Factors Affecting the Investigation of Heavy Metal Speciation in Forest Soils Using Thin-Channel Ultrafiltration

Abstract

In order to evaluate the usefulness of thin channel ultrafiltration for heavy metal speciation studies in litter extracts, the leaching and adsorption phenomena associated with this separation process were examined using different membranes. Using membranes with low heavy metal adsorption capacity (e.g. polycarbonate, polysulfone), thin-channel ultrafiltration was a reliable method for speciation studies based on size fractionation. However, changes in organic-metal association which occur in the separation step could be minimized using thin-channel ultrafiltration technique. This fact has to be taken into consideration when interpreting ultrafiltration data. After washing the ultrafiltration assembly successively with dilute HNO₃ and H₂O, no leaching was detectable. On the other hand, losses up to 100% of initial Pb- and Cu-content were detected on filtering an ionic standard solution. The retention of heavy metals decreased in the following order Pb > Cu°Cd > Zn. Membranes composed of regenerated cellulose had the highest retention potential for heavy metal ions and were therefore not suitable for speciation studies. The size distributions in various water extracts obtained by using thin-channel ultrafiltration are compared and discussed.