Dissolved humic substances (HS) may increase the dissolution of heavy metals (HM) and hence augment HM mobility in soils by forming dissolved HM–HS complexes, but the role of HS composition, which depends on the origin of HS, is uncertain. In order to determine the influence of HS composition / origin on HM dissolution, two HM-polluted soil samples were extracted in the batch mode by three different HS solutions containing ~25 mM dissolved organic carbon (DOC) in 0.05 mol/l KNO₃ at pH = 6 for totally 10 weeks with weekly replacements of HS solutions. Two natural HS solutions (MEMO(beech)-HS and MEMO(spruce)-HS) were obtained from litter layers under beech and Norway spruce. Processed HS (Bio-HS) is cow slurry, which has been treated by oxidative, hydrolytic destruction in order to dissolve organic compounds and remove inorganic compounds such as HM. DOC, Cu concentrations and pH were determined in clear extracts obtained each week by centrifugation.

The investigation showed that natural as well as processed HS possess a high capacity to dissolve HM in polluted soils. In fact, up to 50% of total Cu was extracted after 10 weeks from strongly contaminated soil (CRC), indicating that HS has a substantial potential as a cleaning agent for remediation of HM-polluted soils. The origin of HS infl uenced the extracted amounts, which decreased in the order MEMO(spruce)-HS > ME MO(beech)-HS > Bio-HS, but only in the extraction of the CRC soil, while the three HSs extracted similar percentages from moderately Cu-polluted agricultural soil (Cu-P). Accordingly, HM nature, maybe HS origin and soil composition affect HM mobility and must be taken into consideration if HS is to be used as a cleaning agent in the remediation of HM-polluted soils.

Keywords: copper, dissolved humic substances (HS), dissolved organic carbon (DOC), heavy metal mobilisation, soil