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

Abstract

Apatites of composition (Ca10−xPbx)(VO4)6F2 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 0.0018x2+ 0.0279x+ 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 {1010} faces. These leach losses were augmented through the more rapid removal of cryptocrystalline regions and intergranular thin films.

The Rietveld refinement studies of pyromorphite-vanadinite and mimetite-vanadinite solid solution series [abstract].

“…They form in oxidation zones of lead ore deposits. These minerals have high thermal stability (Dong et al 2002) and low solubility (Flis et al 2011) thus they have many applications. Pyromorphite and mimetite are especially used to immobilize lead in contaminated soils and hazardous industrial wastes (Ma et al 1993, Kim et al 2005, Bajda et al 2007, it is therefore important to know the impact of various factors on their properties.…”

“…Pyromorphite, mimetite and vanadinite crystallize in hexagonal symmetry (the space group P6 3 /m) (Dong et al 2002, Pan & Fleet 2002. They form continuous isomorphic series.…”

Structural and vibrational behaviour of pyromorphite-vanadinite solid solution series

“…Characteristic properties of apatite structure cause that these minerals are successfully used in many fields, especially for the immobilization of toxic waste and lead-contaminated soil (Ma et al 1993, Chen et al 1997, Dong et al 2002, Kim et al 2005. So far, pyromorphite and mimetite are the most known and used for the immobilization of lead.…”

“…They are secondary minerals formed in the oxidation zones of lead ore deposits. Both crystallize in hexagonal symmetry with the space group P6 3 /m (Dong et al 2002). The crystal structure of these two minerals allows to accommodate both metal cations and anionic complexes.…”

“…It is the reason, why pyromorphite and vanadinite forms solid solution series. Isovalent replacement of P with V is one of the most common anionic substitution.Lead apatites are one of the least soluble along apatites group minerals and characterized by high thermal stability (Dong et al 2002, Flis et al 2011. Characteristic properties of apatite structure cause that these minerals are successfully used in many fields, especially for the immobilization of toxic waste and lead-contaminated soil (Ma et al 1993, Chen et al 1997, Dong et al 2002, Kim et al 2005.…”