Removal of Zinc Ions Using Hydroxyapatite and Study of Ultrasound Behavior of Aqueous Media

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Article Removal of Zinc Ions Using Hydroxyapatite and Study of Ultrasound Behavior of Aqueous Media

Simona Liliana Iconaru 1, Mikael Motelica-Heino 2,Régis Guegan 3, Mihai Valentin Predoi 4, Alina Mihaela Prodan 5,6 and Daniela Predoi 1,* 1 National Institute of Materials Physics, Atomistilor Street, No. 405A, P.O. Box MG 07, Magurele 077125, Romania; [email protected] 2 Institut des Sciences de la Terre d’Orléans (ISTO), UMR 7327 CNRS Université d’Orléans, 1A rue de la Férollerie, 45071 Orléans CEDEX 2, France; [email protected] 3 Faculty of Science and Engineering, Global Center for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan; [email protected] 4 Department of Mechanics, University Politehnica of Bucharest, BN 002, 313 Splaiul Independentei, Sector 6, Bucharest 060042, Romania; [email protected] 5 Emergency Hospital Floreasca Bucharest, 8 Calea Floreasca, Sector 1, Bucharest 014461, Romania; [email protected] 6 Carol Davila University of Medicine and Pharmacy, 8 Eroii Sanitari, Sector 5, Bucharest 050474, Romania * Correspondence: [email protected]; Tel.: +40-212418154

Received: 6 June 2018; Accepted: 1 August 2018; Published: 3 August 2018

Abstract: The present study demonstrates the effectiveness of hydroxyapatite nanopowders in the adsorption of zinc in aqueous solutions. The synthesized hydroxyapatites before (HAp) and after the adsorption of zinc (at a concentration of 50 mg/L) in solution (HApD) were characterized using X-ray diffraction (XRD), and scanning and transmission electron microscopy (SEM and TEM, respectively). The effectiveness of hydroxyapatite nanopowders in the adsorption of zinc in aqueous solutions was stressed out through ultrasonic measurements. Both Langmuir and Freundlich models properly fitted on a wide range of concentration the equilibrium adsorption isotherms, allowing us to precisely quantify the affinity of zinc to hydroxyapatite nanopowders and to probe the efficacy of hydroxyapatite in removal of zinc ions from aqueous solutions in ultrasonic conditions.

Keywords: hydroxyapatite; ultrasonic measurements; water depollution; zinc; adsorption

1. Introduction The most pressing issue today is the increase in contaminated sites in both numbers and area as a result of rapidly growing industrial activities that have led to ecosystem damage due to excessive pollution. Moreover, many industries have the potential to pollute water resources with different wastes such as organic and inorganic compounds. The most frequent pollutants due to the industrial sites are heavy metal ions and aromatic compounds [1,2]. Due to an increased awareness regarding the well-being of the planet, tremendous attention has been paid to heavy metals such as cadmium, lead, zinc, and arsenic, which are reported to have a detrimental impact on the environment all over the world. These particular heavy metals are not biodegradable and can be accumulated in organisms, thus representing a serious health threat to plants, animals and human beings. Indeed, metals, such as zinc, cadmium, led, and arsenic, show a high toxicity even at low concentrations. Even though human do not have direct contact with these pollutants, human health can be indirectly inﬂuenced by multiple ways, particularly through drinking water and the food chain. In addition, in agriculture, the presence of heavy metals pollution in soils can cause great harm to crop growth, yield and quality. Therefore, the removal of heavy metals, from natural waters or soils, has attracted considerable attention [3,4].

Materials 2018, 11, 1350; doi:10.3390/ma11081350 www.mdpi.com/journal/materials Materials 2018, 11, 1350 2 of 16

Various conventional technologies for the removal of heavy metal ions from aqueous solution, such as chemical precipitation, electrochemical treatment, ion exchange, reverse osmosis and electrodialysis, have been described [5]. Recently, considerable attention has been given to alternative methods, one of the most promising being the use of cheap materials as potential sorbents for heavy metals removal [6–8]. The most popular materials used as sorbents have been carbons, zeolites, clays, biomass and polymeric materials [9]. However, in the literature, it has been reported that these materials exhibit low adsorption capacities towards heavy metals ions and suffers from separation inconvenience. Therefore, considerable efforts are still needed for the development of newly materials that can be used as adsorbents in decontamination applications. Several studies conducted in recent years have reported the applications of mineral materials in the disposal of wastewaters contaminated with heavy metals [9]. Materials such as zeolite [10], clay minerals [11,12] and other organo-derived clay materials [13–15] have been reported to be promising candidates in the treatment of wastewater due to their excellent surface characteristics. Recent studies have reported that phosphate minerals represent promising materials in the treatment of wastewaters for the removal of fluoride and heavy metals [16,17]. Incentive results on the efficacy of phosphate minerals have been reported for the activity of hydroxyapatite (HAp) with the chemical formula Ca10(PO4)6(OH)2 as an excellent material for removing long-term pollutants from contaminated water due to its high affinity for heavy metals, low water solubility, high stability and low cost [18]. One of the most popular and widely used adsorbents in water treatment especially for heavy metal removal has been activated carbon since it was introduced [19]. Even though activated carbon [20], silica gel [21] and activated alumina [22] are highly effective for the adsorption of heavy metals from water, their use is restricted, as these materials remain expensive materials. Therefore, they cannot be widely used in small-scale industries because of cost inefficiency. Recently, the interest in the research of alternative adsorbents to replace the costly activated carbon, silica gel and activated alumina has increased. Considerable attention has been focused on adsorbents, with metal-binding capacities and able to remove heavy metals from contaminated water at low cost [23,24]. Natural materials such as chitosan, zeolites, chitosan, clay, or waste products from industrial operations such as fly ash, coal, and oxides have been classified as low-cost adsorbents [25]. It was estimated that chitosan could be obtained using fish and crustaceans as raw materials at a market price of US$ 15.43/kg [26]. Similar estimations have been made regarding the price of zeolites which are considered very cheap (about US$ 0.03–0.12/kg) [27]. On the other hand, apatite materials have been depicted as ideal materials for long-term containment of pollutants due to their high sorption capacity for actinides and heavy metals [28], low water solubility [29], high stability under reducing and oxidizing conditions, availability and low cost [30]. Moreover, natural hydroxyapatite obtained from bovine cortical bone ash [30] has been considered as an environmentally benign functional material and has been widely investigated for water treatment applications due to its high capacity for the removal of heavy metal ions, low water solubility, high stability under reducing and oxidizing conditions, availability and low cost [31–33]. Zinc is considered one of the most important elements and plays an essential role in biological functions. Nevertheless, zinc ions at a certain concentration display a great toxicity towards human organism [34]. Thus, the Canadian Water Quality Guidelines [35] and WHO [36] defined that the maximum admissible level of zinc in drinking water be ≤5 mg/L. The contamination of zinc in effluents often results from industry discharges. Since zinc is not biodegradable, it travels through food chain via bioaccumulation and possess a high toxicity risk to humans [37]. In the last years, the use of ultrasound measurements to characterize suspensions has become a large and growing application, as reviewed by McClements [38]. Several studies have stated that ultrasonic measurements could offer significant insight about materials properties [39,40]. Even though there have been reported only a handful of few basic and quantitative investigations, the common conclusion of the reported papers has been that ultrasound methods are very rigorous and much less sensitive to contamination Materials 2018, 11, 1350 3 of 16 providing accurate information about the microstructural properties as well as deformation processes of the investigated samples [41,42]. In this context, this study focused on using hydroxyapatite nanopowders for the removal of zinc ions from contaminated waters. It principally aimed at pointing out the interests of hydroxyapatites as potential adsorbents for zinc (Zn2+) in aqueous solution. Pure hydroxyapatite obtained by an adapted coprecipitated method was characterized by XRD, SEM and TEM before and after removing zinc from aqueous solutions. Adsorption kinetics of zinc ions (Zn2+) on hydroxyapatite was studied by the Langmuir and Freundlich models. The effectiveness of decontamination was correlated with ultrasonic measurements. The advantages of ultrasonic techniques are synthetically presented in the book of Povey [43] and an updated review was presented by Povey [44]. More recently, Galaz [45] presented an experimental verification of ultrasonic methods applied to suspensions of small particles. There are even ultrasonic studies dedicated to the estimation of the suspension particle sizes, from which we mention here on of the most recent [46]. In this paper, the ultrasonic technique is simple and directly applicable, based on acoustic signature of the signal in different suspensions. The only limitation is that the reference fluid and the depolluted suspension must be tested with the same experimental setup.

2. Experimental Section

2.1. Synthesis of Hydroxyapatite Nanoparticles (HAp)

The hydroxyapatite, Ca10(PO4)6(OH)2, nanopowders were obtained using a previously described adapted coprecipitation method [47,48]. The molar ratio Ca:P was maintained at 1.67 [36].

2.2. Structural and Morphological Characterizations The structure and morphology of the recovered powders were characterized by XRD, SEM and TEM. X-ray diffraction was performed for HAp nanopowders before and after zinc ions adsorption from aqueous solution using a Bruker D8 Advance diffractometer (Bruker, Karlsruhe, Germany) with nickel filtered CuKα (λ = 1.5418 Å) radiation and a high efficiency one-dimensional detector (Lynx Eye type, Bruker, Karlsruhe, Germany) operated in integration mode. The data were collected over the 2θ range of 25–55◦, using a step size of 0.02◦ and 34 s measuring time per step. The morphology of the HAp nanopowders before and after zinc ions adsorption from aqueous solution was investigated by scanning electron microscopy (SEM) using a HITACHI S4500 microscope (Hitachi High Technologies America, Inc., Schaumburg, IL, USA). The size and morphology of HAp nanopowders before and after zinc ions adsorption from aqueous solution were analyzed by transmission electron microscopy (TEM) using a CM 20 (Philips—FEI, Eindhoven, The Netherlands), equipped with a filament Lab6 that works at 200 kV. For TEM investigations, the nanopowders were dispersed in ethanol using an ultrasonic bath (Retsch GmbH, Haan, Germany) and a drop of the resulting solution was deposited on a carbon-coated Cu grid.

2.3. Batch Adsorption Experiments The effectiveness of zinc adsorption onto hydroxyapatite nanopowders was investigated by batch adsorption experiments. The experiments were conducted in 40 mL silicon tubes with aqueous solutions containing zinc ions in a concentration range of 5–150 mg L−1. The amount of the HAp nanopowder used as adsorbent was 0.2 g and the solution pH was adjusted to 5 with a 0.1 M hydrochloric acid (HCl) solution. During the experiments, the solution volume was kept at 20 mL and the mixture was stirred using a Mixer SRT1 Roller (Stuart Scientiﬁc, Staffordshire, UK) for 24 h. After 24 h, the tubes were centrifuged for 1 h at 10,000 rpm. The supernatant was ﬁltered, recovered and analyzed by Atomic Absorption Spectrometry (AAS) [49]. The batch experiments were carried out at room temperature, in triplicate. The AAS measurements were performed using a 213.9 nm wavelength for zinc. Materials 2018, 11, 1350 4 of 16

2.4. UltrasonicMaterials 2018 Measurements, 11, x FOR PEER REVIEW 4 of 16 Ultrasonic2.4. Ultrasonic measurements Measurements were performed on water (Wi), water contaminated with zinc (50 mg/L zincUltrasonic ions) measurements (WZn) and water were afterperformed decontamination on water (Wi), (Wd). water Solutions contaminated analyzed with byzinc ultrasonic (50 measurements,mg/L zinc ions) water (WZn) (Wi), and zinc water contaminated after decontamination water (50 (Wd). mg/L Solutions zinc ions) analyzed (WZn) by and ultrasonic water after decontaminationmeasurements, (Wd) water were (Wi), placed zinc contaminated in the thermally water controlled(50 mg/L zinc container. ions) (WZn) The and temperature water after of the solutionsdecontamination was kept constant (Wd) were at 21placed◦C. in During the therma thelly measurements, controlled container. the WZn The andtemperature Wd samples of the were stablesolutions and no precipitationwas kept constant of nanoparticles at 21 °C. During was the observed measurements, at the endthe WZn of the and experiment. Wd samples The were chain of stable and no precipitation of nanoparticles was observed at the end of the experiment. The chain of measurement used to ultrasonic characterization of Wi, WZn, and Wd is shown in Figure1. measurement used to ultrasonic characterization of Wi, WZn, and Wd is shown in Figure 1.

Figure 1. Overview of the chain of measurement. Figure 1. Overview of the chain of measurement. 3. Results and Discussions 3. Results and Discussions To highlight the effectiveness of hydroxyapatite in the removal of zinc ions from aqueous Tosolutions, highlight XRD, the SEM, effectiveness EDX and TEM of hydroxyapatiteanalysis were performed in the on removal prepared of hydroxyapatite zinc ions from (HAp) aqueous solutions,and hydroxyapatite XRD, SEM, EDX recovered and TEM after analysis adsorption were of performedzinc ions from on aqueous prepared solutions hydroxyapatite (HApD). Figure (HAp) and 2a shows the XRD patterns of the HAp and HApD after zinc adsorption from aqueous solutions hydroxyapatite recovered after adsorption of zinc ions from aqueous solutions (HApD). Figure2a containing zinc ions in a concentration of 50 mg/L. The XRD peaks from X-ray diffraction patterns of showsprepared the XRD HAp patterns and ofHApD the HAp corresponds and HApD to the after stan zincdard adsorption XRD peaks from of hexagonal aqueous solutionshydroxyapatite containing zinc ions(JCPDS in a 9-432). concentration XRD analysis of 50 did mg/L. not reveal The other XRD calcium peaks fromphosphate X-ray phases diffraction or mineral patterns formation of prepared in HAp andthe solid HApD of Zn corresponds reaction with to phosphate the standard in agreem XRDent peaks with of previous hexagonal studies hydroxyapatite conducted by Cao (JCPDS et al.9-432). XRD analysis[50]. The didcrystal not size reveal was other determined calcium based phosphate on Scherer’s phases formula or mineral (D = 0.9 formationλ/βcosθ). inIn theScherer’s solid of Zn reactionformula, with phosphateλ is the wave in length agreement of X-rays, with β previousis full width studies at halfconducted height of peak by in Cao radians et al. and [50 ].θ Theis the crystal size wasangle determined of diffraction. based To estimate on Scherer’s the mean formula crystal (Dlite = size 0.9 λ(D)/β ofcos theθ ).HAp InScherer’s and HApD formula, samples, λtheis line the wave broadening of the peaks at 2θ = 25.8° assigned to the crystalline plane (002) was used. The crystal size length of X-rays, β is full width at half height of peak in radians and θ is the angle of diffraction. of HAp determined using the Scherer’s formula was 18.8 ± 1 nm. The D value of the HApD was 15.12 To estimate± 1 nm. theThe meancharacteristic crystallite diffraction size (D)peaks of such the HApas (002), and (210), HApD (211), samples,(300), (202), the (310), line (222), broadening (213), of ◦ the peaksand (004) at 2 θcan= 25.8be clearlyassigned seen, indicating to the crystalline the formation plane of HAp (002) crystalline was used. phase. The The crystal XRD patterns size of HAp determinedof HApD using showed the Scherer’s a similar formulacurve. We was observed 18.8 ± that1 nm. the Thepeak D associated value of to the (002) HApD diffraction was 15.12 of the± 1 nm. The characteristicHApD shifted diffractiontoward the high peaks diffraction such as angle (002), (Fig (210),ure 2b). (211), According (300), to (202), Hayakawa (310), et (222),al. [51], (213), this and (004) canbehavior be clearly suggested seen, that indicating the Zn2+ ions the were formation substituted of HApfor thecrystalline Ca2+ ion. phase. The XRD patterns of HApD showedSEM images a similar of curve.the HAp We powders observed before that and the peakafter zinc associated adsorption to (002) from diffraction aqueous solutions of the HApD containing zinc ions in a concentration of 50 mg/L are presented in Figure 3a,b. The SEM images shifted toward the high diffraction angle (Figure2b). According to Hayakawa et al. [ 51], this behavior revealed that all investigated samples presented nanoparticles with elongated morphology and a suggested that the Zn2+ ions were substituted for the Ca2+ ion. tendency of forming agglomerates due to their nanometric size. Furthermore, SEM images suggested SEMthat the images morphology of the of HAp the samples powders was before slightly and influenced after zinc by the adsorption presence of from the zinc aqueous ions in solutionsthe containingpowders. zinc ions in a concentration of 50 mg/L are presented in Figure3a,b. The SEM images revealed thatIn the all EDX investigated spectra of Hap, samples the presence presented of peaks nanoparticles for Ca, P and O, with the major elongated constituent morphology elements and a tendencyof hydroxyapatite, of forming agglomerateshave been highlighteddue totheir (Figure nanometric 4a). In addition size. Furthermore, to HAp’s constituents, SEM images the EDX suggested that thespectrum morphology of HApD of the reveals samples the presence was slightly of zinc influenced (Figure 4b). by By the the presence presence of of the the zinc maxima ions attributed in the powders. Into the zinc EDX in the spectra EDX spectrum of Hap, of the HApD, presence we can of peaksconclu forde that Ca, zinc P and ions O, were the majorefficiently constituent adsorbed elementsby synthetized HAp. of hydroxyapatite, have been highlighted (Figure4a). In addition to HAp’s constituents, the EDX spectrum of HApD reveals the presence of zinc (Figure4b). By the presence of the maxima attributed to zinc in the EDX spectrum of HApD, we can conclude that zinc ions were efﬁciently adsorbed by synthetized HAp. Materials 2018, 11, 1350 5 of 16 Materials 2018, 11, x FOR PEER REVIEW 5 of 16 Materials 2018, 11, x FOR PEER REVIEW 5 of 16

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FigureFigure 2.2. XRDXRD patternspatterns ofof hydroxyapatite (HAp)(HAp) andand hydroxyapatitehydroxyapatite recoveredrecovered afterafter zinczinc adsorptionadsorption Figure 2. XRD patterns of hydroxyapatite (HAp) and hydroxyapatite recovered after zinc adsorption fromfrom aqueousaqueous solutions solutions (HApD) (HApD) containing containing concentration concentration of of 50 50 mg/L mg/L zinc zinc ions. ions. (a) ( 2aθ) range2θ range of 25–55 of 25–◦; from aqueous solutions (HApD) containing concentration of 50 mg/L zinc ions. (a) 2θ range of 25– (55°;bFigure) 2 θ(brange) 2.2θ XRD range of 25–27patterns of 25–27°.◦. of hydroxyapatite (HAp) and hydroxyapatite recovered after zinc adsorption 55°; (b) 2θ range of 25–27°. from aqueous solutions (HApD) containing concentration of 50 mg/L zinc ions. (a) 2θ range of 25– 55°; (b) 2θ range of 25–27°.

Figure 3. SEM images of HAp (a); and hydroxyapatite recovered after zinc adsorption from aqueous Figure 3. SEM images of HAp (a); and hydroxyapatite recovered after zinc adsorption from aqueous Figuresolutions 3. SEM(HApD) images containing of HAp concen (a); andtration hydroxyapatite of 50 mg/L recovered zinc ions ( afterb). zinc adsorption from aqueous solutionsFigure 3. (HApD)SEM images containing of HAp concen (a); andtration hydroxyapatite of 50 mg/L recovered zinc ions af(bter). zinc adsorption from aqueous solutions (HApD) containing concentration of 50 mg/L zinc ions (b). solutions (HApD) containing concentration of 50 mg/L zinc ions (b).

Figure 4. EDX spectrum and elemental mapping of synthetized HAp (a); and EDX spectrum and Figure 4. EDX spectrum and elemental mapping of synthetized HAp (a); and EDX spectrum and FigureelementalFigure 4.4. EDXEDXmapping spectrumspectrum of hydroxyapatite and elemental mapping mappingrecovered of of aftersynthetized synthetized zinc adsorption HAp HAp (a ();a ); andfrom and EDX EDXaqueous spectrum spectrum solutions and and elemental mapping of hydroxyapatite recovered after zinc adsorption from aqueous solutions elemental(HApD)elemental containing mapping mapping of concentratio of hydroxyapatite hydroxyapatiten of 50 recovered mg/Lrecovered zinc after ionsafter zinc (b zinc). adsorption adsorption from aqueousfrom aqueous solutions solutions (HApD) (HApD) containing concentration of 50 mg/L zinc ions (b). containing(HApD) containing concentration concentratio of 50 mg/Ln of 50 zinc mg/L ions zinc (b). ions (b). In addition, the elemental mapping of HAp and HApD, as shown in Figures 4 and 5, provided In addition, the elemental mapping of HAp and HApD, as shown in Figures 4 and 5, provided informationIn addition, about the the elemental homogeneity mapping of the of samplesHAp and an HApD,d the uniform as shown distribution in Figures 4of and the 5, Ca, provided P and O. informationinformation aboutabout thethe homogeneityhomogeneity ofof thethe samplessamples anandd the the uniform uniform distribution distribution of of the the Ca, Ca, P Pand and O. O.

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In addition, the elemental mapping of HAp and HApD, as shown in Figures4 and5, provided Materials 2018, 11, x FOR PEER REVIEW 6 of 16 information about the homogeneity of the samples and the uniform distribution of the Ca, P and O. The uniform distribution of Zn was also observed in HApD. These results were fully consistent with X-ray diffraction analysis. Figure 55 presentspresents thethe transmissiontransmission electronelectron micrographsmicrographs ofof purepure hydroxyapatitehydroxyapatite (HAp)(HAp) andand hydroxyapatite recovered after zinc adsorption from aqueous solutions (HApD) containing zinc ions ± in a concentration ofof 5050 mg/L.mg/L. The The average average particle size ofof thethe purepure HApHAp waswas 2020 ± 11 nm. The The average ± particle size of the HApD decreased toto 1717 ± 11 nm, nm, in in agreement with the X-ray results. The particles are less elongated. Furthermore, it should be not noteded that more agglomeration of the particles after Zn ion adsorption was observed. The The particle size and morphology changed little after zinc adsorption from aqueous solutions containing 50 mg/L zinc zinc ions. ions.

Figure 5. Transmission electron microscopy (TEM) micrographs of the HAp powders before (a) and Figure 5. Transmission electron microscopy (TEM) micrographs of the HAp powders before (a) and after zinc adsorption from aqueous solutions containing zinc ions (HApD) in a concentration of 50 after zinc adsorption from aqueous solutions containing zinc ions (HApD) in a concentration of mg/L (b). Scale bar: 50 nm. 50 mg/L (b). Scale bar: 50 nm.

The adsorption data obtained from the batch equilibrium experiments were used in the kinetic studiesThe and adsorption the applicability data obtained of different from theadsorption batch equilibrium isotherms to experiments Zn2+ were studied. were used The in percentage the kinetic 2+ removalstudies and efficiency the applicability of the adsorbents of different R(%) adsorption and the sorption isotherms capacity to Zn atwere equilibrium studied. were The percentagecalculated usingremoval the efﬁciency following of Equations the adsorbents (1) and R(%) (2): and the sorption capacity at equilibrium were calculated using the following Equations (1) and (2): C C R% = × 100 (1) C C− C R(%) = 0 e × 100 (1) C0 V q = C C × (2) mV qe = (C0 − Ce) × (2) where C0 (mg/L) is the initial metal ion concentration, Ce (mg/L)m is the equilibrium concentration of whereZn (II), C V0 (L)(mg/L) is the is volume the initial of the metal solution ion concentration, and m (g) is Cthee (mg/L) mass of is the the adsorbent. equilibrium concentration of Zn (II),The V adsorption (L) is the volume of zinc of ions the solutionfrom aqueous and m solutions (g) is the massusing ofHAp the adsorbent.nanopowders was studied in batchThe experiments adsorption and of zinc the ionsremoval from efficiency aqueous solutions of adsorption using HApof zinc nanopowders ions from aqueous was studied solution in batch by HApexperiments is presented and thein Figure removal 6. efﬁciencyThe initial ofzinc adsorption ions concentration of zinc ions was from set aqueousin the range solution of 5–150 by HAp mg/L. is Inpresented Figure 6, in the Figure adsorption6. The initial capacity zinc ionsincreases concentration with the wasincrease set in of the the range initial of 5–150zinc ions mg/L. concentration. In Figure6, Bythe increasing adsorption the capacity initial increases concentration with the of increasezinc, the of interaction the initial zincbetween ions concentration.adsorbent and By zinc increasing ions is enhanced,the initial concentrationleading to an of increase zinc, the in interaction the adsorpti betweenon uptake adsorbent of zinc and ions zinc onto ions HAp is enhanced, nanopowders. leading Figureto an increase 6 shows in that the adsorptionthe percentage uptake zinc of zincions ionsremoval onto HApincreased nanopowders. from 30.88% Figure to 641.57% shows with that thethe increasepercentage of the zinc initial ions removalzinc concentration increased from 5 30.88% mg/L to to 150 41.57% mg/L. with It was the also increase noticed of the that, initial after zinc the concentration fromof 110 5 mg/L, mg/L the to 150 removal mg/L. perc It wasentage also stabilized noticed that, around after the the value concentration of 40%. of 110 mg/L, the removalThe values percentage corresponding stabilized to the around percentage the value of zi ofnc 40%. ions removal and the adsorption capacity as a function of zinc concentrations at equilibrium are presented in Table 1. The results show that the adsorption capacity of HAp nanopowders increases with the increase of the zinc initial concentration.

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FigureFigure 6. 6. RemovalRemoval efficiency efﬁciency of of zinc zinc ions ions on on HAp HAp de dependingpending on the initial zinc concentration.

Table 1. The percentage of zinc ions removal and adsorption capacity of HAp upon varying the zinc The values corresponding to the percentage of zinc ions removal and the adsorption capacity as concentrations. a function of zinc concentrations at equilibrium are presented in Table1. The results show that the adsorptionZinc Concentration capacity of HAp (mg/L) nanopowders % Removal increases of Zinc with Ions the increaseAdsorption of the Capacity zinc initial qe concentration. (mg/g) 10 34.65 3.47 Table 1. The percentage20 of zinc ions removal35.4 and adsorption capacity of HAp 7.08 upon varying the zinc concentrations.30 39.97 11.99 50 40.36 20.18 Zinc Concentration70 (mg/L) % Removal40.77 of Zinc Ions Adsorption Capacity 28.54 qe (mg/g) 10 34.65 3.47 The equilibrium20 interactions between adsorbent 35.4 and adsorbate are usually 7.08 described with the 30 39.97 11.99 aid of sorption isotherms50 [52]. The experimental 40.36 data obtained from 20.18 the batch equilibrium experiments were fitted70 using Langmuir and Freundlich 40.77 isotherm equations. 28.54 Langmuir isotherm is generally constructed with the assumption that a fixed number of adsorption sites are available and that the adsorption is reversible; therefore, this model can be used only when the adsorbent surface The equilibrium interactions between adsorbent and adsorbate are usually described with the aid is homogeneous. The Langmuir isotherm is expressed as described by Langmuir [53]: of sorption isotherms [52]. The experimental data obtained from the batch equilibrium experiments C C 1 were ﬁtted using Langmuir and Freundlich= isotherm+ equations. Langmuir isotherm is generally(3) constructed with the assumption that aq ﬁxedq numberK ×q of adsorption sites are available and that wherethe adsorption Ce is the equilibrium is reversible; concentration therefore, this of heavy model metal can be ions used (mg/L), only q whene is the the equilibrium adsorbent adsorption surface is uptakehomogeneous. of heavy The metal Langmuir ions (mg/g), isotherm qm is is the expressed amount asadsorbed described to byform Langmuir a complete [53 ]:monolayer on the surface (mg/g) and KL is the Langmuir constant (L/mg). Ce Ce 1 Meanwhile, the Freundlich model is based= on+ the assumption that the adsorption takes place on(3) q q KL × q a heterogeneous surface. The Freundlich isotherme m is describem by the following equation [54]: where Ce is the equilibrium concentration of heavy metal1 ions (mg/L), qe is the equilibrium adsorption n uptake of heavy metal ions (mg/g), qm islnq the amount=lnK + adsorbed to form a complete monolayer on(4) the lnC surface (mg/g) and KL is the Langmuir constant (L/mg). whereMeanwhile, qe is the amount the Freundlich that was adsorbed model isbased at the onequilibrium the assumption concentration that the (mg/g), adsorption KF is takes an empirical place on constanta heterogeneous of Freundlich surface. isotherm The Freundlich (L/mg), C isotherme is the equilibrium is describe by concentration the following of equation zinc ions [ 54in]: solution (mg/L) and n is a parameter correlated with the intensity of adsorption. For a better understanding of the adsorption capacity1 of HAp nanopowders, the equilibrium = + n lnqe lnKF (4) adsorption isotherm for zinc ions onto HAp at room temperaturelnCe is presented in Figure 7. Figure 7 shows the adsorption capacity of zinc ions upon varying the zinc ions concentration in the aqueous where q is the amount that was adsorbed at the equilibrium concentration (mg/g), K is an empirical solutions.e F constant of Freundlich isotherm (L/mg), Ce is the equilibrium concentration of zinc ions in solution (mg/L) and n is a parameter correlated with the intensity of adsorption.

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For a better understanding of the adsorption capacity of HAp nanopowders, the equilibrium adsorption isotherm for zinc ions onto HAp at room temperature is presented in Figure7. Figure7 shows the adsorption capacity of zinc ions upon varying the zinc ions concentration in the aqueous solutions. Materials 2018, 11, x FOR PEER REVIEW 8 of 16

Materials 2018, 11, x FOR PEER REVIEW 8 of 16

Figure 7. Equilibrium adsorption isotherm for zinc ions onto HAp at room temperature. Figure 7. Equilibrium adsorption isotherm for zinc ions onto HAp at room temperature. Figure 7. Equilibrium adsorption isotherm for zinc ions onto HAp at room temperature. The Langmuir and Freundlich linearized fits for the adsorption of zinc ions on HAp are shown The Langmuir and Freundlich linearized ﬁts for the adsorption of zinc ions on HAp are shown in in FigureThe 8. Langmuir The constants and Freundlich calculated linearized from the linearizedfits for the plotsadsorption are reported of zinc inions Table on HAp 2. are shown Figure8. The constants calculated from the linearized plots are reported in Table2. in Figure 8. The constants calculated from the linearized plots are reported in Table 2.

Figure 8. Langmuir (a); and Freundlich (b) linearized fits for the adsorption of zinc ions on HAp. Figure 8. Langmuir (a); and Freundlich (b) linearized fits for the adsorption of zinc ions on HAp. Figure 8. Langmuir (a); and Freundlich (b) linearized fits for the adsorption of zinc ions on HAp. Table 2. Isotherms parameters for zinc adsorption onto HAp. Table 2. Isotherms parameters for zinc adsorption onto HAp. Table 2. Isotherms parametersLangmuir for zinc adsorption ontoFreundlich HAp. Pollutant Sample Langmuir Freundlich Pollutant Sample qm (mg/g) KL (L/mg) R2 N KF R2 qm (mg/g) Langmuir KL (L/mg) R2 N FreundlichKF R2 PollutantZn2+ SampleHAp 57.504 0.052 0.995 0.884 0.408 0.997 2+ 2 2 Zn HAp qm 57.504(mg/g) KL 0.052(L/mg) 0.995R 0.884NK 0.408F 0.997R At room temperature,Zn2+ HAp the correlation 57.504 coefficient 0.052 of Freundlich 0.995 0.884 isotherm 0.408 for 0.997Zn2+ removal by At room temperature, the correlation coefficient of Freundlich isotherm for Zn2+ removal by HAp was 0.997 and had a higher value than the correlation coefficient of Langmuir isotherm, which HAp was 0.997 and had a higher value than the correlation coefficient of Langmuir isotherm, which was 0.995. On the other hand, the maximum adsorption capacity for the solid phase2+ calculated from wasAt 0.995. room On temperature, the other hand, the correlationthe maximum coefficient adsorption of Freundlich capacity for isotherm the solid for phase Zn calculatedremoval by from HAp the linearized form of Langmuir equation, qm, for Zn2+ was 57.504 mg/g and the Langmuir constant wasthe 0.997linearized and hadform a of higher Langmuir value equation, than the correlationqm, for Zn2+ coefficientwas 57.504 ofmg/g Langmuir and theisotherm, Langmuir which constant was KL for the adsorption of Zn2+ was 0.052 L/mg. The linear plot of Ce/Qe against Ce for the adsorption of 0.995.KL for On the the adsorption other hand, of Zn the2+ was maximum 0.052 L/mg. adsorption The linear capacity plot of for Ce/Q thee against solid phase Ce for calculated the adsorption from of the zinc ions on HAp exhibited in Figure 8a revealed 2+that the adsorption is in good agreement with the linearizedzinc ions on form HAp of Langmuirexhibited in equation, Figure 8a q mrevealed, for Zn thatwas the 57.504 adsorption mg/g is and in good the Langmuir agreement constant with the K L LangmuirLangmuir model. model. ByBy fitting fitting the the data data usingusing thethe FreundlichFreundlich adsorption isothermisotherm model,model, a a linear linear relationship relationship was was obtained where KF has been equal to 0.408, and the 1/n was equal to 1.13 (Figure 8b). Following these obtained where KF has been equal to 0.408, and the 1/n was equal to 1.13 (Figure 8b). Following these results,results, the the inverse inverse of of the the empirical empirical parameterparameter related to thethe intensityintensity of of adsorption adsorption has has values values greater greater thanthan 1 1(1/n (1/n > > 1) 1) which which shows shows thatthat thethe absorptionabsorption coefficient increasesincreases with with increase increase in in concentration concentration resultedresulted as as effect effect ofof increaseincrease inin hydrophobichydrophobic surface characteristicscharacteristics afterafter monolayer.monolayer. The The results results

Materials 2018, 11, 1350 9 of 16

2+ for the adsorption of Zn was 0.052 L/mg. The linear plot of Ce/Qe against Ce for the adsorption of zinc ions on HAp exhibited in Figure8a revealed that the adsorption is in good agreement with the Langmuir model. By fitting the data using the Freundlich adsorption isotherm model, a linear relationship was obtained where KF has been equal to 0.408, and the 1/n was equal to 1.13 (Figure8b). Following these results, the inverse of the empirical parameter related to the intensity of adsorption has values greater than 1 (1/n > 1) which shows that the absorption coefficient increases with increase in concentration resulted as effect of increase in hydrophobic surface characteristics after monolayer. The results suggest that HAp nanopowders exhibited a good affinity towards zinc ions, rendering HAp a suitable promising adsorbent for water treatment technologies. For the first time, the effectiveness of hydroxyapatite in water decontamination was highlighted by ultrasonic studies on water (Wi), zinc-contaminated water (50 mg/L zinc ions) (WZn) and water after decontamination (Wd). The parameters confirming the decontamination are the signal velocity in the suspension and the similarity of the acoustic signals of decontaminated water and pure water. For the interpretation of the ultrasonic signals, specialized programs have been used, able to provide essential information on the parameters characteristic of each studied material. For the ultrasonic characterization of the dispersions presented above (Wi, HZ2 and Wf) it was considered that the propagation velocity of the acoustic waves in homogeneous liquids can be calculated with the relation: s 1 c = (5) κρ where the adiabatic compressibility κ is the inverse of the compression elastic modulus (K = 1/κ). The ultrasound velocity through solutions and suspensions, in the linear approximation of small disturbances, depends on the average density and average compressibility. These averaged values are calculated from the Urick equations [55]:

κ = ∑ Φiκi; ρ = ∑ Φiρi (6) i i in which Φi represents the volumetric fraction of component of index i. The volumetric fraction can be transformed into mass fraction by:

Φρ w = 2 (7) Φρ2 + (1 − Φ)ρ1 in thecase of two phases, marked by index 1 for the solvent and 2 for the dispersed phase [43,44]:

κ = Φκ2 + (1 − Φ)κ1; ρ = Φρ2 + (1 − Φ)ρ1 (8)

It is known that water has a different ultrasonic acoustic behavior in many aspects from other fluids. For example, below 74 ◦C, the gradient of ultrasound propagation velocity in water is positive, while for most liquids it is negative. Another special property is the negative dilatation coefficient below 4 ◦C. However, many experimental results are available for water, which facilitates studies based on this solvent. Moreover, the ultrasound speed in water strongly depends on the temperature. This paper presents the experimental results of ultrasonic measurements on water (Wi), zinc contaminated water with 50 mg/L zinc ions (WZn) and water after decontamination (Wd). The time intervals between echoes make it possible to accurately determine the distance between the transducer face and the flat bottom surface of the used aluminum container. The obtained signals were processed using a specialized algorithm implemented in Matlab (Matlab2016). The modeling of the ultrasound wave effect on the analyzed samples was performed with an original model implemented in Comsol–Multiphysics finite elements program. The specialized program for temporal signal analysis Materials 2018, 11, 1350 10 of 16 allows determining the time differences between equivalent echoes in different fluids, with an accuracy of 1 ns. For all tested fluids, the reference was bi-distilled water. The small signals observed before the second and third echoes are signals of multiple reflections on the flat bottom of the aluminum container used for the fluids tested. These signals do not affect the results obtained in the signal interpretation program. Figure9 shows the signals acquired for Wi, WZn and Wd. Figures 10–12 present the first, second and third echoes for the three studied samples, respectively. Materials 2018, 11, x FOR PEER REVIEW 10 of 16 Materials 2018, 11, x FOR PEER REVIEW 10 of 16 Materials 2018, 11, x FOR PEER REVIEW 10 of 16

FigureFigure 9. 9.Acquired Acquired signals for: Wi Wi ( (aa);); WZn WZn (b (b);); and and Wd Wd (c ().c ). Figure 9. Acquired signals for: Wi (a); WZn (b); and Wd (c). Figure 9. Acquired signals for: Wi (a); WZn (b); and Wd (c).

Figure 10. Zoomed first echo for: Wi (a); WZn (b); and Wd (c). FigureFigure 10. 10.Zoomed Zoomed ﬁrstfirst echo for: Wi Wi ( (aa);); WZn WZn (b (b);); and and Wd Wd (c ().c ). Figure 10. Zoomed first echo for: Wi (a); WZn (b); and Wd (c).

Figure 11. Zoomed second echo for: Wi (a); WZn (b); and Wd (c). Figure 11. Zoomed second echo for: Wi (a); WZn (b); and Wd (c). FigureFigure 11. 11.Zoomed Zoomedsecond second echoecho for: Wi ( a);); WZn WZn ( (bb);); and and Wd Wd (c ().c).

Materials 2018, 11, 1350 11 of 16 MaterialsMaterials 20182018,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 1111 ofof 1616

FigureFigure 12. 12. ZoomedZoomed third third echo echo for: for: Wi Wi ( (aa);); WZn WZn ( (bb);); and and Wd Wd ( (cc).).

TheThe time time differences differences betweenbetween eacheach of of the the three three echoes echoes obtained obtained for for WiWi ee andand WZnWZn werewere were obtained.obtained. obtained. Thus,Thus, inin thethethe casecasecase of ofof the thethe first firstfirst echo echoecho recorded recordedrecorded for forfor Wi WiWi and andand WZn, WZn,WZn, there therethere was waswas a temporal aa temporaltemporal difference differencedifference of 1.67ofof 1.671.67µs. μForμs.s. ForFor the thethe second secondsecond echo, echo,echo, the thethe temporal temporaltemporal difference differencedifference was waswas 3.255 3.2553.255µ μs,μs,s, which whichwhich corresponds correspondscorresponds to toto aaa differencedifferencedifference in in equivalentequivalent signalssignals of ofof 1.628 1.6281.628µ μμs.s.s. Similarly, Similarly,Similarly, for forfor the thethe third thirdthird echo, echo,echo, the thethe time timetime difference differencedifference to to thisto thisthis echo echoecho was waswas 5.17 5.175.17µs, μthusμs,s, thusthus obtaining obtainingobtaining a temporal aa temporaltemporal difference differencedifference of of 1.723of 1.7231.723µs. μμs.s. Using UsingUsing the thethe mean meanmean of ofof these thesethese time timetime differences differencesdifferences andand thethe distancedistance traveled traveled byby thethe the signals,signals, signals, thethe the ultrasonicultrasonic ultrasonic velovelo velocitycitycity waswas was obtainedobtained obtained byby by WZnWZn WZn (1454.17(1454.17 (1454.17 ±± 1.061.06± 1.06 m/s)m/s) m/s) andand anandan attenuationattenuation an attenuation ofof 9.110559.11055 of 9.11055 Np/mNp/m Np/m (1.04889(1.04889 (1.04889 dB/m).dB/m). dB/m). InIn addition,addition, In addition, thethe timetime the differencesdifferences time differences betweenbetween between eacheach ofof each thethe threeofthree the echoesechoes three echoes obtainedobtained obtained forfor WiWi for andand Wi WdWd and werewere Wd were obtained.obtained. obtained. ComparingComparing Comparing thethe firstfirst the firstrecordedrecorded recorded echoecho echo forfor forbi-bi- distilledbi-distilleddistilled waterwater water andand and WdWd Wd dispersiondispersion dispersion,,, aa time atime time differencedifference difference ofof of 0.7030.703 0.703 μμµss s waswas was obtained.obtained. obtained. ForFor For thethe the second second echo,echo, thethe time time difference difference was was 0.701 0.7010.701 μµμs.s. Similarly, Similarly, for for the the third third echo, echo, the the temporal temporal differencedifference waswas 0.6910.691 μμµs.s. ConsideringConsidering thesethese temporaltemporal differences, differences,differences, ultrasonic ultrasonicultrasonic velocity velocityvelocity was waswas obtained obtainedobtained as asas 1476.02 1476.021476.02± ±± 0.15m/s 0.15m/s0.15m/s for for Wd Wd andand a a corresponding corresponding attenuationattenuation ofof 3.874193.87419 Np/mNp/m (0.446033(0.446033 dB/m).dB/m). FiguresFigures 13–151313–15–15 showshow overlappingoverlapping ofof thethe threethree echoesechoes obtainedobtained forfor WiWiWi andandand WZn.WZn.WZn. TheThe smallersmaller signals,signals, due due to to multiple multiple reflections reflectionsreflections on on the the flat flatflat bottombottom ofof thethe aluminumaluminum containercontainer superposedsuperposed over over thosethose of of Wi Wi (blue(blue signals),signals), cancan bebe seen.seen. The The almost almostalmost disappearance disappearance of of these these low low intensity intensity signals signals in in the the casecase of of WZn WZn dispersions dispersions (orange (orange signals) signals) is is determi determineddeterminedned by by the the increased increased ultrasonic ultrasonic signal signal attenuation forfor this this medium. medium. In InIn overlapping overlappingoverlapping these these signals, signals, the the time time delay delay is is optimized optimized for for overlapping overlapping the the signal signal modules,modules, avoidingavoidingavoiding problems problemsproblems of phaseofof phasephase jumps jumpsjumps of the ofof signal ththee signal atsignal the successiveatat thethe successivesuccessive reflections reflectionsreflections on the aluminum onon thethe aluminumbottomaluminum wall. bottombottom wall.wall.

FigureFigure 13.13. OverlappingOverlapping of of the the first ﬁrstfirst echo echo of of Wi Wi (blue)(blue) overover signalssignals from:from: WZn WZn ( (aa);); and andand Wd WdWd ( ((bb))) (orange). (orange).(orange).

The intensities of WZn signals are much weaker than those associated with Wi. This is due to the increased attenuation through the investigated dispersions. In the case of decontamination

Materials 2018, 11, 1350 12 of 16 water (Wd), the three signals resemble those of the Wi-associated signals, even if they are slightly attenuated. This behavior reveals that the amount of zinc in the Wd sample dropped significantly, since the decontaminated water has ultrasonic characteristics (velocity, signal aspect) closely resembling those of pure water. A great advantage of ultrasound measurements that were first used in such a study is that this method is reproducible and non-destructive. This method offers a quick analysis with low costs, as the consumption of resources such as water, electricity, etc. is low. On the other hand, the analyzed material (liquid or solid) is not damaged during the analysis and can be examined in the operative condition. The results obtained by ultrasound studies confirms the effectiveness of Materials 2018, 11, x FOR PEER REVIEW 12 of 16 hydroxyapatiteMaterials 2018, 11, x in FOR the PEER decontamination REVIEW of zinc-polluted water. 12 of 16

Figure 14. Overlapping of the second echo of Wi (blue) over signals from: WZn (a); and Wd (b) Figure 14. Overlapping of the second echo of Wi (blue) over signals from: WZn (a); and Wd (b) Figure(orange). 14. Overlapping of the second echo of Wi (blue) over signals from: WZn (a); and Wd (b) (orange). (orange).

Figure 15. Overlapping of third echo of Wi (blue) over signals from: WZn (a); and Wd (b) (orange). FigureFigure 15.15. OverlappingOverlapping of third echo of Wi (blue) over signals from: WZn WZn ( (aa);); and and Wd Wd ( (bb)) (orange). (orange). The intensities of WZn signals are much weaker than those associated with Wi. This is due to TheThe present intensities research of WZn is part signals of the are recent much concern weaker about than thethose use associated of hydroxyapatite with Wi. asThis an adsorbentis due to the increased attenuation through the investigated dispersions. In the case of decontamination water forthe metals increased in water. attenuation Due to through its high the affinity investigated for divalent dispersions. heavy metals In the ions, case hydroxyapatiteof decontamination is a uniquewater (Wd), the three signals resemble those of the Wi-associated signals, even if they are slightly inorganic(Wd), the compound three signals and hasresemble been used those to remove of the variousWi-associated heavy metals signals, such even as nickel if they [56 ]are lead slightly [56–58], attenuated. This behavior reveals that the amount of zinc in the Wd sample dropped significantly, cadmiumattenuated. [59 This], cooper behavior [60], reveals zinc [8, 61that] and the arsenicamount [ 62of]. zinc In theirin the study, Wd sample Ramesh dropped et al. [4] significantly, reported that since the decontaminated water has ultrasonic characteristics (velocity, signal aspect) closely HApsince had the a decontaminated greater affinity towards water cooperhas ultrasonic ions compared characteristics to zinc ions,(velocity, estimating signal a removalaspect) capacityclosely resembling those of pure water. A great advantage of ultrasound measurements that were first used ofresembling 125 mg of those Cu/g of and pure 30.3 water. mg ofA great Zn/g. advantage Meanwhile, of ultrasound Chen et al. measurements [62] highlighted that that were the first sorption used in such a study is that this method is reproducible and non-destructive. This method offers a quick in such a study is that this method is reproducible and non-destructive. This method offers a quick affinityanalysis of with HAp low for leadcosts, ions as the in single-metalconsumption sorption of resources systems such was as water, significantly electricity, higher etc. than is low. for On copper the analysis with low costs, as the consumption of resources such as water, electricity, etc. is low. On the andother cadmium hand, the ions. analyzed Regarding material the adsorption (liquid or capacitysolid) is ofnot zinc damaged ions onto during hydroxyapatite, the analysis studiesand can have be other hand, the analyzed material (liquid or solid) is not damaged during the analysis and can be reportedexamined values in the in theoperative range ofcondition. 19.76–109.37 The mg/gresults [ 63obtained,64]. The by studies ultrasound presented studies above confirms are in good the examined in the operative condition. The results obtained by ultrasound studies confirms the agreementeffectiveness with of thehydroxyapatite results obtained in the in decontamination this research regarding of zinc-polluted the use of water. hydroxyapatite powders in effectiveness of hydroxyapatite in the decontamination of zinc-polluted water. the removalThe present of zinc research ions from is contaminated part of the recent aqueous concern solutions. about the use of hydroxyapatite as an The present research is part of the recent concern about the use of hydroxyapatite as an adsorbentIn conclusion, for metals we in could water. say Due that to theits high hydroxyapatite affinity for divalent has proven heavy to beme atals good ions, absorbent hydroxyapatite for zinc adsorbent for metals in water. Due to its high affinity for divalent heavy metals ions, hydroxyapatite ionsis a andunique could inorganic also be usedcompound for other and pollutants. has been used Furthermore, to remove hydroxyapatite various heavy has metals additional such as benefits nickel is a unique inorganic compound and has been used to remove various heavy metals such as nickel such[56] aslead low [56–58], costsand cadmium being easy[59], cooper to obtain. [60], zinc [8,61] and arsenic [62]. In their study, Ramesh et al. [56] lead [56–58], cadmium [59], cooper [60], zinc [8,61] and arsenic [62]. In their study, Ramesh et al. [4] reported that HAp had a greater affinity towards cooper ions compared to zinc ions, estimating a [4] reported that HAp had a greater affinity towards cooper ions compared to zinc ions, estimating a removal capacity of 125 mg of Cu/g and 30.3 mg of Zn/g. Meanwhile, Chen et al. [62] highlighted that removal capacity of 125 mg of Cu/g and 30.3 mg of Zn/g. Meanwhile, Chen et al. [62] highlighted that the sorption affinity of HAp for lead ions in single-metal sorption systems was significantly higher the sorption affinity of HAp for lead ions in single-metal sorption systems was significantly higher than for copper and cadmium ions. Regarding the adsorption capacity of zinc ions onto than for copper and cadmium ions. Regarding the adsorption capacity of zinc ions onto hydroxyapatite, studies have reported values in the range of 19.76–109.37 mg/g [63,64]. The studies hydroxyapatite, studies have reported values in the range of 19.76–109.37 mg/g [63,64]. The studies presented above are in good agreement with the results obtained in this research regarding the use presented above are in good agreement with the results obtained in this research regarding the use of hydroxyapatite powders in the removal of zinc ions from contaminated aqueous solutions. of hydroxyapatite powders in the removal of zinc ions from contaminated aqueous solutions. In conclusion, we could say that the hydroxyapatite has proven to be a good absorbent for zinc In conclusion, we could say that the hydroxyapatite has proven to be a good absorbent for zinc ions and could also be used for other pollutants. Furthermore, hydroxyapatite has additional benefits ions and could also be used for other pollutants. Furthermore, hydroxyapatite has additional benefits such as low costs and being easy to obtain. such as low costs and being easy to obtain.

Materials 2018, 11, 1350 13 of 16

4. Conclusions This study investigated the efficiency of hydroxyapatite obtained by an adapted coprecipitation method in the removal of zinc from aqueous solutions. During the process of removing zinc ions from aqueous solutions, zinc ions were introduced into the HAp lattice. After introducing zinc ions into the HAp lattice, a slight change in crystallinity and a decrease in particle size was observed. The adsorption of zinc ions from aqueous solutions was evidenced by XRD, SEM, and TEM. Hydroxyapatite has good affinity for Zn. The effectiveness of hydroxyapatite in the decontamination of zinc-polluted water was confirmed for the first time by ultrasound studies. The decontaminated water has ultrasonic characteristics such as velocity and signal aspect closely resembling those of pure water. The sorption of Zn onto HAp was well characterized by the Langmuir and Freundlich models. The results revealed that the maximum adsorption capacity for the solid phase calculated from the linearized form of 2+ Langmuir equation, qm, for Zn was 57.504 mg/g and the Langmuir constant KL for the adsorption of Zn2+ was 0.052 L/mg. Furthermore, for the first time, the effectiveness of hydroxyapatite in the adsorption of zinc ions from contaminated solutions was highlighted by ultrasonic measurements. The results obtained from ultrasonic measurements were also in good agreement with those obtained from adsorption experiments. The results obtained in the present study show that HAp nanopowders are good adsorbents for zinc ions and can therefore be considered for applications for removal of zinc ions from wastewater.

Author Contributions: Conceptualization, M.V.P. and D.P.; Formal analysis, S.L.I. and M.V.P.; Investigation, S.L.I., M.M.-H., R.G., M.V.P. and A.M.P.; Methodology, D.P.; Resources, M.M.-H. and R.G.; and Writing—original draft, M.V.P. and D.P. Funding: This research was funded by [Unitatea Executiva pentru Finantarea Invatamantului Superior, a Cercetarii, Dezvoltarii si Inovarii (UEFISCDI)], grant number [PN-III-P1-1.2-PCCDI-2017-0629, Contract No. 43PCCDI/2018]. Acknowledgments: We thank A. Richard and A. Sauldubois from the “Centre de Microscopie Electronique” of University of Orléans for assistance in SEM and TEM data acquisition. This work was supported by the Romanian Ministry of Research and Innovation, UEFISCDI, through the projects PN-III-P1-1.2-PCCDI-2017-0134, Contract Nr. 23PCCDI/2018, PN-III-P1-1.2-PCCDI-2017-0629, Contract No. 43PCCDI/2018 and Contract No. T-IS 251801/04.05.2018. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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