SIMULTANEOUS DISSOLUTION OF HYDROXYLAPATITE AND PRECIPITATION OF HYDROXYPYROMORPHITE - DIRECT EVIDENCE OF HOMOGENEOUS NUCLEATION

Results of in situ atomic force microscopy (AFM), ex situ AFM, optical microscopy, scanning electron microscopy, energy dispersive spectrosc opy, and x-ray diffraction were combined with previous macroscopic inv estigations to characterize aqueous Pb(NO3)(2) reaction with hydroxyla patite (Ca-5(PO4)(3)OH) (HAP). Experiments were conducted by immersing particulate HAP crystals in the AFM fluid cell in solutions with init ial Pb concentrations = 100 mg L-1, at pH 6 and 22 degrees C under sta tic conditions. Experimental results showed that transport-controlled HAP dissolution provided phosphate for precipitation of hydroxypyromor phite (Pb-5(PO4)(3)OH) (HPY), which in turn sequestered aqueous Pb. Th e combination of in situ and ex situ techniques provided direct eviden ce that HPY formed primarily by homogeneous nucleation in solution. Ne vertheless, HPY needles were found in close association with HAP surfa ces, most probably due to diffusional controls on phosphate concentrat ions. Diffusion of phosphate away from dissolving HAP appeared to be t he rate-limiting step in the overall reaction sequence. Copyright (C) 1998 Elsevier Science Ltd.