Surfactant supported chitosan for efficient removal of Cr(VI) and anionic food stuff dyes from aquatic solutions

In order to develop a novel and cost-effective adsorbent with outstanding adsorption capacity and excellent recyclability for anionic pollutants, the chitosan-modified cetyltrimethylammonium bromide sorbent (CS@CTAB) was fabricated. Fourier-transform infrared spectroscopy, N2 adsorption–desorption isotherm, elemental analysis, Thermogravimetric analysis, X-ray diffraction, and Scanning electron microscopy have been applied to evaluate both raw and surfactant modified chitosan (CS@CTAB). Azorubine, Sunset Yellow, and hexavalent chromium were used to study the adsorption behavior of CS@CTAB under various parameters such as adsorbent dose, initial dye and metal ion concentration, contact time, and temperature. Adsorption equilibrium, kinetics models and thermodynamic parameters were investigated. The adsorption isotherm fitted well with the Langmuir isotherm model, with a maximum adsorption capacity of 492.6 mg/g, 492.6 mg/g, and 490.196 mg/g for Azorubine, Sunset Yellow, and Hexavalent Chromium, respectively. The kinetic studies showed that the pseudo-second-order model provided a better correlation between experimental data. Furthermore, the calculated thermodynamic parameters confirmed that the adsorption of Cr(VI), E110, and E122 by CS@CTAB material is a spontaneous and exothermic process. The fabricated CS@CTAB adsorbent was employed for the efficient elimination of Azorubine, Sunset Yellow, and hexavalent chromium from real water samples, synthetic mixtures, and colored soft drinks, with a percentage of recovery of ~ 96%. The plausible adsorption mechanisms of Azorubine, Sunset Yellow, and hexavalent chromium on the surface of CS@CTAB are elucidated. The adsorption anticipated to be due to electrostatic interaction and hydrogen bond formation for hexavalent chromium; while the adsorption of Azorubine and Sunset Yellow, was assumed to be due to electrostatic interaction, hydrogen bonding, and n-π interaction. Finally, the study demonstrates the efficiency of CS@CTAB for the removal of anionic species from several samples, including natural water and colored beverages.


Characterization.
A Perkin Elmer 550 spectrophotometer was used for the determination of the concentration of K 2 Cr 2 O 7 , anionic dyes in single and multi-components systems over a range of 200-900 nm using quartz cells.The λ max was found to be 486 nm for E110, 518 nm for E122 and 427 nm for K 2 Cr 2 O 7 .The specific surface area of the CS and CS @CTAB materials was obtained using the Brunauer Emmet Teller (BET) analysis (Size Analyzer (QUANTACHROME-NOVA 2000 Series).Elemental analysis of CS, CS@CTAB was performed using a Costech (ECS-4010) elemental analyzer.Surface morphology of native CS and CS @CTAB was investigated using a JSM-6510LV model.The IR spectra of native CS and CS@CTAB, before and after adsorption of anionic species, were investigated by a Perkin-Bhaskar-Elmer Co., USA.Samples were grounded and mixed with KBr and then pressed in pellet form.X-ray diffraction (XRD) patterns of the CS and CS@CTAB materials were attained by means of PAN analytical X'Pert PRO diffractometer using the 2-theta (2θ) which ranged from 4° to 70°.The thermal stability of CS and CS@CTAB was inspected using thermogravimetric analysis (Berkin Elmer TGA 4000) on a heating rate from 30 to 900 °C of 15 °C/min.The concentrations of analytes before and after adsorption were measured spectrophotometrically using a Jasco UV-Vis spectrophotometer (model V-530, Japan).The point of zero charge (pH PZC ) of CS@CTAB was determined as follows: 0.1 g of the CS@CTAB adsorbent was added to a 25 ml of pH-adjusted NaCl (0.01 M) solution that varied from 2 to 12 and the mixtures were allowed to shake at the equilibrated shaker for 48 h.0.1 M of HCl and 0.1 M of NaOH were utilized for NaCl pH adjustment.After the shaking, the final pH was recorded and ΔpH was measured as in the following equation (ΔpH = pH i − pH f ) and was plotted against the initial pH (pH i ).The pH pzc value is the cross point where the curve ΔpH versus pH i crosses the line ΔpH = 0.
Adsorption and regeneration procedures.Batch tests.1000 ppm of stock solution of each investigated pollutant (E110, E122, and Cr(VI)) was prepared by dissolving 1.0 g of the dye or metal salt in 1000 mL of double-distilled water (DDW) to attain the required concentration.The adsorption experiments were carried out using 0.005 g of CS@CTAB in 25 mL dye solution at various experimental parameters such as concentration (50-250 ppm), pH (2.0-9.0),temperature (25-45 °C), adsorbent dose (0.005-0.03 g), contact time (15-300 min), and ionic strength.The amount of remaining pollutant in the solution was detected by UV-vis spectra at the specified wavelengths of 486, 518, and 427 nm for E110, E122, and K 2 Cr 2 O 7, respectively.
The removal percentage (R,%) and adsorption capacity (q e ) of anionic species were calculated as represented in Eq. (1) and Eq. ( 2), respectively: where C f and C i, (ppm) are the equilibrium and the initial dye concentrations, respectively.While V is the volume of analyte solution (L) and m is the weight (g) of CS@CTAB 52 .
(1) www.nature.com/scientificreports/Batch experiments for pollutants in the binary system.To investigate the efficiency CS@CTAB in the studied anionic species in binary systems, batch experiments were performed using 0.005 g of CS@CTAB in 25 ml of each binary system solutions with 50 ppm of each pollutant at pH 3 with shaking at 120 rpm for specified periods.Maximum adsorption of each binary systems was recorded every hour over a period of 6 h.
Desorption studies.Pollutants (E110 and E122) and Cr(VI) were adsorbed on the CS@CTAB surface.Then, desorption of such pollutants was investigated by applying a variety of eluents including sodium hydroxide (0.1 mol/L), Na 2 CO 3 (1 mol/L) with and without heating, sodium acetate (0.1 mol/L), ethanol, and hydrochloric acid (0.2 mol/L).The batch technique was employed to study the regeneration of CS@CTAB over five repeated cycles of the adsorption-desorption procedure.For (Cr(VI), E110, and E122), 0.005 g of CS@CTAB was shaken in 25 ml of (100 ppm for E110, E122, and Cr(VI)) for 240 min.After that, the CS@CTAB was filtered and washed.Consequently, desorption step was carried out by utilizing 0.005 g of CS@CTAB-E110, CS@CTAB-E122, and CS@CTAB-Cr(VI) and in 25 ml Na 2 CO 3 (1 mol/L); then shaking for 2 h at 45 °C.This process was performed for four more times.Using Eq. ( 3), the desorption efficiency[D,%)] of pollutants under investigation, was estimated: Isothermal, kinetics and thermodynamics studies.Initial concentration and isothermal studies.Isothermal studies for E110, E122, and Cr(VI) were investigated by the addition of 0.005 g CS@CTAB adsorbent to a set of bottles containing E110, E122, and Cr(VI) solutions to be adsorbed.They initially contain between 50 and 250 ppm.For E110, E122, and Cr(VI), the latter bottles were shaken in a thermostated shaker at 150 rpm for 240 min at 25 °C.The isothermal models of Langmuir and Freundlich were used in linear form and estimated as given in Eqs. ( 4) and (5).A crucial parameter is the Langmuir separation factor (R L ), which is stated in Eq. ( 6), (3) Desorption% = amount desorbed to the solution (mg/l) amount adsorbed on CS@CTAB (mg/l) × 100 www.nature.com/scientificreports/ is utilized to determine adsorbent-sorbate affinities.The meaning of R L values can be illustrated as follows: if its value is larger than 1, it shows that the researched adsorbent is not suitable, while if its value lies between 0 and 1, it shows that the used adsorbent is suitable.
As, C e (ppm) is the concentration of pollutant (E110, E122, and Cr(VI)) at equilibrium, q e (mg/g) is the adsorption capacity of pollutant at equilibrium, q m (mg/g) adsorption maximum amount, 1/n is the heterogeneity factor, while K L (L/mg), K F (mg/g), and K are Langmuir, Freundlich constants.
Two kinetic models, pseudo-1st-order and pseudo-2nd-order, given in (Eqs.( 7) and ( 8) respectively, were used in kinetic studies for the evaluation of the adsorption rate-limiting step.The experiments were carried out utilizing 25 ml (100 ppm of E110, E122, and Cr(VI)) and 0.005 g of CS@CTAB at the optimum pH for each pollutant.The mixture was shaken at room temperature with a constant speed of 150 rpm for different contact times ranging from 15 to 300 min.
The adsorption efficiency for examined pollutants at equilibrium and at a specific time t (min) are stated as q e (mg/g) and q t (mg/g), respectively.Besides K 1 and K 2 are constants for pseudo-1st-order and pseudo-2ndorder, sequentially.
Temperature and thermodynamics.In a balanced shaker running at a constant speed of 150 rpm for 240 min, a set of 50 ml stoppered bottles holding 25 ml of pollutant solution (100 ppm) for [E110, E122, and Cr(VI)], 0.005 g of CS@CTAB at varying temperatures (298-318 K) and ideal pH value for each pollutant, were shaken.After adsorption and filtering, the concentration of remaining (E110, E112, and Cr(VI)) was detected.The following equations (Eqs.( 9) and (10) describe the thermodynamic parameters viz.adsorption enthalpy (ΔH o ), free energy (ΔG o ), and entropy (ΔS o )); ΔS o and ΔH o were estimated from intercept of Eq. (10), which equals ΔS o /R, and slope, which equals -ΔH o /R of Ln K c vs. 1/T.The universal gas constant (R) is equivalent to 8.314 J/ mol K.
Applications in real samples.Natural water samples.To investigate the applicability of CS@CTAB, different water samples including (Tap and seawater) were employed for the adsorption of E110, E122, and Cr(VI) in (100 ppm).In advance of the spiking of the pollutants, the natural water samples were digested by the addition of 0.5 g of K 2 S 2 O 8 and 5 ml H 2 SO 4 98% (w/w) to 1000 ml of water sample and heated for 120 min at 90 °C for complete digestion of organic materials.After cooling to room temperature, 0.005 g of CS@CTAB was introduced to these samples, and the appropriate pH value for each pollutant was set with continuous shaking for 240 min for E110, E122, and Cr(VI).The solutions were centrifuged and another 0.005 g of CS@CTAB was introduced to the supernatant to guarantee the complete separation of analytes.The remaining of each of E110, E122 and Cr(VI) was determined using a Unicam UV 2100 UV/Visible spectrometer at appropriate wavelengths.
Industrial samples and colored soft drinks.In order to remove E110 and E122 from degassed carbonated beverages and jelly, CS@CTAB was utilized.Initially, the carbonated beverages (an orange beverage with E110 dye and a pomegranate beverage with E122 dye) were degassed by leaving them in the air at 25 °C for 120 min.After being digested (which means the breakdown of unwanted substances in the sample) using acetic acid (4%), the strawberry and orange-flavored jelly was dissolved in DDW 53,54 .For E110 and E122, CS@CTAB (0.005 g) was introduced to each sample, and the optimum pH for each dye was set with constant shaking for 240 min.Using the Unicam UV 2100 UV/Visible spectrometer at the proper wavelengths, the residual E110 and E122 was identified.
(4) C e q e = 1 k L q m + C e q m (5)

Result and discussion
Materials' design.Figure 2 represents the CS@CTAB synthesis steps, adsorption of the three investigated anionic pollutants and their determination.Moreover, the application of the prepared CS@CTAB on real (soft drinks) samples is included.
Characterization.N 2 adsorption/desorption isotherm Brunauer-Emmett-Teller (BET) analysis.The surface area of the pure chitosan and CS@CTAB were analyzed using the BET method, Fig. 3.The results, presented in Table 1, show that the BET surface area of pure chitosan increased from 7.876 to 492.133 m 2 /g in CS@CTAB.The increase in the surface area of an adsorbent leads to enhancing the chitosan adsorption capacity for the pollutants.
Elemental analysis.The elemental analysis was evaluated for CS and CS@CTAB materials to prove the modification step.The results, presented in Table 2, show a decrease in the nitrogen percentage from 7.5 to 6.3606% in the CS@CTAB sample with an increase in carbon atom percentage from 44.1% to 57.0178% and hydrogen atom percentage from 6.0377 to 9.1308%.The CTAB surfactant, with chemical formula C 16 H 33 N(CH 3 ) 3 , its reaction www.nature.com/scientificreports/with the chitosan resulted in the insertion of a long carbon chain with one nitrogen group, which led to a decrease in the nitrogen percentage in the modified material.So, these results prove the formation of the CS@ CTAB material by reacting chitosan with CTAB in the presence of glutaraldehyde.
Morphology. Figure 4 demonstrates SEM micrographs for CS (Fig. 4a,b) and CS@CTAB (Fig. 4c,d).These micrographs show that CS@CTAB has a dissimilar morphology to native CS.SEM micrographs of CS show smooth and regular surface topology, while CS@CTAB offers a much rougher and irregular surface structure.
The SEM outcomes prove that the modification of CS increased the number of pores and produced a rougher surface.Based on SEM micrographs, the aromatic rings on the CS structure cause adverse surface topology along with high porosity and surface roughness when compared to native CS 55 .
FT-IR spectra.The IR spectrum of CS, Fig. 5a, demonstrates a broad peak centered at 3450 cm −1 assigned to the stretching vibration of the -OH, -NH 2 groups and to intramolecular and intermolecular hydrogen bonding.The peaks at 2930 cm −1 , 2884 cm −1 , 1427 cm −1 , 1381 cm −1 , and 1321 cm −1 are due to stretching and bending vibration of C-H of -CH 3 , and -CH 2 groups.The peak at 1653 cm −1 is due to stretching vibration of -C=O in -NHCO, whereas the peak at 1603 cm −1 is due to -NH bending vibration of NH 2 .Other peaks at 1155 cm −1 , and 1090 cm −1 correspond to the -C-O bending.www.nature.com/scientificreports/On the other hand, the IR spectrum of CS@CTAB, Fig. 5b, shows the following peaks: a peak at 3450 cm −1 appears to be less broad and sharper due to the reaction of the -NH 2 group with CTAB and glutaraldehyde so this peak only describes the -OH group.The peaks at 2856 cm −1 and 2923 cm −1 are due to the -CH groups, those peaks increased in intensity and sharpness due to the increase in -CH 2 groups as a result of introducing CTAB molecule which contains 16 additional -CH 2 groups.Whereas the peak at 1748 cm -1 is due to -C=N that appeared as a result of addition of glutaraldehyde.The peak at 1462 cm −1 appeared to be due to the presence of -CH alkane groups introduced by the addition of CTAB surfactant.The other peak at 1257 cm −1 is due to the -C-N amine interaction 56 .
Thermogravimetric analysis (TGA). Figure 6 reveals the several stages of weight loss characteristic of CS; the initial one was between 35 and 100 °C denoting the dehydration of the polymer.The subsequent stage was between 235 and 400 °C, which is characteristic of the breakage of the CS chain, depolymerization, and decomposition of the acetylated and deacetylated parts.The last phase was between 410 and 710 °C; marking the degradation of the products created via the preceding incident [57][58][59] .
The chief loss presented at temperatures of 175 °C to 400 °C was due to the decomposition of fresh CTAB (which is around 240%) and the liberation of boundless water [60][61][62] .

X-ray diffraction analysis (XRD).
Figure 7 shows the XRD of both CS and CS@CTAB.The XRD of CS reveals that CS has a low-crystalline composition that can be established via two peaks: one at 10.26°, which is linked www.nature.com/scientificreports/ to the hydrated low-crystalline composition, and the other at 20.165°, which is a result of the amorphous state of the material.These peaks were found to be distinctive to the raw CS diffractogram; they also suggested the formation of intra-and intermolecular hydrogen bonding in addition to the existence of the free NH 2 groups in the raw CS composition 61 .
The XRD of CS@CTAB displayed no considerable diffraction peak in XRD, with the exception of that at 20.885°.Consequently, CS@CTAB displays amorphous features that are dissimilar to those of its main constituents 60 .
Scherrer's equation is represented in Eq. ( 11): where D is the nano crystallite size.K is a constant related to crystallite shape, normally taken as 0.9.β is the peak width of the diffraction peak profile at half maximum height (FWHM) resulting from small crystallite size in radians.λ is the X-ray wavelength in nanometer (nm).By calculating the value of D for both raw and modified chitosan the values were found to be 1.734 and 17.23, respectively 63 .

Adsorption studies. Point of zero charge (pH PZC ).
The point of zero charge (pH pzc ) is generally described as the pH at which the net charge of the adsorbent's surface is equal to zero in an aqueous solution during the adsorption of ionic species.In this study, a plot for pH pzc determination of CS@CTAB is shown in Fig. 8.The CS@CTAB (pH pzc ) has been attained by applying the previously published studies.Figure 8 demonstrates the variation of ΔpH value (pH initial _pH final ) of CS@CTAB as a function of the pH initial .The value of the pH pzc of CS@ (11)  www.nature.com/scientificreports/CTAB is found to be 6.85.This illustrates that at pH less than 6.85, the CS@CTAB surface is considered to have positive charges 64 .
Effect of pH.The pH effect, which is demonstrated in Fig. 9, is a crucial factor in the removal of environmental pollutants.The effect of pH was carried out by varying the pH in the range of 2.0-9.0 keeping other parameters constant (shaking for 4h, at room temperature, and using a concentration of 100 ppm).It is expected that treatment of CS with the CTAB will lead to the formation of potentially several nitrogen moieties on the CS surface 65 .
Elemental composition evidence (Table 1) indicates a nitrogen of 6.36% (wt%) on the CS@CTAB material, most likely indicating the formation of a variety of more basic nitrogen moieties on the CS@CTAB surface.Nitrogenbased surface functionalities can be seen with the presence of a broad FTIR peak with the midpoint at approximately 3450 cm −1 potentially indicating the presence of a basic amide surface functionality 65 .Following the adsorption process in the current work, in the CS@CTAB FTIR spectrum after (E122, E110, and Cr(VI)) adsorption the specific peak at 1650 cm −1 disappeared for the three pollutants accompanied by the appearance of S-O and S=O specific peaks for the CS@CTAB-E110 and C@CTAB-E122.Based on experimental data, it was found that the maximum removal efficiency (R %) of Cr(VI) was 99.5 at pH 2 and 99.5 for E110 and E122 at pH 3, but decreased sharply and converged as the solution pH increased (Fig. 9).This is consistent with other batch studies which also reported maximum Cr(VI) adsorption in acidic environments 37,66,67 and can be explained by considering the surface charges of the adsorbents and the Cr(VI) adsorbate species 68 .The surface charge of an adsorbent is determined by pHpzc where an adsorbent surface is neutral at pH = pHpzc, but develops a positive charge at pH < pHpzc and a negative charge at pH > pHpzc.Therefore at pH 2 CS@CTAB had positive surface charges; however, as solution pH increased, the positive charges are reduced and became negative 69 once the pH values increased above its pHpzc (pH 6.85), (Fig. 8).In addition, the main ionic forms of chromate at acidic pH (pH 1.0-6.8)are hydrogen chromate (HCrO 4 -) and dichromate (Cr 2 O 7 2− ) with chromate (CrO 4 2− ) being predominant at pH > 6.8 and this results in an electrostatic attraction to the positively charged groups present on the adsorbent surface.In contrast, increasing solution pH led to a decrease in the Cr(VI) uptake due to the deprotonation of surface functional groups, along with competition between the Cr(VI) ions and the OH − ions to occupy the active sites.Given the enhanced adsorption of Cr(VI) onto CS@CTAB at pH 2 (Fig. 3A), the remainder of this study focused on Cr(VI) removal by CS@CTAB using a pH 2 solution as optimal.Based on the evidence, it appears that the principal binding process is one of an electrostatic interaction between the sulfonyl moieties and the binding Cr(VI) species.
Because of the strong protonation, the adsorbent surface becomes positively charged at low pH.The Cr(VI) adsorption was enhanced due to the electrostatic force between negatively charged HCrO 4 − and Cr 2 O 7 -2 and SO 3 − of (E110 and E122) and the positively charged adsorbent surface 37 .Thus, this, in turn, enhances the affinity of CS@CTAB material towards attracting positively charged metal ions and dye molecules depending on pH control, causing the improvement of Cr(VI), E110, and E122 adsorption as shown in the equations below (12)  and (13), respectively.
Effect of dose.The influence of dose of CS@CTAB, Fig. 10, was examined in the range of (0.005-0.01 g), keeping the other parameters constant (shaking for 4 h, at room temperature, using a 25 ml of 100 ppm (E110, E122, and Cr(VI)) and an appropriate pH value for each pollutant).The maximum adsorption capacity for all pollutants took place using 0.005 g of CS@CTAB in 25 ml pollutant.www.nature.com/scientificreports/Effect of initial concentration.The effect of the initial concentrations of pollutants on the adsorption of the studied analytes, was investigated in the range of (50-200 ppm) as shown in Fig. 11, using 0.005 g CS@CTAB adsorbent and appropriate pH value for each pollutant while keeping the other parameters constant (shaking for 4 h, at room temperature).From the graph, the adsorption capacity of CS@CTAB increases with the increase in the initial concentration of pollutants and then becomes constant at higher concentration values.The adsorption capacity increased with initial pollutant concentration due to availability of large number of binding sites of the surface of CS@CTAB.At high pollutant concentrations, the adsorption capacity reached maximum value suggesting that the binding sites on the CS@CTAB surface reached saturation.
Adsorption isotherm.The Langmuir and Freundlich isotherm are presented in Fig. 12 and 13 and the calculated coefficients are shown in Table 3.The Langmuir isotherm model (R 2 = 0.999) seemed to describe better the adsorption process of Cr(VI), E110, and E122 by the CS@CTAB than the Freundlich isotherm model (R 2 (Cr(VI) = 0.543, R 2 (E110) = 0.962, and R 2 (E122) = 0.5435).As can be noticed, the Langmuir model accurately fits the adsorption of E110, E122, and Cr(VI) on CS@CTAB.This shows that the Cr(VI), E110, and E122 molecules bind to the active sites on the biosorbent surface as a monolayer 31 .The R L values obtained for all initial concentrations of Cr(VI), E122, and E110 lie between 0 and 1 (Table 3), indicating that biosorption of Cr(VI) ions and the two dyes by CS@CTAB is a favorable process.
Effect of oscillation time and adsorption kinetics.For the investigation of reaction time influence on the adsorption of pollutants, the time was changed in the range of (30-300 min) using 0.005 g CS@CTAB and appropriate amount for each pollutant and keeping other parameters constant (using 25 ml of 100 ppm pollutant at room temperature).Figure 14 showed that the adsorption was rapid about 80% in the first 30 min, then, by increasing  www.nature.com/scientificreports/time from 30 to 240 min the adsorption capacity increased to about 95% after which the adsorption capacity remained constant indicating that it reached equilibrium 73 .Adsorption kinetic models can determine the adsorption mechanism and even the physicochemical properties of the adsorbent 74 .Therefore, two models, pseudo-second-order and pseudo-first-order, were employed to investigate the adsorption of E110, E122 and Cr(VI).The kinetic models were explained in their nonlinear form, as illustrated in Figs. 15 and 16.The attained parameters of each model are given in Table 4.The pseudo-secondorder and pseudo-first-order models frequently reveal the surface-controlled adsorption process.In the present study, the pseudo-second-order model fits better than the pseudo-first-order model where the correlation coefficient (R 2 ) is very close to unity (Table 4).Therefore, the adsorption of E110, E122 and Cr(VI) on the surface of CS@CTAB is controlled by a chemisorption mechanism.

Effect of temperature and thermodynamic studies.
To examine temperature influence on the adsorption capacity of CS@CTAB, isotherms were attained in the range of (298-318 K).Equilibrium isotherms for investigated temperatures were demonstrated in Fig. 17.From Fig. 17, it was noted that the adsorption capacity on CS@ CTAB decreased by increasing temperature 31 .
To explore the adsorption process of Cr(VI), E110, and E122 dyes onto the CS@CTAB surface in terms of spontaneity and feasibility and to determine the degree of randomness at the solid/liquid interface, adsorption thermodynamic parameters were determined.
The adsorption of Cr(VI), E110, and E122 was studied at different temperatures from 298 to 318 K at pH 2 for Cr(VI) and pH3 for the E110 and E122 for 4 h.Free energy parameter (ΔG o ads), adsorption entropy parameter (ΔS • ads), and heat of enthalpy parameter (ΔH • ads) of Cr(VI) metal ion, and E110 and E122 dyes adsorption by CS@CTAB adsorbent were calculated.ΔG o ads parameter was calculated from the following Eqs.( 9) and (10).
From the experimental data that present in Table 5, it was noticed that the negative ΔG o adsn value which confirms that the adsorption of Cr(VI), E110 and E122 by CS@CTAB adsorbent is a spontaneous process.It was also observed that the negative ΔH o ads value confirms that the adsorption of Cr(VI), E110, and E122 by CS@ CTAB material is exothermic.From ΔH o values that are higher than 80kJ/mole, it was proven that the adsorption   Table 4. Kinetic parameters for (E122, E110 and Cr(VI)) adsorption on to CS@CTAB.
Effect of ionic strength.For the investigation of the ionic strength effect several species have been used.This procedure is crucial due to the presence of adverse ions in industrial wastewater with high concentrations.So, to study the effect of ionic strength the following species are used: CH 3 COONa, EDTA, and Ca(NO 3 ) 2 at optimum adsorption parameters.The influence of different anions on the adsorption of E110, E122, and Cr(VI) was presented in Fig. 19.The effect of different anions having a concentration of 0.1 M is insignificant on the adsorption of pollutants under investigation.
Regeneration experiments.To test the reusability of CS@CTAB, the desorption procedure was performed under optimum parameters.Subsequently, we examined the result of applying adverse eluents like HCl, NaOH, ethanol, and Na 2 CO 3 (with and without heating) as presented in Fig. 20a and it was observed that the effect of Na 2 CO 3 with heating at 45 °C achieved the best results for the desorption of E122, while Na 2 CO 3 alone was the best eluent for E110 and eventually EDTA presented the best desorption for Cr(VI).Hence HCl, ethanol, and NaOH are poorly affecting the desorption of the three investigated pollutants.Figure 20b shows the influence of Na 2 CO 3 at room temperature, Na 2 CO 3 at 45 °C, and EDTA eluents on the desorption of E110, E122, and Cr(VI), respectively through five repeated cycles of adsorption-desorption 75 .
Removal of different pollutants from binary systems.To examine the effect of CS@CTAB on the removal efficiency of pollutants in binary systems, the λ max for each pollutant and that for each binary system were measured as follows: E110 (486 nm), E122 (518 nm), and Cr(VI) (427 nm), while λ max of binary systems are: E110 + E122 (508 nm), E110 + Cr(VI) (478 nm), E122 + Cr(VI) (525 nm), and E110 + E122-Cr(VI) (505 nm).New overlapped peaks appeared after the formation of binary systems then adsorption studies took place at each new λ max appeared.Adsorption took place by applying 0.005 g of CS@CTAB in 25 ml of binary system solutions with 50 ppm of each pollutant to form the binary system at pH 3, then undergoes shaking at 120 rpm.Maximum adsorption of binary systems took place after 4 h as shown in UV-visible data in Figs.21 and 22.The adsorption efficiency of E110, E122, and Cr(VI) was estimated from Eq. (2).
Application.In natural water samples.To assess the effectiveness of CS@CTAB for the adsorption of different pollutants, ideal experimental circumstances were fitted to natural samples.Standard solutions were used to create calibration curves.Under the above-optimized experimental circumstances, standard solutions (1.0 L) were handled.Several water samples, including tap water from our lab, and seawater from Ras Elbar City, Egypt, were used as analytical samples.Table 6 displays the analysis outcomes.The recoveries of spiked samples with a known amount of each pollutant were investigated.The recovered amounts ranged from 96.00 to 99.69%.These findings suggest that the CS@CTAB can be used to accurately to identify E110, E122, and Cr(VI) in natural water samples.www.nature.com/scientificreports/ In colored soft drinks and food industrial.To assess the CS@CTAB performance for anionic dye adsorption, different samples containing E110 and E122 were subjected to the optimum experimental factors for E110 and E122 adsorption.The standard solutions were used to create the calibration curves.Degassed carbonated beverages and jelly made up the food samples.Figure 23 shows that more than 98% of the E110 and E122 extraction from the tested samples was accomplished.These findings suggest that the removal of E122 and E110 from various samples could be accomplished using the CS@CTAB adsorbent 76,77 .
Reasonable mechanism of Cr(VI), E110, and E122 adsorption onto CS@CTAB.To investigate the possible mechanism of metal ion adsorption onto CS@CTAB, the morphology, surface charge, optical images and FTIR of the adsorbent were evaluated.www.nature.com/scientificreports/Optical images of the chitosan, CS@CTAB, CS@CTAB-E122, CS@CTAB-Cr(VI), and CS@CTAB-E110 are represented in Fig. 24A(a-e).A great change appeared in the color of the chitosan, CS@CTAB, CS@CTAB-E122, CS@CTAB-Cr(VI), and CS@CTAB-E110.The beige color of chitosan (Fig. 24A.a) changed to brown in CTAB-modified chitosan (CS@CTAB) after the reaction of chitosan with CTAB in the presence of glutaraldehyde (Fig. 24A.b).After adsorption of each pollutant, the color of the CS@CTAB was changed from brown to pink in the adsorption of E122 (Fig. 24A.c), to yellow in the adsorption of Cr(VI) (Fig. 24A.d), and to orange in the adsorption of E110 (Fig. 24A.e).Those outcomes show the adsorption ability of CS@CTAB towards the pollutants.
The FT-IR spectra of CS@CTAB after loading of E122, E110, and Cr(VI) are shown in Fig. 24B A change in the stretching vibration of the hydroxyl group can be observed in the IR spectra of CS@CTAB-Cr(VI), CS@ CTAB-E122, and CS@CTAB-E110 as the peak of the OH group of CS@CTAB undergoes a slight shift as the peaks at 3683 cm −1 and 3462 cm −1 are shifted to 3750 cm −1 and 3518 cm −1 at CS@CTAB-Cr(VI), CS@CTAB-E122, and CS@CTAB-E110, and the inhomogeneous broadening of the peak after adsorption indicates the formation of medium H-bonding 78 .Additionally, the fork shape of the (N-H) bending peak at 1650 cm −1 disappeared in the www.nature.com/scientificreports/IR spectra of CS@CTAB-Cr(VI), CS@CTAB-E122, and CS@CTAB-E110, indicating its reaction.In the IR spectra of CS@CTAB-E122 and CS@CTAB-E110, new peaks appeared at 1315 cm −1 and 1075 cm −1 , attributed to (S-O) bending vibration and strong (S=O) stretching of the (SO 3 ) group.On top of a shift in (C-O) bending vibration from 1155 to 1162 cm −1 in the IR spectra of both CS@CTAB-E122 and CS@CTAB-E110 [79][80][81][82] .
The adsorption mechanism of the investigated anionic species was predicted by taking into consideration the functional groups on the surface of CS@CTAB, as shown in Fig. 25a,b.These functional groups include amino (-NH 2 ) and hydroxyl (-OH) groups.As illustrated in Fig. 25a, For Cr(VI), in an acidic medium, the CS@CTAB adsorbs Cr(VI) through the electrostatic interactions between the negatively charged oxygen on the Cr(VI) and the positively charged groups (-NH 3 + , -OH 2 + ) of the CS@CTAB adsorbent.Furthermore, hydrogen bonding has a vital role in the adsorption process for the three anionic species via the interaction between hydrogen on the surface of the adsorbent and atoms including oxygen and nitrogen in the structure of the anionic species, respectively 83 .Eventually, Fig. 25b illustrates the n-π's interactions are also attributed to the adsorption of the www.nature.com/scientificreports/anionic dyes (E122 and E110) by the interaction of the electron-donating groups, which can be presented by the nitrogen and oxygen groups of the adsorbent and the aromatic rings of both dyes 84 .
Performance of CS@CTAB.Table 7 demonstrates the performance of CS@CTAB in comparison with other adsorbents.As noted from comparison studies, several variables should be taken into consideration, for example, adsorption dose, adsorption capacity, equilibration time, the sorbent type, and the initially applied concentration.From the studies, CS@CTAB proved to have a higher capacity towards Cr(VI), E122, and E110, as represented in Table 7.  www.nature.com/scientificreports/

Conclusion
CS@CTAB was fabricated via facile methodology followed by adsorption of these anionic species (E122, E110, and Cr(VI)).The CS@CTAB adsorbent contains a functional quaternary ammonium group, providing a large number of active adsorption sites.Hence, this adsorbent shows convenient recycling properties and notable practicality.We noted an efficient role for CS@CTAB in the adsorption of sunset yellow FCF, Azorubine, and hexavalent chromium (individually and in combination) at the optimum conditions of 0.005 g dose, 100 ppm initial concentration, and 240 min for all pollutants, while a pH of 3 was used for both dyes and 2 for Cr(VI).The adsorption kinetics of anionic pollutants on the surface of CS@CTAB was well-fitted to the pseudo-second-order model, and the isotherms were in agreement with the Langmuir isotherm with a maximum adsorption capacity 2) q e = C i − C f × V m https://doi.org/10.1038/s41598-023-43034-9

Figure 11 .
Figure 11.Effect of initial concentration of E110, E122 and Cr(VI) on adsorption.

Table 1 .
BET analysis of CS and CS@CTAB.

Table 2 .
Elemental analysis of CS and CS@CTAB.

Table 5 .
Thermodynamic parameters for the E122, E110 and Cr(VI) adsorption on to CS@CTAB:

Table 6 .
Analytical results of adsorption of E110, Cr(VI) and E122 in natural water samples employing CS@ CTAB as an adsorbent.(n = 5).