[Metal logarithmic scale titration as a tool for complexing ligand distribution determination: an application by DPASV]. / Titration en mode logarithmique comme methode de determination d'une distribution de ligands: une application par DPASV.

A new logarithmic scale titration is proposed for the characterisation of natural organic matter-trace metals interactions in natural systems. The Suwannee River Fulvic Acid complexation with Cd and Pb ions has been analysed by this technique, using Differential Pulse Anodic Stripping Voltammetry (DPASV) for labile metal concentrations measurement. Results of titrations have been modelled by four ligands. Their complexing properties (CLi, Ki(Cd), Ki(Pb)) have been determined by a non-linear optimisation based on the speciation program MINEQL, coupled with a simplex. These ligands were called "very weak, weak, strong and very strong" with respect to their complexing constants (Ki(Cd), Ki(Pb)) ranging from 10(4) to 10(11). The obtained ligands concentrations (CLi) are respectively 1.9 microM, 150 nM, 25.1 nM and 21.1 nM for a 7.9 ppm carbon content. Moreover, this model takes account of the pH dependency and metals competition respectively by experiments at pH 7.8 and 4.6, and by definition of stability constants for each ligand toward each analysed metal. As a conclusion, the authors suggest a systematic use of the logarithmic scale titrations when full characterisation of the metal-organic matter interactions is necessary.

Effects of UV-visible irradiation on natural organic matter from the Amazon basin.

The aim of this work was to evaluate the effects of UV-visible irradiation on organic carbon concentration, fluorescence intensity and metal transport ability of the natural organic matter (NOM) at fixed pH values and under several types of atmosphere (air, N(2), O(2)). The water samples were obtained from various sampling sites along the Rio Negro (Amazon basin) and the NOM metal transport ability was determined with regard to Cu(2+). As a first step we measured the concentration of total organic carbon (TOC) and its fluorescence intensity before and after an irradiation for a given time. The results demonstrated that when photodegradation happened, the dominant reactions involved O(2) and the fluorescent sites were among the first to be altered. Then we determined by spectrofluorometry the complexing capacity (C(L)) before and after irradiation. Fluorescence and complexing site densities (C(L)/TOC ratio) increased after a short irradiation time (20 min, i.e. before the photodegradation happened) and decreased after longer irradiation times. Therefore, we conclude that a change of the macromolecular structure activates previously inactive fluorophores and complexing sites prior to photodegradation.

Excitation-emission fluorescence matrix to study pH influence on organic matter fluorescence in the Amazon basin rivers.

A study of the three-dimensional excitation-emission fluorescence matrix (EEFM) of organic matter from the Amazon basin rivers is reported in this paper. The EEFM, applied to the fractions obtained from sequential tangential ultrafiltration (STUF), give spectroscopic informations on the fluorescent properties of particulate (>0.22 microm), colloidal and dissolved (< 5 kD) organic matter. STUF process does not seem to alter the characteristic peaks position of humic material. Two characteristic excitation-emission peaks were observed in all fractions of the samples investigated, one stimulated by UV excitation (peak A) and one by visible excitation (peak C). Increasing the pH, fluorescence intensities of the fluorophores A and C, respectively, I(A) and I(C), responded in the same manner to pH, increasing with increasing pH. However, the fluorophore A seems more sensitive to this pH variation because its fluorescence intensity enhancement is always higher. Thus the chemical behaviour of the two fluorophores must be somewhat differentiated. The evolution of the I(A)/I(C) ratio with pH is studied for all samples. This investigation showed a linear relation between I(A)/I(C) and pH. Moreover, this behaviour is water type (black or white) and molecular size independent. Indeed, this linear relationship is observed for all samples and all fractions.