Speciation of heavy metals in landfill-leachate polluted groundwater

Anaerobic leachate polluted groundwater samples were collected downgradient of a Danish landfill. The groundwater samples were spiked with heavy metals at concentrations within the range of metal concentrations found in landfill leachates (100 μg Cd l−1, 500 μg Ni l−1, 1000 μg Zn l−1, 1000 μg Cu l−1 and 1000 μg Pb l−1) and divided under anaerobic conditions into colloidal (screen-filtration and cross-flow ultrafiltration), organic (anion-exchange) and dissolved inorganic species of the heavy metals. The distribution of the heavy metals between the different size-fractions showed that a substantial, but highly varying part of the heavy metals was associated with the colloidal fractions (Cd: 38–45%, Ni: 27–56%, Zn: 24–45%, Cu: 86–95%, Pb: 96–99%). These colloidal bound metals were furthermore associated primarily with the organic fraction, except for Zn, which was associated mainly with the inorganic fractions. Dissolved Cd, Cu and Pb were mainly associated with dissolved organic carbon, while Ni was also present as carbonate complexes and Zn as carbonate complexes and free divalent Zn. Most of the metals present in the colloidal fractions would have been sampled as dissolved if the commonly used filter size of 0.45 μm had been used, since very little metal was found in colloids larger than 0.40 μm (Pb<15%, Cd<10%, Zn<5%, Ni and Cu<1%). The results indicate that heavy metals in leachate-polluted groundwater are strongly associated with small-size colloidal matter and organic molecules. The behavior of heavy metals in the environment is thus expected to differ from that of truly dissolved heavy metals.