Pyromorphite

Abstract: Considerable evidence, both theoretical and experimental, support the assertion that lead phosphates are the most stable environmental soil lead form and that they may form rapidly when adequate phosphate is present. This study demonstrated that the weathering of galena to insoluble lead phosphates in soils at a port facility historically used for shipment of ore concentrates is due to the presence of adequate soil phosphate. Geochemical modeling indicated that the addition of a phosphate amendment to Pb-bearing soil would result in lead phosphate formation with an estimated aqueous lead solubility of 0.1 μg/L in soil

Abstract: In its soluble ionic forms, lead (Pb) is a toxic element occurring in waters and soils mainly as the result of human activities. The bioavailability of lead ions can be decreased by complexation with various materials in order to decrease their toxicity. Pb chemical immobilization using phosphate addition is a widely accepted technique to immobilize Pb from aqueous solution and contaminated soils. The application of different P amendments cause Pb in soils to shift from forms with high availability to the most strongly bound Pb fractions. The increase of Pb in the residual or insoluble fraction results from formation of pyromorphite Pb5(PO4)3X where X = F, Cl, Br, OH, the most stable environmental Pb compounds under a wide range of pH and Eh natural conditions. Accidental pyromorphite ingestion does not yield bioavailable lead, because pyromorphite is insoluble in the intestinal tract. Numerous natural and synthetic phosphates materials have been used to immobilize Pb: apatite and hydroxyapatite, biological apatite, rock phosphate, soluble phosphate fertilizers such as monoammonium phosphate, diammonium phosphate, phosphoric acid, biosolids rich in P, phosphatic clay and mixtures. The identification of pyromorphite in phosphate amended soils has been carried out by different non destructive techniques such as X-ray diffraction, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, X-ray absorption fine structure, transmission electron microscopy and electron microprobe analysis. The effectiveness of in situ Pb immobilization has also been evaluated by selective sequential extraction, by the toxicity leaching procedure and by a physiologically based extraction procedure simulating metal ingestion and gastrointestinal bioavailability to humans. Efficient Pb immobilization using P amendments requires increasing the solubility of the phosphate phase and of the Pb species phase by inducing acid conditions. Although phosphorus addition seems to be highly effective, excess P in soil and its potential effect on eutrophication of surface water, and the possibility of As enhanced leaching remains a concern. The use of mixed treatments may be a useful strategy to improve their effectiveness in reducing lead phyto- and bioavailability.

Abstract: Phosphate compounds of Pb [e.g., pyromorphite Pb5(PO4)3(X) where X = OH, F, or Cl] are comparatively insoluble, and inducing their formation in contaminated soils may be a means of reducing the bioavailability and chemical lability of Pb in soil. Previous research has documented the formation of pyromorphite subsequent to the addition of phosphates, as soluble phosphate (Cotter-Howells, J.; Caporn, S. Appl. Geochem. 1996, 11, 335) and as apatite (Laperche et al. Environ. Sci. Technol. 1996, 30, 3321), to Pb-contaminated soils. In the present study, the effect of apatite amendments on the bioavailability of Pb in contaminated soil and the stability of pyromorphite were examined. A Pb-contaminated soil was treated with natural and synthetic apatites, and the bioavailability of Pb was determined in plant uptake studies with sudax (Sorghum bicolor L. Moench). The Pb content in shoot tissue decreased as the quantity of added apatite increased. However, Pb and P contents in the plant roots increased as the quan...