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Zn(I1) with humic and tannic acids were studied to evaluate the possibility of metal speciation using a combination of dialysis/atomic absorption spectroscopy, and the p H characteristics of the metal-organic species. The stability constants of metal-organic complexes were found to decrease in the order Pb(I1) > Cu(I1) > Cd(I1) > Zn(I1) 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 metalorganic 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 > Mn02 > 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 p H levels (6-8), 20pg/ml solutions of the organic acids released 20 pmol manganese per 100 ml solutions whereas at p H 2, 60 pmol manganese were released. The pH behaviour of the organic acids has been explained in terms of formation of oxalic acid at p H < 5, which then reduces the hydrous oxide. Chem. 54, 2600Chem. 54, (1976. Faisant appel a une combinaison de spectroxopie atomique, de dialyse ainsi qu'aux caract6 ristiques de p H des esp6es organometalliques, on a 6tudie les interactions de Cu(II), de Pb(II), de Cd(I1) et de Zn(I1) avec les acides humique et tannique dans le but d'kvaluer la possibilitk de spkiation des metaux. On a trouvC que les constantes de stabilitk des complexes organom6talliques dkroissent dans I'ordre Pb(1I) > Cu(1l) > Cd(I1) > Zn(l1) et EDTA > acide humique > acide tannique. Les courbes de Scatchard pour les systtmes acide humique/metal indiquent qu'il y a deux types de sites de couplage et que la difference entre leurs constantes de stabilitk est d'environ 10. L'ultrafiltration de solutions de mCtal/acide humique indique que les ions metalliques sont concentres dans les fractions de grands poids molkulaires ( > 3.1 nm). On a utilisC la separation par dialyse pour examiner la liberation, par les acides organiques (les complexes organometalliques ne sont pas dialysables), des metaux attaches a des composants modkles de suiments. Pour la liberation du cuivre de la bentonite, on a trouvC que la quantitk de cuivre libere depend la fois de la concentration des ligands et de la constante de stabilite du complexe mCtallique avec un rapport de liberation du metal par rapport aux sites complexants qui diminue dans I'ordre EDTA > aci...

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