A Green Approach to the Removal of Malachite Green Dye from Aqueous Medium Using Chitosan/Cellulose Blend

The usage of a cost-effective, eco-friendly, and highly efficient adsorbent for the removal of dye from an aqueous solution is presented in this paper. This work focuses on the prospective use of chitosan/cellulose blend to remove malachite green (MG) from an aqueous medium. Results revealed that the maximum adsorption of the dye occurs within 30 min of the experiment. The SEM images exhibited a change in their surface morphology upon the adsorption of dye. The adsorption isotherm of MG fits the Langmuir and Freundlich equations and follows the pseudo-second-order rate equation. Freundlich isotherm exhibited the maximum adsorption capacity of 115.1 mg/g when compared to Langmuir isotherm. 98.65% of dye degradation is observed at room temperature for an adsorbent dosage of 0.2 g per 30 ml initial dye concentration. RL, dimensionless constant separation factor evaluated at room temperature in the present study is 0.1488, which specifies that the adsorption is favorable. Current analysis and its comparison studies with other reports on adsorbents conclude that the chitosan/cellulose blend can be considered a cost-effective choice for the removal of MG.


Introduction
Increasing industrialization has led to serious ecosystem concerns due to the intake of excessive toxic pollutants into either underground or exposed water bodies. The textile industry solely witnesses two-thirds of the overall dyestuff production [1,2]. Disposal of colored wastes into aquatic sources not only disturbs the esthetic nature, but also hinders diffusion of light into streams, consequently reducing the photosynthetic act. Further, chemicals present in the colored wastes inhibit the lethal impacts on microbial populations and could remain toxic to mammals [3,4]. Conventional biological treatment practices were ineffective in dye removal as dyes are non-biodegradable [5].
The recent advancements in dye removal technologies include physiochemical, biological, and chemical techniques such as flocculation and coagulation [6], adsorption [7], ozonation [8], electrochemical [9], and fungal decolonization [10]. Among these methods, adsorption is an efficient and cost-effective method in the removal of dyes from effluents and hence has gained more prominence [11]. Adsorption is a surface process that leads to the accretion of a substance (an ion or atom, a molecule) on a solid surface from its liquid or gaseous environs [12][13][14]. Usually, adsorption exhibits high treatment efficiency and adsorbents can be reused multiple times after regeneration.
The ability of adsorption is mainly dependent on the properties of the adsorbent; hence, developing an effective adsorbent is very important for its broad application in water treatment [15]. The molecular structure of both chitosan and cellulose consists of β-glycoside linkages. Therefore, there is a similarity in their structures. At C-2 positions, chitosan has primary amino groups, whereas cellulose has hydroxyl groups [16][17][18]. Chitosan is a deacetylated form of a biopolymer called chitin in the presence of a highly concentrated sodium hydroxide solution. Chitin and its derivatives have many intrinsic features, making them effective adsorbents for dye removal [19]. Chitosan/modified chitosan adsorbents 1 3 have shown large adsorption capacities for acidic, basic, and reactive dye in natural solutions [20,21].
Basic malachite green (MG) has been generally used in the coloring of leather articles, wool, silk, and jute, and in the aquaculture industry in controlling fungal infections of fish as an antiseptic [22]. It is very difficult to remove MG from aqueous solutions due to its chemical properties. This reduces growth, fertility, and food consumption, resulting in liver damage, kidney, and heart and imposing abrasions on the eyes, lungs, skin, lungs, and bones. Rats exposed to MG have encountered the prevalence of tumors in the lungs, ovary, and breasts. Exposure to MG reduces the RBC count and Hb and increases the WBC count which delays the blood coagulation [23][24][25].
Several works are available on chitosan-cellulose blend being used as adsorbent for adsorption studies. Manzoor et al. have reported the removal of Cu (II) metal ion using chitosan-carboxymethyl cellulose-based adsorbent and obtained an adsorption capacity of 142.95 mg/g [26]. Wang et al. have reported the use of chitosan-cellulose composite aerogel exhibiting the highest adsorption capacity for Congo red, with the maximum capacity reaching 381.7 and 580.8 mg/g at 303 K and 323 K [27]. Johns et al. have reported the removal of methylene blue from its aqueous solution using natural rubber-chitosan blends as adsorbent [28] and much more. In the present study, chitosan/cellulose blend is used as an adsorbent to check the ability of its MG dye adsorption and the results revealed that the adsorption occurs in a short interval of agitation. The impact of several system variables such as the concentration of dye and the contact time is examined and the optimum experimental conditions are determined. The adsorption mechanism was studied using an adsorption isotherm and also kinetic models.

Preparation of Adsorbent and Dye Solution
Chitosan was dissolved in distilled water consisting of 5% (v/v) acetic acid to get a homogeneous solution. Secondly, a solution of CMC was made by dissolving cellulose in a 10% NaOH solution. The blends were made by mixing chitosan and cellulose in the weight ratio of 30:70 to get a uniform solution and were cast into a Petri dish for 48 h at 60 °C. Finally, the casted samples were peeled out and kept in a desiccator to avoid moisture absorption. The resulting sample thickness is 2.38 mm.
Cationic dye malachite green (MG) belongs to triphenylmethane. To abide by a positive charge density, its autochrome group is protonated in water at lower pH (pK a = 10.3). The molecular formula of dye is C 23 H 25 N 2 Cl (mol. wt.: 364.92, maximum absorption wavelength, λ max = 624 nm) and Fig. 1 represents the structure.
1000 mg/l stock dye solution was made using distilled water. An appropriate diluting of the stock to a pre-determined concentration was carried out to get a working solution. Analytical-grade reagents were used in the present study.

Methods
Isotherm studies were performed for altered initial concentrations of MG (50-200 mg/l) at room temperature with 0.2 g of adsorbent dosage. The adsorption isotherm was studied at different intervals of time varying from 0 to 45 min. The measurement of absorbance of color was done by using a spectrophotometer (Systronics 171) at λ max = 619 nm (Fig. 2).

SEM Analysis
The morphological study provides information about the adsorption of dye onto the surface of adsorbents. SEM (scanning electron microscopy) helped in examining the surface morphology of the samples and the microphotographs are shown in Fig. 3. From the SEM analysis, a rough surface can be seen in the blend of chitosan and cellulose ( Fig. 3a and b). The rough surface on the material indicates that more area is offered for adsorption [29]; nevertheless, it became much smoother upon dye adsorption ( Fig. 3c and

FTIR Analysis
The FTIR spectra of chitosan, cellulose, and chitosan-cellulose blend are shown in Fig. 4. The saccharide group was identified by a resonance peak at 1038 cm −1 . Moreover, the peak at 1397 is due to the coupling of the O-H in-plane vibrations. The peaks at 1540, 1630, and 2910 cm −1 were attributed to N-H bending, C-O-NHR, and CH 2 , respectively, and the broad band at 3222 cm −1 was attributed to the stretching vibration of the -OH group, which are characteristic peaks of chitosan [32].
For the cellulose sample, a band group in the 1200-1400 cm −1 region, assigned to C-H and O-H bending and CH 2 wagging motions, appeared to relate to crystallinity and not to lattice type. The broad band centered near 3319 cm −1 is due to -OH stretching, while the absorption band at 2886 cm −1 is due to -CH 2 stretching vibration. These bands are characteristic of cellulose. For chitosan-cellulose blend, a peak at 1568 cm −1 is seen with chitosan and composite aerogels corresponding to secondary amine bending and the peak at 1411 cm −1 is likely due to asymmetrical stretching of C-O of the carboxylate group. The peak at 1024 cm −1 relates to C-O stretching in the glycoside linkage [27].

Role of Dye Concentration
The impact of primary dye concentration on adsorption rate onto chitosan-cellulose blend was studied. The experimentations were done for a constant adsorbent dose (0.2 g/30 ml) using varied concentrations of MG dye (50, 100, 150, and 200 mg/l) at different time intervals up to equilibrium, and Fig. 5 represents the variation of absorbance. It shows that the adsorption rate is rapid during the early 20 min of the sorption study in the case of all the initial concentrations.
The degradation efficiency of MG was estimated using Eq. (1).
where C 0 and C t (mg/l) are the concentration initially and at time t, respectively. Percentage dye adsorptions are found to be 98.65, 97.84, 97.12, and 96.08%, respectively, for 50,  (Fig. 6). Initial dye concentration and adsorption are exponentially correlated. During adsorption, at first the dye molecules need to face the boundary layer effect and further diffuse onto the adsorbent surface. Lastly, the dye molecules infiltrate into the porous structure of the adsorbent taking a moderately extensive contact time. A smooth, solitary, and continuous curve was observed in the profile of dye uptake representing probable monolayer coverage of dye on the surface of the adsorbent [2]. The isotherm study was investigated for initial concentrations of MG (50-200 mg/l). 0.2 g of the blend as the adsorbent was kept in 30 ml aqueous solutions of MG. Using a UV-Vis spectrophotometer, at λ max = 619 nm, the absorbance of solutions was measured for different contact duration till the equilibrium state of adsorption.
The amount of adsorption at time t, q t was evaluated using Eq. (2). (2) The quantity of dye adsorbed at different time intervals with various initial concentrations of MG is shown in Fig. 7. Adsorption equilibrium is attained in 30 min and, further, no dye gets adsorbed from the solution.

Intraparticle Diffusion Models
The intraparticle diffusion model is presented by the following equation [33]: where K t is the intraparticle diffusion rate constant (g/mg/ min 1/2 ), which is derived from the slope of the linear part of the equation for this model. q t is the quantity of the dye adsorbed at "equilibrium" and at time "t".
The introduction of Eq. (3) to the experimental data indicates that the plot of q t versus t 1∕2 exhibits two separate regions with two straight lines, implying the existence of several steps that may have controlled the adsorption reaction process. The intraparticle diffusion constants were observed to have decreasing values, i.e., K t1 = 1.578 (g/mg/ min 1/2 ) and K t2 = 0.200 (g/mg/min 1/2 ), successively, which are directly related to the nature of the internal porosity of chitosan-cellulose blend [34].
In Fig. 8, two plateaus are observed, indicating that the diffusion of malachite green onto the chitosan-cellulose blend surface is carried out in two stages, suggesting that the intraparticle diffusion was not the only rate-limiting mechanism in the adsorption mechanism.  The behavior may be due to the dimensionality of malachite green dye, since it is a voluminous molecule with a calculated polarizability value of 42 Å 3 . It can be deduced from here that the dye diffused slowly into the macropores of phosphate particles during the adsorption reaction. The phenomenon, therefore, confirms that the mechanism of malachite green adsorption onto the activated phosphate rock could have been due to the contribution of both surface adsorption and intraparticle diffusion [35].
The adsorption capacity at equilibrium (dye adsorbed per unit quantity of sorbent), q e , is determined using Eq. (4): where W is the dry weight of the adsorbent (g), V is the volume of the dye solution (l), and C e is the dye concentration at equilibrium.
The adsorption capacity upsurges with the rise in the initial dye concentration at equilibrium (Fig. 9). Nonetheless, the adsorption rate increases by increasing the concentration of dye, indicating that the initial dye concentration has a vital part for adsorption of MG by the chitosan/cellulose blend.
The MG dye is a positively charged cationic dye. Chitosan has four groups/molecule of OH − and two groups/molecule of NH 2 − , whereas cellulose has six groups/molecule of OH. The presence of oxygen is more polar than nitrogen because oxygen is more electronegative than nitrogen. When compared to chitosan, cellulose is more electronegative. Therefore, the addition of cellulose into chitosan increases the ability of MG dye adsorption.

Adsorption Isotherms
Adsorption isotherm offers useful information on the adsorption capacity, binding affinity, and also surface properties of the biosorbent, helping to understand the binding mechanism of the adsorbate with the biosorbent. As a function of equilibrium concentration in bulk solution at a constant temperature, it is described as the number of adsorbate molecules/unit mass of adsorbent [36]. In this study, Langmuir and Freundlich isotherms are meant for examining the adsorption behavior. Langmuir isotherm explains the monolayer adsorption onto the surface of the adsorbent consisting of a determinate count of adsorption sites [37], while Freundlich isotherm is associated with adsorption occurring on the heterogeneous surface of the adsorbent.

Langmuir Isotherm
Langmuir model was employed initially for systems having monolayer adsorption on the surface of the adsorbent [38] and derived based on the hypothesis that all adsorption sites are alike, not dependent on surface coverage, and happen with no lateral interaction of adsorbate molecules [39].
where q max is the maximum adsorption capacity (mg/g) and K L (l/mg) is the Langmuir's constant. Equation (6) represents the Langmuir's isotherm in linear form for determining the adsorption parameter. If the sorption course is defined by the Langmuir isotherm equation, a plot of 1∕q e vs. 1∕C e would be linear as shown in Fig. 10. K L and q max are taken from the slope and intercept of the straight line, respectively. The maximum adsorption capacity graphically is obtained as 115.1 mg/g. Equation (7) represents the Langmuir isotherm with respect to R L , dimensionless constant separation factor [40].
where C 0 is the highest initial dye concentration (mg/l). The isotherm shape might be inferred as follows based on R L : R L evaluated at room temperature in the present study is 0.1488, which specifies that the adsorption is favorable.

Freundlich Isotherm
Freundlich adsorption isotherm is an empirical equation with uneven distribution of heat for adsorption on the heterogeneous surface over the surface and multilayer adsorption [41]. Equation (8) represents the Freundlich isotherm.
Equation (9) is the linear form of the Freundlich isotherm.
where K f and n are adsorptions constants for measuring adsorption capacity, and 1∕n is the intensity of adsorption. 1∕n value represents the adsorption process to be unfavorable ( 1∕n > 2) or favorable (0.1 < 1∕n < 0.5). The slope 1∕n ranging from 0 and 1 measures the surface heterogeneity, and values closer to zero indicate high heterogeneity. Figure 11 represents the Freundlich isotherm for MG on chitosan-cellulose blend. logK f and 1∕n are 0.5866 mg/g and 0.8025, respectively. Adsorption intensity, n, is found to be 1.25, which is greater than 1, signifying that the adsorption of MG happened on the surface of the blend. Lesser n(< 1) indicates larger heterogeneity in energy distribution favoring adsorption at low concentrations [28]. Table 1 shows the isotherm parameters of both Langmuir and Freundlich equations for the isothermal adsorption of MG onto a chitosan-cellulose blend.

Kinetic Study of Adsorption
The kinetics of MG dye adsorption onto the blend essentially is for optimizing the operative circumstances for a full-scale batch process. The kinetic parameters that assist in estimating the adsorption rate provides significant data for modeling and designing the progressions of adsorption [42]. Adsorption mechanisms comprising kinetics-based models are described. In the present study, two familiar models are  examined to identify a best-fitted model for the obtained experimental data. Thus, the kinetics of MG adsorption onto chitosan blend was studied using pseudo-first- [43] and pseudo-second-order [44] kinetic models. The correlation coefficients (R 2 closer or equal to one) were used to obtain conformity among the model-predicted and experimental values, and a comparatively larger value is the most relevant kinetic model for MG adsorption on the blend.

Pseudo-First-order Equation
The adsorption kinetics was analyzed by Lagergren pseudofirst-order model [43], an equation relating the adsorption rate based on the adsorption capacity. The differential equation is: where k 1 is the rate constant of the pseudo-first-order adsorption (l/min). Integrating Eq. (10) gives Eq. (11).
The linear form of Eq. (11): The values of log q e − q t were linearly correlated with t giving a linear relationship from which k 1 and q e are identified from the slope and intercept of the plot, respectively (Fig. 12). The relationship between the initial solute concentration and rate of adsorption may not be linear when pore diffusion limits the adsorption process. Table 2 comprises the kinetic parameters determined using pseudo-first-order kinetics.

Pseudo-Second-order Equation
Using pseudo-second-order model, the adsorption kinetic could be explained [44]. The differential equation is written as: log q e q e − q t = k 1 2.303 t.
In the above equation, k 2 (g/mg/min) is the second-order rate constant of adsorption. Integrating Eq. (13) and then linearizing gives Eq. (14).
The initial sorption rate was evaluated from secondorder rate constants and is given by: k 2 and q e are found from the intercept and slope of the plots of t∕q t vs. t (Fig. 13). Table 2 consists of the experimental and calculated q e values at different initial MG and adsorbent concentrations. The initial sorption, h, representing the rate of initial adsorption, is nearly augmented from 1.30 to 6.17 mg/g/ min with an upsurge in initial MG concentrations from 50 to 200 mg/l onto the sample. It is noted that the pseudosecond-order rate constant ( k 2 ) lessened from 0.0183 to 0.00575 with increased initial MG concentration from 50 to 200 mg/l.
As MG is a cationic dye, it can be electromagnetically engrossed by the polar functional groups existing in the structures of chitosan and cellulose. The combination of chitosan and cellulose with a substantial number of -NH 2 , -OH, and -CH 3 OH rapidly adsorbs MB dye molecules when compared to the blend constituents.

Adsorbent Reusability
The reusability of adsorbents is a crucial assessment factor in defining the perspective of their uses in the industry from economic and technological views [45,46]. Thus, the reusability of the chitosan-cellulose blend was inspected in the adsorption of cationic MG dye to evaluate the regeneration capacity of the adsorbent for the second cycle of adsorption-desorption. For the next adsorption step, the adsorbents were washed with distilled water and dried overnight at 50 ℃. Outcomes were that the chitosan-cellulose blend could be used again without major activity diminution. About 89.27% (Fig. 14a, b) efficacy was perceived after two sequential series. Table 3 gives us the comparison of maximum adsorption capacity q m (mg/g) for the removal of malachite green using various adsorbents comprising the results of the present study [30,45,[47][48][49][50][51][52][53][54][55][56]. It clearly shows that the blends of chitosan-cellulose exhibited moderate adsorption capacity when compared to other adsorbents. Environmental friendliness, low cost, and reusability are the additional advantages of this system.

Conclusion
The outcome of this research shows that the chitosan/cellulose blend has an appropriate adsorption capacity to remove MG from aqueous solutions and was determined to be 98.65%. In 30 min duration, the equilibrium adsorption was easily achieved. Langmuir and Freundlich isotherm models were fitted to investigate the experimental outcomes. The maximum  adsorption capacity was observed to be 115.1 mg/g and exhibited a monolayer Langmuir-type isotherm. Pseudo-first-and second-order equations were used to complete the kinetic study of MG adsorption on the blend. It can be seen that the adsorption kinetics followed the pseudo-second-order rate. Finally, it can be concluded that the chitosan/cellulose blend serves to be a cost-effective adsorbent for the removal of dyes from an aqueous solution effectively.
Author Contributions All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by BS, JJ, and YSR. The first draft of the manuscript was written by YSR and all authors commented on previous versions of the manuscript. The final revision of the manuscript was done by JJ. All authors have read and approved the final manuscript.
Funding The authors declare that no funds, grants, or other support was received during the preparation of this manuscript.
Data availability Not applicable.