Assessment of the Feasibility of Modified Chitosan Beads for the Adsorption of Nitrate from an Aqueous Solution

. Bead-shaped adsorbents made of biopolymers are the most effective in removing


INTRODUCTION
Nitrate nitrogen is a substance it can be found in a variety of produced as a byproduct of the oxidation and corrosion of nitrogen-containing compounds (Ghadiri et al., 2017). Nitrate is an essential product of chemical fertilizers, sanitary sewage, and industrial wastewater. Additionally, the amount of nitrate in groundwater has increased yearly, raising concerns recently (Daneshvar et al., 2018). It has been reported that the people exposed to nitrate can develop several diseases, such as alimentary canal cancer, which is permeated by nitrate in particular (Afkhami, 2003). The blue-baby syndrome, which happens when hemoglobin transforms into methemoglobin, which is incapable of carrying oxygen, may be brought on by too much nitrate in drinking water (Golden et al., 1998). Consequently, the permissible levels of nitrate in water have been suggested in many countries to prevent these health risks. The World Health Organization has set a 50 mg/L nitrate limit for drinking water (Herschy, 2012).
As a result, many methods for eliminating nitrate from water samples have been documented; reverse osmosis (Raval et al., 2015), electro dialysis (Wang et al., 2007), chemical reduction (Hu et al., 2001), and ion exchange (Bae et al., 2002) are examples of these. Adsorption has been used to remove nitrate from water more and more frequently in recent years compared to other technologies, because of its high efficiency, ability to conserve energy, and ability to protect the environment (Liu et al., 2018;Hammadi et al., 2019). It is well-known that the type of adsorbent significantly affects the adsorption process. Numerous studies have recommended a variety of adsorbents, including organic adsorbents, inorganic adsorbents, and agricultural wastes, such as activated carbon (Sulaymon et al., 2010), silica gel (Choudhary et al., 1982), and dates pits (Hummadi, 2021). Bead-shaped adsorbents made of biopolymers are the most effective in removing pollutants from water. It takes little effort to regenerate the beads or to extract them from the treated water afterwards (Hasmath Farzana et al., 2015). Chitosan has recently been mentioned as one of the adsorbents that can be utilized successfully to remove harmful ions (Karthikeyan et al., 2019a). Deacetylated chitin is a linear polysaccharide, well recognized as a superior biopolymeric substance. Compared to other adsorbents, this type of adsorbent has an advantage that may be summed up as follows: it is a biomaterial that can be formed into various shapes, characterized by non-toxicity, biodegradability, and biocompatibility (Hasmath Farzana et al., 2015). The main benefit of chitosan is that it contains many amino and hydroxyl groups, and the amino groups are easily protonated in acidic media (Keshvardoostchokami et al., 2021). However, certain disadvantages are associated with its use, such as poor selectivity and adsorption capacity, limited mechanical strength, and low solubility rates in acid media (Saheed et al., 2021;Ahmed et al., 2020). To improve the physicochemical properties of chitosan, various modification techniques have been proposed in recent decades. These techniques can be divided into physical and chemical modifications, such as cross-linking (AZLAN et al., 2009), metal loading (Karthikeyan et al., 2019b), and composite blend forms (Rajeswari et al., 2015).
Chitosan is cross-linked to give it mechanical strength, so it can be used in water treatment applications (Sowmya et al., 2014). Chitosan derivatives with improved characteristics are created through cross-linking reactions, which connect the macromolecular chains (enhancement of adsorption capacity and resistance under extreme media conditions, respectively) (Kyzas et al., 2015). Cross-linking of chitosan uses several common cross-linking chemicals, such as glutaraldehyde, epichlorohydrin, and ethylene glycol diglycidyl ether.
Many different alternative methods have been used to enhance the removal effectiveness and adsorption capacity of adsorbents. To solve this issue, it can be filled with various materials like multivalent metal ions. The anions are more likely to be eliminated by ions with a higher positive charge among different types of ions, such as calcium, magnesium, aluminium, and zirconium ions (Kumar et al., 2017). Zirconium is one of the many multivalent metal ions which can be combined with the materials to improve the selectivity of the adsorbent. Furthermore, Zr(IV) can be loaded onto the materials' appropriate functional groups, such as carboxyl, hydroxyl, and amino groups. Zr (IV) could bind anions like nitrate and phosphate by forming stable complexes with vacant orbits (Purbasari et al., 2022). Bentonite is a non-metallic clay mineral with a 2:1 unit cell structure of two tetrahedral silica sheets and one octahedral alumina sheet inserted between the tetrahedral sheets (Wu et al., 2001). Due to its mechanical stability, availability, affordability, and environmental friendliness, bentonite is frequently used in wastewater treatment facilities. In addition to its superior cation exchange capacity and large surface area, it also has a high removal capability for organic and inorganic contaminants (Hamid et al., 2017).
A specific kind of adsorbent was created for this study. Chitosan-bentonite-zirconium chloride composite beads are the name of the product (Cs-Bn-Zr). The primary goal of the current research is to investigate the adsorption of nitrate ions utilizing modified adsorbents. Chitosan was altered for this purpose by being combined with bentonite, and the resulting combination was loaded with zirconium chloride. The improved synthesized adsorbents were evaluated by FTIR and FE-SEM analysis and applied in a batch adsorption method. The viability of using modified chitosan adsorbents in the adsorption process was assessed by considering the removal efficiency and adsorption capacity. Therefore, this work investigated the impact of essential variables, specifically contact time, pollutant concentration, and adsorbent dosages. The study also highlighted the adsorption isotherm for the process and found the best kinetic model to describe the results obtained.

Chemicals
Chitosan powder (90% deacetylated) was purchased from chemsavers (USA). Bentonite was obtained from Iraqi Geological survey. Zirconium chloride (ZrOCL 2 ·8H 2 O) originated from BDH chemicals Ltd England. Glacial acetic acid is 99.5% from Macron Company (China). Glutaraldehyde solution 25% was bought from Thomas Baker. Sodium hydroxide pellets 98% were obtained from Orbital Company. Sodium nitrate (NaNO 3 ) is supplied by CDH. Distilled water was used to prepare all solutions.

Preparation chitosan-bentonitezirconium(CS-Bn-Zr) beads
The chitosan-bentonite beads were created using a previously reported method by (Sowmya et al., 2014) with modifications. Two grams of chitosan were dissolved in (100 ml) of glacial acetic acid (2% v\v) in a (250 ml) beaker. The mixture was stirred by a mixer (OS20-S) for three hours at 300 rpm at ambient temperature to obtain the homogenous gel. Then two grams of bentonite were added to the chitosan mixture and mixed for (6 hrs.) to create a uniform dispersion.
The chitosan and bentonite solution was dropped into a beaker (1 L) containing (1 M) of sodium hydroxide by burette (100 ml) and left for (20 h) to obtain viscous beads. Thereafter, the beads were washed with distilled water until the pH reached between (6.5-7.0); after that the wet beads were immersed in a Glutaraldehyde solution of 25% for (48 h) and washed with distilled water to neutralize the pH. Finally, the (CS-Bn) wet beads were placed in the zirconium chloride solution (ZrOCL 2 ·8H 2 O) (5%w\v) for (24 h) to increase the selectivity for nitrate ions and then washed several times with distilled water to remove the excess zirconium chloride. The beads were dried at room temperature. A schematic of the Cs-Bn-Zr preparation is shown in Figure 1.

The Batch adsorption experiments
The batch adsorption experiments were performed in a 250 ml glass bottle in the shaker (1990 Germany). A specific amount of the prepared adsorbents was placed in 250 ml of sodium nitrate solution. The mixture was shaken at room temperature at a constant rotation speed of 150 rpm.
The effect of contact time (0-180 min), adsorbent dosage (0.05-0.3 g), and initial nitrate ions (50-800 mg/L) on removal efficiency and adsorption capacity were investigated throughout the study. Samples were taken periodically and filtered before analysis. After that, the samples were analyzed by a spectrophotometer at a wavelength of 220 according to Ultraviolet Spectrophotometric Screening Method described in (Rice et al., 2012).
The removal efficiency (% removal) is estimated according to the following Equation: The adsorption capacity was calculated as follows: where: c i -the initial concentration of NO 3 (ppm); c e -the equilibrium concentration of NO 3 (ppm); m -the mass of adsorbent (g); v -the volume of solution (L).

FTIR studied adsorbent
FTIR spectra of chitosan (Cs), bentonite, and chitosan-bentonite-zirconium (Cs-Bn-Zr) bead before and after adsorption are shown in Figure 2. A broad band peak was observed in Figure 2a, which was designated for the pure chitosan overlapped stretching vibrations of the -OH and NH 2 groups at 3444 cm -1 . The asymmetric C-H stretching vibrational peak of the -CH 2 group was presented at 2862 cm -1 . Moreover, it can be noticed that the N-H bending vibration of NH2 and -OH was 1577 cm -1 and 1381 cm -1 (Karthikeyan et al., 2019c). Furthermore, the C-O-C skeletal vibrations caused the peak at 1076 cm -1, while the C-O stretching vibration of chitosan caused the peak at 1026 cm -1 (Elanchezhiyan et al., 2016). The bands are observed at 3618 cm -1 , 3406 cm -1 , 1627 cm -1 , 1033 cm -1 , and 786 cm -1 in the FTIR spectra of BENT as shown in Figure 2b. The broad band at Additionally, it can be observed from Figure 2c that the Zr 4+ ions were embedded into the composite beads by the Cs-Bn-Zr before adsorption peaks observed at 520 and 451 cm -1 (Jiang et al., 2013) attributed to the Zr-N and Zr-O bonds, respectively. The bands at 1670 cm -1 in chitosan shifted to 1651 cm -1 in Cs-Bn-Zr beads, which was attributed to intermolecular interaction between chitosan's -NH 2 group. The band appeared at 3444 cm -1 with chitosan reduced to 3383 cm -1 in the Cs-Bn-Zr bead. However, Figure 2d demonstrated that this peak reduced to 3367 cm -1 in Cs-Bn-Zr beads after adsorption, which may contribute to metal coordination with the chitosan functional groups (Banu et al., 2019). In addition, the spectra of beads after adsorption showed a new peak at 1381 cm -1 (Nur et al., 2015), which was related to the N-O stretching frequency; this demonstrated that nitrate ions had been adsorbed on the adsorbent.

Characterization by FESEM
FESEM images of Cs-Bn-Zr composite beads before and after nitrate ion adsorption are shown in Figure 3 (a to d). Zirconium was visible on the surface, as shown in Figure 3a. Before adsorption, the Cs-Bn-Zr beads in Figures (3b and 3c) showed high porosity and heterogeneity as an uneven rock-like structure resulting from chitosan intercalation on Bentonite surfaces. Because the nitrate-coated beads covered the porosity, they became less porous and smoother after adsorption, as shown in Figure 3d. Figure 4 (a, b) shows the EDX characterization of Cs-Bn-Zr beads before and after nitrate ion adsorption. Bentonite was responsible for  Figure 4a. It can be observed from Figure 4b that the nitrogen and oxygen peaks increased in comparison with those obtained in Figure 4a, indicating that nitrate ions were adsorbed onto Cs-Bn-Zr beads. The peak of Zr +4 emerged, demonstrating that the Zr +4 had been successfully loaded onto the chitosan-bentonite beads. Furthermore, the chloride peak in Figure 4b was decreased, indicating that the sorption system also influenced the ion exchange mechanism.

BET surface area analysis
The surface area of particles is evaluated using the (Brunauer, Emmett, and Teller) method. The surface area is a crucial parameter for enhancement of the mass transfer and provides more active sites. Table 1 displays that the surface areas of chitosan are (18.15 m 2 /g) and the produced adsorbent (Cs-Bn-Zr) is (63.80 m 2 /g). From the obtained values of surface area results, it can be concluded that the addition of bentonite increased the surface area of the chitosan-based bead.

Effect of contact time
With an initial NO 3 concentration of 50 mg/L and an adsorbent dosage of (0.1 g), the impact of contact time on removal effectiveness and adsorption capacity was studied. According to Figure 5, contact time increases removal efficiency and adsorption capacity. By expanding the adsorption time from 30 minutes to 120 minutes, the removal efficiency grew considerably from 22% to 87%. However, the efficiency began to level off, and when the contact time was increased to 180 minutes, the efficiency only increased to 90.5%.
Adsorption capacity displayed a similar pattern of behavior, where increasing the contact time from 30 minutes to 120 minutes caused the adsorption capacity to rise steadily from 5.5 mg/g to 21.8 mg/g. The results show that when the contact period exceeded 120 minutes of contact time, the improvement in adsorption capacity   . (2019). The results of this series of studies suggest that 120 minutes is the ideal operating duration for the adsorption process. At this point, both removal efficiency and adsorption capacity are at appropriate levels. Increasing the operating time beyond that point would be impracticable for the process. As a result, the remaining findings will be presented after 120 minutes of operation. Figure 6 shows how the initial concentration affects the elimination of nitrate. It is clear that raising the initial concentration from 50 to 800 mg/g at the adsorbent dosage of 0.1 g, rotation speed of 150, a contact time of 120 minutes and ambient temperature. the results domenstrated that the removal percentage droped from 87% to 26.65%. this could be due to the fact that at lower concentrations, the majority of the nitrate was adsorbed by Cs-Bn-Zr. Whereas at greater concentrations, the Cs-Bn-Zr was quickly saturable due to limited dosage and capacity.

Effect of initial concentration
On the other hand, the adsorption capacity exhibits a reversal reaction to that achieved with removal efficiency. With an initial NO 3 concentration of 600 mg/L, the adsorption capacity increased significantly from 21.8 mg/g with an initial concentration of 50 mg/g to roughly 110.46 mg/g. This increase may be explained by a boost in the driving force produced by the gradient in nitrate concentration between the aqueous and solid phases. The results align with the work of Liu et al. (2016). At 600 mg/L, the adsorption

Effect of adsorbent dosage
The best adsorbent dosage might be obtained by varying the dosage (0.05, 0.1, 0.2, and 0.3 g) under the study conditions, such as a contact time of 120 minutes, 150 rpm, and 50 ml of nitrate concentration of 50 mg/L. The findings presented in Figure 7 showed that when the dosage of Cs-Bn-Zr was increased from (0.05 to 0.2 g), the removal was enhanced from 11 to 97 percent. The availability of increased surface area and adsorption functional sites with higher Cs-Bn-Zr dosage may cause this effect. Beyond 0.2 g, an additional increase in adsorbent dosage has no discernible impact on the removal efficiency. This behavior was in agreement with the results obtained by Liu et al. (2016). Therefore, 0.2 g was selected as the optimal dose for this study.

Adsorption isotherm
The adsorption isotherm represents the equilibrium between the quantity of adsorbate that is adsorbed and that is in solution.
The experiments produced the most accurate adsorption statics. Nitrate adsorption statics onto Cs-Bn-Zr beads were determined by comparing Temkin's isotherm models to well-known isotherm models like Freundlich and Langmuir.

Langmuir isotherm model
According to the Langmuir adsorption model, maximal adsorption corresponds to a saturated monolayer (chemical adsorption) of solute molecules on the adsorbent surface, with no transverse contact between the adsorbed molecules (Vijayaraghavan et al., 2006).
where: q e -the amount of nitrate adsorbed by adsorbent mass at equilibrium (mg/g); c e -the concentration of nitrate (mg/L); k L -Langmuir constant (L/mg); q max -the maximal monolayer adsorption capacity (mg NO 3 /g adsorbent).
Additionally, it was possible to explain the Langmuir isotherm using the separation factor (R L ) (Weber et al., 1974) that was determined using Equation (4): where: c i -the initial concentration of nitrate (mg/L). R's value indicates the sort of isotherm, with R L = 0 being irreversible, (0 < R L < 1) being favorable, and R L = 1 being unfavorable. was illustrated in Figure 8. The q max and k L were computed from the slope and intercept, respectively.

Freundlich isotherm model
The Freundlich model presupposed that the adsorption process takes place in multilayer sorption and that the molecular distribution on the surface of the adsorbent is heterogeneous (Shavandi et al., 2012). Freundlich's Equation is illustrated below in its linear form: where: q e (mg/g) -the amount of nitrate adsorbed by adsorbent mass at equilibrium; c e -the concentration of nitrate (mg/L); Freundlich empirical constants k f (mg/g) and n serve as an indicator of adsorption capacity and favorability.
The plot of log q e opposite log c e is illustrated in Figure 9. The , and k f were computed from the slope and intercept, respectively.

Temkin isotherm model
One characteristic of the Temkin isotherm makes good quality of the interaction between the adsorbent and adsorbate. Forms using this model were used and provided as Equation 6: where: B -the Temkin constant; k T -the Temkin isotherm constant (l/mg).
The plot of q e opposite ln c e is illustrated in Figure 10. Table 2 shows the values obtained for the Langmuir, Freundlich, and Temkin isotherm constants. With a correlation coefficient R 2 of (0.9728), it amply demonstrates the goodness of fit of experimental data of nitrate ion adsorption by Cs-Bn-Zr with the Freundlich model which indicate to multilayer adsorption. The size of the Freundlich coefficient n provides a favorability measurement of the adsorption process. In the range, n values between 2 and 10 indicate acceptable physical adsorption, between 1 and 2 moderate difficulties, and below 1 poor adsorption (Li et al., 2017;Radhi et al., 2021). This was supported by the results of the n (3.2071) values obtained in the current study. This indicates significant physical adsorption.

Adsorption kinetic
The kinetic analysis determines the solute uptake rate, which also establishes the amount of time that must pass before the adsorption reaction is complete (Sanyangare, 2016).
Two well-known kinetics adsorption models, the pseudo-second-order Equation and the pseudo-first-order Equationcan be used to determine the adsorption rate. The trials were carried out in various contact situations time. The linear form of the pseudo-first-order equation is as follows: where: k 1 -the pseudo-first-order constant (L/min); q e and q t -the amounts of nitrate ions adsorbed at equilibrium and at any time, respectively (mgNo 3 /g adsorbent). Figure 11 shows the ln (q e -q t ) vs time graph.
The linear form of the pseudo-second-order equation demonstrates: In Eq. (7), where q e and q t (mg/g) are defined, k 2 (g/mg. min) is the pseudo-second-order constant. Figure 12 shows the graph of According to Table 3, the correlation coefficients for the pseudo-first-order and pseudo-secondorder were, respectively, (0.9755) and (0.816). In conclusion, the adsorption of nitrate ions by Cs-Bn-Zr was best characterized by the pseudo-first-order model. The pseudo-first order q e .cal (18.1 mg/g) was much closer to the experimental value (12.16 mg/g)  Figure 11. Pseudo-first-order kinetic model of nitrate ions than the pseudo-second-order value. The pseudofirst-order kinetic process has been applied for reversible reactions, and an equilibrium between the liquid and solid phases has been achieved. At the same time, the pseudo-second-order kinetic model predicts that mass transfer is not the rate-limiting step but rather the adsorption mechanism.

Regeneration process
The possibility of reuse of adsorbents is a significant consideration in the evaluation of adsorbents performance. To remove the nitrate from the adsorbent, 0.2 g of used (Cs-Bn-Zr) beads were immersed in 0.1 M NaCl solution in 50 ml for 90 minutes and shakingat 150 rpm. Then, filtered and rinsed several times with distilled water. Thereafter, the beads were air-dried at room temperature. The dried beads were subjected to an adsorption, adding 50 mL of a 600 mg/L nitrate solution and spinning the mixture at 150 rrpm for 120 minutes. The remaining nitrate concentration was analyed using a spectrophotometer. From the Figure 13, it can be observed that the  nitrate adsorption capacity of Cs-Bn-Zr has not been significantly affected. After the first cycle of regeneration, the nitrate adsorption capacity of bead was found to be 58.99 mg/g. The second regeneration revealed that the adsorption capacity decreased to 55.92 mg/g. The results of regeneration test demonstrated the economic feasibility of Cs-Bn-Zr adsorbent in removal of nitrate.

Adsorption mechanism
As shown in Figure 14, the proposed mechanism for removing nitrate by Cs-Bn-Zr is driven by electrostatic attraction and an ion exchange mechanism. Electrostatic forces caused Zr(IV), which is attached to amino groups in chitosan, to draw the oxy anions. Since nitrate ions exchange the Cl ions present in the Cs-Bn-Zr composite beads, which can be approved by the EDX spectrum, an ion exchange process was also involved. Table 4 shows that the adsorption capacity produced with Cs-Bn-Zr toward the nitrite was sufficient to conduct the removal process satisfactorily compared to the compression of modified chitosan adsorbent obtained in the prior research.

CONCLUSIONS
In order to expand the surface area and selectivity of the adsorbent, Cs-Bn-Zr composite beads were produced. Zirconium was loaded onto the beads to improve the selectivity for nitrate anions, and bentonite was utilized to expand the surface area. FTIR, FESEM, EDX, and BET were used to analyze the (Cs-Bn-Zr) bead via the appropriate functional groups of beads (such as hydroxyl and amine). At 0.2 g of Cs-Bn-Zr optimal dosage at 2 hours with an initial concentration of 50 mg/l, the highest clearance percentage was 97.28%. The Freundlich isotherm model was  Figure 14. The proposed mechanism of removing nitrate by Cs-Bn-Zr beads compatible with the nitrate adsorption isotherm on the surface of the Cs-Bn-Zr beads, demonstrating the multilayer adsorption. Nitrate ions behaved in pseudo-first order according to kinetic analyses, proving that physical adsorption was the rate-limiting step. Ion exchange, hydrogen bonding, and electrostatic attraction played significant roles in the adsorption mechanism.