BazTech - Yadda

1

The effect of Ca-Pb and P-As substitutions on the solubulity of hydroxylapatites

Młynarska M., Puzio B., Kwaśniak-Kominek M., Manecki M.

Geology, Geophysics and Environment

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2016

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Vol. 42, no. 1

100--101

EN

Hydroxylapatites are important biomaterials. Substitutions of Pb for Ca and As for P in hydroxylapatites are recently intensively studied due to their significance in the environmental immobilization of Pb and As (Lee et al. 2009, Chlebowska et al. 2015, Motyka et al. 2015). The general chemical formula of minerals in the apatite group is expressed by A 5 (XO 4 ) 3 Z, where A are bivalent cations (e.g., Ca 2+ or Pb 2+ , cations are distributed on two distinct crystallographic sites), XO 4 is a trivalent oxyanion (e.g., PO 4 3, AsO 4 3 - ), and Z is a monovalent anion (OH, F, Cl, or O). Positions Z and X may be partly filled with carbonate CO 3 2−. The structure of hydroxylapatite allows for unlimited substitutions of Pb 2+ for Ca 2+ and AsO 4 3− for PO 4 3−. The ability of lead and arsenic apatites to immobilize these toxic elements result from their high durability and low solubility at the conditions on the Earth surface. Various apatites possess different thermodynamic properties including different solubility in aqueous solutions but the systematic variation of these properties in solid solution series is poorly understood. The main objective of this research is determination of systematic variation in the solubilities of hydroxylapatites resulting from cationic substitutions of Pb 2+ for Ca 2+ and anionic substitutions of AsO 4 3− for PO 4 3− in their structure. Three solid solution series were synthesized: - HPY hydroxypyromorphite Pb 5 (PO 4 ) 3 OH – HAP hydroxyapatite Ca 5 (PO 4 ) 3 OH, HMi hydroxymimetite Pb 5 (AsO 4 ) 3 OH – JBM johnbaumite Ca 5 (AsO 4 ) 3 OH, - HAP hydroxyapatite Ca 5 (PO 4 ) 3 OH – JBM johnbaumite Ca 5 (AsO 4 ) 3 OH. The phases were synthesized from aqueous solutions at high pH above 8, at ambient temperature, by dropwise mixing of chemical reagents. The products are white, fine, homogeneous crystalline powders. Chemical composition determined by SEM/EDS is close to theoretical. X-ray diffraction confirms their crystalline structure and systematic changes in unit cell parameters with ionic substitution. Dissolution experiments were run in thermostatic bath at 25°C. An aliquot of 0.5 g of apatite was dissolved in 250 mL of 0.05M NH 4 NO 3 background solution at pH in the range of 3.5–5.0. Background solution was used to keep the ionic strength constant. The dissolution was carried out for 3 months. The bottles were manually stirred at least two times a week. The solution was syringe-sampled periodically and filtered through 0.2 μm polycarbonate filter to remove the suspended solids. The concentration of Pb and Ca was determined by atomic absorption spectroscopy. The concentration of dissolved phosphates and arsenates was determined by UV-vis colorimetry using a molybdenum blue method. The plateau on concentration evolution patterns from the dissolution experiments was observed to determine equilibrium in the suspensions. Dissolution of all the phases at the conditions of these experiments is incongruent. An increase in solution pH resulting from dissolution was observed in all cases. The system was considered in equilibrium when at least three consecutive samples showed identical concentration of Ca 2+ or AsO 4 3−. The equilibrium in HPY series was assumed by analogy. All the concentrations were recalculated to activities using PHREEQC model with Llnl database. The solubility K sp determined for the endmembers at 25°C conform with the literature data and equal to: HPY-K sp = 10 −7 7. 31 , HAP-K sp = 10 −55.66 , HMi-K sp = 10 −71. 56 , and JBM K sp = 10 −37.76. This confirms that, despite the incongruence of dissolution, the experimental procedure and the calculation scheme provide reliable approximation of the solubilities. The most soluble phase is johnbaumite Ca 5 (AsO 4 ) 3 OH. The solubility of all hydroxylapatites decreases linearly with the increase of Pb and P content. These trends do not correlate with the changes in Gibbs free energy of formation of the phases in question. This indicates that structural (e.g. the size of the ions) and the chemical factors (e.g. electronegativity) play the dominant role in the solubility of substituted hydroxylapatites.

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Arsenate substitution in synthetic hydroxylapatite – structural characterization of the Ca5(PO4)x(AsO4)3-xOH solid solution

Chlebowska P., Manecki M., Młynarska M., Puzio B.

Geology, Geophysics and Environment

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2015

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Vol. 41, no. 1

66--67

EN

Occurences of dissolved As in surface and ground waters and observed adverse health effects have emphasized the need for better understanding of reactions that govern As mobility. Precipitation of Ca apatites is quite often used for immobilization of toxic forms P and As in the environment. Hydroxylapatite – Ca5(PO4)3OH (HAP) and johnbaumite – Ca5(AsO4)3OH (AsHAP) are isostructural apatite minerals naturally occurring in the oxidation zones.The main objective of this research is identification of systematic relation between structural and spectral properties and As content in the Ca5(PO4)3OH, Ca5(PO4)2(AsO4)OH, Ca5(PO4)1,5(AsO4)1,5OH, Ca5(PO4)(AsO4)2OH, Ca5(AsO4)3OH. Recent studies of this isomorphic series provide inconsistent results (Lee et al. 2009, Zhu et al. 2009) indicating controversies and the need for further studies. Awet method of synthesis from aqueous solutions was used to precipitate precisely defined phases from the series: HAP, AsHAP and three intermediate compositions. For the synthesis of pure HAP solution of Na2HPO4 was mixeddrop wise using a syringe pump with aCa(NO3)2 solution.The pH was set at 11 with 1M NH4OH. Precipitated powders were washed five times in redistilled water and air dried. The other phases were precipitated in the presence of KH2AsO4 at various proportions. Johnbaumite was synthesized at 75°C. Ca5(PO4)3OH, Ca5(PO4)2(AsO4)OH, Ca5(PO4)1,5(AsO4)1,5OH, Ca5(PO4)(AsO4)2OH, Ca5(AsO4)3OH synthesized phases were characterized using scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), powder X-Ray diffraction (XRD), Infrared absorption spectroscopy (FTIR) and Raman spectroscopy. SEM-EDS analysis confirms that the products of the synthesis are homogeneous. The crystals size varies between 0.1–1.0 μm and increases with the increase of Ascontent. Semi-quantitative elemental analysis by EDS indicates that Ca, O, P and Asare the only elements detected and that the molar proportions correspond to theoretical composition of HAP, Ca5(PO4)2(AsO4)OH, Ca5(PO4)1,5(AsO4)1,5OH, Ca5(PO4)(AsO4)2OH and finally AsHAP. Systematic shift of peaks is apparent on the diffraction patterns. Hexagonal system and P63/m group of symmetry were used for unite cell parameters calculation. The results show systematic increase in the unit cell parameters with substitution of As for P in the series: parameter a from 9.41 to 9.43 and parameter c from 6.85 to 6.87. Crystal volumes also increase with increasing amount of As in synthesized phases. Calculated values are within the error similar to these reported by Zhu et al. (2009). In contrary to their results, a linear increase of unit cell parameters w it h As content is observed. This is in accordance with Lee et al. (2009). The intensity and the position of PO4 and AsO4 absorption bands varies systematically in the FTIR spectra being shifted towards higher wavenumber with increasing As content. The bands originating from CaO and OH (3400 cm–1) are practically not affected in all five spectra. The Raman spectra have shown systematic variation in stretching and bending vibrations in the series. Observed shifts in positions of Raman effects correlate linearly with As increase. This can be explained by the fact that atomic mass of As is higher than of P and that As-O bonds are longer than P-O bonds. Bands originating from P-O vibration modes shift in less regular manner. In the range of 610–550 cm−1 we observe shift of bands originating from CaO bond. The results correlate well with and expand upon the observations by Lee et al. (2009).Taken together, in this research we have explored possibility of HAP-AsHAP isomorphic series synthesis and we found it structural properties. Obtained results suggest that further insights may be obtained from more research on these minerals. It may find potential applications inremoval of toxic arsenic from the environment and this is a goal of future research.

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Structural and Raman spectroscopy studies of schultenite - phosphoschultenite isomorphic series

Młynarska M., Manecki M., Bajda T.

Geology, Geophysics and Environment

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2014

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Vol. 40, no. 1

110--111

EN

In many countries, there is a problem of lead and arsenic soils contamination. Lead can be immobilized using phosphate amendments (Cao et al. 2003). This technique, however, results sometimes in arsenic migration. Mineral phases, which can precipitate in this conditions include schultenite PbHAsO4 and phosphoschultenite PbHPO4. Only recently it has been experimentally shown that these minerals can form a continuous solid solution series in Earth surface environments (Zembal 2012). Therefore, it is important to extend our knowledge on the properties of schultenitephos-phoschultenite solid solution series in order to solve certain environmental problems. For the first time, Raman spectroscopy was used to characterize schultenite, phosphoschultenite and their solid solutions. The main objective of this research is the identification and interpretation of systematic changes in the results of X-ray diffractometry and Raman spectroscopy applied to synthetic schultenites with various P to As ratio. Six samples were synthesized from aqueous solutions at ambient conditions. The ratio of P/(P + As) varied from 0 to 1 by 0.2 (Zembal 2012). Schultenite PbHAsO4 and phosphoschultenite PbHPO4 belong to monoclinic crystallographic system. The standard model of the P2/c symmetric group was used in the calculation of unit cell parameters. The results show systematic increase in unit cell parameters with substitution of P for As in the series: parameter a from 4,6737 to 4,8588, parameter b from 6,6381 to 6,7528 and parameter c from 5,7617 to 5,8495. The Raman spectra have also shown systematic variation in stretching and bending vibrations in the series. The ν1 stretching As-O band shifts from 463 cm-1 in schultenite PbHAsO4 to 469 cm-1 in P-substituted schultenite while ν 4 bending As-O band shifts form 824 cm-1 to 841 cm-1. Similar is observed in phosphoschultenite: the v1 stretching P-O band shifts from 555 cm-1 to 569 cm-1 and v bending P-O band shifts form 905 cm-1 to 940 cm-1 with increasing P content Observed shifts in position of Raman effects correlate linearly with As-P substitution: the bands originating from vibration modes shift to higher wave numbers with the substitution of P for As from schultenite PbHAsO4 towards phosphoschultenite PbHPO4. This can be explained by the fact that atomic mass of As is higher than that of P and that As-O bonds are longer than P-O bonds. The results extend our knowledge about schultenite-phosphoschultenite isomorphic series. Systematic shifts of Raman effects correlate strongly with chemical composition of analyzed phases. This is associated with systematic linear variation in the height (between 4,403 and 411 units for As-0 ν 4 bending band and between 825 and 70 units for P-0 v4 bending band) and area under certain peaks on Raman spectra (between 71,423 and 30,805 units for As-O ν 4 bending band and between 28,982 and 2,095 units for P-O ν 4 bending band). Current findings suggest that it is possible to estimate the overall P/(P+As) ratio in mineral samples by precise determination of the position and area under the respective Raman spectra and comparison with standard measurements.