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Original Article
DOI: 10.1016/j.jmrt.2018.07.020
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Preparation and characterization of chitosan/clay composite for direct Rose FRN dye removal from aqueous media: comparison of linear and non-linear regression methods
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Abida Kausara, Kashaf Naeema, Muhammad Tariqb, Zill-i-Huma Nazlia, Haq Nawaz Bhattic, Farhat Jubeena, Arif Nazird,
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anmalik77@gmail.com

Corresponding author.
, Munawar Iqbald
a Department of Chemistry, Govt. College Women University, Faisalabad, Pakistan
b CVAS, University of Veterinary and Animal Sciences, Jhang Campus, Pakistan
c Department of Chemistry, University of Agriculture, Faisalabad, Pakistan
d Department of Chemistry, University of Lahore, Lahore, Pakistan
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Tables (8)
Table 1. Kinetic parameters for Rose FRN dye sorption as function of time.
Table 2. Isotherm parameters obtained by using linear methods.
Table 3. Isotherm parameters obtained by using non-linear methods.
Table 4. Optimization of equilibrium and kinetic models for sorption by error functions.
Table 5. Thermodynamic parameters for dye sorption as a function of temperature.
Table 6. Comparison of the effects of different interfering cations on dye sorption.
Table 7. Expected functional groups of raw clay and chitosan/clay composite.
Table 8. BET surface area analysis of raw and chitosan/clay composite.
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Abstract

In the present study, sorption efficacy of chitosan (β-(1→4)-linked d-glucosamine and N-acetyl-d-glucosamine) composite for synthetic direct Rose FRN dye removal from aqueous media was investigated. Chitosan and clay were subjected to chemical modifications to prepare chitosan/clay composite. Batch sorption affecting parameters like pH, composite dose, volume, initial dye concentration, time and temperature were optimized. Maximum sorption capacity (17.18mg/g) was found within first 40min of contact. Point of zero charge was found to be 7.0. Linearized and non-linearized regression forms of pseudo 1st and 2nd order kinetic models were used to predict the nature of rate limiting steps involved in the sorption process. Sorption equilibrium data was revealed by applying linear and non-linear equilibrium Langmuir, Freundlich and Redlich–Peterson isotherm models. Calculated values of thermodynamic factors showed that sorption process is exothermic, spontaneous and feasible. Desorption studies were performed for the regeneration of chitosan/clay composite by using different eluting agents. The synthesized composite were characterized by X-ray diffraction (XRD), surface analysis (Brunauer, Emmett and Teller: BET), scanning electron microscopy (SEM), Fourier transforms infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The developed method was also applied on the real textile effluent for the efficient removal of dyes.

Keywords:
Chitosan
Composite
Batch sorption
Linear regression
Nonlinear regression
Full Text
1Introduction

Risk free water is needed by all living organisms for their proper metabolic and temperature regulation processes as it comprises about two third of human body. Water contamination is mainly caused by the anthropogenic activities that result in terrible environmental problems. There has been an irresistible growth in the fabrication and usage of synthetic chemicals. Water supplies have been contaminated by most of these chemicals over the years. In textile industries, it is estimated, 10% of the chemical in textile processing will remain on the fabrics 90% will be discharged in textile effluent. The emission of wastewater from these industries constituting dyes is a severe menace for receiving water bodies around the industrial zones. These effluents can destroy the entire ecosystem because plant life is affected by these toxic organic dyes [1].

The textile dyeing wastes comprise partially used organic compounds, strong color and chemical oxygen demand (COD) and biological oxygen demand (BOD). The chemical oxygen demand is increased by the presence of dyes in water bodies [2]. The changes in physical, chemical and biological properties of water are caused by the discharge of dye loaded wastewater to rivers and basins that cause a possible risk to local ecosystem and occupant's health [3].

These dyes were found to be toxic and sometimes cancer-causing, disturbing the whole ecosystem. These dyes have been found to be inert because of their complex aromatic structure that make them unaffected to light, biodegradation and oxidation process and thus are difficult to remove [4]. The low concentration of dye molecules in wastewater is found to be another difficulty. Removal by sorption onto activated carbons, activated sludge, ozonation, coagulation, microbial action, flocculation, reverse osmosis, chemical oxidation and physical approaches like membrane filtration and electrochemical methods are either ineffective or expensive techniques at low dye concentration at large scale [5]. The sorbents may be of organic, inorganic or biological source, industrial byproducts, zeolites, polymeric compounds, biomasses and agrarian wastes. Many natural sorbents including clays, siliceous material and agricultural waste products because of their abundance and little expenses have been used for wastewater treatment effectively and economically [6].

Chitosan is a de-acetylated derivative of chitin, after the cellulose a second most abundant polymer in nature. Among the biopolymers, chitosan has been found to have maximum sorption capacity. It can be extracted from crawfish shell and other crustaceans [7–10]. Chitosan gained interest of scientists not only because of its various properties but also due to its unique applications in various fields such as in biomedical industries, in food industries for food preparations, as soothing, binding, condensing and crystallizing agent, in chemical plants for wastewater handling. Chitosan is a cationic polymer that shows high affinity for most dyes especially for anionic dyes [11–13].

Chitosan is an attractive polymer to be developed into multifunctional adsorbent depending upon the field it needs to be applied. Composite comprises of two or more ingredients with distinguishable interface and diverse physical and chemical characteristics, have several benefits such as corrosion resistance, high toughness, thermal insulation, low density and flame retardancy. Previously different types of clay have been used for chitosan modification [11,14].

The accumulation of clay into polymer backbone was found to lead improved strength and heat resistance [15]. Due to their ecological friendly properties, an attractive way to develop composite has been found by the combination of clay and biopolymer with properties of high sorption capacity. The binding mechanism of local natural clay composites for the removal of dyes have been understand and utilized in this study. So this work is observed to be cost effective and innovational technique for treatment of selected dyes loaded water using chitosan/clay composites as sorbent.

2Material and methods2.1Collection and preparation of sample

The raw clay with composition of SiO2: 49–52%, Al2O3: 10–12%, Fe2O3: 5–6%, CaO: 8–8.5%, MgO: 2.7–2.8%, SO3: 0.02%, K2O: 2–2.3% and Na2O: up to 1.00% was collected from Best way cement industry, Hattar, Pakistan. Rose FRN dye was collected from Masood textile industry, Faisalabad, Pakistan. Firstly, clay was extensively washed with tap water and then three times with double distilled deionized water (DDW) to remove water soluble surface contaminants. After washing, clay was air dried at ambient temperature then ground and sieved to obtain a homogenous material of uniform size. The prepared clay was then stored in desiccators until use.

2.2Preparation of aqueous dye solutions and synthesis of biopolymer composite

Stock solution (1000mg/L) of Rose RFN dye was prepared by dissolving 1g of dye in 1000mL of double distilled water. The experimental solutions of different concentrations ranging from 10 to 100mg/L were prepared by further dilutions of stock solution. The solution was scanned from 190 to 900nm for λmax measurement (541nm) of dye standard curve was developed through the measurement of the absorbance of dye solution by double beam spectrophotometer [16]. The modified chitosan (MW 50,000Da; Sigma–Aldrich)/clay composite was obtained by following the method [17] and is shown in Fig. 1 and sieved to a powder form for use. The point of zero charge (pHpzc) was determined by solid addition method [18]. The final pH of the solution was recorded and difference between initial and final pH (ΔpH) (Y-axis) was plotted against initial pH (X-axis). The point of intersection of this curve yielded point of zero charge.

Fig. 1.
(0.16MB).

Texture of prepared chitosan/clay composite.

2.3Batch sorption

Dye containing effluents have a variety of chemical composition and their binding interactions with sorbent depend on the chemical structure, the specific chemistry and morphology of the sorbent surface and properties of the dyes in solution or wastewater. Therefore, it is necessary to see effects of parameters like pH, composite dose, contact time, initial dye concentration, volume of dye solution and temperature on reaction to investigate true mechanism of reaction as well experimental conditions optimization. The equilibrium sorption uptake, qe (mg/g), was calculated using the following relationship:

The percent removal was calculated by using following formula:
where C0 is the initial dye concentration (mg/L), Ce is the equilibrium dye concentration (mg/L), V is the volume of the solution (L) and W is the mass of the composite (g).

The experiment was performed using 0.05g of composite at different pH 2–11 by using 0.1M solutions of NaOH and HCl to adjust the pH. The effect of chitosan/clay composite amount on sorption of the Rose FRN dye was studied by using different amounts (0.05–0.25g) in 25mL. The experiment was performed using 25mg/L of dye initial concentration at optimized pH, at 30°C and shaking speed of 125rpm for 1h. The equilibrium time required by the composite to bind to dye was determined by adding composite (0.05g/25mL) in solution of 25mg/L of dye for different time period (10–160min). The experiments were carried out at different initial dye concentrations (10–100mg/L) by adding 0.05g of chitosan/clay composite to Rose FRN dye at previously optimized conditions. The sorption experiments were performed at different temperatures (30–60°C) to find the effect of time. The effect of volume of dye solution on sorption capacity of chitosan/clay composite was studied by using 0.05g of composite dose in different volume (20–70mL). The experiments were performed using 25mg/L of initial dye concentration at optimized pH, at 30°C and shaking speed of 125rpm till equilibrium time.

2.4Sorption kinetics

The pseudo-first order kinetic model [19] based on solid capacity, expresses the mechanism of removal as a sorption preceded by diffusion through a boundary. It considers that the sorption is partial first ordered depending on the concentration of free sites. Pseudo-first order kinetic model is based on the fact that the change in dye concentration with respect to time is proportional to the power one. The non-linear and linear forms of model are given below

where qe and qt are the amount of dye adsorbed (mg/g) at equilibrium and at time t (min), respectively, and k1 (min−1) is the pseudo-first-order rate constant. Values of k1 are calculated from the plots of log (qeqt) versus t.

Pseudo-second order kinetic [20] model is based on the assumption that sorption follows a second rate kinetic mechanism. So, the rate of occupation of sorption sites is proportional to the square of the number of unoccupied sites. Linear and nonlinear forms of pseudo-first and second-order expressions used in this study are presented below.

where qe and qt in pseudo-second order equations are the amount of dye adsorbed on composite (mg/g) at equilibrium and at time t (min), respectively, and k2 is the pseudo-second order rate constant (g/mgmin). Based on the experimental data of qt and t, the equilibrium sorption capacity (qe) and the pseudo-second-order rate constant (k2) can be determined from the slope and intercept of a plot of t/qe versus t.

2.5Equilibrium, thermodynamic and desorption studies

The Freundlich [21] equation is an empirical equation employed to describe heterogeneous systems, in which it is characterized by the heterogeneity factor 1/n. Hence, the empirical equation can be written:

1/n is the heterogeneity factor. When n=1, the Freundlich equation reduces to Henry's Law. A linear form of the Freundlich expression can be obtained by taking logarithms of Eq. (2.7).
The values of KF and 1/n are calculated from the intercept and slope respectively in linear regression method.

Langmuir [22] proposed a theory to describe the adsorption of gas molecules onto metal surfaces. The Langmuir adsorption isotherm has found successful application to real sorption processes of monolayer adsorption. Theoretically, the composite has a finite capacity for the sorbate. Therefore, a saturation value is reached beyond which no further sorption can take place. The saturated or monolayer (as Ce∞) capacity can be represented by the expression:

The Langmuir equation degenerates to Henry's Law at low concentration. Langmuir isotherm can be linearized into at least four different types and simple linear regression will result in different parameter estimates

A linear expression of the Langmuir-1 equation is:

Therefore, a plot of Ce/qe versus Ce gives a straight line. Where qe is the amount of dye sorbed on the chitosan/clay composite (mg/g) at equilibrium, Ce is the equilibrium concentration of dye (mg/L), qm is the maximum sorption capacity describing a complete monolayer adsorption (mg/g) and Ka is adsorption equilibrium constant (L/mg) that is related to the free energy of sorption.

A linear expression of the Langmuir-2 equation is:

A linear expression of the Langmuir-3 equation is:
A linear expression of the Langmuir-4 equation is
Redlich–Peterson [23] incorporated three parameters into an empirical isotherm. The Redlich–Peterson isotherm model combines elements from both the Langmuir and Freundlich equation and the mechanism of adsorption is a hybrid one and does not follow ideal monolayer adsorption. The Redlich–Peterson equation is widely used as a compromise between Langmuir and Freundlich systems.
where A, B, and g are Redlich–Peterson parameters. When g=1 it becomes Langmuir equation
When g=0 it reads like the Henry's law equation:

Further non-linear form (2.9) can be converted into linear form by taking logarithms:

Redlich–Peterson's constant B and g can be calculated from linear plot of ln(A(Ce/qe)1) vs. ln(Ce). The values of Redlich–Peterson constant A can be calculated by maximizing R2 using trial and error method in Microsoft excel solver ads in function.

The optimization procedure requires the error functions in order to evaluate the best fit isotherm to explain the experimental kinetic and equilibrium data [24,25]. In this study, six non-linear error functions were examined using statistical software, i.e. R-Version 2.15.1, by minimizing the respective error function across the time and concentration range studied. The value of coefficient of determination (R2) for non-linear regression was evaluated by following formula [26].

The thermodynamic parameters for the adsorption process namely Gibbs energy (ΔG°), enthalpy of adsorption (ΔH°) and entropy of adsorption (ΔS°) were determined by carrying out the adsorption experiments at different temperatures and using the following equations [27]:

Thermodynamic parameters ΔH° and ΔS° were computed from linear plot of Log (qe/Ce) and 1/T from slope and intercept respectively.

Desorption study to regenerate the composite were done using eluting agents such as distilled water, H2SO4, HCl, NaOH and MgSO4, to compare their capacity to elute sorbed dye. To regenerate the composite, first Rose FRN (25mg/L) was sorbed under optimized conditions and the dye loaded residues dried in oven at 40°C for 24h. The loaded composite was then desorbed in 50mL of 0.1M solution of each selected eluting agent, by shaking at 125rpm till equilibrium time. Percentage desorption was calculated by formula:

where qdes is eluted dye content (mgg−1) and Cdes (mgL−1) is dye concentration in eluent solution of volume V (L) and composite weight (W) in gram.

2.6Chitosan/clay composite characterization

X-ray diffraction was used to determine the chemical composition of the composite. X-ray diffraction analyses of modified chitosan/clay were performed using a Bruker D8 X-ray diffractometer equipped with a Cu anode. X-ray diffractograms were collected in the 2θ range from 5° to 90°, using a step size of 0.02° and a counting rate of 1s per step. Specific studies of modified chitosan/clay composite determined by BET (Brunauer, Emmett and Teller), performed on a surface area analyser. Surface composition of the chitosan/clay composite was examined using a JSM-5910, JEOL model scanning electron microscope. These analyses were conducted on each sample under optimized conditions using Pt coating to avoid charge indulgence during SEM scanning in an Ar atmosphere at 10kV. Thermal analysis was done using a Perkin Elmer Pyris-1 in nitrogen (N2) atmosphere; N2 flowed at 20.0mL/min. The heating rate was set at 5.00°C/min and temperature started from 40 to 800°C.

FT-IR analysis modified chitosan/clay composite and loaded with Rose FRN dye were carried out to identify the chemical functional groups, responsible for sorption of dye molecules. FTIR data were observed by preparing KBr disk of chitosan/clay composite material and spectra were recorded on software Bio-Rad Merlin All, unloaded and loaded samples were recorded in a Fourier Transform Infrared Spectrometer (Shimadzu, IR Prestigue-21) using a silver gate apparatus by measuring percentage of transmittance against wavenumber in the range of 4000–650cm−1 and number of scans were 32 and resolution 2cm−1.

All results were discussed by reporting means along with standard deviations (±SD) for doublets.

The coefficients of equilibrium, kinetic and thermodynamic models were determined by using regression techniques [28].

3Results and discussion3.1Determination of point zero charge (pHpzc)

Point zero charge (pHpzc) was determined to obtain the information about the charge nature on the composite surface. Surface of composite will be negatively charged at pH above pHpzc and positively charged at pH below pHpzc[1]. Point zero charge (pHpzc) of chitosan/clay composite was determine by solid addition method [18], it was found to be at pH 7.0 (Fig. 2). The sorption of Rose FRN dye onto chitosan/clay composite was carried at pH above the pHzpc. So, it is estimated that surface of composite is negatively charged due to deprotonation of function groups at higher pH.

Fig. 2.
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Determination of point zero charge (pHpzc) of chitosan/clay composite.

Charri et al. [29] reported that pHzpc for smectite-rich clayey rock (AYD) as 6.5 in their research work. Anirudhan and Ramachandran [30] analyzed the pHzpc of modified clay and it was observed to be 7.8. The pHzpc for magnetic chitosan/clay beads had been reported at pH 7.3 [31].

3.2Effect of pH

The effect of pH on Rose FRN dye sorption onto chitosan/clay composite was studied at different pH range 2–11 as shown in Fig. 3a. It clearly illustrates that sorption capacity of Rose FRN dye first continuously increases with increasing pH till pH 10 and after that it starts decreasing due to precipitation. Maximum sorption capacity (13.27mgg−1) was observed at pH 10 for chitosan/clay composite. The point zero charge (pHzpc) of prepared chitosan/clay composite was found at 7.0 pH. Surface of composite will be negatively charged at pH above 7.0 and will be positively charged at pH below 7.0. The adsorption of Rh 6G onto chitosan/g-(N-vinylpyrrolidone)/montmorillonite composite was found to increase with an increase in pH of the solution [11]. The similar behavior was reported for adsorptive removal of methylene blue (MB) onto ATP/CCS, Crosslinked chitosan/bentonite composite (CCSB) composite and MBC/CH was found to be increased at relatively high pH and approaches the maximum value at basic pH [3,7].

Fig. 3.
(0.25MB).

Effect of parameters (pH, sorbent dose, time, initial dye concentration, temperature and volume) onto sorption capacity of chitosan/clay composite for Rose FRN dye.

3.3Effect of composite amount

The effect of composite amount was studied by varying the sorbent dose from 0.05g to 0.25g in 25mL solution of 25mg/L dye concentration at optimized pH and results are presented in Fig. 3b. The results indicate that as the composite dose increases, the sorption capacity of chitosan/clay composite decreases. Maximum sorption capacity (mg/g) was attained with minimum sorbent dose (0.05g). The sorption capacity of chitosan/clay composite decreased from 13.76 to 2.68mg/g as composite amount increased from 0.05 to 0.25g for Rose FRN dye.

The reason behind the decrease in sorption capacity with the increase in sorbent dose can be explained as the overlapping or accumulation of active sites takes place which results in decrease in the total composite surface area available for the attachment of dye molecules. Another important reason is that at high sorbent dosage, the available dye molecules are deficient to completely cover the available binding sites on the composite, which resulted in low solute uptake [1].

The adsorption capacity of CS-MMT hydrogel for NY dye was reported to be decreased with increasing the adsorbent [32].

3.4Effect of contact time

The experiment for analyzing the optimized time was conducted to determine the maximum adsorptive removal of Rose FRN dye onto chitosan/clay composite. The effect of contact time on the sorption of Rose FRN dye onto chitosan/clay composite was investigated over different time intervals 10–160min (Fig. 3c). From results it is clear that 13.51mg/g was the maximum sorption capacity, which was obtained during the initial stages of the sorption process. Maximum sorption was increased at 40min of sorption process. With further increase in contact time, no change in the dye removal was observed, so 40min was considered as equilibrium time.

3.5Effect of initial dye concentration

The effect of changing initial dye concentration was studied in the range of 10–100mg/L by keeping the other parameters same like pH 10, sorbent dose 0.05g, temperature 30°C, shaking speed at 125rpm and contact time 40min is presented in Fig. 3d. The uptake capacity of the composite sharply increased from 9.48 to 14.93mg/g, as the initial dye concentration increased from 10 to 30mg/L. After that sorption capacity slightly increased beyond those concentrations and then slowly attained the equilibrium. The maximum sorption capacity (17.18mg/g) of chitosan/clay composite for Rose FRN dye was obtained at 100mg/L. This is because at lower initial Rose FRN dye concentrations, the ratio of the initial moles of dye to the available surface area is low. Initial dye concentration found to have substantial impact on sorption, with a higher concentration resulting in a high solute uptake [33]. Similar results were reported by Auta and Hameed [7] for MB dye removal onto modified ball clay chitosan composite.

3.6Effect of temperature

The variation in sorption capacity as function of temperature was shown in Fig. 3e. The results clearly illustrate that decrease in sorption capacity of chitosan/clay composite was observed with the increase in temperature. It shows that sorption process of Rose FRN dye onto chitosan/clay composite was an exothermic process. The effect variation in temperature from 30°C to 60°C on dye uptake was small and maximum sorption capacity for Rose FRN dye was achieved at 30°C temperature. The reason may be the deactivation of available active sites on composite at higher temperature which resulted in the decreased sorption capacity at higher temperatures. Ngwabebhoh et al. [32] quoted same results for the variation in adsorption capacity of CS-MMT hydrogel for NY dye as function of temperature.

3.7Effect of volume

Effect of volume on sorption capacity of chitosan composite was analyzed by varying the volume in range of 20–70mL of Rose FRN dye solution. The results indicate that sorption capacity increases as the volume of dye solution (25mg/L concentration) increases as displayed in Fig. 3f. Adsorption capacity sharply increased from 2.4 to 10.2mg/g in accordance with volume variation from 10mL to 20mL and then slowly attained the equilibrium. The reason behind this trend is the maximum opportunity of dye molecules to saturate the available active sites of composite occupying the surface and inside pores too. With lower volume of dye solution, chances of dye molecules to interact with sorbent surface and to absorb in their pores was declined and hence, dye uptake was only 2.4mg/g at 10mL of Rose FRN dye.

3.8Sorption kinetics

The experimental data for Rose FRN dye sorption process by linear and nonlinear methods fits well to pseudo second order kinetics. The values of constant K1 and K2 rate constants of pseudo first order and second order kinetic models respectively, qe calculated, qe experimental and R2 for the sorption of Rose FRN dye are summarized in Table 1.

Table 1.

Kinetic parameters for Rose FRN dye sorption as function of time.

Kinetic parameters  Linear method  Nonlinear method 
Pseudo-first order
K1 (Lmin−1)  0.0173  0.177 
qe calculated (mg/g)  0.829  12.01 
qe experimental (mg/g)  12.12  12.12 
R2  0.489  0.914 
Pseudo-second order
K2 (Lmin−10.0566  0.0358 
qe calculated (mg/g)  12.475  12.255 
qe experimental (mg/g)  12.12  12.12 
R2  0.999  0.956 

The value of K1 and qe (mg/g) at equilibrium was obtained by plotting of log (qeqt) versus t (min), a straight line was obtained with very poor correlation coefficient (R2). The value of K2 was calculated from the slope and intercept of straight line by plotting t/qt versus t. The values of correlation coefficient R2 from pseudo-second-order plots were high and comparison of the theoretical values of qe were in good contract with experimental qe value. The comparison between experimental and predicted by pseudo-first and second-order model values is shown in Fig. 4a. Both linear and non-linear regression analysis showed the good fitness of the pseudo-second order kinetic model to the experimental kinetic data of Rose FRN dye sorption onto chitosan/clay composite.

Fig. 4.
(0.1MB).

Comparison of nonlinear kinetic and equilibrium models for Rose FRN dye sorption onto chitosan/clay composite.

3.9Equilibrium modeling

The most widely used two parameter sorption isotherms, i.e. Langmuir, Freundlich and three parameter equation i.e. Redlich–Peterson have been used to compare the experimental equilibrium adsorption data by linear and non-linearized form of these isotherm models [34].

Langmuir isotherm can be linearized into four different types [35]. Out of four different types of linearized forms, Langmuir-1 and Langmuir-2 are most widely used and the relatively higher R2 was obtained from Langmuir-1 as compared to other three forms. Langmuir-3 and Langmuir-4 gave almost equal R2 which confirm that these two types are in same error distribution. The Freundlich constant KF, exponent 1/n and value of R2 are shown in Table 2. From results it can be seen that linearized form of Freundlich isotherm have lower R2 (0.755) value. Similarly, the experimental data were further analyzed by three parameter equation Redlich–Peterson isotherm. Evaluation of three unknown parameters A, B and g of Redlich–Peterson by linearized method are not possible or efficient. So a minimization method to get maximize coefficient of determination R2 is implemented. The calculated Redlich–Peterson constants and R2 are sown in Table. 2. The very higher R2 value (0.9988) by linearized form of Redlich–Peterson shows that it is suitable to use Redlich–Peterson isotherm for Rose FRN dye sorption. The comparative values of experimental sorption capacity qe are shown in Fig. 4b and predicted equilibrium curve using non-linearized form three equilibrium isotherm models and their corresponding parameters are shown in Table 3. Different outcomes revealed complexities when coefficient of determination was compared from Langmuir-1, Langmuir-2 and Redlich–Peterson, all of them were found to have higher R2 values [35]. Similarly by of comparison of R2 from Langmuir-3, Langmuir-4 and Freundlich isotherms suggest them not be optimum isotherms. Thus it is more efficient to use non-linear methods to predict that which isotherm model is well fitted.

Table 2.

Isotherm parameters obtained by using linear methods.

Linear regression method
Langmuir type 1
qm (mg/g)  17.42 
Ka (L/mg)  1.089 
RL  0.011 
R2  0.9995 
Langmuir type 2
qm (mg/g)  17.69 
Ka (L/mg)  0.946 
RL  0.013 
R2  0.9723 
Langmuir type 3
qm (mg/g)  17.62 
Ka (L/mg)  0.977 
RL  0.006 
R  0.9413 
Langmuir type 4
qm (mg/g)  15.07 
Ka (L/mg)  0.9206 
RL  0.0133 
R2  0.9413 
Freundlich
KF  10.957 
n  7.440 
R2  0.755 
Redlich–Peterson
A (L/g)  14.46 
B (dm3/mg)g  0.724 
g  1.037 
R2  0.9988 
Table 3.

Isotherm parameters obtained by using non-linear methods.

Non-linear regression method
Non-linear Langmuir
qm (mg/g)  18.774 
Ka (L/mg)  0.248 
RL  0.048 
R2  0.941 
Non-linear Freundlich
KF  8.162 
n  0.203 
R2  0.847 
Non-linear Redlich–Peterson
A (L/g)  3.564 
B (dm3/mg)g  1.094 
g  1.746 
R2  0.950 

Six error functions were analyzed to investigate the optimum kinetic and equilibrium model for Rose FRN dye sorption onto chitosan/clay composite (Table 4). The comparatively small value of MPSD error function and higher value of correlation coefficient R2 for pseudo second order suggested the fitness of model. Other error functions for both models were found to be almost same. From linear and non-linear regression analysis the sorption process of Rose FRN dye onto chitosan/clay composite was also best described by pseudo second order kinetic model. The comparatively lower values of six error functions and higher value of correlation coefficient R2 confirmed Redlich–Peterson model as best fitted isotherm model to explain the sorption process of Rose FRN dye onto chitosan/clay composite.

Table 4.

Optimization of equilibrium and kinetic models for sorption by error functions.

Error function  Pseudo first order  Pseudo second order  Langmuir  Freundlich  Redlich–Peterson 
Sum of square error (EARSQ)  0.2652449  0.1334830  3.3771992  8.7716927  2.8539815 
Sum of absolute error (EABS)  0.9256441  0.6064623  3.7468979  7.8990682  3.5713484 
Average relative error (ARE)  1.6378815  1.0254851  2.7797995  5.8344885  2.5200033 
Hybrid fractional error function (HYBRID)  0.7883964  0.3698399  3.0624260  8.6076578  2.8834726 
Marquardt's percent standard deviation (MPSD)  2.6537019  1.7545317  4.7735012  8.4494310  4.5480826 
Chi-square test (Chi-Sq/χ20.022986766  0.011202299  0.25888289  0.67915122  0.21188626 
3.10Thermodynamic study

The thermodynamic study was carried out from the thermal data of sorption of direct Rose FRN dye onto chitosan/clay composite. The thermodynamic parameters like entropy change (ΔS), enthalpy change (ΔH) and Gibbs free energy change (ΔG) were calculated to investigate the spontaneity, feasibility of sorption process as function of temperature and to determine the effect of temperature on sorption capacity presented in Table 5. The decrease in disorder during the dye Rose FRN sorption process was suggested by the negative value of entropy change (ΔS°). The negative value of ΔH° from results confirms the exothermic nature of sorption process of Rose FRN on chitosan/clay composite. The negative value of ΔG° at optimized temperature confirm the spontaneous nature of sorption process [36].

Table 5.

Thermodynamic parameters for dye sorption as a function of temperature.

Temperature (°C)  ΔG° (kJmol−1ΔS° (Jmol−1K−1ΔH° (kJmol−1
30  −181.166     
40  11.734  −19.29  −759.866 
50  204.634     
60  397.534     
3.11Desorption studies

For commercial feasibility of sorbent for water purification, it is necessary to regenerate the used sorbent. Desorption of the sorbed Rose FRN dye by different desorbing agents such as HCl, MgSO4, NaOH and distilled water at fixed dye concentration was studied in a batch process. Desorption efficiency (qdes) of chitosan/clay composite for the Rose FRN dye was found to be maximum with HCl. Efficiency of the selected desorbing agents decrease in following order for chitosan/clay composite.

HCl>MgSO4>NaOH>Water

Desorption capacity of chitosan/clay composite was found to be maximum with 0.1M of HCl. The weak adsorptive forces between sorbent surface and dye molecules in acidic medium resulted in higher desorption of Rose FRN dye in the presence of HCl than other desorbing agents. This is due to the fact that, the surface of chitosan/clay composite was observed to be positively charge at pH below pHpzc which leads to detachment of dye.

3.12Effect of interfering ions

The effect of cations on the sorption capacity of chitosan composite is reported in Table 6. The effects of different interfering cations Cd2+, Pb2+, Ni2+ and Cu2+, on the sorption process was checked at different metal ion concentration such as 5, 10 and 15mg/L. From results it is clear that the sorption process of selected Rose FRN dye onto chitosan/clay composite is suppressed in the presence of all the above cations even at low concentration of metal cations. This decrease in sorption capacity in the presence of metal cations may be due to the interaction of cations on active sites of chitosan/clay composite and compete with dye molecules to bind on the available sorbent sites [37,38].

Table 6.

Comparison of the effects of different interfering cations on dye sorption.

Cations  5mg/L (qmix/qo10mg/L (qmix/qo15mg/L (qmix/qo
Cd2+  0.587  0.761  0.890 
Pb2+  0.710  0.735  0.826 
Ni2+  0.935  0.916  0.923 
Cu2+  0.782  0.657  0.566 
3.13Characterization of chitosan/clay composite

Thermogravimetric analysis of raw clay, chitosan/clay composite and Rose FRN dye loaded composite are shown in Fig. 5a–c, respectively. The 10–15% weight loss below 200°C is attributed to removal of interlayer water molecules labeled as stage 1. From TGA analysis of raw clay in Fig. 5a, the continuous weight loss in temperature region of 200–450°C was observed may be due to dehydration of exchangeable cations upon heating [39]. Then the complete weight loss of raw clay was observed till 800°C as displayed.

Fig. 5.
(0.13MB).

Thermogravimetric analysis of raw clay (a), unloaded (b) and Rose FRN dye loaded (c) chitosan/clay composite.

The 20–22% weight loss in stage 2 in composite from 200 to 300°C occurred due to deacetylation of chitosan followed by weight loss in region of 300–450°C may ascribed to splitting of main chain and decomposition of organic compound as shown in Fig. 5b [40]. The weight loss of Rose FRN loaded composite in stage 2 at temperature 220–360°C is attributed to deacetylation of chitosan chain and 27–30% weight loss in temperature region of 370–500°C which accounts for splitting of main chain and degradation of organic compound [14].

FTIR spectra for functional groups of raw clay, chitosan/clay composite and dye loaded composite are presented in Fig. 6. The peaks appeared at 1395, 1007, 874 and 714cm−1 may be due to SO3H group [41], Si–O group [42], Al–OH–Mg bond [43] and Si–O bond [30]. Peaks at 2514, 2164, 1800cm−1 are related to dolomite bands, associated with the stretching vibration of the carbonate group which were disappeared after composite formation [44]. Peak at 1422cm−1 is characteristic peak of pure chitosan due to CH-OH bond in chitosan/clay composite. Shift in peak at 1418cm−1 is attributed due to binding of Rose FRN dye. Peak at 2929cm−1 was observed due to the C–H stretching vibration. Peaks arising in the region 1200 to 950cm−1 may be due to Si–O–Si. The shift of O–H peak of water at 3734cm−1 is attributed to the association of the hydroxyl with the other components within the structure. Peak at 2850 and 2853cm−1 may be attributed to the stretching of –CH2 in chitosan or due to impregnated CTAB. The band 676cm−1 are attributed to Si–O–Mg bonds [43]. The peak at 1007cm−1 corresponding to stretching vibration of Si–O bond in clay is slightly shifted at 1031cm−1 due to composite formation. Peak due to Si–O bond appeared at 987cm−1 in dye loaded composite. These shifting indicate the strong interaction between corresponding functional groups. Appearance of peaks at 1532cm−1 and 1538cm−1 in the 1800–1200cm−1 range of wavenumbers are due to the amine and amide groups of chitosan in chitosan/clay composite shifted at 1541cm−1 and 1559cm−1 after dye binding. The peak at 1603cm−1 (i.e. amide I band) is assigned to CO group of chitosan. Various peaks and their expected functional groups are summarized in Table 7.

Fig. 6.
(0.22MB).

FTIR spectra of raw clay (c), chitosan/clay composite (b) and Rose FRN dye loaded chitosan/clay composite (a).

Table 7.

Expected functional groups of raw clay and chitosan/clay composite.

Expected functional groups  Raw Clay  Chitosan/clay composite  Rose FRN dye loaded composite 
Al–O–Mg bond  —  676cm−1; 88.07  — 
Al–OH–Mg  874cm−1; 43.43     
Si–O group vibrations  1007cm−1; 59.52
714cm−1; 60.14 
1031cm−1; 72.20  987cm−1; 72.48 
SO3H group  1395cm−1; 43.08  —  — 
Stretching vibration of the group (CO2−)3  1800cm−1; 93.91
2164cm−1; 90.58
2514cm−1; 93.77 
—  — 
CH–OH of pure chitosan  —  1410cm−1; 63.21  1420cm−1; 79.49 
N–H vibration of amine and amide group  —  1532cm−1; 82.37
1538cm−1; 82.23 
1541cm−1; 73.05
1559cm−1; 72.84 
CO group of chitosan  —  1603cm−1; 73.89  — 
C–H stretching of alkanes  —  2918cm−1; 82.20  2921cm−1; 93.01 
O–H stretching vibration  —  —  3734cm−1; 90.72 
–CH2 stretching vibration  —  2850cm−1; 89.42
2853cm−1; 90.19 
— 

The SEM micrographs of prepared chitosan composite and selected Rose FRN dye loaded composite are shown in Fig. 7. It can be interpreted from these figures that surface texture of composite changed considerably after loading the Rose FRN dye, a bough like surface was observed due to presence of dye molecules. Brunauer–Emmett–Teller surface area analysis of raw and chitosan/clay composite are given in Table 8. In this study the specific surface area and pore volume of raw clay increased after modification with chitosan. Removal of color from diluted real effluent using chitosan/clay composite and comparison of different desorbing agents on dye sorption onto composite was shown in Figs. 8 and 9.

Fig. 7.
(1.57MB).

SEM analysis of raw clay (Top), unloaded (Middle) and Rose FRN dye loaded (Bottom) chitosan/clay composite.

Table 8.

BET surface area analysis of raw and chitosan/clay composite.

Samples  Raw clay  Chitosan/clay composite 
Surface area  8.14153m2/g  10.4071m2/g 
Pore volume  0.004191cm3/g  0.006729cm3/g 
Pore size  19.9229Å  25.8639Å 
Nanoparticle size  7129.835Å  5765.305Å 
Fig. 8.
(0.08MB).

Removal of color from diluted real effluent using chitosan/clay composite.

Fig. 9.
(0.04MB).

Comparison of different desorbing agents on dye sorption onto composite.

XRD pattern of chitosan/clay composite and Rose FRN dye loaded chitosan/clay composite are shown in Fig. 10. Chitosan shows two distinct crystalline peaks at around 10° and 20°. The presence of plenty of hydroxyl and amino groups in the chitosan structure form hydrogen bonds, leading to generation of regular crystalline structure in the chitosan molecules [11]. In chitosan/clay composite, the peak at 10° disappeared may be attributed to the destruction of the intermolecular hydrogen bonds and the crystalline regions of chitosan, indicating successful modification of chitosan with raw clay [45]. A specific crystalline peak of chitosan at 21.7° in unloaded chitosan/clay composite is shifted to 21.0° with the decreased intensity in Rose FRN dye loaded composite. The results indicate the further decreased crystallinity of chitosan after sorption. XRD pattern of chitosan/clay composite indicated the presence of calcite (2θ: 43.07), quartz (2θ: 32.4) and dolomite (2θ: 53.8) may be attributed to presence of raw clay in composite [42]. The diffraction peaks placed between 20° and 60° (2θ) seen to be shifted to some different angles in Rose FRN dye loaded composite, which could be attributed to compression and expansion of layers after sorption of dye molecules [14]. Under the current scenario of environmental pollution [46–57], there is a need to adopt eco-friendly materials for the remediation of pollutant of environmental concerns.

Fig. 10.
(0.07MB).

XRD pattern of (a) chitosan/clay composite and (b) Rose FRN dye loaded composite.

4Conclusions

Chitosan/clay composite was synthesized and used for the sorption of Rose FRN dye from aqueous solutions. The results showed that pH of the medium, composite amount, initial dye concentration and volume strongly affected the removal efficiency of composite as compared to time of contact and temperature of the solution. The maximum sorption capacity was 17.18mg/g obtained at higher pH. The sorption capacity of composite for Rose FRN dye was found to increase with rise in initial dye concentration and volume of dye solution. The sorption process of Rose FRN followed the pseudo-second order kinetic model and Redlich–Peterson model. Kinetic models and equilibrium models were optimized by comparing R2 value of both linear and non-linear regression and six different non-linear error functions. The characteristic functional group of chitosan like amine and of clay like Si–O and Al–O were found to be involved in sorption of dye. Presence of different heavy metal ions (Cu, Ni, Pb and Cd) resulted in decreased removal of direct Rose FRN dye. Desorption study was conducted for the recovery of composite with 0.1M HCl as an eluting agent. The developed method was applied on real industrial effluent at different dilutions. Maximum removal of 87% was obtained with diluted effluent by using synthesized chitosan/clay composite. So, we can suggest that sorption technology using modified chitosan/clay composite at large scale would be effective, precise and inexpensive method for treatment of textile wastewater.

Conflicts of interest

The authors declare no conflicts of interest.

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