Model apatite systems for the stabilization of toxic metals: II, cation and metalloid substitutions in chlorapatites

Environmental chlorapatites, of the form A(10)(BxC1-xO4)(6)Cl-2 where A = Ca, Cd, or Pb; B/C = P, V, or Cr and 0 <= x <= 1, were synthesized by solid-state reactions. Crystal structure variations, primarily lattice parameters, were examined through powder X-ray diffraction methods, while performance and long-term durability of these waste form apatites were assessed by a combination of the toxicity characteristic leaching procedure (TCLP) and the American Nuclear Society (ANS) method. As expected, an overall dilation of unit cell edges was observed with increasing ionic substitutions. However, a discontinuity in linearity was often observed, generally for x >= 0.5. The correlation of microstructural changes and leach testing shows that lead chlorapatite is found to be more suitable for waste stabilization than calcium and cadmium analogues.