Attenuation of metal contamination in landfill leachate by dewatered waterworks sludges

• Waterworks sludges shown to be good adsorbents for Cr, Cd and Pb.
• Maximum adsorption obtained at pH 4 in most cases.
• Metals sorbed were strongly bound to waterworks sludges.

Excess heavy metals in landfill leachate pose considerable environmental risk. The reduction of this risk using a by-product (dewatered waterworks sludges) is the main aim of this study. Bench-scale experiments were conducted towards understanding the key factors for the uptake of Pb, Cr and Cd by the sludges. Results indicate the maximum uptake for most of the sludges occurred at pH 4. Of the six models used for fitting the adsorption data for Pb, Cr and Cd; Freundlich, Temkin and Langmuir models gave the best fit in that order. Kinetic data was well fitted with the pseudo first order model for Pb and pseudo second order model for Cr; whilst the intraparticle diffusion model was found to adequately describe the kinetics for Cd. Principal component and canonical correlation analysis, FT-IR spectroscopy and sequential extraction revealed that: (i) organic carbon plays a key role in Cr adsorption and Pb through the carboxylate group, (ii) Cd adsorption is correlated to iron or aluminium (oxy) hydroxides, and (iii) the greatest amount of metals sorbed by the sludges were tightly bound and not extractable. These findings show that the sludges are potential adsorbents for Cr, Cd and Pb removal in practical applications.