Characteristics of different molecular weight fractions of organic matter in landfill leachate and their role in soil sorption of heavy metals

We have characterised two kinds of municipal landfill leachates derived fro m 'old' and 'young' municipal waste landfills on the basis of the molecular weight distribution of the constituents, taking into account that the grea t variety of leachate constituents prevents any evaluation of the fate and of the role played by each component in the environmental impact. In the sa mple S1 told leachate), the constituents were distributed over a wider rang e of molecular weights; high molecular weight fractions were present. In sa mple S2 (young leachate), the fractions are actually narrower at the lower molecular weights. The high molecular weight fractions of old leachates are found to be complex structures formed by condensed nuclei of carbons subst ituted by functional groups containing nitrogen, sulphur and oxygen atoms; the low molecular weight fractions of leachates are, instead, characterised by linear chains substituted by oxygenated functional groups such as carbo xyl and/or alcoholic groups. After characterising each fraction we studied the role played by these fractions in the soil's capability for retaining h eavy metals [copper (Cu) and cadmium(Cd)]. The Cd uptake increases only on the soil treated with sample S1 characterised by a higher pH value and by t he presence of high molecular weight fractions. The Cu uptake also increase s on the soil treated with sample S2, characterised by the sole presence of low molecular weight fractions. On the other hand, the metal adsorption te sts performed on soil treated with the single fractions show that the amoun t of Cu and Cd retained by soil treated with the high molecular weight frac tions of sample does not increase after 72 h of treatment and that the amou nt of Cu retained by the low molecular weight fractions of sample S1 and by the fractions of sample S2 increases, but does not justify the amount reta ined by soil treated with the total leachates. (C) 2001 Elsevier Science Lt d. All rights reserved.