GEOCHEMICAL OPTIONS FOR IMMOBILIZING LEAD (PB) CONTAMINATION IN SOIL
In the Contaminated Urban-yard Restoration: Testing Apatite II Immobilization of Lead in Soil (CURTAILS) - A New Orleans Field Trial, we have been assessing the possibility of reducing the amount of bioaccessible Pb in residential soils using a biogenic phosphate product (Apatite II) obtained from fish bones (Ca10-xNax(PO4)6-x(CO3)x(OH)2 where x < 1). We have evaluated how effective Apatite II might prove in a series of plot trials with a variety of other phosphate products. Namely: Triple Super Phosphate (Ca(H2PO4)2•H2O), rock phosphate (Ca5(PO4)3F), hydroxyapatite Ca10(PO4)6(OH)2, bone-char, [Ca10(PO4)6OH2], bone-meal fertilizer and phosphoric acid (H3PO4). Trials in New Orleans are underway at 3 residential sites.
Here we present results of solubility tests performed on soils collected 6-months after phosphate addition. Solubility testing used an in vitro bioaccessibility assay that employed a simulated gastric fluid. Initial tests of soil Pb showed levels of >1,000 mg/kg. Each study yard was divided into 8, 10’x10’ plots, 6 of which were amended with the phosphate products and the other 2 left as controls. The pre-amendment soils did not exhibit 100% Pb bioaccessibility. Microscopic analysis of the pre-amendment soil revealed Pb in the form of Pb-paint particles, and a fraction of the Pb in the also associated with phosphorus. This suggested that a transformation of the soil Pb to a Pb-phosphate form was occurring in the soils prior to the project. Tests of the soils at 6-months post addition showed that all the phosphate products had some effect, but in one soil the effect was less; we posit this was because there had already been long term change to a Pb-phosphate at this location.