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The sorption onto kaolinite of Pb, Cd, Ni and Cu from a landfill leachate was studied in relation to the metal speciation in the liquid phase. Metal speciation was determined by two different experimental procedures based on the exchangeability on a cation chelating resin (Chelex100) and on the separation by dialysis with membranes at different molecular weight (MW) cut off. The speciation procedures were applied on the leachate before and after equilibration with clay, in order to determine the contribution of the different fractions to the total sorption. As determined by the MW-based procedure, large fractions of dissolved metals were associated to substances with high MW (>1000 and >12000 daltons), even if these substances represent only 18% of the total organic content (as determined by chemical oxygen demand, COD). These high-MW fractions contribute to metal sorption onto kaolinite, as also confirmed by …