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PH16 Metals in the Environment: Chemistry and Fate Issues
(PH160) Competing effect of metal ions sorbed onto natural organic matter.
Twardowska, I.1, Kyziol, J.1, 1 Polish Academy of Sciences, Institute of Environmental Engineering, Zabrze, Poland
ABSTRACT- The knowledge of the mechanisms of metal binding onto organic matter is crucial both for understanding the processes affecting metal mobility and bioavailability in soils, and for pollution control and remediation of contaminated sites practice. In the most such cases, not a single, but several metal ions are involved. The competing effect of Zn, Cd, Cu and Cr for sorption sites in binary systems, studied in the batch sorption experiments on peat as almost pure organic matter, was found to be highly specific. Despite similar sorption behavior and predominant affinity to the weakly bound ion exchange sites, labile Zn and Cd ions appeared to be weak competitors to each other, enriching total ion exchange capacity of the organic matter to the saturation state, while in the monometallic system it was for each metal undersaturated. Particularly strong competing effect on Zn and Cd binding exert Cu and Cr ions, which predominantly form chelating complexes with carboxylic COOH and phenolic OH groups of humic substances and are bound in stable organometallic complexes. These ions displaced Cd from all the sorption sites proportionally, which resulted in almost 5- and 6-fold lower sorption of Cd onto organic matter in the presence of Cu and Cr, respectively. Simultaneously, "weak competitor" Cd affected deeply sorption capacity of organic matter for "strong competitors" Cu and Cr that resulted in a significant reduction of total sorption capacity, as well as of binding the individual Cu and Cr ions in the binary system with Cd. Moreover, it caused dramatic alteration of their "sorption pools", generally in the direction of weaker binding sites. This change was mostly due to the screening effect of Cd than its successful competing for sorption sites. In turn, in Cu-Cr binary system, Cr appeared to be a stronger competitor for binding sites, occupying mostly stable "insoluble organic" fraction and weakly bound carbonate sites that caused 2-fold reduction of Cu sorption. Thus, the competing affects not only "weak competitors", but "strong" competing metal ions as well, due to different interaction mechanisms.
Key words: organic matter, heavy metals, sorption, metal partitioning
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