engleski

PROSECE as a tool for investigation of Natural Organic Matter geochemical changes

In natural systems, trace metals speciation is greatly influenced by the complexation of dissolved natural organic matter (NOM). Thus, bioavailability and toxicity of these elements are mostly governed by NOM properties. Due to the various origins and modification process of NOM in aqueous media, its chemical structure and reactivity is highly heterogeneous. So, NOM properties with regard to trace metals, proton and major cations have to be correctly determinated to estimate their impact on the environment [1]. There are various techniques for analysing NOM binding sites, as electrochemical or spectroscopic measurements. From this perspective, a new software has been developed, named PROSECE (PRogramme d'Optimisation et de SpEciation Chimique dans l'Environnement), in order to perform characterisation of NOM properties from analytical data. NOM modelling is based on a discrete model, using a set of sites only defined by complexing parameters (concentration, binding and/or acidity constants, ... ). PROSECE is composed of two modules: (1) speciation calculus, (2) complexing parameters optimisation. The first one solves the equilibrium by calculating the chemical speciation of the defined system, using a Newton-Raphson subroutine to minimize deviation from the mass balance equation [2]. The second module allows the optimisation of unknown complexing parameters (concentration, binding and/or acidity constants, ... ) by the way of a modified simplex process which adjusts these parameters to minimize the deviation between measured data (for example, concentrations of labile cadmium measured by voltammetry) and calculated ones (for example, concentrations of free and inorganic cadmium determined by the speciation module). PROSECE has been theoretically compared to different usual mathematical treatments (Ruzic, Chau-Buffle and Scatchard linearisations, non-linear fitting on transformed data) on data resulting from different modes of titration (linear, logarithmic and decade standard additions) for the determination of metal complexing parameters of single and multi-ligand systems measured by stripping voltammetry. Use of PROSECE clearly increases the results accuracy, especially if data are issued from logarithmic additions [3]. This titration mode/data treatment combination is advised for correct characterisation of NOM complexing properties towards trace metals. The use of this program on Ceara Rivers (Rio Coco, Rio Jagaribe, Rio Ceara) allowed determination of geochemical transformations of the NOM properties due to land use. The characterisation of acidity properties of different NOM (model, standard and natural ones) at different dissolved organic (DOC) contents carried out with PROSECE, showed that there is a limit for a correct modelling of NOM acid-base properties which is defined as 0.04 meq of total analysed acidic sites concentration. Above this limit, PROSECE allows modelling of standard and natural organic matters properties by using 6 different acidic sites [4]. References: [1] J. Buffle, Analytical Chemistry, Ellis Horwood, 1988, 692pp. [2] F.M.M. Morel, Principles of aquatic chemistry, Wiley-InterScience, New York, 1983 [3] C. Garnier, I. Pižeta, S. Mounier, J.Y. Bena&iuml ; ; m, M. Branica, Analytica Chimica Acta 505 (2004) 263-275 [4] C. Garnier, S. Mounier, J.Y. Bena&iuml ; ; m, Water Research 38 (2004) 3685-3692

chemical speciation; complexing parameters; PROSECE; NOM; trace metals; proton

nije evidentirano