Chitosan-Functionalized Hydroxyapatite-Cerium Oxide Heterostructure: An Efficient Adsorbent for Dyes Removal and Antimicrobial Agent

The current research intended to employ a facile and economical process, which is also ecofriendly to transform camel waste bones into novel heterostructure for cleansing of diverse waste waters. The bones of camel were utilized for preparation of hydroxyapatite by hydrothermal method. The prepared hydroxyapatite was applied to the synthesis of cerium oxide-hydroxyapatite coated with natural polymer chitosan (CS-HAP-CeO2) heterostructure. Being abundant natural polymer polysaccharide, chitosan possesses exceptional assets such as accessibility, economic price, hydrophilicity, biocompatibility as well as biodegradability, therefore style it as an outstanding adsorbent for removing colorant and other waste molecules form water. This heterostructure was characterized by various physicochemical processes such as XRD, SEM-EDX, TEM, and FT-IR. The CS-HAP-CeO2 was screened for adsorption of various industrially important dyes, viz., Brilliant blue (BB), Congo red (CR), Crystal violet (CV), Methylene blue (MB), Methyl orange (MO), and Rhodamine B (RB) which are collective pollutants of industrial waste waters. The CS-HAP-CeO2 demonstrated exceptional adsorption against CR dye. The adsorption/or removal efficiency ranges are BB (11.22%), CR (96%), CV (28.22%), MB (47.74%), MO (2.43%), and RB (58.89%) dyes. Moreover, this heterostructure showed excellent bacteriostatic potential for E. coli, that is liable for serious waterborne diseases. Interestingly, this work revealed that the incorporation of cerium oxide and chitosan into hydroxyapatite substantially strengthened antimicrobial and adsorption capabilities than those observed in virgin hydroxyapatite. Herein, we recycled the unwanted camel bones into a novel heterostructure, which assists to reduce water pollution, mainly caused by the dye industries.


Introduction
The ecosphere encounters pure water crisis even in the 21st century. Unpolluted water is essential for the ecosystem; however, inattention and maladministration of natural water assets had considerably endangered the availability of pure water. Annually, water contamination and allied ailments prerogative affect approximately 2.1 million lives. Predominantly, various biological pollutants are accountable for water-related infections. The incidence of Coliforms is considered as indicator of fecal contagion. The eruptions of water-related microbial infections and inorganic materials are the main causes of loss of precious lives [1]. Admitting, not severely damaging in minute amounts, but they are the main pollutants once concentrated in water. Similarly, organic materials and prescription drugs etc., enter through different sources into water bodies [2]. Besides being aesthetic, the colored dyes are cancer-causing, and block light from entering aquatic systems. Numerous The camel bones were assembled from abattoir of Albaha city of Saudi Arabia. Extensive cleaning of bones was done following our earlier procedures [24], primarily, these mandibles were swept away extensively using water, also with acetone to exclude fat and superfluous impurities. Later on, bones were put in an oven at 170 • C for 48 h. Furthermore, it was calcined in a furnace at an elevated temperature of 700 • C to get HAP.

Synthesis of Chitosan Coated Cerium Oxide-HAP Heterostructure (CS-HAP-CeO 2 )
Cerium sulfate (5.6 g, anhydrous, 97%, Alfa Aesar, India) was added to 80 mL of water in a beaker. Then s sodium hydroxide solution (2 M) was added dropwise to attain pH~11. HAP (2 g) powder was mixed in the same solution with continuous stirring. About 10 mL of chitosan solution (500 mg of chitosan was liquefied in acetic acid solution, 1% v/v in water) was dissolved to it. At that time the final solution was shifted to the hydrothermal reactor and heated at 180 • C for 15 h. The prepared CS-HAP-CeO 2 heterostructures were filtered and rinsed quite a few times with water and then with ethanol and kept at 90 • C overnight in an oven.

Characterization
The crystallinity of HAP and CS-HAP-CeO 2 heterostructures were examined by X-ray diffraction (XRD, Rigaku (Tokyo, Japan) D/Max-2550, λ = 0.154 18 nm). The morphology of the samples was depicted by scanning electron microscopy equipped with energydispersive X-ray (SEM-EDX, JEOL, Tokyo, Japan). The presence of samples functional group was performed by Fourier transform infrared (FTIR, Thermo Scientific, Waltham, MA, USA) spectrometer. The microscopic images of samples were obtained by transmission electron microscopy (TEM, Hitachi, Tokyo, Japan).

Adsorption Studies
The adsorption study was conducted by batch method. The impact of various factors such as adsorbent dosage, contact time, pH, dyes concentration, and temperature on the removal efficiency of CR by CS-HAP-CeO 2 heterostructure in the range (0.0025-0.1 g), (5-1440 min), (3-11 pH), (50-500 mg/L), and (25-45 • C) respectively, was tested. For adsorptive dye removal, 0.02 g of CS-HAP-CeO 2 heterostructure adsorbent solution was thoroughly mixed with 50 mL aqueous dye solution of 50 mg/L in 100 mL conical flask. Dye pH was changed by the addition of 0.1 M HCl or 0.1 M NaOH and then placed on a shaker at 100 rpm at ambient temperature, and the samples were tested at specific time intervals. After equilibrium period, the solution was filtered and the concentration of CR, RB, MB, CV, BB, and MO dyes were observed by UV-visible spectrophotometer at maximum absorption wavelengths (λ max ) of 500, 603, 665, 590, 592, and 468 nm, respectively. The removal efficiency (%) and adsorption capacity (mg/g) were determined using the following Equations (1) and (2), respectively [25].
where C o is the initial concentration of dye and C e is the concentration of MB equilibrium time), V is the volume of dye solutions, L, and m is the adsorbent dosage, g.

Antimicrobial Activity
The antibacterial activity of pristine HAP and novel CS-HAP-CeO 2 heterostructure was verified in liquid broth [26]. The E. coli culture was maintained (10 6 CFU) with known amount of pristine HAP and CS-HAP-CeO 2 heterostructure (0, 50, 100, and 200 µg/mL) to regulate the minimum inhibitory concentration (MIC). The expansion kinetics was supervised at every 4 h by reading the OD in a spectrophotometer. A persistent incubation temperature of 37 • C and rpm of 150 were sustained in a rotary shaker. Growth inhibition in presence of HAP and HAP-Ce coated with natural polymer CS was upheld to 16 h and the shift in absorbance was considered at 600 nm by ultra-violet [27] spectrophotometer.

Sythesis and Characterization
The formation of CS-HAP-CeO 2 heterostructure can be hypothesized as follows: the bone does have 65-70% HAP and 30-35% organic compounds (on dry weight basis). In our work, we extracted HAP by simple thermal treatment of natural bone. Cerium oxide was prepared using cerium sulfate precursors with HAP in solution. The use of sodium hydroxide resulted in a white precipitate, Ce(OH) 3 , from cerium sulfate solution. Oxidation of Ce 3+ to Ce 4+ in solution occurs at high pH and forms Ce(OH) 4 [28]. Then it is converted to cerium oxide at high temperature. Furthermore, chitosan as a linear polysaccharide consists of (1,4)-linked 2-amino-deoxy-b-d-glucan with two free hydroxyl and one primary amino group. The positive and negative charge of amino and hydroxyl groups could make it available to attached with the surface of HAP and cerium oxide, thus resulting in the formation of novel CS-HAP-CeO 2 heterostructure [28]. Figure 1 displays the XRD spectra of untainted HAP and CS-HAP-CeO 2 heterostructure. All diffraction peaks of pure HAP correspond to single phase hexagonal structure of [Ca 10 (PO 4 ) 6 (OH) 2 ] (JCPDS no. 09-0432). There was no contamination peaks present in the spectrum, it confirms the formation of pure HAP [29] (Figure 1a). The XRD patterns of CS-HAP-CeO 2 heterostructure exhibited all characteristic diffraction peaks of HAP, in addition, it also shows the diffraction peaks corresponding to the crystalline plane (111), (200), (220), and (311) which confirms the cubic fluorite assembly of CeO 2 (JCPDS no. 81-0792) [30]. In the same spectrum, a weak peak at around 2θ of 20 • corresponds to pure chitosan [31] (Figure 1b). Figure 2 presents the surface structure and morphology of pure HAP and CS-HAP-CeO 2 heterostructure. Figure 2 illustrates the micro-structural study of HAP particles, which shows somewhat spherical in shape, displaying small and large particles with maximum size of around 1 µm (Figure 2a,b). Whereas, in the heterostructure, the HAP particles together with nanotube-like structure having diameter of around 100 nm with varying lengths (Figure 2c,d) were visible.
The close inspection of the TEM image confirmed the presence of CeO 2 nanotubes having diameter of around 100 nm over the surface of HAP particles, which is in good agreement with SEM results (Figure 3a). The high-resolution image clearly shows different crystal structures on the heterostructure surface, the fringes of d = 0.34 nm and d = 0.27 nm, observed in Figure 3b matched well to those of (002) and (200) crystallographic planes of HAP and CeO 2 particles, respectively. A thin amorphous uniform coating layer on the crystal surface can also be seen confirming the chitosan presence in the heterostructure (Figure 3b). This confirms that CS-HAP-CeO 2 heterostructure was successfully prepared.   Figure 2 illustrates the micro-structural study of HAP particles, which shows somewhat spherical in shape, displaying small and large particles with maximum size of around 1 µm (Figure 2a,b). Whereas, in the heterostructure, the HAP particles together with nanotube-like structure having diameter of around 100 nm with varying lengths (Figure 2c,d) were visible. The close inspection of the TEM image confirmed the presence of CeO2 nanotubes having diameter of around 100 nm over the surface of HAP particles, which is in good agreement with SEM results (Figure 3a). The high-resolution image clearly shows different crystal structures on the heterostructure surface, the fringes of d = 0.34 nm and d = 0.27 ent crystal structures on the heterostructure surface, the fringes of d = 0.34 nm and d = nm, observed in Figure 3b matched well to those of (002) and (200) crystallographic p of HAP and CeO2 particles, respectively. A thin amorphous uniform coating layer o crystal surface can also be seen confirming the chitosan presence in the heterostru (Figure 3b). This confirms that CS-HAP-CeO2 heterostructure was successfully prep  The EDX spectrum of CS-HAP-CeO2 showed the presence of all the expected elements in heterostructure. The EDX analysis confirms the presence of calcium (Ca), phosphorus (P), cerium (Ce), and oxygen (O) (Figure 4).

Adsorption Studies
The obtained adsorbent was tested for removal of various anionic dyes such as CR, RB, BB, and MO and cationic dyes such as CV and MB in aqueous medium at the param-

Adsorbate Selectivity
The obtained adsorbent was tested for removal of various anionic dyes such as CR, RB, BB, and MO and cationic dyes such as CV and MB in aqueous medium at the parameters (dye = 20 mg/L, agitation = 100 rpm, dose = 0.02 g; and 25 • C) as shown in Figure 5. The experimental observations revealed that removal efficiency of the different dyes were BB (11.22%), CR (96%), CV (28.22%), MB (47.74%), MO (2.43%), and RB (58.89%). This indicates that the removal efficiency of CR dye was higher than the other tested dyes and order of affinity built on the amount of dye uptake was found to be: CR > RB > MB > CV > Bb > MO dye. This is owing to the fact that different dyes will undergo different physical and electrostatic forces, thus the difference in removal efficiency of dyes is due to their structure, molecular size, and functional groups which react with the functional groups of adsorbent surfaces with different physical and electrostatic forces [32,33]. Consequently, CR molecules occupy larger area over the CS-HAP-CeO 2 heterostructure surface.

Effect of pH
The initial pH solution shows an important role in the adsorption process owing t its impact on both, the active sites of ionization process of the CR molecule and the CS HAP-CeO2 heterostructure in the solution. Therefore, the effect of pH solution on the ad sorption process at (CR = 100 mg/L, agitation = 100 rpm, dose = 0.02 g; and 25 °C) wa tested at the varying initial pH (3-11) as presented in Figure 6a. The CR is a dipolar mo ecule and it represents a cationic form at acidic pH and an anionic form at basic pH [34 Moreover, the normal red color of CR at pH around 7 changes to dark blue in a highl acidic medium and to red in basic medium ~pH (11)(12), this color in an alkaline medium is slightly different from the original red at pH = 7 [35]. The results reveal that the remova efficiency and adsorption capacity increased from (242.84 mg/g and 97.13%) to (248.4 mg/g and 99.63%) with the increase in pH from 3 to 7, respectively, and then reduced. Th dissociation constant (pKa) of CR is 4.0 [36]. According to this value when the pH < (pK = 4), the surfaces of both CS-HAP-CeO2 heterostructure and CR dye are positivel charged, showing an electrostatic repulsion between them. On the other hand, when th pH > (pKa = 4), the CR dissociates into polar groups (SO3 − ) along with the positive charg of -N=N + -groups. The SO3 − group over the CR surface adsorbs on positively charged CS HAP-CeO2 adsorbent by electrostatic attraction along with H-bonding. As pH increase to 7, the removal efficiency of CR was increased to 99.84% owing to the number of positiv charges in CS-HAP-CeO2 heterostructure surface, decreasing which led to creation of elec trostatic attraction between the -N=N + -group of CR molecules and CS-HAP-CeO2 heter ostructure surface. At an alkaline medium, at high pH, the existence of excess -OH ion compete with the CR anions (sulfonate groups) for the adsorption sites on the CS-HAP CeO2 heterostructure surface, leading to a decrease in the removal efficiency of CR dy

Effect of pH
The initial pH solution shows an important role in the adsorption process owing to its impact on both, the active sites of ionization process of the CR molecule and the CS-HAP-CeO 2 heterostructure in the solution. Therefore, the effect of pH solution on the adsorption process at (CR = 100 mg/L, agitation = 100 rpm, dose = 0.02 g; and 25 • C) was tested at the varying initial pH (3-11) as presented in Figure 6a. The CR is a dipolar molecule and it represents a cationic form at acidic pH and an anionic form at basic pH [34]. Moreover, the normal red color of CR at pH around 7 changes to dark blue in a highly acidic medium and to red in basic medium~pH (11)(12), this color in an alkaline medium is slightly different from the original red at pH = 7 [35]. The results reveal that the removal efficiency and adsorption capacity increased from (242.84 mg/g and 97.13%) to (248.40 mg/g and 99.63%) with the increase in pH from 3 to 7, respectively, and then reduced. The dissociation constant (pKa) of CR is 4.0 [36]. According to this value when the pH < (pKa = 4), the surfaces of both CS-HAP-CeO 2 heterostructure and CR dye are positively charged, showing an electrostatic repulsion between them. On the other hand, when the pH > (pKa = 4), the CR dissociates into polar groups (SO 3 − ) along with the positive charge of -N=N +groups. The SO 3 − group over the CR surface adsorbs on positively charged CS-HAP-CeO 2 adsorbent by electrostatic attraction along with H-bonding. As pH increases to 7, the removal efficiency of CR was increased to 99.84% owing to the number of positive charges in CS-HAP-CeO 2 heterostructure surface, decreasing which led to creation of electrostatic attraction between the -N=N + -group of CR molecules and CS-HAP-CeO 2 heterostructure surface. At an alkaline medium, at high pH, the existence of excess − OH ions compete with the CR anions (sulfonate groups) for the adsorption sites on the CS-HAP-CeO 2 heterostructure surface, leading to a decrease in the removal efficiency of CR dye [37,38]. Therefore, at pH = 7, probably, electrostatic attraction and H-bonding become dominant in the adsorption mechanism. Thus, pH 7 was chosen as the optimum pH value in further experiments. The reported effect of pH on adsorption of CR dye by MgO composite [39] is in agreement with our study. Furthermore, a recent review report based on the applications of artificial intelligence has captured incredible attention owing to solve future water-related problems [40]. Moreover, the cellulose nanofibril/rectorite composite sponges has displaced an efficient dye adsorption and selective separation in recent investigation [41].

Effect of Adsorbent Dose
The influence of CS-HAP-CeO2 heterostructure amount on CR dye adsorption at parameters conditions (CR = 20 mg/L, pH = 7; and 25 °C) in the range (0.0025-0.1 g) was studied as mentioned in Figure 6b. It was noticed that the removal efficiency of CR increases from 60.16% to 99.43% with the increase in amount of CS-HAP-CeO2 heterostructure adsorbent from 0.0025 to 0.02, and then became constant as the amount of CS-HAP-CeO2 heterostructure adsorbent increased to 0.1 g. An increase in the removal efficiency with the increase in amount of CS-HAP-CeO2 heterostructure adsorbent can be ascribed to the increasing accessibility of more adsorption sites [42]. On the other hand, the adsorption capacity was reduced from 240 mg/g to 20 mg/g with an increase in an adsorbent amount from 0.0025 to 0.1 g. This might be due to the aggregation events at high dosage [43]. Similar results were reported to the effect of adsorbent dose on adsorption of CR by Teucrium polium L. [44] and polypyrrole-modified red mud [45]. Therefore, 0.02 g was cho-

Effect of Adsorbent Dose
The influence of CS-HAP-CeO 2 heterostructure amount on CR dye adsorption at parameters conditions (CR = 20 mg/L, pH = 7; and 25 • C) in the range (0.0025-0.1 g) was studied as mentioned in Figure 6b. It was noticed that the removal efficiency of CR increases from 60.16% to 99.43% with the increase in amount of CS-HAP-CeO 2 heterostructure adsorbent from 0.0025 to 0.02, and then became constant as the amount of CS-HAP-CeO 2 heterostructure adsorbent increased to 0.1 g. An increase in the removal efficiency with the increase in amount of CS-HAP-CeO 2 heterostructure adsorbent can be ascribed to the increasing accessibility of more adsorption sites [42]. On the other hand, the adsorption capacity was reduced from 240 mg/g to 20 mg/g with an increase in an adsorbent amount from 0.0025 to 0.1 g. This might be due to the aggregation events at high dosage [43]. Similar results were reported to the effect of adsorbent dose on adsorption of CR by Teucrium polium L. [44] and polypyrrole-modified red mud [45]. Therefore, 0.02 g was chosen as the optimal adsorbent dose under the examined conditions. Figure 6c shows the effect of contact time on the removal of CR dye at an initial concentration of 100 mg/L onto CS-HAP-CeO 2 adsorbent under various conditions (dose-0.02 g; pH-7; T-25 • C). The present outcome exhibited that the adsorption capacity of the CS-HAP-CeO 2 heterostructure and removal efficiency of CR dye increased rapidly from (120.91 to 233.65 mg/g) and (48.36% to 96%) at 100 mg/L with the increase in time from 1 to 15 min at the initial concentration of CR dye, respectively. As a result, the ideal time for equilibrium for future work was evaluated to be 15 min. The CR adsorption on graphene-chitosan composite hydrogel has shown a comparable contact time effect [46].

Effect Temperatures
The effect of temperature on CR adsorption of dye on the CS-HAP-CeO 2 adsorbent was investigated at different temperatures (25,35, and 45 • C) and at different concentrations (50-400 mg/L). Figure 6d shows the initial CR dye concentration effect on the adsorption process at various concentrations (50 to 400 mg/L) and temperatures (25,35, and 45 • C). The conditions are as follows: (dose-0.02 g; pH-7). When the CR dye concentration increased from 50 to 250 mg/L at 25 • C, the adsorption capacity of the CS-HAP-CeO 2 adsorbent increased from 121.87 to 289.21 mg/g. The increase in the driving force for mass transfer from solution to the CS-HAP-CeO 2 heterostructure adsorbent surface results in prominent increase in adsorption capacity at higher initial concentrations [47][48][49]. It was perceived that when the temperature rises, the adsorption capacity decreased marginally, indicating that the CR dye adsorption was an exothermic process. At higher temperatures, the binding forces on the surface with the CS-HAP-CeO 2 heterostructure adsorbent were destroyed [50]. These results were consistent to that of CR adsorption on a fly ash/CeO 2 composite adsorbent [51], malachite green and CR were removed from aqueous solution using magnetic HAP nanopowder [52], CR adsorption on graphene oxide/chitosan fibers [53], as well as CR adsorption on calcium HAP nanoparticles [54].

Adsorption Kinetics
To determine the CR adsorption mechanism onto CS-HAP-CeO 2 heterostructure, two most common adsorption kinetics namely, pseudo-first-order equation (PFO) (Equation (3)) and pseudo-second-order rate equation (PSO) (Equation (4)) were applied. The equations can be expressed as follows [55]: where k 1 (1/min) and k 2 (g/(mg·min)) represent the PFO and PSO constant, respectively; q e and q t (mg/g) represent the adsorption capacity of the CS-HAP-CeO 2 at equilibrium and at various times, respectively. The linear curves plotted of log (q e − q t ) and (t/q t ) versus t are displayed in Figure 7, and the kinetic parameters are listed in Table 1. The results show the experimental data fitted very well with PSO kinetic model, because the R 2 value for the PFO (R 2 = 0.9999) is higher than the R 2 value for the PFO (R 2 = 0.7967). Moreover, the adsorption capacity (qe,cal = 238.09 mg/g) value derived from PSO is close to that obtained experimentally (qe,exp = 237.95 mg/g). Hence, the results show that the adsorption of CR dye onto CS-HAP-CeO2 heterostructure is chemical adsorption via electrostatic attraction and H-bonding. The same result was observed for adsorption of CR by EG@MnFe2O4 and MnFe2O4 [36].

Adsorption Isotherm
To determine the mechanism of the adsorption process and evaluate the quantity of CR dye adsorption onto the CS-HAP-CeO2 heterostructure, four linear isotherm models namely, Langmuir Equation (5), Freundlich Equation (6), Temkin (Equations (7) and (8)), and Dubinin-Radushkevich (Equations (9)-(11)) were used. The equations can be expressed as follows [36]:  The results show the experimental data fitted very well with PSO kinetic model, because the R 2 value for the PFO (R 2 = 0.9999) is higher than the R 2 value for the PFO (R 2 = 0.7967). Moreover, the adsorption capacity (q e,cal = 238.09 mg/g) value derived from PSO is close to that obtained experimentally (q e,exp = 237.95 mg/g). Hence, the results show that the adsorption of CR dye onto CS-HAP-CeO 2 heterostructure is chemical adsorption via electrostatic attraction and H-bonding. The same result was observed for adsorption of CR by EG@MnFe 2 O 4 and MnFe 2 O 4 [36].

Adsorption Isotherm
To determine the mechanism of the adsorption process and evaluate the quantity of CR dye adsorption onto the CS-HAP-CeO 2 heterostructure, four linear isotherm models namely, Langmuir Equation (5), Freundlich Equation (6), Temkin (Equations (7) and (8)), and Dubinin-Radushkevich (Equations (9)-(11)) were used. The equations can be expressed as follows [36]: log q e = log K f + 1 n log C e (6) ln q e = ln(q s ) − K ad ε 2 (9) where C e (mg/L) represents the equilibrium concentration of CR dye (mg/L), q m is the maximum monolayer adsorption capacity (mg/g), K ad (mol 2 /kJ 2 ), K f (mg/g), and K L (L/mg), represent the Dubinin-Radushkevich, Freundlich, and Langmuir isotherm constant, respectively; n is the adsorption intensity; A T (L/g) and B (J/mol) are the binding constant and the constant related to heat of sorption. ε is the Polanyi potential and E (kJ/mol) is the average free energy. The linear plots of four isotherm models are shown in Figure 8, and the isotherm parameters are listed in Table 2. It was noticed that the R 2 values of the Langmuir adsorption model were the highest (R 2 = 0.9989) compared to the other isotherm models, Dubunin-Radushkevich (R 2 = 0.9332), Temkin (R 2 = 0.6384), and Freundlich (R 2 = 0.5963). That means the adsorption process was well fitted with Langmuir adsorption model, representing monolayer adsorption on homogeneous surfaces. The maximum monolayer adsorption capacity of CS-HAP-CeO 2 nanocomposite toward CR dye was 270.27 mg/g. This value is greater than the adsorption capacities of other adsorbents listed in Table 3 [34,[56][57][58][59][60][61]. From the Dubinin-Radushkevich model, the magnitude of E is used for evaluating the type of adsorption mechanism. If the value of E is between 8 and 16 kJ/mol, it indicates chemisorption, while for values of E < 8 kJ/mol, the sorption process is of a physical nature [62,63]. Our results showed that the adsorption of CR on heterostructure adsorbent is physical adsorption, owing to the values of E being between 0.2672 and 1.118 kJ/mol (25 • C < T < 45 • C).

Thermodynamic Studies
To explain the effect of temperature on the CR adsorption over adsorbent and interpret whether the adsorption process occurs spontaneously or not, thermodynamic parameters including, free energy change (∆G • ), enthalpy change (∆H • ), and entropy change (∆S • ) were calculated using the following Equations (12) and (13) [64]: where k is Kc = q e /C e . The values of ∆H • and ∆S • were calculated from the slope and intercept of the plot ln kc versus 1/T (Figure 9). The thermodynamic parameters are summarized in Table 4. The negative values of ∆G • and ∆H • imply that the CR adsorption over CS-HAP-CeO 2 heterostructure are spontaneous and exothermic [65]. The values of ∆G • slightly increased from −11.63 to −5.69 (kJ mol −1 ) with the rising temperature from 298 to 318 K, indicating the adsorption process is favored at lower temperatures [65]. The ∆S • negative value implies a decrease in the randomness of the solid-solution interface.

CR Dye Adsorption Mechanism
CS-HAP-CeO2 heterostructure adsorbent contains functional groups on their surfaces such as phosphate, amino, and hydroxyl groups which are responsible for the binding of the CR dye. According to the dissociation constant (pKa) of CR dye, when the pH < (pKa = 4), the surfaces of both CS-HAP-CeO2 heterostructure and CR dye are positively charged, representing an electrostatic repulsion between them. On the other hand, when the pH > (pKa = 4), the CR dissociates into polar groups (SO3 − ) along with the positive charge of the -N=N + -group. The SO3 − group over CR surface adsorbs on positively charged CS-HAP-CeO2 adsorbent by electrostatic attraction along with H-bonding. As pH increases to 7, the removal efficiency of CR increased due to the decrease in the number of positive charges in the CS-HAP-CeO2 heterostructure surface which led to the creation of electrostatic attraction between the -N=N + -group of CR molecules and the negative charge of CS-HAP-CeO2 heterostructure surface. The functional groups of CS-HAP-CeO2 heterostructure before/and after adsorbing CR dye were identified through FTIR analysis, and the obtained results are shown in Figure 10. It was observed that the intensity of O-H/NH2 stretching shifted from 3529 to 3382 cm −1 and the band associated with P-O at 1038 cm −1 decreased in intensity and shifted to 1029 cm −1 after CR dye adsorption. This is due to the interaction of -N=N + -and -NH2, -SO3H groups on the CR surface of dye with PO4 −3 and O-H/NH2 groups on the surface of CS-HAP-CeO2 heterostructure through electrostatic attraction and H-bonding. The appearance of two new bands at 830 and 755 cm −1 after CR dye adsorption indicates the CR dye loaded successfully onto the CS-HAP-CeO2

CR Dye Adsorption Mechanism
CS-HAP-CeO 2 heterostructure adsorbent contains functional groups on their surfaces such as phosphate, amino, and hydroxyl groups which are responsible for the binding of the CR dye. According to the dissociation constant (pKa) of CR dye, when the pH textless (pKa = 4), the surfaces of both CS-HAP-CeO 2 heterostructure and CR dye are positively charged, representing an electrostatic repulsion between them. On the other hand, when the pH > (pKa = 4), the CR dissociates into polar groups (SO 3 − ) along with the positive charge of the -N=N + -group. The SO 3 − group over CR surface adsorbs on positively charged CS-HAP-CeO 2 adsorbent by electrostatic attraction along with H-bonding. As pH increases to 7, the removal efficiency of CR increased due to the decrease in the number of positive charges in the CS-HAP-CeO 2 heterostructure surface which led to the creation of electrostatic attraction between the -N=N + -group of CR molecules and the negative charge of CS-HAP-CeO 2 heterostructure surface. The functional groups of CS-HAP-CeO 2 heterostructure before/and after adsorbing CR dye were identified through FTIR analysis, and the obtained results are shown in Figure 10. It was observed that the intensity of O-H/NH 2 stretching shifted from 3529 to 3382 cm −1 and the band associated with P-O at 1038 cm −1 decreased in intensity and shifted to 1029 cm −1 after CR dye adsorption. This is due to the interaction of -N=N + -and -NH 2 , -SO 3 H groups on the CR surface of dye with PO 4 −3 and O-H/NH 2 groups on the surface of CS-HAP-CeO 2 heterostructure through electrostatic attraction and H-bonding. The appearance of two new bands at 830 and 755 cm −1 after CR dye adsorption indicates the CR dye loaded successfully onto the CS-HAP-CeO 2 heterostructure surface. Therefore, the mechanism of adsorption of CR dye onto CS-HAP-CeO 2 heterostructure occurred by two mechanisms namely, electrostatic interaction and H-bonding ( Figure 11).
Nanomaterials 2022, 12, x FOR PEER REVIEW 16 of 21 heterostructure surface. Therefore, the mechanism of adsorption of CR dye onto CS-HAP-CeO2 heterostructure occurred by two mechanisms namely, electrostatic interaction and H-bonding ( Figure 11).    heterostructure surface. Therefore, the mechanism of adsorption of CR dye onto CS-HAP-CeO2 heterostructure occurred by two mechanisms namely, electrostatic interaction and H-bonding ( Figure 11).

Antimicrobial
The virgin HAP and CS-HAP-CeO 2 heterostructure were also evaluated using varying amounts against E. coli [66,67]. The produced heterostructure proved reputable antimicrobial activity against E. coli (MIC-50 µg/mL). The effects are shown in Figure 12. Alternatively, moderately trifling antibacterial achievement was detected with HAP at equal concentrations. Obviously, less bacteriostatic outcome was found at small amounts of HAP against E. coli, nonetheless improved activity was found with CS-HAP-CeO 2 heterostructure. The greater activity of CS-HAP-CeO 2 heterostructure was endorsed to morphological topographies with large surface [68] and synergism [69] between HAP and CeO 2 . The coating of CS also augmented in antimicrobial performance. The supposition about the antimicrobial action may be as: possibly at first HAP and CS-HAP-CeO 2 heterostructure are interrelated with bacterial membrane, further disseminating into the inner cell prompting an outflow by disruption of the content of cell. Previous studies have described that nanocomposites via electrostatic attraction result in disruption of membrane, quash enzymes, break bacteria, ultimately hampering protein synthesis [70]. By and large, the bacteriostatic effect of HAP is correlated to liberation of OH − ions in culture broth. Hydroxyl ions are tremendously oxidant free radicals which depict pronounced reactivity toward biomolecules. Furthermore, their lethal influence on bacterial cells are collectively allocated to aforesaid mechanisms [70].

Antimicrobial
The virgin HAP and CS-HAP-CeO2 heterostructure were also evaluated using ing amounts against E. coli [66,67]. The produced heterostructure proved reputable microbial activity against E. coli (MIC-50 µg/mL). The effects are shown in Figure 1 ternatively, moderately trifling antibacterial achievement was detected with HAP at concentrations. Obviously, less bacteriostatic outcome was found at small amoun HAP against E. coli, nonetheless improved activity was found with CS-HAP-CeO2 h ostructure. The greater activity of CS-HAP-CeO2 heterostructure was endorsed to phological topographies with large surface [68] and synergism [69] between HAP CeO2. The coating of CS also augmented in antimicrobial performance. The suppo about the antimicrobial action may be as: possibly at first HAP and CS-HAP-CeO2 h ostructure are interrelated with bacterial membrane, further disseminating into the cell prompting an outflow by disruption of the content of cell. Previous studies hav scribed that nanocomposites via electrostatic attraction result in disruption of memb quash enzymes, break bacteria, ultimately hampering protein synthesis [70]. By and the bacteriostatic effect of HAP is correlated to liberation of OH − ions in culture b Hydroxyl ions are tremendously oxidant free radicals which depict pronounced reac toward biomolecules. Furthermore, their lethal influence on bacterial cells are collec allocated to aforesaid mechanisms [70]. . Considerable difference (* p ≤ 0.05) was perceived between control and treatments. Su tial variance in bacteriostatic effect with HAP and CS-HAP-CeO2 at high quantity was envisa p ≤ 0.05, *** p ≤ 0.001 suggestively different from virgin culture.

Conclusions
In this paper, HAP and CS-HAP-CeO2 heterostructures were primed by hydro mal method. The physicochemical data confirms efficacious formation of CS-HAPheterostructure. The adsorption results of CR dyes indicated outstanding efficiency o Figure 12. Bar illustrations demonstrating E. coli supplemented with different amounts of HAP and CS-HAP-CeO 2 heterostructure (peak enlargement indicates E. coli deprived of HAP and CS-HAP-CeO 2 ). Considerable difference (* p ≤ 0.05) was perceived between control and treatments. Substantial variance in bacteriostatic effect with HAP and CS-HAP-CeO 2 at high quantity was envisaged. * p ≤ 0.05, *** p ≤ 0.001 suggestively different from virgin culture.

Conclusions
In this paper, HAP and CS-HAP-CeO 2 heterostructures were primed by hydrothermal method. The physicochemical data confirms efficacious formation of CS-HAP-CeO 2 heterostructure. The adsorption results of CR dyes indicated outstanding efficiency of CS-HAP-CeO 2 adsorbent. Additionally, the synthesized pristine and CS-HAP-CeO 2 heterostructures unveiled good recyclable use and stability. The enhanced adsorption and antibacterial action of CS-HAP-CeO 2 was mainly attributed to combined influence of CS, HAP and CeO 2 . Conclusively, outcomes of this study endorse an exceptionally capable adsorbent for decontamination of effluents; moreover, helps in reutilization of wastes of Baha city and meritoriously remove the environmental burden and govern the remediation procedure of the pathogenic diseases.

Data Availability Statement:
The data is included in the main text.