Synthesis, Adsorption Isotherm and Kinetic Study of Alkaline- Treated Zeolite/Chitosan/Fe3+ Composites for Nitrate Removal from Aqueous Solution—Anion and Dye Effects

In the present study, alkaline-treated zeolite/chitosan/Fe3+ (ZLCH-Fe) composites were prepared and analyzed using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) and pH of zero point of charge (pHzpc) to remove nitrates from water. The process was carried out using an adsorption method with a varied initial pH, adsorbent dosage, initial nitrate concentration and contact time. The pHzpc demonstrated that the ZLCH-Fe surface had a positive charge between 2 and 10, making it easier to capture the negative charge of nitrate. However, the optimal pH value is 7. After 270 min, the maximum adsorption capacity and percent removal reached 498 mg/g and 99.64%, respectively. Freundlich and pseudo-second-order were fitted to the adsorption isotherm and kinetic models, respectively. An evaluation was conducted on the effects of anions—SO42− and PO43−—and dyes—methylene blue (MB) and acid red 88 (AR88)—upon nitrate removal. The results indicated that the effect of the anion could be inhibited, in contrast to dye effects. However, the optimal pH values were changed to 10 for MB and 2 for AR88, resulting in a hydrogel formation. This might be indicated by the protonation of hydroxyl and amino groups resulting from a chitosan nitrate reaction in the AR88 solution.


Background
Excess nitrate in aquatic ecosystems has seriously endangered human health [1]. High nitrate concentrations in drinking water may cause methemoglobinemia or baby blue syndrome for infants and cancer [2][3][4][5][6]. The primary sources of nitrate are agricultural runoff [7], animal manure [8], leakage from septic tank systems [9], and industrial waste [10]. Due to its poor affinity for soil adsorption and high water solubility, nitrate is classified as the most widespread groundwater fomite in the world, presenting a danger to the safety of drinking water. As a result of the potential health hazards, the nitrate levels in drinking water are rigorously controlled in all nations. In Japan and the USA, the limit is 10 mg/L, but in China, it is 20 mg/L [1,11]. Additionally, nitrate is the primary cause of eutrophication [12]. Consequently, there is an urgent need to develop techniques and materials for removing excessive nitrate from water. Removing nitrate ions from water is one of the world's biggest challenges.

Characterization
Nitrate ions were determined using a nitrate test kit by spectrophotometry (Kyoritsu Chemical-Check Lab., Corp, Japan). The pH zpc process followed previous research [40]. Before and after adsorption, the functional group of the adsorbent was analyzed by ATR-FTIR in an area of 400-4000 cm −1 (Thermo Scientific Nicolet iS10, Thermo Fisher Scientific Inc., Waltham, MA, USA). The photograph was examined using scanning electron microscopy (SEM) (Hitachi TM3000, Tokyo, Japan). Figure 1 the SEM we used to evaluate the morphology of ZLCH-Fe before and after nitrate adsorption. Figure 1a demonstrates the interlayer, adhesive, porous morphologies, and needle-like structure on the surface. However, after adsorption, the surface texture was rough and needle-like disappeared (Figure 1b). This implies that the nitrate molecule has been absorbed by the ZLCH-Fe adsorbent.  (5-1380 min). The adsorption capacity and percent removal were calculated using Equations (1) and (2), respectively.

Characterization
Nitrate ions were determined using a nitrate test kit by spectrophotometry (Kyoritsu Chemical-Check Lab., Corp, Japan). The pHzpc process followed previous research [40]. Before and after adsorption, the functional group of the adsorbent was analyzed by ATR-FTIR in an area of 400-4000 cm −1 (Thermo Scientific Nicolet iS10, Thermo Fisher Scientific Inc., Waltham, MA, USA). The photograph was examined using scanning electron microscopy (SEM) (Hitachi TM3000, Tokyo, Japan). Figure 1 the SEM we used to evaluate the morphology of ZLCH-Fe before and after nitrate adsorption. Figure 1a demonstrates the interlayer, adhesive, porous morphologies, and needle-like structure on the surface. However, after adsorption, the surface texture was rough and needle-like disappeared (Figure 1b). This implies that the nitrate molecule has been absorbed by the ZLCH-Fe adsorbent.  Figure 2 shows the FTIR spectra of ZLCH-Fe before and after nitrate adsorption. Before the adsorption, the peak was at 3359 cm −1 and decreased to 3374 cm −1 after the adsorption process. This is because chitosan contains amine (-NH2) and hydrogen (-OH)  Figure 2 shows the FTIR spectra of ZLCH-Fe before and after nitrate adsorption. Before the adsorption, the peak was at 3359 cm −1 and decreased to 3374 cm −1 after the adsorption process. This is because chitosan contains amine (-NH 2 ) and hydrogen (-OH) groups in ZLCH-Fe and interacts with nitrate [41]. The other peak decreased after adsorption from 1647 to 1644 cm −1 , corresponding to amide carbonyl and hydrogen bonding [42]. A Gels 2022, 8, 782 4 of 15 peak at 1567 cm −1 corresponds to C=C stretching [43]. The increased band peak occurred after nitrate adsorption from 1377 to 1378 cm −1 was assigned to -CH bending to OH > -CHOH [44]. After nitrate adsorption, additional peaks formed at 2165, 2822, and 2878 cm −1 , which are indicated by N-H stretching and S-CN (strong) and correlate to the band at 991 cm −1 (Si-O-Al or Al-O-Fe or Si-O-Fe). groups in ZLCH-Fe and interacts with nitrate [41]. The other peak decreased after adsorption from 1647 to 1644 cm −1 , corresponding to amide carbonyl and hydrogen bonding [42]. A peak at 1567 cm −1 corresponds to C=C stretching [43]. The increased band peak occurred after nitrate adsorption from 1377 to 1378 cm −1 was assigned to -CH bending to OH > -CHOH [44]. After nitrate adsorption, additional peaks formed at 2165, 2822, and 2878 cm −1 , which are indicated by N-H stretching and S-CN (strong) and correlate to the band at 991 cm

The pH Effects and pH of Zero-Point Charge (pHzpc)
The pH values significantly affect the adsorption capacity and percent of adsorbent removal because they are strongly connected to the ionization-induced electronic charges of functional groups on ZLCH-Fe [45]. From 2.5 to 10, the impact of initial pH on nitrate adsorption was investigated ( Figure 3a). The surface charges of ZLCH-Fe were also assessed to explain the results (Figure 3b). The adsorption capacity and percent removal of nitrate increased as pH increased when the initial pH was less than 7 and decreased as pH increased when the initial pH was more than 7. This indicates that hydrogen, carboxyl, and amino groups are protonated at neutral pH (7) in ZLCH-Fe. These findings are similar to those reported by [1,45] for nitrate removal by FeMgMn-LDH and modified corn stalks, respectively.

The pH Effects and pH of Zero-Point Charge (pH zpc )
The pH values significantly affect the adsorption capacity and percent of adsorbent removal because they are strongly connected to the ionization-induced electronic charges of functional groups on ZLCH-Fe [45]. From 2.5 to 10, the impact of initial pH on nitrate adsorption was investigated ( Figure 3a). The surface charges of ZLCH-Fe were also assessed to explain the results (Figure 3b). The adsorption capacity and percent removal of nitrate increased as pH increased when the initial pH was less than 7 and decreased as pH increased when the initial pH was more than 7. This indicates that hydrogen, carboxyl, and amino groups are protonated at neutral pH (7) in ZLCH-Fe. These findings are similar to those reported by [1,45] for nitrate removal by FeMgMn-LDH and modified corn stalks, respectively.

The Effect of Adsorbent Dosage
Adsorbent dosage provides active adsorption sites for nitrate removal. Figure 4 shows that the percentage of nitrate removal increased as the dosage increased from 10 mg/50 mL to 20 mg/50 mL, which may be attributed to the increase in accessible adsorption sites [39,46]. At a high adsorbent dosage, the value of adsorption capacity had the opposite result. This is due to overlapping adsorption sites decreasing the surface area [34,47]. At a low adsorbent dosage, all active sites were completely exposed, and the surface adsorption was rapidly saturated, indicating a high adsorption capability [48]. Based on the adsorption capacity, 10 mg ZLCH-Fe was selected as the optimal dosage.

The Effect of Adsorbent Dosage
Adsorbent dosage provides active adsorption sites for nitrate removal. Figure 4 shows that the percentage of nitrate removal increased as the dosage increased from 10 mg/50 mL to 20 mg/50 mL, which may be attributed to the increase in accessible adsorption sites [39,46]. At a high adsorbent dosage, the value of adsorption capacity had the opposite result. This is due to overlapping adsorption sites decreasing the surface area [34,47]. At a low adsorbent dosage, all active sites were completely exposed, and the surface adsorption was rapidly saturated, indicating a high adsorption capability [48]. Based on the adsorption capacity, 10 mg ZLCH-Fe was selected as the optimal dosage.

The Effect of Initial Concentration
The initial concentration is a key factor in determining the adsorption capacity and removal percentage of ZLCH-Fe. Figure 5 represents the adsorption capacity and percent removal of nitrate in the range of 10 to 100 mg/L initial nitrate concentration. We can see that the initial concentration increased; the adsorption capacity increased from 26.98 to 193.81 mg/g, contrary to the percent removal from 53.97 to 38.76%. This is because an increase in concentration would lead to an increase in the number of molecules, hence increasing the adsorption capacity. However, it would decrease the adsorbate's mass transfer resistance. Consequently, the percent removal is decreased [39,[49][50][51].

The Effect of Adsorbent Dosage
Adsorbent dosage provides active adsorption sites for nitrate removal. Figure 4 shows that the percentage of nitrate removal increased as the dosage increased from 10 mg/50 mL to 20 mg/50 mL, which may be attributed to the increase in accessible adsorption sites [39,46]. At a high adsorbent dosage, the value of adsorption capacity had the opposite result. This is due to overlapping adsorption sites decreasing the surface area [34,47]. At a low adsorbent dosage, all active sites were completely exposed, and the surface adsorption was rapidly saturated, indicating a high adsorption capability [48]. Based on the adsorption capacity, 10 mg ZLCH-Fe was selected as the optimal dosage.

The Effect of Initial Concentration
The initial concentration is a key factor in determining the adsorption capacity and removal percentage of ZLCH-Fe. Figure 5 represents the adsorption capacity and percent removal of nitrate in the range of 10 to 100 mg/L initial nitrate concentration. We can see that the initial concentration increased; the adsorption capacity increased from 26.98 to 193.81 mg/g, contrary to the percent removal from 53.97 to 38.76%. This is because an increase in concentration would lead to an increase in the number of molecules, hence increasing the adsorption capacity. However, it would decrease the adsorbate's mass transfer resistance. Consequently, the percent removal is decreased [39,[49][50][51].

The Effect of Initial Concentration
The initial concentration is a key factor in determining the adsorption capacity and removal percentage of ZLCH-Fe. Figure 5 represents the adsorption capacity and percent removal of nitrate in the range of 10 to 100 mg/L initial nitrate concentration. We can see that the initial concentration increased; the adsorption capacity increased from 26.98 to 193.81 mg/g, contrary to the percent removal from 53.97 to 38.76%. This is because an increase in concentration would lead to an increase in the number of molecules, hence increasing the adsorption capacity. However, it would decrease the adsorbate's mass transfer resistance. Consequently, the percent removal is decreased [39,[49][50][51].

Isotherm Studies
At equilibrium in an adsorption mechanism, the adsorption isotherm adequately describes the distribution of adsorbate molecules between the solid and the liquid phases [52]. The adsorption data were analyzed using the Langmuir and Freundlich adsorption isotherms. Important information may be collected, such as the adsorption mechanism, the favorability of the adsorption process, and the adsorbate-adsorbent affinity. The Langmuir isotherm assumes a surface with uniform binding sites, comparable sorption energies, and no interaction between adsorbed species [39,53]. While the Freundlich isotherm revealed heterogeneous binding sites, this concept may be used to explain multilayer adsorption [54].

Isotherm Studies
At equilibrium in an adsorption mechanism, the adsorption isotherm adequately describes the distribution of adsorbate molecules between the solid and the liquid phases [52]. The adsorption data were analyzed using the Langmuir and Freundlich adsorption isotherms. Important information may be collected, such as the adsorption mechanism, the favorability of the adsorption process, and the adsorbate-adsorbent affinity. The Langmuir isotherm assumes a surface with uniform binding sites, comparable sorption energies, and no interaction between adsorbed species [39,53]. While the Freundlich isotherm revealed heterogeneous binding sites, this concept may be used to explain multilayer adsorption [54].
Linear correlation of the Langmuir in Equation (3).
qe: the amount of the adsorbent (mg/g); Kl: equilibrium constant of adsorption (L/mg); qmax: maximal adsorption capacity; and Ce: equilibrium concentration (mg/L). The essential characteristics of the Langmuir isotherm may be represented in terms of the equilibrium parameter RL, a dimensionless constant also known as the separation factor in the following Equation (4).
ln q e = lnK f + 1 n x lnC e Kf: adsorption capacity (mg/g); 1/n: intensity of adsorption. Ce and qe are the same as in the Langmuir model. Figure 6 and Table 1 illustrate the correlation and linear isotherm plots, respectively. The findings indicate that nitrate adsorption onto ZLCH-Fe was suitable with the Freundlich model (R 2 = 0.9881) and favorable (RL = 0.0011). Linear correlation of the Langmuir in Equation (3).
q e : the amount of the adsorbent (mg/g); K l : equilibrium constant of adsorption (L/mg); q max : maximal adsorption capacity; and C e : equilibrium concentration (mg/L). The essential characteristics of the Langmuir isotherm may be represented in terms of the equilibrium parameter R L , a dimensionless constant also known as the separation factor in the following Equation (4).
C o : initial concentration (mg/L); R L : separation factor, indicating the adsorption is either R L > 1 (unfavorable), R L = 1(linear) or 0 < R L (favorable). Linear correlation of the Freundlich model in Equation (5).
ln q e = lnK f + 1 n × ln C e K f : adsorption capacity (mg/g); 1/n: intensity of adsorption. C e and q e are the same as in the Langmuir model. Figure 6 and Table 1 illustrate the correlation and linear isotherm plots, respectively. The findings indicate that nitrate adsorption onto ZLCH-Fe was suitable with the Freundlich model (R 2 = 0.9881) and favorable (R L = 0.0011).

Kinetic Studies
The kinetics of nitrate adsorption onto ZLCH-Fe were evaluated. Contact time is usually a factor in adsorption transformation processes. The interaction was investigated between 5 and 1380 min with 100 mg/L of nitrate concentration and 10 mg/50 mL at pH 7. As seen in Figure 7, adsorption capacity and percent removal rapidly increase in the first five minutes. Then, the gradual increase reached equilibrium at 270 min, at 498 mg/g and 99.64%, respectively.

Kinetic Studies
The kinetics of nitrate adsorption onto ZLCH-Fe were evaluated. Contact time is usually a factor in adsorption transformation processes. The interaction was investigated between 5 and 1380 min with 100 mg/L of nitrate concentration and 10 mg/50 mL at pH 7. As seen in Figure 7, adsorption capacity and percent removal rapidly increase in the first five minutes. Then, the gradual increase reached equilibrium at 270 min, at 498 mg/g and 99.64%, respectively.

Kinetic Studies
The kinetics of nitrate adsorption onto ZLCH-Fe were evaluated. Contact time is usually a factor in adsorption transformation processes. The interaction was investigated between 5 and 1380 min with 100 mg/L of nitrate concentration and 10 mg/50 mL at pH 7. As seen in Figure 7, adsorption capacity and percent removal rapidly increase in the first five minutes. Then, the gradual increase reached equilibrium at 270 min, at 498 mg/g and 99.64%, respectively.  It is essential to investigate the kinetics of adsorption because it explains the rate at which an adsorbent absorbs an adsorbate [55]. Pseudo-first-order and pseudo-second-order were used to explore the kinetics of adsorption [56,57]. Equations (6) and (7) provide the calculations for the first-order and second-order kinetic models, respectively. log(q e − q t ) = logq e − K 1 t t/q t = 1/(K 2 q 2 e ) + t/q e where k 1 : rate constant of pseudo-first-order kinetic model (min −1 ); t: contact time (minutes). A linear plot of log t against log (q e − q t ) and t against t/q t to determine K 1 and K 2 from the slope of linear plots, respectively. Based on the correlation data (Table 2) and linear kinetic plot data (Figure 8), the R 2 value of the second-order model (0.9974) was identical to that of the first-order one (0.5049). This suggests that nitrate adsorption onto ZLCH-Fe corresponds well to the pseudo-second-order model. Table 2. Pseudo-first-order and Pseudo-second order kinetics models of nitrate adsorption onto ZLCH-Fe.

Pseudo-First Order
Pseudo-Second Order der were used to explore the kinetics of adsorption [56,57]. Equations (6) and (7) provide the calculations for the first-order and second-order kinetic models, respectively. log (q e − q t ) = log q e − K 1 t (6) t/q t = 1/(K 2 q ) + t/q e (7) where k1: rate constant of pseudo-first-order kinetic model (min −1 ); t: contact time (minutes). A linear plot of log t against log (qe − qt) and t against t/qt to determine K1 and K2 from the slope of linear plots, respectively. Based on the correlation data (Table 2) and linear kinetic plot data (Figure 8), the R 2 value of the second-order model (0.9974) was identical to that of the first-order one (0.5049). This suggests that nitrate adsorption onto ZLCH-Fe corresponds well to the pseudo-second-order model. Table 2. Pseudo-first-order and Pseudo-second order kinetics models of nitrate adsorption onto ZLCH-Fe.

Anion Effect Studies
Since there are several competing anions in an aqueous solution, it is essential to explore the influence of the anions. This study investigated the effect of SO4 2− and PO4 3− on nitrate adsorption by ZLCH-Fe, as shown in Figure 9. The process was carried out under optimal nitrate adsorption conditions (20 mg/50 mL (100 mg/L of NO3), pH 7 for 270 min at 30 °C). The adsorption capacity and percent removal in the absence of SO4 2− and PO4 3− were 498 mg/g and 99.64%, respectively. We can see that these anions affected adsorption capacity and percent removal of nitrate at the range of 240-305 mg/g and 48-61%, respectively. Increased initial SO4 2− and PO4 3− concentrations may inhibit nitrate adsorption. This is similar to the results of [1,20,58,59] used FeMgMn-LDH, calcined Mg-Al-Fe, natural zeolite, and Mg/Fe hydrotalcite, respectively.

Anion Effect Studies
Since there are several competing anions in an aqueous solution, it is essential to explore the influence of the anions. This study investigated the effect of SO 4 2− and PO 4 3− on nitrate adsorption by ZLCH-Fe, as shown in Figure 9. The process was carried out under optimal nitrate adsorption conditions (20 mg/50 mL (100 mg/L of NO 3 ), pH 7 for 270 min at 30 • C). The adsorption capacity and percent removal in the absence of SO 4 2− and PO 4 3− were 498 mg/g and 99.64%, respectively. We can see that these anions affected adsorption capacity and percent removal of nitrate at the range of 240-305 mg/g and 48-61%, respectively. Increased initial SO 4 2− and PO 4 3− concentrations may inhibit nitrate adsorption. This is similar to the results of [1,20,58,59] used FeMgMn-LDH, calcined Mg-Al-Fe, natural zeolite, and Mg/Fe hydrotalcite, respectively.

Dye Effect Studies
The influence of dyes (MB and AR88) on nitrate removal is illustrated in Figure 10. The experiment was carried out using 10 mg of ZLCH-Fe, with an initial NO3 of 100 mg/L, and 25 mg/L of dyes at pH intervals of 2, 7, and 10 for 5 min in 50 mL. We can see that the highest dye percent removals at pH 10 for MB and pH 2 for AR88 were 44% and 99%, respectively. This result is similar to [40] and [49] for MB and AR88 removal, respectively. Interestingly, fact that when the solution was mixed with azo dye (AR88) at pH 2 ( Figure  11a), the solution turned into hydrogels, followed by the maximum nitrate removal of 86.65%. This is due to chitosan solubility in acidic conditions [60]. AR88 is deprotonated, and more interactions are generated between Fe 3+ and dyes. These results agree with [61] using nanoporous silica hydrogel by cross-linking SiO2-H3BO3-hexadecyltrimethoxysilane for azo dye removal. However, no hydrogels appeared to react with cationic dye (MB) in all pH ranges, as shown in Figure 11b.
Thus, we increased the initial AR88 concentration up to 100 mg/L under the same conditions reported above. The results showed that an increased initial AR88 concentration would increase nitrate removal, followed by an increase in the swelling ratio, and there was no significant change in AR88 removal ( Figure 12). The swelling ratio is followed in Equation (8) [49]. This indicates that azo dye group molecules are essential in capturing nitrate through hydrogel via ZLCH-Fe adsorption.
where Ww: weight of swollen (before dry oven) (g); Wd: weight of swollen (after dry oven) (g). For the following steps, we dried hydrogel for 4 days at 60 °C. The dried hydrogel was mixed for 60 min at 30 °C with 0.1 M NaOH. The results showed that an increased initial AR88 concentration would increase absorbance ( Figure 13). As compared, the absorbance peak decreased from 503 to 478 nm for AR88 original (AR88-Ori) and dried hydrogel (DH-ZLCH-Fe), respectively. Then, initial AR88 dye was 100 mg/L for further experiment.

Dye Effect Studies
The influence of dyes (MB and AR88) on nitrate removal is illustrated in Figure 10. The experiment was carried out using 10 mg of ZLCH-Fe, with an initial NO 3 of 100 mg/L, and 25 mg/L of dyes at pH intervals of 2, 7, and 10 for 5 min in 50 mL. We can see that the highest dye percent removals at pH 10 for MB and pH 2 for AR88 were 44% and 99%, respectively. This result is similar to [40] and [49] for MB and AR88 removal, respectively. Interestingly, fact that when the solution was mixed with azo dye (AR88) at pH 2 (Figure 11a), the solution turned into hydrogels, followed by the maximum nitrate removal of 86.65%. This is due to chitosan solubility in acidic conditions [60]. AR88 is deprotonated, and more interactions are generated between Fe 3+ and dyes. These results agree with [61] using nanoporous silica hydrogel by cross-linking SiO 2 -H 3 BO 3 -hexadecyltrimethoxysilane for azo dye removal. However, no hydrogels appeared to react with cationic dye (MB) in all pH ranges, as shown in Figure 11b.      Thus, we increased the initial AR88 concentration up to 100 mg/L under the same conditions reported above. The results showed that an increased initial AR88 concentration would increase nitrate removal, followed by an increase in the swelling ratio, and there was no significant change in AR88 removal ( Figure 12). The swelling ratio is followed in Equation (8) [49]. This indicates that azo dye group molecules are essential in capturing nitrate through hydrogel via ZLCH-Fe adsorption.
where W w : weight of swollen (before dry oven) (g); W d : weight of swollen (after dry oven) (g).    For the following steps, we dried hydrogel for 4 days at 60 • C. The dried hydrogel was mixed for 60 min at 30 • C with 0.1 M NaOH. The results showed that an increased initial AR88 concentration would increase absorbance ( Figure 13). As compared, the absorbance peak decreased from 503 to 478 nm for AR88 original (AR88-Ori) and dried hydrogel (DH-ZLCH-Fe), respectively. Then, initial AR88 dye was 100 mg/L for further experiment.  Figure 14 shows the FTIR spectra of DH-ZLCH-Fe and AR88-Ori. Overall, the peak`s area is almost similar. However, the peak spectra were changed. For example, the decreased peak of AR88-Ori to DH-ZLCH-Fe from 3633 to 3446 cm −1 and 3029 to 2928 cm −1 , ascribed to -OH and C-H stretching, respectively [62]. The peak at 1780 cm −1 appeared in DH-ZLCH-Fe, corresponding to C=O stretching vibrations in COO-or COOCH3 [63]. This indicates that the carboxyl group of ZLCH-Fe reacted with azo dye (AR88). The appearance of the peak between 1650 and 1580 cm −1 is attributed to N-H bending. 1400 to 1600 cm −1 attributed to aromatic ring C=C stretching. The presence of peaks between 1251 and 1342 cm −1 may be attributed to S=O stretching. From 939 to 1195 cm −1 is corresponded to C-O-C stretching with or -CH-OH groups [64][65][66][67]. -CH2-and Si-O-Fe or Si-O-Al or Al-O-Fe groups may be associated with a peak between 681 and 983 cm −1 [63].   Figure 14 shows the FTIR spectra of DH-ZLCH-Fe and AR88-Ori. Overall, the peak's area is almost similar. However, the peak spectra were changed. For example, the decreased peak of AR88-Ori to DH-ZLCH-Fe from 3633 to 3446 cm −1 and 3029 to 2928 cm −1 , ascribed to -OH and C-H stretching, respectively [62]. The peak at 1780 cm −1 appeared in DH-ZLCH-Fe, corresponding to C=O stretching vibrations in COO-or COOCH 3 [63]. This indicates that the carboxyl group of ZLCH-Fe reacted with azo dye (AR88). The appearance of the peak between 1650 and 1580 cm −1 is attributed to N-H bending. 1400 to 1600 cm −1 attributed to aromatic ring C=C stretching. The presence of peaks between 1251 and 1342 cm −1 may be attributed to S=O stretching. From 939 to 1195 cm −1 is corresponded to C-O-C stretching with or -CH-OH groups [64][65][66][67]. -CH 2 -and Si-O-Fe or Si-O-Al or Al-O-Fe groups may be associated with a peak between 681 and 983 cm −1 [63].  Figure 14 shows the FTIR spectra of DH-ZLCH-Fe and AR88-Ori. Overall, the peak`s area is almost similar. However, the peak spectra were changed. For example, the decreased peak of AR88-Ori to DH-ZLCH-Fe from 3633 to 3446 cm −1 and 3029 to 2928 cm −1 , ascribed to -OH and C-H stretching, respectively [62]. The peak at 1780 cm −1 appeared in DH-ZLCH-Fe, corresponding to C=O stretching vibrations in COO-or COOCH3 [63]. This indicates that the carboxyl group of ZLCH-Fe reacted with azo dye (AR88). The appearance of the peak between 1650 and 1580 cm −1 is attributed to N-H bending. 1400 to 1600 cm −1 attributed to aromatic ring C=C stretching. The presence of peaks between 1251 and 1342 cm −1 may be attributed to S=O stretching. From 939 to 1195 cm −1 is corresponded to C-O-C stretching with or -CH-OH groups [64][65][66][67]. -CH2-and Si-O-Fe or Si-O-Al or Al-O-Fe groups may be associated with a peak between 681 and 983 cm −1 [63].   Table 3 shows the nitrate adsorption capacity of different adsorbents. We can see that ZLCH-Fe showed better adsorption than others. This indicates that ZLCH-Fe is a potential adsorbent to remove nitrate from water at either low or high concentrations.

Conclusions
This study examined the adsorption process of ZLCH-Fe as a promising adsorbent for nitrate removal from water. Experimental parameters such as pH, adsorbent dosage, initial nitrate concentration, and contact time were essential and investigated to determine the mechanism of nitrate adsorption. The results demonstrated that pH 7 is optimal. Increasing adsorbent dosage decreases adsorption capacity, while increasing initial nitrate concentration increases adsorption capacity. At 270 min, the adsorption capacity and percent removal were 498 mg/g and 99.64%, respectively. The isotherm and kinetic adsorption were compatible with the Freundlich and second-order models, respectively. The effects of anions (SO 4 2− and PO 4 3− ) and dyes (MB and AR88) were investigated. The results showed that the effect of anions might inhibit nitrate removal, in contrast with dye effects. When ZLCH-Fe interacted with AR88 at an initial pH of 2, hydrogels formed in the solution. In addition, the absorbance peak was decreased from 503 nm to 478 nm compared to the original AR88. Therefore, ZLCH-Fe might be utilized satisfactorily to remove nitrate from water.