Colloidal and dissolved metals in leachates from four Danish landfills

Landfill leachate samples were collected at four Danish landfills. The samples were collected anaerobically for characterization of the leachate in terms of pH, total alkalinity, total solids, total organic carbon, cations, anions and heavy metals. The sample procedure was designed to accurately reflect the in situ mobile colloid and associated contaminant concentrations. The leachate samples were separated into size fractions in order to obtain information about size distribution of colloids and associated heavy metals using filters with the pore sizes: 1.2, 0.40 and 0.001 μm. The general characterization of the four leachate samples showed large variation in the leachate quality for the investigated landfills ranging a factor of 2–60 among the four landfills for individual parameters. The dominant part of the colloidal material was in the smallest colloidal size fraction (0.001–0.40 μm) with total organic carbon (TOC) as one of the major components of the colloidal mass. Also Si, Al, Ca, Fe and S were found in the colloidal fractions. This indicated that both clay type particles, organic matter and precipitates (most likely sulfides) were present in the leachate samples. The distribution of the investigated elements varied from Cl being primarily in the dissolved fraction to Fe which mostly was found in the colloidal fractions. The concentrations of heavy metals in the leachate samples were 0.2–3.6 μg Cd/L, 28–84 μg Ni/L, 85–5310 μg Zn/L, 2–34 μg Cu/L, 0–188 μg Cr/L and 0–16 μg Pb/L with a considerable but highly varying part of the heavy metals associated with the colloidal fractions. However, when the heavy metal content in leachates is low, it is probably not of major environmental concern that part of the heavy metal is associated with the colloids. However, if the metal content is high and, as was the case for Zn in one of the leachate samples, nearly exclusively associated with the colloidal fractions, the behavior of the heavy metal in the leachate treatment facilities and in the environment may differ from the behavior expected for truly dissolved heavy metal.