Les métaux échantillonnés par la DGT sont-ils représentatifs de la fraction biodisponible ?

International audienceBioavailability of trace metals for aquatic organisms in the dissolved compartment of water is usually interpreted with the Free Ion Activity Model (FIAM) and its extension, the Biotic Ligand Model (BLM). Under fame generally hypothesis, the FIAM states that the biological response to a metal exposure is proportional to the activity of the free-ion {Mz+} in solution. The BLM generalizes this concept and includes competition between metal and major cations (Ca2+, Mg2+, Na+, K+ and H+) on specific binding sites at the surface of the biological membrane. As a speciation device, DGT samples labile metal concentration: free ion plus a fraction of organic metal complexes. It is claimed to estimate bioavailable fractions of metals. The aim of this communication is to synthesize the results of some recent work comparing biological response and labile fraction sampled by DGT in order to evaluate the ability of DGT to assess bioavailability. We investigated two types of biological responses: acute toxicity and bioaccumulation. In the first part, DGT were used to measure metals inducing lethality on daphnia magna during acute toxicity tests. Various media were studied: synthetic organic ligands, humic acids, and natural organic matter (algae and river extracts, wastewater), with two types of gels (classical open pores and restrictive pores). Copper and cadmium were studied. Measurements by DGT equipped with restricted gels appear to be in good agreement with inorganic fraction of metal, even if labile fraction could overestimate bioavailable fraction. In the second part, the accumulation of copper by aquatic mosses at environmentally relevant concentrations (1-5 µg.L-1) and with various organic ligands is compared to DGT-labile restricted gels measurements. In this case, DGT tends to underestimate the bioavailable fraction. This suggests that not only inorganic copper but also some weak organic complexes are available for aquatic mosses at realistic metal concentrations. Finally, we discuss the role of major cations in the bioavailability of copper, in the cases of acute toxicity and bioaccumulation at realistic environmental concentrations. Ca2+ and Mg2+ compete indeed with copper and play a protective role, which would not be highlighted by the sampling with DGT

Caractérisation de la matière organique dissoute des rejets d'une importante station d'épuration et influence sur la biodisponibilité du cuivre

National audienceLa matière organique dissoute [MaO) joue un rôle important dans la spéciation et la biodisponibilité des métaux traces. Cependant aucune information n'est actuellement disponible quant à l'influence des Mao issues du rejet de station d'épuration sur la biodisponibilité du cuivre. Une procédure d'extraction et de fractionnement de la MOD sur résines [DAX-8 et XAD-41 a été mise en place afin d'extraire la MaO urbaine dans son intégralité avec un minimum de perte et d'altération. Trois fractions: hydrophobe (HPa], transphiLique (TPHI et hydrophile lHPIl représentant respectivement 34 %, 20 % et 46 % du carbone ont ainsi été isolées d'une eau de sortie de station d'épuration [STEP]. Cette distribution montre l'importance en milieu anthropisé de la fraction HPI souvent négligée de par sa difficile extraction. Différentes techniques analytiques (élémentaires, spectroscopiques: UV, IR] ont permis de caractériser ces fractions mettant ainsi en évidence: une MaO urbaine peu humifiée, des fractions HPO et TPH de nature chimique proche et une fraction HPI différente, notamment de par sa très faible aromaticité. La biodisponibilité du cuivre en présence de chacune de ces trois fractions de MaO a été évaluée à l'aide d'un biotest de toxicité aigüe sur Daphnia magna. Les résultats montrent qu'à l'instar des substances humiques, la MaO issue de rejets traités de STEP, bien que moins humifiée, conserve un pouvoir limitant la toxicité du cuivre

Characterisation of dissolved organic matter in Parisian urban aquatic systems: Predominance of hydrophilic and proteinaceous structures

Understanding the nature of organic matter is a necessary first step in assessing contaminant bioavailability and allowing water supply managers to optimise the treatment train in the aim of providing safe and inexpensive drinking water. This study provides further insight into the composition, structure and functional groups of dissolved organic matter (DOM) (both hydrophobic and hydrophilic) from urban aquatic systems by means of various analytical techniques (DAX-8/XAD-4 fractionation, elemental analysis, UV and FTIR spectroscopies, 13C and 15N isotopic analysis, size exclusion chromatography and Pyrolysis-GC-MS). The analytical range chosen for this study constitutes a powerful tool in the characterisation of DOM in urban water. The inclusion of information from one technique to the next might not only serve as a support to each one, but also as a complement. The DOM fraction from treated effluent and, more generally, DOM from urban water (i. e. receiving treated effluent) display a strong hydrophilic characteristic [i. e. low humic substance (HS) content, low SUVA], along with a high distribution in molecular weights observed by SEC and low average molecular weight. Due to the origin of this DOM, proteinaceous structures constitute the main compounds, as observed by FTIR and Py-GC-MS. Such characteristics (i. e. heterogeneity, low average molecular weight and diverse functional groups, which make up a total of N) could explain that DOM from treated effluent displayed a strong reactive potential metals pollutants as previously demonstrated