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How Humic Substances Dominate Mercury Geochemistry in Contaminated Floodplain Soils and Sediments

Frontier Geosciences Inc., 414 Pontius Ave. North, Seattle, WA 98109,

Madhukar V. M. Desai

Bhabha Atomic Research Center (BARC), Health Physics Div., Bombay-400 085, India,

Markus Spengler

GKSS Forschungszentrum GmbH, Inst. für Physikalische und Chemische Analytik, Abteilung Organische Spurenanalyse, Max-Planck-Str., 21502 Geesthacht, Germany;

Cláudia Carvalhinho Windmöller

UNIJUI, Dep. Biol. Química, R. São Francisco, 501, Ijui-R.S.-CEP 98700 000, Brazil,

Rolf-Dieter Wilken

Johannes-Gutenberg-Univ., Inst. of Geosciences, Johannes-Joachim-Becher-Weg 21, 55099 Mainz, Germany,

^* Corresponding author (dirkw{at}frontier.wa.com).

ABSTRACT

The interaction of mercury (Hg) and humic substances (hs) was studied in floodplain topsoils and surface sediments of the contaminated German river Elbe. An intimate coupling exists between the geochemical cycles of Hg and organic carbon (OC) in this ecosystem. Humic substances exert a dominant influence on several important parallel geochemical pathways of Hg, including binding, transformation, and transport processes. Significant differences exist between the Hg-hs associations in floodplains and sediments. Both humic acids (ha) and fulvic acids (fa) contribute to Hg binding in the sediments. In contrast, ultrafiltration experiments proved that Hg in the floodplain soils is almost exclusively bound to very large humic acids (ha) with a nominal molecular weight (MW) >300 000. Successive cation and anion exchange experiments demonstrated that those Hg-ha complexes are inert toward competition by other cations, and also apparently predominantly electroneutral. Speciation transformation reactions in the solid phase were investigated by sequential extraction and thermal release experiments. Upon addition of Hg model compounds to a sediment matrix, all species were transformed to the same new speciation pattern, regardless of their original speciation. The accompanying alterations in availability and solubility were partially due to interconversion between the different Hg redox states, including Hg(I). Simultaneously, partial transformation of added Hg²⁺ into volatile Hg compounds (35% in 10 d) was observed. Finally, Hg association with water-soluble ha continuously increased downstream, indicating that hs play a key role in both lateral and longitudinal Hg transport in the Elbe ecosystem.

previously at GKSS;

previously at GKSS;

previously at GKSS;

previously at GKSS.

Received for publication September 8, 1997.