Adsorption of congo red onto lignocellulose/montmorillonite ...

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The adsorption of congo red (CR) on LNC/MMT nanocomposite was studied ... words: lignocellulose; montmorillonite; nanocomposite; adsorption; congo red.

Journal of Wuhan University of

Technology-Mater. Sci. Ed.

Oct.2012

931

DOI 10.1007/s11595-012-0576-2

Adsorption of Congo Red onto Lignocellulose/ Montmorillonite Nanocomposite

ZHAO Yahong1, XUE Zhenhua1, WANG Ximing1, WANG Li1, WANG Aiqin2 (1.College of Material Science and Art Design, Inner Mongolia Agricultural University, Hohhot 010018, China; 2. Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China) Abstract: Lignocellulose/montmorillonite (LNC/MMT) nanocomposites were prepared and characterized by FTIR and XRD. The adsorption of congo red (CR) on LNC/MMT nanocomposite was studied in detail. The effects of contact temperature, pH value of the dye solutions, contact time and concentration of dye solutions on the adsorption capacities of lignocellulose (LNC), montmorillonite (MMT) and the nanocomposite were investigated. The adsorption kinetics and isotherms and adsorption thermodynamics of the nanocomposite for CR were also studied. The results show that the adsorption capacity of LNC/MMT nanocomosite is higher than that of LNC and MMT. All the adsorption processes fit very well with the pseudo-second-order and the Langmuir equation. From thermodynamic studies, it is seen that the adsorption is spontaneous and endothermic. Key words: lignocellulose; montmorillonite; nanocomposite; adsorption; congo red

1 Introduction The presence of dyes in effluents is a major concern due to their adverse effect to many forms of life. The discharge of dyes in the environment is worrying for both toxicological and esthetical reasons[1]. Industries including textile, leather, paper and plastics are some of the sources for dye effluents[2]. Many reactive dyes are toxic to some organisms and hence harmful to aquatic animals[3]. Furthermore, some dyes and their reaction products, such as aromatic amines, possess high carcinogenicity. Besides, they also pose a problem because they may be mutagenic and carcinogenic and can cause severe damage to human beings, such as dysfunction of kidney, reproductive system, liver, brain and central nervous system[4, 5]. In addition, due to its high water-solubility, it is estimated that 10-20% of reactive dye remains in wastewater during production and nearly 50% of reactive dyes ©Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2012 (Received: Aug. 18, 2011; Accepted: May 23, 2012) ZHAO Yahong(赵亚红): E-mail: [email protected] *Corresponding author: WANG Li(王丽): Prof.; E-mail: [email protected] Funded by Special Fund for National Forestry Industry Scientific Research in the Public Interest of China (No. 201104004) and the National Natural Science Foundation of China (No. 20867004)

may be lost to the effluents during dyeing process[6, 7]. Various treatment technologies have been used for the remove of dyes from the effluents, such as chemical coagulation-flocculation [8], oxidation [9], biological process[10], membrane-based separation processes[11-13]. All of the above processes have their own benefits and limitations. Among them, adsorption technology is generally considered to be an effective method to quickly decrease the concentration of dissolved dyes in an effluent, and it has been used effectively for the removal of dyes from wastewater[14-17]. Activated carbon is one of the common adsorbents due to its high surface area and high adsorption capacity. However, its high cost makes the process impossible for industrial applications[18]. Therefore, the process of dye removal by adsorption is being diverted to the use of lower cost adsorbents which will make the process economically feasible. Lignocellulose (LNC) which can be obtained from timber through chemical treatment is the organic cellulose and the most plentiful natural biopolymer. What is more, Antoine et al [19] showed that LNC seems to be an efficient adsorbent that retains organic pollutants on the soil surface and avoid the contamination of groundwaters. As a result, it seems that LNC is a promising new cheaper and effective adsorbent which can remove dyes from wastewater.

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Nevertheless, it is difficult to separate LNC from the wastewater because of its low density. Bentonite is the commercial designation of a natural clay mineral with montmorillonite (MMT) as the main component. MMT is widely used as adsorbent due to its high specific surface area. Wang et al[20] compared the adsorption properties of basic dyes onto Ca-MMT and Ti-MMT and found that Ca-MMT possessed larger adsorption capacity than Ti-MMT because Ca2+ is easier to be displaced through the ion-exchange process. However, Ca-MMT carries a permanent negative charge in its structural framework and has little affinity for the anionic dyes. In addition, it is difficult to separate CaMMT from the colored wastewater because of the expansion of MMT. Therefore, several attempts have been made to develop easily-separated and effective adsorbents. Polymer/layered silicate nanocomposites frequently exhibit remarkably improved mechanical and materials properties and are attracting considerable interests in polymer science field. In our previous studies[21-23], chitosan/montmorillonite (CTS-MMT), chitosan/organo-montmorillonite (CTS-OMMT) and N, O-carboxymethyl-chitosan/montmorillonite (N,OCMC–MMT) nanocomposites showed good adsorption ability for CR dye and well flocculation ability in aqueous solution. To the best of our knowledge, there is no literature focusing on the adsorption capacity of CR dye onto lignocellulose/montmorillonite (LNC/MMT) nanocomposite. Therefore, the adsorption behaviors of CR onto the LNC/MMT nanocomposite were studied in this paper. The effect of contact temperature, pH value of the dye solutions, contact time and concentration of dye solutions on adsorption capacity of the nanocomposite for CR were investigated. The adsorption kinetics and isotherms and adsorption thermodynamics of the nanocomposite were also studied.

2 Experimental 2.1 Materials The fiber content and fiber length of LNC (Beijing Qinli Hengtong Co., China) is 85% and 800-1 000 μm, respectively. The cation exchange capacity (CEC) of Ca-MMT (Chifeng Mining Co., China) is 100 meq/100 g. The purity of the CR (made in Czechoslovakia) is 99%. Other agents used were all of analytical grade and all solutions were prepared with distilled water. 2.2 Preparation of the nanocomposite

4.0 g of MMT was dispersed in 120 g of distilled water. The LNC solution was prepared by dissolving 4.0 g of LNC in 120 mL of 20% (m/m) NaOH solution. Subsequently, the LNC solution was slowly added into the MMT suspension followed by stirring at 60 ℃ for 6 h to obtain the nanocomposite. The formed nanocomposite was washed with distilled water until the pH of the supernatant (fluid) reached 7.00, and then dried at 105 ℃ for 5 h. The sample was ground and sieved to 200 mesh sizes. 2.3 Characterization IR spectra of the samples were characterized using a FTIR Spectrophotometer (Thermo Nicolet, NEXUS, TM) in KBr pellets. XRD analyses of the powered samples were performed using an X-ray power diffractometer with Cu anode (PAN alytical Co. X’pert PRO), running at 40 kV and 30 mA, scanning from 3° to 12° at 3°/min. The BET specific surface area and average pore width of the samples were measured using an Accelerated Surface Area and Porosimetry System (Micromeritics, ASAP 2020) by BET-method at 76 K, and the results were shown in Table 1.

2.4 Adsorption experiments For all the batch adsorption experiments, 0.10 g of the adsorbent and 25.00 mL of CR solution were used. The effects of all factors on the adsorption behaviors were studied by experiments following single factor (the adsorption under different temperatures, different pH values, different times and different concentrations). The effect of temperature on dye removal was carried out in 25.00 mL of dye solutions (1 200 mg/L, pH 9.5) with 0.10 g of adsorbent for 6 h. The influence of pH on CR removal was studied by adjusting CR solutions (600 mg/L) to different pH values (4-10) using a pH meter (PB-10) and agitating 25.00 mL of dye solution with 0.10 g of adsorbent at 30 ℃ for 6 h. For kinetic study, 280 mg/L dye solutions (25.00 mL, pH 9.5) were agitated with 0.10 g of adsorbent at 30 ℃ for predetermined intervals of time. Batch equilibrium adsorption experiments were carried out by agitating 25.00 mL of CR solution of different concentrations at pH 9.5 with 0.10 g of adsorbent at

Journal of Wuhan University of

Technology-Mater. Sci. Ed.

30 ℃ until equilibrium was established. The samples were withdrawn from the thermostated shaker (SHA-C, 150 r/min) when reaching adsorption equilibrium, and then the dye solution was separated from the adsorbent by centrifugation (H-2050R) at 8 000 r/min for 5 min. The absorbencies of solution were measured using a UVVis spectrophotometer (TU-1901) at 500 nm where CR has the maximum absorbency. The molecular structure of CR is shown in Fig.1. The concentrations of the solutions were determined by using linear regression equation (y= 0.03344x + 0.00157, R2= 0.9999) obtained by plotting a calibration curve for dye over a rang of concentrations.

The amount of dye adsorbed at equilibrium qe (mg/ g) was calculated from the following equation:

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stretching vibration of MMT, disappears on the spectra of the nanocomposite (b). The absorption band at 3 423 cm1, assigned to -OH stretching vibration of H2O of MMT, strengthened and shifted to higher wavenumber (3 430 cm1) (b). The adsorption band at 1 636 cm1, attributed to -OH bending vibration of H2O of MMT, weakened and shifted to lower wave number 1 634 cm1 (b). The absorption band at 1 033 cm1, assigned to Si-O stretching vibration of MMT, weakened on the FTIR spectra of the nanocomposite (b). In addition, the absorption bands at 1 164 cm1, 1 112 cm1 and 1 032 cm1, corresponding to C-O-C and C-O stretching vibration of LNC, disappeared on the FTIR spectra of the nanocomposite (b). The absorption band at 797 cm1, attributed to Al-O stretching vibration of MMT, disappeared on the FTIR spectra of the nanocomposite (b). It can be concluded from the information of FTIR that -OH, Si-O and Al-O groups of MMT interact with C-O-C and C-O groups of LNC through coordination and complexation. 3.2 X-ray diffraction analysis of the nanocomposite

(1) where, c0 (mg/L) is the initial dye concentration, ce (mg/ L) is the equilibrium concentration of dye solution, v (L) is the volume of dye solution, m (g) is the mass of adsorbent.

3 Results and discussion 3.1 FTIR analysis of the nanocomposite FTIR spectra of MMT (a), LNC/MMT nanocomposite (b) and LNC (c) are shown in Fig.2. Compared with the FTIR spectra of MMT (a), the adsorption band at 3 617 cm1, corresponding to -OH

MMT (a) and LNC/MMT nanocomposite (b) were also analyzed by XRD and the results are presented in Fig.3. A typical diffraction peak of MMT is 5.83°, corresponding to a basal spacing of 1.48 nm. However, after intercalation with LNC, this peak greatly weakens and almost disappears. It can be concluded that the sheet structure of MMT may be opened and LNC intercalated into the separations, forming an intercalated-exfoliated nanocomposite. According to the results of FTIR and XRD, it may be concluded that LNC intercalated into MMT interlayers by destroying the crystalline structure of MMT. The information observed from FTIR and XRD indicates that the changes in the structure of the nanocomposite could influence chemical environment of the nanocomposite, and then may have an influence on absorption properties of the nanocomposite, which will

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be discussed in the following sections in detail. 3.3 Effect of temperature on adsorption

Fig.4 shows the relationship between the contact temperature and the adsorption capacities of LNC, MMT and the nanocomposite for CR. The results show that the adsorption capacities of LNC, MMT and the nanocomposite all increased with increasing the temperature from 30 to 70 ℃. For LNC and MMT, the adsorption capacities increased slowly with increasing the temperature, whereas, the adsorption capacity of the nanocomposite increased evidently from 51.17 to 263.67 mg/g with increasing temperature from 30 to 70 ℃. The phenomenon showed that higher temperature facilitated to the adsorption of CR on the nanocomposite. It could be due to the fact that increasing temperature may produce a swelling effect within the internal structure of the nanocomposite, which facilitates the penetration of dye molecules into the internal structure of the nanocomposite[24]. This result is also in agreement with the adsorption of CR on activated carbon prepared from coir pith[25]and calciumrich fly ash[26]. In addition, it is worth pointing out that the adsorption capacity of LNC/MMT nanocomposite is higher than those of LNC and MMT under the same conditions. The results may be attributed to the following fact. Compared with LNC and MMT, the nanocomposite has well flocculation ability in aqueous solution and results in an increase in the absorption of CR. To further support the explanation mentioned above, we also investigated the BET specific surface area and the average pore width of LNC, MMT and the nanocomposite. The results indicated (see Table 1), compared with MMT (47.83 m 2 /g), the BET specific surface area of the nanocomposite (5.44 m2/g) decreased. These are attributed to the fact that most of

the exchange sites of MMT were occupied by the large LNC molecules and the inaccessibility of the internal surface to nitrogen gas[20]. However, the average pore width of the nanocomposite (14.18 nm) increased comparing with MMT (8.81 nm). These results show that the larger average pore width may facilitate an increase of CR adsorption on the nanocomposite. The synergic effects of these factors such as the changes in the crystalline structure (it also can be seen from Fig.3 and Fig.4), the BET specific surface area and the average pore width may result in the higher adsorption capacity of the nanocomposite. Therefore, LNC/MMT nanocomposite can be effectively used as an adsorbent in treatment of CR wastewaters. 3.4 Effect of pH value on adsorption The pH value of the dye solutions is an important factor for the determination of the adsorption of solutes[27]. The results in Fig.5 show that when the pH value of dye solution raised from 4 to 10 (it is worth pointing out that CR was slightly soluble in water at the pH 2), the adsorption capacity reduced from 46.69 to 13.50, 55.00 to 30.42, 84.17 to 52.75 mg/g for LNC, MMT and the nanocomposite, respectively. Fig.5 also indicates that the adsorption capacity of LNC/MMT nanocomposite for CR was higher than those of LNC and MMT at any pH.

Two possible mechanisms of adsorption of CR should be considered: (a) electrostatic interaction between the protonated groups of adsorbent and the dye, and CR as the anionic dyes could interact with cation of the adsorbent by electrostatic force, which makes the dyes adsorbed, and (b) the chemical reaction between the adsorbate and the adsorbent. That is to say, the stable chemical bonds can be formed between CR and the adsorbent by the chemical reaction. The decreasing tendency of adsorption capacity with

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increasing the pH value may be attributed to the following facts. On the one hand, at low pH, the reaction may be occurred by electrostatic gravitational between cations in the adsorbent and anions of the dye. On the other hand, at pH above 7, although the excessive hydroxyl anions may compete with the dye anions, a slow reduction in dye uptake was observed with increasing the pH further. In fact, significant adsorption of the anionic dye on the adsorbent still occurred at alkaline pH values. This suggests that the chemisorption mechanism might domain the adsorption process. Similar trend have been observed in the adsorption of CR on onwollastonite[28], biogas residual slurry[29], banana pith[30], waste Fe(III)/Congo Red(III) hydroxide[31], waste orange peel[32], waste red mud[33] and CTS/MMT nannocomposite[21]. 3.5 Adsorption kinetics

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adsorption equilibrium in this study. In order to investigate mechanism of the adsorption process of LNC, MMT and LNC/MMT nanocomposite for CR, the pseudo-first-order and pseudo-second-order equations were used to fit the experimental data. The adsorption kinetics can be explained by two simplified kinetics models: pseudofirst-order and pseudo-second-order equations. The pseudo-first-order formula is given as[34]: (2) where, k1 is the pseudo-first-order rate constant (min1), qe and qt are the amounts of dye adsorbed (mg/g) at equilibrium and time t (min). After integration with the initial condition qt = 0 at t = 0, Eq. (3) can be obtained: (3) Pseudo-second-order model is based on adsorption equilibrium. The pseudo-second-order assumes that the rate-controlling step is chemisorption. It can be defined as[35]: (4) When the initial condition is qt = 0 at t = 0, Eq. (5) can be given as:

Contact time is an important parameter of adsorption which can reflect the adsorption kinetics of an adsorbent. Fig.6 indicates the relationship between contact time and the adsorption capacity of CR solution by LNC, MMT and the nanocomposite, respectively. It can be found that the adsorption capacity of LNC, MMT and the nanocomposite increased with the time ranging from 15 to 360 min and remained basically unchanged from 360 to 600 min. It can be seen that the adsorption equilibrium of CR on LNC, MMT and the nanocomposite were reached at 360 min. So, 360 min was chosen as the adsorption time in order to reach the

(5) where k2 is the pseudo-second-order rate constant (g × mg1 × min1). The linear plots of log (qe-qt) versus t and (t/qt) versus t are drawn for the pseudo-first-order and the pseudo-second-order models, respectively. The rate constant of k1 and k2 can be obtained from the experimental data. The experimental data have been analyzed by two correlation coefficients. The linear R2 coefficient (the coefficient of determination) compares the estimated and actual y-axis values, and ranges in value from 0 to 1 when two arrays of data are fitted to linear equation. If it is 1, there is a perfect correlation

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increased from 200 to 360 mg/L, no further increase of the adsorption capacity was observed. However, for the LNC/MMT nanocomposite, there is no further increase of adsorption capacity when the dye concentration increased from 280 to 360 mg/L. That may be attributed to the following facts. Generally, the higher the initial CR concentration, the greater the driving force of the concentration gradient at the solid-liquid interface. The increase of driving force of the concentration gradient at the solid-liquid interface may cause the increase of the amount of dyes ions adsorbed onto the adsorbent. However, the aggregation of CR dye molecules makes it almost impossible for them to diffuse deeper into adsorbent structure with further increasing the initial concentration of dye.

in the sample. That is to say, there is no difference between the estimated and the actual y-axis value. At the other extreme, if the coefficient of determination is 0, the regression equation is not helpful in predicting a y-axis value. The non-linear R2 value is based on the actual deviation between the experimental data and the theoretically predicted data and is a better correlation of the experimental data with the equation. The pseudo-first-order model and pseudo-secondorder model for adsorption of CR on LNC, MMT and the nanocomposite are given in Fig.7 and Fig.8. The rate constants and the correlation coefficients of the two kinetic models are shown in Table 2. It is can be seen that compared with the pseudo-first-order model, LNC, MMT and the nanocomposite are better fitted the pseudo-second-order model, which shows that the assumption of the chemisorptive nature of adsorbate– adsorbent system for the pseudo-second-order model is valid for CR-LNC, CR-MMT and CR-LNC/MMT system investigated. 3.6 Adsorption isotherm It is clear that the initial dye concentration plays an important role in the adsorption processes. Fig.9 shows the relationship between the adsorption capacity of LNC, MMT and the nanocomposite for CR and the concentration of dye solutions. It can be seen that the adsorption capacity increased with increasing the initial dye concentration. For LNC and MMT, when the initial concentration of dye solution was further

Adsorption isotherms are important for the description of how molecules of adsorbate interact with adsorbent surface. Hence, the correlation of equilibrium data using either a theoretical or empirical equation is essential for the adsorption interpretation and prediction of the extent of adsorption. The adsorption process can be expressed by two isotherm equations[36], the Langmuir and the Freundich equation, which are given as: (6)

(7) where, qm (mg/g) and b (L/mg) are Langmuir isotherm coefficients. The value of qm represents the maximum adsorption capacity, kf (mg/g) and n are Freundlich constants. Two adsorption isotherms were constructed by plotting the c e/q versus c e, log q versus logc e, respectively. The values of R2 of Langmuir isotherm for LNC,

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MMT and the nanocomposite are 0.972 4, 0.997 7 and 0.999 9, respectively. The values of R2 of Freundlich isotherm for LNC, MMT and the nanocomposite are 0.7841, 0.7946 and 0.9518, respectively. Obviously, the correlation coefficients (R 2) of the linear form of the Langmuir model are closer to 1 than that of the Freundlich model. Therefore, the adsorption isotherm of LNC, MMT and the nanocomposite follow the Langmuir model, which also means the monolayer coverage of the dye on the surface of the nanocomposite. Similar behavior was also found for the adsorption of CR onto the CTS/MMT [21], CTSOMMT[22] and N,O-CMC-MMT[23] nanocomposites. The qm values of CR on LNC/MMT nanocomposite were compared with those of other absorbents (see Table 3). It can be seen from Table 3 that the qm value of LNC/MMT nanocomposite is larger than those of other absorbents value such as waste Fe(III)/Cr(III) hydroxide, waste orange peel, waste banana pith, and so on. So, the LNC/MMT nanocomposite can be used as an alternative-adsorbing agent in dye wastewaters. 3.7 Adsorption thermodynamics In understanding better the effect of temperature on the adsorption, it is important to study the thermodynamic parameters such as standard Gibbs free energy change △G 0, standard enthalpy △H 0, and standard entropy △S0. The effect of temperature on adsorption processes was determined through the following relation: (8) And the van't Hoff equation as (9)

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where, Kc is the equilibrium constant, which is the ratio of the equilibrium concentration of the dye ions attached to adsorbent compared to the equilibrium concentration of the dye ions in solution. R is the ideal gas constant and T is the adsorption temperature in Kelvin. The plot of lnKc against 1/T should be linear. The slope of the van’t Hoff plot is equal to −△H0/R, and its intercept is equal to △S0/R. Thermodynamic parameters obtained are given in Table 4. As shown in the table, the negative values of △G0 at different temperatures indicate the spontaneous nature of the adsorption process. Positive △H0 reveals endothermic adsorption. The positive value of △S0 suggests the increased randomness at the solid/solution interface during the adsorption of the dye onto LNC/MMT. A similar trend has been reported for the adsorption of congo red onto fly ash[26].

4 Conclusions The adsorption capacity of LNC, MMT and the nanocomposite are affected by contact temperature, pH value of the dye solutions, contact time and concentration of the dye solutions. The results of the adsorption kinetics and isotherms showed that the adsorption processes of the nanocomposite for CR fit better to the pseudo-second-order equation and the Langmuir equation, respectively. The monolayer coverage of CR on the surface of the nanocomposite was in the ascendant. Thermodynamic studies showed CR adsorption on the nanocomposite was spontaneous and endothermic. The adsorption capacity of LNC/ MMT nanocomposite for CR was higher than those of

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LNC and MMT under the same conditions. Besides, the nanocomposite could be easily separated from the wastewater compared with LNC and MMT. In conclusion, the nanocomposite are quite cheaper and effective adsorbents for the removal of CR from wastewater. References [1]

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