Studies of metal–organic interactions in model systems pertaining to natural waters

Abstract

The interactions of Cu(II), Pb(II), Cd(II), and Zn(II) with humic and tannic acids were studied to evaluate the possibility of metal speciation using a combination of dialysis/atomic absorption spectroscopy, and the pH characteristics of the metal–organic species. The stability constants of metal–organic complexes were found to decrease in the order Pb(II) > Cu(II) > Cd(II) > Zn(II) and EDTA > humic acid > tannic acid. Scatchard plots for the metal–humic acid systems indicated two types of binding sites with a difference in stability constants of about 10. Ultrafiltration of metal–humic acid solutions indicated that the metal ions were concentrated in the large molecular size fractions (> 3.1 nm). Separation by dialysis was used to investigate the release of metal from model sediment components by the organic acids (the metal–organic complexes were non-dialyzable). For release of copper from bentonite, it was found that the amount of copper released depended on both the concentration of ligand and the stability constant of metal complex, with the ratio of metal release to complexing sites decreasing in the order EDTA > humic acid > tannic acid. Release of metal from the solids decreased in the order: bentonite > MnO2 > humic acid. Release of metal from the hydrous oxides via a redox decomposition of the oxide by natural organics was investigated using pyrogallol, gallic, and tannic acids. At natural water pH levels (6–8), 20 μg/ml solutions of the organic acids released 20 μmol manganese per 100 ml solutions whereas at pH 2, 60 μmol manganese were released. The pH behaviour of the organic acids has been explained in terms of formation of oxalic acid at pH &([a-z]+); 5, which then reduces the hydrous oxide.