Effects of aging and pH on dissolution kinetics and stability of chloropyromorphite.

The objectives of this research were to understand the effect of aging time on chloropyromorphite stability by dissolution, to examine physical and chemical alterations of the pyromorphite samples, and to model the kinetic data collected from the dissolution experiments. The results of this investigation indicate that chloropyromorphite formation is kinetically rapid and that its dissolution in acid is thermodynamically stable, ideal conditions for Pb immobilization that has emerged as a potential remediation strategy. In terms of aging prior to dissolution, X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD) spectroscopies were unable to distinguish fundamental differences in progressively aged samples; however, high-resolution thermogravimetric analysis (HRTGA) did demonstrate thatthe thermostability of the chloropyromorphite material increased with increasing residence time. The stirred-flow and batch dissolution studies suggest that the aging process ceased within 24 h and that the dissolution rate of the 1-day aged sample was not significantly different than the 1-year aged specimen. The amount of Pb released peaked at 21% (1-h sample, stirred-flow, pH 2.0) and was as low as 0.17% (1-year sample, batch method, pH 6.0). Postdissolution analyses of chloropyromorphite with XAFS, XRD, and HRTGA revealed no detectable chemical alterations of the pyromorphite samples signifying only release of dissolved Pb to solution and no formation of secondary products during dissolution.