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Interactions of hydrophobic metal complexes and their constituents with aquatic humic substances.

Boullemant, Amiel; Gagné, Jean-Pierre; Fortin, Claude; Campbell, Peter G. C. (2007). Interactions of hydrophobic metal complexes and their constituents with aquatic humic substances. Environmental Chemistry , vol. 4 , nº 5. p. 323. DOI: 10.1071/EN07046.

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Résumé

Abstract. The ability of dissolved humic substances (HS: fulvic and humic acids) to complex cationic metals is well known, but their interactions with neutral lipophilic metal complexes are little understood. In the present study, we have examined the behaviour of two such complexes (Cd L02: L = DDC = diethyldithiocarbamate, or L = XANT = ethylxanthate) in the presence of Suwannee River Humic and Fulvic acids. Interactions between the neutral complexes and the humic substances were assessed by excitation-emission matrix (EEM) fluorescence spectroscopy at pH 5.5 and 7.0, and by equilibrium dialysis experiments (500 Da cut-off). The EEM measurements were carried out by titrating the humic substances (6.5 mg C L–1) with Cd, in the absence or presence of ligand L (1 µM DDC or 100 µM XANT). Given the very high stability constants for the complexation of cadmium by DDC and XANT and the excess ligand concentration, virtually all (>96%) of the Cd added to the L + HS matrix was calculated to be present as the neutral CdL20 complex over the entire pH range tested. For both humic substances, addition of DDC or XANT alone led to shifts in the fluorescence spectra at both pH values, indicating that the DDC and XANT anions likely interact by electrostatic or hydrogen bonding within the humic molecules. The subsequent addition of Cd to these L + HS systems resulted in a disproportionately large enhancement of the fluorescence intensities of individual EEM peaks, this fluorescence enhancement being only slightly decreased by the shift from pH 7.0 to 5.5. We interpret this enhancement as evidence that the two neutral complexes associate with the humic substances, presumably by forming ternary complexes (Ln-Cd-HS). Hydrophobic interactions between the humic substances and the neutral complexes may also contribute, but to a lesser extent, as demonstrated by partitioning calculations based on the lipophilicity of the neutral complexes. The association of the neutral complexes with Suwannee River Humic Acid was confirmed by dialysis experiments.

Type de document: Article
Mots-clés libres: Cd; equilibrium dialysis; fluorescence; lipophilic metal complexes; ternary complexes
Centre: Centre Eau Terre Environnement
Date de dépôt: 08 janv. 2021 19:41
Dernière modification: 08 janv. 2021 19:41
URI: http://espace.inrs.ca/id/eprint/10923

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