were greater than 0.999. This fact is also evident from the values of normalized standard deviations. This result indicated that the adsorption of tartrazine fitted well with the pseudo-second-order model, in agreement with the previous studies reporting the removal of tartrazine onto activated carbon of Lantana camara and papaya seeds have been reported by Gautam et al. [18] and Weber et al. [48].
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Intra-particle diffusion study
330
The study is significant from the viewpoint of adsorption process as it indicates about the diffusivity of the adsorbate molecules, which plays important role in ascertaining the overall rate of sorption process. Although, upon investigation of kinetic parameters confirmed that pseudo-second-order equation agrees well with the experimental sorption data, the result obtained from this model failed to predict the diffusion process. Adsorption is a complex process which usually governed by more than one step such as boundary layer diffusion, intra-particle mass diffusion, or both [49]. Therefore, the slowest step which might be either film diffusion or particle diffusion can be designated as the overall rate determining step of the sorption process. The expression for intraparticle diffusion suggested by Weber and Morris [50] is given as:
331
1
qt ¼ ki t2 þ C
Fig. 5. Effect of initial tartrazine concentration and contact time (adsorbent dose = 0.2 g, Shaking speed = 120 rpm, contact time = 60 min, temperature = 30 C.
5
318 319 320 321 322 323 324 325 326 327 328 329
332 333 334 335 336 337 338 339 340 341 342 343
ð7Þ
where ki is the intra-particle diffusion rate constant (mg g1 min1/ 2 ) and C is the thickness of boundary layer (mg g1). The theory suggests that if the fitting of experimental data leads to linear plot and line passes through origin then pore diffusion controls the sorption process but any deviation from linearity indicates that film diffusion dominates the sorption process [51]. Table 2 reports the values of ki, C and R2 and Fig. 6 represents the intra-particle diffusion plots for various initial dye concentrations. The resultant plots obtained for all initial dye concentrations were linear over the whole time range but did not passes through the origin. Therefore the nonzero intercept of the plots advocate that intra-particle diffusion not involved in the adsorption process moreover film diffusion process dictate the overall rate of the adsorption process. This behavior is in accordance with the obtained by Dotto et al. [52]
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P.K. Gautam et al. / Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx Table 2 Kinetic parameters for the adsorption process of tartrazine onto activated carbon of Alligator weed. Tartrazine concentration
Pseudo–first order
(mg L1)
k1 (min1)
qecal (mg g1)
qeexp (mg g1)
R2
ERRSQ
Normalized standard deviation (%)
25 50 100 200
0.044 0.043 0.035 0.041
34.04 60.81 94.14 169.01
119.5 229.1 438.35 835.85
0.933 0.911 0.931 0.959
7.30 103 2.83 104 1.18 105 4.44 105
41.29 42.41 45.34 46.06
Pseudo-second order Tartrazine concentration (mg L1)
k2 (g mg1 min1)
qecal (mg g1)
qeexp (mg g1)
R2
ERRSQ
Normalized standard deviation (%)
25 50 100 200
2.861 103 1.851 103 1.102 103 6.260 104
121.95 232.55 434.78 833.33
119.5 229.1 438.35 835.85
0.9977 0.9982 0.9989 0.9993
0.60 101 1.19 101 1.27 101 0.63 101
1.18 0.87 0.47 0.17
Intra-particle-diffusion
25 50 100 200
ki (mg g min1/2)
C (mg g1)
R2
4.055 7.087 11.689 20.809
87.38 172.03 339.31 628.1
0.9796 0.9819 0.9810 0.9957
capacity of adsorbent from the ratio between the quantity adsorbed at the interface and that remaining in the bulk at the fixed temperature at equilibrium [53]. The isotherm results were analyzed by fitting the equilibrium adsorption data to Langmuir and Freundlich model. Langmuir model is related to the assumption that maximum sorption corresponds to a saturated molecules of solute component with, no lateral interaction between adsorbed molecules [54]. The linear form of Langmuir isotherm model is expressed as: Ce 1 Ce þ ¼ qe ðK L Q m Þ Q m
360
and the tartrazine adsorption onto chitin occurred only by film diffusion.
361
Adsorption isotherms study
RL ¼
362
Study of adsorption isotherms is worthwhile to understand the complex mechanism of interaction between the adsorbate and adsorbent. Additionally, it also helps in predicting the adsorption
where C0 is the initial concentration of adsorbate (mg L1) and KL is Langmuir constant. The value of RL determines that the adsorption process is irreversible (RL = 0), linear (RL = 1), favorable (0 < RL < 1)
359
363 364
1 ð1 þ K L C 0 Þ
30 40 50
Langmuir isotherm
367 368 369 370 371 372 373
375 374 376 377 378 379 380 381 382 383
ð9Þ
Table 3 Isotherm constants for the adsorption of tartrazine onto activated Alligator weed at different temperatures. Temperature ( C)
366
ð8Þ
where qe refers to the amount of dye adsorbed at equilibrium (mg g1), Ce is the equilibrium concentration of dye solution (mg L1), Qm is the maximum adsorption capacity (mg g1) and KL is the Langmuir constant (L mg1) related to the maximum adsorption capacity and energy of sorption. The values of Qm, KL, and R2 are presented in Table 3. The essential characteristic of Langmuir model can be described by a dimensionless separation factor RL which is calculated by following expression:
Fig. 6. Intra-particle diffusion plot for the adsorption of tartrazine on AAW at 30 C.
365
Freundlich isotherm
Qm (mg g1)
KL (L mg1)
Normalized standard deviation (Dqe %)
R2
KF (mg g1)
n
Normalized standard deviation (Dqe %)
R2
77.118 98.765 172.428
15.440 10.850 6.006
22.01 17.71 13.91
0.8933 0.9150 0.9415
107.569 131.671 183.4
1.746 1.936 2.149
5.03 5.42 5.17
0.9968 0.9943 0.9950
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or unfavorable (RL > 1). The values of RL was found to be in the range of 0–1, indicates the favorability of adsorption. In the present study RL values were calculated in the range of 0.0006–0.00032 for 30 C, 0.0036–0.00046 for 40 C and 0.0066–0.00083 for 50 C. All the RL values were found to be exist in the range of 0–1 which further indicates that adsorption of tartrazine on AAW is favorable in nature. The equilibrium data were further studied by Freundlich isotherm which is an empirical relationship proposed to describe heterogeneous systems [55]. The linear form of Freundlich isotherm is represented by following expression: 1 ð10Þ lnC e lnqe ¼ lnK F þ n
418
where KF (mg g1) and n are the constants representing the adsorption capacity and the adsorption intensity, respectively. The values KF, n and R2 were ascertained graphically from Freundlich plot at different temperatures and has been recorded in Table 3. It illustrates that the adsorption of tartrazine obeyed Freundlich model well. Adsorption capacity (KF) and adsorption intensity (n) increased with increasing temperature. The values of n were found greater than unity for all the studied temperature values, confirming that the tartrazine is favorably adsorbed onto AAW. The results clearly revealed that the multilayer coverage of adsorbate molecules on the surface of activated carbon facilitated by heterogeneous sites present at the surface of sorbent. Table 4 presents a comparison of the maximum adsorption capacity of tartrazine onto adsorbents from various sources. From the Table 4, it can be observed that AAW exhibits much higher sorption capacity for tartrazine as compared to other reported adsorbents. However, the difference observed for the sorption capacities mainly attributed to dye structure, functional groups, pHzpc, and surface area and porosity of each sorbent.
419
Thermodynamics
400 399 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417
420 421 422 423 424
Thermodynamic studies of the adsorption of tartrazine onto AAW were undertaken to observe the possible mechanism involved [56]. Thermodynamics parameters such as change in standard free energy (DG ), enthalpy (DH ) and entropy (DS ) were evaluated by following expression: logK c ¼
DS 2:303R
DH 2:303RT
ð11Þ
7
Table 5 Values of different thermodynamic parameters of adsorption for tartrazine onto AAW at various temperatures. Temperature ( C)
DG (kJ mol1)
DH (kJ mol1)
DS (J mol1 K1)
30 40 50
7.73 8.04 8.35
1.66
31.84
DG ¼ DH T DS
ð12Þ 1
where R (8.314 J mol K) is the gas constant, T is the absolute temperature (K) and Kc refers to the standard thermodynamic equilibrium defined by Cad/Ce, where Cad and Ce are equilibrium concentration of dye on the surface of AAW and equilibrium concentration of dye in the solution. The slopes and intercepts of the plot of log Kc versus 1/T were employed to calculate the values of DH and DS (Plot was not shown). The evaluated parameters at all the studied temperature values are listed in Table 5. The values of DG were negative for all the temperatures and decrease with the increase in temperature which exhibits the spontaneity of the sorption process. It also indicates that higher temperature accelerates the extent of adsorption by lowering the viscosity of solution [57]. This reduction in viscosity enhances the rate of mass transfer from the liquid to solid phase and makes adsorption easier [58]. Similar findings were reported by Rozada et al. [59] on the adsorption study of methylene blue and brilliant red from aqueous phase onto discarded tyres. Positive value of DH indicated the endothermic nature of the adsorption. Positive value of DS confirmed increase in randomness at the solid–liquid interface.
426 425 427
Recycling study
445
Reusability and recyclability of the sorbent are highly significant as far cost effectiveness of the adsorption process is concerned. Therefore, desorption study has also been undertaken to examine the possibility to reuse the adsorbent and regenerate the adsorbate. Tartrazine loaded AAW was recovered using an optimized concentration (0.1 M) of aqueous NaOH solutions of 100 mL. The experimental values of adsorption capacities and percent adsorbate recovery in each adsorption-desorption cycle are given in Table 6. The results indicated that the adsorption capacity of the regenerated adsorbent decreased sluggishly after every cycle of 15 min. The adsorbate regeneration efficiency was found to 90.4%
446
Table 4 Comparison of the maximum monolayer adsorption of tartrazine onto adsorbents from various sources. Adsorbents
Maximum adsorption capacity (mg g1)
Ref.
Activated carbon of Alligator weed Activated carbon of Lantana camara Saw dust Mixed-waste activated carbon Chitin Chitosan Cross-linked chitosan coated bentonite Polyaniline nano layer composite Hen feather Bottom ash Deoiled soya Commercial activated carbon Amberlite IRA-900 Amberlite IRA-910 Commercial activated carbon Alligator weed (Malachite green) Alligator weed (Methylene blue)
183.4 90.90 4.71 74.9 30 350 294.1 2.47 6.4 105 1.01 105 2.12 105 121.3 49.88 49.96 4.484 185.54 150.39
Present study [18] [46] [39] [52] [52] [61] [60] [42] [41] [41] [62] [63] [63] [64] [65] [66]
Please cite this article in press as: P.K. Gautam, et al., Preparation of activated carbon from Alligator weed (Alternenthera philoxeroids) and its application for tartrazine removal: Isotherm, kinetics and spectroscopic analysis, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j. jece.2015.08.004
428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444
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G Model
JECE 737 1–9 8
P.K. Gautam et al. / Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx
Table 6 Batch adsorption–desorption cycle of tartrazine. Cycle 1 2 3 4
457
Tartrazine qe (mg g1)
Recovery (%)
168.3 163.1 157.2 152.4
96.11 94.45 91.26 88.70
460
at the fourth cycle. The results showed that basic solution weakened the electrostatic interaction between the adsorbent and tartrazine, suggesting that AAW is a promising material for the treatment of dye laden wastewater.
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Conclusions
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Removal of tartrazine was investigated using AAW from aqueous solutions. The influence of initial adsorbate concentration, solution pH, temperature, and contact time on the adsorptive behavior was carefully examined. Acidic pH (2.0) favored the process of adsorption. The equilibrium data fitted well with Freundlich model, showing multilayer coverage of tartrazine molecules over the surface of AAW. This fact was visually supported by the SEM micrographs. The Freundlich maximum adsorption capacity was found to be 183.4 mg g1. Kinetics of the adsorption followed the pseudo-second order model. The rate of sorption process was controlled by the film diffusion mechanism. Thermodynamic parameters revealed that the adsorption process is endothermic, spontaneous with an increased randomness in nature. Reusability of the adsorbent was satisfactorily observed from desorption experiments. 0.1 M NaOH was considered as most suitable eluent and the regenerated adsorbent can be efficiently used up to four times. The results of the present work indicates that the AAW can be successfully employed as an efficient alternative adsorbent to commercial activated carbon for the removal of tartrazine from aqueous phase.
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Acknowledgements
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P.K. Gautam and R.K. Gautam are grateful to University Grant Commission, India for the financial aid in the form of RGNF Program and Senior Research Fellowship, respectively. S. Banerjee is thankful to Council for Scientific and Industrial Research, New Delhi, India, for providing Senior Research Fellowship. We are thankful to the Director, NCEMP, University of Allahabad, Allahabad for recording SEM images. The authors equally acknowledge the support and provision of the necessary facilities provided by the University of Allahabad, Allahabad, India. The support and encouragement of Prof. V.S. Tripathi from the Department of Chemistry, University of Allahabad is also appreciated. We are thankful to the anonymous reviewers for giving their kind criticisms and comments which fueled the zeal of the manuscript.
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References
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