Selective removal of cationic dye from aqueous solution by low-cost adsorbent using phytic acid modified wheat straw

Colloids and Surfaces A: Physicochem. Eng. Aspects 509 (2016) 91–98

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Selective removal of cationic dye from aqueous solution by low-cost adsorbent using phytic acid modified wheat straw Hui You a , Jiucun Chen a,b,∗ , Chao Yang c , Liqun Xu a a

Institute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, China Key Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, College of Pharmaceutical Sciences, Southwest University, Chongqing 400715, China c State Key Laboratory Breeding Base of Nonferrous Metals and Specific Materials Processing, College of Material Science and Engineering, Guilin University of Technology, Guilin 541004, China b

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• A low-cost adsorbent (PA-WS) was synthesized for methylene blue removal. • Adsorption capacities of PA-WS achieve to 205.4 mg g−1 at pH 10.0. • Adsorption mechanism of PA-WS for methylene blue was studied. • PA-WS presented excellent selective adsorption and regeneration-reuse property.

a r t i c l e

i n f o

Article history: Received 7 July 2016 Received in revised form 27 August 2016 Accepted 31 August 2016 Available online 31 August 2016 Keywords: Wheat straw Phytic acid Adsorption mechanism Selective adsorption Regeneration

a b s t r a c t In this paper, the wheat straw (WS) was modified using phytic acid (PA) to improve adsorption capacity for selective removal of methylene blue dye (MB, cationic dye). The morphology, structure and surface state of the modofied wheat straw using phytic acid (PA-WS) were characterized using scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), respectively. The effects of pH, MB concentration, different temperatures and contact time on adsorption experiments were investigated. The maximum adsorption quantity of PA-WS for MB was up to 205.4 mg g−1 at 25 ◦ C. Equilibrium adsorption isotherm data indicated a good fit to the Langmuir isotherm model and the adsorption kinetic was well-fitted by the pseudo-second-order model and the Elovich model. Regeneration study revealed that PA-WS can be reused effectively. These results indicated that PA-WS was a promising adsorbent for the removal of cationic dyes. © 2016 Elsevier B.V. All rights reserved.

1. Introduction ∗ Corresponding author at: Institute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, China. E-mail address: [email protected] (J. Chen). http://dx.doi.org/10.1016/j.colsurfa.2016.08.085 0927-7757/© 2016 Elsevier B.V. All rights reserved.

Nowadays, water pollution is becoming more and more serious because large quantities of wastewater are discharged into

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the environment. Thus, removal of various pollutants from industrial effluents has become a critical issue. Dyes, as main pollutants in industrial sewage, are widely used in textile, leather, tanning, paper, food processing, plastics, cosmetics, rubber, and printing [1]. Most of dyes are harmful to ecosystem owing to the carcinogenic compounds produced during the process of degradation. Various methods such as adsorption [2], flocculation [3], oxidation [4] and electrolysis [5] have been used in the removal of dyes from wastewater. Among these techniques for dyes removal, adsorption is more efficient and economical way than the others [6]. To date, varieties of materials have been explored as the adsorbent for dyes removal [7–10]. Among these, biological materials, especially agricultural by-products or waste have been the most popular due to its low-cost, biodegradability, such as rice husk [11], bagasse [12] and peanut shell [13]. Wheat straw (WS) is an abundant agricultural by-product and has been used as an adsorbent to remove contaminant from wastewater [14]. However, the raw wheat straw is not an excellent adsorbent for removing ionic dyes because of the lack of functional groups [15]. In order to improve its adsorption capacity, it is effective to introduce ionic groups onto the surface of wheat straw [16–18]. Phytic acid (PA, known as inositol hexakisphosphate), was discovered in 1903 [19]. It is a non-toxic naturally occurring organic acid, and is biocompatible and environment friendly which widely exist in cereals and grains [20,21]. The six phosphate groups of PA provide a variety of viable cross-linking site [22]. Hence, PA can be used to modify the surface properties of many substrates [23]. The objectives of this study are: (1) to modify wheat straw by phytic acid (PA-WS) and apply it as an adsorbent for the removal of cationic dyes from aqueous solutions; (2) to study the effects of pH, temperature, contact time and further explore the possible adsorption mechanism according to adsorption equilibrium and kinetics; (3) to investigate the selective adsorption for cationic dyes and regeneration. 2. Materials and methods 2.1. Materials The wheat straw used in this study was collected from local countryside, Chongqing, China. Methylene blue (MB), orange G (OG), urea, phytic acid (PA, 70 wt% in H2 O) were purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China). Dimethylformamide (DMF), hydrochloric acid (HCl) and sodium hydrate (NaOH) were purchased from commercial sources and used without further purification. 2.2. Preparation of adsorbent The wheat straw (WS) was grated and sieved to produce particles of 140–160 mesh. The sieved WS was then dried at 80 ◦ C for 24 h. The modified wheat straw using phytic acid (PA-WS) was synthesized via esterification as shown in Scheme 1. A certain amount of WS powder was immersed in DMF and the mixture was ultrasonically dispersed for 30 min. Then, a finite amount of PA and urea were added to the mixture under stirring at 60 ◦ C for 3 h. After cooling, the obtained PA-WS was filtered, washed thoroughly with deionized water and ethanol three times, respectively. Finally, PA-WS powder was dried at 60 ◦ C for 24 h.

sample was obtained as KBr disks on a Thermo Nexus 470 FTIR spectrometer. X-ray Photoelectron Spectroscopy (XPS) was performed using 200 W monochromated Al K␣ radiation (Thermo Scientic ESCALAB 250Xi). The binding energies were calibrated based on the graphite C1 s peak at 284.8 eV. The concentration of dyes in aqueous solution was measured by UV–vis spectroscopy (UV-2550, Shimadzu Corporation). 2.4. Batch adsorption equilibrium and kinetics experiments 2.4.1. Preparation of methylene blue (MB) solution Stock solution (1000 mg L−1 ) was prepared by dissolving MB in distilled water. MB solutions with different concentrations were obtained by diluting the stock solution with distilled water. 2.4.2. Effect of pH The effect of pH was studied by varying pH of MB solutions in the range of 2.0–10.0. The solution pH was adjusted by 0.1 M HCl or NaOH. Accordingly, adsorption experiments were measured by adding the PA-WS (0.05 g) and MB solutions (50 mL, C0 = 300 mg L−1 ) to 100 mL conical flask. And then, the mixture was shaked constantly at 30 ◦ C for 12 h. After reaching the adsorption equilibrium, the MB dye solutions were filtered using syringe filters. Then, the filtrate was analyzed by UV–vis spectra, which was detected at wavelength of 664 nm. The adsorption quantity at equilibrium qe (mg g−1 ) was calculated using the following equation: qe =

(C0 − Ce)V m

(1)

where C0 and Ce (mg L−1 ) are the initial and equilibrium concentrations of MB, respectively; V (L) is the volume of MB solution and m (g) is the dried weight of the adsorbent. 2.4.3. Equilibrium isotherms and kinetics The adsorption equilibrium study of PA-WS was conducted at different dye solution temperatures: 25 ◦ C, 35 ◦ C and 45 ◦ C, respectively. MB solutions (50 mL) with different initial concentration ranging from 100 to 900 mg L−1 were separately pipette into 100 mL conical flask, which contained PA-WS (0.05 g) powder. The MB solutions pH was adjusted to 10.0. After shaken for 12 h, the MB dye solutions were filtered by using syringe filters. The analysis method is similar with mentioned above. The adsorption quantity was calculated based on Eq. (1). The kinetics of adsorption was also conducted at different temperatures: 25 ◦ C, 35 ◦ C and 45 ◦ C, respectively. PA-WS (0.2 g) powder and MB dye solutions (200 mL, pH = 10.0; C0 = 300 mg L−1 ) were added into a 500 mL conical flask. Under shaking in a constant temperature oscillator, 3 mL of the sample solutions was taken out at desired intervals to trace the current dye solutions concentration. Meanwhile, 3 mL of distilled water was added into the bulk solution to keep the volume constant. The adsorption quantity q( ti) (mg g−1 ) at time ti was calculated by using the following equation: q(ti) =

(C0 − Cti )V0 − i−1 Ct(i−1) Vs 2 m

(2)

where C0 and Cti (mg L−1 ) are the dye initial concentration and dye concentration at time ti, respectively. V0 (L) is the volume of the dye solution. Vs (L) is the volume of the sample solution taken out each time, which equal to 0.003L in this equation. And m (g) is the mass of adsorbent.

2.3. Characterization of adsorbent 2.5. Selective adsorption The morphologies of WS and PA-WS were observed using a JSM-6510LV scanning electron microscopy (SEM) at an accelerating voltage of 15KV. Fourier transform infrared (FTIR) spectrum of

The selective adsorption experiment was carried out according to a reported method in the literature [24]. Concentrations of MB

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Scheme 1. Synthesis of PA-WS (cellulose as example).

(cationic dye, 50 mg L−1 ) and OG (anionic dye, 50 mg L−1 ) solutions were mixed uniformly at pH 7.0. In a typical procedure, PA-WS (0.05 g) powder was added to mixed dyes solutions (50 mL). After shaking at 25 ◦ C for 2 h, the dyes solutions was filtered by using a syringe filter and the concentration was recorded by UV–vis spectra.

2.6. Regeneration study The MB-loaded adsorbents from adsorption kinetics study were recovered using 0.1 M HCl aqueous solution, and then collected from the solutions via filtration, washed with distilled water and dried. Moreover, the collected adsorbent was reused in the next cycle of adsorption experiments. Four cycles of consecutive adsorption-desorption studies were conducted. All the adsorption-desorption experiments were repeated three times, and the average values were obtained at 30 ◦ C.

3. Results and discussion 3.1. Characterization of the adsorbent The surface morphology of the adsorbent can be examined through SEM images. Fig. 1 represented the SEM micrographs of WS and PA-WS. There was a denser layer structure and smooth surface on WS shown in Fig. 1A. It was found in Fig. 1B that the surface of PA-WS became rough and the specific surface area was increased, which was beneficial to improve the adsorption capacity. This phenomenon may be caused by the hydrolysis of cellulose. FTIR spectra of WS and PA-WS were obtained to compare the difference of functional groups between WS and PA-WS. As shown in Fig. 2A, the peaks around 3390, 2920, 1060 cm−1 appeared in WS and PA-WS, that were originated from the stretch vibrations of O H in the hydroxyl groups, C H in the methylene groups, and C O C in the glucose rings of cellulose and lignin, respectively. Compared to those of WS, the extra peaks at 1240 cm−1 and 1080 cm−1 (shown in Fig. 2A) appeared and corresponded to the stretch vibrations of P O and P OH, respectively. It was conformed that PA introduced into wheat straw after modification. The XPS technique was further applied to test the surface state of PA-WS. The strong C 1s and O 1s peaks in PA-WS spectrum shown in Fig. 2B and C indicated that carbon and oxygen were the main elements. And it also contained a small amount of phosphorus in Fig. 2D, which can be taken as a clear evidence for the phytic acid on the surface of wheat straw. Moreover, the relative concentrations of C, O and P elements were 71.7%, 25.8%, 2.5%, respectively.

Table 1 Adsorption capacities of various adsorbents for MB. Adsorbent

Optimum pH

q (mg g−1 )

Ref.

PA-WS WS MWS EWS Modified WS Rice husk Rice husk ash Sorghum/PPy Bamboo charcoal Actived carbon

10.0 7.05 7 7 10.0 8.0 7.0 10.0 5.0 9.0

205.4 60.66 62.9 129.87 138 40.58 16.26 143.5 29.26 315

This study [25] [26] [27] [28] [11] [29] [30] [31] [32]

3.2. pH effect on adsorption of methylene blue The pH of dye solution is one of the most important parameters that might influence the adsorption performance of ionic-type adsorbents. Thus, the adsorption of MB on PA-WS as a function of pH (2–10) was studied and the results were presented in Fig. 3. As shown in Fig. 3, with the increasing of initial solution pH, the MB adsorption quantity onto PA-WS distinctly increased. Due to the anionic group ( PO4 3− ) introduced onto the surface of WS by PA, the adsorbent may exist a good affinity with cationic dyes. The main reason for this was probably that electrostatic attraction was the primary driving force between adsorbent and dyes. Protonation of PO4 3− was responsible for the unsatisfactory adsorption quantity of MB at low pH range. When the pH increased to 10, the maximum adsorption quantity of PA-WS for MB reached approximately 205.4 mg g−1 . This may be attributed to deprotonation of the PO4 3− . For comparison, the adsorption capacities of MB by the different adsorbents were summarized in Table 1. Most of the maximum MB uptakes by different adsorbents were conducted at neutral or alkaline condition. In this study, the MB adsorptions were mostly favored at pH 10.0, which was similar to those in Table 1. In particular, it was obvious that PA-WS had more superior adsorption quantity comparing with unmodified wheat straw (WS) [25] and wheat straw modified by other modifiers [26–28]. It was also more better than other materials [11,29–31] in maximum adsorption capacity and more low-cost than actived carbon [32]. 3.3. Adsorption equilibrium study The adsorption equilibrium isotherms are essential to describe the adsorption process. According to Fig. 3, the maximum adsorption quantity of PA-WS took place at pH 10.0 for MB. Therefore, the adsorption equilibrium studies were conducted at pH 10.0. The adsorption isotherms for MB onto PA-WS that relied on the values of Ce and qe at three different temperatures (25 ◦ C, 35 ◦ C, 45 ◦ C)

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Fig. 1. SEM images of WS (A) and PA-WS (B).

Fig. 2. FTIR spectra of WS and PA-WS (A), and XPS spectra of PA-WS (O 1 s (B), C 1 s (C) and P 2p (D)).

were shown in Fig. 4a. There were no significant differences for MB in various temperatures from Fig. 4a. It may be due to lack of physical adsorption, which is usually notably affected by temperature. Additionally, adsorption equilibrium provides fundamental data for investigating the applicability of the adsorption process. In the present study, Langmuir, Freundlich and DubininRadushkevich (D-R) isotherm models were used to describe the adsorption mechanism. The Langmuir model [11] is mainly applied to describe the monolayer adsorption equilibrium. The models of Langmuir are expressed in the following:

restricted to the formation of the monolayer. The Freundlich model can be expressed as follow:

1 Ce Ce = + qe qmKL qm

where qe (mg g−1 ), Ce (mg L−1 ) is the adsorption quantity and dyes concentration at equilibrium, respectively. qm (mg g−1 ) is the theoretical saturation capacity, KL (L mg−1 ) is the Langmuir constant. Kf is the Freundlich constant and 1/n is the heterogeneity factor. KDR (mol2 kJ−2 ) is the D-R isotherm constant related to the mean

(3)

The Freundlich model [11] is an empirical model for a heterogeneous system, which describes reversible adsorption and is not

ln qe = ln Kf +

1 ln Ce n

(4)

The D-R model [33] could provide significant information about adsorption mechanisms, physical or chemical processes, from the parameter (E) in the following equations: ln qe = ln qm − KDRε2 ε = RT ln(1 +

1 ) Ce

(5) (6)

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95

Elovich model: qt = A + B ln t

Fig. 3. (a) The effect of pH on MB adsorption by PA-WS at 30 ◦ C.

free energy of adsorption, and ε is the Polanyi potential, the D-R constant can provide valuable information on the mean energy of adsorption through the following equation: 1 E= √ 2KDR

(7)

The adsorption behavior is described as physical adsorption when E < 8 kJ mol−1 or chemical adsorption when E = 8–16 kJ mol−1 . The nonlinear curves of the Langmuir model and the Freundlich model in three temperatures using nonlinear regression were shown in Fig. 4b–d. All relative parameters of each isotherm equation were listed in Table 2, respectively. From Table 2, the corresponding R2 of the Langmuir model all are much higher than those of the Freundlich model. According to the Langmuir isotherm model, the adsorption maybe occurred at a monolayer adsorption mechanism instead of heterogeneous system. Meanwhile, the calculated E for MB was higher than 8 kJ mol−1 at various temperatures. This phenomenon indicated that the adsorption behavior followed the chemical adsorption process. 3.4. Adsorption kinetics study The effect of adsorption time is also very important, especially in practical application. Thus, the relationship between adsorption time and amounts at different temperature: 25 ◦ C, 35 ◦ C and 45 ◦ C were studied and the results were shown in Fig. 5a. As Fig. 5a, the adsorption process could be divided into three stages. The amounts of MB removal increased rapidly at the first stage (0–10 min) because of the rapidly attachment of MB onto PA-WS. The second stage (10–50 min) was slower due to intraparticle diffusion of MB to the internal matrix of PA-WS, while on the third stage (>50 min), the qt (mg g−1 ) stayed stable, which was the indicative of equilibrium. Meanwhile, to further investigate the adsorption behavior, peseudo-first-order model, peseudo-second-order and Elovich equations [34] were applied to fit the kinetics data using nonlinear regression and the nonlinear curves of each kinetic model in three temperatures were shown in Fig. 5b–d, respectively. The representation of kinetics equations were shown in the following: Pseudo-first-order model qt = qe(1 − e−k1t )

(8)

Pseudo-second-order model: qt =

k2qe2 t 1 + k2qet

(9)

(10)

where qt (mg g−1 ) is the amount of dye adsorbed at the time t; qe (mg g−1 ) is the adsorption quantity at equilibrium; k1 (min−1 ) is the pseudo-first-order kinetic rate constant; k2 (mg g−1 min−1 ) is the pseudo-second-order kinetic rate constant; A and B are the constants relating to fraction of the surface covered and chemisorption activation energy. The pseudo-first-order kinetic model is a hypothesis that adsorption rate is proportional to the difference between qe and qt . The pseudo-second-order kinetic model is based on the adsorption quantity of the solid phase, and it is also considered as a chemical adsorption process involving electron sharing or transfer between adsorbent and adsorbate. The Elovich model is used to describe the ion exchange process where the activation energy has a great change. The fitting parameters of each kinetic model were listed in Table 3. It was observed that the value of R2 for the pseudo-secondorder kinetic model was higher than those of the pseudo-first-order kinetic model. The higher correlation coefficients (R2 ) illustrated that the pseudo-second-order kinetic model was more suitable than the pseudo-first-order kinetic for the removal of MB by PA-WS. In addition, all the value of R2 was higher than 0.99 for the Elovich model, which showed the suitability. Thus, the adsorption system of MB on PA-WS was a chemical process with an ion exchange. 3.5. Selective adsorption An adsorbent selective for dyes in some specific applications is very useful. Owing to the phosphate (anionic group) on the surface of PA-WS, it may selective adsorbed cationic dyes from the mixture of dyes. Fig. 6a (1) and (3) were methylene blue (cationic dye, MB) and orange G (anionic dye, OG), respectively. Fig. 6a (2) was the mixture of MB and OG. As shown in Fig. 6b, the original mixture of MB and OG showed two obvious characteristic absorption peaks at 664 nm and 475 nm by the UV–vis spectrophotometry. After adsorption experiment, the adsorption peak of MB disappeared, while the OG peak showed the little change. This phenomenon demonstrated the adsorbent had a better selectivity in removing cationic dye. 3.6. Regeneration study Recovery and regeneration of exhausted adsorbent will reduce environment load [35]. For this purpose, the dyes adsorption and desorption were repeated for four times to examine cycle of application. The MB-loaded adsorbents were recovered from 0.1 M HCl. Fig. 7a was the PA-WS powder after MB adsorption, while Fig. 7b was the PA-WS powder after recovering. The regeneration results were shown in Fig. 7c, the adsorption rates for the first, second, third and fourth regeneration cycles were 93.4%. 80.7%, 73.3% and 68.5%, respectively. This implied that ion exchange and electrostatic attraction were dominant in adsorption process. It also meant that phosphate group on PA-WS was stable enough. 4. Conclusions In conclusion, PA-WS is an excellent adsorbent for the removal of methylene blue, which had definite selectivity for cationic dyes simultaneously. The maximum adsorption capacity was up to 205.4 mg g−1 at experimental condition, and the adsorbent can be recycled. The adsorption process is dependent on pH but showed no significant at varied temperature. In addition, the Langmuir adsorption isotherm model and the pseudo-second-order kinetic model

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Table 2 Parameters of adsorption isotherm models for MB adsorption. T/◦ C

25 ◦ C 35 ◦ C 45 ◦ C

Langmuir model

Freundlich model

D-R model

qm (mg g−1 )

KL (L mg−1 )

R2

Kf

1/n

R2

E(kJ mol−1 )

R2

217.4 220.9 215.9

0.4333 0.2399 0.08813

0.9171 0.9804 0.9708

101.25 96.17 84.97

0.1137 0.1282 0.1468

0.8267 0.8433 0.7933

12.25 8.81 8.69

0.8644 0.8884 0.8807

Fig. 4. (a) Adsorption isotherms of MB on PA-WS at 25 ◦ C, 35 ◦ C and 45 ◦ C; (b), (c) and (d) the fitting curves of the Langmuir model and Freundlich model at 25 ◦ C, 35 ◦ C, 45 ◦ C using nonlinear regresion, respectively.

Fig. 5. (a) Adsorption kinetics of MB on PA-WS at 25 ◦ C, 35 ◦ C and 45 ◦ C; (b), (c) and (d) the fitting curves of each kinetic model at 25 ◦ C, 35 ◦ C, 45 ◦ C using nonlinear regresion, respectively.

H. You et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 509 (2016) 91–98

Fig. 6. (a) MB (1), OG (3) and mixed solutions (2), (b) adsorption spectra of the mixed solution of MB and OG before and after adsorption by PA-WS.

Fig. 7. (a) PA-WS powder after MB adsorption, (b) PA-WS after recovering and (c) regeneration of PA-WS for four cycles.

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Table 3 The kinetic parameters for MB adsorption on PA-WS. T/◦ C

25 ◦ C 35 ◦ C 45 ◦ C

Pseudo-first-order model

Pseudo-second-order model

k1 (s−1 )

R2

k2 (10−3 min−1 )

R2

A

B

R2

0.1210 0.1398 0.1361

0.9693 0.9725 0.9708

4.669 6.751 9.945

0.9903 0.9912 0.9902

94.81 107.45 105.03

21.88 18.96 19.42

0.9902 0.9916 0.9905

fitted well to describe the adsorption behavior, which belonged to the monolayer chemical adsorption with an ion-exchange. Conflicts of interest The authors declare that they have no conflict of interest. Acknowledgment This work is financially supported by the Fundamental Research Funds for the Central Universities (SWU 113075 and XDJK2014B015). References [1] M.T. Yagub, T.K. Sen, S. Afroze, H.M. Ang, Dye and its removal from aqueous solution by adsorption: a review, Adv. Colloid Interface Sci. 209 (2014) 172–184. [2] C. Li, J. Cui, F. Wang, W. Peng, Y. He, Adsorption removal of Congo red by epichlorohydrin-modified cross-linked chitosan adsorbent, Desalin. Water Treat. 57 (2015) 1–7. [3] Y. Tian, B. Ju, S. Zhang, L. Hou, Thermoresponsive cellulose ether and its flocculation behavior for organic dye removal, Carbohydr. Polym. 136 (2016) 1209–1217. [4] N. Serpone, S. Horikoshi, A.V. Emeline, Microwaves in advanced oxidation processes for environmental applications, J. Photochem. Photobiol. C 11 (2010) 114–131. [5] J. Vidal, L. Villegas, J.M. Peralta-Hernández, R. Salazar González, Removal of acid black 194 dye from water by electrocoagulation with aluminum anode, J. Environ. Sci. Health Part A 51 (2016) 1–8. [6] W.W. Ngah, L.C. Teong, M.A.K.M. Hanafiah, Adsorption of dyes and heavy metal ions by chitosan composites: a review, Carbohydr. Polym. 83 (2011) 1446–1456. [7] S. Liu, D. Chen, J. Zheng, L. Zeng, J. Jiang, R. Jiang, G. Ouyang, The sensitive and selective adsorption of aromatic compounds with highly crosslinked polymer nanoparticles, Nanoscale 7 (2015) 16943–16951. [8] A. Negrulescu, V. Patrulea, M. Mincea, C. Moraru, V. Ostafe, The adsorption of tartrazine, congo red and methyl orange on chitosan beads, Dig. J. Nanomater. Biostruct. 9 (2014) 45–52. [9] H. Najafi, E. Pajootan, A. Ebrahimi, M. Arami, The potential application of tomato seeds as low-cost industrial waste in the adsorption of organic dye molecules from colored effluents, Desalin. Water Treat. 57 (2015) 1–11. [10] Z. Xue, S. Zhao, Z. Zhao, P. Li, J. Gao, Thermodynamics of dye adsorption on electrochemically exfoliated graphene, J. Mater. Sci. 51 (2016) 4928–4941. [11] V. Vadivelan, K.V. Kumar, Equilibrium kinetics, mechanism, and process design for the sorption of methylene blue onto rice husk, J. Colloid Interface Sci. 286 (2005) 90–100. [12] Y.Z. Wang, L.Y. Zhang, Y. Mo, G.T. Wei, Z.M. Li, K. Huang, Y. Deng, Preparation of a Low-Cost adsorption material from red mud and bagasse, Mater. Manuf. Process. 31 (2016) 162–167. [13] P. Wang, Q. Ma, D. Hu, L. Wang, Adsorption of methylene blue by a low-cost biosorbent: citric acid modified peanut shell, Desalin. Water Treat. 57 (2015) 1–9. [14] Q. Hou, B. Yang, W. Liu, H. Liu, Y. Hong, R. Zhang, Co-refining of wheat straw pulp and hardwood kraft pulp, Carbohydr. Polym. 86 (2011) 255–259. [15] W. Zhang, H. Li, Kan, L. Dong, H. Yan, Z. Jiang, R. Cheng, Adsorption of anionic dyes from aqueous solutions using chemically modified straw, Bioresour. Technol. 117 (2012) 40–47.

Elovich model

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