The Removal of Brilliant Green Dye from Aqueous Solution Using Nano Hydroxyapatite/Chitosan Composite as a Sorbent

Nanocomposites of natural bone that show some benefits in terms of both composition and microstructure were synthesized by an in situ precipitation method. Hydroxyapatite (Hap) was prepared from cost-effective precursors within chitosan (CS) dissolved in aqueous acetic acid solution. The nanocomposite was synthesized for the removal of brilliant green dye (BG) from a contaminated water solution. The compositional and morphological properties of the nanocomposite were studied by means of FTIR spectroscopy, X-ray diffraction (XRD), SEM, and TEM analysis. Batch experiments were carried out to investigate the effects of pH, contact time, and initial concentration, as well as the adsorbent dosage and zero point charge for the sorbent to determine a suitable medium for the adsorption process. The sorption models using Mories-Weber, Lagrange, and Bangham equations were used to identify the mechanism and reaction order. The isotherm model was carried out using Langmuir, Freundlich, and Dubinin-Radusekevisch-Kanager equations to calculate the adsorption capacity and type of adsorption. Thermodynamic parameters, enthalpy change (∆Ho), entropy change (∆So), and Gibbs free energy (∆Go) were evaluated. All of the results suggest the feasibility of using nanocomposites as a sorbent for brilliant green dye removal.


Introduction
Adsorption with low-cost adsorbents is an effective and economic method for water decontamination. Chitosan is derived by the deacetylation of the naturally occurring biopolymer chitin. Some of the useful features of chitosan include its biocompatibility, biodegradability, nontoxicity, hydrophilicity, and anti-bacterial property [1][2][3]. Currently, it is used in applications in industrial wastewater treatment. Chitosan is an effective material for the sorption of organic compounds such as phenols, metal ions, biphenyls, polychlorinated biphenyls, and proteins. This property is due to the hydroxyl and amino groups on the polymer chains that can act as coordination and electrostatic interaction sites. Chitosan has a high affinity for many types of dyes, except for basic dyes, and it has a greater adsorption capacity compared to other materials.
Hydroxyapatite (Hap, Ca 10 (PO 4 ) 6 (OH) 2 ) is a calcium phosphate that can be employed as an adsorbent for dyes in wastewater treatment. Its properties include high absorption capacity, low cost, and high stability under oxidizing and reducing conditions, as well as low water solubility, availability, excellent bioactivity, biocompatibility, and chemical stability [4,5]. However, the brittleness and weak performance of the mechanical stability of hydroxyapatite limits its use in various applications. The combination of this compound with a polymeric biomaterial is believed to compensate for the poor The disappearance/deformation of the ether bond in the pyranose ring at 1158 cm −1 and the amide III band at 1257 cm −1 are considered as additional evidence for the chemical interconnection of the Hap/CS nanocomposite. However, the broadening of the band around 1050 cm −1 shows the presence of the polymer and its interaction with phosphate groups [23,24]. Therefore, with the analysis of FTIR spectra, we can conclude that there were possible physical interactions (electronic interaction and hydrogen bonds) rather than a chemical reaction between Hap and chitosan. Vibrations of hydroxyl groups show a slight shift towards lower wavenumbers. For pure chitosan, the peak was recorded at 3441 cm −1 , while for composites it appeared at 3428 cm −1 . The slightly lower values of the peak for composites most likely indicate the formation of hydrogen bonds between compounds.

X-ray Diffraction Studies
The X-ray diffraction (XRD) patterns of the Hap/CS nanocomposite are shown in Figure 2. As per the previously reported pattern of chitosan, two main peaks were observed at 10.5° and 22.5°, respectively corresponding to the characteristic peaks of chitosan [22]. The diffractogram presents The disappearance/deformation of the ether bond in the pyranose ring at 1158 cm −1 and the amide III band at 1257 cm −1 are considered as additional evidence for the chemical interconnection of the Hap/CS nanocomposite. However, the broadening of the band around 1050 cm −1 shows the presence of the polymer and its interaction with phosphate groups [23,24]. Therefore, with the analysis of FTIR spectra, we can conclude that there were possible physical interactions (electronic interaction and hydrogen bonds) rather than a chemical reaction between Hap and chitosan. Vibrations of hydroxyl groups show a slight shift towards lower wavenumbers. For pure chitosan, the peak was recorded at 3441 cm −1 , while for composites it appeared at 3428 cm −1 . The slightly lower values of the peak for composites most likely indicate the formation of hydrogen bonds between compounds.

X-ray Diffraction Studies
The X-ray diffraction (XRD) patterns of the Hap/CS nanocomposite are shown in Figure 2. As per the previously reported pattern of chitosan, two main peaks were observed at 10.5 • and 22.5 • , respectively corresponding to the characteristic peaks of chitosan [22]. The diffractogram presents Molecules 2019, 24 FOR PEER REVIEW 3 OH groups and -CH stretching vibrations of CS clearly decrease in intensity in the Hap/CS nanocomposite. The characteristic bands in the range of 1091-1030 cm −1 and at 600 cm −1 can be associated with the stretching and bending vibrations of the PO4 3− group in hydroxyapatite, respectively [22]. The disappearance/deformation of the ether bond in the pyranose ring at 1158 cm −1 and the amide III band at 1257 cm −1 are considered as additional evidence for the chemical interconnection of the Hap/CS nanocomposite. However, the broadening of the band around 1050 cm −1 shows the presence of the polymer and its interaction with phosphate groups [23,24]. Therefore, with the analysis of FTIR spectra, we can conclude that there were possible physical interactions (electronic interaction and hydrogen bonds) rather than a chemical reaction between Hap and chitosan. Vibrations of hydroxyl groups show a slight shift towards lower wavenumbers. For pure chitosan, the peak was recorded at 3441 cm −1 , while for composites it appeared at 3428 cm −1 . The slightly lower values of the peak for composites most likely indicate the formation of hydrogen bonds between compounds.

X-ray Diffraction Studies
The X-ray diffraction (XRD) patterns of the Hap/CS nanocomposite are shown in Figure 2. As per the previously reported pattern of chitosan, two main peaks were observed at 10.5° and 22.5°, respectively corresponding to the characteristic peaks of chitosan [22]. The diffractogram presents  [25]. It was also possible to observe some lower intensity secondary peaks located at 26 • , 40 • , and 47 • , and another less intense peak located at 53.2 • , which corroborate the existence of hydroxyapatite. It was concluded that the Hap/CS nanocomposite is highly crystalline.

Scanning Electron Microscopy
The scanning electron microscopy images are shown in Figure 3. The morphology revealed by the scanning electron microscopy images is a spikey structure that helps in the adsorption of brilliant green dye. The micrograph also shows that the composite surface is rough and has a porous structure with holes and small openings on the surface, indicating that the prepared material may have a good adsorption capacity. This is in a good agreement with the experimental results. The homogeneously distributed pore structure is also supported by the high porosity and high open pore content [26]. Figure 2. X-ray diffraction (XRD) pattern of Hap/CS nanocomposite. Peaks located at 31.4°, 32.2°, and 33°, representing the nanostructured hydroxy apatite [25]. It was also possible to observe some lower intensity secondary peaks located at 26°, 40°, and 47°, and another less intense peak located at 53.2°, which corroborate the existence of hydroxyapatite. It was concluded that the Hap/CS nanocomposite is highly crystalline.

Scanning Electron Microscopy
The scanning electron microscopy images are shown in Figure 3. The morphology revealed by the scanning electron microscopy images is a spikey structure that helps in the adsorption of brilliant green dye. The micrograph also shows that the composite surface is rough and has a porous structure with holes and small openings on the surface, indicating that the prepared material may have a good adsorption capacity. This is in a good agreement with the experimental results. The homogeneously distributed pore structure is also supported by the high porosity and high open pore content [26].

Zeta Potential Distribution Study
The stability of the as-prepared Hap/CS nanocomposite depends on the density of charges existing upon its surface. From the zeta potential measurements shown in Figure 5, the stability of  . X-ray diffraction (XRD) pattern of Hap/CS nanocomposite. Peaks located at 31.4°, 32.2°, and 33°, representing the nanostructured hydroxy apatite [25]. It was also possible to observe some lower intensity secondary peaks located at 26°, 40°, and 47°, and another less intense peak located at 53.2°, which corroborate the existence of hydroxyapatite. It was concluded that the Hap/CS nanocomposite is highly crystalline.

Scanning Electron Microscopy
The scanning electron microscopy images are shown in Figure 3. The morphology revealed by the scanning electron microscopy images is a spikey structure that helps in the adsorption of brilliant green dye. The micrograph also shows that the composite surface is rough and has a porous structure with holes and small openings on the surface, indicating that the prepared material may have a good adsorption capacity. This is in a good agreement with the experimental results. The homogeneously distributed pore structure is also supported by the high porosity and high open pore content [26].

Zeta Potential Distribution Study
The stability of the as-prepared Hap/CS nanocomposite depends on the density of charges existing upon its surface. From the zeta potential measurements shown in Figure 5, the stability of

Zeta Potential Distribution Study
The stability of the as-prepared Hap/CS nanocomposite depends on the density of charges existing upon its surface. From the zeta potential measurements shown in Figure 5, the stability of the Hap/CS nanocomposite can be determined. The zeta potential can greatly influence the nanocomposite's stability in suspension by means of electrostatic repulsion between the particles. Zeta potential values near zero (−43.9 mV) indicate that the particles having a negative zeta potential are expected to interact strongly with cationic additives.
Molecules 2019, 24 FOR PEER REVIEW 5 the Hap/CS nanocomposite can be determined. The zeta potential can greatly influence the nanocomposite's stability in suspension by means of electrostatic repulsion between the particles. Zeta potential values near zero (−43.9 mV) indicate that the particles having a negative zeta potential are expected to interact strongly with cationic additives.

Effect of pH
In order to evaluate the influence of this parameter on the adsorption of BG dye, the experiments were carried out in the pH range of 2.0 to 9.0, with 5 mg•L −1 of BG dye and 0.9 g L −1 of Hap/CS nanocomposite subjected to stirring for 60 min. The variation of the adsorption capacity of the Hap/CS nanocomposite with pH is graphically represented in Figure 6. The maximum uptake of BG dye took place at pH 7.0 and the adsorption capacity decreased with the decrease of pH to a pH value of 2.0. The removal efficiency of the BG dye at pH 7 sharply increased up to 99.5% removal efficiency (optimum) due to the protonation of -NH2 groups of chitosan by H3O + ions in a slightly acidic solution, yielding positively charged -NH3 + groups [26]. Further increases in pH led to a reduction in the removal efficiency of BG dye. This is due to the high amount of OH ions accumulated on the adsorbent surface. Therefore, the electrostatic interaction between the negatively charged adsorbent surface and cationic dye molecules was reduced, and the adsorption of dye molecule on the surface of the Hap/CS nanocomposite decreased. The composite showed a low absorption efficiency in an acidic pH due to the solubility of the composite in the acidic medium; however, in neutral and basic pH solutions it showed a high stability.

Effect of pH
In order to evaluate the influence of this parameter on the adsorption of BG dye, the experiments were carried out in the pH range of 2.0 to 9.0, with 5 mg·L −1 of BG dye and 0.9 g L −1 of Hap/CS nanocomposite subjected to stirring for 60 min. The variation of the adsorption capacity of the Hap/CS nanocomposite with pH is graphically represented in Figure 6. The maximum uptake of BG dye took place at pH 7.0 and the adsorption capacity decreased with the decrease of pH to a pH value of 2.0. The removal efficiency of the BG dye at pH 7 sharply increased up to 99.5% removal efficiency (optimum) due to the protonation of -NH 2 groups of chitosan by H 3 O + ions in a slightly acidic solution, yielding positively charged -NH 3 + groups [26]. Further increases in pH led to a reduction in the removal efficiency of BG dye. This is due to the high amount of OH ions accumulated on the adsorbent surface. Therefore, the electrostatic interaction between the negatively charged adsorbent surface and cationic dye molecules was reduced, and the adsorption of dye molecule on the surface of the Hap/CS nanocomposite decreased. The composite showed a low absorption efficiency in an acidic pH due to the solubility of the composite in the acidic medium; however, in neutral and basic pH solutions it showed a high stability.
Molecules 2019, 24 FOR PEER REVIEW 5 the Hap/CS nanocomposite can be determined. The zeta potential can greatly influence the nanocomposite's stability in suspension by means of electrostatic repulsion between the particles. Zeta potential values near zero (−43.9 mV) indicate that the particles having a negative zeta potential are expected to interact strongly with cationic additives.

Effect of pH
In order to evaluate the influence of this parameter on the adsorption of BG dye, the experiments were carried out in the pH range of 2.0 to 9.0, with 5 mg•L −1 of BG dye and 0.9 g L −1 of Hap/CS nanocomposite subjected to stirring for 60 min. The variation of the adsorption capacity of the Hap/CS nanocomposite with pH is graphically represented in Figure 6. The maximum uptake of BG dye took place at pH 7.0 and the adsorption capacity decreased with the decrease of pH to a pH value of 2.0. The removal efficiency of the BG dye at pH 7 sharply increased up to 99.5% removal efficiency (optimum) due to the protonation of -NH2 groups of chitosan by H3O + ions in a slightly acidic solution, yielding positively charged -NH3 + groups [26]. Further increases in pH led to a reduction in the removal efficiency of BG dye. This is due to the high amount of OH ions accumulated on the adsorbent surface. Therefore, the electrostatic interaction between the negatively charged adsorbent surface and cationic dye molecules was reduced, and the adsorption of dye molecule on the surface of the Hap/CS nanocomposite decreased. The composite showed a low absorption efficiency in an acidic pH due to the solubility of the composite in the acidic medium; however, in neutral and basic pH solutions it showed a high stability.  Figure 7 shows the variation of the amount of adsorbed dye as a function of time, ranging from 5 to 90 min. The experiments were carried out in a solution of pH 7 and with 5 mg·L −1 , 20 mg·L −1 , 50 mg·L −1 , or 80 mg·L −1 of BG dye, as well as 0.9 g·L −1 of Hap/CS nanocomposite. Due to the faster adsorption kinetics achieved with smaller particles, the adsorption was initially rapid and then slow in the later stages. The initial rapid adsorption is presumably due to electrostatic attraction. The slow adsorption in the later stages is related to the decrease in the number of adsorption sites with affinity toward BG dye [27]. The results show that the time required to reach equilibrium was 60 min for all BG dye concentrations.  Figure 7 shows the variation of the amount of adsorbed dye as a function of time, ranging from 5 to 90 min. The experiments were carried out in a solution of pH 7 and with 5 mg•L −1 , 20 mg•L −1 , 50 mgL −1 , or 80 mg•L −1 of BG dye, as well as 0.9 g•L −1 of Hap/CS nanocomposite. Due to the faster adsorption kinetics achieved with smaller particles, the adsorption was initially rapid and then slow in the later stages. The initial rapid adsorption is presumably due to electrostatic attraction. The slow adsorption in the later stages is related to the decrease in the number of adsorption sites with affinity toward BG dye [27]. The results show that the time required to reach equilibrium was 60 min for all BG dye concentrations.  5  45  50  60  65  10  56  60  68  80  20  60  70  75  92  30  71  85  90  100  60  80  89  100  100  90  80  90 100 100

Effect of Adsorbent Dosage on BG dye Removal
The effect of the adsorbent dosage on BG dye removal was determined using dosages of 0.3 gL −1 , 0.5 g•L −1 , and 0.9 g•L −1 of nanocomposite, in addition to 5 mg•L −1 of BG dye at pH 7. The removal efficiency results are displayed in Figure 8. According to Figure 8, the prepared Hap/CS nanocomposite has high potential for dye removal. The removal efficiency of BG dye increased with the increase in the amount of the Hap/CS nanocomposite. This is due to the greater availability of binding sites of the sorbent. The most optimum adsorbent dose for dye removal in the aqueous solution was at 0.9 g•L −1 when the removal efficiency achieved 99.5 %. At this point, the adsorption capacity demonstrated maximum removal efficiency due to the high external surface area of the adsorbent and the available sites for binding dye molecules [28]. A similar results was reported in a study that investigated hydroxyapatite/ chitosan biocomposite for Remazol Blue Dyes Removal that achieved high adsorption capacity for about 95 % dyes removal [17]. Also the same results was reported in a study that investigated Hydroxyapatite-chitosan HAp-CS composite for removal of

Effect of Adsorbent Dosage on BG dye Removal
The effect of the adsorbent dosage on BG dye removal was determined using dosages of 0.3 g·L −1 , 0.5 g·L −1 , and 0.9 g·L −1 of nanocomposite, in addition to 5 mg·L −1 of BG dye at pH 7. The removal efficiency results are displayed in Figure 8. According to Figure 8, the prepared Hap/CS nanocomposite has high potential for dye removal. The removal efficiency of BG dye increased with the increase in the amount of the Hap/CS nanocomposite. This is due to the greater availability of binding sites of the sorbent. The most optimum adsorbent dose for dye removal in the aqueous solution was at 0.9 g·L −1 when the removal efficiency achieved 99.5 %. At this point, the adsorption capacity demonstrated maximum removal efficiency due to the high external surface area of the adsorbent and the available sites for binding dye molecules [28]. A similar results was reported in a study that investigated hydroxyapatite/chitosan biocomposite for Remazol Blue Dyes Removal that achieved high adsorption capacity for about 95 % dyes removal [17]. Also the same results was reported in a study that investigated Hydroxyapatite-chitosan HAp-CS composite for removal of Congo red dye from aqueous solution that exhibited adsorption capacity higher than that of pure chitosan and hydroxyapatite [18].
Molecules 2019, 24 FOR PEER REVIEW 7 Congo red dye from aqueous solution that exhibited adsorption capacity higher than that of pure chitosan and hydroxyapatite [18].

Sorption Model
The study of adsorption kinetics is important because the rate of adsorption (which is one of the criteria for determining the efficiency of an adsorbent) and the mechanism of adsorption can both be concluded from kinetic studies. As a standard parameter for studying the behavior of BG, dye adsorption at the Hap/CS surface is obtained using the Mories-Weber equation [29]. q = Kd (t) 1/2 (1) where q is the amount of dye adsorbed (mg/g), Kd is the intraparticle diffusion rate constant, and t 1/2 is the square root of time. In Figure 9, the Morris-Weber model reveals an initial linear portion which may be due to the boundary layer effect and a second portion which may be due to the intraparticle diffusion effect [30]. The value of the rate constant for the intraparticle diffusion Kd was evaluated as 0.07 (g/g•min −1 ) for BG dye and gives an indication of the mobility of the dye toward the composite.

Sorption Model
The study of adsorption kinetics is important because the rate of adsorption (which is one of the criteria for determining the efficiency of an adsorbent) and the mechanism of adsorption can both be concluded from kinetic studies. As a standard parameter for studying the behavior of BG, dye adsorption at the Hap/CS surface is obtained using the Mories-Weber equation [29].
where q is the amount of dye adsorbed (mg/g), K d is the intraparticle diffusion rate constant, and t 1/2 is the square root of time. In Figure 9, the Morris-Weber model reveals an initial linear portion which may be due to the boundary layer effect and a second portion which may be due to the intraparticle diffusion effect [30]. The value of the rate constant for the intraparticle diffusion K d was evaluated as 0.07 (g/g·min −1 ) for BG dye and gives an indication of the mobility of the dye toward the composite.
Molecules 2019, 24 FOR PEER REVIEW 7 Congo red dye from aqueous solution that exhibited adsorption capacity higher than that of pure chitosan and hydroxyapatite [18].

Sorption Model
The study of adsorption kinetics is important because the rate of adsorption (which is one of the criteria for determining the efficiency of an adsorbent) and the mechanism of adsorption can both be concluded from kinetic studies. As a standard parameter for studying the behavior of BG, dye adsorption at the Hap/CS surface is obtained using the Mories-Weber equation [29]. q = Kd (t) 1/2 (1) where q is the amount of dye adsorbed (mg/g), Kd is the intraparticle diffusion rate constant, and t 1/2 is the square root of time. In Figure 9, the Morris-Weber model reveals an initial linear portion which may be due to the boundary layer effect and a second portion which may be due to the intraparticle diffusion effect [30]. The value of the rate constant for the intraparticle diffusion Kd was evaluated as 0.07 (g/g•min −1 ) for BG dye and gives an indication of the mobility of the dye toward the composite.  The Lagrange equation is employed to determine the order of the adsorption, as cited by Gupta et al. [31].
Log (q e − q) − log q e = −K ads ·t/2.303 (2) where q e is the amount of dye adsorbed at equilibrium (mg·g −1 ) and K ads is the first-order rate constant for dye adsorption onto the sorbent (min −1 ). The linear plot of Log (q e − q) vs. t shows the appropriateness of the above equation and, thus, the first-order nature of the process involved. Pseudo-second-order model: The pseudo-second-order equation based on the adsorption equilibrium capacity can be expressed in the following form [32]: t/q t = 1/K 2 q e 2 + t/q e where k 2 is the rate constant of the second-order adsorption (g mg −1 min −1 ). Similarly, the slope of the plot of t/q t as a function of t was used to determine the second-order rate constant k 2 . The Bangham equation is used to investigate the amount of BG dye that can by introduced into the pores of the nanocomposite [33].
where K o is the proportionality constant and α is the Bangham equation constant. Kinetics parameters for the sorption of BG dye on Hap/CS nanocomposite are shown in Table 1. These results show that the diffusion of dye into composite pores plays a role in the adsorption process [34]. The value of α constants indicates that the sorption of dye is favored to be less than 1.

Langmuir Isotherm
For modeling the equilibrium data, a concentration of 0.9 g/L −1 of composite and different concentration-in equilibrium-of BG dye were applied for the analysis of the isotherm and thermodynamic models. The Langmuir model was widely used to indicate the monolayer of the composite surface, as shown in the following equation [35] and in Figure 10. C e /q e = 1/b Q max + (1/Q max ) C e (5) where b is the monolayer adsorption capacity related to the sorption heat (L·mg −1 ) and Q max is the maximum adsorption capacity (mg·g −1 ). where Kf (mol 1−n L n g −1 ) represents the sorption capacity when the dye equilibrium concentration is equal to 1 and n represents the degree of dependence of sorption on the equilibrium concentration. Favorable adsorption was demonstrated by the fact that the value of n was greater than unity.

Dubinin-Radusekevisch-Kanager Isotherm
The Dubinin-Radusckevisch (D-R) isotherm is more general than the Langmuir model, because it does not assume a homogeneous surface or constant sorption potential. In general, the model is compatible between Gaussian energy distribution and adsorption processes on a heterogeneous surface. The D-R equation is expressed as follows [34]: where q(D-R) is the theoretical adsorption capacity (mg•g −1 ), ß is the activity coefficient related to the mean sorption energy (mol 2 kJ −2 ), ε is the Polanyi potential, R is the ideal gas constant (0.008314 KJmol −1 K −1 ), and T is the absolute temperature in Kelvin (K). E (kJ mol −1 ) is defined as the free energy change required to transfer 1 mole of dye from the solution to the solid surface, which is equal to: The magnitude of E is useful to estimate the type of sorption reaction. If E is in the range of 8-16 kJ mol −1 , the sorption is governed by chemical ion exchange. In the case of E < 8 kJ mol −1 , physical forces may affect the sorption. On the other hand, the sorption may be dominated by particle

Freundlich Isotherm
The Freundlich expression is an empirical equation describing sorption to a heterogeneous surface [36]. The Freundlich adsorption is presented in Equation (6) and shown in Figure 11: ln q e = ln K f + 1/n ln C e (6) where K f (mol 1−n L n g −1 ) represents the sorption capacity when the dye equilibrium concentration is equal to 1 and n represents the degree of dependence of sorption on the equilibrium concentration. Favorable adsorption was demonstrated by the fact that the value of n was greater than unity.
Molecules 2019, 24 FOR PEER REVIEW 9 where Kf (mol 1−n L n g −1 ) represents the sorption capacity when the dye equilibrium concentration is equal to 1 and n represents the degree of dependence of sorption on the equilibrium concentration. Favorable adsorption was demonstrated by the fact that the value of n was greater than unity.

Dubinin-Radusekevisch-Kanager Isotherm
The Dubinin-Radusckevisch (D-R) isotherm is more general than the Langmuir model, because it does not assume a homogeneous surface or constant sorption potential. In general, the model is compatible between Gaussian energy distribution and adsorption processes on a heterogeneous surface. The D-R equation is expressed as follows [34]: ln q = ln q(D-R) − ßε 2 (7) ε = RT ln(1 + 1/Ce) (8) where q(D-R) is the theoretical adsorption capacity (mg•g −1 ), ß is the activity coefficient related to the mean sorption energy (mol 2 kJ −2 ), ε is the Polanyi potential, R is the ideal gas constant (0.008314 KJmol −1 K −1 ), and T is the absolute temperature in Kelvin (K). E (kJ mol −1 ) is defined as the free energy change required to transfer 1 mole of dye from the solution to the solid surface, which is equal to: The magnitude of E is useful to estimate the type of sorption reaction. If E is in the range of 8-16

Dubinin-Radusekevisch-Kanager Isotherm
The Dubinin-Radusckevisch (D-R) isotherm is more general than the Langmuir model, because it does not assume a homogeneous surface or constant sorption potential. In general, the model is compatible between Gaussian energy distribution and adsorption processes on a heterogeneous surface. The D-R equation is expressed as follows [34]: ln q = ln q (D-R) − ßε 2 (7) ε = RT ln(1 + 1/C e ) (8) where q (D-R) is the theoretical adsorption capacity (mg·g −1 ), ß is the activity coefficient related to the mean sorption energy (mol 2 kJ −2 ), ε is the Polanyi potential, R is the ideal gas constant (0.008314 KJmol −1 K −1 ), and T is the absolute temperature in Kelvin (K). E (kJ mol −1 ) is defined as the free energy change required to transfer 1 mole of dye from the solution to the solid surface, which is equal to: The magnitude of E is useful to estimate the type of sorption reaction. If E is in the range of 8-16 kJ mol −1 , the sorption is governed by chemical ion exchange. In the case of E < 8 kJ mol −1 , physical forces may affect the sorption. On the other hand, the sorption may be dominated by particle diffusion if E > 16 kJ mol −1 [37]. From the results of the D-R model simulation shown in Table 2, the E value was 8.2 kJ mol −1 for BG dye in the range of 8-16 kJ mol −1 , indicating that the sorption was governed by physical-chemical adsorption.

Thermodynamic Parameters
In order to investigate the effect of temperature on the adsorption of BG dye onto on the Hap/CS nanocomposite, the distribution coefficient K d (L·g −1 ) was calculated at temperatures of 288, 298, 313, and 323 K using Equation (10). Thermodynamic parameters of the entropy change (∆S o ) and enthalpy change (∆H o ) were calculated from the intercept and slope of the plot of ln K d against 1/T, respectively [38,39]. Ln The other thermodynamic parameter, Gibbs free energy (∆G o ), was calculated by: where R is the universal gas constant (8.314 J mol −1 K −1 ) and T is the temperature (K). The K d value increased with increasing temperature, revealing the adsorption of BG dye onto the Hap/CS nanocomposite to be endothermic. Thermodynamic parameters, specifically the enthalpy change (∆H o ) and the entropy change (∆S o ), were calculated from the data of Equation (10) and are shown in Figure 12. The other thermodynamic parameter, Gibbs free energy (∆G o ), was calculated by Equation (11). A positive ∆H o indicates that the adsorption of BG dye onto the Hap/CS nanocomposite is endothermic. For entropy change (∆S o ), a positive sign means that the adsorption of BG dye onto sorbents is a random reaction, as shown in Table 3. Meanwhile, a negative value of ∆G o indicates that the adsorption of BG dye onto on the Hap/CS nanocomposite is feasible and thermodynamically spontaneous. In addition, the reaction was observed to proceed physically, and these results are in good agreement with the D-R isotherm.

Synthesis of the Hap/CS Nanocomposite
The starting materials included: Ca(NO3)2.4H2O, (NH4)2HPO4, triphenylphosphate, and chitosan. All reagents were of AR grade and used without further purification. Deionized water was used in all synthesis steps. The synthesis of the Hap/CS nanocomposite is shown in Figure 13. For the first step in the preparation of the Hap/CS nanocomposite, chitosan was dissolved in 0.5 % (v/v) acetic acid aqueous solution until a homogeneous chitosan solution was obtained. Appropriate amounts of triphenylphosphate were added and stirred for about 1 h. A gelatinous precipitate of nano chitosan was then formed. In the second step, Ca(NO3)2.4H2O and (NH4)2HPO4 were dissolved in deionized water separately. The pH of each aqueous solution was adjusted to 11 using 25% NH4OH solution.
During the dropwise addition of Ca(NO3)2.4H2O aqueous solution under conditions of vigorous stirring with (NH4)2HPO4 at room temperature for about 1 h [40], nano chitosan was added. This produced a milky and gelatinous precipitate. The mixture was stirred for 1 h and dried at 80 °C for 4 h. Then, calcination occurred at temperatures of 800 °C for 1 h, 1000 °C for 2 h, and 1200 °C for 1 h.

Synthesis of the Hap/CS Nanocomposite
The starting materials included: Ca(NO 3 ) 2 .4H 2 O, (NH 4 ) 2 HPO 4 , triphenylphosphate, and chitosan. All reagents were of AR grade and used without further purification. Deionized water was used in all synthesis steps. The synthesis of the Hap/CS nanocomposite is shown in Figure 13. For the first step in the preparation of the Hap/CS nanocomposite, chitosan was dissolved in 0.5 % (v/v) acetic acid aqueous solution until a homogeneous chitosan solution was obtained. Appropriate amounts of triphenylphosphate were added and stirred for about 1 h. A gelatinous precipitate of nano chitosan was then formed. In the second step, Ca(NO 3

Analytical Instruments
To characterize our product, we used X-ray diffraction (MiniFlex, HyPix-400 MF. Japan) to determine the structure of our composite and scanning electron microscopy (SEM) (JSM-6510 LV JEOL. Japan) to give us an idea of the morphology of the surface of our adsorbent. The identification of functional groups in the Hap/CS nanocomposite as well as the interfacial modification were analyzed by FTIR analysis (Thermo Fisher Nicolete IS10) within the scanning range of 400-4000 cm −1 . Transmission electron microscopy (JEM 2100, HRTEM, JEOL) was also used. Surface area was calculated using an St 4 on NOVA touch 4LX instrument with nitrogen. In order to determine the influencing effects of the parameters on the adsorption in the studied systems, the measurement of the quantity of BG adsorbed on our composite was carried out with an analysis wavelength of λmax = 626 nm on a Cintra 101 double beam spectrophotometer.

Adsorption Studies of BG Dye
The influence of key adsorption parameters (pH, contact time, initial concentration, and adsorbent dosage) on the adsorption behavior of BG on the Hap/CS nanocomposite was explored using batch experiments. Table 4 shows the characteristics of brilliant green dye. The molecular structure of brilliant green dye is displayed in Figure 14. Figure 15 shows the UV-vis absorption spectra of neat/ blank BG in aqueous medium, with max wavelength absorption centered at 625 nm. The absorption measurements were repeated at definite time intervals. It was observed that the neat BG aqueous solution is stable during the time range of the adsorption study [41].

Analytical Instruments
To characterize our product, we used X-ray diffraction (MiniFlex, HyPix-400 MF. Japan) to determine the structure of our composite and scanning electron microscopy (SEM) (JSM-6510 LV JEOL. Japan) to give us an idea of the morphology of the surface of our adsorbent. The identification of functional groups in the Hap/CS nanocomposite as well as the interfacial modification were analyzed by FTIR analysis (Thermo Fisher Nicolete IS10) within the scanning range of 400-4000 cm −1 . Transmission electron microscopy (JEM 2100, HRTEM, JEOL) was also used. Surface area was calculated using an St 4 on NOVA touch 4LX instrument with nitrogen. In order to determine the influencing effects of the parameters on the adsorption in the studied systems, the measurement of the quantity of BG adsorbed on our composite was carried out with an analysis wavelength of λ max = 626 nm on a Cintra 101 double beam spectrophotometer.

Adsorption Studies of BG Dye
The influence of key adsorption parameters (pH, contact time, initial concentration, and adsorbent dosage) on the adsorption behavior of BG on the Hap/CS nanocomposite was explored using batch experiments. Table 4 shows the characteristics of brilliant green dye. The molecular structure of brilliant green dye is displayed in Figure 14. Figure 15 shows the UV-vis absorption spectra of neat/ blank BG in aqueous medium, with max wavelength absorption centered at 625 nm. The absorption measurements were repeated at definite time intervals. It was observed that the neat BG aqueous solution is stable during the time range of the adsorption study [41].   Values of pH, contact time, initial concentration, and adsorbent dosage were varied from pH 1 to 9, 5 to 90 min, 5 to 80 mgL −1 of BG dye, and 0.3, 0.5, or 0.9 mgL −1 of Hap/CS nanocomposite, respectively. The initial and final concentrations of BG were estimated using a Hach Lange spectrophotometer. The equilibrium adsorption capacity qe (mg/g) and the percentage removal were determined using Equations (12) and (13).
where qe (mg/g) denotes the equilibrium adsorption capacity, Co and Ce are the initial and equilibrium concentrations (mg/L) of GB, and V (L) and W (g) are the volume of the solution and weight of the adsorbent, respectively.

Conclusion
Hydroxyapatite/chitosan nanocomposite was prepared and used for the removal of BG dye from an aqueous solution. The following conclusions were made based on the results of the present study: The Hap/CS nanocomposite was characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transition electron microscopy (TEM), and X-ray diffraction analysis (XRD) techniques.
The sorption of BG dye was found to increase with the increase in contact time, and the adsorbent dosage reached equilibrium at 60 min.
The experimental data are best correlated by a first-order kinetic model. The Morris-Weber model showed that the rate constant for the intrapore diffusion Kd was evaluated as 0.07 (g/g•min −1 ). Bangham equation showed that the sorption of dye was favored to be less than 1.
The equilibrium data were fitted to Langmuir, Freundlich, and Dubinin-Radushkevich isotherm models and it was found that the equilibrium data are best described by the Dubinin-Radushkevich  Values of pH, contact time, initial concentration, and adsorbent dosage were varied from pH 1 to 9, 5 to 90 min, 5 to 80 mgL −1 of BG dye, and 0.3, 0.5, or 0.9 mgL −1 of Hap/CS nanocomposite, respectively. The initial and final concentrations of BG were estimated using a Hach Lange spectrophotometer. The equilibrium adsorption capacity qe (mg/g) and the percentage removal were determined using Equations (12) and (13).
Adsorption % = ( ) × 100 (13) where qe (mg/g) denotes the equilibrium adsorption capacity, Co and Ce are the initial and equilibrium concentrations (mg/L) of GB, and V (L) and W (g) are the volume of the solution and weight of the adsorbent, respectively.

Conclusion
Hydroxyapatite/chitosan nanocomposite was prepared and used for the removal of BG dye from an aqueous solution. The following conclusions were made based on the results of the present study: The Hap/CS nanocomposite was characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transition electron microscopy (TEM), and X-ray diffraction analysis (XRD) techniques.
The sorption of BG dye was found to increase with the increase in contact time, and the adsorbent dosage reached equilibrium at 60 min.
The experimental data are best correlated by a first-order kinetic model. The Morris-Weber model showed that the rate constant for the intrapore diffusion Kd was evaluated as 0.07 (g/g•min −1 ). Bangham equation showed that the sorption of dye was favored to be less than 1.
The equilibrium data were fitted to Langmuir, Freundlich, and Dubinin-Radushkevich isotherm models and it was found that the equilibrium data are best described by the Dubinin-Radushkevich Values of pH, contact time, initial concentration, and adsorbent dosage were varied from pH 1 to 9, 5 to 90 min, 5 to 80 mg·L −1 of BG dye, and 0.3, 0.5, or 0.9 mg·L −1 of Hap/CS nanocomposite, respectively. The initial and final concentrations of BG were estimated using a Hach Lange spectrophotometer. The equilibrium adsorption capacity q e (mg/g) and the percentage removal were determined using Equations (12) and (13).
Adsorption Capacity q e = (C 0 − C e ) V W (12) Adsorption % = (C 0 − C e ) C 0 × 100 (13) where q e (mg/g) denotes the equilibrium adsorption capacity, C o and C e are the initial and equilibrium concentrations (mg/L) of GB, and V (L) and W (g) are the volume of the solution and weight of the adsorbent, respectively.

Conclusions
Hydroxyapatite/chitosan nanocomposite was prepared and used for the removal of BG dye from an aqueous solution. The following conclusions were made based on the results of the present study: The Hap/CS nanocomposite was characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transition electron microscopy (TEM), and X-ray diffraction analysis (XRD) techniques.
The sorption of BG dye was found to increase with the increase in contact time, and the adsorbent dosage reached equilibrium at 60 min.
The experimental data are best correlated by a first-order kinetic model. The Morris-Weber model showed that the rate constant for the intrapore diffusion K d was evaluated as 0.07 (g/g·min −1 ). Bangham equation showed that the sorption of dye was favored to be less than 1.
The equilibrium data were fitted to Langmuir, Freundlich, and Dubinin-Radushkevich isotherm models and it was found that the equilibrium data are best described by the Dubinin-Radushkevich isotherm models. The E value was 8.2 kJ mol −1 for BG dye in the range of 8-16 kJ mol −1 , indicating that the sorption was governed by physical-chemical adsorption.
The thermodynamic results showed the feasibility as well as the spontaneous and endothermic nature of the adsorption of BG dye onto Hap/Chitosan nanocomposite. Based on these results, it can be concluded that the Hap/Chitosan nanocomposite is an effective sorbent for the removal of BG from aqueous media.