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 sam ples were calculated by linearization of potentiometric titration data usin g a modified Gran method. Sewage samples were also titrated with cupric ion s 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 effec t in the concentration of free metal caused by the competition between copp er and hydrogen ions for organic ligands present in raw sewage. Particulate matter as well as carbonate did not show a significant contribution to con trol copper speciation in the system. Assuming a two binding sites model, c onditional stability constants (K') of copper complexes were dependent on p H, showing values around log K'(1) = 8.00 and log K'(2) = 6.30 at pH 5.8, w hereas 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' Va lues due to adsorption contribution, and this effect was minimized in dilut ed samples. Ligand concentrations varied between 5 and 10 mu mol l(-1). (C) 2000 Elsevier Science Ltd. All rights reserved.