Etude de la complexation du cadmium par un acide humique de reference

Abstract

A previous structural investigation of a commercial model of humic acids by Ebenga et al. [ Wat. Res. 20, 1383–1392 (1986)] revealed the presence of phenolic, benzoate, phtalate and salicylate groups close to semiquinonic anion radicals, aromatic units being linked by some alcanoïc chains. These structural features are potential sites of interaction with micropollutants and it is necessary to determine the thermodynamical characteristics of these interactions so as to take them into account in water treatment processes. Cadmium was chosen as a model of micropollutant and Fluka humic acids (FHA) were considered. Complexation phenomenon was first studied by u.v.-visible spectrophotometry and 113Cd NMR. The first failed in the characterization of interaction sites but the second revealed that essentially two kinds of complexes are to be considered (Fig. 1). In order to determine the corresponding stability constants, free metal concentrations were measured by the use of differential pulse anodic stripping voltammetry (DPASV) and uncomplexed metal species were determined with an ion selective electrode (ISE). A preliminary calibration with titrated Cd-solutions revealed that over 100 μgl −1, the DPASV peak corresponding to a medium containing FHA is higher than that observed in a medium without ligand! This observation can surely be matched with the known surfactant character of FHA [Kozarac et al., Wat. Res. 20, 295–300 (1986)], property which is known to disturb the linear relationship between diffusion current intensity and free metal ion concentration. Comparison of DPASV and ISE results shows that over 2 μg Cd mg FHA −1, some complexed species are destroyed during DPASV analytical process (Fig. 2). This comforts the previous NMR hypothesis of different kinds of complexes. Computation of conditional stability constants was then performed taking advantage of the complementarity of DPASV and ISE data. For the first time, Scatchard's method was applied to determine Cd-Fluka humic acids interaction; it fits the best for this kind of investigation since it is a promising method for multisite complexing molecules, already used with benefit by Alberts and Giesy [ Aquatic and Terrestrial Humic Materials (1983)] relatively to isolated natural organic matter. Two kinds of complexation sites were detected, for 1 and 10 Cd ions per FHA molecule, with corresponding stability constants of around 10 7 and 10 5 M −1, respectively. This investigation has thus shown that complexation of Cd by Fluka humic acids leads to very stable structures, from which it is rather difficult to eliminate metallic ions. But fulvic acids are also to be considered since their occurrence in the environment induces the formation of metallic complexes the stability constants of which are smaller [Bhat and Weber, Analytica chim. Acta 141, 95–103 (1982)]. FHA thus seems no longer to be a representative model and one had rather to substitute it by a more convenient model, the determination of which is to be performed. A deeper investigation of the actual structure of the interaction sites and of the complexed form stabilities indeed needs the synthesis of a representative model, considering both the toxicological and the water treatment points of view.