Complexation and adsorption of copper in raw sewage

Competition between complexation and adsorption of copper was evaluated in raw sewage samples. Dissociation constants of filtered and non-filtered samples were calculated by linearization of potentiometric titration data using a modified Gran method. Sewage samples were also titrated with cupric ions at three different pH values using an ion selective electrode to quantify the effect of pH, ligand concentration, and particulates in the speciation of copper. Three titratable groups with pKa values of 4.45, 6.42, and 9.31 were detected in non-filtered samples. Changes in pH showed a marked effect in the concentration of free metal caused by the competition between copper and hydrogen ions for organic ligands present in raw sewage. Particulate matter as well as carbonate did not show a significant contribution to control copper speciation in the system. Assuming a two binding sites model, conditional stability constants (K′) of copper complexes were dependent on pH, showing values around log K′1=8.00 and log K′2=6.30 at pH 5.8, whereas at pH 7.8, these constants increased to log K′1=10.60 and log K′2=8.70 for filtered samples. Particulates caused small changes in K′ values due to adsorption contribution, and this effect was minimized in diluted samples. Ligand concentrations varied between 5 and 10 μmol l−1.