Model apatite systems for the stabilization of toxic metals: I, calcium lead vanadate

Apatites of composition (Ca10-xPbx)(VO4)(6)F-2 with 0 less than or equal to x less than or equal to 9 were synthesized by sintering in air at 800degreesC the oxides of calcium, lead, and vanadium that were mixed with calcium fluoride. Variations in lattice parameters, grain size, and durability were monitored as a function of composition. The expected dilation in unit cell edges with increasing replacement of calcium (IR = 1.26 Angstrom) by lead (IR = 1.45 Angstrom) was observed. The a cell edge (in angstroms) varied linearly (0.0458x + 9.7068) with x, while c followed the binomial function 0.0018x(2) + 0.0279x + 7.0118. Average grain sizes increased from approximately 5 mum (x = 0) to 25 mum (x = 9) as grain growth was enhanced due to incipient melting of the lead-rich compositions. For x < 5 the materials were biphasic. Overall dissolution, as determined by the toxicity characteristic leaching procedure (TCLP), was faster in calcium-rich apatites. Etching of well-crystallized apatite grains occurred preferentially on {1010} faces. These leach losses were augmented through the more rapid removal of cryptocrystalline regions and intergranular thin films.