Polymeric Substitution of Triazole Moieties in Cellulosic Schiff Base for Heavy Metal Complexation Studies

The adsorption of metal ions from wastewater using Schiff base cellulose bearing pendulant heterocyclic chelating groups (MC-Tz) as a sorbent is the subject of this paper. Solid state 13C-NMR, FT-IR, SEM, and XRD spectroscopy, as well as TGA and XRD were utilized to examine the adsorbent. The batch sorption process used pH, adsorbent dose, initial adsorbate concentration, temperature, as well as contact time to calculate the metal ion levels. The optimum pH-6.0, with the complexation reaction and ion exchange phase as the mechanisms at work. To investigate the equilibrium concentration and temperature-dependent rate constants, various models, such as the Langmuir, Freund, Temkin, and Redlich–Peterson adsorption isotherm were utilized. Maximum adsorption capacity of the modified cellulose (MC-Tz) towards Lead(II), Copper(II), Nickel(II), and Cadmium(II) were found to be 453.2, 485.5, 473.2, and 455.6 mg/g respectively. A Kinetic study shows that the Langmuir is more in agreement with the Pseudo-second order Kinetic model. Adsorption–Desorption experiments over four cycles demonstrated the feasibility of the sorbent's regeneration potential and the measured values of enthalpy and entropy explain the essence of the adsorption process. The objective of this research is to discover non-toxic, environmentally friendly adsorbent biodegradable components and to conduct evaluations to determine their use in wastewater treatment.


Introduction
Heavy metals, which are made up of a complex group of elements, remain in the atmosphere and have been shown to have significant health effects. They also play an important but less evident role in acute toxicity. It is a term used to identify the collective arrangements of metals and metalloids with an atomic density above 6 g/cm 3 , for example Cadmium, Chromium, Copper, Mercury, Nickel, Lead, and Zinc, which are usually linked through contamination and toxic issues. Metal use in industries, vehicle exhausts, waste incineration, smelting processes, and the combustion of fossil fuels all contribute to the release of vast quantities of possibly harmful heavy metals into the environment. Pb roots encephalopathy, cerebral dysfunction, kidney damage, anemia, reproductive system deadliness, and behavioral abnormalities at elevated exposure levels [1]. Cadmium is also linked to nephrotoxicity and bone damage [2,3]. Wilson's disease is a neurotoxic syndrome caused by copper accumulation in the lenticular nucleus of the brain and kidney failure [4]. Nickel compounds are carcinogenic and readily absorbed by the skin, and exposure to a concentration of 30 mg/L in the atmosphere for half an hour is lethal [5]. To control emissions, it is important to eliminate these pollutants, which necessitates the use of a cost-effective and recyclable method including adsorption. The quest for environmentally safe materials for wastewater treatment with minimal and non-hazardous by-products has become a focal point in recent years [6].
In recent years, the synthetic polymers used in wide range of applications pollute the environment after their disposal. Bio-polymers and bio-fillers are biodegradable used for food packaging materials. Biodegradable polymers can be a feasible solution to these problems since they are produced from renewable sources like cellulose, starch, polysaccharides, proteins, polylactic acid (PLA), polypropylene carbonate (PPC), Poly(3-hydroxybutyrate-co3-hydroxyvalerate) (PHBV) etc. [7] Cellulose is a fascinating solid support that has been used for the immobilization of various ligands. It is a direct-chain macromolecule having an enormous quantity of -OH groups with 3 AGU(anhydro-glucose)/unit present in the four C1 conformations with chemical modification. The extent of the macromolecular cellulosic chain is determined by the quantity of constituents anhydro-glucose units, which differs depending on the basis and dealing of the cellulose(raw) material [8,9]. The cellulose fibers versatility is due to its form, that permits to screw and twist ways other than plane. Due to contact among neighboring cellulose fragments in dehydrated fibers and the existence of -OH(hydroxyl) groups in the chain, strong intermolecular hydrogen bonds were formed [9]. Many chemical treatments may be used to boost the water loving or repelling character, structural sturdiness, elasticity, or ion exchange ability, heat resistance, resistance to microbiological attack, while decreasing water retention, among which are those that have had higher or lower elimination of heavy metal ion characteristics.
Cellulose with chelating groups such as quaternary ammonium groups [10], thio and amine [11], carboxyl [2,3], hydroxyl [12], imidazole, and amino [13] has been identified as possible aid. The series of modified cellulose-bearing pendant groups such as benzalaniline [14], Pyridine [15], and carboxylic acid [16] that serve as chelating sites for the successful removal of metal ions process are planned with reference to previous work.
Cellulose with benzotriazole chelating groups (MC-Tz) was synthesized with a chemical modification that included metal-complexing ligands. The chelating agents anchored to polymeric support and the electron donating methyl groups bonded to N, which increases basicity, form an important chelating site for metal ion removal from wastewater. FTIR and 13C-NMR spectroscopy were used to determine the structure of the Mc-Tz adsorbent. SEM was used to determine surface morphology and elemental composition, while Energy Dispersive X-ray was used to determine particle size (EDX). Before and after the chemical equilibration, the AAS (AA6300-Shimada, Japan) was employed to monitor for heavy metal concentration. The experiment was designed for pH, dosage, contact time, and Initial ion strength, to boot. Although the pseudo first-order and second-order rate equations were used, the Langmuir, Freundlich-Peterson, and Temkin models were all used as well. In order to examine the chemical stability and efficiency of the recovery method, experiments were performed using different eluting agents.

Synthesis of Modified Cellulose Bearing Triazole Backbone (MC-Tz)
To prepare derivatives of reactive cellulose, the cellulose was oxidized and functionalized. In addition to that, the dialdehyde cellulose is formed by the periodate oxidation of cellulose [17,18]. Following the oxidation of the dialdehyde cellulose, the solution was treated with slightly acidified sodium chlorite. The water-soluble 2,3-dicarboxy cellulose oxidised to over 98% of its overall oxidation level while the insoluble 2,3-dicboxy cellulose had just 70% oxidation. Before the coupling step, the oxidation reaction using NaIO 4 (sodium metaperiodate) was conducted out on cellulose. This reaction cuts the bond between C2 and C3 of the glucosidic ring and transforms the compounds into 2, 3-dialdehydic groups. The Malaprade reaction mechanism is involved in this process [15,17,18]. An aqueous solution of sodium metaperiodate was combined with powdered cellulose suspended in distilled water and mixed at laboratory environment in the dark. DAC formed, after completion of reaction, it was separated and neutralized with double distilled water as stated in the procedure. The findings of the survey conducted by [15,16] 4 h at full darkness, under NaIO 4 concentrations of 0.01 M, thus resulting in 30 carbonyl groups per 100 glucose units [19]. When a dialdehyde and cellulose are reactivated, they will react with amines to create amine derivatives. The imaginative use of death that differs drastically [14,20]. (2-Aminotriazole-dialdehyde-cellulose) was synthesized by condensation of 2-aminotriazole dialdehyde with cellulose. After dissolution of the 2-aminotriazole (1.5 g) in 20 ml of N,N-dimethylformamide (DMF), the mixture was diluted with dialdehyde cellulose (0.5 g) and a few drops of hydrochloric acid (concentrated) was applied as a catalytic agent and stirred at 70 °C for 5 h, resulting in the catalyst with MC-Tz shown in the scheme (Fig. 1).

Influence of Solution pH
A strong influence on sorption of metal ions onto the ionic state of functional groups in the sorbent is the pH of the medium. For optimum pH adsorption, the experiments were performed at pH 1-9 with 1 h of equilibration time at 30 + 1 °C. According to the data in Fig. 3a, the highest sorption occurs around pH 6. At lower pH, the protonation of imine (-NH-) (MC-Tz) was easily happened to form -NH + cation with occurring reduced metal adsorption. Also, the heavy metals are cationic, the electrostatic repulsion between metal ions and -NH + (MC-Tz) was enhanced on the surface of MC-Tz. At low pH, Hence, the adsorption capacity of metal ions onto MC-Tz adsorbent reduced with decrease of pH [22]. Other case, as the solution pH increased, the adsorbent surface was deprotonated, leading to the increase of imine (-NH-) (MC-Tz) sites on the surface of adsorbent. Expansion of the number of H+ ions and metal ions at the pH of 6.0 occurs due to both ion exchange and complex formation. These results stated that the pH and electrostatic force played major role in the metal-MC-Tz adsorption system [23]. The adsorption performance of the MC-Tz for heavy metal ions in the higher pH range (beyond 8) was decreased due to metal hydroxides formations in the solution. In order to speed up the process, an optimal pH of 6 was developed for further adsorption experiments. For different adsorbents, comparable optimum pH for Lead(II), Cadmium(II), and Copper(II) binding has been observed [24,25].

Influence of Adsorbent Dose
For metal ion removal, the adsorbent dose has a 3b curve that increases, and then the curve starts to fall (MC-Tz).

Fig. 3 Effect of Adsorption parameters
The increase in adsorption dose results in an increase in the adsorption potential, as there is more active adsorption site surface area and additional sites that are usable for adsorption. Maximum heavy metal ion removal is reached at 80 mg/L of the MC-Tz adsorbent and furthers no considerable increase in adsorption ability beyond this value. Although the equilibrium between the metal ions and the adsorption sites was nearly reached, there was no appreciable further reduction in the metal ion concentration. This may be due to the saturation of adsorption sites of the MC-Tz adsorbents.

Modeling-Effect of Agitation time
Adsorption processes such as mass transfer and chemical reaction, have been analyzed by using the pseudo-first-order, pseudo-second order, and the Intra particle diffusion and Elovich kinetic models [26]. The kinetic activity of the sorbent (MC-Tz) towards the metal ions was investigated by calculating adsorption on a contact time scale.
At initially, the fast removal of metal ions was due to the availability of more number of reactive sites and actively participation of more functional groups which may in the removal of Cu(II) and Pb(II) ions till its equilibrium time (60 min) and after that there was no further adsorption. Therefore, the contact time was optimised 60 min for the subsequent experiments as shown in Fig. 3c [15].
One continuous curve, as if covering a monolayer of metal ions, portrays a single, flat, and nearly instantaneous rate of absorption, with equilibrium reached at a certain point in time regardless of initial concentrations. The results obtained in the present study were compiled into Table 1 in the form of the pseudo-first order rate constant, kad, and correlation coefficients for different concentrations of the Lead(II), Copper(II) and Nickel(II) and Cadmium(II) metal ions. The estimated correlation coefficient value varies greatly from the experimentally observed one, showing a weak pseudo first-order fit to the experimental results.
For Lead(II), Copper(II) and Nickel(II) and Cadmium(II), the pseudo-second-order kinetic model is studied, and the following plot is made: [t(qt)/q] versus [t]. The value of qe, k, and R 2 can be found from the slope and intercept. If this form is true to a pseudo second-order model, the linearized plot should follow a straight line. One piece of evidence that shows a link between the t/qt (or t) versus t plot for the pseudo-second order model for the Lead(II), Copper(II), and Nickel(II) and Cadmium(II) metal ions is that these plots yield strikingly straight lines (correlation coefficient, R 2 > 0.99). Additionally, qe's theoretical values coincide with the experimental value extremely well. While these facts support the hypothesis that adsorption of Lead(II), Copper(II), Nickel(II) and Cadmium(II) metal ions by MC-Tz follows the pseudo-second-order kinetic model, which implies that Chemisorption is the rate-limiting step, it should be noted that the latter theory has not been proven in all cases.
In Chemisorption, metal ions bind to the adsorbent surface by forming a chemical (usually covalent) bond and congregate in areas where there are the most coordination sites [27]. There is a direct relationship between the concentration of metal ions in the solution and the possibility of collisions among these species. The faster at Lead(II), Copper(II), Nickel(II) and Cadmium(II) metal ions bind to the surface of the adsorbent, the lower the concentration of metal ions in the solution [28].
Typical is fitting the kinetic data to the intra-particle diffusion model when an approach to find out the rate-limiting step. Since each of these heavy metal ions (Lead(II),

3
Copper(II), and Nickel(II) and Cadmium(II)) shows the same type of plot with differing characteristics, it can be concluded that transport of heavy metal ions from solution through the particle solution interface, and the pores of the particle, all proceed in a nearly identical manner. When extrapolating the linear part of the plot to the axis, one obtains the intercepts, which are proportional to the thickness of the boundary layer, or the larger the intercept, the greater is the boundary layer effect [29]. Elovich equation describes the heterogeneous distribution of adsorption due to heterogeneous chemisorption kinetics. Data provided in the current study indicates a correlation coefficient of 93% or higher indicates that the Chemisorption process refers to the ion exchange reaction.

Effect of Initial Metal Ion Concentration
Metals are taken up by the adsorbent, which can be represented by the isotherm model of Langmuir, Freundlich, Redlich-Peterson, and Temkin, as shown in Fig. 3d and as shown in the results in Table 2. In sorption equilibrium studies, adsorbate distribution, adsorbent heterogeneity, adsorbent coverage, and adsorbent interaction are defined. Maximal adsorption is usually occurring when a saturated monolayer of the molecules is on the adsorbent. When a constant solute molecule is present, the adsorption energy is present in all areas.
Since the adsorbate molecules are in constant motion, it is impossible to shift them toward the surface. The maximum adsorption capacity for the solid phase loading and the heat of adsorption defined by the Langmuir constants qm and KL is equal to the maximum adsorption capacity of the fluid phase loading and the temperature of adsorption. RL values illustrate the adsorption mechanism because RL > 1 indicates unfavorable adsorption, RL = 1 portrays linear adsorption, and RL < 1 reflects favorable adsorption [30]. The graphical data shown in Fig. 4 shows that the Langmuir equation matches the isotherm data very well (R 2 = 0.997). The values of qm and RL for Lead(II), Copper(II), Nickel(II), and Cadmium(II) were found to be 453.2, 485.5, 473.2, and 455.6 mg/g and 0.0205, 0.01050, 0.0148, and 0.0122 l/ mg. That is, the mechanism of adsorption was depicted as favorable. Ionic radii may be the reason for this preferential sorption conduct. Ionic radii are highest for Lead(II) (1.19°) and then decrease as the other cations have smaller sizes: Cadmium(II) (0.97°), Nickel(II) (0.72°), and Copper(II) (0.69°). The greater the ionic radius, the weaker the ionic binding and the water phase because of the smaller hydration potential of that ion. Due to the fact that Lead(II) and Cadmium(II) have a higher hydration energy than Nickel(II) and Copper(II), adsorption preference may be present for Lead(II) and Cadmium(II) as well [31].
Metal ion adsorption potential (qe) for a given mass of adsorbent is well-defined by Langmuir and Freundlich characteristics involve both homogeneous and heterogeneous structures, which refer to the heavy metal ion adsorption mechanism with the Redlich Isotherm model. The Redlich-Peterson isotherm exponent between 0 and 1, β, is defined as β. For equivalent values of β, the Langmuir isotherm is the better. Otherwise, the Freundlich isotherm is the better. When adsorbing Lead(II),Copper(II), and Nickel(II) ions onto the MC-Tz adsorbent, the results show that the adsorption constant, β, is close to 1, suggesting that Lead(II),Copper(II), and Cadmium(II) ions have strong isotherms while Nickel(II) ions have worse ones. Lead(II) is adsorbed last based on the R 2 values because the adsorption of the other metals goes from the least to most expensive.
The assumptions made in constructing the Temkin isotherm are that all molecules in the layer undergo a linear decrease in heat of sorption as a result of sorbate-sorbent interactions, and the amount of heat of sorption decrease is proportional to the degree of coverage. B, which represents the constant of adsorption in this isotherm, can be defined by the universal gas constant (J/mol/K), R is the universal gas constant (J/mol/K), T is the temperature (K), b is the variation of adsorption energy (J/ mol), and the equilibrium binding constant (l mg −1 ) that corresponds to the maximum binding energy. Applying Qe to the plot of lnCe vs. ln results in the determination of the isotherm constants B and KT. B < 20 kJ/mol. This highlights weak interaction between the metal ions and sorbent (MC-Tz), facilitating ion exchange. In the Temkin isotherm model, the metal ions are ordered according to the R 2 values: Lead(II) > Copper(II) > Cadmium(II) > N ickel(II). The high electro negativity and high softness value of Lead(II) is conducive to this, as it adsorbs over the Copper(II) and Nickel(II) on the binding sites of the MC-Tz.

Thermodynamic Study
Depending on the temperature used, the adsorption equilibrium can either be exothermic or endothermic. Inflating the adsorbate's sorption potential increases with the rise in temperature, as the pore size of the adsorbent and the adsorbent surface are now becoming larger. In Table 3, it is observed that the Gibbs free energy shift ( ΔG • ) is small and negative ΔS • but increases with increased temperature, which means that the adsorption mechanism is thermodynamically unfavorable. The randomness of metal adsorbent solution interface during the adsorption indicates by the negative value of  when the negative value of ( ΔH • ) is taken into consideration, it can be inferred that metal ion adsorption is an exothermic operation. The MC-Tz adsorbent was shown to be efficient for the removal of metal ions such as Lead(II), Copper(II), Nickel(II), and Cadmium(II) from aqueous solutions by the above observations.

Complexation Mechanism
Reduced bond strength was caused by the complexation of MC-Tz with Lead(II), Nickel(II) and Cadmium(II) metal ions, which resulted in a major stretching frequency change, which started at 1677 cm −1 (-CH=N-stretching) and ended at 1646, 1626, and 1644 cm −1 . The C-N peak in the metalchelated MC-Tz changed from 1562 to 1516, 1545, and 1548 cm −1 (Fig. 5). Furthermore, the metal chelated MC-Tz was found to have a drastically different shape and strength.
The following discussion is based on the explanations above and recommends that the chelating groups such as -CH=N-, -OH, and C-N should be considered the primary binding sites for metal ions as shown in Fig. 5.
In Fig. 6, a SEM image of the surface morphology of the MC-Tz, Lead(II), Copper(II), Nickel(II), and Cadmium(II) metal ions loaded MC-Tz is shown. The surface shown in the SEM micrographs is non-particulate and coated in folds. The surface of the polymer mat is rough, creating more gaps that allow the metal solution to mix more easily with the solution, resulting in better metal adsorption. When the metal ions adsorb to the functional group inside the wall of the polymeric region, the surface texture and pore development change. Grooves are less prominent in the SEM micrograph of MC-Tz after metal adsorption because of the Lead(II), Copper(II), Nickel(II) and Cadmium(II) particles adsorbed.
EDX spectra of the metal ions Lead(II), Copper(II), Nickel(II), and Cadmium(II) that were applied (Fig. 7). There are only C, N, and O peaks present in the EDX spectrum of MC-Tz. N content of 13.80 wt% indicates the successful adsorption mechanism, the Schiff base with aminotriazole groups is followed by further experiments. Figure 8A shows the XRD patterns of the chemically modified MC-Tz, which shows the amorphous nature due to the large size of the adsorbent chains. When observing the long, polymeric peaks, it is evident that the amorphous nature of the MC-Tz polymeric adsorbent makes it perfect for Lead(II), Copper(II) and Nickel(II) and Cadmium(II) metal ion adsorption. TGA was used to investigate the thermal stability and degradation behavior of the MC-Tz by rapidly heating and cooling the sample to 60 °C and 10 °C/ min. Figure 8B shows the TGA curves for this test. According to one hypothesis, the first stage of decomposition, to about 200 °C, was triggered by dehydration and volatilization processes, and the next stage, about 15% weight loss, occurred between 220 and 340 °C in MC-Tz. This furthers the results from the MC-Tz show the strong thermal stability, and the components can be used up to 250 °C. Table 4 identifies the adsorbent known as the MC-Tz and compares it to other recorded materials. This study shows that metal ions minimise the thermodynamic stability of this substance.

Desorption and Regeneration Studies
It is essential that the adsorbent is easily regenerated, and the desorbing agent is effective, inexpensive, non-polluting, and non-damaging. For the desorption efficiency shown in Fig. 9, a concentration of 0.1 M HCl, H 2 SO 4 , and CH 3 COOH was used. It is also evident from the results that after numerous regeneration and reuse cycles, the novel polyadsorbent MC-Tz are still successful.

Conclusion
Environmental issues have caused a substantial amount of research into industrial adsorbents. Considering this, we created a new, low-cost sorbent that is made out of cellulose, which is abundant, biodegradable, and structurally stable. Incorporating polymeric chelating moieties that contain a variety of different O, N, S, and P donor atoms into the cellulose backbone results in the high extraction efficiency for metals from aqueous solutions. In addition to a main band shift, there is a distinct shift in the primary stretching frequencies for -C=N-, -C-N-, -N=N °C and -CH=N °C classes in the FTIR spectrum. These heteroatoms can be bound to the metal ions by coordination bonds. The greater probability of bond formation is further shown by the lack of chemical resistance to metal ions when using a 0.1 N H 2 SO 4 solution. As a potent adsorbent, the chemically modified Mc-Tz adsorbs a wide range of pH values and demonstrates good activity over a broad pH range, with adsorption isotherm and adsorption kinetic data fitting the pseudo-second-order model. By conducting thermodynamic experiments, the feasibility and exothermic design of the adsorption method is thoroughly developed. These treated wastewater samples show a strong application opportunity and important future research potential in heavy metal adsorbent research.