A simplified method for detection of lead contamination of soil

The rhodizonate spot test for Pb has been adapted to the detection of Pb contamination of soil. Depending upon the method of extraction chosen, Pb is detectable to 400–700 μg g⁻¹ in the dry soil sample. Extraction is carried out by heating or shaking with nitric acid, followed by filtration. The filtrate is brought to pH 1·5 with citrate using a pH meter or an indicator. The extract is spotted on filter paper and treated with freshly prepared aqueous sodium rhodizonate. Heating extracts more Pb than shaking, but a relatively stable percentage of the Pb present is extracted by shaking for 30 s. This latter method of extraction may be used as part of a rapid, simplified procedure requiring a minimum of apparatus. Use of citrate for pH adjustment eliminates the problem of crystallisation encountered with tartarate, as specified in the original procedure. Soil extracts with pH < 1 showed rapid fading of the pink Pb-rhodizonate spot, and those with pH > 3 gave the intense yellow colour of free rhodizonate, which tended to obscure the pink Pb-rhodizonate colour. Differences in the effect of heating and the effect of extraction time on Pb and Fe levels were observed, with Pb more readily extracted than Fe. Use of extracts with low levels of Fe made possible visual endpoint detection for the adjustment of pH. Using the shaken extraction method with a series of 107 samples, positive results were obtained for 82 samples with > 700 μg Pb g⁻¹, negative results for 10 samples with < 400 μg Pb g⁻¹ and mixed results for 15 samples in the range 400–700 μg Pb g⁻¹. Selection of a cutoff value of 500 μg Pb g⁻¹ eliminated false negative results in a pilot screening trial of 38 samples prepared by the heated extraction method.