Synthesis and characterization of cadmium chlorapatite Cd5(PO4)3Cl

Abstract One of the most effective methods for the immobilization of toxic metals involves the use of minerals from the apatite supergroup. The formation of cadmium chlorapatite may lead to successful entrapping of cadmium; thus, it is important to examine the solubility constant to determine the stability of cadmium in the the apatite structure. Cadmium chlorapatite was synthetized and characterized by X-ray diffraction, infrared spectroscopy, Raman spectroscopy, and scanning electron microscopy. The solubility constant (log) Ksp of cadmium chlorapatite was -65.58. The Gibbs free energy of formation of cadmium chlorapatite reached -3950.48 kJ mol−1. The solubility constant turned out to be low but was enough for cadmium chlorapatiteto be considered a very stable compound..


Introduction
Environment pollution by heavy metals has gained widespread attention because of their toxicity and non-biodegradability (Fu, Wang 2011). One of the critical polluting heavy metals originating from industries is cadmium (Wang et al. 2018). Cadmium is mainly derived from petroleum refining, electroplating and alloying, nickel-cadmium batteries, coal combustion, and plastic stabilizers (Chen et al. 2018). Cadmium can accumulate in living tissues and cause serious health issues like bone and kidney damage, cancer, renal disturbance, and hypertension (Shirkhanloo et al. 2016). The removal of cadmium from the environment is important and various treatment methods such as precipitation, adsorption, ion exchange, and membrane filtration have been used to remove to achieve this (Deng et al. 2017;Lee et al. 2018;Wong et al. 2014). Immobilization by minerals such as apatite is a promising method for cadmium removal (Matusik et al. 2012).
Phosphate-bearing apatite is generally characterized by the formula M 10 (PO 4 ) 6 X 2 (M and X represent cations and anions, respectively) (Drouet 2015). One of the features of apatites is the ability to exchange cations and anions in the structure. Hydroxylapatite (Ca 10 (PO 4 ) 6 (OH) 2 ) and fluoroapatite (Ca 10 (PO 4 ) 6 F 2 ) are the most common minerals in this apatite supergroup; however, chlorinated and bromated counterparts are also present. Calcium ions can be replaced by other cations such as magnesium, cobalt, lead, strontium, or cadmium (Wopenka, Pasteris 2005;Sternlieb et al. 2009). Moreover, tetrahedron PO 4 can be replaced by VO 4 or AsO 4 (Hara et al. 2006). The ability of apatites to exchange cations and anions is very useful in designing specific materials for reducing the mobility and bioavailability of heavy metals, such as cadmium, in polluted water, soils, and sediments (Viipsi et al. 2013).
Many studies addressed the interaction of cadmium with apatites, especially the sorption of cadmium by apatites (Klee, Engel 1970;Jeanjean et al. 1996;Corami et al. 2008;Matusik et al. 2012;Zhu et al. 2017). Possible mechanisms for the sorption process are ion exchange, superficial sorption, and precipitation (Jeanjean et al. 1996). Another studies demonstrated that one of the most effective methods to remove Pb(II) ions from the solution is precipitation of pyromorphite and mimetite, initiating the idea of immobilizing Cd in the apatite structure and examining its stability (Ma et al. 1993;Miretzky, Fernandez Cirelli 2008;Flis et al. 2011).
The purpose of this research was to synthesize cadmium chlorapatite, characterize the mineral and determine its solubility constant based on the results of dissolution experiments.

Synthesis of cadmium chlorapatite Cd 5 (PO 4 ) 3 Cl
To synthesize cadmium chlorapatite (Cd 5 (PO 4 ) 3 Cl), 10 g of CdCl 2 x1.5 H 2 O and 0.4 g of K 2 HPO 4 were grated in an agate mortar, mixed and placed in a platinum melting-pot. The sample was then placed in an oven preheated to 750°C for one hour. To separate the cooled alloy from the melting-pot, the latter was heated in a sand bath until the alloy dissolved and the reaction product was obtained in the form of macroscopically visible transparent needles, sized 3-4 mm, at the bottom of the pot. The obtained mineral was rinsed several times with redistilled water and then placed in an oven and dried at 60°C until the water evaporated completely.

Dissolution experiments
Dissolution experiments were conducted at 25ºC with an initial pH value of 2.5. The pH value of 0.05 M KNO 3 matrix solution was adjusted to the desired value using 0.1 M HNO 3 . The experiment was conducted in triplicate in unbuffered suspensions. To determine that equilibration had been reached, samples of the reaction solutions were taken after 15 min and after 1, 2, 4, 25, 50, 93, 165, and 336 hours. The pH was allowed to drift freely after the initial value was set and the bottles containing the suspensions were always shaken after sampling. For each sample, the solids were allowed to settle before 5 mL of the supernatant was withdrawn without refilling with the solution afterwards. Equilibrium was deemed attained when the concentrations of Cd 2+ , PO 4 3in three consecutive samples were equal.

Methods
The composition of the synthesized samples was determined by X-ray powder diffraction using Philips APD PW 3020 X'Pert diffractomer with CuKα radiation and graphite monochromator. The measurements were done in a 2Θ range of 5° to 65° with a 2Θ step size of 0.05°. The spectra were interpreted using the XRAYAN software and Joint Committee on Powder Diffraction Standards (JCPDS) cards, 29-0254 and 30-0206. The Fourier-transform infrared (FTIR) spectra were obtained by a Nicolet 6700 spectrometer using the DRIFT technique (3% wt. sample/KBr) with 64 scans at 4 cm -1 . The FTIR spectra were analyzed by comparing them with the characteristic spectra of apatites described by Klee and Engel (1970). Raman spectra were obtained using the DXR Raman Microscope from (Thermo Scientific). The registration was carried out in the 100-3000 cm -1 range using a 780 nm laser. The positions and intensities of the bands were determined after decomposition of the spectra into component bands. Spectra decomposition was carried out by following the method proposed by Handke et al (1994). Air-dried, uncoated samples were examined by a variable pressure field emission scanning electron microscopy (SEM, FEI Quanta 200) equipped with an energy dispersive spectrometer for elemental microanalysis. PO 4 3concentrations were determined colourimetrically (molybdene blue method; Lenoble et al. 2003) using a Hitachi 1600 spectrophotometer (Hitachi, Japan) at 870 nm wavelength. The molybdene blue method base, on formation of an antimonylphosphomolybdate complex, gives a blue species when reduced with ascorbic acid (Lenoble et al. 2003). The concentration of Cd 2+ ions was determined by atomic absorption spectroscopy using a GBC SavantAA spectrometer. The concentration of Clions was calculated based on the stoichiometry of the compound as the dissolution was congruent. Solubility constant calculations were performed using the PHREEQC computer program and the MINTEQ.v4 thermodynamic database. The activities of ionic species were calculated from measured concentrations by conversion of the results from molarity to molality and application of the Davies equation or extended Debye-Hückel equation.

Solid characterization
The X-ray pattern of the product of the synthesis reaction is shown in Figure 1. The product was identified as cadmium chloroapatite Cd 5 (PO 4 ) 3 Cl by comparing the peak positions with those of cadmium chloride phosphate reported in the JCPDS cards 29-0254 and 30-0206. All peaks were attributed to cadmium chlorapatite (CdClap) and there were no other phases in the sample composition. Calculated unit cell parameters were: a = 9.601(1) Å and c = 6.395(1) Å. These values agree well with the values of a = 9.633 Å and c = 6.484 Å reported for cadmium chloride phosphate (JCPDS cards 29-0254 and 30-0206).  Figure 2 shows the infrared spectra of cadmium chlorapatite, with the analysis focused on mid-infrared spectra. The band at 3400 cm -1 is due to the adsorbed water and the rest of infrared spectra is poorly defined in the 4000-1400 cm -1 region. The spectra indicates characteristic bands coming from P-O or O-P-O vibrations present in the [PO 4 ] tetrahedra. These bands are attributed to the symmetric stretching vibrations ᴠ 1 (939 cm -1 ), asymmetric stretching vibrations ᴠ 3 (1057, 999 cm -1 ), and asymmetric bending vibrations ᴠ 4 (600, 542 cm -1 ). The weak band at 482 cm -1 may be originating from the symmetric bending vibrations v 2 . The observed spectra are comparable with the spectra of CdClap, which were discovered by Klee and Engel (1970).
The Raman spectrum of CdClap is presented in Figure 3. The spectrum shows the bands due to the stretching mode of PO 4 , as in FTIR analysis. Symmetric stretching vibrations ᴠ 1 (872 cm -1 ) and asymmetric stretching vibrations ᴠ 3 (908, 933, 956, 980, 1021 cm -1 ) are present, while the bands localized in the range of 340-540 cm -1 are assigned to the bending mode.
Synthetic cadmium chlorapatite is presented in Figure 4. SEM images show that the precipitate is mainly composed of idiomorphic crystals in the form of hexagonal prisms with an average length of 300-400 μm, with some crystals eaching even 1.5 mm in length. The crystals are randomly distributed and have a smooth surface.

Dissolution experiments
Cadmium chlorapatite dissolution was most intensive during the first 50 hours of the experiment. Over time, the rate of release of phosphate ions and cadmium into the solution decreased ( Figure 5). The initial pH of the solution was 2.5 and the final pH was 3.34.
The dissolution of cadmium chlorapatite was stoichiometric, i.e. the molar ratio of Cd, PO 4 and Cl ions in the tested solutions was approximately 5:3:1 (due to the mineral formula: Cd 5 (PO 4 ) 3 Cl). The molar ratio Cd/PO 4 in the equilibrium solution was 1.696 ± 0.11. The results of the dissolution experiment are shown in Table 1.   Balance was achieved after two weeks of reaction (336 hours). To determine the state of equilibrium, it was necessary to check whether the differences between Cd and PO 4 concentrations in three consecutive samples were within the error limits. During the experiment, the pH value of the sample changed significantly. For equilibrium, it was 3.34.
In equilibrium, the solubility constant K sp of cadmium chlorapatite is equal to the product of the ionic activity (IAP) which can be determined from the mineral dissolution reaction. For the following dissolution reaction of cadmium chlorapatite, IAP can be written as: where brackets denote activities. The Cd 2+ , PO 4 3and Clion activities were calculated by the PHREEQC program using measured and equilibrated concentrations of Cd 2+ , PO 4 3-, Clions and pH. The solubility constant was calculated based on the dependence of the equality of the solubility constant and the product of ionic activity. The results are presented in Table 2. All compounds contain intrinsic energy, referred as Gibbs free energy. The Gibbs free energy of a system is defined as the enthalpy of the system minus the product of the temperature and entropy of the system and it may be used to predict the spontaneity of a process. A negative value indicates a spontaneous process, a positive value -a nonspontaneous process, and zero indicates that the system is at equilibrium (Huang, Gobran 2005). The relation between the reaction equilibrium constant (K sp ) and the change in Gibbs free energy of reaction (ΔG°r) is given by the equation: where R is the gas constant (8.314472 J mol -1 K -1 ) and T is the temperature (298.15 K). This gives a value for ΔG°r of 374.310 kJ mol -1 . Calculation of the Gibbs free energy for the formation of cadmium chlorapatite is enabled by equation 4: Based on the values provided by Robie et al. (1978): ΔG°f(Cd 2+ ) = -77.58, ΔG°f(PO 4 3-) = -1019.00 and ΔG°f(Cl -) = -131.27 kJ mol -1 , and log K sp = -65.58 as experimentally determined above, Gibbs free energy of formation of CdClap may be calculated as ΔG°f(CdClap) = -3950.48 kJ mol -1 . The obtained log K sp value is higher than the value reported by Eighmy et al. (1997), where log K sp for cadmium chlorapatite was -49.66. This difference is probably caused by the difference in methodology -the synthesis of cadmium chlorapatite at high temperatures may cause the formation of the mineral with slightly different properties than those of the natural mineral or those synthesized at low temperatures. The ΔG°f value describes the amount of energy that is released or consumed when a phase is created from other phases. The mineral with lower energy is more stable. The solubility constant and the Gibbs free energy of formation of cadmium chlorapatite confirm its low solubility and high stability. Comparing the calculated solubility constant of cadmium chlorapatite with the K sp of pyromorphite (Pb 5 (PO 4 ) 3 Cl), it can be seen that K sp is higher for pyromorphite. Log K sp of pyromorphite is -79.6 (Flis et al., 2011) and log K sp of cadmium chlorapatite is -65.58. This proves that Cd substituted pyromorphite is less stable than Pb-pyromorphite. Precipitation of Pb-pyromorphite is one of the effective methods to remove lead ions from contaminated soils and solutions. The similarity between pyromorphite and cadmium chlorapatite allows the assumption that cadmium chlorapatite can be successfully used for the immobilization of Cd.

Conclusions
Cadmium chlorapatite is a mineral that is less stable and more soluble than chloride pyromorphite. However, its solubility constant is low enough for it to be considered a very stable compound. This is confirmed by the theories currently being researched on the use of apatites to remove cadmium impurities from soil and water. It seems that immobilization of cadmium in the structure of the stable apatite will allow for the effective immobilization of this toxic element.