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

molecules

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

Ahmed Ragab, Inas Ahmed * and Dina Bader

Department of Chemistry, Collage of Science, King Khalid University, Abha 61413, Saudi Arabia; [email protected] (A.R.); [email protected] (D.B.) * Correspondence: [email protected]; Tel.: +966-55-823-7133

Received: 7 December 2018; Accepted: 25 February 2019; Published: 28 February 2019

Abstract: 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.

Keywords: nanoparticles; nanocomposites; chitosan; hydroxyapatite; kinetic and isotherm models; brilliant green dye

1. 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 [1–3]. 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, Ca10(PO4)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

Molecules 2019, 24, 847; doi:10.3390/molecules24050847 www.mdpi.com/journal/molecules Molecules 2019, 24, 847 2 of 16 mechanical properties of hydroxyapatite and result in improved properties, such as a better modulus, stiffness, and strength. Chitosan is a potential biopolymer which can be combined with hydroxyapatite to improve its efficiency for contaminant removal in wastewater treatment. Moreover, highly porous nanosized materials with active surface sites have been used in the treatment of wastewater [6–8]. Composites composed of nanomaterials have recently gained increasing interest as sustainable and efficient adsorbents for wastewater treatment. Nanoparticles have a high reactive capacity due to their high surface area. They can be functionalized with various chemical groups to increase their affinity towards a given compound. However, the separation difficulty of these particles always exists in practice due to their ultrafine size [9,10]. Wastewater contamination with harmful dyes is a serious matter in modern industrial societies because of its low degradability, high toxicity, and high stability to photodegradation [11]. Brilliant green (BG) is a typical example of an industrially relevant toxic cationic dye with harmful effects on humans. Brilliant green dye is used for various purposes, e.g., as a biological stain, a dermatological agent, and an additive to poultry feed to prevent the formation of parasites and fungi [12,13]. It is also extensively used in textile dying and paper printing [14,15]. It causes irritation to the gastrointestinal tract in humans; symptoms include nausea, vomiting, diarrhea, and irritation to the respiratory tract resulting in cough and shortness of breath. It may also cause dermatitis upon skin contact, leading to redness and pain [16]. The synthesis and characterized hydroxyapatite/chitosan biocomposite for Remazol Blue Dyes Removal were studied. Hydroxyapatite was extracted from egg shell and incorporated with commercial chitosan to improve its mechanical strength and adsorption capacity. The observed results show that the developed adsorbent achieved the highest adsorption capacity for about 95% dyes removal. The findings perhaps can be used as a fundamental knowledge for the development of dyes wastewater treatment mainly in textile industry [17]. Hydroxyapatite-chitosan HAp-CS composite was developed via embedding of HAp into CS and used for removal of Congo red dye from aqueous solution. The kinetic data were best described by the pseudo-second-order model (R2 > 0.9999), while thermodynamic investigation of CR adsorption by HAp-CS composite confirmed a spontaneous adsorption. FT-IR and X-ray photoelectron spectroscopy studies showed that possible pathways for CR adsorption may include surface complexation, ion exchange and hydrogen bonding. HAp-CS composite containing 50 wt% of CS exhibited adsorption capacity higher than that of pure CS, HA [18]. Therefore, in this work, we attempt to synthesize, characterize, evaluate the Hap/chitosan (CS) nanocomposite and extend the use of HAp-CS nanocomposite to study its efficiency for the removal of brilliant green dye from aqueous solutions through the experimental method, using sorption models and thermodynamic parameters.

2. Results and Discussion

2.1. Structural and Surface Characterization of the Adsorbent

2.1.1. FTIR Study To verify the integrity of the adsorbent, Hap/CS nanocomposites were characterized by FTIR, and the results are shown in Figure1. The FTIR spectrum of pure CS shows a characteristic band around 3441 cm−1, which corresponds to stretching vibrations of hydroxyl groups and overlaps with the −1 -NH2 stretching vibration peak of chitosan [19,20]. The visible bands ranging from 1658 to 1609 cm represent the C-O stretching vibrations and the N-H in-plane bending vibrations characteristic of amide I and II structures [21]. Also, the characteristic peak of the amide III structure is visible at 1267 cm−1. Bands visible around 2925 cm−1 are attributed to -CH vibrations, while a peak around −1 1380 cm is attributed to -CH3 and -CH2 in-plane deformation vibrations. A characteristic peak around 1078 cm−1 most likely corresponds to glucosamine stretching vibrations. The FTIR spectrum of the Hap/CS nanocomposite revealed some important changes. The peak at 3428 cm−1 can be attributed to stretching vibrations of -OH groups. This peak has a lower wave number, which is MoleculesMolecules2019, 201924, 847, 24 FOR PEER REVIEW 33 of 16

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 characteristicMolecules of -OH2019, 24groups FOR PEER subjectREVIEW to intermolecular or intramolecular hydrogen bonds.3 The peaks 3− relatedassociated to the -OHwith groupsthe stretching and -CH and stretching bending vibrationsvibrations of of CSthe clearlyPO4 group decrease in hydroxyapatite, in intensity in the respectivelyOH groups [22]. and -CH stretching vibrations of CS clearly decrease in intensity in the Hap/CS Hap/CS nanocomposite. The characteristic bands in the range of 1091–1030 cm−1 and at 600 cm−1 nanocomposite. The characteristic bands in the range of 1091–1030 cm−1 and at 600 cm−1 can be 3− can be associatedassociated with with thethe stretchingstretching and and bending bending vibrations vibrations of the of PO the43− POgroup4 ingroup hydroxyapatite, in hydroxyapatite, respectivelyrespectively [22]. [22]. 270

220 270 1267 2866 2925 668 1380 1434

170 1658 220 (a)Pure chitosan 1609 1158 1267 1078 3441 2866 2925 668 1380 120 1434

170 1658 (a)Pure chitosan 1609 1158 1078 70 120 3441 1611 3428

70 1611 20 3428 (b) Hap/CS nanocomposite 609 1091

20 (b) Hap/CS nanocomposite 1030 -30 609 1091 3950-30 3450 2950 2450 1950 1450 1030 950 450 3950 3450 2950 2450 1950 1450 950 450 Figure 1. FTIR spectra of (a) pure chitosan and (b) hydroxyapatite (Hap)/chitosan (CS) Figurenanocomposite. 1. FTIRFigure spectra 1. FTIR of (spectraa) pure of chitosan (a) pure and chitosan (b) hydroxyapatite and (b) hydroxyapatite (Hap)/chitosan (Hap)/chitosan (CS) nanocomposite. (CS) nanocomposite. −1 The disappearance/deformationThe disappearance/deformation ofof thethe etherether bond in in the the pyranose pyranose ring ring at at1158 1158 cm− cm1 and andthe the The disappearance/deformation−1 of the ether bond in the pyranose ring at 1158 cm−1 and the amideamide III band III band at 1257 at 1257 cm cm−1are are consideredconsidered as as additional additional evidence evidence for the for chemical the chemical interconnection interconnection of amide III band at 1257 cm−1 are considered as additional evidence for the chemical interconnection of −1 of the Hap/CS nanocomposite. However, the broadening of the band around 1050−1 cm shows the Hap/CSthe Hap/CS nanocomposite. nanocomposite. However, However, the the broadeningbroadening ofof the the band band around around 1050 1050 cm− 1cm shows shows the the the presencepresencepresence ofof thethe of polymerthepolymer polymer and and its its interaction interaction withwith with phosphatephosphate phosphate groups groups groups [23,24] [23,24] [.23 Therefore,,.24 Therefore,]. Therefore, with withthe withthe the analysisanalysis ofanalysis FTIR of FTIR spectra,of FTIR spectra, spectra, we we can we can concludecan conclude conclude that that that there therethere were were possible possible possible physical physical physical interactions interactions interactions (electronic (electronic (electronic interactioninteractioninteraction and hydrogenand andhydrogen hydrogen bonds) bonds) ratherbonds) rather thanrather a thanthan chemical a chemical reaction reaction reaction between between between Hap Hap and Hap and chitosan. andchitosan. chitosan. Vibrations VibrationsVibrations of hydroxyl of hydroxyl groups groups show show a aslight slight shiftshift towards lower lower wavenumbers. wavenumbers. For Forpure pure chitosan, chitosan, of hydroxyl groups show a slight shift−1 towards lower wavenumbers. For pure−1 chitosan, the peak was the peakthe peakwas recordedwas recorded at 3441 at 3441 cm cm−1, while, while for for composites composites it it appeared appeared at 3428at 3428 cm cm. The−1. Theslightly slightly lower lower recorded at 3441 cm−1, while for composites it appeared at 3428 cm−1. The slightly lower values of the valuesvalues of the of peakthe peak for forcomposites composites most most likely likely indicateindicate thethe formation formation of ofhydrogen hydrogen bonds bonds between between peak for compositescompounds. most likely indicate the formation of hydrogen bonds between compounds. compounds. 2.1.2. X-ray2.1.2. Diffraction X-ray Diffraction Studies Studies 2.1.2. X-ray Diffraction Studies The X-ray diffraction (XRD) patterns of the Hap/CS nanocomposite are shown in Figure 2. As TheThe X-rayper theX-ray diffractionpreviously diffraction reported (XRD) (XRD) pattern patternspatterns of chitosan, of of the the Hap/CStw Hap/CSo main nanocomposite peaks nanocomposite were observed are shown at are 10.5° shown inand Figure 22.5°, in 2. Figure As 2. ◦ ◦ As perper the therespectively previously previously corresponding reportedreported pattern to the characteristic of of chitosan, chitosan, peaks tw twoo ofmain mainchitosan peaks peaks [22]. were The were observeddiffractogram observed at 10.5°presents at 10.5 and 22.5°,and 22.5 , respectivelyrespectively corresponding corresponding to to the the characteristic characteristic peakspeaks of chitosan chitosan [22]. [22 ].The The diffractogram diffractogram presents presents

◦ ◦ 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. Molecules 2019, 24 FOR PEER REVIEW 4 Molecules 2019, 24 FOR PEER REVIEW 4 Figure 2. X-ray diffraction (XRD) pattern of Hap/CS nanocomposite. Peaks located at 31.4°, 32.2°, and Figure33°, representing 2. X-ray diffraction the nanostructured (XRD) pattern hydroxy of Hap/CS apatite nanocomposite. [25]. It was also Peaks possible located to observe at 31.4°, some 32.2°, lower and 33°,intensity representing secondary the peaksnanostructured located at hydroxy 26°, 40°, apatiteand 47°, [25]. and It another was also less possible intense to peakobserve located some at lower 53.2°, intensitywhich corroborate secondary thepeaks existence located of at hydroxyapatite. 26°, 40°, and 47°, It wasand concludedanother less that intense the Hap/CS peak located nanocomposite at 53.2°, which corroborate the existence of hydroxyapatite. It was concluded that the Hap/CS nanocomposite Moleculesis highly2019, 24 crystalline., 847 4 of 16 is highly crystalline. 2.1.3. Scanning Electron Microscopy 2.1.3.2.1.3. Scanning Scanning Electron MicroscopyMicroscopy The scanning electron microscopy images are shown in Figure 3. The morphology revealed by the scanningTheThe scanning scanning electron electron electron microscopy microscopy microscopy images images images is a spikey are are sh shown structureown in in FigureFigure that helps 33.. The in morphologythe adsorption revealed of brilliant by thethegreen scanning scanning dye. The electron electron micrograph microscopy microscopy also shows images images that is is athe a spikey spikey comp structure structureosite surface that that ishelps helps rough in in andthe the adsorption has adsorption a porous of of structurebrilliant brilliant greengreenwith holesdye. dye. The and The micrograph micrographsmall openings also also onshows shows the surface,that that the the indicacomp compositeositeting thatsurface surface the isprepared is rough rough and andmaterial has has a a porousmay porous have structure structure a good withwithadsorption holes holes and andcapacity. small small openingsThis openings is in ona on good the the surface, surface,agreement indica indicating withting the that that experimental the the prepared prepared results. material material The may may homogeneously have have a a good good adsorptionadsorptiondistributed capacity. capacity.pore structure This This is isis in inalso a a good good supported agreement agreement by the with with high the the porosity experimental experimental and high results. results. open The Thepore homogeneously homogeneously content [26]. distributeddistributed pore pore structure structure is is also also supported supported by by the the high high porosity porosity and and high high open open pore pore content [26]. [26].

Figure 3. SEM images of Hap/CS nanocomposite: (a) 1-µm view; (b) 50-µm view. FigureFigure 3. 3. SEMSEM images images of of Hap/CS Hap/CS nanocomposite: nanocomposite: (a ()a )1-µm 1-µm view; view; (b (b) )50-µm 50-µm view. view. 2.1.4. Transmission Electron Microscopy 2.1.4. Transmission Electron Microscopy 2.1.4. Transmission Electron Microscopy TEM photographs of the prepared Hap/CS nanocomposite are shown in Figure 4. The TEM TEM photographs of the prepared Hap/CS nanocomposite are shown in Figure4. The TEM imagesTEM indicate photographs that the of crystallitesthe prepared have Hap/CS a sphe nanore-likecomposite shape; theare compositeshown in Figureexhibits 4. noThe serious TEM images indicate that the crystallites have a sphere-like shape; the composite exhibits no serious imagesaggregation indicate and that Hap the nucleates crystallites on chitosan. have a Thesphe sizere-like of theshape; particles the compositeis about 40–70 exhibits nm. Theno seriousspecific aggregation and Hap nucleates on chitosan. The size of the particles is about 40–70 nm. The speciﬁc aggregationsurface area and of the Hap composite nucleates is on 76.39 chitosan. m2/g. The size of the particles is about 40–70 nm. The specific surface area of the composite is 76.39 m2/g. surface area of the composite is 76.39 m2/g.

FigureFigure 4.4. TEMTEM imagesimages ofof Hap/CSHap/CS nanocomposite: ( (aa)) 200 200 nm; nm; ( (bb)) 1 1 µm.µm. Figure 4. TEM images of Hap/CS nanocomposite: (a) 200 nm; (b) 1 µm. 2.1.5. Zeta Potential Distribution Study 2.1.5. Zeta Potential Distribution Study 2.1.5. TheZeta stability Potential of Distribution the as-prepared Study Hap/CS nanocomposite depends on the density of charges existing The stability of the as-prepared Hap/CS nanocomposite depends on the density of charges upon its surface. From the zeta potential measurements shown in Figure5, the stability of the Hap/CS existingThe uponstability its surface.of the as-prepar From theed zeta Hap/CS potential nanocomposite measurements depends shown onin Figurethe density 5, the ofstability charges of existingnanocomposite upon its cansurface. be determined. From the zeta The potential zeta potential measurements can greatly shown inﬂuence in Figure the 5, nanocomposite’s the stability of 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 Molecules 2019, 24 FOR PEER REVIEW 5 the Hap/CS nanocomposite can be determined. The zeta potential can greatly influence the 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. 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 ZetaMolecules potential2019, values24, 847 near zero (−43.9 mV) indicate that the particles having a negative zeta potential5 of 16 are expected to interact strongly with cationic additives. are expected to interact strongly with cationic additives.

FigureFigure 5.5. Zeta potential distribution of Hap/CS nanocomposite. Figure 5. ZetaZeta potential potential distribution distribution of ofHap/CS Hap/CS nanocomposite. nanocomposite. 2.2. Effect of pH 2.2. Effect of pH In order toto evaluate the influenceinfluence ofof thisthis parameterparameter onon thethe adsorption ofof BGBG dye, the experiments In order to evaluate the influence of this parameter on the adsorption−1 of BG dye, the experiments−1 werewere carriedcarried outout inin thethe pHpH rangerange of of 2.0 2.0 to to 9.0, 9.0, with with 5 5 mg mg·L∙L−1of ofBG BG dye dye and and 0.9 0.9 g g L L−1of ofHap/CS Hap/CS 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 subjectedsubjected to to stirring stirring for for 60 min.60 min. The variationThe variation of the of adsorption the adsorption capacity capacity of the Hap/CS of the nanocomposite subjected to stirring for 60 min. The variation of the adsorption capacity of the nanocompositeHap/CS nanocomposite with pH with is graphically pH is graphically represented represented in Figure in 6Figure. The 6. maximum The maximum uptake uptake of BG of dye BG Hap/CS nanocomposite with pH is graphically represented in Figure 6. The maximum uptake of BG tookdye took place place at pH at 7.0pH and7.0 and the adsorptionthe adsorption capacity capacity decreased decreased with with the the decrease decrease of pH of pH to ato pH a pH value value of dye took place at pH 7.0 and the adsorption capacity decreased with the decrease of pH to a pH value 2.0.of 2.0. The The removal removal efficiency efficiency of of the the BG BG dye dye at at pH pH 7 7 sharply sharply increased increased up upto to 99.5%99.5% removalremoval efficiencyefficiency of 2.0. The removal efficiency of the BG dye at pH 7 sharply increased up+ to 99.5% removal efficiency (optimum)(optimum) duedue to to the the protonation protonation of -NHof -NH2 groups2 groups of chitosan of chitosan by H3 Oby ionsH3O in+ ions a slightly in a acidicslightly solution, acidic (optimum) due to the protonation of+ -NH2 groups of chitosan by H3O+ ions in a slightly acidic yieldingsolution, positivelyyielding positively charged charged -NH3 groups -NH3+ groups [26]. Further [26]. Further increases increases in pH in led pH to led a reductionto a reduction in the in solution, yielding positively charged -NH3+ groups [26]. Further increases in pH led to a reduction in removalthe removal efficiency efficiency of BG of dye. BGThis dye. is This due tois due the high to the amount high amount of OH ions of accumulatedOH ions accumulated on the adsorbent on the the removal efficiency of BG dye. This is due to the high amount of OH ions accumulated on the surface.adsorbent Therefore, surface. theTherefore, electrostatic the electrostatic interaction betweeninteraction the between negatively the charged negatively adsorbent charged surface adsorbent and adsorbent surface. Therefore, the electrostatic interaction between the negatively charged adsorbent cationicsurface and dye moleculescationic dye was molecules reduced, was and reduced, the adsorption and the of dyeadsorption molecule of ondye the molecule surface ofon the the Hap/CS surface surface and cationic dye molecules was reduced, and the adsorption of dye molecule on the surface nanocompositeof the Hap/CS nanocomposite decreased. The compositedecreased. showedThe compos a lowite absorption showed a efficiencylow absorption in an acidic efficiency pH due in an to of the Hap/CS nanocomposite decreased. The composite showed a low absorption efficiency in an theacidic solubility pH due of to thethe compositesolubility of in the the composite acidic medium; in the however,acidic medium; in neutral however, and basic in neutral pH solutions and basic it acidic pH due to the solubility of the composite in the acidic medium; however, in neutral and basic showedpH solutions a high it stability.showed a high stability. pH solutions it showed a high stability.

120 120 99.5 100 91.7 99.5 95 95 100 88.3 91.7 95 95 80 88.3 80 80 80 60 60 55 60 60 55

40 40 Adsorbance, % Adsorbance, % 20 20

0 0 23456789 23456789pH pH Figure 6. Inﬂuence of pH on the adsorption of brilliant green (BG) dye by Hap/CS nanocomposite.

Molecules 2019, 24 FOR PEER REVIEW 6 Molecules 2019, 24, 847 6 of 16 Figure 6. Influence of pH on the adsorption of brilliant green (BG) dye by Hap/CS nanocomposite. 2.3. Effect of Contact Time and Initial BG Dye Concentration 2.3. Effect of Contact Time and Initial BG Dye Concentration Figure7 shows the variation of the amount of adsorbed dye as a function of time, ranging from 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, 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 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 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 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 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 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 toward BG dye [27]. The results show that the time required to reach equilibrium was 60 min for all BG dye concentrations. BG dye concentrations.

110

100

70 Adsorption % Adsorption

60 80 mgL-1 of BG dye 50 mgL-1 of BG dye 50 20 mgL-1 of BG dye 5 mgL-1 of BG dye 40 0 20406080100 Time, min Removal, % Time (min) 5 mg∙L−1 of BG dye 20 mg∙L−1 of BG dye 50 mg∙L−1 of BG dye 80 mg∙L−1 of BG dye 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

Figure 7. 7. InfluenceInﬂuence of ofstirring stirring time time on the on adsorption the adsorption of various of various concentrations concentrations of BG dye of BGby Hap/CS dye by nanocomposite.Hap/CS nanocomposite.

2.4. Effect of Adsorbent Dosage on BG dye Removal −1 The effect of the adsorbent dosagedosage onon BGBG dyedye removalremoval was was determined determined using using dosages dosages of of 0.3 0.3 g ·gLL −1,, −1 −1 −1 0.5 g·∙L−1, ,and and 0.9 0.9 g g∙·LL−1 ofof nanocomposite, nanocomposite, in in addition addition to to 5 5 mg mg∙·LL−1 ofof BG BG dye dye at at pH pH 7. The removal efficiencyefficiency resultsresults are are displayed displayed in Figure in Figure8. According 8. According to Figure8 ,to the Figure prepared 8, Hap/CSthe prepared nanocomposite Hap/CS nanocompositehas high potential has forhigh dye potential removal. for Thedye removalremoval. efficiencyThe removal of BG efficiency dye increased of BG dye with increased the increase with thein the increase amount in ofthe the amount Hap/CS of nanocomposite.the Hap/CS nanocomposite. This is due to This the is greater due to availability the greater of availability binding sites of bindingof the sorbent. sites of Thethe mostsorbent. optimum The most adsorbent optimum dose adsorbent for dye dose removal for indye the removal aqueous in solutionthe aqueous was −1 solutionat 0.9 g· Lwas whenat 0.9 theg∙L− removal1 when the efficiency removalachieved efficiency 99.5 achieved %. At 99.5 this %. point, At this the point, adsorption the adsorption capacity capacitydemonstrated demonstrated maximum maximum removal efficiencyremoval efficiency due to the due high to externalthe high surface external area surface of the area adsorbent of the adsorbentand the available and the sites available for binding sites for dye binding molecules dye [molecules28]. A similar [28]. resultsA similar was results reported was in reported a study thatin a studyinvestigated that investigated hydroxyapatite/chitosan hydroxyapatite/ biocomposite chitosan biocomposite for Remazol for BlueRemazol Dyes Blue Removal Dyes thatRemoval achieved that achievedhigh adsorption high adsorption capacity forcapacity about 95for %about dyes 95 removal % dyes [17 removal]. Also the[17]. same Also results the same was results reported was in reporteda study thatin a investigatedstudy that investigated Hydroxyapatite-chitosan Hydroxyapatite-chitosan HAp-CS composite HAp-CS forcomposite removal for of removal Congo red of

Molecules 2019, 24, 847 7 of 16 Molecules 2019, 24 FOR PEER REVIEW 7 Molecules 2019, 24 FOR PEER REVIEW 7 dyeCongo from red aqueous dye from solution aqueous that solution exhibited that adsorption exhibited capacityadsorption higher capacity than higher that of than pure that chitosan of pure and Congo red dye from aqueous solution that exhibited adsorption capacity higher than that of pure hydroxyapatitechitosan and hydroxyapatite [18]. [18]. chitosan and hydroxyapatite [18].

110 110 100 100 90 90 80 80 70 70 Adsorption % Adsorption

Adsorption % Adsorption 60 60 0.9 gL-1 composite 50 0.90.5 gL-1 composite 50 0.50.3 gL-1 composite 40 0.3 gL-1 composite 40 -5 15 35 55 75 95 -5 15 35Time, min 55 75 95 Time, min Figure 8. Effect of the adsorbent dosage of Hap/CS nanocomposite on BG dye removal. Figure 8. Effect of the adsorbent dosage of Hap/CS nanocomposite on BG dye removal. 2.5. Sorption ModelFigure 8. Effect of the adsorbent dosage of Hap/CS nanocomposite on BG dye removal. 2.5. Sorption Model 2.5.The Sorption study Model of adsorption kinetics is important because the rate of adsorption (which is one of the criteriaThe study for determining of adsorption the kinetics efﬁciency is important of an adsorbent) because the andrate theof adsorption mechanism (which of adsorption is one of the can The study of adsorption kinetics is important because the rate of adsorption (which is one of the bothcriteria be concluded for determining from kineticthe efficiency studies. of an As adsorbent) a standard and parameter the mechanism for studying of adsorption the behavior can both of be BG, criteriaconcluded for determiningfrom kinetic the studies. efficiency As aof standard an adsorbent) parameter and the for mechanism studying ofthe adsorption behavior canof BG, both dye be dye adsorption at the Hap/CS surface is obtained using the Mories-Weber equation [29]. concludedadsorption fromat the kineticHap/CS studies. surface Asis obtain a standarded using parameter the Mories-Weber for studying equation the behavior[29]. of BG, dye adsorption at the Hap/CS surface is obtained using the1/2 Mories-Weber equation [29]. qq = = K Kdd (t)(t)1/2 (1) (1) q = Kd (t)1/2 (1) where q is the amount of dye adsorbed (mg/g), Kd is the intraparticle diffusion rate constant, and t1/21/2 where q is the amount of dye adsorbed (mg/g), Kd is the intraparticle diffusion rate constant, and t whereis the square q is the root amount of time. of dyeIn Figure adsorbed 9, the (mg/g), Morris-Weber Kd is the model intraparticle reveals diffusion an initial rate linear constant, portion and which t1/2 is the square root of time. In Figure9, the Morris-Weber model reveals an initial linear portion which ismay the be square due to root the of boundary time. In Figure layer effect 9, the and Morris-Weber a second portion model whichreveals may an initial be due linear to the portion intraparticle which may be due to the boundary layer effect and a second portion which may be due to the intraparticle maydiffusion be due effect to the [30]. boundary The value layer of the effect rate and constant a second for theportion intraparticle which may diffusion be due K tod was the evaluatedintraparticle as diffusion effect [30]. The value of the rate constant for the intraparticle diffusion K was evaluated as diffusion effect−1 [30]. The value of the rate constant for the intraparticle diffusion Kd wasd evaluated as 0.07 (g/g∙min−1) for BG dye and gives an indication of the mobility of the dye toward the composite. 0.070.07 (g/g (g/g·min∙min−1) for for BG BG dye dye and and gives gives an an indication indication of of the the mobility mobility of ofthe the dye dye toward toward the thecomposite. composite.

5 5 4.9 4.9 4.8 4.8 4.7 4.7 4.6

Adsorbed amount(g/g) Adsorbed 4.6

Adsorbed amount(g/g) Adsorbed 4.5 4.5 4.4 4.4 024681012 024681012Time1/2, min Time1/2, min

Figure 9. Adsorbed amount of BG dye onto Hap/CS nanocomposite as a function of the square root FigureFigureof time. 9. 9.Adsorbed Adsorbed amount amount of of BG BG dyedye ontoonto Hap/CSHap/CS nano nanocompositecomposite as as a afunction function of ofthe the square square root root ofof time. time.

Molecules 2019, 24, 847 8 of 16

The Lagrange equation is employed to determine the order of the adsorption, as cited by Gupta et al. [31]. Log (qe − q) − log qe = −Kads·t/2.303 (2) −1 where qe is the amount of dye adsorbed at equilibrium (mg·g ) and Kads is the ﬁrst-order rate −1 constant for dye adsorption onto the sorbent (min ). The linear plot of Log (qe − 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]:

2 t/qt = 1/K2qe + t/qe (3)

−1 −1 where k2 is the rate constant of the second-order adsorption (g mg min ). Similarly, the slope of the plot of t/qt as a function of t was used to determine the second-order rate constant k2. The Bangham equation is used to investigate the amount of BG dye that can by introduced into the pores of the nanocomposite [33].

Log log [Ci/(Ci − q m)] = log (Ko m/2.303 V) + α log t (4) where Ko 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 Table1.

Table 1. Kinetics parameters for the sorption of BG dye on Hap/CS nanocomposite.

−1 2 Lagrange (Pseudo-First-Order) Kads (min ) qe (mg/g) R BG dye 0.02 30.2 0.998 (pseudo-second-order) k (g mg−1 2 q (mg/g) R2 min−1) e BG dye 2.05 30.2 0.995 Bangham 2 AKo R BG dye 0.02 9.2 0.985

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.

2.6. Isotherm Model

2.6.1. 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.

Ce/qe = 1/b Qmax + (1/Qmax)Ce (5)

−1 where b is the monolayer adsorption capacity related to the sorption heat (L·mg ) and Qmax is the maximum adsorption capacity (mg·g−1). Molecules 2019, 24 FOR PEER REVIEW 9 where Kf (mol1−n Ln 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. Molecules 2019, 24, 847 9 of 16 Favorable adsorption was demonstrated by the fact that the value of n was greater than unity.

14 Molecules 2019, 24 FOR PEER REVIEW 9 12

1−n n −1 -4 where Kf (mol L g ) represents10 the sorption capacity when the dye equilibrium concentration is x10 e /q equal to 1 and n represents e the degree of dependence of sorption on the equilibrium concentration.

C 8 Favorable adsorption was demonstrated by the fact that the value of n was greater than unity. 6

4 18 2 16-5 0 5 10 15 20 25 30 35 C x10-3 e 14 −1 −1 FigureFigure 10. 10.Langmuir Langmuir12 adsorption adsorption for for 20 20 mg mgL·L −1 BGBG dyedye removalremoval onon 0.90.9 ggL·L−1 Hap/CS.Hap/CS. -4 10 x10

2.6.2. Freundlich Isotherm e

/q e

C 8 The Freundlich expression10.0 is an empirical equation describing sorption to a heterogeneous 6 surface [36]. The Freundlich9.5 adsorption is presented in Equation (6) and shown in Figure 11:

4 9.0 ln qe = ln Kf + 1/n ln Ce (6) 2 8.5-5 0 5 10 15 20 25 30 35 -3

1−n n −1 e C x10 where Kf (mol L g ) represents the sorption capacitye when the dye equilibrium concentration is

ln q ln equal to 1 and n represents8.0 the degree of dependence of sorption on the equilibrium concentration. −1 −1 Favorable adsorptionFigure 10. was Langmuir demonstrated7.5 adsorption by for the 20 factmgL that BG the dye value removal of n on was 0.9 greatergL Hap/CS. than unity.

7.0

10.0 6.5 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 9.5 ln C e 9.0 Figure 11. Freundlich adsorption for 20 mgL−1 BG dye removal on 0.9 gL−1 Hap/CS. 8.5 e

2.6.3. Dubinin-Radusekevisch-Kanagerq ln 8.0 Isotherm

The Dubinin-Radusckevisch7.5 (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 7.0 compatible between Gaussian energy distribution and adsorption processes on a heterogeneous 6.5 surface. The D-R equation is expressed-0.5 0.0 as 0.5follows 1.0 [34]: 1.5 2.0 2.5 3.0 3.5 4.0 ln C e ln q = ln q(D-R) − ßε2 (7) · −1 · −1 FigureFigure 11. 11.Freundlich Freundlich adsorption adsorption forε for= 20RT 20 mg ln(1mgLL +−1 BG1/CBG dyedyee) removalremoval onon 0.90.9 ggLL−1 Hap/CS.Hap/CS. (8)

2.6.3.where Dubinin-Radusekevisch-Kanager q(D-R) is the theoretical adsorption Isotherm capacity (mg∙g−1), ß is the activity coefficient related to the 2.6.3. Dubinin-Radusekevisch-Kanager Isotherm mean sorption energy (mol2 kJ−2), ε is the Polanyi potential, R is the ideal gas constant (0.008314 The Dubinin-Radusckevisch (D-R) isotherm is more general than the Langmuir model, because KJmolThe−1 K Dubinin-Radusckevisch−1), and T is the absolute (D-R)temperature isotherm in isKelvin more (K). general E (kJ than mol− the1) is Langmuirdefined as model, the free because energy it does not assume a homogeneous surface or constant sorption potential. In general, the model itchange does notrequired assume to transfera homogeneous 1 mole of surface dye from or thconstae solutionnt sorption to the potential.solid surface, In general, which is the equal model to: is is compatible between Gaussian energy distribution and adsorption processes on a heterogeneous compatible between Gaussian energy distribution and adsorption processes on a heterogeneous surface. The D-R equation is expressed as followsE = 1/(2ß) [34]:1/2 (9) surface. The D-R equation is expressed as follows [34]: The magnitude of E is useful to estimate the type of− sorptionε2 reaction. If E is in the range of 8–16 lnln q q = = ln ln q q(D-R)(D-R) − ßßε2 (7)(7) 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 εεother= =RT RT ln(1 haln(1nd, + + 1/C 1/Cthe e))sorption may be dominated by particle(8) (8) where q(D-R) is the theoretical adsorption capacity (mg∙g−1), ß is the activity coefficient related to the mean sorption energy (mol2 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:

E = 1/(2ß)1/2 (9) 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

Molecules 2019, 24, 847 10 of 16

−1 where q(D-R) is the theoretical adsorption capacity (mg·g ), ß is the activity coefﬁcient related to the mean sorption energy (mol2 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: E = 1/(2ß)1/2 (9)

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 Table2, 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.

Table 2. Isothermal parameter for the sorption of BG dye onto Hap/CS nanocomposite.

Langmuir −1 −1 2 b (L·mg )Qmax (mg·g )R BG dye 10.3 49.1 0.987 Freundlich n−1 n −1 2 Kf (mol L g ) n R BG dye 1.4 1.2 0.980 D-R model E (kJ mol−1) q(D-R) (mg·g−1) R2 BG dye 8.2 29.9 0.985

2.7. Thermodynamic Parameters In order to investigate the effect of temperature on the adsorption of BG dye onto on the Hap/CS −1 nanocomposite, the distribution coefﬁcient Kd (L·g ) was calculated at temperatures of 288, 298, 313, and 323 K using Equation (10). Thermodynamic parameters of the entropy change (∆So) and o enthalpy change (∆H ) were calculated from the intercept and slope of the plot of ln Kd against 1/T, respectively [38,39]. Ln Kd = ∆So/R − ∆Ho/RT (10) The other thermodynamic parameter, Gibbs free energy (∆Go), was calculated by:

o ∆G = −RT ln Kd (11) where R is the universal gas constant (8.314 J mol−1 K−1) and T is the temperature (K). The Kd 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 (∆Ho) and the entropy change (∆So), were calculated from the data of Equation (10) and are shown in Figure 12. The other thermodynamic parameter, Gibbs free energy (∆Go), was calculated by Equation (11). A positive ∆Ho indicates that the adsorption of BG dye onto the Hap/CS nanocomposite is endothermic. For entropy change (∆So), a positive sign means that the adsorption of BG dye onto sorbents is a random reaction, as shown in Table3. Meanwhile, a negative value of ∆Go 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. Molecules 2019, 24, 847 11 of 16 Molecules 2019, 24 FOR PEER REVIEW 11

7.2 80 mgL-1 of BG 50 mgL-1 of BG

6.9

6.6 ln Kd ln

6.3

6.0

3.0 3.1 3.2 3.3 3.4 3.5 1/T x 10-3

−1 −1 Figure 12. ThermodynamicThermodynamic adsorption adsorption for for 50 50and and 80 mgL 80 mg−1 ·BGL dyeBG removal dye removal on 0.9 ongL− 0.91 Hap/CS g·L nanocomposite.Hap/CS nanocomposite. Table 3. Thermodynamic data for the adsorption of BG dye onto Hap/CS nanocomposite. Table 3. Thermodynamic data for the adsorption of BG dye onto Hap/CS nanocomposite. o −1 o −1 o −1 −1 2 BG Dye T (K) lnKd ∆G (kJ·mol ) ∆H (J·mol ) ∆S (J·mol ·K )R BG dye T (K) lnKd ∆Go (kJ·mol−1) ∆Ho (J·mol−1) ∆So (J·mol−1·K−1) R2 288288 6.7 6.7 − 16.2 −16.2 0.9980.998 298 6.9 −17.1 0.997 50 mg·L−1 298 6.9 −17.1 20.1 29.9 0.997 50 mgL−1 313 7.1 −8.5120.1 29.9 0.988 313 7.1 −18.5 0.988 323 7.3 −19.6 0.989 323 7.3 −19.6 0.989 288 5.9 −14.1 0.995 288 5.9 −14.1 0.995 298 6.3 −15.6 0.985 80 mg·L−1 298 6.3 −15.6 19.8 29.5 0.985 80 mgL−1 313 6.9 −17.919.8 29.5 0.958 323313 7.1 6.9 −19.0 −17.9 0.958 0.997 323 7.1 −19.0 0.997 3. Experimental Methods 3. Experimental Methods 3.1. Synthesis of the Hap/CS Nanocomposite 3.1. Synthesis of the Hap/CS Nanocomposite The starting materials included: Ca(NO3)2.4H2O, (NH4)2HPO4, triphenylphosphate, and chitosan. All reagentsThe starting were ofmaterials AR grade included: and used withoutCa(NO3 further)2.4H2O, puriﬁcation. (NH4)2HPO Deionized4, triphenylphosphate, water was used and in chitosan.all synthesis All steps.reagents The were synthesis of AR ofgrade the Hap/CSand used nanocompositewithout further ispurification. shown in Figure Deionized 13. For water the ﬁrstwas usedstep inin theall synthesis preparation steps. of theThe Hap/CSsynthesis nanocomposite, of the Hap/CS nanocomposite chitosan was dissolved is shown in in Figure 0.5 % (13.v/v )For acetic the firstacid step aqueous in the solution preparation until of a homogeneousthe Hap/CS na chitosannocomposite, solution chitosan was obtained. was dissolved Appropriate in 0.5 % amounts (v/v) acetic of acidtriphenylphosphate aqueous solution were until added a homogeneous and stirred for chitosan about1 solution h. A gelatinous was obtained. precipitate Appropriate of nano chitosan amounts was of triphenylphosphate were added and stirred for about 1 h. A gelatinous precipitate of nano chitosan then formed. In the second step, Ca(NO3)2.4H2O and (NH4)2HPO4 were dissolved in deionized water was then formed. In the second step, Ca(NO3)2.4H2O and (NH4)2HPO4 were dissolved in deionized separately. The pH of each aqueous solution was adjusted to 11 using 25% NH4OH solution. During water separately. The pH of each aqueous solution was adjusted to 11 using 25% NH4OH solution. the dropwise addition of Ca(NO3)2.4H2O aqueous solution under conditions of vigorous stirring with During the dropwise addition of Ca(NO3)2.4H2O aqueous solution under conditions of vigorous (NH4)2HPO4 at room temperature for about 1 h [40], nano chitosan was added. This produced a milky stirringand gelatinous with (NH precipitate.4)2HPO4 Theat room mixture temperature was stirred for for about 1 h and 1 h dried [40], atnano 80 ◦ Cchitosan for 4 h. was Then, added. calcination This producedoccurred at a temperaturesmilky and gelatinous of 800 ◦ Cprecipitate. for 1 h, 1000 The◦C mixture for 2 h, was and stirred 1200 ◦C for for 1 1h 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.

Molecules 2019, 24, 847 12 of 16 Molecules 2019, 24 FOR PEER REVIEW 12

Figure 13. Synthesis of the Hap/CS nanocomposite. Figure 13. Synthesis of the Hap/CS nanocomposite. 3.2. Structural and Surface Characterization of the Hap/CS Nanocomposite 3.2. Structural and Surface Characterization of the Hap/CS Nanocomposite Analytical Instruments Analytical Instruments To characterize our product, we used X-ray diffraction (MiniFlex, HyPix-400 MF. Japan) to determineTo characterize the structure our of product, our composite we used and X-ray scanning diffraction electron (MiniFlex, microscopy HyPix-400 (SEM) (JSM-6510MF. Japan) LV to JEOL.determine Japan) the to structure give us an of idea our of composite the morphology and scan ofning the surface electron of ourmicroscopy adsorbent. (SEM) The identification(JSM-6510 LV ofJEOL. functional Japan) to groups give us in an the idea Hap/CS of the morphology nanocomposite of the as surface well as of the our interfacial adsorbent. modification The identification were analyzedof functional by FTIR groups analysis in the (Thermo Hap/CS Fisher nanocomposite Nicolete IS10) as within well theas the scanning interfacial range modification of 400–4000 cm were−1. Transmissionanalyzed by FTIR electron analysis microscopy (Thermo (JEMFisher 2100, Nicole HRTEM,te IS10) within JEOL) the was scanning also used. range Surface of 400–4000 area cm was−1. calculatedTransmission using electron an St 4microsco on NOVApy (JEM touch 2100, 4LX instrumentHRTEM, JEOL) with was nitrogen. also used. In order Surface to determine area was thecalculated influencing using effects an St of 4 theon parametersNOVA touch on 4LX the adsorptioninstrument inwith the nitrogen. studied systems, In order the to measurementdetermine the ofinfluencing the quantity effects of BG of the adsorbed parameters on our on composite the adsorp wastion carriedin the studied out with systems, an analysis the measurement wavelength ofof λthemax quantity= 626 nm of on BG a adsorbed Cintra 101 on double our composite beam spectrophotometer. was carried out with an analysis wavelength of λmax = 626 nm on a Cintra 101 double beam spectrophotometer. 3.3. Adsorption Studies of BG Dye 3.3. Adsorption Studies of BG Dye The influence of key adsorption parameters (pH, contact time, initial concentration, and adsorbent dosage)The on influence the adsorption of key behavior adsorption of BG parameters on the Hap/CS (pH, nanocompositecontact time, initial was explored concentration, using batch and experiments.adsorbent dosage) Table 4on shows the adsorption the characteristics behavior ofof brilliantBG on the green Hap/CS dye. nanocomposite The molecular was structure explored of brilliantusing batch green experiments. dye is displayed Table in 4 Figure shows 14 the. Figure characteristics 15 shows theof brilliant UV-vis absorption green dye. spectra The molecular of neat/ blankstructure BG inof aqueousbrilliant medium,green dye with is displayed max wavelength in Figure absorption 14. Figure centered 15 shows at 625 the nm. UV-vis The absorptionabsorption measurementsspectra of neat/ were blank repeated BG in aqueous at definite medium, time intervals.with max Itwavelength was observed absorption that the centered neat BG at aqueous 625 nm. solutionThe absorption is stable measurements during the time were range repeated of the adsorptionat definite time study intervals. [41]. It was observed that the neat BG aqueous solution is stable during the time range of the adsorption study [41]. Table 4. Characteristics of brilliant green dye. Table 4. Characteristics of brilliant green dye. Chemical Formula Molecular Weight Color Index λmax Chemical Formula Molecular Weight Color Index λmax C27H33N2.HO4S 482.64 g/mol 42040 626 nm C27H33N2.HO4S 482.64 g/mol 42040 626 nm

Molecules 2019,, 24, FOR 847 PEER REVIEW 13 of 1613 Molecules 2019, 24 FOR PEER REVIEW 13

Figure 14. Molecular structure of brilliant green dye. Figure 14. Molecular structure of brilliant green dye.

3 3 20 min 2060 min 6090 min 2 90 min 2 Absorption

Absorption 1 1

0 0 400 450 500 550 600 650 700 750 800 400 450 500 550Wavelength(nm) 600 650 700 750 800 Wavelength(nm) FigureFigure 15.15. TheThe UV-visUV-vis absorptionabsorption spectraspectra ofof neat/neat/ blank BG in aqueous medium. Figure 15. The UV-vis absorption spectra of neat/ blank BG in aqueous medium.

Values ofof pH,pH, contactcontact time, time, initial initial concentration, concentration, and and adsorbent adsorbent dosage dosage were were varied varied from from pH pH 1 to 1 Values of pH, contact time,−1 initial concentration, and adsorbent −dosage1 were varied from pH 1 9,to 59, to 5 90to min,90 min, 5 to 5 80to mg80 ·mgLL −of1 of BG BG dye, dye, and and 0.3, 0.3, 0.5, 0.5, or or 0.9 0.9 mg mgL·L −1 ofof Hap/CSHap/CS nanocomposite, 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 The initial initial and and final ﬁnal concentrations concentrations of of BG BG were estimated using a HachHach LangeLange respectively. The initial and final concentrations of BG were estimated using a Hach Lange spectrophotometer. The The equilibrium equilibrium adsorption adsorption capacity capacity qe (mg/g) qe (mg/g) and andthe percentage the percentage removal removal were spectrophotometer. The equilibrium adsorption capacity qe (mg/g) and the percentage removal were weredetermined determined using using Equations Equations (12) and (12) (13). and (13). determined using Equations (12) and (13). ( ) Adsorption Capacity qe = ((C0 − ) C e) V (12) AdsorptionAdsorption Capacity Capacity q q=e = (12)(12) e W ( ) Adsorption % = ( ) × 100 Adsorption % = × 100 (13) (C − C ) (13) Adsorption % = 0 e × 100 (13) where qe (mg/g) denotes the equilibrium adsorption capacity,C0 Co and Ce are the initial and equilibrium 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 whereconcentrations qe (mg/g) (mg/L) denotes of theGB, equilibrium and V (L) and adsorption W (g) are capacity, the volume Co and of C thee are solution the initial and and weight equilibrium of the adsorbent, respectively. adsorbent,concentrations respectively. (mg/L) of GB, and V (L) and W (g) are the volume of the solution and weight of the adsorbent, respectively. 4. Conclusion 4. Conclusion 4. Conclusions Hydroxyapatite/chitosan nanocomposite was prepared and used for the removal of BG dye from 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: an aqueousHydroxyapatite/chitosan solution. The following nanocomposite conclusions was were prepared made based and used on the for results the removal of the of present BG dye study: from The Hap/CS nanocomposite was characterized by Fourier transform infrared spectroscopy an aqueousThe Hap/CS solution. nanocomposite The following was conclusions characterized were madeby Fourier based ontransform the results infrared of the presentspectroscopy study: (FTIR), scanning electron microscopy (SEM), transition electron microscopy (TEM), and X-ray (FTIR),The scanning Hap/CS nanocompositeelectron micros wascopy characterized (SEM), transition by Fourier electron transform microscopy infrared spectroscopy(TEM), and (FTIR),X-ray diffraction analysis (XRD) techniques. diffractionscanning electron analysis microscopy (XRD) techniques. (SEM), transition electron microscopy (TEM), and X-ray diffraction The sorption of BG dye was found to increase with the increase in contact time, and the analysisThe (XRD)sorption techniques. of BG dye was found to increase with the increase in contact time, and the adsorbent dosage reached equilibrium at 60 min. adsorbentThe sorption dosage ofreached BG dye equilibrium was found toat increase60 min. with the increase in contact time, and the adsorbent The experimental data are best correlated by a first-order kinetic model. The Morris-Weber dosageThe reached experimental equilibrium data atare 60 best min. 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). 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. 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 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 models and it was found that the equilibrium data are best described by the Dubinin–Radushkevich

Molecules 2019, 24, 847 14 of 16

The experimental data are best correlated by a ﬁrst-order kinetic model. The Morris-Weber −1 model showed that the rate constant for the intrapore diffusion Kd was evaluated as 0.07 (g/g·min ). 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.

Author Contributions: Investigation, A.R. and I.A.; Methodology, A.R. and D.B.; Project administration, A.R. and I.A.; Writing-review & editing, I.A. Funding: This research was funded by the Deanship of Scientific Research at King Khalid University. The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through a General Research Project under grant number 109/2018. Acknowledgments: The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through a General Research Project under grant number 109/2018. Conflicts of Interest: The author declares no conflict of interest.

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Sample Availability: Samples of the compounds and composite all are available from the authors.

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