SCREENING FOR BIOACCESSIBLE PB IN SOILS USING A RAPID XRF ANALYZER METHOD

Bioavailable Pb, instead of total Pb, in soils is a better measure of actual health risk. The EPA method 9200.2-86 (Standard Operating Procedure for an In Vitro Bioaccessibility Assay for Lead in Soil) uses a 0.4-M glycine acid at pH=1.5 under controlled temperature and time to leach the Pb from soil samples, and use the obtained Pb concentrations as a measure of the “bioavailable Pb.” There are also many other in vitro assay used to estimate soil Pb bioavailability, but much of the data are not very consistent. One of the challenges of these in vitro assay (a.k.a., bioaccessibility) methods is that they need to be done in well equipped laboratories that are capable of measuring Pb concentrations in leachate solutions. In this study we experiment with an approach for rapid assessment of soil Pb bioaccessibility that can be conducted in the field. This will take advantage of an X-Ray Fluorescence Environmental Scanner that is being widely used for the field screening of total Pb in soils. The idea is that, after extracting the Pb from the soil with an assay, a small volume of the leachate containing high Pb concentrations is mixed with a lime powder – so as to neutralize the acidity. From this mixture a paste can be made and then examined by the XRF scanner for Pb content. Six urban garden soil samples, total Pb ranged 376 – 3430 ppm and percent bioaccessible Pb ranged 25-48%, were tested in an initial trial. Ten grams of a soil sample were extracted with 10-mL 50% HCl for 30 minutes. Then 1.5-mL extracted solution was thoroughly mixed with 10 g of limestone powder. The extracted Pb was then calculated based on the XRF readings on the paste material. The calculated extractable Pb with this method correlates well to the bioaccessible Pb determined previously with the EPA standard protocol for these samples. More research is needed to evaluate the variability with different assays, but this rapid XRF method has great potential for use in the rapid assessment soil Pb risk from urban areas, brownfield, industrial and military sites.