Diffusion and solid speciation of Cd and Pb in clay liners

Abstract

For the assessment of the long-term sorption behavior of landfill clay liners towards contaminants the information given by distribution coefficients or retardation factors is not sufficient. Information regarding the solid speciation, which describes the actual bonding sites of sorbed contaminants, is required. The objective of the study presented in this paper was to analyze the retardation and solid speciation of Cd and Pb migrating into an illitic/smectitic loess loam by diffusion. Apparent diffusion coefficients as obtained from diffusion experiments on compacted loess loam with heavy metal solutions containing 0.001 M Cd or Pb chloride were in the range of 4.9-6.7 · 10-8 cm2/s for Cd and 3.6-7.4 · 10-9 cm2/s for Pb. The solid speciation was analyzed using a sequential extraction procedure giving operationally defined fractions (i.e. exchangeable, carbonatic, oxidic, organic, and residual fraction) of the sorbed heavy metals. The amounts of Cd and Pb migrated into the loess loam by diffusion clearly showed weaker bonding than the geogenic fraction. The speciation itself was significantly dependent on the heavy metal load showing weak bonding at high concentrations and strong bonding at low concentrations. Chemical extraction procedures can be used to analyse the sorption capacity of individual phases of the liner material towards heavy metals. Langmuir adsorption isotherms could be constructed for the individual operationally defined fractions, and the adsorption maxima and affinity parameters of these fractions could be determined. The results show that the specific sorption sites only have a limited capacity for heavy metals compared to nonspecific adsorption sites as represented particularly by the exchangeable fraction. Set in relation to the total sorption capacity of the investigated loess loam towards Cd and Pb, nonexchangeable bound species amount to only 7% (Cd) and 28% (Pb) for the carbonatic fraction, 1% (Cd) and 4% (Pb) for the oxidic fraction, and 0.5% (Cd) and 1% (Pb) for each the organic and residual fraction. The results emphasise that chemical extraction procedures can give important indications about the way heavy metals are actually fixed to the components of a landfill clay liner material. © 1998 Elsevier Science B.V.