Studies on some physico-chemical aspects of lead vanadate apatite

Abstract

The toxicity of lead and vanadium and their salts [1][2][3][4][5] is well known. The mechanism [6] of their incorporation in the humal skeletal system is based on the isomorphous substitution of Ca 2+ by Pb 2+ (ionic radii 0.099 and 0.120nm respectively [7]) and of PO 3-by VO]-(covalent radii 0.110 and 0.122 nm respectively [7]) in calcium phosphate apatite, Cal0(PO4)6(OH)2 , the principal inorganic constituent [8,9] of bones and teeth, resulting in the formation of lead vanadate apatite (LVA), Pb~0(VO4)6(OH)2. The preparation, characterization and determination of the solubility of LVA are reported in this letter; the method adopted for the preparation of LVA is the first of its kind.The preparation of LVA brought about by a modification of Hayek's method [10,11] is based on the following equation, with ethylenediamine being used instead of ammoniaStock solutions of Pb 2+ and VO 3-obtained by dissolving lead acetate and vanadium pentoxide in water and sodium hydroxide, respectively, were preserved in polyethylene containers and their amounts of lead and vanadium were estimated [12,13]. Details of the method adopted for the preparation and purificiation of the sample were the same as those described elsewhere for an allied system [14,19]. A part of the sample washed with acetone and air dried was heated to ~ 300°C for about 2 h and cooled in a desiccated atmosphere for use in chemical [15], X-ray and infrared analyses, as well as for density determinations [16,17], whereas electron-microscopic and thermogravimetric investigations were performed on the air-dried sample. The sample was characterized by X-ray, infrared, electron-microscopic and thermogravimetric analyses and the experimental details are the same as those described elsewhere [14]. The solubility studies carried out at 37°C in buffered aqueous media, raised to 0.165 M with respect to sodium chloride in the pH range 2 to 3, were based on complexometric [12] and spectrophotometric [18] determinations, respectively, of lead and vanadium. The buffer comprised potassium acid phthalate and hydrochloric acid. Investigations on the dissolution kinetics proved the optimum period of equilibration for the formation of a saturated solution of the sample to be 3 h. In order to be quite sure that the condition of saturation had been achieved, the period was extended to 12 h; the remaining experimental details for solubility studies are the same as those described before [19] for allied systems.The effect of common ions on the PKsp of the sample was also investigated at a convenient pH value.The formation of tertiary phosphates/vanadates of heavy metals through a conventional precipitation from aqueous media was complicated because of co-precipitation of their hydroxides, primary and secondary species [20]. Among the methods available for elimination of such a co-precipitation [21, 22], that of Hayek [22], involving complexing of the heavy metal ions with an appropriate ligand, was found to be suitable because of the ability of such ions to form well-defined soluble...