Tetraethyl orthosilicate coated hydroxyapatite powders for lead ions removal from aqueous solutions

The goal of this study was to synthetize and characterize a porous material based on tetraethyl orthosilicate (TEOS) coated hydroxyapatite (HApTh) after removal experiments of Pb2+ ions from aqueous solutions. In order to study the morphology and composition, the samples obtained after removal experiments of Pb2+ ions from aqueous solution with the initial Pb2+ ion concentrations of the aqueous solutions were 0.1 gċL-1 (HApTh-50) and 0.9 gċL-1 (HApTh-450) have been investigated by scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). Removal experiments of Pb2+ ions were carried out in aqueous solutions with controlled concentration of Pb2+. After the removal experiment of Pb2+ ions from solutions, porous hydroxyapatite nanoparticles were transformed into HApTh-50 and HApTh-450 due to the adsorption of Pb2+ ions followed by a cation exchange reaction. The obtained results show that the porous HApTh nanopowders could be used for Pb2+ ions removal from aqueous solutions.


Introduction
One of the major problems encountered in the public health area worldwide is the poisoning with various heavy metals.Researchers around the world have turned their attention to inding new efective and cost-eicient methods for depollution, considering the fact that heavy metals are nonbiodegradable, having the tendency to accumulate in living bodies, leading to disorders of diferent functions and to serious diseases [1,2].Lead (Pb) is a very toxic heavy metal, found in the earth's crust with an average concentration of 16 mg/kg in soils [3, ].On the other hand, lead has been extensively used in various industries, being a constituent in building materials, pipes, lead-acid batteries, bullets, and paints.In a report released by the Agency for Toxic Substances and Disease Registry (ATSDR), in 2007, the harmful efect of lead on the human nervous and reproductive systems was emphasized.Contamination of wastewaters represents a major concern because this is one way of lead bioaccumulation in the food chain [3].Recent studies have revealed that children are more susceptible to lead poisoning, their bodies being able to absorb around 50% of inhaled or ingested lead and the efectsbeingmorepronouncedandlong-lasting,comparedto adults [5].Moreover, the efects of chronic lead exposure to various functions of the human body have already been reported [3,6].In this context, many researchers have focused their attention to developing diferent methods for dangerous heavy metals removal from wastewaters.Among the methods already used, chemical precipitation, membrane iltration, ion-exchange, and adsorption could be mentioned [7][8][9].Of all these methods, the most preferred one is the adsorption of heavy metals ions from aqueous solution using cost-eicient materials [7][8][9].Previous studies have revealed that apatites a r ea b l et os u c c e s s f u l l yr e m o v el e a df r o ma q u e o u ss o l utions [ , 10]. he best known member of the apatite family is hydroxyapatite (HAp).Synthetic hydroxyapatite, Ca 10 (PO 4 ) 6 (OH) 2 , has been used in the last decades in many biomedical applications as coating for orthopedic or dental implants or as illing material for various injuries or defects, being similar to the natural mineral component of the human bones and teeth [11].Furthermore, two of the most appealing properties for environmental applications that HAp possesses are its ability to adsorb complex organic materials and the capacity of heavy metals ion-exchanging .Previous studies have shown that Pb 2+ ions exchange rapidly with Ca, this process inducing morphological changes in the surface region [15][16][17].In order to increase the adsorption ability of hydroxyapatite, an increase in its porosity must be obtained.herefore, doping hydroxyapatite with silicon ions may be the key to increase the porosity, thus creating a better material for lead removal from aqueous solutions.Recent studies have shown that the addition of tetraethoxysilane (TEOS) can induce the formation of large textural pores [18].Consequently, in order to increase the lead adsorption capacity of hydroxyapatite, it must be doped with a silicon based compound, such as TEOS.
he goal of this study was to synthetize and characterize new porous material based on tetraethyl orthosilicate (TEOS) and hydroxyapatite (HAph) ater removal experiment of Pb 2+ ions from aqueous solution.he obtained powders ater removal experiment of Pb 2+ ions from aqueous solution when the initial Pb 2+ ion concentrations of the aqueous solutions were 0.1 g⋅L −1 (HAph-50) and 0.9 g⋅L −1 (HAph-50) have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR).Another objective was to investigate the removal of Pb 2+ ions from aqueous solutions with diferent pH values using HAph nanopowders.

Synthesis of Hydroxyapatite/TEOS Nanocomposites.
Hydroxyapatite/TEOS nanocomposites (HAph) were prepared using tetraethyl orthosilicate and hydroxyapatite.he hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ) nanoparticles were prepared by setting the atomic ratio of Ca/P at 1.67 in accord with [19,20]. he hydroxyapatite (HAp) was immobilized into a tetraethyl orthosilicate foam using the technique r e p o r t e di nt h el i t e r a t u r e [ 20].HAph was obtained when tetraethyl orthosilicate solution (1 g HAp/10 mL) was dropped on the HAp powder.he mixtures were then stirred vigorously for 30 min until homogeneity was achieved.Ater forming stable structures, HAph composites were allowed to dry at 80 ∘ C for 2 h in a vacuum oven for the excess solvent to evaporate.Finally, HAph composite samples were then ground in order to obtain powders.

Samples Characterization.
In order to investigate the composition and morphology of the samples an equipment FESEMHITACHI 700coupledwithanenergydispersiveXray attachment (EDAX/2001 device) was used.TEM studies were carried out using a FEI Tecnai 12 (FEI Company, Hillsboro, OR, USA) equipped with a low-dose digital camera from Gatan Inc. (Pleasanton, CA, USA).Small quantities of HAph powder were dispersed in deionised water and depositedonacoppergridcoatedwithcarbonilm.histechniqueallows a good deinition of crystal morphology.
he FTIR spectra were acquired using a Spectrum BX spectrometer.Pellets of 10 mm diameter for FTIR measurements were prepared (1% of the powder was mixed and ground with 99% KBr) by pressing the powder mixture at a load of 5 tons for 2 min.he spectra were registered in the range from 00 to 000 cm −1 with a resolution of and 128 times scanning.
he Raman spectra were registered with a Renishaw InVia dispersive Raman spectrometer (2012), equipped with a Leica DM microscope and one laser source at 51 nm (gas-type), Spectra Physics Ar ion laser (20 mW). he samples were analyzed using the 51 nm laser with a power at 0.2 mW and 1800L/mmgratings.hespectralrangecoveredforallspectra is 100-2000 cm −1 with a resolution below 2 cm −1 .P r i o rt o each reference measurement, the instrument was calibrated on the internal Si-reference standard (520.6 ± 0.1 cm −1 ).
he removal performance of Pb 2+ ions by the HAph powders was investigated by batch experiments, monitoring t h ec h a n g eo fP b 2+ ion concentration in the aqueous solutions.For these experiments, 5 g of HAph nanocomposites were added to 500 mL aqueous solution with various initial Pb 2+ ion concentrations and pH values in accord with Jang et al. [21]. he initial Pb 2+ ion concentrations of the aqueous solutions were controlled and the values were set in the range 0.1-1.5 g⋅L −1 by dissolving lead nitrate [Pb(NO 3 ) 2 ] in deionized water.he pH values of aqueous solutions with controlled initial Pb 2+ ion concentrations were adjusted from 3 to 6 by adding small amounts of 0.1 M HCl standard solution.For all experiments the solution was stirred constantly for 2 h by a mechanical stirrer at room temperature.

Results and Discussions
Scanning electron microscopy (SEM) was used to characterize the morphology and elemental composition of synthesized powders ater lead removal experiments.SEM images presented in Figure 1 showed that lead incorporation into HAph samples afects the particle shape. he crystal clusters with needle or rod-like shapes were observed in SEM micrographs for the HAph-50 and HAph-50 samples. he clusters with needle or rod-like shapes were distinguished when the Pb 2+ ion concentration from the aqueous solutions was 0.9 g⋅L −1 (HAph-50 samples).
he EDX analysis indicates that calcium, phosphorus, silicon, and oxygen are the major constituents of the samples.he presence of Pb is also observed.
Figure 2 shows the TEM image of HAph powders ater the sorption experiment.hese nanoparticles exhibit  a needle-like morphology usually observed on HAp powders obtained by coprecipitation method [22].Moreover, in TEM images it is easy to observe the formation of a new phase (pyromorphite) with a plate-like morphology (especially in thecaseofHAph-50samples).hisbehaviorsuggeststhat the incorporation of lead in the HAph matrix stimulates local calcium enrichment.
hese images also conirm the hypothesis that the main mechanismofleadimmobilizationisirstofallbasedonthe dissolution of HAph followed by the precipitation of phases with higher lead content [23].
he Fourier transform infrared spectroscopy provides valuable information about the short-range ordering of materials.In Figure 3  his behavior can be explained by the reaction mechanism which involves the HAph dissolution in acid environment andHAphPbcrystallisation [2 ].
In all the spectra the presence at around 6 0 cm −1 of OH vibration peak could be noticed.he broad peaks in the regions 1600-1700 cm −1 and 3200-3 00 cm −1 are attributed to the hydroxyl groups [25][26][27].

Journal of Nanomaterials
he speciic bands of (PO 4 3− ) phosphate groups characteristic to hydroxyapatite structure were observed at 568 cm −1 ,6 3 7 c m −1 ,6 0 5 c m −1 ,9 6 c m −1 , and 1000-1100 cm −1 [28,29]. he 96 cm −1 ba n dca nbea s soc ia t edt o the ] 1 nondegenerated symmetric stretching mode of P-O. he vibrational bands at 605 cm −1 and 568 cm −1 are attributed to the triply degenerated ] 4 vibration of O-P-O bond, and the band at 72 cm −1 maybeattributedtothe] 2 bending of O-P-O bond.Moreover, the weak band observed around 1500 cm −1 c o u l db ea s s i g n e dt ot h eA -t y p ec a r b o n a t ei o n substitutions, whereas the bands at 1 22-1 56 cm −1 (] 3 )can be attributed to B-type CO 3 2− substitutions [30][31][32].
Moreover, it is well known that the vibrations bands (stretching and bending) assigned to the Si-O-Si bonds appear in the 1100-900 cm −1 and 550-00 cm −1 spectral regions.In the FTIR spectra (Figure 3) it is obvious that the stretching and bending bands of SiO 4 groups are overlapping with the band of the PO 4 group [30,31,[33][34][35][36][37].
According to Ping et al. [38]t h ev i b r a t i o nb a n da t 806 cm −1 (HAph-50 sample) could be attributed to Si-O stretching of dimer silicate chains, indicating the start of silicate polymerization.
h eR a m a ns p e c t r ao ft h eH A p h -5 0a n dH A p h -5 0 powders are reported in Figure .he Raman vibrations bands at 959 and 92 cm −1 are attributed to the ] 1 PO 4 vibration modes.In HAph-50 samples, only the 959 cm −1 band appears, whereas the HAph-50 samples exhibit two bands, one strong at 959 cm −1 and the other band at 92 cm −1 .hisbehavior is caused by the A g -E 2g splitting in the hexagonal 6h symmetry [38,39].For the all samples, the vibrational spectra exhibit a strong molecular character associated with the internal modes of the tetrahedral PO In order to evaluate the impact of the lead concentration in the aqueous solution, absorption experiments were performed.HAph solutions with concentration varying from 0.1 to 1.5 g⋅L −1 at pH 5 were used. he measurements were performed on 500 mL solution (pH 5) with an initial Pb 2+ ion concentration of 63 mg⋅L −1 .I nF i g u r e5 the adsorption eiciency of Pb 2+ ions as a function of the Pb 2+ concentration in the solution is presented.It was observed that the removal eiciency is dependent on the initial Pb 2+ concentration.For a lead concentration of 0.2 g⋅L −1 , the removal eiciency reached 98.6%, showing that the adsorbent material (HAph) has a strong ainity to Pb 2+ ions.For Pb 2+ concentration ranging from 0. g⋅L −1 to 1.5 g⋅L −1 , the removal eiciency was a r o u n d1 0 0 % .hi sb e h a v i o rc o u l db ee x p l a i n e db yt h ef a c t that the Pb 2+ ions were completely removed from the solution.
Figure 5 shows the measured lead concentration in the solution ater the reaction with HAph has taken place.For the studies on the efect of the solution pH, a solution containing 0.9 g⋅L −1 of lead was selected (Figure 6).A solution containing 563 mg⋅L −1 of Pb 2+ ions was obtained from 0.9 g of Pb(NO 3 ) 2 in 1 L of distilled water.
A removal eiciency of 100% was achieved for pH 6.On the other hand, the removal eiciency of Pb 2+ ions was 98.5% at pH 3. he removal eiciency of Pb 2+ ions was 99% for pH .When the pH value was set to 5 the removal eiciency of Pb 2+ ions was 99.5%. he present studies have shown that the removal of Pb 2+ ions from aqueous solution by hydroxyapatite/TEOS nanocomposites was greater than 98.6% at all pH values of the aqueous solution used for the removal of Pb 2+ ions.A ccordingtotheseresults,itcanbehighlightedthefact that the HAph can really remove Pb 2+ ions from aqueous solutions.
One of the most important parameters which must be considered in the absorption experiments is the pH of the solutions due to the fact that the pH of groundwater and surface waters varies between 5 and 7 [ 0]. he results of our studies have shown that the solid reaction products of aqueous Pb 2+ with the HAph are mainly pH-dependent, this being in good agreement with other results reported in literature [ 0].
For the assimilation of diferent metals from aqueous solutions by hydroxyapatite [17,[1][2][3] diferent processes have been proposed, such as metal complexation on the HAp surface, apatite dissolution followed by a new metal phase precipitation, and cation exchange.In previous studies [ ], it has been shown that the formation of a stable lead apatite such as Ca (10−) Pb (PO 4 ) 6 (OH) 2 by ion exchange mechanism where Pb 2+ present in the solution replaces Ca 2+ ions from the hydroxyapatite lattice is possible.Furthermore, the Pb (10−) Ca (PO 4 ) 6 (OH) 2 crystals with higher content of calciumareunstable [ 5]. herefore, the HAp should be subject of a permanent process of dissolution and precipitation in order to obtain more stable materials with a higher lead percentage.
ar ep r esen tedtheFTIRspectrao fthe obtained powders ater Pb 2+ removal experiments.he intera c t i o nb e t w e e nl e a d( I I )a n dH A p hs t r u c t u r el e dt os o m e modiications of infrared vibrations of the functional groups.

Figure 5 :Figure 6 :
Figure 5: Efect of the Pb 2+ concentration on the removal of the Pb 2+ ions by HAph.