Efficient removal of noxious methylene blue and crystal violet dyes at neutral conditions by reusable montmorillonite/NiFe2O4@amine-functionalized chitosan composite

The jeopardy of the synthetic dyes effluents on human health and the environment has swiftly aggravated, threatening human survival. Hence, sustained studies have figured out the most acceptable way to eliminate this poisonous contaminant. Thereby, our investigation aimed to fabricate montmorillonite/magnetic NiFe2O4@amine-functionalized chitosan (MMT-mAmCs) composite as a promising green adsorbent to remove the cationic methylene blue (MB) and crystal violet (CV) dyes from the wastewater in neutral conditions. Interestingly, MMT-mAmCs composite carries high negative charges at a wide pH range from 4 to 11 as clarified from zeta potential measurements, asserting its suitability to adsorb the cationic contaminants. In addition, the experimental study confirmed that the optimum pH to adsorb both MB and CV was pH 7, inferring the ability of MMT-mAmCs to adsorb both cationic dyes in simple process conditions. Furthermore, the ferromagnetic behavior of the MMT-mAmCs composite is additional merit to our adsorbent that provides facile, fast, and flawless separation. Notably, the as-fabricated composite revealed an auspicious adsorbability towards the adsorptive removal of MB and CV, since the maximum adsorption capacity of MB and CV were 137 and 118 mg/g, respectively. Moreover, the isotherm and kinetic investigatins depicted that the adsorption of both cationic dyes fitted Langmuir and Pseudo 2nd order models, respectively. Besides, the advanced adsorbent preserved satisfactory adsorption characteristics with maximal removal efficacy exceeding 87% after reuse for ten consecutive cycles. More importantly, MMT-mAmCs efficiently adsorbed MB and CV from real agricultural water, Nile river water and wastewater samples at the neutral pH medium, reflecting its potentiality to be a superb reusable candidate for adsorptive removal cationic pollutants from their aquatic media.

www.nature.com/scientificreports/ Preparation of AmCs. The AmCs was prepared for our previous work with slight modifications 44 . Firstly, chitin (7 g) was immersed into a solution of PBQ (6.8 mM, 100 mL) at pH 9, under mixing at 60 °C for 6 h.  Batch experiments. The current manuscript is designed to treat two organic dyes, such as MB and CV, through an easy and economical method using MMT-mAmCs to produce MB/CV-free water for human use. A batch-system procedure was applied for the adsorption investigations to achieve dye-removal data. Stock solutions (1000 ppm) of MB and CV were made by dissolving 1 g of MB and CV into 1 L double distilled water, then stored in dark bottles. The desired concentrations of MB and CV were prepared by dilution by double distilled water. In a typical experiment, 50 mL of MB/CV solution was agitated with 50 mg of MMT-mAmCs at an optimum pH, time, and temperature. pH values of solutions were adjusted in the batch operation experiment by adding small amounts of 0.1 M solutions of NaOH and HCl. The effects of pH, stirring time, MB/CV concentration, and temperatures on the MB and CV adsorption efficiency and capacity were systematically investigated to achieve optimum dye-removal conditions. MMT-mAmCs was filtered from equilibrated MMT-mAmCs-dye solution via centrifugation (5000 rpm). The MB and CV concentrations were assessed via UV-Vis spectrophotometer (Evolution 300, Thermo Scientific, England) at λ max = 658, and 580 nm, respectively. The MB/ CV removal efficiency % and the adsorption capacity of the MMT-mAmCs (q e , mg/g) were estimated through the next equivalences 12 : C i and C f are MB and CV dyes' initial and final concentrations (i.e., before and after the removal process). V is the MB/CV solution volume of (L), and w is the MMT-mAmCs (g) quantity. To assess the performance of the MMT-mAmCs in removing MB and CV from aqueous solution and understanding the possible adsorption mechanism, isotherm, kinetic and thermodynamic investigations have been investigated. The regeneration of the used MMT-mAmCs was performed according to the elution treatment protocol using HNO 3 as eluent agents. The regenerated MMT-mAmCs was reused for several repeated cycles through the batch-system technique. The real applicability of MMT-mAmCs was done by removing MB and CV dyes from real water samples.

Results and discussion
Fabrication mechanism. The fabrication mechanism of MMT-mAmCs composite was divided into three consecutive stages as delobrated in Scheme 1. The first stage involves the activation of -OH groups of chitin biopolymer under alkaline conditions using PBQ, which act as an activator. The reason for the activation process is to create active sites on the surface of chitin which facilitate its chemical modification. Next, EDA molecules were easily reacted with the activated -OH groups to produce amine-functionalized chitin, which followed by deacetylation process to convert the acetyl groups (-NHCOCH 3 ) to the primary active NH 2 groups. The second stage includes combination of AmCs with NiFe 2 O 4 through various electrosatatic and chelation interactions between the generated extra positively charged -NH 2 groups of AmCs with the negatively charged NiFe 2 O 4 . The third stage involves the reaction of -OH and -NH 2 groups of mAmCs with Si-O and -OH groups on MMT clay surface through hydrogen bonding interactions.
Characterization of MMT-mAmCs. FTIR. Figure 1 represents the FTIR spectra of NiFe 2 O 4 , MMT, AmCs, and MMT-mAmCs composite. The FTIR curve of NiFe 2 O 4 illustrates the absorbance peaks at 604 and  55 . In addition, the weight loss between 229 and 696 °C is due to the corresponding metal oxide growth 56 . The TGA profile of MMT clarified the weight loss between 36 and 263 °C, which is assigned to the moisture removal and the dehydration of the hydrated cation. Furthermore, the slight weight loss after 263 °C is due to MMT's de-hydroxylation. Moreover, the TGA profile of AmCs shows a mass deficiency between 36 and 150 °C, corresponding to water evaporation. In addition, the weight loss is caused by the dehydration of the saccharide rings, and de-polymerization of AmCs occurs between 150 and 350 °C. After 600 °C occurred, complete decomposition of the AmCs skeleton. TGA profile of MMT-mAmCs composite reveals a significant  www.nature.com/scientificreports/ Optimization of MB and CV adsorption. This study mainly explains how the adsorption effectiveness of MB and CV dyes is affected by the parameters mentioned above. The batch methodology was employed to attain the efficient purpose of MMT-mAmCs in terms of the removal and adsorption of MB and CV dyes. Here, bench-top trials were completed to adjust the adsorption conditions, such as pH, stirring time, MMT-mAmCs dose, and dye-initial concentration.
Effect of pH and adsorption mechanism. One of the key parameters that can be applied to remove MB and CV is the pH of a solution. pH is the greatest aspect influencing the charge type of MMT-mAmCs surface-active sites, causing either the boost or diminution of dye-removal effectiveness. In bench-top trials, 50 mg of MMT-mAmCs was agitated with 50 mL of MB/CV solution (25 ppm) at diverse pH conditions (pH 2-10). Figure 5A indicated that the MB/CV removal % was enhanced with pH growth. At pH 7, the MB and CV removal % were 99 and 98%, respectively. MB and CV dyes are cationic dyes and can be ionized into Clions and dye + ions (i.e., positively MB + and CV + ions). Thus, both MB and CV exist as MB + and CV + ions at pH > 7 20,57 . At pH < 7, active exterior sites of MMT-mAmCs become positively charged sites. These positively functional positions decrease the attraction of MB + and CV + ions onto MMT-mAmCs surface, thus decreasing the dye removal efficiency. The smaller removal % at pH < 7 may be due to the rivalry between H + ions and MB + and CV + ions at surface-active sites 12 . At pH ≥ 7, the MMT-mAmCs surface is changed to negative positions as verified by ZP measurements, which boosts the adsorption of cationic MB and CV dyes. Thus, the removal of MB and CV dyes using MMT-mAmCs was found favorable at the higher pH values. Attributable to the insignificant variation between the removal % at pH 7 and pH > 7, pH 7 was selected as an optimal pH condition for the next investigations. The extreme removal % of the MMT-mAmCs at pH 7 may be owing to the following; (i) a huge quantity of surface-active positions interior/exterior grooves/pores and outside surface of the MMT-mAmCs adsorbent, and (ii) rapidity diffusion of MB and CV through the grooves/pores along with the MMT-mAmCs matrix. Hence, the adsorbent surface would participate in cation attraction and exchange reactions and the extraordinary electrostatic attraction between the cationic dye and the anionic adsorbent sites. Accordingly, the barrier to MB and CV molecules diffusion decreases, resulting in a high adsorption capacity. Table 1 shows the comparison between MMT-mAmCs adsorbent and other adsorbents used to remove MB and CV from wastewater, which were reported elsewhere, based on adsorption capacity (mg/g), and adsorption conditions.  Figure 5B indicated that the MMT-mAmCs adsorbent adsorbs more than 99% and 98% of MB and CV dyes, respectively. The findings explained that the removal of MB and CV was relatively fast at the early stages of the removal procedure and slackened before reaching the stability point 70 . This result confirmed that removing MB and CV is a rapid and time-dependent process. The low efficiency at stirring time < 30 min may be caused by inadequate contact time between MB&CV dyes and the MMT-mAmCs and the controlled diffusion of the MB&CV fragments from the bulk solution to the surface-active sites of the MMT-mAmCs adsorbent. Growing the stirring time enhances the MB&CV-removal %, which is considered an acceptable outcome for the real elimination of MB and CV dyes from actual wastewater models.
Effect of MMT-mAmCs dosage. The dose of MMT-mAmCs can influence the removal % of MB and CV dyes. The MMT-mAmCs quantity was adjusted from 25 to 200 mg at pH 7, room temperature, and 25 ppm MB/CV concentration. As revealed in Fig. 5C, the dye-removal % was improved with an expanded MMT-mAmCs dose. At smaller MMT-mAmCs quantities, the dye-adsorption % was insignificant due to the deficiency of surfaceactive sites 71 . In this case, 50 mg of the MMT-mAmCs has the maximum dye-removal ability (> 99 and 98% for both MB and CV). Therefore, future expanded the MMT-mAmCs amount is unnecessary, where the removal % remained almost constant. Hence, 50 mg of the MMT-mAmCs was enough to remove MB&CV dyes.
Effect of MB&CV concentration. The preliminary MB&CV concentration plays a significant role in offering a guiding force to conquer the unwillingness of mass gradient between the dye solution and the solid MMT-mAmCs adsorbent. Therefore, bench-top trials were performed by varying preliminary concentrations (5-200 ppm) to evaluate the maximum adsorption capability (q e , mg/g) of the proposed MMT-mAmCs. At the same time, the rest factors remain constant such as MMT-mAmCs dose (50 mg), pH of the solution (pH 7), stirring time (30 min), and temperature (room temp.). The result in Fig. 5D suggested that the adsorbed amount of   www.nature.com/scientificreports/ Q o and K L are the utmost capabilities of the MMT-mAmCs (mg/g) and the adsorption equipoise constant (L/mg). K F and n are the constants relative to the sorption capability of MMT-mAmCs and adsorption strength. These parameters were defined through the slope and intercept of obtained linear figures and then recorded in Table 2. Per correlation coefficients (R 2 ) in Fig. S2, the Langmuir isotherm model fitted perfectly with the investigational data obtained (0.993 and 0.998), contrasted with the Freundlich isotherm model which displayed a small R 2 value. The theoretical maximum adsorption capacities Q o were 125 and 140.8 mg/g. The Q o values are near the experimental values of 118 and 137 mg/g of both MB and CV, respectively. Hence, our outcomes suggested that 1 g of used MMT-mAmCs adsorbent can adsorb around 0.125 and 0.141 g of MB and CV from the real wastewater. The obtained data indicated that the K L values are smaller than 1, indicating the reversibility of MB&CV adsorption. Values of 1/n are additionally lower than 1, suggesting that the adsorption nature of MB and CV are chemisorption and favorable. The obtained value of R L between 0 and 1 indicates the favorability of the adsorption.
In conclusion, the investigation confirmed that MB/CV dyes were adsorbed and formed a monolayer on the homogeneous MMT-mAmCs surface. The adsorbed MB&CV attached to the active sites of MMT-mAmCs through chemical bonds. The great sorption capacity of our MMT-mAmCs adsorbent qualifies it to be a suitable effective adsorbent to remove MB&CV from dyes-rich wastewater.
Kinetic study of MB/CV-removal. The kinetic study of MB and CV adsorption was explored to define the sorption performance regarding (i) stability with time, (ii) type of MB and CV interaction and binding mechanism with MMT-mAmCs, and (iii) adsorption rate. The removal process of MB and CV was performed at 25 ppm as initial concentration, room temperature, pH 7, and time intervals of 2.5-30 min. The residual MB and CV content was determined by UV-Vis spectrophotometer. The obtained data suggested that for the early 30 min, the sorption was quick because of the abundance of active surface positions. In comparison, the adsorption/removal % became comparatively stable after 30 min, as explained before in the study of stirring time parameter. Therefore, two usually kinetic versions (the pseudo 1st and 2nd order versions) were employed in the following linear forms to fit the obtained results and investigate the possible rate-controlling of MB and CV removal mechanism under optimal removal circumstances. Pseudo 1st order type is generally utilized to discover the sorption behavior in a solid-solution system, while the pseudo 2nd order paradigm is used to evaluate the nature of sorbent-sorbate interaction and a rate-limiting stage 74 : k 1 and k 2 are the 1st and 2nd rate invariable. q e and q t are the adsorbed MB&CV amounts at steadiness step and time (t). These constants can be determined through the slope and intercept of Log(q e -q t ) and t/q t against time plots. As revealed in Fig. S3 and Table 3, the adsorption of MB and CV dyes fits the pseudo 2nd order, according to R 2 values. Thus, the adsorption/removal mechanism of MB and CV using MMT-mAmCs is chemically adsorption. This behavior may be due to the rapid binding effect of MB and CV with MMT-mAmCs surface-active sites and the increasing MB&CV transfer/diffusion rate. The rate constant of the MB and CV similarly seems to be (4) www.nature.com/scientificreports/ controlled by a chemosorption interaction, where the large value of k 2 also confirms the quicker sorption rate. The linear fitting of the pseudo 1st order is satisfactory (R 2 ≥ 0.75). Therefore, the achieved q e values are near the experimental adsorption capacity of the MMT-mAmCs for MB&CV dyes.
Thermodynamic studies of MB/CV-removal. Batch trials were performed to investigate temperature's effect on adsorption and removal of MB and CV dyes. The temperature of the adsorption process can impact the adsorption efficiency via the diffusion rate of MB and CV molecules along with the surface-active sites and into MMT-mAmCs's pores. The temperature during the MB&CV-removal process gives info regarding variations in enthalpy (ΔH°) and entropy (ΔS°). MB and CV adsorption showed a slight variation from 98 to 99% at temperatures (25 ± 2 °C to 60 ± 2 °C) using 25 ppm initial concentration and pH 7, as shown in Fig. 6A. In general, expanding temperature may improve the mobility and diffusion of MB and CV molecules due to the formation of surface monolayers. Furthermore, high temperature decreases the intermolecular influences between water and MB&CV molecules, allowing for easier and faster diffusion of the MB and CV into the MMT-mAmCs matrix. Because of the insignificant variation between the removal efficiencies at 25 °C and 60 °C, 25 °C was chosen as an optimal temperature for our study.  R is the universal gas constant (8.314 J/mol.k). T is the temperature in kelvin. K d is the equilibrium constant, where K d = q e /C e . Values of ΔH° and ΔS° were defined from a linear plot (ln K d vs. 1/T), as displayed in Fig. 6B. The acquired values of ΔH° and ΔS° are registered in Table 4. The positive value of ΔH° suggested that the MB&CVadsorption using MMT-mAmCs is an endothermic reaction. The increase in temperature was advantageous by accelerating the adsorption rate and dye diffusion. The negative ΔG° value suggests the spontaneity of the MB&CV-adsorption. Spontaneous MB&CV-adsorption is favorable and preferred. The ΔG° value converted to more negative values with increasing temperature conditions, telling that high temperature was advantageous for MB&CV adsorption. A positive value of ΔS° during MB&CV adsorption onto the MMT-mAmCs shows randomness expansion on the MMT-mAmCs-dye interface, caused by the change of MMT-mAmCs surface or surface movement of adsorbed MB&CV molecules along the adsorbent surface.

Regeneration and reusability of MMT-mAmCs.
The proposed MMT-mAmCs should display efficient adsorption efficiency and reusability during numerous adsorption-elution procedures to decrease the full cost of the wastewater treatment operation. The regeneration and reusability of MMT-mAmCs for MB and CV adsorption were explored in ten repeated reuse/cycles (Fig. 7). For every experiment, 50 mg of MMT-mAmCs was stirred with 50 mL of 25 ppm MB/CV solutions at pH 7, and room temperature for 30 min. After the adsorption process, the adsorbed MB and CV onto MMT-mAmCs were eluted using 0.1 mol/L HNO 3 solutions for 30 min. The treated MMT-mAmCs was washed, filtered, dried at 70 °C, and then used in successive MB and CV adsorption. The gained results signified that the developed adsorbent still retains acceptable adsorption profiles even after reuse for ten repeated adsorption-desorption cycles since the removal efficiency still exceeded 87%. Overall, the results indicated that the MMT-mAmCs is an effective adsorbent in wastewater purification as a consequence of (i) its excellent removal %, (ii) cost-efficiency, (iii) reusability without considerable change in the  www.nature.com/scientificreports/ MMT-mAmCs aptitude even after multiple reuse cycles, and (iv) simple desorption operation of the adsorbed MB and CV dyes at the neutral medium.
Adsorption of MB and CV from actual water samples. The actual application of the proposed MMT-mAmCs to remove MB and CV from polluted water samples, such as farming wastewater, Nile River water, and industrial wastewater, under optimal removal conditions was explored. In actual application tests, 50 mg of MMT-mAmCs was stirred with 50 mL of water samples at pH 7 and room temperature for 30 min. The farming wastewater, Nile River water, and industrial wastewater samples were filtered through filter paper and centrifuge to eliminate large particles and apparent impurities. The finding results proved that the used MMT-mAmCs adsorbent is considered an efficient adsorbent to remove MB and CV dyes from actual wastewater. Table 5 suggested that the removal % of MMT-mAmCs toward MB&CV dyes was reduced because of further co-existing ions. The primary considerations for the high removal % of MMT-mAmCs may be due to: (i) abundant surface active positions and (ii) rapid and simple diffusion of MB and CV along the MMT-mAmCs surface. Accordingly, the proposed approach can be utilized efficiently to remove MB and CV from actual water to produce healthy water using efficient, non-toxic, and low-priced adsorbent.

Conclusion
Our investigation provides a sophisticated green adsorbent with remarkable merits, including high adsorption capacity, costless, easy separation, and excellent reusability for removing the cationic MB and CV dyes in a neutral medium. The ZP measurements demonstrated the presence of high negative charges onto the MMT-mAmCs surface reached − 20.5 mV at pH 7, enhancing its ability to grasp the cationic MB and CV from wastewater via the electrostatic interactions. In addition, the experimental study confirmed that the optimal pH to adsorb both cationic dyes was pH 7. These findings elucidated that MMT-mAmCs composite exhibits efficient adsorption processes to MB and CV and a quite simple process that could be implemented in the neutral pH medium and room temperature. Besides, the good magnetic behavior of MMT-mAmCs composite facilitated its separation after the adsorption processes and endowed it with a high ability to recycle and reuse many adsorption cycles. Moreover, the as-fabricated composite showed a promising adsorption behavior since the Q o of MB and CV were 137 and 118 mg/g, respectively. Notably, MMT-mAmCs revealed efficient removal for both MB and CV in the real agricultural water, Nile river water, and wastewater samples at the neutral pH medium.

Data availability
The data presented in this study are available on request from the corresponding author.