Use of the diffusive gradients in thin films technique (DGT) with various diffusive gels for characterization of sewage sludge-contaminated soils

The diffusive gradients in thin film technique (DGT) was used for characterization of South Moravian arable soils (sampling sites Zlín, Tuřany, and Chrlice) amended by sewage sludge in the 1980s. Two types of polyacrylamide diffusive gel with different pore size (APA gels—cross-linked with agarose and RG gels—cross-linked with bis-acrylamide) were employed. The (bio)available parts of Cd, Cu, and Ni and the proportions of inorganically and organically complexed species of these metals were assessed. The degree of metal resupply from the soil solid phase to the soil solution was also determined. Metal concentrations obtained by the DGT technique were lower by almost 4 to 5 orders of magnitude in comparison with those obtained by extraction with aqua regia. DGT concentrations of metals were also lower by approximately 1 to 2 orders of magnitude in comparison with those obtained by extraction with sodium nitrate (commonly used for assessment of the (bio)available part of metals). Results obtained by DGT measurement were expected to be closer to the actual content of available metal species than results obtained by extraction with sodium nitrate. Using RG gels together with APA gels provided resolution of inorganically and organically complexed metal species and their proportional representation. Inorganic metal species (particles smaller than 1 nm) formed a predominant part of assessed metal content in all studied soil samples and horizons. However, there was the exception of the cadmium content in the middle profile of Chrlice sandy soil sample. Ratio R values indicated that resupply of Cd, Cu, and Ni from the solid phase to the soil solution varied for individual soil samples and individual depth profiles. Mobile and labile species of Cd, Cu, and Ni were much more closely related to upper rather than deeper horizons. This observation correlates very well with the mechanical treatment and amendment of the studied soils.