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INFLUENCE OF AGING ON PYROMORPHITE FORMATION AND DISSOLUTION
Citation:
Scheckel*, K G. AND J A. Ryan*. INFLUENCE OF AGING ON PYROMORPHITE FORMATION AND DISSOLUTION. Presented at 222nd American Chemical Society National meeting, Chicago, IL, 8/26-30/2001.
Description:
Published literature has clearly demonstrated that the reaction of a lead (Pb) source as either Pb-minerals (angelesite, cerrusite, or galena), goethite adsorbed lead, Pb-contaminated soils, or an in-vitro bioavailable assay with a phosphate reserve (apatite or hydroxyapatite) results in the rapid kinetic formation of pyromorphite. Pyromorphites are the most stable Pb minerals found under normal environmental conditions nd thus other solid phase Pb species should b transformed to pyromorphite by a dissolution-precipitation mechanism to immobilize soluble soil Pb in-situ by simply adding phosphate to the soil. Work to date has demonstrated that the addition of (hydroxy)apatite to contaminated soil reduces dissolved Pb to concentrations below federal drinking water limits. However, the long-term fate of immobilized Pb as precipitated pyromorphite is not fully understood. Crystal dynamics dictate that as aging time increses, the relative stability of a precipitate increases thus resulting in reduced solubility and decreased bioavailability. Nevertheless, the effect of residence time has not been explored in relation to the formation of pyromorphite and its potential as an in-situ remediation technique.
The purpose of this study was to investigate the kinetics of dissolution and crystallization dynamics of precipitated synthetic pyromorphite as influenced by increasing aging times ranging from one hour to 12 months. The use of spectroscopic and microscopic techniques regarding the formation of the aged synthetic pyromorphite showed that the crystal structure of the material changed very little after 24 hours of aging indicating minute differences between one-day and one-year aged samples. The dissolution agent employed for the stirred-flow and replenishment techniques was nitric acid at pH values of 2, 4, and 6 to represent the wide range of possible soil environments in that Pb is found. The results of the dissolution experiments demonstrate that 1) the stirred-flow technique is more effective in dissolving Pb from pyromorphite than the replenishment method because of removal of reaction products that inhibit dissolution, 2) as pH decreased, the amount of Pb released into solution increased, 3) the pyromorphite material was identical before and after dissolution, and 4) the influence of time on synthetic pyromorphite showed little effect on the stability of the solid material after 24 hours of aging. Additionally, the rate of dissolution for pyromorphite followed first-ordered kinetics regardless of aging time or dissolution agent. Overall, the results of this research illustrates that once pyromorphite forms in Pb-contaminated soils treated with phosphorus, the newly formed pyromorphite crystals quickly transform to stable mineral phases that become resistant to dissolution under natural soil and sediment environmental conditions.