INFLUENCE OF METAL SPECIATION IN LANDFILL LEACHATES ON KAOLINITE SORPTION MAURO MAJONE 1 * * M , MARCO PETRANGELI PAPINI 1 and ENRICO ROLLE 2* M 1 Department of Chemistry, University of Rome ``La Sapienza'', P.le Aldo Moro, 5-00185, Rome, Italy and 2 Department of Environmental Engineering, University of Rome ``La Sapienza'', Via Eudossiana, 18-00184, Rome, Italy (First received December 1995; accepted in revised form June 1997) AbstractÐThe sorption onto kaolinite of Pb, Cd, Ni and Cu from a land®ll leachate was studied in re- lation to the metal speciation in the liquid phase. Metal speciation was determined by two dierent ex- perimental procedures based on the exchangeability on a cation chelating resin (Chelex100) and on the separation by dialysis with membranes at dierent molecular weight (MW) cut o. The speciation pro- cedures were applied on the leachate before and after equilibration with clay, in order to determine the contribution of the dierent 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 con®rmed by the concurrent removal of COD and phenolic substances. As determined by the exchange-based procedure, the main contribution to metal sorption derived from free/labile (rapidly exchangeable) or slowly exchangeable fractions. However, Pb and Cd were also removed from the stable/inert (not exchangeable) fraction. Because metal sorption is partially due to species that are not exchangeable on Chelex100 and partially to species that have MW more than 1000 daltons, these exper- imental evidences strongly support that free metals are not the only species participating in the sorption onto kaolinite and that some metal species are adsorbed without previous dissociation. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐland®ll leachates, heavy metals, speciation, soil sorption INTRODUCTION Land®lling is the most common method for dispos- ing of solid wastes. One of the major problems as- sociated with land®lling is the generation of large quantities of heavily polluted leachate. Land®ll lea- chate is identi®ed as a potential source of ground and surface water contamination, as it may perco- late through soils and subsoils causing extensive pollution of streams, creeks and water wells. Moreover, migration of leachates is often slow and the environmental impact of an improper manage- ment of the land®ll plant might become evident only after a long time. Among the dierent pollutants occurring in lea- chates, heavy metals can be particularly dangerous. Their movement through soils is mainly related to the ¯uid dynamics and to the sorption on the solid phase, the latter being in turn controlled by the composition of both solid and liquid phases. With regard to this, metal distribution among the individ- ual physico-chemical forms in the liquid phase, i.e. speciation, has to be considered more than their total concentration. As a matter of fact, the complexing ability of land®ll leachates has been demonstrated to be cru- cial in determining metal mobility (Garcia- Miragaya and Page, 1976; Elliott and Denneny, 1982; Christensen and Kjeldsen, 1989). Because of the high variability in the leachate composition (Johansen and Carlson, 1976; Fuller et al., 1979; Ghassemi et al., 1984; Lema et al., 1988), the use of measurable broad parameters such as total organic carbon (TOC), pH or concentration of soluble com- mon salts could be misleading in the assessment of the transport of metals through soils. In particular, the organic fraction of land®ll leachates is highly variable in composition, ranging from simple ligands, like acetate or aminoacids, to high molecu- lar weight compounds with many phenolic, car- boxylic and amminic chelating groups, like fulvic and humic substances (Chian, 1977; Chian and De Walle, 1977; Sawhney and Kozlosky, 1984; Millot et al., 1987; Gourdon et al., 1989; Oman and Hynning, 1993). Metal speciation in leachates has been studied by both experimental and theoretical methods, either Wat. Res. Vol. 32, No. 3, pp. 882±890, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00 PII: S0043-1354(97)00288-1 *Author to whom all correspondence should be addressed. 882