Equilibrium, Kinetic, and Thermodynamic Studies of Azo Dye

Article pubs.acs.org/IECR

Equilibrium, Kinetic, and Thermodynamic Studies of Azo Dye Adsorption from Aqueous Solution by Chemically Modified Lignocellulosic Jute Fiber Aparna Roy, Basudam Adhikari, and S. B. Majumder* Materials Science Centre, Indian Institute of Technology Kharagpur, West Bengal 721302, India S Supporting Information *

ABSTRACT: The lignocellulosic biomass jute fiber (JF) was chemically modified with polyphenolic tannin in aqueous medium by epoxy activation under mild conditions and applied as a potential adsorbent for the removal of Congo Red, a model azo dye, from aqueous solution. The virgin and tannin-modified JF samples were characterized by solid-state nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, and scanning electron microscopy. Within the studied range of dye concentrations, the adsorption equilibrium was found to follow the Langmuir isotherm model well, with R2 > 0.99. The rate of adsorption of the dye onto treated JF was very high, and equilibrium was attained within 15−30 min of contact. The efficiency of modified JF for the spontaneous and exothermic adsorption of azo dye is attributed to the copious availability of hydroxyl and other polar functional groups on the fiber surface. The present adsorption studies of azo dye from aqueous solution revealed the potential of modified JF to be utilized as an alternative, inexpensive, and environmentally benign adsorbent for water purification.

1. INTRODUCTION With its progress in society, science, and technology, humankind faces a serious problem with water pollution. The major water pollutants that seriously affect biodiversity, ecosystem functioning, and the natural activities of aquatic systems include a variety of hazardous organic and inorganic materials. One such pollutant is synthetic dyes.1 Dye-producing industries as well as major dye-consuming industries, such as textiles, dyeing, paper and pulp manufacturing, tanneries, paints and printing, cosmetics, rubber manufacturing, plastics, foods and pharmaceuticals, commonly use substantial quantities of water and thus generate huge volumes of dye-containing effluents that are drained to local water resources. The increasing use of a wide variety of dyes and dyed products has alarmingly increased pollution by colored wastewater. The dyes not only deteriorate the aesthetic properties of water by imparting significant color but also increase the chemical oxygen demand (COD), biological oxygen demand (BOD), and dissolved and suspended solids of water. Moreover, they hamper the photosynthesis of water-borne plants by impeding light transmission through water; they interfere with biological metabolism processes; and they also create microtoxicity to fish and other organisms, resulting in the disruption of aquatic ecosystems. Azo dyes are of most concern because of their potentiality to induce genotoxicity, namely, carcinogenicity or mutagenicity in humans and animals, causing adverse effects such as dysfunction of the kidneys, reproductive system, liver, brain, and/or central nervous system.2 Thus, in view of the toxicological effects of dyes on the environment and living beings, to protect the ubiquitous and increasing degradation of aquatic ecosystems, the treatment of wastewater containing these toxic compounds before they are discharged into freshwater bodies is imperative. Cellulosic and lignocellulosic biomasses have attracted increasing worldwide interest as abundant, inexpensive, and © 2013 American Chemical Society

environmentally benign biopolymers. Aside from their major use as the raw materials for textiles, paper and paperboard, barrier packaging, and composites and other industrial applications, researchers have attempted to expand their domain of application to include use as biofilters for the remediation of water contaminated with different hazardous pollutants.3 Mainly, the presence of three hydroxyl groups at the C2, C3, and C6 atoms of the anhydroglucose units of the cellulose backbone in cellulosic/lignocellosic biomass provides the possibility for use as reusable bioadsorbents. Despite various advantages, such as renewability, reusability, and low cost, unconventional cellulosic/lignocellulosic bioadsorbents in their native form lack high adsorption capacities toward organic and metallic contaminants as compared to commercial adsorbents (e.g., activated carbon or zeolite).4 With the aim of enhancing the adsorption potential, researchers have chemically modified the original biomaterials, using diverse solvents under different synthetic conditions, by introducing different types of new functionalities, such as esters,5,6 amines,7 amino-terminated hydrocarbons,3 triethylenetetramine,8,9 Schiff bases,10 anhydrides,11,12 aspartic acid,13 poly(methacrylic acid),14 thiols,15 phosphates, and oxides.16 In a previous study,17 we explored the significant potential of untreated jute fiber (JF) for dye absorption. However, JF is not highly effective for bioadsorption in its virgin form because of its low specific surface area and low density of active anchoring sites per unit surface area. Accordingly, in the present study, we successfully enhanced the adsorption capacity of JF by surface functionalization with condensed tannin18 and evaluated the adsorption performance of surface-modified JF for dye removal Received: Revised: Accepted: Published: 6502

January 21, 2013 April 3, 2013 April 20, 2013 April 20, 2013 dx.doi.org/10.1021/ie400236s | Ind. Eng. Chem. Res. 2013, 52, 6502−6512

Industrial & Engineering Chemistry Research

Article

Figure 1. (a) Plausible reaction scheme for the surface functionalization of JF with tannin. (b−d) Solid-state untreated, (c) pretreated, and (d) tannin-modified JF.

13

C CP-MAS NMR spectra: (b)

solution, and a 0.5 g L−1 solution of sodium borohydride was added to the suspension as a catalyst. The solution was constantly stirred at 60 °C for 6 h under a nitrogen atmosphere. The treated JF was separated by filtration, extensively washed with acetone and distilled water until no color was observed in the filtrate, and subsequently dried. Finally, tannin-immobilized dark-brown-colored JF was obtained. The treated JF was vacuum-dried, weighed after cooling, and preserved in a desiccator. A plausible reaction scheme for tannin immobilization on JF, using epichlorohydrin as the cross-linking agent, is shown in Figure 1a. 2.3. Characterization of Adsorbent. 2.3.1. Elemental Analysis. Elemental analysis of the JF samples was performed using a CHNS analyzer (Euro EA). Each sample was analyzed three times, and the average values are reported. 2.3.2. Solid-State 13C Cross-Polarization Magic-AngleSpinning Nuclear Magnetic Resonance (CP-MAS NMR) Spectroscopy. Solid-state 13C NMR spectra of untreated, pretreated, and treated JF samples were recorded at a magnetic field strength of 7.01 T under cross-polarization magic-anglespinning (CP-MAS) conditions on a Bruker AV-300 FT NMR spectrometer. Experimental data were acquired at ambient temperature with a contact time of 2.5 ms. The 13C spectra were referenced externally to the chemical shift of the methylgroup carbons of tetramethylsilane. 2.3.3. Fourier Transform Infrared (FTIR) Spectroscopy. FTIR spectroscopic studies of unmodified, modified, and dye-

in a series of batch experiments. The presence of uniformly distributed phenolic hydroxyl groups in the flavonoid units of condensed tannin allows these units to exhibit strong potentiality to adsorb different pollutants.19

2. MATERIALS AND METHODS 2.1. Reagents. Analytical-grade sodium hydroxide (NaOH) and epichlorohydrin were procured from Merck, Mumbai, India. Commercial-grade tannin (wattle extract) was purchased from the local market. The tannin content of the wattle extract used in this study was determined to be 57% according to the Prussian Blue test.20 JF (Corchorus olitorius) was obtained from Gloster Jute Mill, Kolkata, India. Congo Red (CR), used as a model azo dye, was purchased from Loba Chemicals, Mumbai, India. 2.2. Preparation of Adsorbent. The surface functionalization of JF with tannin was carried out by employing an approach that was optimized in our previous study.18 First, the fiber surface was activated by pretreatment with 200 mL of 1.0% (w/v) NaOH solution at ambient temperature for 0.5 h. Suspension of pretreated JF in 0.25% (w/v) NaOH solution was followed by sequential addition of epichlorohydrin (20 mL L−1). The reaction mixture was stirred continuously for 2 h at 60 °C, filtered, and washed thoroughly with acetone and distilled water. Tannin solution (15 g L−1) was prepared in distilled water, and the pH of the solution was adjusted to 7. The epoxy-activated JF was then immersed in the tannin 6503

dx.doi.org/10.1021/ie400236s | Ind. Eng. Chem. Res. 2013, 52, 6502−6512


Article

py. The signals for methyl and carboxylic carbons in the acetyl groups of hemicellulose appeared at 19.4 and 170.6 ppm, respectively (Figure 1b). The less intense signals at 54.4, 124.4, 126.6, 144.1, and 151.1 ppm are assigned to the methoxyl and aromatic carbons of lignin in untreated JF. After pretreatment of JF with alkali solution, some of the signals for hemicellulose (19.4 ppm) and lignin (124.4, 126.6, 144.1 ppm) were found to disappear as a result of the partial removal of such amorphous materials.22 However, no considerable changes were detected in the chemical shifts of the cellulosic carbons of pretreated JF (Figure 1c). The major change observed upon grafting tannin onto pretreated JF surface was the appearance of two signals for aromatic carbons at 142.8 and 146.6 ppm (Figure 1d). Zhang et al.23 reported that the 13C NMR spectra of condensed tannins show signals at 70−90 ppm (C2−C4), 106−115 ppm (C8, C6, C2′, C5′ and C6′), and 150−160 ppm (C5, C7, and C8a) and that the typical signal for C3′, C4′, and C5′ appears at around 142−146 ppm. Accordingly, the presence in Figure 1d of two peaks at 142.8 and 146.6 ppm might be due to C3′, C4′ and C5′ of the tannin unit. This provides strong evidence of the surface modification of JF by tannin.18 3.1.3. FTIR Study. The explanation of the FTIR spectral data of biomacromolecules by the conventionally practiced baseline method is difficult and erroneous because of the complex, broad, and overlapping peaks of such molecules. Hence, for more accurate data interpretation, the FTIR spectra of JF were analyzed by applying the Voigt function to the IR spectra with the help of PeakFit software, and the peak areas were determined by a deconvolution method (Figure 2).24 The prominent peak for the β-glucosidic linkage of cellulose in JF at 898 cm−1 was chosen as the internal standard; this linkage was assumed to remain unaffected during chemical treatment,25 and the absorbance areas of the peaks of interest were normalized

adsorbed treated JF were performed to determine the surface functional groups of the adsorbent and to identify the functional groups responsible for dye adsorption. Analyses were performed in a humidity-free atmosphere at room temperature using a Thermo Nicolet Nexus 870 spectrophotometer. 2.3.4. Scanning Electron Microscopy (SEM). The surface topographies of unmodified, modified, and CR-adsorbed modified JF were investigated with a TESCAN VegaLSV scanning electron microscope. 2.4. Batch Adsorption Studies. Batch adsorption studies were accomplished by suspending JF (2−10 g L−1) in CR solutions of different initial concentrations (10−250 mg L−1) and different initial pH values (3−12). The suspensions were continuously agitated (140 rpm) at constant temperature (303−323 K), and aliquots were withdrawn at regular intervals of time until equilibrium was reached. The concentration of CR in the supernatant was estimated by UV/visible spectroscopy (Perkin-Elmer, Lambda 750) at 497 nm, and the amount of CR adsorbed by JF from aqueous solution at equilibrium (qe, mg g−1) was calculated as21 qe =

(Co − Ce)V m

(1)

where Co and Ce are the initial and equilibrium dye concentrations (mg L−1), respectively; V is the volume of solution (L); and m is the dry adsorbent dosage (g). For a complete investigation of CR adsorption on treated JF, four sets of experiments were conducted varying one parameter while keeping the others constant. The results presented herein are the means of triplicate determinations. 2.5. Batch Desorption and Regeneration Studies. For desorption studies, the JF was loaded with 50 mg L−1 dye solution, washed gently with water to remove any unadsorbed dye, and dried. The spent adsorbent (10 g L−1), suspended in eluting solvents (i.e., neutral deionized water, 0.1 M HCl, 0.1 M NaOH, and 0.1 M CH3COOH), was agitated (130−140 rpm) at 303 K for 3 h. The desorbed CR concentration was quantified by UV/visible spectroscopy. The desorption efficiency of the adsorbent was calculated as6 desorption efficiency (%) amount of dye desorbed from adsorbent = × 100 amount of dye adsorbed on adsorbent

(2)

After desorption, the recovered adsorbent was thoroughly washed with deionized water until a neutral pH was obtained. The regenerated material was then dried and suspended in dye solution, to be reused in the next cycle of adsorption experiments. The amount of regenerated adsorbent and other experimental conditions were the same as in the aforementioned adsorption studies.

3. RESULTS AND DISCUSSION 3.1. Characterization of Adsorbent. 3.1.1. Elemental Analysis. The elemental analyses of untreated and tannintreated jute fiber revealed that the carbon content of the fiber increased from 41.988% ± 0.13% to 46.963% ± 0.15% after tannin treatment. This indicates the effective incorporation of tannin onto the surface of the jute fiber. 3.1.2. Solid-State 13C CP-MAS NMR Spectroscopy. Structural studies of untreated, pretreated, and tannin-modified JF were performed by solid-state 13C CP-MAS NMR spectrosco-

Figure 2. FTIR spectra of (a) untreated and (b) tannin-grafted JF. (c) Fitting of FTIR absorption spectra of surface-modified JF using the Voigt function and (d) corresponding deconvoluted results in the range of 1800−800 cm−1. 6504



Article

Figure 3. SEM images of (a) untreated, (b) tannin-grafted, and (c) dye-adsorbed modified JF.

Figure 4. Effects of (a) initial dye-solution pH and (b) adsorbent dose (●, 2; ■, 4; ▲, 6; ⧫, 8; ▼, 10 g L−1) on the adsorption capacity of treated jute fiber for CR.

0.214 to 0.662, respectively, after immobilization with tannin onto pretreated JF (Figure 2b).18 A considerable variation in the peak intensities of different functional groups of the adsorbent was also detected after dye adsorption. The peak at 2920 cm−1 for the CH stretching vibration of methyl and methylene groups on the fiber surface was selected as the internal standard because it was anticipated that this peak would remain unaffected during dye adsorption.17 The absorbance intensities of the peaks for OH and CO stretching were normalized with respect to the intensity of this internal standard peak. The decrement of the ratios of the absorbance maxima of the two peaks to the internal standard peak of modified JF after adsorption suggests that polar functional groups, such as the hydroxyl and carbonyl groups of the modified fiber, are likely to be responsible for CR adsorption. Similar observations were also reported by Feng et al.27 for basic dye adsorption onto sesame hull. 3.1.4. SEM Study. Significant changes in the surface topography of JF after chemical modification and dye adsorption were revealed by SEM (Figure 3). The surface of raw JF was smooth with a multicellular nature, whereas after the tannin treatment, the surface morphology was observed to be rougher (Figure 3a,b).18 Hence, the coarser surface features of chemically modified JF might provide a larger surface area for dye molecules to be adsorbed onto the fiber.27

with respect to the area of this internal standard peak. In the FTIR spectra of JF, the bands for hydrogen-bonded OH stretching and ester-group CO stretching appeared at 3200− 3600 and 1735 cm−1, respectively (Figure 2a). A reduction in intermolecular/intramolecular hydrogen bonding between the hydroxyl groups of cellulose and hemicellulose and removal of hemicellulose upon alkali treatment resulted in a decrement in the absorbance area ratios of both of these peaks to the internal standard peak after alkali treatment.26 On the other hand, upon tannin treatment, the absorbance peak area ratio for the OH stretching frequency increased from 19.069 to 26.776. For treated JF, because of the presence of aromatic CH/methylene (CH2) bridging units in tannin, the intensity ratio of the peak at 2920 cm−1, associated with the CH stretching in methyl and methylene groups, was found to increase to 20.976. The chemical modification of JF also augmented the peak area ratios corresponding to aromatic CC stretching (1600 cm−1) and asymmetrical C OC stretching (1161 cm−1) from 2.389 to 3.135 and from 2.817 to 3.241, respectively. The absorbance area ratios of the peaks at 1452 cm−1 (CH2 bending), 1374 cm−1 (CH asymmetric stretching), 1035 cm−1 (aromatic CH in-plane deformation), and 840 cm−1 (aromatic CH out-of-plane vibration) of JF were observed to be enhanced from 1.994 to 3.047, from 2.574 to 3.166, from 3.089 to 3.547, and from 6505



Article

Figure 5. (a) Effect of initial dye concentration (■, 10; ●, 50; ▲, 100; ⧫, 150; ▼, 200; ◀, 250 mg L−1) on the adsorption capacity of chemically modified JF for CR adsorption. (b) Concentration decay profile of dye during adsorption on treated JF. (c) Effect of temperature on adsorption capacity. (d) Plot of ln KC versus 1/T for the removal of CR by treated JF.

deprotonation of the dye molecules and the surface negativity of the adsorbent encumbered the adsorption process through the electrostatic repulsion between the negatively charged adsorbent and the negatively charged adsorbate molecules, which, in turn reduced the removal of dye by modified JF. 3.2.2. Effect of Adsorbent Mass. The necessity to investigate the effect of adsorbent dose comes from the economic viewpoint, which requires the minimum amount of adsorbent for effective dye removal. The dye removal percentage increased with increasing amount of adsorbent up to 10 g L−1, and for higher doses, no dramatic change in removal efficiency was observed despite the increasing amount of JF. The increase in the amount of adsorbent enhanced the surface area, which, in turn, increased the number of available adsorption sites and, thus, also boosted the interaction between the adsorbent and CR, leading to the augmentation in the extent of dye removal by JF.29 However, the amount of adsorbent inversely affected the adsorption capacity of JF. Thus, as the adsorbent dose increased from 2 to 10 g L−1, the amount of dye adsorbed per unit weight of adsorbent decreased, causing a decrease in adsorption capacity from 24.23 to 4.96 mg g−1 (Figure 4b). This result can be attributed to the significant unsaturation of adsorption sites at higher adsorbent dosages for constant dye concentration and volume.30

After dye adsorption, the adherence and surface coverage of the dye on the spent adsorbent was observed (Figure 3c). This image evidences dye adsorption by chemically treated JF. 3.2. Batch Adsorption Study. 3.2.1. Effect of pH on Adsorption. The initial pH of the solution is one of the most significant factors affecting the adsorption capacity for water treatment. The variation of adsorption capacity for the adsorbent at different pHs can be attributed to the chemistry of both the dye and the adsorbent, specifically the surface charge of the adsorbent in aqueous solution at a certain pH. The effect of pH on the adsorption efficiency of modified JF toward CR is presented in Figure 4a. It can be observed from this figure that the removal of dye from aqueous solution by modified JF decreased from 99.87% to 56.16% as the pH of solution was changed from 3 to 12. Because CR is a dipolar molecule, it exists in cationic form below pH 4.5−5.5, whereas it exists in anionic form above pH 4.5−5.5 .28 On the other hand, the point of zero charge of tannin-treated JF is 2.8 (see the Supporting Information). Consequently, when the solution pH was 3, the protonated dye molecules were strongly attracted by the negatively charged adsorbent surface. Although the dye molecules remained positively charged below the pH range of 4.5−5.5, above pH 2.8, the surface negativity of JF increased with increasing solution pH. This resulted in a minute decrement in the dye removal percentage when the solution pH was increased from from 3 to 5. Above pH 5, the 6506



Article

ΔGo = ΔH o − T ΔS o

3.2.3. Effect of Initial Dye Concentration on Adsorption. The initial dye concentration in solution had a pronounced effect on the adsorption capacity of modified JF. It is evident from Figure 5a that the adsorption capacity of modified JF increased from 9.92 to 22.63 mg g−1 as the initial dye concentration was increased from 10 to 250 mg L−1. Conversely, the percentage removal of dye showed the opposite trend and decreased from 99% to 56% as the initial dye concentration was increased from 10 to 250 mg L−1. During adsorption, the initial dye concentration, adsorption sites, and available sorption surface provide the main driving force to overcome the mass-transfer resistance of dye between the aqueous and solid phases. Consequently, for constant adsorbent dosage, the lower availability of adsorption sites of JF at higher initial dye concentrations led to lower removal percentages, and the dye adsorption became dependent solely on the initial concentration. Thus, the increase in initial concentration increased the adsorption capacity of the adsorbent by enhancing the interaction between adsorbent and dye.29 3.2.4. Effect of Contact Time on Adsorption. As shown in Figures 4b and 5a, irrespective of the amount of adsorbent and the initial dye concentration, dye adsorption from aqueous solution by treated JF was rapid during the initial period of contact and then gradually became slower and leveled out with increasing contact time. The adsorption attained equilibrium within 15−30 min of contact, and after reaching equilibrium, the adsorption rate remained almost constant with a very slow increment in dye adsorption (Figure 5a,b). The adequate accessibility of vacant adsorption sites cause rapid adsorption onto the exterior surface of the fiber during the early phase of adsorption, as represented by the curved portions in Figures 4b and 5a. After a lapse of time, the majority of the exterior sites of the adsorbent surface were occupied by dye molecules, and the remaining vacant sites became unavailable due to the repulsive forces between the solute molecules in the solid and bulk phases. At this stage, the dye molecules were transported to internal sites of the adsorbent through pore diffusion, which requires a longer contact time and is represented by the linear portions of Figures 4b and 5a.31 It is noteworthy that the amount of adsorbent and initial dye concentration had minor influences on the adsorption rate and the contact time required to reach the equilibrium. 3.2.5. Effect of Temperature on Dye Adsorption. The influence of temperature on the adsorption capacity of modified JF toward CR adsorption is depicted in Figure 5c. The adsorption capacity of the modified JF was found to be reduced slightly from 4.96 to 3.59 mg g−1 as the temperature was increased from 30 to 50 °C, thereby indicating the exothermic nature of the adsorption. As suggested by Chatterjee et al.,32 this phenomenon can be attributed to the weakening of the physical interactions, for example, hydrogen bonds and van der Waals forces, between the adsorbed dye and JF. The thermodynamic parameters were calculated to evaluate the feasibility and the nature of dye adsorption by modified JF. The thermodynamic parameters, namely, the Gibb’s free energy (ΔG°, kJ mol−1), enthalpy (ΔH°, kJ mol−1), and entropy (ΔS°, J mol−1 K−1) changes associated with CR adsorption, were determined using the Gibb’s free energy (eq 3) and van’t Hoff (eq 4) equations33 ln K C =

ΔS o ΔH o − R RT

(4)

where KC is the distribution coefficient of adsorption, R is the universal gas constant (8.314 J mol−1 K−1), and T is the adsorption temperature (K). The values of ΔH° and ΔS° were calculated from the slope and intercept of the plot ln KC versus 1/T (K−1) (Figure 5d). The exothermic nature of CR adsorption by modified JF was confirmed by the negative value of ΔH° (−32.397 kJ mol−1). The negative values of ΔG° (−6.025, −5.154, and −4.284 kJ mol−1) at all temperatures (30, 40, and 50 °C, respectively) verified the thermodynamic feasibility and spontaneity of the adsorption process. The favorability of CR adsorption onto modified JF at low temperature was also revealed by the shifting of ΔG° to more negative values with decreasing temperature. The negative ΔS° value (−87.035 J mol−1 K−1) suggests that the disorder at the solid−liquid interface decreased during dye adsorption and that no significant changes occurred in the internal structure of the adsorbents upon adsorption.33 3.3. Adsorption Isotherm. Equilibrium adsorption isotherms are of fundamental importance in the design of adsorption systems for practical applications, as the isotherms can be used to interpret the specific relationship between the concentration of adsorbate and its extent of adsorption onto the adsorbent surface at a constant temperature. An estimation of the adsorption capacity of modified JF for the removal of CR from aqueous solution was obtained by modeling the experimental data with the Langmuir and Freundlich isotherm equations (Figure 6).

Figure 6. Langmuir and Freundlich isotherm fits for CR adsorption onto treated JF at different temperatures (initial pH, 6.2; adsorbent dose, 10 g L−1).

3.3.1. Langmuir Isotherm. The Langmuir isotherm model (eq 5) describes the adsorption that occurs at specific, energetically equivalent sites of the adsorbent with monolayer coverage of the adsorbate over a homogeneous adsorbent surface.34

qe =

qmbCe 1 + bCe

(5)

The Langmuir parameters qm (maximum monolayer capacity of adsorbent, mg g−1) and b (Langmuir constant, L mg−1) were evaluated at different temperatures and are presented in Table 1. The qm value of modified JF calculated from Langmuir model

(3) 6507



Article

mechanism of dye adsorption. The pseudo-first-order, pseudosecond-order, intraparticle diffusion, and liquid film diffusion models were fitted to the experimental data for the determination of potential rate-controlling steps of the CR adsorption kinetics. 3.4.1. Pseudo-First-Order Model. The linearized form of the Lagergren pseudo-first-order rate equation is given by35

Table 1. Langmuir Isotherm Parameters for CR Adsorption by Treated JF at Different Temperatures temperature (°C)

qm (mg g−1)

b (L mg−1)

R2

30 40 50

27.12 24.71 20.11

0.507011 0.029331 0.016541

0.9949 0.9908 0.9932

was observed to be close to the experimental adsorption capacity. However, the values of both qm and b for modified JF decreased from 27.12 to 20.11 mg g−1 and from 0.5070 to 0.0165 L mg−1, respectively, as the temperature was increased from 30 to 50 °C. This again confirms the exothermicity of CR adsorption by treated JF. Furthermore, the high R2 values (0.99) indicate the applicability of the Langmuir isotherm for CR adsorption onto surface-functionalized JF (Figure 6). Thus, from the viewpoint of the adsorption mechanism at the molecular level, the Langmuir model suggests the facile anchoring of dye to the abundant functional groups of modified JF with the formation of monolayer surface coverage. The list of the adsorption capacities of various chemically treated bioadsorbents for azo dyes in Table 2 can be useful for

log(qe − qt ) = log qe −

adsorbent

dye

phosphoric acid treated sugar cane bagasse mercerized rice husk

Methyl Red

10.98

40

Malachite Green Methylene Blue Methylene Blue Reactive Red 120 Sunset Yellow FCF Basic Blue

17.98

38

16.21

41

14.95

42

5.61

43

12.72

44

7.59

45

Congo Red

27.12

hydrolyzed wheat straw HCHO/HCOOH-treated brown macroalga octadecylamine- and chitosanmodified montmorillonite formaldehyde-modified mangrove barks quarternized sugar cane bagasse treated jute fiber

t 1 1 = + t qt qe k 2qe 2

ref

(9)

where k2 (g mg−1 min−1) is the rate constant of second-order adsorption. Analysis of the experimental data with the pseudosecond-order kinetic model provided linear plots of t/qt versus t. The fittings of the adsorption data with the pseudo-secondorder kinetic model for varying initial dye concentrations are presented in Figure 7b. In contrast to the pseudo-first-order model, the pseudo-second-order model fitted the experimental data well, with exceptionally high correlation coefficients (R2 > 0.99) and insignificant χ2 values. The very similar values of the adsorption capacities predicted by the pseudo-second-order model and obtained experimentally indicate the good agreement of this kinetic model with the process of CR adsorption onto treated JF. The suitability of the pseudo-second-order model for interpreting the kinetic profile of this adsorption process was further confirmed by the regularity in the change in rate constant with the variation of the initial dye concentration. It can be observed from Table 3 that the pseudo-second-order rate constant (k2) decreased as the initial CR concentration increased. The reason for the decrement of k2 with increasing initial dye concentration might be the higher competition of the dye molecules for the surface active sites. In contrast, at lower dye concentrations, higher adsorption rates would be obtained because of the lower competition for the adsorption sites.37 Plots of t/qt versus t at different temperatures (Figure 7c) also showed good linearity, which implies the adequate fitting of pseudo-second-order kinetic model to the experimental adsorption data. The rate constant (k2) was found to decrease from 0.2648 to 0.0380 g mg−1 min−1 as the temperature was increased from 30 to 50 °C. This again confirms the exothermic nature of CR adsorption onto treated JF.38 Thus, in view of these results, it can be concluded that the present adsorption system predominantly follows the pseudo-second-order kinetic

present study

comparison, although the published values were obtained under different experimental conditions. As can be observed from Table 2, the adsorption capacity of modified JF was comparable to those reported in previous studies of other bioadsorbents. 3.3.2. Freundlich Isotherm. Unlike the Langmuir isotherm, the Freundlich isotherm assumes multilayer adsorption of adsorbate on a heterogeneous adsorbent surface. The Freundlich isotherm equation is34 qe = KFCe1/ n

(8)

where qt is the amount of adsorbent (i.e., CR) adsorbed at time t (mg g−1) and k1 is the rate constant of pseudo-first-order adsorption (min−1). Plots of log(qe − qt) versus t (Figure 7a) at different initial dye concentrations obtained by modeling the experimental data with the Lagergren pseudo-first-order equation provided poor fits to straight lines with very poor R2 values and large χ2 values. The high degree of nonlinearity indicates the inapplicability of the pseudo-first-order model for analyzing the adsorption kinetics of CR by treated JF. This inapplicability was further corroborated by the disagreement between the experimental and model-predicted adsorption capacities. An irregularity in the change of pseudo-first-order rate constants with increasing initial dye concentration was also observed. 3.4.2. Pseudo-Second-Order Model. The linear form of the pseudo-second-order kinetic model is expressed as36

Table 2. Comparison of the Maximum Adsorption Capacities of Treated JF and Other Low-Cost Modified Bioadsorbents for Azo Dye Removal adsorption capacity (mg g−1)

k1 t 2.303

(7)

where KF is the Freundlich constant and n is the heterogeneity factor. The unsuitability of the Freundlich isotherm is indicated by the poor fit of the experimental data, which gave low R2 values (0.92−0.85) compared to that obtained for the Langmuir model (Figure 6). 3.4. Adsorption Kinetics. The kinetic behavior of CR removal by modified JF was studied to evaluate the rate of adsorbate uptake from aqueous solution, which controls the 6508



Article

Figure 7. (a) Pseudo-first-order and (b) pseudo-second-order kinetic plots for CR adsorption by treated jute fiber at different initial dye concentrations (●, 50; ▲, 100; ■, 150; ⧫, 200 mg L−1) (initial pH, 6.2; adsorbent dose, 10 g L−1; temperature, 30 °C). (c) Pseudo-second-order kinetic plot for dye adsorption by treated JF at different temperatures (initial dye concentration, 50 mg L−1; initial pH, 6.2; adsorbent dose, 10 g L−1).

model and that the mechanism of the overall process appears to be dependent on both the adsorbate and the adsorbent.33 3.4.3. Intraparticle and Liquid Film Diffusion Model. The diffusivity of the solute molecules plays an important role in determining the overall rate of an adsorption process. Although the pseudo-second-order equation was found to fit the experimental data very well, the results obtained from this model are not sufficient to predict the diffusion mechanism.6 During a solid/liquid adsorption process, adsorbate transfer is usually governed by liquid-phase mass transport (boundary-

Table 3. Pseudo-Second-Order Kinetic Parameters for the Adsorption of CR onto Treated Jute Fiber at Different Initial Dye Concentrations Co (mg L−1)

qe,exp (mg L−1)

qe,cal (mg L−1)

k2 (g mg−1 min−1)

R2

χ2

50 100 150 200 250

4.955 9.897 14.764 19.538 22.672

4.9771 9.9492 14.892 19.646 22.686

0.2648 0.1028 0.0555 0.0545 0.0171

0.9999 0.9998 0.9997 0.9999 0.9995

0.0866 0.1701 0.0076 0.0368 0.0108

Figure 8. Plots of (a) intraparticle diffusion and (b) liquid film diffusion models for CR adsorption by JF at different initial dye concentrations (■, 50; ▲, 100; ●, 150; ▼, 200; ◀, 250 mg L−1) (initial pH, 6.2; adsorbent dose, 10 g L−1; temperature, 30 °C). 6509



Article

Linear fits of plots of ln(1 − F) versus t for the liquid film diffusion model (Figure 8b) had R2 values within the range of only 0.1795−0.8647 (Table 4). The poor R2 values as compared to that of the intraparticle diffusion model and the deviation of the straight lines from the origin in the plots for the liquid film diffusion model indicate the inability of this model to describe the present adsorption system accurately.38 3.5. Activation Parameters. The Arrhenius activation energy (Ea, kJ mol−1) for the adsorption of CR by tannintreated JF was evaluated using the pseudo-second-order rate constant (k2) in the Arrhenius equation as follows6

layer diffusion), or intraparticle mass diffusion, or both. The slowest step, which might be either film diffusion or pore diffusion, would obviously be the overall rate-controlling step of the adsorption process.37 The intraparticle and liquid film diffusion models are represented by eqs 10 and 11, respectively38 qt = k idt 0.5

(10)

ln(1 − F ) = −k fdt

(11)

where kid (mg g−1 min−0.5) and kfd (min−1) are intraparticle and liquid film diffusion rate constants, respectively; and F = qt/qe. Figure 8a presents the fitting of the experimental data with the intraparticle diffusion model for various initial dye concentrations. It can be clearly observed from this figure that plots of qt versus t0.5 are not linear over the whole time range and can be separated into two linear regions. The multilinearity of the plots confirms the involvement of multistage adsorption during the uptake of CR by treated JF. The initial linear portion of the plot is due to external mass transfer, which allows the adsorbate molecules to be transported to the external surface of the treated JF through film diffusion. At this stage, the instantaneous adsorption with a high uptake rate was probably due to a strong electrostatic attraction between the dye and the external surface of the adsorbent. After boundary-layer diffusion, the solute molecules entered the interior of the adsorbent by intraparticle diffusion through pores, as reflected by the second linear portion of the plot. The stage of gradual adsorption whose rate was controlled by intraparticle diffusion was followed by a final equilibrium stage during which the intraparticle diffusion started to slow and become stagnant as the adsorbate molecules occupied all of the active sites of the adsorbent and the maximum adsorption was attained.29 Although all of the intraparticle diffusion plots for different initial dye concentrations provided a linear relationship, none of the line segments passed through the origin. The nonzero intercepts of the plots indicate that intraparticle diffusion is involved in the adsorption process but is not the sole rate-controlling step for the adsorption of CR.39 The increment of kid with increasing initial CR concentrations implies that the increased driving force at high initial concentrations promotes intraparticle diffusion of the adsorbate onto the adsorbent (Table 4).

ln k 2 = ln A −

initial dye concentration (mg L−1)

kid1 (mg g−1 min−0.5) kid2 (mg g−1 min−0.5) intercept1 intercept2 R2 kfd (×10−3 min−1) intercept R2

50

100

Intraparticle Diffusion 0.4703 1.0958 0.0017 0.0071 2.3621 3.8299 4.9278 9.7920 0.9468 0.9291 Liquid Film Diffusion −0.517 −14.08 −0.064 −2.013 0.1795 0.8647

150 Model 1.4352 0.0428 6.3091 14.268 0.9367 Model −18.69 −3.606 0.4985

200

250

1.7278 0.0268 9.8492 19.195 0.9300

0.8291 0.1966 16.474 19.922 0.9989

−28.32 −2.940 0.8335

−38.25 −2.686 0.8056

(12)

where A is the Arrhenius constant, R is the universal gas constant (8.314 J mol−1 K−1), and T is the temperature (K). The value of Ea was calculated from the slope of the plot ln k2 versus 1/T (R2 = 0.99024). Ea represents the minimum energy that the reactants must acquire for the reaction to proceed. According to the literature, when the value of Ea is below 40 kJ mol−1, physisorption is the predominant adsorption mechanism, whereas a value of Ea greater than 40 kJ mol−1 reflects an activated chemical adsorption process. The value of Ea for CR adsorption on treated JF as obtained from the linear plot of ln k2 versus 1/T was 9.31 kJ mol−1, which suggests that the present adsorption system involves a diffusion-controlled physisorption process.6 3.6. Batch Desorption and Adsorbent Regeneration Studies. Desorption studies of a spent adsorbent can provide important insight into the overall adsorption mechanism. Desorption of adsorbate by water suggests the involvement of weak physisorption, whereas desorption by organic acids implies that the attachment of the adsorbate to the adsorbent is through chemisorption. If the dye desorption occurs upon use of strong mineral acids or bases, the adsorption is due to ion exchange. Very low desorption (1.8%) of CR was obtained with 0.1 M HCl, whereas water and 0.1 M CH3COOH desorbed 20.7% and 11.2%, respectively, of the dye. The maximum desorption of dye (80.1%) was achieved by 0.1 M NaOH, indicating that the dye molecules were most probably attached to the adsorbent by ion exchange.17 The adsorption efficiency of JF deteriorated during repeated adsorption− desorption cycles of CR onto JF. The adsorption of dye diminished slightly from 99.1% to 92.3% in the first reuse cycle and then further decreased to 72.6%, 68.3%, and 65.5% in the second, third, and fourth cycles, respectively. 3.7. Adsorption Mechanism. The physical or chemical characteristics of adsorbents and also the system conditions influence the mechanism of the adsorption process. The mechanism of adsorption largely depends on the physical and chemical characteristics of both the adsorbent and the adsorbate. The elucidation of the mechanism of adsorption of CR onto treated JF was performed on the basis of the experimental findings. The FTIR study of treated JF confirmed the major presence in JF of oxygen-containing functional groups such as hydroxyl, ether, and carbonyl groups, whereas CR molecules contain amine, sulfonate, and azo functional groups. Hence, considering the structures of the adsorbate and adsorbent, there exist multiple possibilities for physical bond formation, such as hydrogen bonding between N, S, and O atoms of CR and OH groups of the treated JF surface and van der Waals forces, during adsorption process. Figure 9 presents a plausible mechanism for the physiadsorption of azo

Table 4. Comparison of Intraparticle Diffusion and Liquid Film Diffusion Model Parameters for the Adsorption of CR onto Treated Jute Fiber at Different Initial Dye Concentrations

model parameter

Ea RT

6510



Article

beneficial for practical purposes. The rate of the adsorption process was found to conform reasonably well with the pseudosecond-order kinetic model. The low value determined for the Arrhenius activation energy (9.31 kJ mol−1) suggests that CR adsorption on treated JF is a physisorption process. In addition, the effective desorption of CR from the spent adsorbent with 0.1 M NaOH solution implies the involvement of an ionexchange mechanism during adsorption. Thus, the present investigation has provided a new efficient, stable, low-cost, economical, and environmentally safe adsorbent with potential for practical application in the treatment of dye-contaminated wastewater.

■

ASSOCIATED CONTENT

S Supporting Information *

Determinations of surface area and point of zero charge of modified jute fiber. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 3222 283986. E-mail: [email protected]. in. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■ Figure 9. Schematic representation of dye-modified JF interaction during adsorption: hydrogen bonding between hydroxyl groups of treated JF and electronegative atoms of dye molecules.

■

dye onto treated JF. On the other hand, the desorption study suggests that ion exchange might also play a role in dye adsorption on treated JF. Thus, it can be proposed that both physical interactions and ion exchange are responsible for CR adsorption by treated JF.

ABBREVIATIONS CP-MAS NMR = cross-polarization magic-angle-spinning nuclear magnetic resonance CR = Congo Red FTIR = Fourier transform infrared JF = jute fiber SEM = scanning electron microscopy REFERENCES

(1) Simonič, M.; Lobnik, A. The efficiency of a hybrid flocculation/ UF process for a real dye-house effluent using hydrophilic and hydrophobic membranes. Desalination 2011, 271, 219−224. (2) Christie, R. Environmental Aspects of Textile Dyeing; Woodhead Publishing: Cambridge, U.K., 2007. (3) Alila, S.; Boufi, S. Removal of organic pollutants from water by modified cellulose fibers. Ind. Crops Prod. 2009, 30, 93−104. (4) Boufi, S.; Belgacem, M. N. Modified cellulose fibers for adsorption of dissolved organic solutes. Cellulose 2006, 13, 81−94. (5) Vieira, A. P.; Santana, S. A. A.; Bezerra, C. W. B.; Silva, H. A. S.; Melo, J. C. P.; Filho, E. C. S.; Airoldi, C. Copper sorption from aqueous solutions and sugar cane spirits by chemically modified babassu coconut (Orbignya speciosa) mesocarp. Chem. Eng. J. 2010, 161, 99−105. (6) Sajab, M. S.; Chia, C. H.; Zakaria, S.; Jani, S. M.; Ayob, M. K.; Chee, K. L.; Khiew, P. S.; Chiu, W. S. Citric acid modified kenaf core fibers for removal of methylene blue from aqueous solution. Bioresour. Technol. 2011, 102, 7237−7243. (7) Anirudhan, T. S.; Jalajamony, S.; Divya, L. Efficiency of aminemodified poly(glycidyl methacrylate)-grafted cellulose in the removal and recovery of vanadium(V) from aqueous solutions. Ind. Eng. Chem. Res. 2009, 48, 2118−2124. (8) Gurgel, L. V. A.; Gil, L. F. Adsorption of Cu(II), Cd(II) and Pb(II) from aqueous single metal solutions by succinylated twice-

4. CONCLUSIONS In the present investigation, chemically modified JF was evaluated as a novel bioadsorbent with enhanced adsorption capacity for the removal of an azo dye. The study of influence of pH revealed that the adsorbent exhibited better adsorption performance under acidic conditions and achieved a maximum adsorption at a pH of ∼3. The adsorbent dose inversely affected the adsorption capacity of modified JF, whereas the adsorption capacity increased with increasing initial dye concentration. The negative ΔG° values and negative ΔH° value, as obtained from the thermodynamic study, established the spontaneous and exothermic nature of CR adsorption. The adsorption equilibrium data were fitted very well by the Langmuir isotherm model, with R2 > 0.99. The adsorption of the dye onto modified JF reached equilibrium in a short contact time, resulting in fast and efficient dye removal, which would be 6511



Article

hull (Sesamum indicum L.). Bioresour. Technol. 2011, 102, 10280− 10285. (28) Al-Degs, Y. S.; El-Barghouthi, M. I.; El-Sheikh, A. H.; Walker, G. M. Effect of solution pH, ionic strength, and temperature on adsorption behavior of reactive dyes on activated carbon. Dyes Pigm. 2008, 77, 16−23. (29) Ahmad, R. Studies on adsorption of crystal violet dye from aqueous solution onto coniferous pinus bark powder (CPBP). J. Hazard. Mater. 2009, 171, 767−773. (30) Bhattacharyya, K. G.; Sharma, A. Azadirachta indica leaf powder as an effective biosorbent for dyes: A case study with aqueous Congo Red solutions. J. Environ. Manage. 2004, 71, 217−229. (31) Luo, P.; Zhao, Y.; Zhang, B.; Liu, J.; Yang, Y.; Liu, J. Study on the adsorption of Neutral Red from aqueous solution onto halloysite nanotubes. Water Res. 2010, 44, 1489−1497. (32) Chatterjee, S.; Chatterjee, S.; Chatterjee, B. P.; Guha, A. K. Adsorptive removal of congo red, a carcinogenic textile dye by chitosan hydrobeads: Binding mechanism, equilibrium and kinetics. Colloids Surf. A: Physicochem. Eng. Aspects 2007, 299, 146−152. (33) Vimonses, V.; Lei, S.; Jin, B.; Chow, C. W. K.; Saint, C. Kinetic study and equilibrium isotherm analysis of Congo Red adsorption by clay materials. Chem. Eng. J. 2009, 148, 354−364. (34) Das, S. K.; Bhowal, J.; Das, A. R.; Guha, A. K. Adsorption Behavior of Rhodamine B on Rhizopus oryzae Biomass. Langmuir 2006, 22, 7265−7272. (35) Ho, Y. S. Adsorption of heavy metals from waste streams by peat. Ph.D. Thesis, University of Birmingham, Birmingham, U.K., 1995. (36) Ho, Y. S. Second-order kinetic model for the sorption of cadmium onto tree fern: A comparison of linear and non-linear methods. Water Res. 2006, 40, 119−125. (37) Dawood, S.; Sen, T. K. Removal of anionic dye Congo red from aqueous solution by raw pine and acid-treated pine cone powder as adsorbent: Equilibrium, thermodynamic, kinetics, mechanism and process design. Water Res. 2012, 46, 1933−1946. (38) Chowdhury, S.; Mishra, R.; Saha, P.; Kushwaha, P. Adsorption thermodynamics, kinetics and isosteric heat of adsorption of malachite green onto chemically modified rice husk. Desalination 2011, 265, 159−168. (39) Rubin, E.; Rodriguez, P.; Herrero, R.; Vicente, E. S. Adsorption of methylene blue on chemically modified algal biomass: Equilibrium, dynamic, and surface data. J. Chem. Eng. Data 2010, 55, 5707−5714. (40) Saad, S. A.; Isa, K.Md.; Bahari, R. Chemically modified sugarcane bagasse as a potentially low-cost biosorbent for dye removal. Desalination 2010, 264, 123−128. (41) Batzias, F.; Sidiras, D.; Schroeder, E.; Weber, C. Simulation of dye adsorption on hydrolyzed wheat straw in batch and fixed-bed systems. Chem. Eng. J. 2009, 148, 459−472. (42) Kousha, M.; Daneshvar, E.; Sohrabi, M. S.; Jokar, M.; Bhatnagar, A. Adsorption of acid orange II dye by raw and chemically modified brown macroalga Stoechospermum marginatum. Chem. Eng. J. 2012, 192, 67−76. (43) Kittinaovarat, S.; Kansomwan, P.; Kondru, N. J. Chitosan/ modified montmorillonite beads and adsorption Reactive Red 120. Appl. Clay Sci. 2010, 48, 87−91. (44) Seey, T. L.; Noordin, M. J.; Kassim, M. Removal of Acidic and Basic Dyes from Aqueous Solution by Adsorption on Chemically Treated Mangrove Barks. Int. J. Appl. Sci. Technol. 2012, 2, 270−276. (45) Wong, S. Y.; Tan, Y. P.; Abdullah, A. H.; Ong, S. T. The Removal of Basic and Reactive Dyes Using Quartenised Sugar Cane Bagasse. J. Phys. Sci. 2009, 20, 59−74.

mercerized sugarcane bagasse functionalized with triethylenetetramine. Water Res. 2009, 43, 4479−4488. (9) Gurgel, L. V. A.; Gil, L. F. Adsorption of Cu(II), Cd(II), and Pb(II) from aqueous single metal solutions by succinylated mercerized cellulose modified with triethylenetetramine. Carbohydr. Polym. 2009, 77, 142−149. (10) Filho, E. C. S.; Melo, J. C. P.; Fonseca, M. G.; Airoldi, C. Cation removal using cellulose chemically modified by a Schiff base procedure applying green principles. J. Colloid Interface Sci. 2009, 340, 8−15. (11) Karnitz, O.; Gurgel, L. V. A.; Gil, L. F. Removal of Ca(II) and Mg(II) from aqueous single metal solutions by mercerized cellulose and mercerized sugarcane bagasse grafted with EDTA dianhydride (EDTAD). Carbohydr. Polym. 2010, 79, 184−191. (12) Melo, J. C. P.; Filho, E. C. S.; Santana, S. A. A.; Airoldi, C. Maleic anhydride incorporated onto cellulose and thermodynamics of cation-exchange process at the solid/liquid interface. Colloids Surf. A 2009, 346, 138−145. (13) Gong, R.; Ni, S.; Zhao, J.; Li, N. Epichlorohydrin and aspartic acid incorporated into sawdust and application as sorbent for methylene blue. Wood Sci. Technol. 2009, 43, 183−193. (14) Anirudhan, T. S.; Sreekumari, S. S. Synthesis and characterization of a functionalized graft copolymer of densified cellulose for the extraction of uranium(VI) from aqueous solutions. Colloids Surf. A: Physicochem. Eng. Aspects 2010, 361, 180−186. (15) Wu, Z.; Cheng, Z.; Ma, W. Adsorption of Pb(II) from glucose solution on thiol-functionalized cellulosic biomass. Bioresour. Technol. 2012, 104, 807−809. (16) Elizalde-Gonzalez, M. P.; Mattusch, J.; Wennrich, R. Chemically modified maize cobs waste with enhanced adsorption properties upon methyl orange and arsenic. Bioresour. Technol. 2008, 99, 5134−5139. (17) Roy, A.; Chakraborty, S.; Kundu, S. P.; Adhikari, B.; Majumder, S. B. Adsorption of Anionic-Azo Dye from Aqueous Solution by Lignocellulose-Biomass Jute Fiber: Equilibrium, Kinetics, and Thermodynamics Study. Ind. Eng. Chem. Res. 2012, 51, 12095−12106. (18) Roy, A.; Chakraborty, S.; Kundu, S. P.; Adhikari, B.; Majumder, S. B. Surface grafting of Corchorus olitorius fiber: A green approach for the development of activated bioadsorbent. Carbohydr. Polym. 2013, 92, 2118−2127. (19) Huang, X.; Liao, X.; Shi, B. Tannin-immobilized mesoporous silica bead (BT−SiO2) as an effective adsorbent of Cr(III) in aqueous solutions. J. Hazard. Mater. 2010, 173, 33−39. (20) Price, M. L.; Butler, L. G. Rapid visual estimation and spectrophotometric determination of tannin content of sorghum grain. J. Agric. Food Chem. 1977, 25, 1268−1273. (21) Huang, Z. H.; Zheng, X.; Lv, W.; Wang, M.; Yang, Q. H.; Kang, F. Adsorption of Lead(II) Ions from Aqueous Solution on LowTemperature Exfoliated Graphene Nanosheets. Langmuir 2011, 27, 7558−7562. (22) Martins, M. A.; Forato, L. A.; Mattoso, L. H. C.; Colnago, L. A. A solid state 13C high resolution NMR study of raw and chemically treated sisal fibers. Carbohydr. Polym. 2006, 64, 127−133. (23) Zhang, L. L.; Lin, Y. M.; Zhou, H. C.; Wei, S. D.; Chen, J. H. Condensed Tannins from Mangrove Species Kandelia candel and Rhizophora mangle and Their Antioxidant Activity. Molecules 2010, 15, 420−431. (24) Stancik, A. L.; Brauns, E. B. A simple asymmetric lineshape for fitting infrared absorption spectra. Vib. Spectrosc. 2008, 47, 66−69. (25) Rosa, M. F.; Medeiros, E. S.; Malmonge, J. A.; Gregorski, K. S.; Wood, D. F.; Mattoso, L. H. C.; Glenn, G.; Orts, W. J.; Imam, S. H. Cellulose nanowhiskers from coconut husk fibers: Effect of preparation conditions on their thermal and morphological behavior. Carbohydr. Polym. 2010, 81, 83−92. (26) Roy, A.; Chakraborty, S.; Kundu, S. P.; Basak, R. K.; Majumder, S. B.; Adhikari, B. Improvement in mechanical properties of jute fibres through mild alkali treatment as demonstrated by utilisation of the Weibull distribution model. Bioresour. Technol. 2011, 107, 222−228. (27) Feng, Y.; Yang, F.; Wang, Y.; Ma, L.; Wu, Y.; Kerr, P. G.; Yang, L. Basic dye adsorption onto an agro-based waste materialSesame 6512


Equilibrium, Kinetic, and Thermodynamic Studies of Azo Dye

Recommend Documents