Complexation capacity of freshwater from Saguenay river, Canada
The Saguenay River is a major affluent of the St. Lawrence River, Quebec, Canada. The Saguenay River which drains a heavily industrialized region can be subdivided into two sections: the upper section is rather shallow and contains freshwater as the lower one is a deep fjord characterized by a thermohalocline at about 25 m. This work aimed at identifying the possible modifications brought up by anthropogenic sources upon the complexation capacity of the freshwater of this River. Five different stations were sampled for surface water the same day on the upper section of the River. The samples were filtered on 0,4 μm membrane (pre-cleaned with HNO3). A portion was analyzed and other ones were fractionnated as a function of the nominal molecular mass (NMM) of dissolved ligands by using in series four gel permeation chromatographic (GPC) columns filled with Sephadex G-10, G-15, G-25 and G-50 respectively, the elution being dope by purified 18MOhms water. The complexation capacity (CC) and critical stability constant (CSC) of the different fractions have been characterized using a method based on free Cu2+ back-titration by Differential Pulsed Anodic Stripping Voltammetry (DPASV) and a 1:1 complexation scheme. Because copper was giving two unresolved peaks on the tailing of the oxygen peak, all polarograms have been deconvolved by a PASCAL computer program based on a least-sqares nonlinear fit using the Taylor differential correction technique. All results compiled were from the peak centered at - 60 mV against an Ag/AgCl reference. By manipulating the usual equations to determine CC and CSC with the free Cu2+ back-titration, we were able to calculate CC by three different routes and CSC by two different routes ; when enough reliable data were available for each route, all values obtained were concordant. So we observed that, going downstream, the CC decreased from 0,32 to 0,14 μM for whole samples. At this point, we cannot identity the cause of this decrease wether it is due to simple dilution or by addition of new dissolved metallic ions into the stream. Once fractionnated, CC measured was seen increasing with NMM but normalized CC per unit of carbon has been found to be greater for ligands with small NMM (normalized CC decreased with increasing NMM). The CSC obtained were all similar, about 5 x 107 L mol-1, excepted for ligands with NMM between 700 and 1 800 g mol-1, the CSC being 27 x 107 L mol-1 from the inverse linearized method.