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 the actual health risk posed by this element. U.S. EPA method 1340 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 assays (bioaccessibility) used to estimate soil Pb bioavailability, but much of the data are not very consistent. One of the challenges of these methods is that they need to be done in well-equipped laboratories that are capable of measuring Pb in leachate solutions using advanced instrumentation. X-ray fluorescence (XRF) analysis is a powerful analytical technique to determine a large number of elements present in a sample, spectrochemically. In this study, we propose a new approach for rapid assessment of soil Pb bioaccessibility that can be conducted in the field. Fifteen urban garden soil samples and SRM 2710a were chosen with total Pb ranged 254 – ~5500 mg kg⁻¹ and percent bioaccessible Pb ranged 14 - 48% (pH 2.5) or 14 – 86 (pH 1.5) based on EPA method 1340. Five grams of a soil sample were extracted with 0.4 M glycine at pH 1.5 for 1 hour. The extraction was passed through a 0.45 um filter. The filtered liquid was directly measured in XRF cups with the XRF analyzer. Preliminary results showed that the extracted Pb concentrations with the XRF method are consistent with those determined previously with EPA Method 1340 (glycine extractant of pH 1.5) for these samples. More research is needed to evaluate the variability with different assays, but this XRF method has great potential for use in the rapid assessment of soil Pb risk from urban areas, brownfield, industrial and military sites.