Model Apatite Systems for the Stabilization of Toxic Metals: I, Calcium Lead Vanadate

Apatites of composition (Ca 10− x Pb x )(VO 4 ) 6 F 2 with 0 ≤ x ≤ 9 were synthesized by sintering in air at 800°C 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 Å) by lead (IR = 1.45 Å) was observed. The a cell edge (in angstroms) varied linearly (0.0458 x + 9.7068) with x , while c followed the binomial function 0.0018 x 2 + 0.0279 x + 7.0118. Average grain sizes increased from approximately 5 μm ( x = 0) to 25 μm ( 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 {10 1 0} faces. These leach losses were augmented through the more rapid removal of cryptocrystalline regions and intergranular thin films.