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Synthesis of Hydroxyapatite nanorods as heavy metal adsorbent via L-Arginine assisted ultrasonicated route
1S.Saranya, 2S.J.Samuel Justin, 3Jerrin Anto and 4P.Wilson
1,2,3,4Department of Chemistry, Madras Christian College, Chennai, 600059 Tamil Nadu, India
Email: 1[email protected], 2[email protected], 3[email protected], 4[email protected]
[Recived:1st Jun.2016; Revised: 5thJun. 2015; Accepted: 10th Jun. 2016; Available online:15th Jun.2016]
Abstract— Hydroxyapatite, Ca10 (PO4)6(OH)2, a main inorganic component of bone material is widely used in various biomedical applications due to its excellent bioactivity and biocompatibility. It has been established that the existence of biomolecules like peptides or amino acids during the synthesis of bone affects the biomineralization process differently in “in vitro” systems. In order to comprehend the role of a given amino acid on the morphology of the Hydroxyapatite formed, L-arginine assisted synthesis of hydroxyapatite by ultrasound irradiation has been attempted and comparison has been made with that of chemical precipitation technique. The samples prepared were characterized by various techniques such as FT-IR, XRD, and FE-SEM. XRD peaks were attributed to the stoichiometric hydroxyapatite and no other calcium phosphate peaks were detected. Formation of apatite in both the cases was confirmed by the observed fundamental vibrational modes of the phosphate and hydroxide peaks . FE-SEM images indicate that hydroxyapatite synthesis using L-arginine by ultra sound irradiation technique reduced the agglomeration apart from enhancing the c-axis growth of the crystals. Heavy metal adsorption studies have been carried out using hydroxyapatite thus synthesized as an adsorbent and a suitable adsorption isotherm has been proposed.
Index Terms— Adsorption of heavy metal, Hydroxyapatite, L-arginine, Ultrasonication
I. INTRODUCTION
Calcium phosphate based bio ceramics have proven to be attractive materials for biomedical applications [1].
Among these bio ceramics, particular attention has been given to hydroxyapatite with an ideal chemical formula being Ca10 (PO4)6(OH)2 for its excellent biocompatibility, bone bonding ability, structural and compositional similarity to that of the mineralized matrix of human bone. These materials are extensively used in the form of porous granules, sintered and porous blocks, and powders for different surgical applications in the field of dentistry and orthopedics. Hydroxyapatite also finds application in other fields of industrial or technological
interest such as catalyst in chromatography or gas sensor [2], water purification, fertilizers production, drug carrier [3] and as adsorbents in removal of heavy metals such as Pb, Zn, Cu, Cd, Co, and Sb from waste water [4-8].
Several researchers have shown that the mechanical properties of fabricated hydroxyapatite could be significantly enhanced by controlling the important parameters like particle size, shape, its distribution and agglomeration [9]. The properties of hydroxyapatite can also be influenced by morphology, purity, stoichiometry and structure [10, 11]. Considering the extensive applications of hydroxyapatite in biomedical fields, numerous techniques for the synthesis of hydroxyapatite have been developed. Over the past decades, a variety of methods have been proposed for synthesis of hydroxyapatite such as solid-state route [12], wet chemical precipitation [13], hydrothermal treatment[14], mechano chemical–hydrothermal synthesis [15] sol–gel synthesis[16,17] and microwave synthesis[18] have been developed to produce hydroxyapatite crystals with a variety of shapes and morphologies.
Living organisms synthesize a variety of inorganic minerals under the direct influence of bio molecules, usually proteins and peptides, which remarkably control and regulate their nucleation and growth under conditions that are much milder than those used in conventional processing techniques [19-23]. Likewise, charged amino acids can either inhibit or induce Hydroxyapatite mineralization depending on whether they are dissolved in a solution or bound to a surface. Amino acids are also effective in modifying the morphology and crystalline structure of hydroxyapatite owing to the electrostatic and stereo chemical effects of their charged residues [24-28].
Koutsopoulos and Dalas et.al studied the effect of various amino acids on the crystallization of hydroxyapatite by constant composition technique under physiological concentrations and conditions and have identified the use of amino acids as effective inhibitors in many undesirable cases of pathological calcification [29-33.] Gonzalez et.al [34] have synthesized amino acid functionalized hydroxyapatite nanorods using a range of amino acids like alanine, valine, glycine, asparagines, serine, lysine,
arginine, aspartic acid or glutamic acid by hydrothermal technique.
Mining operations, metal plating, textile industries, tanneries etc., pollute the ground and surface water with heavy metal pollutants which include arsenic, cadmium, cobalt, chromium, copper, mercury, manganese, nickel, lead, tin, and thallium, causing a serious environmental issue [35]. Heavy metals can easily be absorbed by the living organisms due to its high solubility in the aquatic environment and enters the food chain and gets accumulated in the body [36]. If they are ingested beyond the permitted concentration, they can cause serious health disorders [37]. Therefore removal of such metals from water is of great concern in the field of water pollution control. Although some heavy metals (e.g. Iron, Selenium, Cobalt, Copper, Manganese, Molybdenum, and Zinc) are essential to human beings at varying amounts, excessive levels can result in a number of adverse health effects. Among the commonly encountered metals of concern, copper and zinc are hazardous and have been included in the Priority Pollutants List by United States Environmental Protection Agency (USEPA)[38]. Heavy metals can accumulate in living organism and cause health disorders and various diseases.
An excessive intake of copper can cause neurotoxicity, jaundice, and liver toxicity [39,40].
Conventionally, heavy metal removal has been carried out by different methodologies like oxidation, reduction, precipitation, membrane filtration, ion exchange, electrochemical operation, biological treatment, and adsorption [41, 42]. Among the various available methods, adsorption onto a solid substrate has been found to be one of the most suitable processes for the removal of heavy metals from solutions. Several adsorbents such as biomass, activated carbons, fishbone, zeolites, clays have been employed for this purpose [43,44,45-47]. Generally, calcium-hydroxyapatite Ca10(PO4)6(OH)2, has shown the best removal efficiency due to its moderate solubility–between highly insoluble and highly soluble phosphate bearing materials such as phosphate rock and phosphate fertilizers, respectively [48, 49].The present work reports the hydroxyapatite nano rods as the adsorbent in the removal of copper from water using visible spectrophotometry.
II. MATERIALS AND METHODS
Materials
Calcium nitrate tetra hydrate Ca(NO3)2·4H2O, diammonium hydrogen phosphate (NH4)2HPO4), L-Arginine and ammonia solution, cupric sulphate, ammonia solution were all of analytical grade chemicals and used without further purification.
Chemical Precipitation method
Aqueous solutions of 0.4M calcium nitrate tetra hydrate Ca(NO3)2·4H2O) and 0.2395M diammonium hydrogen phosphate (NH4)2HPO4 were prepared using deionized water. Typically, calcium/amino acid solution was prepared by dissolving an appropriate amount of the amino acid (6.968g for L-Arginine) into calcium solution under gentle stirring. The ratio of calcium salt and L-arginine was chosen to be 1:1. Calcium-amino acid solution thus prepared was slowly added into diammonium hydrogen phosphate solution, by maintaining pH at 11 using 1:1 ammonia solution. Upon completion of the addition of reactants, the white suspension as obtained was centrifuged, washed repeatedly with deionized water, and allowed to dry at 120oC in a hot air oven for 2 hrs followed by calcination at 600 oC for 2hrs. The sample thus prepared was designated as PApI.
Ultrasonically aided Chemical Precipitation method The above chemical precipitation method was repeated under ultrasonication and the sample was designated as UApI. For comparison reasons and in an effort to understand the effect of ultrasonication and L-arginine on the crystal nucleation and growth process, control experiments by chemical precipitation and chemically assisted ultrasonication method in the absence of L-arginine were also performed and designated as PB and UB respectively.
Adsorption experiments
Metal salt of cupric sulphate was used to prepare metal ion Cu (II) solution. The stock solution (1,000 mg/L) was prepared by dissolving appropriate amounts of metal salt in double distilled water. The working solution (60-240 mg/L) was prepared by diluting the stock solution to appropriate volume. The samples were agitated using a mechanical shaker, to enhance the contact between hydroxyapatite and Cu (II) ions. The samples were taken out at different time intervals. The sorbent solution mixtures were then centrifuged for 5 min and the supernatant was complexed with ammonia solution and analyzed for the Cu (II) concentration, at 620nm using visible spectrophotometer. The adsorption of Cu (II) ions on hydroxyapatite was studied using different variables, namely initial concentration of Cu (II), contact time and hydroxyapatite dose with pH fixed at 7. The data obtained using each of these variables were analyzed and the adsorption data were fitted using Langmuir and Freundlich models to predict the adsorption behavior of Cu (II) ions on hydroxyapatite.
Characterization studies
Phase composition and crystallinity of the synthesized hydroxyapatite nanoparticles were determined by X-ray
diffraction with Cu Kα radiation where λ = 1.5406 ˚. Data were collected over the 2θ range 20–90◦ with a step size of 0.010◦ and a count time of 0.2 s. FT-IR spectrum was recorded for samples prepared by chemical precipitation and ultrasonically aided chemical precipitation method in the range, 400 to 4000cm-1 to confirm the structure of hydroxyapatite. The size and morphology of the as-synthesized hydroxyapatite nanoparticles were examined by field emission scanning electron microscopy and High resolution transmission electron microscopy.
Hydroxyapatite nano particles were dispersed in ethanol by 30 min ultrasonication, prior to FESEM and HRTEM recording. Adsorption studies for Cu (II) ions were carried out using visible spectrophotometer at the wavelength of 620nm.
III. RESULTS AND DISCUSSION
XRD analysis
XRD patterns of hydroxyapatite derived from chemical precipitation as well as ultrasonically aided chemical precipitation are shown in Fig. 1 and 2. All the peaks in the XRD [2θ values of HAP - 25.88(0,0,2), 31.77(2,1,1), 32.2(1,1,2), 32.9(3,0,0),34.05(2,0,2)] pattern shown in Fig. 1 and 2 are attributed to stoichiometric HAP and no other calcium phosphate peaks were detected. A high consistency between the data was observed with JCPDS No. 09-0432. All peaks show highly crystalline nature of hydroxyapatite.
Fig.1 XRD patterns of a) PB and b) PApI
Fig.2 XRD patterns of a) UB and b) UApI
FTIR analysis
FTIR spectroscopy was used to confirm the chemical structure of the hydroxyapatite nanorods. From Fig.3, it is evident that the formation of apatite in all the cases, by the observed fundamental vibrational modes of the phosphate peaks at around 1093 and 1032 cm−1, 603 and 560 cm−1, 474 cm−1 and 962 cm−1 respectively. The peaks at 3571 cm−1 and 632 cm−1 corresponding to the stretching and bending vibration of the hydroxyl (OH−) group are considered as the characteristic peaks of stoichiometric hydroxyapatite. Apart from that, the peaks observed at around 3400 cm−1 and 1654 cm−1 is due to the stretching and bending modes of adsorbed water. Moreover the carbonate peaks were observed approximately at 1457 cm−1 and 1411 cm−1.
Fig.3 FTIR spectra of a) PApI and b) UApI Morphological analysis
Hydroxyapatite synthesized by chemical precipitation method and ultrasonically assisted chemical precipitation technique are shown in FESEM micrographs from Fig.4a-d. Agglomerated spherical particles in the absence of L-arginine, whereas less agglomerated, short nanorods in the presence of L-arginine, could be seen from Fig. 4a and 4b respectively. It is clear, that L-arginine has decreased the agglomeration and also as reported in literature, it has directed the crystal growth in one direction namely c-axis inhibiting along a and b directions.Fig.4c. shows less agglomerated but rod like morphology in the case of ultrasonication whereas elongated rods in Fig.4d. shows the synergetic effect of L-arginine and ultrasonic irradiation effectively in preventing agglomeration and further increasing the length of the crystal along c-axis. This could be clearly seen from HRTEM image of sample UApI in Fig5. Ca/P ratio was maintained at 1.67 which can be seen from Fig.6.
Fig.4a. FESEM image of PB
Fig.4b. FESEM image of PApI
Fig.4c. FESEM image of UB
Fig.4d. FESEM image of UApI
Fig.5. HRTEM image of UApI
Fig.6 EDAX of UApI
Effect of hydroxyapatite synthesized by different experimental conditions on Cu (II) ions removal The removal of Cu (II) ions by adsorption on hydroxyapatite samples namely PB, PApI, UB and UApI was studied by taking 50mL of 60mg/L of initial concentration of Cu (II) using 0.02g hydroxyapatite dosage at various time intervals maintaining the pH and temperature at 7 and 303K respectively. The adsorption isotherm in Fig.7 reveals that all the adsorbents reached saturation after 45min. Among the various hydroxyapatite samples, UApI seemed to be effective in the removal of Cu (II) ions. Removal of copper by hydroxyapatite follows adsorption over PO4H− surface and also ion-exchange mechanism, in which the metal ion replaces the surface calcium ions [50, 51]. In case of sample UApI, crystal growth has occurred along the c-axis or a,(b)- planes which is rich in surface calcium ions [52-56]. This resulted in enhanced Cu (II) ion removal by ion exchange mechanism. Hence, all the adsorption experiments in this work were performed using UApI sample.
Fig.7. Effect of adsorption on various hydroxyapatite
samples with respect to time.
Fig.8. Effect of hydroxyapatite dosage on removal of Cu
(II) ions.
Effect of contact time
The time- dependent behavior of removal of Cu (II) ions by adsorption on hydroxyapatite was studied at 15, 30, 45, 60 min, at fixed 0.02g hydroxyapatite dosage in 50mL of 60mg/L of Cu(II) ion concentration. pH and temperature were maintained at 7 and 303K respectively. From Fig.9 it can be seen that the removal efficiency of Cu(II) ions increased with time due to the availability of more active sites whereas it reached saturation after 45 min due to lack of active sites for further adsorption.
Fig.9. Effect of contact time on removal of Cu (II) ions.
Effect of hydroxyapatite dosage on removal of Cu (II) ions.
The effect of hydroxyapatite on removal of Cu (II) ions was analyzed by varying the hydroxyapatite dosage from 0.02 to 0.1g in 50mL of 60mg/L Cu (II) ion concentration.
The results obtained from Fig.8 reveals that the removal efficiency of Cu (II) ions increases with increase in hydroxyapatite dosage. This is due to increase in surface area, thereby increasing the number of active sites for adsorption.
Effect of Cu (II) ions initial concentration
The removal of Cu (II) ions by adsorption on hydroxyapatite was studied at different initial concentrations containing 50mL of 60 to 240 mg/L, at fixed hydroxyapatite dose of 0.05g, 60 min as contact time, maintaining the pH and temperature at 7 and 303K respectively. The percentage removal of Cu (II) ions and adsorption capacity are listed in table.1. It shows that, the adsorption capacity of hydroxyapatite increased from 60 mg/g to 64 mg/g, when the initial concentration of copper ions increased from 60 to 240mg/L. The percentage removal of Cu (II) ions is found to decrease with increasing initial concentration. This phenomenon can be attributed to the reduction in immediate adsorption due to the lack of available active site on hydroxyapatite for high initial concentrations.
Table1. Removal of Cu (II) ions by hydroxyapatite at different initial Cu (II) ions concentration
S.
No
Initial Cu(II) ions
concentration mg/L
Percentage removal of Cu(II) ions
%
Adsorption capacity qe (mg/g)
1 60 97.80 60.00
2 120 51.28 61.53
3 180 35.07 63.15
4 240 30.00 64.00
Adsorption isotherms
The adsorption data obtained for Cu (II) ions was analyzed using Langmuir and Freundlich models with 60 to 240mg/L of Cu (II) concentration, 0.02g hydroxyapatite dosage, 60 min as contact time, pH and temperature maintained at 7 and 303K respectively. The adsorption isotherms as showed in fig 10 and 11 follow both Langmuir and Freundlich isotherm respectively. As seen in Table 2, the value of R2 concludes that Langmuir isotherm has the best fit for the Cu (II) ion adsorption on hydroxyapatite. The maximum Cu(II) adsorption capacity was obtained to be 66.7 mg/g from Langmuir model.
Fig.10. Langmuir adsorption isotherm for Cu(II) ions on hydroxyapatite.
Fig.11. Freundlich adsorption isotherm for Cu(II) ions on hydroxyapatite.
Table 2.Isotherm constant and correlation coefficient for Cu (II) ions on hydroxyapatite
Langmuir adsorption model
Qm (mg/g) 66.7
KL (L/mg) 0.246
R2 0.999
Freundlich adsorption model
Kf (mg/g) 52.48
1/n 0.036
R2 0.959
IV. CONCLUSION
In this current work, phase pure hydroxyapatite has been synthesized using chemical precipitation and ultrasonically assisted chemical precipitation method involving calcium nitrate and diammonium hydrogen phosphate as precursors and L-arginine as crystal growth modifier. Synergetic effect of L-arginine and ultrasonic irradiation decreased the agglomeration without affecting the crystallinity of the hydroxyapatite, as confirmed from XRD and FT-IR results. In addition, morphological change was also observed from FE-SEM images as
L-arginine in conjunction with ultrasound irradiation could result in elongated nanorods.
Synthetic hydroxyapatite can efficiently remove Cu (II) ions to the tune of 66.7 mg/g of the adsorbent according to the Langmuir model. This suggests the potential of hydroxyapatite as an adsorbent for Cu (II) species in water. More specifically, among the various samples investigated in the current study, hydroxyapatite synthesized via L-arginine by ultrasonically assisted chemical precipitation method was observed to be effective in removal of Cu (II) ions because crystal growth has occurred along c-axis or a (b) plane, which is rich in calcium ions resulting in enhanced Cu (II) ion removal by ion exchange mechanism. Removal efficiency of Cu (II) ions increased with increase in time and reaches saturation after 45min. The percentage removal of Cu (II) ions is found to decrease with increasing initial concentration. Adsorption of Cu (II) ions increases with increase in hydroxyapatite dosage. Langmuir isotherm has the best fit for the Cu (II) ion adsorption on hydroxyapatite.
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