Application of Box–Behnken Design to Optimize Phosphate Adsorption Conditions from Water onto Novel Adsorbent CS-ZL/ZrO/Fe₃O₄: Characterization, Equilibrium, Isotherm, Kinetic, and Desorption Studies

Abstract

Phosphate (PO₄³⁻) is an essential nutrient in agriculture; however, it is hazardous to the environment if discharged in excess as in wastewater discharge and runoff from agriculture. Moreover, the stability of chitosan under acidic conditions remains a concern. To address these problems, CS-ZL/ZrO/Fe₃O₄ was synthesized using a crosslinking method as a novel adsorbent for the removal of phosphate (PO₄³⁻) from water and to increase the stability of chitosan. The response surface methodology (RSM) with a Box–Behnken design (BBD)-based analysis of variance (ANOVA) was implemented. The ANOVA results clearly showed that the adsorption of PO₄³⁻ onto CS-ZL/ZrO/Fe₃O₄ was significant (p ≤ 0.05), with good mechanical stability. pH, dosage, and time were the three most important factors for the removal of PO₄³⁻. Freundlich isotherm and pseudo-second-order kinetic models generated the best equivalents for PO₄³⁻ adsorption. The presence of coexisting ions for PO₄³⁻ removal was also studied. The results indicated no significant effect on PO₄³⁻ removal (p ≤ 0.05). After adsorption, PO₄³⁻ was easily released by 1 M NaOH, reaching 95.77% and exhibiting a good capability over three cycles. Thus, this concept is effective for increasing the stability of chitosan and is an alternative adsorbent for the removal of PO₄³⁻ from water.

1. Introduction

Phosphate (PO₄³⁻) is a macronutrient needed for plant growth and is frequently applied as a fertilizer on agricultural lands. The increasing demands of food supply nowadays have led to the excessive application of fertilizer. However, excessive fertilizer use can cause PO₄³⁻ to leach into waterways, leading to eutrophication and harmful algal bloom. These blooms diminish oxygen levels [1,2,3], interfere with aquatic life, and adversely affect the quality of drinking water (taste and odor) [4]. According to [5], PO₄³⁻ decontamination must be performed efficiently while having a minimal impact on the surrounding ecosystem. Many methods have been reported to be effective in removing PO₄³⁻ from water, including biological [6] methods, electrochemical [7,8] methods, precipitation [9], ion exchange [10], and adsorption [11,12]. Each strategy has advantages and disadvantages. Biological techniques are more economical; however, the residue of dead bacteria left behind after the process is inconvenient [13]. Electrochemical techniques are expensive but have a lower effectivity toward PO₄³⁻ removal [14]. The precipitation process is simple and effective for chemical treatment but is inefficient for sewage sludge and waste disposal [15]. Ion exchange may also be used to remove anions by exchanging sulfates (SO₄²⁻) for PO₄³⁻ ions; however, this would make the solution more corrosive, and it requires a costly clean-up (Blaney et al. [16]). Adsorption is the best option and is the most widely used method for water contaminants including PO₄³⁻ ions [17,18]. This is because the technique is environmentally safe, the operation is easy and fast, and the technology is highly efficient.

Chitosan is currently gaining popularity as a potential adsorbent for water contaminants because it contains hydroxyl (–OH) and amino (–NH₂) functional groups, which can easily react with other materials and are environmentally friendly [19]. This material, which cannot be accessed readily from nature, is synthesized through the chemical deacetylation of chitin. However, because of its low tensile strength and dissolution under acidic conditions, the use of chitosan directly in wastewater treatment technologies is not recommended. Therefore, chitosan must be modified to increase its chemical stability and adsorption capability [20]. The selection of an appropriate modification method and modifying agent is crucial for assessing the quality and functionality of the product created during the modification process. Crosslinking is one of the most frequently used procedures to enhance the physicochemical characteristics of chitosan [21,22]. Crosslinking is the process of combining two or more molecules via covalent bonds.

Zeolites are crystalline aluminum silicate (Al₂O₃·2SiO₂) minerals with a porous and highly stable structure, and they could enhance the adsorption of chitosan onto their surface, leading to the improved stability of chitosan. These materials can be obtained from natural sources, such as shrimp, or can be synthesized using various methods [23]. Several reports have proven the use of chitosan and zeolite to remove dyes [24,25], pharmaceuticals [26], nitrate [27], and humic acid [28]. On the other hand, the fabrication of chitosan–metal oxides has attracted the attention of a lot of scientists owing to their numerous beneficial characteristics, such as chemical stability, a large surface area, and favorable adsorptive characteristics [29]. Magnesium oxide (MgO) [30], titanium oxide (TiO) [31], zinc oxide (ZnO) [32,33], zirconium oxide (ZrO) [34], and copper oxide (CuO) [35] are examples of metal oxides. ZrO was selected for this study owing to its strong affinity for anions [36].

The separation of the adsorbents is another issue of concern since the usual separation procedures result in the loss of the adsorbents as well as possible dangers to the environment [37,38]. Magnetite (Fe₃O₄) is one of the most magnetic particles that can be used in the manufacture of magnetic adsorbents for water purification because of its biodegradability, thermal stability, and large surface area [39,40]. The use of the crosslinking method to combine magnetite, zeolite, ZrO, and chitosan is a viable strategy. This is because the magnetic particles allow for easy separation when subjected to an external magnetic field, while the chitosan, zeolite, and ZrO provide many adsorption sites [41]. Therefore, the amalgamation of chitosan/zeolite/ZrO, and Fe₃O₄ (CS-ZL/ZrO/Fe₃O₄) may result in the development of novel composite materials with multifunctional constituents.

This study synthesized CS-ZL/ZrO/Fe₃O₄ with the target of using it as a novel adsorbent for PO₄³⁻ removal from water. The response surface methodology (RSM) with the Box–Behnken design (BBD) optimization strategy was used to acquire insight into the effect of process factors such as pH, adsorbent dosage, temperature, and time to achieve the maximal adsorptive removal of PO₄³⁻. This process was performed to obtain the highest PO₄³⁻ adsorptive removal. The adsorption isotherms and kinetic models were also calculated to figure out the adsorption mechanism.

2. Results and Discussion

2.1. Characterization of CS-ZL/ZrO/Fe₃O₄

The experimental results of BBD are listed in

Table 1. Experimental data results from 4 factors of BBD for PO₄³⁻ removal onto CS-ZL/ZrO/Fe₃O₄.

Run	pH	Dosage	Temperature	Time	% Removal
1	2	0.02	45	35	58.95
2	10	0.02	45	35	51.75
3	2	0.10	45	35	72.77
4	10	0.10	45	35	54.54
5	7	0.06	30	10	61.83
6	7	0.06	60	10	56.68
7	7	0.06	30	60	64.95
8	7	0.06	60	60	64.94
9	2	0.06	45	10	59.09
10	10	0.06	45	10	54.34
11	2	0.06	45	60	72.75
12	10	0.06	45	60	59.57
13	7	0.02	30	35	56.02
14	7	0.10	30	35	61.46
15	7	0.02	60	35	56.69
16	7	0.10	60	35	59.69
17	2	0.06	30	35	66.84
18	10	0.06	30	35	60.16
19	2	0.06	60	35	64.70
20	10	0.06	60	35	55.08
21	7	0.02	45	10	54.40
22	7	0.10	45	10	58.87
23	7	0.02	45	60	56.72
24	7	0.10	45	60	69.58
25	7	0.06	45	35	53.48
26	7	0.06	45	35	55.47
27	7	0.06	45	35	53.47

. It can be concluded that a pH of 2 offers the best conditions for PO₄³⁻ removal. The pH_ZPC findings revealed that, at a pH of 2, the surface of CS-ZL/ZrO/Fe₃O₄ had a positive charge (pH < pH_zpc) (Figure 1a). This might indicate the protonation of the -NH₂ groups to -NH₃⁺ groups on the surface. These attract negatively charged H₂PO₄⁻ ions to CS-ZL/ZrO/Fe₃O₄, resulting in the construction of a surface complex between PO₄³⁻ ions and CS-ZL/ZrO/Fe₃O₄. This study was similar to that reported by [42,43], which used SCBC-La and leftover coal, respectively, for PO₄³⁻ removal under acidic conditions. The other possible reaction that could occur is shown in Equation (1).

Fe₃O₄ + 4Zr(OH)₄ + 6H₂PO₄− → FeZr(PO₄)₃ + 12H₂O

(1)

Table 2. Physical properties of the adsorbent.

Specific Surface Area	Value
BET-specific surface area (m2/g)	88.1
Pore volume (mL/g)	0.572
Average diameter (µm)	43.9
Porosity (%)	59

summarizes the physical characteristics of these adsorbents. The results show that the BET-specific surface area was 88.1 m²/g, with a pore volume of 0.572 mL/g, an average diameter of 43.9 µm, and a porosity of 59%. These parameters show that the adsorbent had a substantial surface area for the adsorption of PO₄³⁻ ions.

Figure 1b shows the XRD data used to verify the crystalline structure of the composite material. The XRD pattern shows a huge hump around 2θ = 21.22, which is a chitosan-specific peak [20]. Furthermore, the sharp peaks at 30.11, 35.49, 43.21 are mostly composed of crystalline phases, such as quartz, hematite, and alumina, which are all formed from zeolite- and zirconium-based materials. Magnetite corresponds to the peaks at 53.52, 57.08, and 62.78 [44]. Figure 1c shows a photograph of CS-ZL/ZrO/Fe₃O₄. It can be seen that the adsorbent is attached to the external magnet.

The SEM-EDS characterization of CS-ZL/ZrO/Fe₃O₄ was carried out to explore the surface properties and chemical components of the material. Figure 2 and

Table 3. EDS data before and after PO₄³⁻ adsorption.

Element	Weight %		Atomic %
	Before	After	Before	After
N	3.27	2.06	13.43	7.66
Al	0.78	0.93	1.65	1.79
Si	6.35	1.70	12.98	3.15
Fe	38.92	56.39	40.03	52.69
Zr	50.68	27.76	31.91	15.88
P		11.17		18.82

compare the SEM images and EDS data before and after PO₄³⁻ adsorption. Before adsorption, the surface morphology of the adsorbent was sticky, rough, and porous. The surface became smooth and compact after PO₄³⁻ adsorption, and this indicates that PO₄³⁻ ions were trapped on the adsorbent surface. The primary objective of the EDS data analysis was to identify the components of the adsorbent materials. The weight percentages of Zr and Fe were the highest at 50.68 and 38.92%, respectively. The N value was derived from the chitosan materials [45,46,47]. Al, Si, and Fe were derived from zeolite and magnetite, respectively. Furthermore, the presence of P after the adsorption process indicates that PO₄³⁻ was successfully adsorbed.

Figure 3 shows the functional groups in CS-ZL/ZrO/Fe₃O₄ before and after PO₄³⁻ adsorption through an FTIR-ATR analysis. A CS-ZL/ZrO/Fe₃O₄ band was detected following PO₄³⁻ adsorption from 3326 cm⁻¹ to 3320 cm⁻¹. This shows that PO₄³⁻ ions interact with the stretching vibrations of hydrogen and amine in chitosan [48]. After PO₄³⁻ adsorption, a decrease in the peak from 1634 to 1627 cm⁻¹ was observed, which is associated with carboxyl groups (–COOCH₃) [49]. An increased peak and a more curved and newer peak appeared after PO₄³⁻ adsorption from 951 to 1006 cm⁻¹ and at 2161 cm⁻¹, which were assigned to Si-O-Al, Fe-O-Si, or Zr-O-Fe and CN stretching, respectively. This indicated a strong interaction with PO₄³⁻ ions.

2.2. Mechanical Stability

The mechanical stability of the CS-ZL/ZrO/Fe₃O₄ composite was determined through the percentage of the initial mass that was preserved after drying. Figure 4a shows that increasing the concentration of the solution led to a higher WR%. Compared to the HCl-containing solution, the H₂SO₄-containing solution exhibited a higher WR%. Consequently, the crystalline structure of CS-ZL/ZrO/Fe₃O₄ was deformed, indicating that H₂SO₄ had significant contact with the chitosan group. Figure 4b shows the IR spectra after treatment. The positions of the peaks were consistent for all the samples. According to [50], the broad band visible at 3176–3345 cm⁻¹ is assigned to the -NH₂ groups changing to –NH₃⁺ groups. The peaks between 1611 and 1630 cm⁻¹, which were ascribed to the carboxyl (–COOCH₃) and –NH₂ groups, were generated through H⁺ generation by HCl and H₂SO₄. The peak shifted to 1068 cm⁻¹, and expansion occurred when treated with 0.1 M H₂SO₄. SO₄²⁻ ions have been shown to be associated with Si, Al, Fe, and Zr [51]. According to these results, the physical and chemical characteristics of the CS-ZL/ZrO/Fe₃O₄ composites did not change significantly.

2.3. ANOVA and Equations for Fitting Empirical Models

Table 4. ANOVA results for PO₄³⁻ removal.

Source	DF	Sum of Squares	Mean of Squares	F-Value	p-Value	Remarks
Model	14	839.751	59.982	16.68	<0.0001 *	Significant
A	1	296.610	296.610	51.8093	<0.0001 *
B	1	149.672	149.672	12.5582	<0.0001 *
C	1	15.143	15.143	1.2835	0.063
D	1	156.241	156.241	0.6342	<0.0001 *
A2	1	74.285	74.285	12.8151	0.001 *
B2	1	9.642	9.642	1.4722	0.127
C2	1	67.008	67.008	7.1279	0.001 *
D2	1	90.952	90.952	1.9374	<0.0001 *
A × B	1	30.415	30.415	4.2734	0.013 *
A × C	1	2.161	2.161	0.3036	0.453
A × D	1	17.766	17.766	8.2118	0.046 *
B × C	1	1.488	1.488	0.2091	0.532
B × D	1	17.598	17.598	0.0540	0.047 *
C × D	1	6.605	6.605	1.84	0.200
Error	12	43.157	3.596
Lack-of-Fit	10	40.504	4.050	3.05	0.272
Pure Error	2	2.653	1.3267
Total	26	882.908
R2		95.11
R2 adj		89.41

* Significant.

shows the results of the statistical analysis, using the ANOVA test to evaluate the relationship between the input effective variables (A, B, C, and D) and their responses (Y). Table 4 shows that the F-value of the quadratic model was 16.68 and that the p-value was < 0.0001, indicating that this model was significant. Models A, B, D, A², C², D², A × B, A × D, and C × D, marked with an asterisk (*), were found to be significant parameters of the model. The statistical variables obtained from the ANOVA test (Equation (2)) provide a full quadratic regression model for PO₄³⁻ removal (%).

PO₄³− removal (%) = 99.2 − 1.72 A + 63 B − 1.478 C − 0.472 D + 0.2333 A² + 840 B² + 0.01575 C² + 0.00661 D² − 17.23 A*B − 0.0123A*C − 0.02107 A*D − 1.02 B*C + 2.098 B*D + 0.00343 C*D

(2)

The coefficients in the equation with positive and negative signs describe the additive and multiplicative effects of the variables on the response. The “Lack of Fit” was determined by comparing the residual error to the pure error in close proximity to the repeatedly designed points. F = 3.05 and p = 0.272 for the “Lack of Fit” revealed that it was not significant for the model (p < 0.05). It can be assumed that the model was adequately fitted and that there was no lack of fit.

The R² value of the calculated second-order response model was 95.11, which is also known as the coefficient of determination. Consequently, it can be applied to reliably calculate the response at any given parameter level regardless of their values. In addition, a regression model was used to calculate the standardized influence of the independent factors on PO₄³⁻ removal. A response surface plot was generated to investigate the influence of two components at initial the PO₄³⁻ concentration of 20 mg/L (Figure 5). This plot was used to determine the standardized effects of all the independent variables. A surface plot is an easier way to display the response behavior that occurs when two parameters are simultaneously altered at the same time. It is more beneficial to select the quantity of various ingredients to obtain the desired response. Figure 5a displays a Pareto chart that compares the relative magnitude and the corresponding main, square, and interaction effects of the selected variables. The square effects of all four factors were found to be very significant (p ≤ 0.05) in addition to the main effects of the factors, which were also found to be highly significant (p ≤ 0.05). The ANOVA results reported in Table 4 led to the same conclusions. PO₄³⁻ removal continuously increased in response to the pH, adsorbent dosage, and time. Figure 5b,c show that pH is a key factor in the removal of PO₄³⁻, and Figure 5d shows that increasing the contact time increases the percentage removal.

2.4. Initial Concentration and Isotherm Studies

The effects of the initial PO₄³⁻ concentrations ranging from 20 to 500 mg/L, 0.06 g of adsorbent (CS-ZL/ZrO/Fe₃O₄), and pH (2.0) were investigated. Figure 6 shows that the PO₄³⁻ adsorption capacity rose from 30.64 to 682.31 mg/g; however, the percentage of PO₄³⁻ removal decreased from 91.91% to 81.88%. The adsorption capacity increased with the concentration because the total number of molecules increased. Consequently, the mass transfer resistance of adsorbate decreased. As a result, the percentage of removal decreased [52].

Adsorption isotherms are necessary to assess the capabilities of an adsorbent and the interactions between an adsorbate (a solution containing PO₄³⁻ ions) and an adsorbent (CS-ZL/ZrO/Fe₃O₄). The acquired isotherm parameters can be used to conduct an accurate analysis while constructing an effective adsorption system. Both the equilibrium concentration and the adsorption capacity were estimated. The Langmuir model describes the monolayer adsorption processes at the designated homogenous surfaces on the adsorbent (Equation (3)). The essential property of the Langmuir model can be described as a dimensionless constant also known as the separation factor (R_L), which is shown in Equation (4). By contrast, the Freundlich model is based on heterogeneous surfaces and multilayer sorption (Equation (5)). This is a linear equation, which is shown as follows:

$${\mathrm{C}}_{\mathrm{e}}{/\mathrm{q}}_{\mathrm{e}}=(\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\max }})+1/\left({\mathrm{K}}_1{\mathrm{q}}_{\max }\right)$$

(3)

$${\mathrm{R}}_{\mathrm{L}}=(\frac{1}{{1+\mathrm{bC}}_{\mathrm{o}}})$$

(4)

$${\mathrm{Ln} \mathrm{q}=\mathrm{lnK}}_{\mathrm{f}}+\frac{1}{\mathrm{n}}\mathrm{x} {\mathrm{lnC}}_{\mathrm{e}}$$

(5)

q_e (mg/g) is the adsorption capacity; K₁ (L/mg) is the equilibrium constant of adsorption; q_max (mg/g) is the maximal adsorption capacity; C_e (mg/L) is the equilibrium concentration; C_o (mg/L) is the initial concentration; R_L is the separation factor; 0 < R_L is favorable; R_L > 1 is unfavorable; R_L = 1 is linear; and K_f (mg/g) and 1/n are the adsorption capacity and the intensity of adsorption, respectively.

Figure 7 shows the isotherm model curves, and

Table 5. Isotherm model parameters for PO₄³⁻ removal onto CS-ZL/ZrO/Fe₃O₄.

Isotherms	Parameters	Value
Langmuir	qmax	1259.79
	KL	14.27
	R2	0.7409
	RL	0.0007
Freundlich	Kf	1135.07
	1/n	0.7555
	R2	0.9970

shows the fitting results corresponding to these curves. The Freundlich model’s linear correlation coefficient (R²) was 0.9970, indicating that it provided the best fit compared to the other models. More importantly, the Langmuir and Freundlich parameters, known as R_L and 1/n, indicate that the PO₄³⁻ ion has a type of < 1. According to these data, the PO₄³⁻ adsorption method is favorable.

2.5. Adsorption Kinetic Studies

This study investigated the influence of the contact time (35–2880 min) on PO₄³⁻ adsorption at 30 °C. Figure 8 shows that the percentage of PO₄³⁻ removal and the capacity for adsorption increased rapidly from 35 to 60 min and then gradually increased up to 90 min. This is because the adsorbent includes carboxyl, amine, hydrogen, and magnetite groups, all of which cause the adsorbent surface to become active and trap PO₄³⁻ ions. Subsequently, the adsorption capacity decreased and increased, resulting in fast/slow PO₄³⁻ adsorption, and it finally reached equilibrium at 1440 min, with an adsorption capacity and percent removal of 732.56 mg/g and 87.91%, respectively.

Adsorption kinetic studies are important because they deliver information on the adsorption mechanism, which is necessary to assess the effectiveness of the process [53]. Two kinetic models were used in this study: pseudo-first-order (PFO) (Equation (6)) and pseudo-second-order (PSO) (Equation (7)) models were investigated. The linear form can be obtained by calculating the following equation.

$$\mathrm{Log}\left({\mathrm{q}}_{\mathrm{e}}-{ \mathrm{q}}_{\mathrm{t}}\right)=\log {\mathrm{q}}_{\mathrm{e}}-{\mathrm{K}}_1\mathrm{t}$$

(6)

$$\mathrm{t}/{\mathrm{q}}_{\mathrm{t}}=1/\left({\mathrm{K}}_2{{\mathrm{q}}_{\mathrm{e}}}^2\right)+\mathrm{t}/{\mathrm{q}}_{\mathrm{e}}$$

(7)

where k₁ (min⁻¹) is the rate constant of the PFO model, t (min) is the time, and a linear plot of log t against log (q_e − q_t) and t against t/q_t was used to determine K₁ and K₂ from the slope of the linear plots, respectively.

Figure 9 shows the fitting curves for the kinetic models, and

Table 6. Kinetic model parameters for PO₄³⁻ removal onto CS-ZL/ZrO/Fe₃O₄.

Kinetics	Parameters	Value
PFO	qe	2.5165
	K1	1.42857 × 10−6
	R2	1.00 × 10−4
PSO	qe	510,204.1
	K2	0.000119
	R2	0.9979

lists the fitting results corresponding to those curves. The findings confirm that the PSO model provided better results than the PFO model in terms of the linear correlation coefficient R² value (0.9979). These findings imply that chemical processes control the adsorption rate.

2.6. Effect of Anions and Cations on PO₄³⁻ Removal onto CS-ZL/ZrO/Fe₃O₄

Wastewater contains various substances, including anions and cations, which can affect the adsorption process [54]; it is essential to investigate the effect of ionic strength in an aqueous solution. This is because wastewater is made up of numerous components that might be found together. Figure 10 depicts the effect of different anions and cations on the PO₄³⁻ adsorption capacity of CS-ZL/ZrO/Fe₃O₄. The experimental data indicate that there was no significant influence on PO₄³⁻ removal. It revealed that the fabrication of CS-ZL/ZrO/Fe₃O₄ was effective in eliminating PO₄³⁻ from water.

2.7. Desorption Studies

Figure 11a shows the desorption percentage of PO₄³⁻ at different NaOH concentrations from 0.01 M to 1 M for 30 min at 30 °C. The results indicate that increasing the concentration increased the desorption percentage to 95.77%. Then, subsequent experiment at different contact times, from 30 to 150 min, using 1 M NaOH (Figure 11b). The desorption percentage increased and then decreased up to 150 min, which is similar to the results of the adsorption studies. The highest desorption percentage was observed after 30 min. The desorption mechanism may cause the hydroxide ions (OH-) in the sodium hydroxide solution to react with the CS-ZL/ZrO/Fe₃O₄-P surface and replace the PO₄³⁻ groups, resulting in the release of PO₄³⁻ into the liquid solution (Equation (8)). The reusability studies of PO₄³⁻ adsorption onto CS-ZL/ZrO/Fe₃O₄ showed good performance for three cycles (Figure 11c).

H₂PO₄⁻ + OH⁻ → HPO₄²⁻ + H₂O

(8)

2.8. Adsorption Performance Comparison

Table 7. List comparing PO₄³⁻ adsorption amounts.

Adsorbent	pH	qe (mg/g)	References
Magnetic iron oxide nanoparticles	11	5.03	[1]
Fe-HNT	4	5.46	[18]
Halloysite	4	3.56	[18]
20MMSB	4	121.25	[55]
Amine-functionalized nano magnetic Fe3O4 polymer	3.0	102.04	[56]
MFB-MCs	3.0	487.99	[57]
Fe3O4@SiO2 core/shell magnetic nanoparticles	-	27.8	[58]
AgNPs-TAC	3	13.62	[59]
Ce0.8Zr0.2O2	6.2	112.23	[60]
Zr/Al-Mt	5.0	17.2	[61]
PZC 7.3	11	2.41	[62]
Zeolite	11	0.69	[62]
Biochar	11	3.60	[62]
CS-ZL/ZrO/Fe3O4	2	732.56	Present study

compares the equilibrium and maximum adsorption capacity of CS-ZL/ZrO/Fe₃O₄ with those of various adsorbents. It can be seen that the pH is one of the main factors for PO₄³⁻ removal onto the adsorbent, and the surface charge can become either positive or negative over a wide pH range, which influences the interaction between the adsorbent and PO₄³⁻ ions. It is clear that the CS-ZL/ZrO/Fe₃O₄ adsorbent has a higher capacity than the other adsorbents. It is feasible to conclude that these adsorbents are viable alternatives for removing PO₄³⁻ from water.

3. Materials and Methods

3.1. Materials

Chitosan (CH) (C₆H₁₁NO₄) with molecular weight of 100,000–300,000 Da was bought from Acros Organics, Belgium. Zeolite (ZL) (Al₂O₃·2SiO₂) was obtained from Tosoh Co. Ltd., Japan. Sodium hydroxide (NaOH), acetic acid (CH₃COOH), disodium hydrogen phosphate (Na₂HPO₄), ferric chloride (FeCl₃), ferrous sulfate (Fe₂SO₄), ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O)), antimony potassium tartrate (K₂Sb₂(C₄H₂O₆)₂), ascorbic acid (C₆H₈O₆), hydrochloric acid (HCl), and sulfuric acid (H₂SO₄) were bought from Kanto Chemical Co., Inc., Tokyo, Japan. ZrClO was purchased from Fujifilm Wako Chemical, Tokyo, Japan.

3.2. Synthesis of CS-ZL/ZrO/Fe₃O₄

CS-ZL/ZrO/Fe₃O₄ was synthesized through crosslinking method; chitosan (1 g) was dissolved in 100 mL of acetic acid (1%), and the resulting viscous solution was maintained at ambient temperature (25–30 °C) with magnetic stirring for 24 h (Equation (9)). Subsequently, 25 mL of the resulting chitosan solution was mixed with 0.5 g of zeolite and 20 mL of 1 M FeCl₃ + 0.5 M Fe₂SO₄ + 0.5 M ZrClO. The mixture solution was then heated to 60 °C and was stirred for 1 h. The pH of the solution was adjusted to 10 using 3 M NaOH over 24 h with magnetic stirring at ambient temperature (25–30 °C), and the solution was filtered and washed multiple times with acetone and distilled water (DW) to remove any remaining NaOH. Subsequently, the materials were dried for 48 h in an oven at 60 °C (Equation (13)). The adsorbents are referred to as CS-ZL/ZrO/Fe₃O₄.

(CH₃COOH)n + (C₆H₁₁NO₄)m → (CH₃COO⁻)n(C₆H₁₁NO₄H⁺)m

(9)

(C₆H₁₁NO₄H⁺)n + Al₂O₃·2SiO₂ → (C₆H₁₁NO₄ − Al₂O₃·2SiO₂)n + 2H₂O

(10)

5FeCl₃ + 15Fe₂(SO₄)₃ + 12NaOH → 5Fe₃O₄ + 15Na₂SO₄ + 6H₂O + 36NaCl

(11)

FeCl₃ + 3Fe₂(SO₄)₃ + ZrClO + 14NaOH → 5Fe₃O₄ + Zr(OH)₄ + 2Na₂SO₄ + 6NaCl + 7H₂O

(12)

2(CH₃COO⁻)n(C₆H₁₁NO₄H⁺)m + 3Al₂O₃·2SiO₂ + 3FeCl₃ + Fe₂SO₄ + ZrClO₄ + 14NaOH → [3Al₂O₃·2SiO₂ −(C₆H₁₁NO₄)]2m/3·Fe₃O₄·xH₂O + 3Fe(OH)₃ + 2Zr(OH)₄ + 6NaCl + (2n + 2m)CH₃COONa + (2n + m)H₂O

(13)

Following this reaction, the negatively charged surface of the zeolite (Al₂O₃.2SiO₂) may interact with the positively charged chitosan to produce chitosan–aluminosilicate complex. Electrostatic interactions between Fe³⁺ and Zr⁴⁺ ions and chitosan are another mechanism by which chitosan combines with metal ions to form chitosan–metal complexes. Fe(OH)₃ and Fe₃O₄ are formed when Fe²⁺ and Fe³⁺ ions react with hydroxide ions (OH⁻) from NaOH.

3.3. The Design of the Experiment

Experiments were conducted using response surface methodology (RSM) in combination with Box–Behnken design (BBD), and statistical analysis was performed using Minitab 21.3.1 software. (A) The pH (2–10), (B) dosage (0.02–0.1 g), (C) temperature (30–60 °C), and (D) contact time (10–60 min) were the independent variables examined in the BBD, with three levels and four parameters (

Table 8. Variables and levels.

Symbol	Factor	Level 1 (− 1)	Level 2 (0)	Level 3 (+ 1)
A	pH	2	7	10
B	Dosage (g)	0.02	0.06	0.10
C	Temperature (°C)	30	45	60
D	Time (min)	10	35	60

). In total, 27 different sets of experiments were performed to determine the optimal conditions for PO₄³⁻ removal. The data obtained were assessed using an equation for a quadratic polynomial response surface, which was calculated using Equation (14), to identify the relationships between independent variables and response.

$${\mathrm{Y}=\mathrm{E}}_0{+\mathrm{E}}_1{\mathrm{A}+\mathrm{E}}_2{\mathrm{B}+\mathrm{E}}_3{\mathrm{C}+\mathrm{E}}_4{\mathrm{D}+\mathrm{E}}_{11}{\mathrm{A}}^2{+\mathrm{E}}_{22}{\mathrm{B}}^{2 }{+\mathrm{E}}_{33}{\mathrm{C}}^2{+\mathrm{E}}_{12}{\mathrm{AB}+\mathrm{E}}_{13}{\mathrm{AC}+\mathrm{E}}_{23}\mathrm{BC}+\mathsf{\epsilon }$$

(14)

The coefficients of the polynomial model are represented as follows: E0 is constant expression, E1–E3 are linear effects, E11–E33 are second-order effects, E12–E23 are interactive effects, and ε is error term. An analysis of variance (ANOVA) was performed to calculate the F- and p-values of the model to measure its statistical significance and appropriateness. The statistical significance of the model is shown through the model’s F-value and p-value, and a lack-of-fit study of the proposed model was executed using Minitab 21.3.1 software. In addition, a 3D response surface plot and Pareto chart of standardized effects were developed to figure out the cooperative quantitative impact of the independent variables on the response and overall value of the model [63].

3.4. Batch Adsorption Study and Response Determination (PO₄³⁻ Removal %)

To evaluate the efficiency of PO₄³⁻ removal, batch adsorption approach was used in this study. In total, 100 mL of PO₄³⁻ (20 mg/L) was placed in a 300 mL conical flask. After the adsorption procedure was completed, external magnetite was placed in the conical flask to separate the adsorbent and adsorbate. PO₄³⁻ removal was calculated using Equation (15).

$${{\mathrm{PO}}_4}^{3-} \mathrm{removal} \%=\frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{o}}} \times 100$$

(15)

where C_o and C_e are the initial and equilibrium PO₄³⁻ concentrations (mg/L), respectively.

The data from run 17 of the BBD were used for subsequent experiments (isotherm and kinetic models). However, 30 min was not used because the results were far from equilibrium. The amount of PO₄³⁻ adsorbed was determined using Equation (16).

$${\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}}{\mathrm{W}} \times \mathrm{V}$$

(16)

where q_e (mg/g) is the adsorption capacity, W (g) is the amount of CS-ZL/ZrO/Fe₃O₄, and V (L) is the volume of adsorbate (PO₄³⁻ solution).

3.5. Adsorption Isotherm Studies

The isotherm model was studied with PO₄³⁻ solutions ranging from 20 mg/L to 500 mg/L with pH of 2. These examinations were performed for 60 min at 30 °C, and adsorbent dosage of 0.06 g was placed in the flask. In this work, Langmuir and Freundlich models were used to assess PO₄³⁻ adsorption onto CS-ZL/ZrO/Fe₃O₄ [64].

3.6. Adsorption Kinetic Studies

Pseudo-first-order (PFO) and pseudo-second-order (PSO) models were used to investigate the model of adsorption kinetics. The following parameters were used in the experiment: an adsorption temperature of 30 °C, an initial PO₄³⁻ concentration of 500 mg/L at pH of 2, an adsorbent dosage of 0.06 g, and contact time ranging from 35 to 2880 min.

3.7. Influence of Coexisting Ionic Strength

The experiment was conducted under optimum conditions with a dosage of 0.06 g, an initial PO₄³⁻ concentration of 500 mg/L, and a contact time of 1440 min at 30 °C. The coexisting ion was prepared with cationic and anionic ions at a concentration of 20 mg/L (Mg²⁺, Ca²⁺, CO₃²⁻, SO₄⁻, and Na⁺).

3.8. Desorption and Reusability Studies

In most practical applications, it is essential to employ adsorbents with high level of reusability. NaOH was chosen as desorbing agent to release PO₄³⁻ ion from CS-ZL/ZrO/Fe₃O₄. Firstly, 0.06 g of CS-ZL/ZrO/Fe₃O₄ was loaded with 500 mg/L of PO₄³⁻ ion at pH of 2.0, which was called CS-ZL/ZrO/Fe₃O₄-P. Then, 0.01 g of CS-ZL/ZrO/Fe₃O₄-P was dispersed in 60 mL of NaOH at 30 °C. The desorption capacity and desorption percentage are shown in Equations (17) and (18), respectively. Reusability was assessed using the same treatment as described above.

$${\mathrm{q}}_{\mathrm{des}}=\frac{\mathrm{C}}{\mathrm{W}} \times \mathrm{V}$$

(17)

$$\% \mathrm{Desorption}=\frac{{\mathrm{q}}_{\mathrm{des}}}{{\mathrm{q}}_{\mathrm{e}}} \times 100$$

(18)

where q_des (mg/g) is the desorption capacity; C (mg/L) is the PO₄³⁻ concentration of desorption; % Desorption (%) is the percentage desorption; and W, V, and q_e are the same as above.

3.9. PO₄³⁻ Measurements

PO₄³⁻ ions were measured using the molybdate blue method. A total of 12 g of (NH₄)₆Mo₇O₂₄·4H₂O was mixed with 100 mL of DW. K₂Sb₂(C₄H₂O₆)₂ (0.277 g) was added followed by 140 mL of 18 M H₂SO₄. Afterward, it was adjusted to 1 L with distilled water (solution A). A total of 1.06 g of C₆H₈O₆ was added to and mixed with 100 mL of solution A, 25 mL of 4 N H₂SO₄ was added, and the solution was adjusted to 1 L with DW (solution B). Note: This solution must be prepared in every experiment. The procedure for the mixed solution was as follows: 2 mL of liquid sample/standard was mixed with 10 mL of solution B. Afterwards, we waited for 30 min and then analyzed the solution using a UV-Vis spectrophotometer (Jasco V-530) at a wavelength of 693 nm. A standard curve for PO₄³⁻ was constructed using Na₂HPO₄.

3.10. Mechanical Stability

The mechanical stability of the CS-ZL/ZrO/Fe₃O₄ composite was evaluated based on the responses of the samples to a water bath shaker at 80 °C. For one hour, dried CS-ZL/ZrO/Fe₃O₄ was soaked in HCl and H₂SO₄ concentrations ranging from 0.01 to 0.1 M. Following that, the sample was dried in an oven at 60 °C for twenty-four hours. The calculation of the dry weight retention (WR) was performed using Equation (19).

$$\mathrm{WR} (\%)=\frac{{\mathrm{w}}_{\mathrm{i}}}{{\mathrm{w}}_{\mathrm{a}}} \times 100$$

(19)

where w_i and w_a are the dry weights of CS-ZL/ZrO/Fe₃O₄ before and after treatment, respectively.

3.11. Characterization of CS-ZL/ZrO/Fe₃O₄

The crystalline structure of CS-ZL/ZrO/Fe₃O₄ was analyzed using a powder X-ray diffractometer (XRD) equipped with Cu/Kα radiation (Hypix-3000). Fourier transform infrared spectra (FTIR) of CS-ZL/ZrO/Fe₃O₄ were measured before and after PO₄³⁻ adsorption using a Thermo Scientific Nicolet iS10 instrument (Thermo Fisher Scientific Inc., Waltham, MA, USA). The ATR-FTIR approach was used to analyze samples with a resolution of 4 cm⁻¹ throughout the wavenumber spectrum spanning 400–4000 cm⁻¹. To determine the specific surface area (SSA), the BET approach was combined with a surface area analyzer (MicroActive AutoPore V 9600 2.03.00, Micromeritics, Norcross, GA, USA). SEM-EDS (JIED-2300, Shimadzu, Kyoto, Japan) was used to examine the SEM images and the elemental distributions of CS-ZL/ZrO/Fe₃O₄. The initial (pHi) and final (pHf) pH values of the solutions were measured to determine the surface charge over a range of pH values (pH_zpc). The pHi was adjusted from 2.0 to 10.0 in 0.01 M NaCl solution. Following that, 0.1 g of CS-ZL/ZrO/Fe₃O₄ was added and stirred for 24 h at 30 °C, and pHf was measured. A plot of ΔpH = pHf − pHi vs. pHi was used to determine pH_pzc, which corresponds to the neutral surface charge.

3.12. Data Analysis

All results were noted and edited using Microsoft Excel. The effects of coexisting ions on PO₄³⁻ removal were examined using a completely randomized design (CRD). Data were analyzed using ANOVA with Tukey’s test (p ≤ 0.05) using Minitab 21.3.1.

4. Conclusions

In this study, a novel adsorbent, CS-ZL/ZrO/Fe₃O₄, was prepared from chitosan (CS), zeolite (ZL), ZrO, and magnetite (Fe₃O₄) via a crosslinking approach. The Box–Behnken design (BBD) and the response surface methodology (RSM), with their corresponding four separate factors (pH, dosage, temperature, and time), were used to develop the best experimental conditions for PO₄³⁻ removal. Weight retention (WR) was measured in a batch reactor under acidic conditions (HCl and H₂SO₄) at 80 °C for 1 h to determine the mechanical stability. The results indicate that CS-ZL/ZrO/Fe₃O₄ was stable and did not change in the functional group peak area after treatment. The best conditions were at a pH of 2.0, with an adsorption capacity and percentage removal of 732.56 mg/g and 87.91%, respectively. The Freundlich isotherm and pseudo-second-order (PSO) kinetic models were fitted to PO₄³⁻ removal, indicating heterogeneous and chemical sorption. In addition, the results suggest that PO₄³⁻ adsorption occurred via the electrostatic interactions between the positive charge of CS-ZL/ZrO/Fe₃O₄ and the negative charge of H₂PO⁴⁻ as well as ion exchange and hydrogen bonding. The presence of coexisting ions (Mg²⁺, Ca²⁺, CO₃²⁻, SO₄²⁻, and Na⁺) had no effect on the removal of PO₄³⁻ (p ≤ 0.05). The desorption studies revealed that 1 M NaOH was better at releasing PO₄³⁻, reaching 95.77% after 30 min of treatment at 30 °C. The reusability of CS-ZL/ZrO/Fe₃O₄ showed good performance over three cycles. These findings imply that CS-ZL/ZrO/Fe₃O₄ is the best way to improve the stability of chitosan under acidic conditions, and it is a good adsorbent for removing PO₄³⁻ and other potential water pollutants from water.