Heavy-metal ion complexation by particulate matter in the leachate of solid waste: a multi-method approach

Abstract

The metal ion binding characteristics of particulate matter obtained from column experiments on the anaerobic digestion of solid waste were studied using a titrimetric approach. The experimental set-up allowed us to study the dynamics of particle bound ligand concentrations during digestion processes typically found in landfills. We developed a continuous titration method by simultaneously using a Cd-sensitive and p H electrode and combining metal and acid/base titrations. This technique allows for a more precise determination of p K a-log K M pairs for each ligand than metal titrations alone. The results were compared with titration methods using differential pulse anodic stripping voltammetry (DPASV) and atomic absorption spectroscopy (AAS) with longer equilibration times in order to further characterize ligand properties such as reaction kinetics, the electrochemical lability of the respective complex during DPASV, the distinction between metal adsorption to particulate matter and metal complexation by soluble ligands adhered to particles, reversibility of the binding process by competition studies, and resistance against purging with nitrogen gas. The properties of seven major metal binding ligands were identified and assignments to the most likely functional groups were made. The most important ligand properties are for ligand A: p K a ≈ 9.2, log K cd ≈ 7.0 fast reaction kinetics (mercapto groups); ligand B: p K a = 4.8, log K Cd ≈ 6.0, slow reaction kinetics (chelates with 3 or 4 carboxylic groups); ligand C: p K a ≈ 6.0, log K Cd ≈ 13.0, irreversible metal binding at basic p H-values (uptake inside bacterial cells); ligand D: p K a = 7.7, log K Cd = 4.0, runs parallel to N content of particulate matter with digestion time (primary amines neighboring oxo groups); ligand E: p K a ≈ 12.0, log K Cd = 9.0, runs parallel to P content of particulate matter (phosphate); ligand F:p K a ⪢ > 9.0, log K Cd f = p K a + 0.4 , runs parallel to N content of particulate matter (primary amines neighboring SH groups); and ligand G: p K a ≤ 4.8, log K Pb ≈ 4.3, strong Pb 2+ ligand, even at low p H-values. Metal ions were found to be irreversibly bound by ligand C at low heavy-metal concentratins, whereas at higher concentrations the binding is reversible and can be predicted using the mass of the digestion process (methanogenic phase). All other ligands have their concentration maximum in the transition phase between acetogenic and methanogenic phase.