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 HNO3, 0.43 mol l-1 CH3COOH, 0.05 mol l-1 EDTA, Mehlich III extraction procedure (0.2 mol l-1 CH3COOH + 0.25 mol l-1 NH4NO3 + 0.013 mol.l-1 HNO3 + 0.015 mol.l-1 NH4F + 0.001 mol.l-1 EDTA), 0.01 mol.l-1 CaCl2, 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 cadmium 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 determinations of free and complex portions of Cd and Pb where a different behavior was observed for water extracts of soil and soil solution obtained using suction cups. Moreover, a changing extent of lead complexation was determined with prolonged storage of the samples. The results confirmed that soil and/or soil solution sampling under immediate soil conditions and limitations of pre-extraction operations are necessary.
Highlights
Soil properties, as well as root exudates, substantially affect plant-availability of potentially toxic elements in soil and their mobility and chemical forms in soil solution
HNO3 (2 mol l-1) was able to extract cadmium portions ranging between 25 and 100% while a higher extractability was observed for highly contaminated soils confirming an anthropogenic origin of the soil pollution
Diluted nitric acid is able to dissolve the element portion comparable to the sum of labile soil element fractions [22], and in the case of cadmium, it can be accepted as the approximation of the total element content in soil, especially in anthropogenically contaminated soils
Summary
As well as root exudates, substantially affect plant-availability of potentially toxic elements in soil and their mobility and chemical forms in soil solution. Soil solution represents essential electrolytic water solution containing dissolved organic and inorganic compounds (coloids, complexes, free salts and ions of these salts), atmospheric gases and exudates of plant roots and microorganisms. The knowledge of the composition of a soil solution is substantial for the elucidation of element uptake processes by plants, as well as for plant growth. Mechanisms of released organic acids, such as malate, citrate and oxalate, and their activity in soil (sorption, complex formation, decomposition by soil microorganisms) represent complex and yet to be fully elucidated processes [3,4]
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