Hydroxyapatite interactions with copper complexes

Materials Science and Engineering C 30 (2010) 1060–1064

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Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c

Hydroxyapatite interactions with copper complexes F. Fernane a, M.O. Mecherri a, P. Sharrock b,⁎, M. Fiallo b, R. Sipos c a b c

Labo de Chimie Analytique, Université Mouloud Mammeri, Tizi-Ouzou, Algérie LERISM, IUT Paul Sabatier, Université de Toulouse, Castres, France Department of Inorganic Chemistry, Slovak Technical University, Radlinskeho 9, Bratislava 81237, Slovakia

a r t i c l e

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Article history: Received 2 February 2010 Received in revised form 4 April 2010 Accepted 18 May 2010 Available online 1 June 2010 Keywords: Hydroxyapatite Copper(II) ions Complexation Speciation Adsorption

a b s t r a c t Hydroxyapatite was used to remove dilute copper(II) from aqueous solutions containing organic ligands. Both synthetic apatite and natural apatite originating from animal bones retained copper(II) in a flowthrough column experiment but lost much of this ability when glycine, ethylenediamine or ethylenediaminetetraacetic acid (EDTA) was present in the solution. The amounts of copper(II) withheld by apatite was compared to the concentrations of free cupric ions present in equilibrium with complexed forms as a function of pH using complex formation constants derived from the IUPAC data base. It was found that most of the copper retained on the apatite was correlated with free metal ion concentrations. The strongest ligand, EDTA, formed a soluble stoichiometric copper(II) complex yet small amounts of copper could be sorbed on the apatite particles. The role of surface complexes remains to be determined to fully explain the metal uptake mechanism. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Pentacalcium hydroxyorthophosphate, also known as hydroxyapatite (HA), has been widely studied as an ideal biomaterial because of its close resemblance to the mineral phase present in mammal bones [1]. The stoichiometric formula for Ha is Ca5(PO4)3 (OH) but in natural bones many impurities are included as ionic substitutions, for example fluoride, carbonate or magnesium ions replacing hydroxyl, phosphate or calcium ions in the crystal structure [2]. Analysis of lead and cadmium levels in human teeth was reported to reflect exposure to local heavy metals [3]. The spatial distribution of lead in the dentine of human primary teeth may be used to obtain temporal information of environmental lead exposure during the pre- and neonatal periods [4]. HA has a large affinity for heavy metals and many reports have investigated the interactions between HA particles and aqueous solutions contaminated with divalent or trivalent metals [5]. The removal of lead from aqueous solution by contact with HA is very efficient and has been examined in detail [6,7]. When the metal concentration is low, hydrargyrite forms by epitaxial growth on HA surfaces, but when the rate of lead uptake is too fast, an amorphous phase precipitates and prevents further metal removal from solution. The initial rapid step has been described as metal surface complexation while the subsequent slower step has been related to ionic exchange with calcium ions [8].

⁎ Corresponding author. E-mail address: [email protected] (P. Sharrock). 0928-4931/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2010.05.010

In previous work we reported that the sizes of the lead phosphate particles precipitating on HA depended on the metal ion concentrations and that turbulent flow caused the release of lead from HA loaded cartridges as invisible microscopic solids which sediment very slowly but can be removed by ultrafiltration [9]. Under acidic conditions which favor HA dissolution, lead phosphates form by both heterogeneous and homogeneous nucleation in the reaction medium [10]. Despite accumulated evidence that HA reacts extensively with many heavy metal ions there are few industrial applications illustrating the use of HA for water decontamination. So called Apatite II originating from fish bone wastes was successfully used on a large scale in remediating contaminated soil [11]. Synthetic HA embedded in an organic sponge demonstrated ability to capture metals from aqueous solutions. The organic phase in fish bone did not seem to interfere with heavy metal ion uptake, and only moderately decreased the HA capacity in sponges [12]. However, recent findings showed that organic acids retain metals in solution according to the logarithm of the overall stability constants of the complexes formed and impede their adsorption on HA [13]. We previously demonstrated that organic ligands could modify the rate of metal uptake by HA when subjected to electromigration. Results were explained as related to metal complex charges with neutral complexes showing less mobility than free metal ions and negatively charged metal complexes migrating in the opposite direction to cationic complexes [14]. In this work our aim was to compare a synthetic HA with a natural HA originating from duck bone wastes. We examined the influence of organic ligands on cupric ion uptake by these HA particles with the

F. Fernane et al. / Materials Science and Engineering C 30 (2010) 1060–1064

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working hypothesis that complexed copper would not react. The stability constants for the complexes were derived from the IUPAC database and used to compute the free cupric ion concentrations under various experimental conditions which affect metal adsorption. 2. Materials and methods The copper solutions were prepared by dissolving cupric nitrate in distilled water at a concentration of 1.1 × 10−3 M. Ethylenediamine and glycine were from Aldrich and EDTA from Prolabo and reagent grade. The ligands were added in equimolar amounts except for ethylenediamine which was also introduced in the copper(II) solutions at a ligand to metal ratio of 2/1. The starting solutions were not buffered but all adjusted to a pH value of 5.3 to prevent copper hydroxide formation before being introduced on the apatite columns. Synthetic HA was prepared using the standard procedure published in Inorganic Synthesis [15]. The resulting filter cake was oven dried, and then broken into chunks that were heated to 800 °C in a muffle furnace. The heating rate was set at 5°/mn and the final temperature maintained 1 h before overnight self-cooling inside the oven. Following gentle pestle and mortar grinding, particles of sizes included between 0.5 mm and 1.5 mm were separated by sieving. Natural HA was obtained as described in detail previously, by heating duck bones to 800 °C as above, followed by grinding and sieving [16,17]. The HA particles were packed in glass columns of 1 cm in diameter and 25 cm in height, plugged with loosely compacted glass wool. The HA particles were suspended in water to fill the columns and allowed to sediment and make an air-free bed. The flow rate of copper(II) solutions was maintained between 2.5 and 3 ml/min during the experiments. The HA was renewed for each experiment. Solutions exiting the columns were collected in weighed polyethylene vials approximately every 5 min. The copper(II) concentrations were measured by atomic absorption spectroscopy with a UNICAM instrument using air-acetylene flame. Metal concentrations were expressed as a percentage of the starting solution concentrations set at 100%. The complexation equilibria were computed using the solEq software (Academic Software, Yorks, U.K.) and the IUPAC stability constants database of 2004. X-ray diffraction analysis was carried out with a Plillips powder diffractometer operating with Cu Kalpha radiation (λ = 1.5418 · 10−10 m)at 40 KVA and 20 mA, with 0.1°steps from 2Θ = 25 to 45° with 1 s step times. Scanning electron micrographs were made with a Phillips ESEM after gold sputtering of the HA surfaces. Specific surface areas were measured by the BET method with a Micromeritics ASAP2000 using nitrogen gas. 3. Results and discussion The synthetic HA presented the X-ray diffraction peaks expected for a crystalline material, and the natural HA had some peak shifts due to the presence of some NaCaPO4 as illustrated in Fig. 1. The specific surface area of the natural HA was 15 m2 g m−1 while that of the synthetic product was higher, at 35 m2 g m−1. This could be due to the fact that natural HA can be sintered more easily because of the small initial crystallite size. Heywood et al. reported that crystals of synthetic apatite prepared at 37 °C were platelike and, although generally much larger, had length to width ratios comparable with the natural (turkey) apatite. The textures of the solids were also different, with the natural product showing large pores and relatively dense trabeculae while the synthetic product consisted of micron sized crystallites with micron sized pores left by evaporation of the synthesis water. These differences are shown in Fig. 2. Cupric ions were sorbed by both solids when the dilute solution passed through the HA columns. Fig. 3a,b compares the percentages of copper species withheld by synthetic and natural HA. For the free

Fig. 1. X-ray powder patterns of natural HA (Nat.) and synthetic HA (Syn.). The lower chart is from the JCPDS card for hydroxyapatite.

cupric ion, synthetic HA captured all the metal present in the first 250 ml of solution before saturating while natural HA could only withhold the first 100 ml completely before losing metal uptake capacity, dropping to near 80% when 200 ml of solution had passed through the HA bed. When organic ligands were present the percentages of copper retained dropped sharply with EDTA having the strongest effect. Despite the fact that the HA could not have saturated sorption sites during the first 100 ml of liquid flow, the percentages of metal uptake still decreased gradually. It should be noted that the first 10 ml of solutions collected contained little or no copper because distilled water had first to be replaced by the copper solutions therefore these samples appear as having near 100% copper removal. Nevertheless, organic ligands interfered with copper uptake from the beginning of the experiments. The first step in metal sorption on HA is rapid metal adsorption on the solid surfaces forming a surface complex and expelling protons from surface hydrogeno-phosphate groups. The lower capacity of the natural HA is related to the smaller surface area [18,19]. The second slower step correlates with calcium ion exchange and was reported for many metal ions including copper [20] and adsorbents including bone [21]. It has been reported that patients with metal implants have a significant increase of metal levels in serum and synovial fluid [22]. The presence of metal ions increased the crystal size as well as the crystallinity of HA and reduced the lattice parameter c of the HA framework. The effects or particle size and apatite sintering temperature have been discussed previously [9]. To facilitate thermodynamic modeling and avoid problems involved with calcium binding to the ligands, we restricted our study to the first 200 ml of solutions passing through the HA columns. Indeed, very little calcium ions were found leaching from either synthetic or natural HA during the first 150 ml and calcium progressively started to flow out thereafter. For natual HA, the calcium concentration reached 0.15 mmol l−1whereas the calcium

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Fig. 3. a. Plot of copper concentrations adsorbed on synthetic HA as a function of the amount of copper introduced in the columns and the presence of ligands. En1 and en2 stand for ligand to metal ratios of 1 and 2 respectively. b. Copper adsorbed on natural HA columns.

Fig. 2. a. Texture of the natural HA seen by scanning electron micrography. b. Texture of the synthetic HA. c. Surface structure of the synthetic HA.

concentration was only 0.6 mmol l−1 for synthetic HA. The average pH of the solutions exiting the columns was 7.3 for synthetic HA and 8.0 for natural HA. We believe the pH was modified by trace calcium hydroxide present in the samples. In any case, the natural HA withheld less copper than the synthetic HA no matter what the ligand was. EDTA almost completely blocked the metal scavenging ability of HA, as was previously reported for other metals [23,24]. Ethylene-

diamine (en) had more effect when present in the ligand to metal ratio of 2/1 (en2) than when in the 1/1 ratio (en1). Glycine decreased copper uptake by nearly 50% in both cases. In Fig. 4 the calculated distribution of copper species are illustrated as a function of pH. The free cupric ion concentrations decreased sharply above pH values of 6.5. No copper precipitates formed in solution because of the dilution and presence of ligands exchanging rapidly with the coordination centers, however copper hydroxide could have formed inside the HA particles. Free cupric ions were found in solution because of rapid ligand distribution and formation of a 2/1 complex in small amounts even when only a 1/1 ligand/metal ratio was used for glycine or ethylenediamine. Because the pH values were closer to 7 with synthetic HA, the concentrations of free cupric ions were higher than for natural HA. In the case of EDTA, practically no free cupric ions were found and the EDTA–copper complex formed at more than 99.9%. However, computing the equilibrium concentrations revealed that free metal ions were still present at 10−9 M concentrations. As illustrated in Fig. 5 the free cupric ion concentration follows the protonated H2EDTA concentration profile between pH values of 2 and 6, then decreases to values near 10−11 M, close to the concentration of the monoprotonated HEDTA anion. We feel the small but not negligible free cupric ion concentrations found near neutral pH could be sufficient to bind and accumulate on HA and displace the equilibrium to release more free metal ions. This could explain the fact that some copper is retained, particularly in the case of synthetic HA. The calculated amounts of free cupric ions for the four ligand systems used were plotted as a function of the amounts of copper uptake at 50, 100 and 200 ml solution flow. In both HA cases, the slope of the plot was 1 as expected for sorption of all the copper present in ionic form. Fig. 6 shows the slopes calculated for the 100 ml volumes


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Fig. 5. Calculations of the ion distribution concentrations on a logarithmic scale for the copper(II) EDTA system, 1:1 metal to ligand ratio, c(Cu2+) = 10–3 M. The various species concentrations are indicated by arrows.

Fig. 6. Plots of the amounts of copper(II) adsorbed on synthetic HA (a) and natural HA (b) as a function of the computed free cupric ion concentrations in presence of the organic ligands at pH 7.3 (a) and pH 8.0 (b).

and shows the intercepts were not zero. This can be interpreted as indicating that some copper may be adsorbed as a molecular species different from the free hydrated form. Mixed complex formation of the organic–metal–phosphate type [25] could also explain the anomalously high value of copper sorbed on natural HA in the case of the 2/1 ethylenediamine/copper ratio where twice as much copper

Fig. 4. a. Copper(II)–glycine speciation as a function of pH. b. Copper(II)–ethylenediamine speciation for equimolar ligand to metal concentrations. c. Copper(II)– ethylenediamine speciation for 2/1 ligand/metal ratio.

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is retained (40%) as cupric ions are present (20%). This could also be due to the fact that ethylenediamine is a neutral ligand and forms a dipositive copper complex that could be attracted to the negatively charged HA surface at pH 8. 4. Conclusion The results obtained verify that organic ligands prevent cupric ion uptake by HA. Organic chelating ligands form soluble complexes which remain in solution even in the presence of HA. Our working hypothesis is verified. However, in dilute solutions, copper is retained by HA in proportions mostly related to the free ion metal concentrations. Copper complexes are partly dissociated in aqueous solutions, and the capture of the free cupric ions by hydroxyapatite leads to equilibrium displacement and slow removal of copper from solution. More investigations on the role of pH and complex forming reactions of different stoichiometries with various metals will be needed to evaluate the influence of organic matter on heavy metal clean up of polluted waters with HA. Acknowledgment We thank the Algerian scholarship program for providing a travel grant for F.F. References [1] A. Ravaglioli, A. Krajewski, G.C. Celotti, A. Piancastelli, B. Bacchini, L. Montanari, G. Zama, L. Piombi, Mineral evolution of bone, Biomaterials 17 (1996) 617. [2] J.F. Osborn, H. Newsley, The material science of calcium phosphate ceramics, Biomaterials 1 (1980) 108. [3] G. Fosse, N.P. Berg-Justesen, Cadmium in deciduous teeth of Norwegian children, Inter.J.Environ. Stud. 11 (1) (1977) 17. [4] M. Arora, B.J. Kennedy, S. Elhlou, N.J. Pearson, D. Murray Walker, P. Bayl, S.W.Y. Chan, Spatial distribution of lead in human primary teeth as a biomarker of preand neonatal lead exposure, Sci. Tot. Environ. 371 (1) (2006) 55. [5] A. Nzihou, P. Sharrock, Role of phosphate in the remediation and reuse of heavy metal polluted wastes and sites, Waste Biomass Valorization 1 (2010) 163. [6] S. Baillez, A. Nzihou, D. Bernache-Assolant, E. Champion, P. Sharrock, Removal of Aqueous lead ions by hydroxyapatites: equilibria and kinetic processes, J. Hazard. Mater. 139 (3) (2007) 443.

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