Determination of bioavailable species of lead and cadmium in soils by differential pulse anodic stripping voltammetry

The mobility, transformation and potential bioavailability of elements and their entry into the food chain depends on their chemical forms. Single-step extraction procedures are used prevailingly for assessing bioavailable species in soils. Two single-step extraction procedures validated by EC Bureau of Reference, extraction with 0.05M acetic acid and 0.05M EDTA have been used for selected soil samples. Extractable Pb and Cd were determined by differential pulse anodic stripping voltammetry (DPASV) with hanging mercury drop electrode. This method allows direct determination of both metals in extracts. Interference of humic substances can be decreased by dilution of extracts. Pb(II) and Cd(II) were released from complexes in EDTA extracts by adjustment of pH to 1 with 1M HC1. The precision and accuracy of pretreatment procedure and DPASV technique were evaluated by analyzing synthetic soil extracts spiked with known amounts of Pb(II) and Cd(II). The developed procedure is simple and reliable for determination of very low contents of Pb and Cd in soil extracts as evidenced by low values of relative standard deviation (1.0-7.9 %). Exchangeable forms make 0.2-3.3 % of total Pb and 13.0-63.6 % of total Cd. EDTA extraction released from soils 7.9-29.9 % of total Pb and 33.5-59.6 % of total Cd. Our results obtained by DPASV show reasonable agreement with those of AAS and flow galvanostatic stripping chronopotentiometry.