A novel thermodynamic approach for the complexation study of toxic metal cations by a landfill leachate

Abstract

Landfill leachates can contaminate nearby aquifers. The hazards deriving from this contamination also depend on the chemical speciation of various contaminants. A novel approach is proposed here to face this problem from a chemical thermodynamics point of view. The complexing ability of the soluble fraction of a landfill leachate (collected from Bellolampo, Palermo, Italy) towards Pb²⁺, Cd²⁺ and Cu²⁺ has been investigated at T = 298.15 K in NaCl_aq at I = 0.1 mol dm⁻³. The soluble fraction of the landfill leachate was first characterized by different analytical techniques. Then, its acid–base properties were studied by ISE-H⁺ potentiometric titrations and modelled by the so-called diprotic-like model. Differential Pulse Anodic Stripping Voltammetry (DP-ASV) titrations of metal ion aqueous solutions with a diluted landfill leachate were carried out, successively, in order to determine the stability constants of the leachate-metal complexes. The stability of the Pb²⁺/OH⁻ and Pb²⁺/Cl⁻ complexes was also studied by the same technique. Finally, the sequestering ability of the leachate towards the investigated metal cations has been quantified by the calculations of various pL_0.5 values under different pH conditions. The results proved that the landfill leachate is a good sequestering agent toward those cations, reducing the fraction of the free cations in solution, and that it can be a selective sequestrant at different pH.