Porous methyltrimethoxysilane coated nanoscale-hydroxyapatite for removing lead ions from aqueous solutions

The aim of this study was to synthetize new porous nanoparticles based on methyltrimethoxysilane coated hydroxyapatite (MTHAp) for lead removal form aqueous solutions. The morphological and compositional analysis of MTHAp were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). Removal experiments of Pb2+ ions were carried out in aqueous solutions with controlled concentration of Pb2+ at a fixed pH value of 3 and 5 respectively. After the removal experiment of Pb2+ ions from solutions, porous hydroxyapatite nanoparticles were transformed into PbMTHAp_3 and PbMTHAp_5 via the adsorption of Pb2+ ions followed by a cation exchange reaction. The X-ray diffraction spectra of PbMTHAp_3 and PbMTHAp_5 revealed that the powders, after removal of the Pb2+ ions, were a mixture of Ca2.5Pb7.5(PO4)6(OH)2, Pb2Ca4(PO4)2(SiO4), and Ca10(PO4)6(OH)2. Our results demonstrate that the porous hydroxyapatite nanoparticles can be used as an adsorbent for removing Pb2+ ions from aqueous solutions.


Introduction
O n eo ft h em a j o re n v i r o n m e n t a lp r o b l e m si sr e p r e s e n t e d by the global contamination with potentially toxic trace elements (PTTE).Lead has been widely used in the industrial ield, for lead-based batteries, ammunition, paints, and building materials [1-].Due to their nonbiodegradable behaviour and their incapacity of metabolization and decomposition, PTTE like Pb, Cu, Cd, Zn, and Hg are the main contaminants of soils and ground or surface waters.heir progressive accumulation in the human body can cause signiicant health problems, inducing chronic illness which, when untreated, canleadtoapainfuldeath.
Among a large variety of PTTE, one of the most dangerous is lead.Because of its unique properties, such as resistance to corrosion, malleability, and poor conductibility, it has been used since ancient times for many applications being found in pipes, pottery, or pigments [5].Over time, it has become more and more clear that lead exposure for a long period of time may cause many health problems, afecting the human reproductive, nervous, gastrointestinal, immune, renal, cardiovascular, skeletal, muscular, and hematopoietic systems [5,6].Furthermore, it impairs the development process [5,6].Recently, he International Agency for Research on Cancer (IARC) has included lead in the list of possible human carcinogens (IARC, 1987) together with its inorganic compounds (IACR 2006) [5].herefore, researchers w o r l d w i d eh a v ef o c u s e do nd e v e l o p i n gn e wa n di m p r o v e d methods for removing PTTE from diferent environmental compartments such as soils and waters.Nowadays, diferent 2 Journal of Nanomaterials methods such as chemical precipitation [7,8], nanoiltration [7][8][9], reverse osmosis [7][8][9][10], and adsorption [7,a r e used for depollution of water.
In the last decades, a major attention has been given to a special material, hydroxyapatite (HAp) [Ca 10 (PO 4 ) 6 (OH) 2 ], due to its remarkable properties.Being the main inorganic constituent of bone and teeth and a natural source of phosphate, hydroxyapatite is widely used in the medical ield for many orthopaedic, dental, and maxillofacial applications.On the other hand, hydroxyapatite has a high sorption capacity for actinides and divalent metals [15,16].Furthermore, previous studies have revealed a high capacity for removing divalent ions from aqueous solutions [17]andcontaminated soils [15] .F o rt h er e m o v a lo fP T T Ef r o mp o l l u t e dm e d i a by synthetic HAp diferent mechanisms have been reported, like ion exchange [18] and substitution of Ca ions in HAp by metals ions [15,18].In order to improve the capacity of adsorption, it was shown that there are several factors that must be taken into account, among them the type of divalent metal, the physicochemical properties of HAp, the metal concentration, the solution pH, and so forth [19].
hus, the aim of this study was to prepare methyltrim e t h o x y s i l a n ec o a t e dh y d r o x y a p a t i t e( M T H A p )c o m p o s i t e powders at nanoscale and to investigate the removal of Pb 2+ ions from aqueous solutions using MTHAp samples with diferent pH values of the solution.

Materials and Methods
he morphological and compositional analysis of MTHAp were investigated by X-ray difraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS).Batch experiments at a ixed pH of 3 and 5 were conducted with the powders and solutions of lead.

Experimental Section.
All the reagents for the HAp synthesis, including ammonium dihydrogen phosphate [(NH 4 ) 2 HPO 4 ]( A l p h aA e s a r )a n dc a l c i u mn i t r a t e [Ca(NO 3 ) 2 ⋅ H 2 O] (Alpha Aesar), were purchased without further puriication.Methyltrimethoxysilane (MTMS, H 3 C-Si-(OCH 3 ) 3 ) was purchased from Merck (Darmstadt, Germany).Lead nitrate [Pb(NO 3 ) 2 ] and extra pure HCl solution acquired from Merck were used for adjusting the initial Pb 2+ ion concentration and pH value in the aqueous solutions.

Preparation of Methyltrimethoxysilane Coated Hydroxyapatite (MTHAp).
Methyltrimethoxysilane coated hydroxyapatite (MTHAp) was synthesized using as precursors methyltrimethoxysilane and hydroxyapatite.he hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ,H A p )w a si m m o b i l i z e di n t oa methyltrimethoxysilane foam using the technique reported in literature [20,21].In accord with [21], the atomic ratio Ca/P was set at 1.67 for obtaining pure HAp. he MTHAp was achieved by dropping the solution containing methyltrimethoxysilane (1 g HAp/10 mL) onto HAp powders ater the last centrifugation.he mixture was then stirred vigorously for 1 h until homogeneity was achieved.he inal MTHAp composites were dried in a vacuum oven at 80 ∘ Cfor 72 h.To obtain the powders, the MTHAp composite samples were ground ater drying.For these experiments, 5 g of MTHAp composite sample was added to 500 mL aqueous solution with various initial Pb 2+ ion concentrations and pH values in accord with Jang et al. [17]. he initial Pb 2+ ion concentrations of the aqueous solutions were controlled and the values were set in the range 0.1-0.9g⋅L −1 by dissolving lead nitrate [Pb(NO 3 ) 2 ]i n deionized water.he pH values of aqueous solutions with a controlled initial Pb 2+ ion concentration were adjusted from 5 to 3 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.

Characterization
. he functional groups present in the prepared powders were identiied by FTIR (Spectrum BX Spectrometer).For this, 1% of the powder was mixed and ground with 99% KBr.Pellets of 10 mm diameter for FTIR measurements were prepared by pressing the powder mixture a taloadof5tonsfor2minandthespectrumwastakenin the range of 00 to 000 cm −1 with a resolution of and 128 times scanning.he X-ray difraction spectra for the MTHAp samples were recorded using a Bruker D8 Advance difractometer, with nickel iltered Cu ( =1.5 05 Å) radiation.he difraction patterns were collected in the 2 range 20 ∘ -60 ∘ ,w i t has t e po f0 .0 2 ∘ and 3 s measuring time per step.Scanning electron microscopy (SEM) study was performed on a HITACHI S2600N-type microscope equipped with an energy dispersive X-ray attachment (EDAX/2001 device). he concentration of Pb 2+ ions remaining in the solution ater the MTHAp reacted with the Pb 2+ ion-containing solution was determined by lame atomic absorption spectroscopy (Hitachi Z-8100 spectrophotometer).
he amount of retained lead ion concentration was obtained using atomic absorption. he removal eiciency of lead ions was calculated using the following formula: where and aretheinitialandtheequilibriumconcentration of Pb 2+ (g/L). he adsorption isotherm was also obtained by mixing a solution with diferent initial concentrations of Pb 2+ with a known amount of hydroxyapatite coated with methyltrimethoxysilane powder until the equilibrium was achieved.he sorption capacity representing the amount of metal retained on the unit mass of sorbent at the equilibrium was also estimated.he amount adsorbed at equilibrium, , was calculated with the following formula: where 0 is initial concentration, (mg/L); is equilibrium concentration, (mg/L); is volume of solution, (L); is adsorbent quantity, (g).

Results and Discussions
he FTIR spectra of the methyltrimethoxysilane coated hydroxyapatite powder before (MTHAp) and ater the removal experiment of Pb 2+ ions at pH 5 (PbMTHAp 5) and pH 3 (PbMTHAp 3) are shown in Figure 1.h em o s t noticeable feature in these spectra concerns the presence of the typical ] 4 PO 4 3− and ] 3 PO 4 3− bands [22,23]. he bands present in the ranges 530-650 cm −1 and 900-1200 cm −1 correspond to IR vibrations of phosphate group belonging to apatite [2 , 25]. he band at 75 cm −1 c a nb ea t t r i b u t e dt ot h e] 2 PO 4 3− vibrational mode.According to Gibson et al. [26]t h e incorporation of silicon in the HAp lattice afects the FTIR spectra of HAp, in particular the P-O vibrational bands.
In previous studies realised by Karakassides et al. [27]a n d Bogya et al. [28] it was shown that the distortion is caused by the stretching vibrations assigned to the Si-O-Si bonds that should appear in the range of 950-1200 cm −1 .D u et o the presence of the phosphate groups in the range of 950-1200 cm −1 ,t h ep e a k sa s s o c i a t e dt oS i -O -S ib o n d sc a n n o t be observed [29,30].In addition, the band which appears at 631 cm −1 is assigned to the vibrational stretching mode of OH in the apatite lattice.Moreover, the band at around 3 35 cm −1 is assigned to the water adsorbed on the surface.
Another characteristic peak of the adsorbed water appears at 16 0 cm −1 , due to O-H bending.According to Zhai et al. [31] the bands around 3 00 cm −1 are due to O-H stretching of water associated to crystallization.
Figure 2 presents the X-ray difraction patterns of the methyltrimethoxysilane coated hydroxyapatite powder before (MTHAp) and ater the removal experiment of Pb 2+ ions at pH 5 (PbMTHAp 5) and pH 3 (PbMTHAp 3), respectively.All the difraction peaks of MTHAp powders could be assigned to the standard characteristic peaks of hexagonal hydroxyapatite and no secondary phases were detected, indicating that the phase of the samples was of pure HAp [20]. he difraction patterns support the fact that HAp nanoparticles were successfully coated with methyltrimethoxysilane without any structural changes.he hydroxyapatite coated with methyltrimethoxysilane composite was likely to be transformed into PbMTHAp via the adsorption of Pb 2+ ions followed by the cation exchange reaction [32].
he spectra of PbMTHAp 5 and PbMTHAp 3 revealed that the powders ater the removal of Pb 2+ were a mixture of Ca 2.5 Pb 7.5 (PO 4 ) 6 (OH) 2 ,P b 2 Ca 4 (PO 4 ) 2 (SiO 4 ) (represented by * intheXRDspectra),andCa 10 (PO 4 ) 6 (OH) 2 .Inprevious studies on the removal of lead ions from aqueous solution by a phosphosilicate glass, Kim et al. [33]s h o w e dt h a t ,a t pH 3 and 5, only Pb 10 (PO 4 ) 6 (OH) 2 crystals formed on the glass when the glass reacted with the solution containing Pb 2+ ions.On the other hand, Zhang et al. [3 ], in their recently study on the eicient and selective immobilization of Pb 2+ in a highly acidic wastewater using strontium hydroxyapatite nanorods, showed that the inal solids were a mixture of SrHAp, PbHPO 4 ,andPb 5 (PO 4 ) 3 (OH).
he hydroxyapatite coated with methyltrimethoxysilane before and ater the reaction with the Pb 2+ ion-containing solution with pH 3 and 5 for 2 h was investigated by SEM and theresultsarepresentedinFigure 3. he SEM investigations suggest that the solid reaction products of aqueous Pb with Journal of Nanomaterials  the MTHAp were dependent on the pH value.Distinct diferences between the sample morphologies were observed in the SEM micrographs.he presence of Ca 2.5 Pb 7.5 (PO 4 ) 6 (OH) 2 and Pb 2 Ca 4 (PO 4 ) 2 (SiO 4 ) crystals was identiied by crystal clusters with needle or rod-like shapes.he quantity of Ca 2.5 Pb 7.5 (PO 4 ) 6 (OH) 2 and Pb 2 Ca 4 (PO 4 ) 2 (SiO 4 )c ry s t a l si s more substantial for the solution with pH 3 consistent with the XRD analysis.hese results are in good agreement with preliminary studies [3 ].
In order to investigate the Pb absorption by the MTHAp samples, the EDAX mapping technique was used.In concentration in the solution.It was noticed that the removal eiciency increased proportionally with the Pb concentration.When a Pb concentration of 0.1 g⋅L −1 was used, the removaleiciencyreached98.%,showingthattheadsorbent composite, MTHAp, had a strong ainity to Pb 2+ ions.At Pb concentration from 0.5 g⋅L −1 to 1.5 g⋅L −1 ,t h er e m o v a l eiciency was nearly 100%.In this case, the Pb 2+ ions in the solution were completely removed.For the studies on the efect of the solution pH, the lead concentration of 0.9 g⋅L −1 was chosen.
A solution containing 563 mg⋅L −1 of Pb 2+ ions was prepared from 0.9 g of Pb(NO 3 ) 2 in 1 L of distilled water.Figure 6 shows the measured Pb 2+ ions concentration in the solution ater the reaction with MTHAp has taken place.
At pH 3, a removal eiciency of 100% was achieved.At pH 6, the removal eiciency of Pb 2+ ions was 82%.For intermediate pH values, the removal eiciency of Pb 2+ ions was 95% and 91% for pH and pH 5, respectively.
Asshownbefore,theinluenceofpHontheadsorption eiciency of lead is small.hese results are in good agreement with the outcome of XRD and SEM analysis.
he theoretical Langmuir isotherm is oten used to describe adsorption of a solute from a liquid solution as follows [35,36]: where and are the Langmuir constants, which represent the maximum adsorption capacity for the solid phase loading and the energy constant related to the heat of adsorption, respectively.he two constants from the Langmuir isotherm can be determined by plotting (1/ )versus(1/ ). he experimental data and the Langmuir theoretical model are shown in Figure 7.hegraphofPb 2+ adsorbed per unit mass of MTHAp, , against the concentration of Pb 2+ remaining in the solution, ,i ss h o w n .hec o e i c i e n to f Langmuir isotherm at room temperature is 2 = 0.97305.In agreement with previous studies [37,38]t h e r ew a sn o problem with the transformation of nonlinear isotherm equation to linear form, by using nonlinear method.
herefore, by plotting (1/ )v e r s u s( 1/ ), the two constants from the Langmuir isotherm were determined.Figure 8 shows the plots of the Langmuir isotherm model.It can be seen that the isotherm data its the Langmuir equation with the highest value of the regression coeicients 2 = 0.9979.On the other hand, the maximum adsorption capacity for the solid phase ( ) is 105.85 mg(Pb)/g(MTHAp).he Langmuir constant was found to be 9.856 L/mg.
F o l l o w i n gt h e s er e s u l t s ,i tc a nb ee m p h a s i z e dt h a t MTHAp can efectively remove Pb 2+ ions from aqueous solutions.Because the pH of the groundwater and surface waters varies in the range of 5-7, the pH value in the aqueous solution used for the removal of Pb 2+ ions is an important parameter that must be considered [39].Along with previous studies, such as [39], our study suggests that the solid reaction products of aqueous Pb with the apatite are pH-dependent.In order to describe the uptake of diferent metals from aqueous solutions by synthetic hydroxyapatite [ 0-3]v a r i o u sp r ocesses have been proposed, such as cation exchange, metal complexation on the HAp surface, and apatite dissolution followed by a new metal phase precipitation.Suzuki et al., in their previous study [ ], suggested the formation of a more stable lead apatite, such as Ca (10−) Pb (PO 4 ) 6 (OH) 2 via an ion exchange mechanism where Pb 2+ present in the solution replaces Ca 2+ ions from the hydroxyapatite lattice.
According to [ 5], Pb (10−) Ca (PO 4 ) 6 (OH) 2 crystals with higher content of calcium are unstable.As a consequence, the HAp would be subject to a continuous process of dissolution and precipitation leading to more stable structures with a higher lead content.
Ater the reaction of lead uptaken by MTHAp, dissolution of Ca 10 (PO 4 ) 6 (OH) 2 was involved, followed by the precipitation of solid solution of Pb and Ca as Ca 2.5 Pb 7.5 (PO 4 ) 6 (OH) 2 and Pb 2 Ca 4 (PO 4 ) 2 (SiO 4 ).In agreement with previous studies [3], the formation of the new crystals was identiied.Distinct morphologies were observed in the SEM micrographs.Scattered needle-or rod-shaped crystals were observed in the sample obtained from the reaction performed at low pH.Ma et al. [ 6] reported similar crystals in previous studies.In agreement with former experiments [3,18,5,6], the diferences were detected under SEM between the unreacted apatite and those reacted under conditions of low pH.

Conclusions
h eo b j e c t i v eo ft h i ss t u d yw a st os y n t h e s i z ean e wp o r o u s nanocomposite material based on methyltrimethoxysilane coated hydroxyapatite.Its ability to remove Pb 2+ ions from aqueous solutions with a variety of initial Pb 2+ ion concentrations and pH values from 3 to 5 was investigated.When MTHAp reacted with the solution containing Pb 2+ ions, the lead ions were completely removed from the solution.he dissolution of Ca 10 (PO 4 ) 6 (OH) 2 occurred, forming Ca 2.5 Pb 7.5 (PO 4 ) 6 (OH) 2 and Pb 2 Ca 4 (PO 4 ) 2 (SiO 4 ). he MTHAp composite material exhibited the higher removal eiciency of Pb 2+ ionsatlowpH.Itsremovalcapacitywasthe h i gh e s ta tp H3a n dth er e m o v a lc a p a c i t yo fl e a dd e c r e a se d slo wl ywhenthepHincr eased .At er2 h,mos to fthePb 2+ ions were eliminated from the aqueous solution at various pH values and for an initial Pb concentration of 563 mg⋅L −1 .hisresearch showed that the MTHAp nanocomposite material is a promising adsorbent for Pb 2+ ions from aqueous solution a tv a r i o u sp Hv a l u e sa n dc o u l db eu s e da sap u r i i e rf o r wastewaters.

2. 3 .
Removal Experiment of Pb 2+ Ions in Aqueous Solution.Removal performance of Pb 2+ ions by the MTHAp composite powders was investigated by batch experiments, monitoring the change of Pb 2+ ion concentration in the aqueous solution.

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

Figure 7 :
Figure 7: Langmuir isotherm obtained using the nonlinear method for the adsorption of Pb 2+ onto hydroxyapatite coated with methyltrimethoxysilane.