Adsorptive Removal Of Cationic Methylene Blue Dye.pdf

Accepted Manuscript Title: Adsorptive removal of cationic methylene blue dye using carboxymethyl cellulose/k-carrageenan/activated montmorillonite composite beads: Isotherm and kinetic studies Authors: Chao Liu, A.M. Omer, Xiao–kun Ouyang PII: DOI: Reference:

S0141-8130(17)31560-X http://dx.doi.org/10.1016/j.ijbiomac.2017.08.084 BIOMAC 8076

To appear in:

International Journal of Biological Macromolecules

Received date: Revised date: Accepted date:

1-5-2017 29-7-2017 13-8-2017

Please cite this article as: Chao Liu, A.M.Omer, Xiao–kun Ouyang, Adsorptive removal of cationic methylene blue dye using carboxymethyl cellulose/k-carrageenan/activated montmorillonite composite beads: Isotherm and kinetic studies, International Journal of Biological Macromoleculeshttp://dx.doi.org/10.1016/j.ijbiomac.2017.08.084 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Adsorptive removal of cationic methylene blue dye using carboxymethyl cellulose/ k-carrageenan/activated montmorillonite composite beads: Isotherm and kinetic studies Chao Liua, A. M. Omera,b, Xiao–kun Ouyanga* a

School of Food and Pharmacy, Zhejiang Ocean University, Zhoushan 316022, P.R.

China b

Polymer Materials Research Department, Advanced Technology and New Materials

Research Institute, SRTA-City, New Borg El-Arab City, P.O. Box: 21934, Alexandria, Egypt *Corresponding Author: Xiao–kun Ouyang Tel.:

+86–580–2554781.

Fax:

+86–580–2554781.

[email protected] (Xiao–kun Ouyang).

E–mail

address:

Highlights 

CMC/kC/AMMT composite beads were developed for the adsorption of MB dye.



The adsorption process has been conducted under different adsorption conditions.



CMC/kC/AMMT composite beads adsorb more MB dye than plain CMC and kC beads.



Results have been tested by using isotherm and classical kinetic models.

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CMC/kC/AMMT composite beads were successfully reused for five adsorption-desorption cycles.

Abstract In the present study, new composite beads based on carboxymethyl cellulose (CMC)/k-carrageenan (kC)/activated montmorillonite (AMMT) were prepared for adsorptive removal of cationic methylene blue (MB) as a dye model. The structure and morphology of the composite beads were investigated by FT-IR and SEM, while the thermal properties were tested using TGA. Factors affecting the removal percent of MB such as CMC/kC/AMMT ratios, initial MB concentration, pH medium, adsorbent dosage, solution temperature, and agitation speed were also explored. Results demonstrated that MB removal (%) exceeded 92% after 120 min using

CMC/kC/AMMT (1:1:0.4 ratio) compared to 69% in case of free AMMT beads. Moreover, data obtained from isotherm studies were fitted well to Langmuir model (R2=0.999), and the kinetics of adsorption followed pseudo-second order model. Finally, the composite beads showed good reusability for MB dye removal with high efficiency. Results obtained from this study suggest that the prepared composite beads could be applied effectively for removing cationic dyes from aqueous solutions. Keywords: Carboxymethyl cellulose, k-Carrageenan, Activated montmorillonite, Dye removal, Isotherm, Kinetics

1. Introduction Many chemical industries such as pharmaceutical, textile, polymers, refineries, plastic, and leather use diverse types of dyes. Usually, these industrial processes discharge 30-40 % of dyes which remain in the wastewaters [1]. Dyes molecules are chemically stable and difficult to biodegradable naturally [2]. However, these organic contaminants are carcinogenic, hazardous and highly-toxic [3] and may cause solemn threat not only for human health but also for marine life even at low concentrations. Methylene blue (MB) has been considered as a common cationic dye model in the adsorption studies owing to its planar form [4]. This structure makes MB readily

aggregates and highly soluble in solutions even at micro molar concentrations causing harmful effects. Additionally, a revelation to MB can lead to breathing difficulties, eyes burn, vomiting, nausea, and mental bewilderment [5]. That’s why; the treatments of these dyes effluents before its release into water resources are crucial [6]. Several conventional techniques have been used for treating wastewaters containing dyes and heavy metals including coagulation, flocculation, oxidation, adsorption, membrane separation, biological, and electrochemical methods [4, 7, 8]. Adsorption method is considered one of the most superior methods adopted for removing toxic dyes from their aqueous solutions [9]. Also, it is a highly efficient technique without the formation of any harmful by-products. Therefore, seeking for renewable, efficient and low-cost adsorbents materials is still in progress. Regarding the economic feasibility and environmental importance both synthetic and natural biopolymers are attracting much interest [10]. Moreover, the development of natural, cheap, biocompatible, and biodegradable polymeric sorbents [11] are an extraordinarily growing field in adsorbent materials research as sorbents based on synthetic polymers are highly-priced and have difficulties in regeneration. Polysaccharides such as alginate (Alg), chitin, chitosan (CS), cellulose, and carrageenan (kC) are widely used in wastewater treatment [12-14]. Recently, researchers have been committed to developing polymeric hydrogel beads containing numerous functional groups in its molecular structures as the most promising adsorbent materials for dyes molecules. These beads show adequate mechanical stability, specific large surface area, uniform shape and particle size [15, 16]. Carboxymethyl cellulose (CMC) is anionic polyelectrolyte material which is sensitive to a change in solution pH, temperature, and ionic strength, etc. On the other hand, CMC beads may be crosslinked with epichlorohydrin [17], bifunctional epoxy [18], and acids [19]. However, purely

crosslinked crystalline CMC beads have a weak mechanical stability and limited swelling degree. Thus, it was favorable to modify CMC via blending [20], grafting [21], forming an interpenetrating network (IPN) and composites with other polymers, or immobilizing clays [22] and metals. Due to the existence of carboxylic groups in the CMC structure, it has been effectively used in wastewaters treatment processes for removing cationic dyes through the electrostatic- interactions forces between the negatively charged carboxylate ions and positively charged cationic dyes [23]. Where, CMC has been prepared from sugarcane bagasse and used for the removal of Methylene blue (MB) dye [24]. Furthermore, CMC has been grafted with acrylic acid and applied as an adsorbent for disperse blue 2BLN, methyl orange, and malachite green chloride dyes, and the removal (%) reached to 79.6%, 84.2%, and 99.9%, respectively [25]. Similar to CMC, carrageenan is a linear sulfated polysaccharide obtained from red seaweed. It is classified according to the number and position of ester sulfate group which determine their outstanding properties to three forms namely; lambda (none gelling), kappa (strong gelling), and iota (weak gelling). Also, the gelling of kappa-carrageenan (kC) may occur chemically or physically [26, 27]. The electrostatic interaction between sulfate anions on carrageenan backbone with metal cations, poly cationic polyelectrolytes especially chitosan [28], alginate [29], and CMC [30, 31] are considered classes of physical crosslinking. As a result, it may be widely used as cost effective stabilizer, thickener, texture modifier, moisture retainer, drug delivery applications [32]. Kappa-carrageenan beads have been applied for removing of crystal violet dye [33]. In addition, carrageenan based nanocmoposite, magnetic carrageenan/silica hybrid nano-adsorbent and poly (sodium acrylate)- carrageenan/Na-montmorillonite nanocomposite superabsorbents have been developed and used for the removal of cationic dyes [34-36]. Natural clays and their

modified forms have been conducted for removing of metal ions from their aqueous solutions. The great advantage in the modified clays as adsorbent materials is mostly related to the large specific areas accompanying with their layered structure. In addition, acid activation of clays can generate large number of active sites, increasing porosity, and eliminates mineral impurities, increasing their adsorption affinity toward organic pollutants and cationic species [37-39]. Where, modified montmorillonite and kaolinite clays have been applied for removing Zn (II) [40], Cu (II) [41] and Pb (II) [42] ions from their aqueous solutions. However, due to their colloidal dimensions, regeneration and reusability of these clays are difficult. So, encapsulation of modified clays into the polymeric matrix seems as a good solution for this problem to get suitable dimensions with low cost composite beads. Among these clays, montmorillonite (MMT) which have been used in the preparation of polymer composites as reinforcing fillers [43]. Dispersion of inorganic layered silicates into polymeric networks can enhance strength of the resultant composite beads and reduce the final production cost. Montmorillonite also have been used with polysaccharides such as PVA, kC, and CS as composite polymers for dye adsorption [44, 45]. The aim of this work was to prepare non-toxic and eco-friendly CMC/ k-carrageenan/ activated MMT composite beads for adsorption of methylene blue (MB). The developed composite beads were verified their structures, morphologies, and thermal stability using different characterization tools. The impact of different adsorption conditions on the removal percentage and the adsorption capacity was studied. Furthermore, reusability, adsorption isotherms, and kinetic studies were evaluated. 2. Experimental 2.1. Materials

Carboxymethyl cellulose sodium salt (CMC; Mw 90000, DS 0.7), Kappa-Carrageenan (kC. Montmorillonite K-10 (nitrogen (external) specific surface area=240m2/g), H2SO4, FeCl3. 6H2O, and KCl were purchased from Aladdin Industrial Corporation (Shanghai, China). Methylene blue tri-hydrate (MB) was supplied from (NICE CHEMICALS Pvt. Ltd., COCHIN) as a model of organic dye to evaluate the adsorption process, and its characteristics are presented in Table 1. All other reagents (analytical grade) used in this study were also supplied from Aladdin Industrial Corporation and were used without further purification. 2.2. Preparation of activated montmorillonite (AMMT) Activation of montmorillonite (AMMT) clay was achieved using acid treatment process [38] for replacing the exchangeable cations with H+ ions of acid solution. In addition, Al3+ and the other cations releases during this process from the octahedral and tetrahedral layers of MMT clay leaving SiO4 groups mostly intact. According to the standard procedure [46], 10 g of MMT clay were refluxed for 3 h with 100 mL of 0.25M H2SO4. Then, the resulting acidified clays were centrifuged and washed several times with distilled water until they were free of SO42- and dried at 110 oC in an oven until reach to a constant weight. Finally, the prepared activated montmorillonite (AMMT) clay was thermally treated using calcination at 500 oC for 10 h before its use. 2.3. Preparation of composite beads CMC and kC biopolymer solutions were firstly prepared separately by dissolving each of them in distilled water at 50 oC (for CMC) and 70 oC (for kC) with continuous stirring for 1 h to reaches complete solubilization. Different weights of CMC: kC with the ratios of 2:0, 1.5:0.5, 1:1, 0.5:1.5 and 0:2 (w/w) were mixed and stirred at 70 oC for 30 min to have final concentration 2 % for both biopolymers. Then, a known

amount of AMMT (0-0.6 %) was dispersed in distilled water under stirring at 70 oC for 10 min, then added to the polymers solution with a highly stirring rate at the same temperature for another 30 min. Thereafter, the composite mixture was sonicated at 70 oC for 20 min to obtain a homogenous solution. To obtain composite beads, the temperature of the composite mixture decreased firstly to 40 °C, then dropped into a gelling salt solution mixture containing 3 % FeCl3 – 3 % KCl using a fine glass syringe (5 cm3) and allowed to harden for 30 min at 40 °C with a constant gentle stirring (50 rpm). It may be noted that the gelling salt solution in the case of CMC: kC (2:0) was only 3 % FeCl3, while in the case of CMC: kC (0:2) was only 3 % KCl. After complete of gelation, the resultant spherical beads were collected and washed several times to remove the un-participated constituents. Finally, the composite beads were left to dry at room temperature for constant weight. The prepared composite beads display a moderately smooth and uniform surface. Scheme for synthesis of CMC/kC/AMMT composite beads, freshly prepared wet and dried composite beads was represented in Fig.1.

2.4. Physicochemical characterization The chemical structure of the developed adsorbent was investigated using Fourier Transform Infrared Spectroscopy (FTIR, Nicolet 6700 spectrometer, Japan). Their thermal degradation behaviors were studied using thermal gravimetric analyzer (TGA, Model 50/50H, Shimadzu, Japan). Additionally, the changes in the surface morphology for the developed composite beads were investigated using scanning electron microscopy (SEM, Hitachi Limited, Japan,). 2.5. Dye adsorption studies

The adsorption experiments of MB dye experiments were achieved; a stock solution of MB dye (1000 mg/ L) was firstly prepared. Known amounts of dried adsorbent beads (0.05-0.25 g) were thoroughly added to 50 mL of MB dye solutions which prepared with final concentrations ranged from 10 to 500 ppm. pH medium was initially adjusted using HCl and NaOH (0.1 M) to have final values ranged from 2 to 10. The adsorption process was conducted in a shaking water bath at different temperatures (25-45 °C), and different shaking rate (50-200 rpm). At every 15, 30, 60, 120, 240, and 300 min, samples were collected and filtered regularly, and the remaining MB concentration was determined at wavelength 664 nm using UVspectrophotometer. Digital photos for MB dye before and after adsorption were also shown in Fig. 1. Before the measurement, a calibration curve of MB with known concentrations was obtained. The percent removal and the adsorbed capacity (q) of MB were calculated as follow: Removal (%) 

q t (mg / g ) 

C 0 C t  100 C0

C C V t

0

W

(1)

(2)

Where qt is the adsorption capacity (mg/g) at time t, Co is the initial concentration of MB, Ct is the final concentration of MB at time t (ppm), V is the solution volume of MB (L), and W is the weight of the adsorbent sample (g). 2.5.1. Adsorption isotherm models Adsorption isotherms get their beneficial use from their applicability to identify the interaction between the adsorbent materials and adsorbate of a given system. Several models can be applied to provide information about the adsorption mechanism as well as the adsorbent surface properties and affinities [47, 48]. The most accepted models

for single solute systems are Freundlich and Langmuir models [49]. Adsorption isotherms study was achieved by adding 0.1 g of the composite beads into 50 mL of MB solutions with final concentrations 10, 25, 50, 100, 200, and 500 mg L-1. Where q and Ct are replaced in Eq. 2 with the equilibrium adsorption capacity (qe) and equilibrium concentration (Ce) of MB in the solution respectively. Thus, data were applied to Langmuir and Freundlich isotherm equations. Where Langmuir model originally suggest that the adsorption process occurs in a monolayer or occurs at the identical surface sites with a fixed number [50], this model can be investigated as the following equation: Ce C 1  qmax  e qe K L qmax

(3)

where Ce is MB concentration at equilibrium (mg/L), qe is the adsorption capacity at equilibrium (mg/g), qmax is the monolayer capacity of adsorbent (mg/g), and KL is Langmuir constant (L/mg). The linear plot of (Ce/qe) against Ce gives the values of qmax and KL from the slope and intercept of the plot. The effect of isotherm profile has been deliberated to predict whether an adsorption system is favorable or unfavorable by means of ‘RL’, a dimensionless constant denoted to the separation factor RL and can be determined as follow [51, 52] RL 

1 1  K L Co

(4)

On the other hand, Freundlich isotherm is a fairly acceptable empirical isotherm model can describe the heterogeneity of sorbent surface [53], and can be expressed as follow: qe= KF Ce 1/n

(5)

The linear form is written as: ln qe  ln K F 

1 ln Ce n

(6)

Where qe is the adsorbed MB amount (mg/g) at equilibrium, Ce is MB concentration at equilibrium (mg/L), KF is Freundlich constant. The magnitude of the exponent (n) provides an indication about the capacity of the adsorbent- adsorbate system [54]. 2.5.2. Adsorption kinetic models A kinetic study has a great significant consideration in the adsorption process as it offers information about adsorption and mass transfer mechanism. Three different models were applied as following: a- The pseudo-first order model The pseudo-first order model can be expressed by the following integrated linear form [55]:

ln qe  qt   ln qe  k1t

(7)

where qe is the amount of dye adsorbed at equilibrium, mg/g, qt is the adsorbed amount at time t (mg/g), and k1 is the rate constant (min−1). The plot of ln (qe − qt) against t was used to test this model. b- The pseudo-second order model The equation of this kinetic model can be written as follow: dqt 2  k 2 qe  qt  dt

(8)

By rearranged equation (9) the linear form is obtained as follows [56]:

t 1 t   2 qt k 2 qe qe

(9)

where k2 is the rate constant of pseudo-second order (gmol-1min-1) as determined from the t/qt vs. t plot. The initial rate of adsorption rate, h (molg-1min-1) can be described as follow:

h  k2 qe2

(10)

c- Intraparticle diffusion model Rate constant (kp) is given by the following equation [57]:

qt  k p t 0.5  C

(11)

where kp is the rate constant of intraparticle diffusion model (mol/g min1/2). The value of C gives a good impression about the boundary layer thickness i.e.; the boundary layer effect is greater at a higher value of C. 2.6. Reusability study Reusability of the prepared composite beads for MB adsorption was achieved via adsorption–desorption cycles. The indicator for reusability of the developed composite beads is related to the ability of these composite beads for regeneration. Since we can use it repeatedly for the adsorption of MB dye molecules using a suitable desorption solution. After complete the adsorption process, the dye adsorbed composite beads were separated from the batch adsorption runs and immersed in a solution of desorption agent (50 mL) containing HCl (1M)/ethanol (98%). The desorption process was carried out using shaking water bath with shaking rate 50 rpm for 3 h at 30 °C. Where, a protonation reaction occurs between H+ ions that existing in the regenerative solution and the present anionic groups at the surface of CMC/kC/AMMT composite beads. Consequently, the ionic interaction between these

anionic groups and cationic MB dye is demolished and followed by composite beads regeneration. Therefore, the ability of these beads to retain good adsorption properties after several adsorptions-desorption cycles is the monitor for their reusability. Finally, the ability of the composite beads to reuse was studied for 5 consecutive cycles. 3. Results and discussion 3.1. Beads characterization 3.1.1. FTIR analysis FTIR spectra for native CMC, kC, AMMT clay, and CMC/kC/AMMT composite beads were obtained to get clear information regarding their chemical structures as shown in Fig. 2. The spectra of CMC showed abroad absorption at 3450 cm -1 and 2920 cm-1, due to the stretching frequency of the O-H and C–H. Besides, broad asymmetrical and symmetrical vibrations at 1630 cm-1 and 1420 cm-1 were present due to the characteristic stretching of the carboxylates anions. The bands around 1326 cm-1and 1060 cm-1 were assigned to –OH bending vibration and sugar ring absorption,

respectively.

Compared

with

the

spectra

of

CMC

the same peaks in kC spectra not only appeared, but also the basic characteristic functional groups, where bands at 1380 cm-1, 1261 cm-1, 925 cm-1, and 846 cm-1 were investigated. These bands are attributed to the presence of sulfate groups and corresponding to the sulfonic acid group, C–O stretching band，3, 6-anhydro-Dgalactose and glycosidic linkages of kC backbone, respectively. For AMMT clay, the peak at 3750 cm-1and 3450 cm-1 could be attributed to the surface structural O-H groups of layered aluminosilicates and adsorbed water [39]. Moreover, bands at 2920 cm-1, 1630 cm-1, 1060 cm-1，1030 cm-1, and 798 cm-1 due to the asymmetric stretching vibration of C–H bonds , the bending vibration of water, Si–O–Si stretching

vibration for silicates, and Si–O bonds respectively. On the other hand, from the IR spectra of CMC/kC/AMMT composite beads, it was observed that the signal hydroxyl peak at about 3450 cm-1 became stronger, that could be explained by the superposition of the stretching vibration of O–H groups in all materials. Also, the bands at 2920 cm-1, 1630 cm-1, and 1420 cm-1 represent C-H, C=O asymmetric and symmetric stretching. The observed peaks at 1380 cm-1and 1260 cm-1 attributed the sulfate groups. Additionally, sharpening of the bands at about 1060 cm-1, 920 cm-1, 850 cm-1, and 720 cm-1 were observed and can be denoted as Si–O and Al-O stretching vibration in the presence of quartz and aluminosilicates. 3.1.2. TGA Thermal stability of the prepared beads was studied by thermal gravimetric analysis and as shown in Fig. 3 and Table 2. As can be seen, the apparent weight loss from all studied samples was assigned at the first degradation stage (0-150 °C). This could be attributed to the loss of moisture content as hydrogen-bound water to the polysaccharide structure (i.e. CMC and kC) as well as dehydration of AMMT clay at the ambient temperature (150 °C). With increasing temperature, the rate of weight loss increased and both second and third degradation stages took place beyond 200 °C. The observed weigh loss could be related to the release of water more firmly bound through the polar interactions with the carboxylate groups (in case of CMC) and sulfate groups (in case of kC), in addition to the loss of CO 2 from the polysaccharides (CMC and kC) and decomposition of the cyclic products. On the other hand, the developed CMC/kC/AMMT composite beads were found more thermally stable with increasing temperature than the native polymer beads. Where, the temperature required for CMC/kC/AMMT composite beads to loss their half weights (T50%) was 445.89 °C compared to 310.3, 399.59, and 388.31 °C for CMC,

kC, and CMC/kC beads respectively. These observations could be related to the stability of AMMT clay at higher temperatures, since the dehydroxylation of the aluminosilicates within the AMMT clay layers occurs at temperatures beyond 500 °C. As a result, AMMT enhanced the stability of the developed composite beads. The obtained results are consistent with other studies which focused on studying the effect of clays encapsulation on the thermal behavior of beads prepared from other natural polysaccharides [39, 58]. 3.1.3. SEM The morphological structure of the prepared adsorbents and the native used polymers was investigated by SEM as shown in Fig. 4. It can be seen that the fracture surface of native CMC (Fig. 4A) and kC (Fig. 4B) were exhibited irregular granular surfaces, while the surface of CMC/kC beads was significantly affected and changed to a curly texture surface as shown in Fig. 4C1. Furthermore, Fig. 4D1 shows that CMC/kC/AMMT composite beads depict a crusty and coarse surface. With comparing the whole beads images for CMC/kC (Fig. 4C2) beads and CMC/kC/AMMT composite beads (Fig. 4D2) at a low magnification, it was observed that the surface of CMC/kC beads was a curly and rough moon-like surface (spherical shape). While surface of the developed composite beads was not completely spherical and rougher due to incorporation of AMMT clay with CMC/kC matrix. 3.2. Factors affecting MB adsorption 3.2.1. Effect of CMC/kC ratio Effect of CMC/kC ratio on the removal behavior was clarified as shown in Fig. 5 using different mass ratios of CMC: kC (2:0, 1.5:0.5, 1:1, 0.5:1.5 and 0:2 (w/w) respectively at constant 0.4 % AMMT clay and other adsorption conditions (MB dye conc. (25 ppm), adsorbent dosage (0.1g), (pH6), solution temperature (30°C) and

agitation speed (50 rpm)). Results showed that the removal (%) of MB dye for all studied ratios increased gradually with increasing the adsorption contact time from 15 to 300 min. Also, the composite beads containing both CMC and kC showed high removal (%) exceeded 90 % compared to 67 % and 81 % for individual CMC and kC composite beads (2:0 and 0:2 ratios) respectively. Moreover, CMC: kC (1:1) ratio recorded the highest removal (%) of MB dye and reached 98% after 300 min from the initial contact time. These results indicated that both CMC and kC were involved in the MB adsorption process. In addition, amount of –COOH and –SO3H groups for CMC and kC play an important role in the process of adsorption. Where, presence of both CMC and kC with an equal ratio increases the exposed surface area with large number of active sites for further MB molecules attachment, and enhances the adsorption process to a great extent. The negatively charged carboxylate and sulfate ions on the surface of CMC and kC are able to bind at the optimum pH range with the positively charged groups in MB dye through electrostatic interaction. As a result, the following factors affecting the adsorption behavior were studied based on CMC and kC with the mass ratio 1:1.

3.2.2. Effect of AMMT amount Effect of AMMT clay content on the removal (%) of MB is shown in Fig. 6. It was clear from results that the removal (%) was improved by the addition of AMMT clay compared to the free AMMT beads (CMC/kC beads). Where, it increased gradually and reached 92 % using CMC/kC/ 0.4 % AMMT composite beads after 120 min from the initial contact time compared to 69 % in case of free AMMT beads. However, at a constant mass ratio of CMC/kC (1:1) and the other adsorption conditions, a slight increase in the removal (%) was noticed with increasing AMMT content up to 0.6 %.

The increase in MB removal (%) in case of CMC/kC/AMMT composite beads compared to the free AMMT beads could be ascribed to the large specific surface area and the high density of the negative surface charges of AMMT clay. Similar observations were also reported by other researchers [37]. 3.2.3. Effect of initial MB concentration Effect of initial MB concentration on the removal (%) was studied in the range 10-500 ppm at constant CMC/kC (1:1), AMMT (0.4%), adsorbent dosage (0.1g), (pH6), solution temperature (30°C) and agitation speed (50 rpm). As shown in Fig. 7, it was obvious that lower concentrations of MB dye provide higher and rapid removal (%). Where, the removal (%) reached 90 % and 83 % using 10 and 25 ppm after 60 min from the initial contact time compared to 41 % using 500 ppm respectively. Thereafter, the removal (%) of MB was slow for all studied MB concentrations with increasing time up to 300 min, where it reached about 99 % and 98 % using 10 and 25 ppm compared to 77 % using 500 ppm respectively. The presented results could be explained by the availability of vacant surface sites of CMC/kC/AMMT composite beads at lower MB concentrations during the first 60 min of adsorption, and after a certain time period these sites get occupied by MB molecules [59, 60]. Also, at high initial concentrations of MB dye, the lower removal (%) is probably attributed to saturation of the adsorptive surface sites. As a result, the repulsion forces between molecules of the solute in the solid phase and the bulk phase took place with increasing MB concentration up to 500 ppm, and this in turn leads to a decrease in the removal (%). 3.2.4. Effect of initial pH medium Generally, initial pH medium has the greatest effect on the adsorption process at water adsorbent interface, and plays an important role to get the adsorbate degree of

ionization depending on the adsorbent sites nature. Also, pH variation of the dye medium could also affects the molecular structure of dye, and consequently the ionic dyes removing can be influenced significantly. As shown in Fig. 8, consequence of pH variation on the removal (%) was examined over a pH range from 2 to 10 with respect to all other conditions (CMC/kC (1:1), AMMT (0.4%), MB dye conc. (25 ppm), adsorbent dosage (0.1g), solution temperature (30°C) and agitation speed (50 rpm)). The results clearly indicated that pH considerably affected MB adsorption performance. Where, the removal (%) of MB was increased significantly from 68 % to 93 % after 120 min with increasing pH value from 2 to 6, thereafter became nearly constant until pH 10. Although kC exhibit pH-independent behavior, this anionic polysaccharide carrying sulfate (-OSO3-) groups which can participate in the adsorption process through the electrostatic interaction with the cationic dye molecules [61]. With increasing pH from 2 to 6, the deprotonation process is occurred and most of carboxylic groups of CMC are ionized to carboxylate anions (-COO-), whereas pKa of carboxylic groups is 4.6. Additionally, silanol groups of AMMT clay on the adsorbent surface also become increasingly deprotonated at the higher pH values. So, the number of negative charges on the adsorbent surface increased, and two probable mechanisms of MB adsorption could be occurred with increasing pH. The first is the strong electrostatic interaction occurred between negatively charged groups on the adsorbent and positively charged MB molecules. The second is the hydrogen bonding interaction between imines groups of MB molecules (RCH=NR) and the reactive OH groups of the polymers used [62], in addition to a certain cation exchange reactions between AMMT layers and MB molecules can take place simply. As a result, the removal (%) of MB molecules increases at high pHs. On the other hand, the decrease in the removal (%) at lower pHs may be attributed to the presence

of more hydrogen ions (H+) that compete with MB dye cations for the free active sites (i.e. charge screening effect) [63]. This in turn decreases the vacant sites for MB molecules, and a low electrostatic attraction can occur between the cationic dye and the anionic (carboxyl, hydroxyl, and also sulfate) groups leading to a decrease in the removal (%). Also, at lower pHs both carboxyl and hydroxyl groups are mostly nonionized (protonation process) and the generated repulsive forces between the positive charges hindered the adsorption process. 3.2.5. Effect of initial adsorbent dosage Influence of variation the initial dosage of CMC/kC/AMMT composite beads was studied in the range 0.05-0.25 g as shown in Fig. 9, while all other parameters were kept constant. Results showed that the removal (%) increased quickly with increasing the adsorbent dosage up to 0.25 g. Where, maximum removal (%) was recorded 93 % using 0.25 g of adsorbent after only 30 min from the initial contact time compared to 53 % using 0.05 g at the same contact time. Furthermore, the lowest amount of adsorbent (0.05 g) needed about 180 min to reach the same percent removal (93 %). These results could be clarified by increasing the available sites for the adsorption with increasing the adsorbent dosage, this leads to increasing the attached MB molecules and consequently the MB removal (%) increases. Results indicated also that increasing adsorbent dosage beyond 0.15 g didn't have a significant effect on the removal (%) after 180 min for all studied amounts beyond 0.15 g, which reached the highest value (almost 100 %). Similar results have been stated by other authors [64]. 3.2.6. Effect of solution temperature Effect of solution temperature on MB removal (%) was investigated by performing the adsorption experiments in the temperature range 25–45 °C for a period time 300 min as displayed in Fig. 10. The results clearly indicated that the removal (%)

increased gradually from 76 % to 91 % after 60 min from the initial contact time with increasing temperature from 25 °C to 35 °C. These observations may be attributed to increasing the diffusion rate of MB molecules through the external boundary layer of the composite beads. In addition to larger number of active sites may be generated on the composite beads surface with increasing temperature causing an increase in the MB removal (%). A slight increase in the removal (%) was noticed with rising temperature up to 45 °C. This may be ascribed by saturation most sites on the adsorbent surface with MB molecules, whereas the removal (%) reached about 95 % after 120 min with increasing temperature beyond 35 °C. 3.2.7. Effect of agitation speed Fig. 11 shows the effect of agitation speed variation on the removal (%) was studied in the range 0-200 rpm, while all other conditions were kept constant. The obtained results clearly demonstrated the removal (%) increased from 65 % to 95 % after 60 min from the initial contact time with increasing speed up to 200 rpm. Actually, increasing the agitation speed enhances the solute distribution and improves the diffusion of MB molecules towards the adsorbent surface, where large numbers of unoccupied surface sites are available for MB adsorption with higher removal (%). However, at the higher agitation speed (200 rpm), the removal (%) decreased from 95 % to 77 % with further increasing the contact time up to 300 min. This can be explained by increasing the desorption tendency of MB molecules resulting from deformation of the stable film after a lapse of time at the higher agitation speed. 3.3. Adsorption isotherms Langmuir and Freundlich isotherms were studied as shown in Fig. 12 (a and b), while values of isotherm parameters were displayed in Table 3. It was noticed that R2 of

Langmuir model equal to 0.999 (very close to 1) and larger than Freundlich model. Besides, the highest monolayer adsorption capacity that obtained from Langmuir isotherm was 10.75 mg/g, which approaches the experimental data (12.5 mg/g). Moreover, The separation factor (RL) values were <1 as stated in Table 4, which related to Langmuir isotherm and reflect favorable adsorption [14, 52]. 3.4. Adsorption kinetics Fig. 13 (a, b, and c) shows the pseudo-first order, pseudo-second order, and intraparticle diffusion models respectively. The kinetic parameters such as correlation coefficients (R2), k1, k2, and calculated qe,cal values were displayed in Tables 5-7. It was clear that the process of MB adsorption follows the pseudo-second order model. Where, values of qe,cal in case of pseudo-second order kinetic model was very close to the experimental qe (12.5 mg/g), and R2 values were obviously more than the pseudofirst order model (Tables 5 and 6). On the other hand, stages of the process were investigated using intraparticle diffusion model (Fig. 13c). Results indicated that the process of adsorption occurred in two stages. The first stage describes the diffusion of MB molecules into the external surface of the composite beads or the boundary layer diffusion of solute molecules. The second stage could be ascribed to the gradual adsorption stage or the coarse surface of the adsorbent. As displayed in Table 7, the larger intercept (C) values, the greater the involvement of the surface adsorption in the rate-controlling step. From the results, it can be concluded that the intraparticle diffusion was not the rate controlling step, where C≠ 0 [58]. 3.5. Reusability Indeed, reusability of an adsorbent is very important from an economic point of view. Since it control the cost production, in addition to investigate the ability of the

adsorbent materials to be reused. Fig. 14 displays the influence of adsorption– desorption cycles on the adsorption capacity and MB removal (%). Results indicated that the developed CMC/kC/AMMT composite beads still retain good adsorption properties after 5 repeated cycles. Where, the adsorption capacity slightly decreased from 12.257 to 11.891 mg/g, while the removal (%) still exceeded 95 %. This demonstrate the reusability of the developed adsorbent, where it could be effectively used as a reliable adsorbent for MB dye with a highly removal percent. 4. Conclusion In this study, CMC/kC/AMMT composite beads were prepared and optimized for removing of cationic methylene blue dye (MB). The composite beads were characterized using different characterization tool, and their ability for MB removal was evaluated under different adsorption conditions. Results revealed that the optimum ratio for the composite beads used was 1:1:0.4 for CMC:kC:AMMT respectively, and the maximum MB removal (%) was 98 % with adsorption capacity equal 12.25 mg/g. Moreover, the experimental data were applicable to the pseudosecond order model and follow Langmuir isotherm model (R2=0.999). Reusability study showed good adsorption properties after 5 consecutive cycles.

From the

obtained results, it can be concluded that the prepared CMC/kC/AMMT composite beads could be applied as a reusable adsorbent materials for MB dye from aqueous solutions. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21476212) and the Foundation of Science and Technology Department of Zhejiang Province (2017C33126).

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Figure captions Fig.1. (a) scheme for the synthesis of CMC/kC/AMMT composite beads, (b) freshly prepared (1) wet and (2) dry composite beads and digital photos for MB dye before and after adsorption. Fig.2. FTIR spectra of native CMC, kC, AMMT and CMC/ kC/ AMMT composite beads. Fig.3. TGA of (1) native CMC, (2) native kC, (3) CMC/kC beads, and (4) CMC/kC/AMMT composite beads. Fig.4. SEM images of (A) native CMC, (B) native kC, (C1 and C2) CMC/kC beads at x50 and x10.0 k magnification, and (D1 and D2) CMC/kC/AMMT composite beads at x50 and x10.0 k magnification respectively. Fig.5. Effect of CMC/kC ratio on the removal (%) of MB dye at constant AMMT (0.4%), MB dye conc. (25 ppm), adsorbent dosage (0.1g), (pH6), solution temperature (30°C) and agitation speed (50 rpm). Fig.6. Effect of AMMT amount on the removal (%) of MB dye at constant CMC/kC (1:1), MB dye conc. (25 ppm), adsorbent dosage (0.1g), (pH6), solution temperature (30°C) and agitation speed (50 rpm). Fig.7. Effect of initial MB dye concentration on the removal (%) at constant CMC/kC (1:1), AMMT (0.4%), adsorbent dosage (0.1g), (pH6), solution temperature (30°C) and agitation speed (50 rpm).

Fig.8. Effect of pH medium on the removal (%) of MB dye at constant CMC/kC (1:1), AMMT (0.4%), MB dye conc. (25 ppm), adsorbent dosage (0.1g), solution temperature (30°C) and agitation speed (50 rpm). Fig.9. Effect of initial adsorbent dosage on the removal (%) of MB dye at constant CMC/kC (1:1), AMMT (0.4%), MB dye conc. (25 ppm), (pH6), solution temperature (30°C) and agitation speed (50 rpm). Fig.10. Effect of solution temperature on the removal (%) of MB dye at constant CMC/kC (1:1), AMMT (0.4%), MB dye conc. (25 ppm), adsorbent dosage (0.1g), (pH6) and agitation speed (50 rpm). Fig.11. Effect of agitation speed (rpm) on the removal (%) of MB dye at constant CMC/kC (1:1), AMMT (0.4%), MB dye conc. (25 ppm), adsorbent dosage (0.1g), (pH6) and solution temperature (30°C). Fig.12. Isotherm models of (a) Langmuir and (b) Freundlich for the adsorption of MB dye with different initial concentrations (10, 25, 50, 100, 200 and 500 ppm) using 0.1g of CMC/kC/AMMT (1:1:0.4%), (pH6), solution temperature (30°C) and agitation speed (50 rpm). Fig.13. Kinetics models (a) pseudo-first order, (b) pseudo-second order, and (c) intraparticle diffusion model for the adsorption of MB dye with different initial concentrations (10, 25, 50, 100, 200 and 500 ppm) using 0.1g of CMC/kC/AMMT (1:1:0.4%), (pH6), solution temperature (30°C) and agitation speed (50 rpm). Fig.14. Reusability cycles of CMC/kC/AMMT composite beads for MB dye adsorption using 0.1g of CMC/kC/AMMT (1:1:0.4%), MB dye conc. (25 ppm), (pH6) solution temperature (30°C) and agitation speed (50 rpm).

Table 1 Characteristics of MB dye

Chemical name

Molecular formula

Methylene Blue C16H18N3SCl tri-hydrate

.3H2O

Chemical structure

Mw

λmax.

(g mol-1)

(nm)

373.9

665

Table 2 TGA data for native CMC, native kC, CMC/kC beads, and CMC/kC/AMMT composite beads

Sample

T50% (oC)

Weight loss (%) at 150 (oC)

CMC

310.3

9.56

kC

399.59

13.1

CMC/kC

388.31

8.92

CMC/kC/AMMT

445.89

7.62

Table 3 Parameters of Langmuir and Freundlich models for the adsorption of MB dye onto CMC/kC/AMMT composite beads Model Langmuir

Freundlich

Constants qmax (mg/g)

KL (L/mg)

R2

10.75

0.326

0.999

KF (mg/g) (L/mg)1/n

1/n

R2

11.63

0.052

0.641

Table 4 RL values for each MB dye concentrations Co (mg/L)

RL

10

0.2347

25

0.109

50

0.057

100

0.029

200

0.015

500

0.006

Table 5 Parameters of pseudo-first order model at each initial MB concentrations used Co (mg/L)

K1 (min -1)

qe,cal (mg/g)

qe,exp (mg/g)

R2

10

0.011

3.457

12.5

0.942

25

0.011

4.381

12.5

0.95

50

0.011

4.68

12.372

0.952

100

0.009

5.067

12.3375

0.923

200

0.01

5.822

12.229

0.947

500

0.005

5.624

10.901

0.831

Table 6 Parameters of pseudo-second order model at each initial MB concentrations Co (mg/L)

K2 (g/mg)

qe,cal (mg/g)

qe,exp (mg/g)

R2

10

0.004259

12.82

12.5

0.999

25

0.004259

12.82

12.5

0.999

50

0.008206

12.987

12.372

0.999

100

0.011904

13.157

12.3375

0.996

200

0.17596

13.698

12.229

0.978

500

0.05181

10

10.901

0.89

Table 7 Parameters of intraparticle diffusion model at each initial MB concentrations KP (mg/g t0.5)

Intercept (C; mg/g)

R2

10

0.295

7.918

0.73

25

0.397

6.231

0.855

50

0.439

0.837

0.837

100

0.518

3.987

0.85

200

0.583

2.583

0.909

500

0.338

2.807

0.814

Co (mg/L)

FIGURES

Fig.1. (a) scheme for the synthesis of CMC/kC/AMMT composite beads, (b) freshly prepared (1) wet and (2) dry composite beads and digital photos for MB dye before and after adsorption.

Transmittance(%)

(1)

1326 1420 850 1380 1260 920 720

(2) (3)

3750

3450 4000

(1)--CMC/kC/AMMT (2)--CMC 1630 (3)--AMMT (4)--kC 3000 2500 2000

2920

(4)

3500

798 1380 1060 1260 1500 1000 500

-1

Wavenumber(cm )

Fig.2. FTIR spectra of native CMC, kC, AMMT and CMC/ kC/ AMMT composite beads.

100 (1)

Weight(%)

80

(3) (4)

60

(2)

40 (1)--CMC (2)--kC (3)--CMC/kC (4)--CMC/kC/AMMT

20 0

0

100

200

300

400

500

600

700

800

o

Temperrature( C)

Fig.3. TGA of (1) native CMC, (2) native kC, (3) CMC/kC beads, and (4) CMC/kC/AMMT composite beads.

Fig.4. SEM images of (A) native CMC, (B) native kC, (C1 and C2) CMC/kC beads at x50 and x10.0 k magnification, and (D1 and D2) CMC/kC/AMMT composite beads at x50 and x10.0 k magnification respectively.

100 90

Removal (%)

80 70 60

CMC:kC (2:0) CMC:kC (1.5:0.5) CMC:kC (1:1) CMC:kC (0.5:1.5) CMC:kC (0:2)

50 40

0

50

100

150

200

250

300

Time(min)

Fig.5. Effect of CMC/kC ratio on the removal (%) of MB dye at constant AMMT (0.4%), MB dye conc. (25 ppm), adsorbent dosage (0.1g), (pH6), solution temperature (30°C) and agitation speed (50 rpm).

100

Removal (%)

90 80 70 60 50 0

50

100

150

200

AMMT(0%) AMMT(0.1%) AMMT(0.2%) AMMT(0.4%) AMMT(0.6%) 250 300

Time(min)

Fig.6. Effect of AMMT amount on the removal (%) of MB dye at constant CMC/kC (1:1), MB dye conc. (25 ppm), adsorbent dosage (0.1g), (pH6), solution temperature (30°C) and agitation speed (50 rpm).

100 90

Removal (%)

80 70 10 ppm 25 ppm 50 ppm 100 ppm 200 ppm 500 ppm

60 50 40 0

50

100

150

200

250

300

Time(min)

Fig.7. Effect of initial MB dye concentration on the removal (%) at constant CMC/kC (1:1), AMMT (0.4%), adsorbent dosage (0.1g), (pH6), solution temperature (30°C) and agitation speed (50 rpm).

100

Removal (%)

90 80 70

pH=2 pH=4 pH=6 pH=8 pH=10

60 50

0

50

100

150

200

250

300

Time(min)

Fig.8. Effect of pH medium on the removal (%) of MB dye at constant CMC/kC (1:1), AMMT (0.4%), MB dye conc. (25 ppm), adsorbent dosage (0.1g), solution temperature (30°C) and agitation speed (50 rpm). .

100

Removal (%)

90 80 70 0.05 g 0.1 g 0.15 g 0.2 g 0.25 g

60 50 40

0

50

100

150

200

250

300

Time(min)

Fig.9. Effect of initial adsorbent dosage on the removal (%) of MB dye at constant CMC/kC (1:1), AMMT (0.4%), MB dye conc. (25 ppm), (pH6), solution temperature (30°C) and agitation speed (50 rpm).

.

100

Removal (%)

90 80 70

o

25 C o 30 C o 35 C o 40 C o 45 C

60 50 0

50

100

150

200

250

300

Time(min)

Fig.10. Effect of solution temperature on the removal (%) of MB dye at constant CMC/kC (1:1), AMMT (0.4%), MB dye conc. (25ppm), adsorbent dosage (0.1g), (pH6) and agitation speed (50 rpm). .

100 90 Removal (%)

80 70 60

0 rpm 50 rpm 100 rpm 150 rpm 200 rpm

50 40 30

0

50

100

150

200

250

300

Time(min)

Fig.11. Effect of agitation speed (rpm) on the removal (%) of MB dye at constant CMC/kC (1:1), AMMT (0.4%), MB dye conc. (25 ppm), adsorbent dosage (0.1g), (pH6) and solution temperature (30°C).

2.56

(b) 2.52

ln qe

2.48 2.44 2.40 -4

-3

-2

-1

0

1

2

ln Ce

Fig.12. Isotherm models of (a) Langmuir and (b) Freundlich for the adsorption of MB dye with different initial concentrations (10, 25, 50, 100, 200 and 500 ppm) using 0.1g of CMC/kC/AMMT (1:1:0.4%), (pH6), solution temperature (30°C) and agitation speed (50 rpm).

2

(a)

ln(qe-qt)

1 0

10 ppm 25 ppm 50 ppm 100 ppm 200 ppm 500 ppm

-1 -2 0

50

100

150

200

250

300

Time(min)

35

25 t/qt (min*mg/g)

(b)

10 ppm 25 ppm 50 ppm 100 ppm 200 ppm 500 ppm

30

20 15 10 5 0

0

50

100

150

200

250

300

Time (min) 14

10

qt(mg/g)

(c)

10 ppm 25 ppm 50 ppm 100 ppm 200 ppm 500 ppm

12

8 6 4

0

2

4

6

8

10

0.5

0.5

t (min.

12

14

16

18

)

Fig.13. Kinetics models (a) pseudo-first order, (b) pseudo-second order, and (c) intraparticle diffusion model for the adsorption of MB dye with different initial

concentrations (10, 25, 50, 100, 200 and 500 ppm) using 0.1g of CMC/kC/AMMT (1:1:0.4%), (pH6), solution temperature (30°C) and agitation speed (50 rpm).

12.50

Removal(%)

80

.

12.25

.

60

.

40

.

.

q (mg/g)

100

12.00

11.75 20 0

1

2

3

4

5

11.50

Reusability cyles

Fig.14. Reusability cycles of CMC/kC/AMMT composite beads for MB dye adsorption using 0.1g of CMC/kC/AMMT (1:1:0.4%), MB dye conc. (25 ppm), (pH6), solution temperature (30°C) and agitation speed (50 rpm).