Simultaneous Removal of Cationic Crystal Violet and Anionic Reactive Yellow Dyes using eco-friendly Chitosan Functionalized by Talc and Cloisite 30B

Designing adsorbent materials that can effectively remove many types of organic dyes is crucial because of the wide diversity of synthetic dyes found in wastewater. Thus, this study presents the synthesis of economic and eco-friendly adsorbent composites composed of chitosan (Cs), talc (T), and Cloisite 30B clay (C) to remove both cationic crystal violet (CV) and anionic reactive yellow 145 (RY) dyes for the first time. Cs was functionalized with T and subsequently the CsTC1 and CsTC2 composites were prepared by sensitizing the obtained CsT with different weight ratios of Cloisite 30B (C). X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), N2 adsorption–desorption isotherm, and zeta potential analysis were employed to characterize the materials. The addition of T to Cs matrix improves the thermal stability, pore size, and pore volume with respect to pure Cs. The effect of C inclusion was examined by measuring the affinity of the prepared composites towards adsorption of cationic CV and anionic RY dyes in comparison to pure Cs and CsT composite. The adsorption results in a single dye solution revealed that the CsTC1 composite is the most effective adsorbent for removal of RY dye with 76.9 mg/g adsorption capacity, whereas the CsTC2 composite exhibited the highest adsorption activity for CV dye (37.03 mg/g). Furthermore, RY and CV co-adsorption on CsTC composites was tested from a mixed dye solution. The adsorption kinetics of RY and CV adsorption followed the pseudo second order model. Langmuir isotherm model described the experimental adsorption data better than the Freundlich, Dubinin–Radushkevich, and Temkin isotherm models, indicating a monolayer sorption process for both dyes. The proposed mechanism for RY and CV adsorption using CsTC composites was investigated.


Introduction
Nowadays, remediation of contaminated water from synthetic dyes is considered one of the most important challenges facing all countries due to the harmful effects of these dyes on the environment [1,2]. Synthetic dyes are found in wastewater as a result of improper discharge of dyes from the textile, leather, paper, rubber, plastics, and pharmaceuticals industries [3,4]. The presence of these dyes in the wastewater stream can cause a variety of environmental issues because they are carcinogenic and mutagenic to living organisms even at low concentrations [5,6]. For example crystal violet (CV) dye, belongs to cationic dyes, which is frequently employed in textile factories to color cotton and silk, as well as in the manufacture of paints and inks. CV dye is employed to recognize bloody fingerprints [7]. CV may cause mild irritation of the eyes and serious damage to the conjunctiva and cornea. Exposure to CV in excess may cause harm to the digestive tract and skin, as well as kidney and respiratory problems. In addition, CV is non-biodegradable and is hardly ever digested by microorganisms. As a result, it can persist in a variety of conditions [8]. Consequently, the elimination of CV from solution is crucial and needed as will be discussed in this study in addition to reactive yellow 145 dye. Since it was frequently used in textile industries' dying procedures and can be detected in industrial effluent. Therefore, the removal of these dyes before they are discharged into various water bodies has become an essential step [9]. Several methods have been tested for removing toxic dyes from wastewater, such as biological, ion exchange, advanced oxidation, coagulation, flocculation, ultra-filtration through membranes, and adsorption methods [3,6,10]. Adsorption method has received great attention among other methods due to its cost-effective, highly efficient, easy operation, simple design, and availability of a wide range of adsorbent materials [1,3,9]. Adsorption techniques that use biopolymers have recently been made to remove heavy metals and dyes from wastewater.
Among the biopolymers that have attracted the attention of researchers for being developed for water treatment is chitosan (Cs). This was attributed to the advantages that Cs has over the other adsorbents. It is characterized by its biocompatibility, biodegradability, antibacterial, non-toxicity, and possessing adsorption properties [10,11]. The adsorption properties of Cs were attributed to the presence of amine and hydroxyl groups in its structure, which allowed it to adsorb contaminants from wastewater via complex formation [10][11][12]. Cs has a high effectiveness for anionic dye adsorption over cationic dyes because of its cationic nature. With progress in science and technology, innovative strategies for improving Cs adsorption performance towards cationic dyes have emerged [13][14][15]. Additionally, pH sensitivity and swelling susceptibility limit the practical use of pure chitosan. In order to resolve these problems, Cs was modified with other organic and/or inorganic materials that have affinity for cationic dyes such as graphene oxide, manganese dioxide, sodium alginate, clay, magnetic nanoparticles, silica, carbon materials, cellulose, and karaya gum [10,13,[16][17][18][19][20][21][22]. Furthermore, these materials provide Cs with good thermal and chemical stability [10,23]. Clay minerals, for example, have been used to make CS composites due to their environmentally friendly and inexpensive manufacturing methods, as well as their promising physical and chemical properties. Talc (T) is a type of natural clay with the chemical formula Mg 3 Si 4 O 10 (OH) 2 . It is composed of three layers: a monolayer of magnesium hydroxide (MgO⋅H 2 O) sandwiched between two layers of silicate (SiO 2 ) [24,25]. Layered talc particles are hydrophobic on their basal planes due to the presence of siloxane groups (-Si-O-Si-) but edge hydroxyl groups (-SiOH & -MgOH) are hydrophilic [26]. It was characterized by its abundance, low cost, high surface area, low electrical conductivity, good thermal stability, and chemical inertness [27,28]. Owing to all these characteristics, T was used in numerous industrial applications, including water treatment, paper, plastic, rubber, composites, paper, cosmetics, and medicines [28,29]. Recently, Abil and Arefinia [25] studied the effect of talc particles on the ability of a polyurethane coating to protect carbon steel from corrosion. Kalantari et al. [30] integrated various amounts of talc into polysulfone polymer, with the goal of separating carbon dioxide from methane. T has been utilized as an adsorbent in water treatment to remove contaminants from water, either alone or in conjunction with other inorganic/organic materials [24,[26][27][28][29][31][32][33][34][35]. According to the research findings, T has high adsorption affinity for cationic dyes and positively charged metal ions because its structure contains a large number of functional groups such as Mg − O, − OH, Si − O − Si, and O-Si-O linkages [35]. Sprynskyy et al. [26] used talc to eliminate hexavalent uranium. Wenlei et al. [36] presented the adsorption of methyl orange dye on the surface of 50% (w/w) HNO 3 acid modified talc. In another study, talc was modified with microwave and acetic acid to adsorb methylene blue (MB) dye [37]. Alizadeh et al. [28] developed Fe 3 O 4 @SiO 2 -IL/Talc nanocomposite for sorption of MB dye. FeSx/talc composite was prepared for the removal of 2,4,6-trichlorophenol under microwave [38]. There are few reports for the synthesis of chitosan talc (CsT) composite. For example Lima et al. [39] prepared CsT using glutaraldehyde as a crosslinker for drug delivery systems. Chitosan/ polyvinyl alcohol/talc composite was synthesized to remove anionic dyes (Congo red & Methyl orange) and heavy metals (Pb 2+ &Cr 6+ ) [24]. They reported that to prevent swelling during the experiment, talc was added to the composite as reinforcement.
Cloisite 30B is composed of a naturally occurring montmorillonite clay [(Ca,Na) 0.3 Al 2 (Si,Al) 4 O 10 (OH) 2 xH 2 O] that has been modified with the organic surfactant methyl tallow bis(2-hydroxyethyl) quaternary ammonium salt [6]. It made up of tetrahedral silicate sheets sandwiched between octahedral alumina sheets. This type of organoclay is of interest as it can provide the adsorbent with additional positive charge arising from the presence of the quaternary ammonium salt surfactant in its composition [40,41]. At the same time the organic modifier in Cloisite 30B is a bi-functional amphiphilic surfactant (ammonium bearing alkyl chains and hydroxyl groups) [6]. Chitosan/polylactide composite was sensitized with different wt% of Cloisite 30B to control release of the anticancer drug [42]. Recently, Safarzadeh et al. [4] used Cloisite 30B to improve of the adsorption properties of poly(methacrylic acid-co-acrylamide) hydrogel towards MB dye. Recently, Sujithra and Arthanareeswaran modified Cloisite 30 B with PEO after that polyether sulfone/f-(PEO-Cloisite) nanocomposite membranes were prepared for the removal of dyes and salts from waste water [43]. Literature search has revealed that, there are limited studies on synthesis of chitosan/Cloisite 30B based composites for dye removal. Vanaamudan and Sudhakar [6] prepared chitosan/ Cloisite 30B composite for sorption of anionic dyes, reactive blue-21 and reactive red-141. Also, chitosan/Cloisite 30B composite was used for the adsorption of Rhodamine-6G [5]. Daraei et al. [44] prepared chitosan/Cloisite 30B nanocomposites with different quantities of Cloisite 30B weight percentage (0, 0.5, 1, and 2%) to be coated over the surface of commercial polyvinylidene fluoride (PVDF) to be used as microfiltration membrane for dye removal.
To our knowledge, there has been no report of the synthesis of ternary composites consists of chitosan (Cs), talc (T), and Cloisite 30B organoclay (C) as will be presented here neither for dye adsorption nor for any applications. In this work, Cs was firstly modified with talc for improving its thermal stability. After that the obtained CsT composite was sensitized different amounts of C. The prepared Cs, CsT composite, and CsTC hybrid composites were characterized by XRD, FTIR, SEM, TGA, surface area, and zeta potential. It is expected that incorporating C into the CsT composite would impart the resulting composite with additional positive charges besides those of Cs and this in turn could further enhance the adsorption performance of the resulting composite towards anionic dyes as well. In a single dye solution, the removal efficiency of the developed CsTC composites in comparison with pure Cs and CsT composite was evaluated using reactive yellow 145 (RY) dye (an anionic dye) and crystal violet (CV) dye (a cationic dye). According to the presented findings, CsTC composites exhibited simultaneous removal of anionic and cationic dyes in a mixed dye solution. Also the ability of CsTC composites for simultaneous removal for both dyes was presented. In addition, the effect of C mass ratio, initial dye concentration, contact time, adsorbent dosage, and pH on the adsorption performance was evaluated. Kinetic and isotherm studies for adsorption of CV and RY dyes using CsTC composites were also studied. Finally, the adsorption mechanism for CV and RY dyes was proposed based on FTIR analysis.

Materials
High molecular weight chitosan (Cs), glacial acetic acid, sodium hydroxide, crystal violet (CV), and reactive yellow 145 (RY) were obtained from Sigma-Aldrich. Some properties of CV and RY are listed in Table 1.Talc powder (T) was purchased from Five Fives company, Egypt. Cloisite 30B (C) was bought from Southern Clay Products Inc.

Methods
Chitosan/talc/Cloisite 30B (CsTC) composites, which contain different weight ratios of Cloisite 30B (C) and fixed amounts of chitosan (Cs) and talc (T), were prepared via a simple solution mixing method. The mass ratio of Cs, T, and C was 1:0:0, 1:2:0; 1:2:1, and 1:2:2 to obtain Cs, CsT, CsTC1, and CsTC2 composites, respectively. 1 g of Cs was dissolved in 50 ml of 1% (v/v) acetic acid to get solution A. In another beaker T and C was dispersed in 5 ml distilled water to form solution B. Then, solution B was added to solution A slowly under vigorous stirring for 2 h. Finally, 100 ml of NaOH (2 M) was slowly added to the resulting mixture solutions to separate the obtained white precipitates, which were left in NaOH solutions overnight. After that, they were filtrated, washed with distilled H 2 O until pH became neutral, and oven dried at 60 °C for 24 h. For comparison, pure Cs was prepared by the same procedure except the step of adding T and C dispersion.

Characterization
The microstructure of the various samples, including Cs, CsT, CsTC1, and CsTC2 composites was detected by scanning electron microscopy (SEM, Quanta FEG-250). The X-ray analysis (XRD, Panalytical X'Pert PRO) measurement was used to identify the crystal structure. The functional groups of the prepared materials were detected by fourier transform infrared spectrometry (FT-IR, 8400 Shimadzu, Japan) in the range of 4000-400 cm −1 . The thermal stability was evaluated by thermogravimetric analysis (TGA-50Shimadzu) that measured under nitrogen atmosphere from 25 to 700 °C at 10 °C/min heating rate. The value of specific surface area was estimated by measuring N 2 adsorption-desorption isotherm using Quanta chrome apparatus and pore size distribution was determined using the DFT method. Zeta potential analysis was carried out using Nano-ZS (Malvern Instruments Ltd., UK).

Adsorption Experiments
In a batch mode reactor, crystal violet (CV) and reactive yellow 145 (RY) were chosen as examples of cationic and anionic dyes, respectively, to study the adsorption properties of the prepared samples. The experiments for CV or RY adsorption were conducted by adding the adsorbent to dye solution under constant stirring and every 30 min, 3 ml of this dispersion was taken out and centrifuged at 5000 rpm for 2 min. The remaining concentration of CV and RY dye was estimated by the absorbance of the major peak at 586 nm and 419 nm, respectively. All experiments were performed twice, with the average results presented. The following equations are used to calculate the dye removal percentage (R%, Eq. 1) and adsorption capacity (qt, Eq. 2) for all samples.
where C 0 is the initial dye concentration and C t is the dye concentration at a certain time (t).The volume of the dye solution is V, and the sample weight is w. For the samples that exhibited the highest dye removal efficiency, the effect of varying contact duration, adsorbent does, pH, and initial dye concentration was investigated in a separate dye solution.

Freundlich isotherm
Temkin isotherm Where C e is the dye concentration at equilibrium and its corresponding adsorption amount is q e . K L , K F , and b T are the Langmiur, Freundlich, and Temkin isotherm constants, respectively. Equation 9 is used to estimate the separation factor, R L , and n corresponds to the intensity parameter in Freundlich model. A is related to the equilibrium binding constant. R and T are the universal gas constant and working temperature, respectively. The Polanyi sorption potential is , which determined using Eq. 13. E ads is the adsorption free energy that is estimated by using Eq. 14.
Finally, the activity of the prepared composites (CsTC1 and CsTC2) for simultaneous removal of both cationic crystal violet (CV) and anionic reactive yellow 145 (RY) dye were investigated in a mixed dye solution.

XRD Analysis
The XRD technique was used to study the crystalline structure. Figure 1 displays the XRD patterns of pristine Cloisite 30B (C) as well as CsT composite before and after inclusion of C. XRD pattern of pure cloisite 30B, Fig. 1a, exhibits a peak at 2θ = 4.67° [5,6,53]. For CsT composite, Fig. 1b, the peaks related to T were observed at 2θ = 9.34°, 18.83°, 28.5°, 35.12°, and 48.42° while the peak related to Cs was observed at 19.29° [24,27]. Bragg's formula, Eq. (15), was used to estimate the distance between the talc sheets (d) in different adsorbents based on the XRD patterns [28].
where n, λ, and θ denote a positive integer, the incident wave's wavelength, and the Bragg angle of the diffraction peak, respectively. As depicted in Fig. 1 (B), the d-spacing of talc in CsT at 2θ = 9.34 degrees was 0.95 nm, and it dropped gradually to 0.94 nm at 2θ = 9.41 degrees due to the inclusion of Cloisite 30B clay particles on the surface of talc layers.
Comparing the XRD pattern of CsT composite with that of CsTC hybrid composites, it was found that a new peak related to Cloisite 30B (C) had developed at 4.84° and 4.94° for CsTC1 (Fig. 1c) and CsTC2 (Fig. 1d), respectively [6]. Additionally, it was noticed that this peak become broader and its intensity was reduced with increasing loading of clay as shown in Fig. 1 (B) in the range of 2θ = 4-11 indicating intercalation between polymer and clay. The inter layer basal spacing at 4.67° was 1.89 nm for pure Cloisite 30B clay which agree with literature [54]. This value was decreased to 1.82 nm for CsTC1 and 1.78 nm for CsTC2. This result may be due to layer compression of clay. Also, Daraei et al [44] found that by raising the amount of Cloisite 15A, the characteristic peak shifted to a larger angle due to a decrease in interlayer spacing. Furthermore, the intensities of the diffraction peaks of talc were dramatically reduced in the XRD pattern of composites when compared to that of CsT. The crystalline peak of T was shifted to higher degree, 9.37° for CsTC1 and 9.41° for CsTC2. Therefore, these results indicate a considerable interaction between chitosan, talc, and Cloisite 30B [24].
Using the Debye Scherrer equation, XRD revealed that the particle size of C, CsTC1, and CsTC2 was 5.57, 3.34, CsT, c CsTC1, and d CsTC2 composites from 2θ = 2-55 degree (A) and from 2θ = 4−11 degree (B) and 3.07 nm, respectively. The decrease in the particle size of Cloisite 30B clay after incorporation in polymer matrix agrees with previous reports [5,6].

SEM Analysis
The surface morphology of all the prepared samples such as Cs, CsT, CsTC1, and CsTC2 composites was studied by SEM analysis as shown in Fig. 2. Pure chitosan (Cs) is free of pores and it has relatively homogenous and smooth surface, Fig. 2a. From SEM figure of CsT composite, Fig. 2b, it was observed that there was a good dispersion of T within the Cs matrix. This might be due to the good compatibility and interaction between OH groups on the surface of the talc and OH and/or NH 2 groups on Cs, which resulted in a good adhesion between them. It was also observed that the addition of T significantly increased the surface roughness relative to unmodified Cs. For the CsTC1 and CsTC2 hybrid composites in Fig. 2c and d, respectively, there was a dramatic change in the microstructure after the incorporation of clay material. The composites exhibited more cracks and voids as well as a rise in the surface roughness relative to pure Cs and CsT composite. The nanocomposite's surface roughness may be an indication that silicate platelets have been incorporated into the polymer matrix. In addition, this rough surface indicates higher surface area as will illustrated later in BET section. Consequently, this can improve the composite's ability to adsorb dyes on its surface. However, the surface of CsTC composites, even with high loading of clay, is free of any agglomeration demonstrating uniform distribution of clay particles within the chitosan matrix. This could be due to the presence of hydroxyl groups in Cloisite 30B, which gives it a hydrophilic character, facilitates better interaction between clay particles and chitosan.

FTIR Analysis
The FTIR spectroscopy was used to analyze the interaction between composite constituents, including Cs, T and C. Figure 3 shows the FTIR spectra of CsTC1 and CsTC2 hybrid composites in addition to pure Cs and CsT composite for comparison. Spectrum in Fig. 3a, shows the characteristic bands of Cs that appeared at 3200 cm −1 (− OH  [26,28,57]. In addition, the characteristic bands of Cs were appeared with some alterations for example the intensity of O − H stretching, − NH stretching, and C − O stretching bands of Cs increased and there was a slight shift. The composite formation may take place as a result of interactions between OH groups on the surface of the talc and OH and/or NH 2 groups on Cs. The bands of amide and amine of Cs that appeared at 1630 and 1533 cm −1 in Cs spectrum were shifted to 1658 and 1550 cm −1 . This shift was attributed to the presence of an electrostatic interaction between the negatively charged sites of T and amine groups of Cs [58]. The spectra of CsTC hybrid composites (  [44]. The strong band between 1190 and 850 cm −1 is made up of the overlapping bands of the talc, Cs, and C, as well as the new Si-O-C and C-NH-C vibrations. The Al − OH and Si − OH vibrations of Cloisite 30B are appeared at 995 & 795 cm −1 , respectively, as clearly shown for high doping CsTC composite [6]. These results verify the inclusion of Cloisite 30B clay in the CsT matrix. As the loading of Cloisite 30B clay increases, it can be seen that the intensities of the peaks become stronger, suggesting more loading of Cloisite 30B.

TGA Analysis
Thermogravimetric (TGA) analysis was used to evaluate the thermal stability of samples (Cs, CsT, CsTC1, and CsTC2) as shown in Fig. 4. In this test, the weight loss percentage was determined as a function of temperature in N 2 atmosphere. The thermal degradation of Cs occurred in three stages, as can be seen in Fig. 4a. The first stage occurred approximately at 100 °C as a result of moisture evaporation. The second stage observed at around 220 °C and could be attributed to the degradation of Cs chains. The third stage was noticed in the range of 365-531 °C due to the decomposition of pyranose rings of Cs with ring-opening reaction [10]. It is noticed that modification of Cs with T significantly enhanced its thermal stability as revealed in Fig. 4b. At 700 °C the weight loss was 45.5% for Cs and 14.4% for CsT. This enhancement in the thermal stability of the prepared CsT relative to pure Cs could be attributed to the interaction via hydrogen bonding between Cs and hydroxyl groups of talc as well as the high thermal stability of T [24,60,61].The prepared composites CsTC1 (Fig. 4c) and CsTC2 (Fig. 4d) exhibited a higher thermal stability than the unmodified Cs. This demonstrates that the incorporation of Closite 30B improves the thermal stability of polymer matrix in accordance with previous results [4,6]. The hybrid composites showed approximately the same weight loss and there were no significant differences in the weight loss curves. The small reduction in thermal stability is most likely due to the creation of Cloisite 30B clay platelet agglomerates and phase separation at higher concentrations of Cloisite 30B, which promotes local stress generation and weakens the interaction bonds between CsT and Cloisite 30B particles. Poddar et al. [62] have proposed that a lower amount of Cloisite 30B clay improves thermal stability, whereas, higher clay concentrations seem to reduce thermal stability. Furthermore, it was noticed that with incorporating C into CsT composite, a decrease in the degradation temperature of the resulting composites (CsTC1 and CsTC2) occurred. This was ascribed to the low thermal stability of the organic modifier (surfactant) in the C composition. In conclusion, the weight loss in case of composites (20-22%) was very lower than that of Cs (45.5%) at 700 °C. Table 2 provides the mass loss for all samples at 700 °C. This demonstrated that the thermal stability of CsTC composites was improved after the incorporation of Cloisite 30B, as recently reported by Safarzadeh et al. [4].

Zeta potential Analysis
To explore the surface charge of the prepared samples (Cs, CsT, CsTC1, and CsTC2 composites), the zeta potential (ZP) was determined as revealed in Fig. 5. Zeta potential value of pure Cs was − 9.98 mV. The addition of talc to Cs increased the negative charges on the surface and the ZP value increased to − 11.4 mV. This could be due to the negative charge on the talc's surface. As a result of the results discussed above, it is predicted that CsT composite may show higher adsorption affinity towards cationic dyes (CV) and on the otherwise lower adsorption affinity towards anionic dyes (RY) in comparison with pure Cs, as will be confirmed in the dye adsorption section. The inclusion of Cloisite 30B clay to CsT composite decreased the value of the ZP to − 10.9 mV and − 5.95 mV for CsTC1 and CsTC2 composites, respectively. Similar observations in zeta potential values have also been reported for PMMA/Cloisite 30B composites containing varying concentrations of cloisite 30B clay [62]. They found that increasing clay content decreased the zeta potential to less native values. Bhanvase et al. [63] observed a consistent trend of zeta potential drop with nanocomposite formation and increasing clay concentration. This decrease suggests an increase in the latex stability due to the uniform distribution of silicate layers in the polymer matrix [62]. Figure 6 A reveals the N 2 adsorption-desorption isotherms of various samples including Cs, CsT, CsTC1, and CsTC2 composites. It is clear that the isotherms of the formed composites (CsTC1 and CsTC2) after the incorporation of Cloisite 30B clay exhibited a hysteresis loop that displaying their mesoporous character. The type of the isotherm is type IV. The specific surface area value was 13.53 m 2 /g for Cs and after combination with talc it was increased to 14.60 m 2 /g. For CsTC1 and CsTC2 composites, the surface area reached 15.16 and 17.65 m 2 /g, respectively. Figure 6 B shows the estimated pore size distribution using DFT model for all samples. It is noticed that the addition of Cloisite 30B clay resulted in the development of larger pores. The size of the pores is less than 50 nm that confirmed the mesoporous properties. The average pore size was 2.81 nm for Cs, 5.32 nm for CsT, 7.86 nm for CsTC1, and 9.62 for CsTC2 composite. In addition, the total pore volume was 0.019, 0.039, 0.060, and 0.085 cc/g for Cs, CsT, CsTC1, and CsTC2, respectively. Table 2 lists the values the textural parameters for all samples. It is concluded that the first modification for Cs by talc followed by a second modification using Cloisite 30B results in a stepwise enhancement for the textural parameters, including surface area, average pore size, and total pore volume, in addition to the development of mesoporous character. These results predicted the improved dye adsorption properties of the prepared composites as previously reported [48].

Single System Dye Removal
Dye removal efficiency of the developed chitosan/talc/ Cloisite 30B hybrid composites (CsTC1 & CsTC2) in comparison with pure chitosan (Cs) and chitosan/talc composite (CsT) towards cationic dye and anionic dye was evaluated by examining the adsorption performance of the prepared samples in the CV and RY dye solutions separately. Figure 7 shows the UV-Vis spectra of cationic CV dye solution (C 0 = 30 mg/L) at 0 min and after 240 min of adsorption time using several adsorbents. These spectra indicate that after 240 min, both pure Cs (Fig. 7a) and CsT (Fig. 7b) caused a small decrease in the main absorption band of CV at 586 nm. However, once C was incorporated to get CsTC1, there was a significant decline in the intensity of the CV dye, Fig. 7c. Increasing concentrations of clay resulted in a faster decline in the CV peak. For the CsTC2 (Fig. 7d) the peak of CV dye was almost diminished, indicating the highest adsorption affinity for CV dye. The removal percentage for CV dye using Cs, CsT, CsTC1, and CsTC2 adsorbents was estimated as shown in Fig. 7e. Cs had the lowest removal efficiency of only 20%. The addition of T to give CsT composite, caused a small increase in the removal efficiency to 22.7%. On the other hand, the modification with C significantly increased the removal efficiency up to 68.8% and 96.5% for CsTC1 and CsTC2, respectively. These results might be explained based on the charge of Cs, T, and C in addition to the dye molecules. Because of its cationic nature, chitosan exhibited low adsorbing activity towards cationic dyes (CV). Thanks to the hydrophilic O-H groups on the backbone of talc thus it is expected that cationic dye molecules can be adsorbed onto talc via electrostatic interactions [26,28]. But here, it was noticed that the adsorption activity of CsT towards cationic dye wasn't so high. This was attributed to that the Cs has a positively charged surface owing to the presence of protonated amine in its structure. Hence, when T was incorporated into Cs matrix, there might be an electrostatic interaction between the positively charged surface of Cs and negatively charged surface of T [64]. This might lead to suppression in the adsorption activity of CsT composite for cationic CV dye. However, with the incorporation of C, the adsorption activity of the resulting composites (CsTC1 and CsTC2) towards cationic CV dye was enhanced with increasing clay content. This may be owing to that Cloisite 30B clay has negative active sites (Si-O-and OH-groups) that can interact with dye molecules via forming hydrogen bonds. Reddy et al. [65] reported that Cloisite 30B has a net negative charge on the surface. This in accordance with the obtained dye removal efficiency results, so as clay content in the composite increased as the adsorption activity of CsTC composite increased. Based on these results, CsTC2 composite was selected to investigate its kinetics and isotherms for CV dye adsorption as it exhibited the highest adsorption activity towards the CV dye. Additionally, the influence of pH solution, adsorbent dose, contact time and initial concentration of dyes on the removal of CV by CsTC2 composite was investigated.
The activity of samples including, Cs, CsT, CsTC1, and CsTC2 towards adsorption of anionic reactive yellow 145 dye (RY, C 0 = 35 mg/L) was also studied. Figure 8 presents the time dependent UV-vis spectra of RY dye solution in the presence of several adsorbents. The maximum absorption  CsT, c CsTC1, and d CsTC2 composites band of RY dye at 419 nm gradually decreased with increasing adsorption time. It can be seen that the CsTC1 sample, Fig. 8c, exhibited the highest reduction of the anionic dye concentration. The removal percentage for RY dye using Cs, CsT, CsTC1, and CsTC2 adsorbents was estimated as shown in Fig. 8e. Cs showed 81.4% removal efficiency. After the addition of T to form CsT composite, the removal for RY decreased to 69.3%. On the other hand, it was observed that with the inclusion of C, the removal efficiency showed an increase up to 98.4% for CsTC1. A further increase in the amount of C caused a decrease in the RY removal efficiency to 84.7%. The charge of the Cs, T, and C, as well as the dye molecules, could account for these findings. Cs has positive charges, so it has high adsorption activity towards the anionic RY dye and this was confirmed by the obtained result in Fig. 8a and e. However, it was found that when T was incorporated into the Cs matrix, the affinity of the resulting composite was reduced. To clarify the reason for this finding, the adsorption of raw talc was studied and it was found that pure talc powder has no any adsorption ability for RY (not shown) due to repulsion between negative function groups of talc and anionic dye molecules. Thus, it could be speculated that the addition of talc to Cs led to a clear decrease towards the RY removal performance when using the obtained CsT (Fig. 8 b and e) in comparison with unmodified Cs. On the other hand, it was observed that with the inclusion of C, the adsorption activity of the resulting composite (CsTC1) towards anionic (RY) dye significantly increased. This might be explained as follows the organic modifier in cloisite30B is a bifunctional amphiphilic surfactant (ammonium bearing alkyl chains and hydroxyl groups) [6]. Thus, the presence of ammonium cations in its structure would provide strong electrostatic interaction between C and the anionic dye molecules, which resulted in a high percentage removal for RY ( Fig. 8c and e). But with a further increase in the amount of C, a decrease in the removal of the dye was observed ( Fig. 8d and e). This could be explained by the tendency of clay to form agglomerates in the polymer matrix when used in a high amount. Clay particles that are aggregated may not absorb dye molecules as effectively as those that are scattered. The presence of such agglomerates might have an influence in blocking the effect of ammonium groups on the adsorption of anionic dye, as shown by the decrease in RY removal by CsTC composite with high clay content. The same phenomenium was observed by Safarzadeh et al. [4]. They found that high concentration of Cloisite 30B in the poly(methacrylic acid-co-acrylamide) decreased the adsorption efficiency for MB dye, owing to reducing the penetration of MB-dye molecules into the nanoclay composite adsorbent structure. But this assumption isn't supported by SEM photos. Therefore, we suggest another explanation for these results. In comparison with CsTC2, CsTC1 composite contains higher amounts of Cs, which has high adsorption capacity for anionic RY dye. As a result of the synergistic effect of Cs and C in the CsTC1 composite, the removal efficiency of the produced CsTC1 composite towards anionic dye was greatly improved. Whereas, increasing clay content in CsTC2 composite made the adsorption characteristics of clay more effective. Pure chitosan (Cs) exhibited good adsorption properties towards RY dye (81.4%, Fig. 8e) whereas pure Cloisite 30B (C) showed intermediate adsorption ability for RY dye under identical conditions (62.5%, data not shown). This means that Cs is substituted with less active component, consequently the activity of CsTC2 composite towards adsorption of anionic RY dye was decreased.
Owing to the highest adsorption affinity of CsTC1 towards anionic RY dye, the kinetics and isotherms for RY adsorption onto CsTC1 were also investigated. Additionally, the influence of contact time, adsorbent dosage, pH, and initial concentration of dyes on the removal of RY by CsTC1 composite was investigated. In conclusion, the obtained results revealed that Cs, T, and C had synergistic effects on the removal efficiency for CV and RY.

Binary System Dye Removal
The activity of CsTC2 and CsTC1 composites towards simultaneous removal of CV and RY was studied. Figure 9a and b shows the absorption spectra of RY-CV mixed solution as a function of time in the presence of CsTC1 and CsTC2, respectively. These results indicated that the developed composites effectively adsorb both cationic and anionic dyes giving our manufactured composites an additional advantage. Pure Cloisite 30B clay (not shown) exhibited adsorption properties for cationic CV dye (100% removal), due to hydroxyl function groups in its structure, and also its adsorption ability for anionic RY dye may be owing to its ammonium cations (62.5% removal). Thus, the incorporation of clay (C) into CsT composite increases the adsorption properties towards CV and RY dyes. Finally, the developed CsTC hybrid composites acquired advantages relative to its constituent parts.

Influence of Solution pH
The pH could have an effect on the surface charge and functional groups ofthe adsorbent material, which would have a direct impact on the overall adsorption capacity. In order to determine the optimal pH, CV removal by CsTC2 composite and RY removal by CsTC1 was performed in the pH range of 4 to 10. NaOH and HCl were used to change the pH of dye solutions. Figure 10a demonstrates a clear improvement in CV removal as pH increases. Because of the low pKa value, CV is almost completely ionized under all conditions and exists as positively charged dye ions. At lower pH, the high concentration of hydrogen ions in the solution would compete with CV + for the adsorbent's insufficient active sites. As a result, CV removal was reduced in an acidic environment [8]. Also, Cloisite 30B particles become protonated in acidic conditions, resulting in a greater accumulation of positive charge [44], which decreases CV adsorption. At high pH values, the hydroxyl and amido groups in composites would release H + , resulting in an increase in negative charge. The improved electronegative composite would speed the absorption and combination of additional electropositive CV molecules. The data shown in Fig. 10a indicate that the CV removal efficiency increased significantly by increasing initial pH of dye solution from 4 to 8.5. After that increasing pH from 8.5 to 10 had no significant impact on the amount of CV that could be adsorbed. Similar behavior was reported by Kyi et al. [66]. These findings prompted the decision to conduct further tests at a pH of 8.5, which is the pH at which CV solution exists in nature. What's more, this result is relevant to the implementation of the adsorption process on large scale because it eliminates the requirement to alter the pH of the effluent prior to treatment.
For RY adsorption on CsTC1 composite, Fig. 10b, when the pH goes up, the amount of dye that sticks to composite surface goes down. The dye removal efficiency is almost the same in pH ranged from 4 to 8. At pH 10 the adsorption significantly decreased. This decrease may be caused by the repulsion force between the negatively charged dye molecule and the negative surface of composite. Excess hydroxyl ions decreased protonation of amine groups of chitosan and also reduced the positive charge impact of modifying surfactants included in the Cloisite 30B structure, consequently poor dye removal for anionic RY. As previously mentioned [67], the adsorption efficiency for RY 145 is decreased by increasing pH. Thus, for the rest of our studies on RY adsorption, we chose to start with a solution with a pH of 8.

Influence of Composite Dosage
Cost-effective color removal from wastewater has stimulated research into the smallest effective dose of adsorbent for maximizing dye removal. Various amounts of CsTC2 varied from 0.5 to 1.25 g/L and CsTC1 ranged from 0.1 to 0.8 g/L for removal of CV and RY, respectively. As indicated in Fig. 11, the dye elimination was improved with increased composite dose. This was due to an increase in valid adsorption sites [48]. Adsorption might be sped up at higher doses since there would be additional adsorption sites to interact and combine dye with the composite, leading to faster removal [48]. Testing showed that CsTC2 dosage of 1 g/L and CsTC1 dosage of 0.57 g/L was effective in removing CV and RY, respectively. Therefore, these doses were employed in the subsequent testing.

Influence of Contact Time
The influence of contact time on the removal of RY dye (40 mg/L) using CsTC1 and CV dye (30 mg/L) using CsTC2 was studied and the obtained results are presented in Fig. 12. From this figure, it is indicated that whatever the investigated adsorbent material (CsTC1 or CsTC2) and adsorbate molecules (CV or RY dye), by increasing the contact duration from 30 to 240 min, the removal percentage for both dyes increased until the adsorption rate achieved equilibrium state. This was owing to the natural adsorption process. Because all adsorbing sites are free, the dye molecules can interact with them, and the amount of residual dye molecules drops dramatically, causing higher removal efficiency. Afterwards, as the adsorption sites were saturated with pollutant molecules, the removal efficiency becomes almost constant at 270 min [68]. Similar findings were observed in other studies [69,70]. The optimal adsorption time was 240 min.

Influence of Initial Concentration of Dyes
In these experiments, the impact of varying the concentration of CV dye (10,15,20,25,30,35, and 40 mg/L) on the removal percentage was studied using CsTC2 as adsorbent sample (Fig. 12). In the case of RY dye, the initial dye concentrations were (30,35,40,50, and 60 mg/L) and the respective adsorption percentage onto CsTC1 was estimated, as shown in Fig. 12. Both adsorbents, CsTC1 and CsTC2, exhibited the same trend, that is, as the concentration of adsorbate dye (CV or RY) increased, the percentage of dye removal reduced. This was owing to the fact that raising the dye concentration increases the number of dye molecules, but the number of active adsorption sites on the adsorbent material is fixed, hence increasing the initial dye concentration decreases the decolorization effectiveness. Other studies reported similar results [48,70]. The optimal concentration for CV and RY dyes was 30 mg/L and 40 mg/L, respectively.

Adsorption Kinetics
CsTC2 composite was the best adsorbent for CV dye, and CsTC1composite was the best adsorbent for RY dye. Therefore, the adsorption kinetics of CV dye was examined onto CsTC2 sample, and the adsorption kinetics of RY dye was examined onto CsTC1 sample. Figure. 13 shows the linear fitting of PFO, PSO, IPD, and Elovich kinetic models to the experimental adsorption data. For CV, 1 g/L of CsTC2 was added to CV solution with initial concentration of 30 mg/L. The adsorbent dose was 0.57 g/L for CsTC1 using RY solution with initial concentration of 40 mg/L. The value of the correlation coefficient (R 2 ) is used to evaluate the validity of each kinetic model. Table 3 summarizes the kinetics calculation results and the correlation coefficient (R 2 ) values for PFO, PSO, IPD, and Elovich model for CV and RY adsorption onto CsTC2 and CsTC1, respectively. Owing to the extraordinarily high correlation value, 0.994 for CV and 0.997 for RY, the PSO kinetic model was ideal for the adsorption processes. This demonstrated that the CV adsorption onto CsTC2 and RY adsorption onto CsTC1 were controlled by a chemical adsorption step [48,70].  Figure. 14 illustrates the adsorption isotherm models, including Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich for adsorption of CV and RY onto CsTC2 and CsTC1, respectively. The sorption equilibrium studies were carried out using CV with initial concentrations of 10,15,20,25,30,35, and 40 mg/L using CsTC2 composite. For RY, the initial concentrations were 30,35,40,50, and 60 mg/L using CsTC1 composite. According to the correlation coefficients (R 2 ), we can select the best fitting model. The correlation coefficient (R 2 ) value and model parameters for each isotherm were estimated as shown in Table 4. From these results, it could be seen that the R 2 value of the Langmuir isotherm model was very high (0.992 for CV and 0.992 for RY), indicating it was the most suitable model to describe the adsorption processes. The adsorption capacity (q m ) values for CsTC2 and CsTC1 composites were  [71]. The sorption process was unfavorable at R L > 1, favorable at 0 < R L < 1, and irreversible at R L = 0 (37,55). The value of the separation factor (R L ), was equal to 0.012 for CV (C 0 = 30 mg/L) and 0.005 for RY (C 0 = 40 mg/L). The obtained results demonstrated that the adsorption of CV and RY on CsTC2 and CsTC1composites, respectively, were desirable.

FTIR After Adsorption
FTIR spectra of CsTC1 and CsTC2 composites after the adsorption of RY and CV, respectively, relative to that before dye adsorption are displayed in Fig. 15. It was observed that the adsorption of RY dye on the surface of CsTC1 and also the adsorption of CV on the surface of CsTC2 were accompanied by a small shift of the absorption peaks to higher wavenumber in addition to a clear decrease in their intensities, including the characteristic peaks at 3673 cm − [26,28,44,57,59], the − OH and − NH of Cs, and the overlapping bands of the talc, Cs, and C, respectively. These results indicate that these functional groups participated in the dye adsorption process (See Fig. 16)

Proposed Adsorption Mechanism
The proposed mechanism for simultaneous adsorption of CV and RY on the surface of the developed CsTC hybrid composites is based on the structure, function groups, and charge of their constituent components. CsTC composites consist of chitosan (Cs), talc (T), and Cloisite 30B clay. Chitosan contain hydroxyl groups as well as amino groups that make chitosan a cationic polyelectrolyte. Talc is made up of magnesium hydroxide (MgO⋅H 2 O) layers sandwiched between two silicate (SiO 2 ) layers, and it is hydrophobic on their basal planes due to the presence of siloxane groups (-Si-O-Si-) but hydrophilic at edges due to the presence of hydroxyl groups (-SiOH & -MgOH) [26]. Cloisite 30B is a modified montmorillonite clay, and the organic modifier in Cloisite 30B is a bifunctional amphiphilic surfactant (ammonium bearing alkyl chains and hydroxyl groups) [6]. Based on the FTIR results, CsTC composites possess a large number of functional groups such as − OH, − NH 3 + , Si − O, Mg − OH, and AL − OH. After dye adsorption, the intensity of the peaks of these groups were significantly decreased suggesting that these functional groups were involved in the dye adsorption. There are different types of interaction forces can be occurred between the dye molecules and adsorbents such as electrostatic attraction and hydrogen bonding. The electrostatic attraction force arose as a result of the difference in charges between the dye molecules and the surface of the adsorbent. This interaction can occur between the molecules of cationic CV dye and negatively charged groups of T present in CsTC [72,73]. At the same time the molecules of RY dye have negatively charged groups (− SO 3 − ), so strong electrostatic attraction would occur between RY molecules and positively charged groups of Cs (− NH 3 + ) and C (ammonium cations), which are present in CsTC composites. In addition to the electrostatic attraction force, another type of force can be involved in the dye adsorption process is the hydrogen-bonding attraction. Hydrogen-bonding attraction arose as a result of formation of H-bonds between amine groups in the structure of dye molecules and − OH groups present in the structure  of the adsorbent material [72,73]. Thus hydrogen bonds may be formed between hydroxyl group of talc or chitosan or Cloisite 30B and CV or RY dye. All these forces are illustrated in Scheme 1.

Comparison with Other Adsorbents
To evaluate the usefulness of using the prepared hybrid composites to remove both cationic and anionic dyes,the maximum adsorption capacities of the prepared composites were compared with different adsorbents previously reported in the literature for removal of CV and RY dyes, Table 5. From this comparison, it was concluded that the maximum adsorption capacity (q max ) values of the prepared composites in this work were in line with the adsorbents reported in the literature used for the removal of CV or RY dyes. This indicates that the composite prepared in this work is good candidates for the effectively removal of cationic CV and anionic RY dyes from aqueous solution. In addition, no research has been conducted to investigate the adsorption properties of the same adsorbent towards CV and RY dyes as presented here.

Conclusion
Eco-friendly hybrid composites have been successfully developed from chitosan (Cs), talc powder (T) and Cloisite 30B (C) for the first time for simultaneous removal of cationic (CV) and anionic (RY) dyes. The prepared samples were characterized by XRD, SEM, FTIR, TGA, surface area, and zeta potential. The addition T to Cs polymer matrix resulted in higher thermal stability and increased textural parameters of the produced CsT composite. After that Cloisite 30 B clay was incorporated to CsT with weight ratio of 1:2 and 2:2 to obtain CsTC1 and CsTC2 composites, respectively. These composites exhibited higher thermal stability and larger surface area in comparison with unmodified chitosan. XRD and FTIR analyses confirmed the effective interaction between composite constituents in addition their surfaces were free of agglomeration as presented in SEM photos. The adsorption results showed that the Cloisite 30B (C) amount strongly affected the removal efficiency of the prepared composites towards CV and RY removal; as the amount ratio of C increased, the adsorption activity of the resulting composite towards cationic CV increased whereas Scheme 1 Proposed mechanism for simultaneous adsorption of CV and RY dyes on the surface of the developed chitosan/talc/Closite 30B (CsTC) hybrid composites the adsorption of anionic RY showed a different trend. The composite with a weight ratio of 2:2 exhibited the highest adsorption activity for CV dye with adsorption capacity of 37.03 mg/g, on the other hand the composite with a weight ratio of 2: 1 was the best one for the removal of RY dye with adsorption capacity of 76.9 mg/g. The adsorption process of RY and CV on these composites followed the pseudo second order model. The langmuir isotherm model described the experimental adsorption data better than the Freundlich, Dubinin-Radushkevich, and Temkin isotherm models, indicating a monolayer sorption process for both dyes. The proposed mechanism for simultaneous adsorption of CV and RY was discussed. The prepared composites in this work can be considered as eco-friendly and economical candidates for simultaneous adsorption of different types of dyes from aqueous solution.