Effect of soil properties and sample preparation on extractable and soluble Pb and Cd fractions in soils

Abstract

The effect of soil extraction procedures and/or sample pretreatment (drying, freezing of the soil sample) on the extractability of cadmium and lead was tested in a model experiment, with an employment of optical emission and atomic ab-sorption spectrometry methods. In the first part, 6 extraction procedures were compared: 2 mol l -1 HNO 3, 0.43 mol l -1 CH 3 COOH, 0.05 mol l -1 EDTA, Mehlich III extraction procedure (0.2 mol l -1 CH 3 COOH + 0.25 mol l -1 NH 4 NO 3 + 0.013 mol.l -1 HNO 3 + 0.015 mol.l -1 NH 4 F + 0.001 mol.l -1 EDTA), 0.01 mol.l -1 CaCl 2 , and deionised water. Addi-tionally, two methods of soil solution sampling were compared, and the centrifugation of satu-rated soil and the use of suction cups and dif-ferential pulse anodic stripping voltametry was applied to assess free and complexed metals portions. The results showed that different soil sample extraction methods and/or sample pre-treatments including soil solution sampling can lead to different absolute values of mobile ca-dmium and lead content in soils. However, the interpretation of the data can lead to similar conclusions as are apparent from the compari-son of the soil solution sampling methods where fairly good correlation was observed (for Cd r = 0.76, and for Pb r = 0.74). The ambiguous results were reported for voltammetric deter-minations of free and complex portions of Cd and Pb where a different behavior was observed for water extracts of soil and soil solution ob-tained using suction cups. Moreover, a chang-ing extent of lead complexation was determined with prolonged storage of the samples. The re-sults confirmed that soil and/or soil solution sampling under immediate soil conditions and limitations of pre-extraction operations are ne-cessary.