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Abstract: The computations of metal flux in aquatic systems, at consuming interfaces like oganism membranes are of major importance in ecotoxicology and dynamic risk assessment. In this paper, the flux of Pb(II), at a planar consuming interface in natural waters, is studied. The system includes (a) simple ligands (OH−, ); (b) fulvics and (c) aggregates, as complexants, i.e. those which may play the major roles in controlling the metal flux in aquatic media. The effects of various physico-chemical factors, in particular, the diffusion layer thickness, the stability constants of fulvic and aggregate complexes, the complexing site distribution of fulvics and the size distribution of aggregates, are studied in details. [Copyright &y& Elsevier]

Abstract: The computations of metal flux in aquatic systems, at consuming interfaces like microorganism surfaces are of major importance in ecotoxicology and dynamic risk assessment. In this paper, the flux of Zn(II) and Ni(II), at a planar consuming interface in a typical natural freshwater, are studied. The system includes (a) simple ligands (OH−, ); (b) fulvics; (c) aggregates, as complexants, i.e., those which play the major roles in controlling the metal distribution and/or metal flux in aquatic media. The above two metals are chosen because they participate, respectively, to intermediate and very slow chemical reactions with complexing sites, and are thus complementary to Pb(II) and Cu(II) (two metals with very fast reactions) studied in Parts III–V of this series. The effects of the various physico-chemical factors, in particular, the diffusion layer thickness, the stability constants and complexing site distribution of fulvics and the size distribution of aggregates, are studied in details. The contribution to the flux, of each complex type, is computed. This paper also compares the dynamic behaviour of Pb(II), Zn(II) and Ni(II) as well as the labilities and flux contributions of their various complexes at planar and microspherical interfaces. This enables to make predictions on biouptake by microorganisms. [Copyright &y& Elsevier]

Abstract: In environmental processes, the contribution of macromolecular or colloidal metal complexes to the overall diffusion flux of metal may be of major importance. For labile complexes, the computation of such flux is based on the concept of average diffusion coefficient, which has only been checked systematically with small size ligands and in aqueous solution. This concept is checked here, both in aqueous solution and in agarose hydrogel, with large size macromolecules: dextrans of molar masses 10000, 40000 and 464000, modified to introduce fluorescent or complexing (aspartate) moieties in their structure have been used. Their diffusion properties in water and agarose gel are determined by fluorescence correlation spectroscopy (FCS). The complexing and diffusive properties of their Pb(II) complexes in water and agarose hydrogel are also determined by scanning stripping chronopotentiometry (SSCP). It is shown that both in water and gel, the concept of average diffusion coefficient is applicable to compute the flux of these labile complexes at consuming interfaces. The results also show that the three ligands and their Pb-complexes diffuse as spherical coils, in water and in the gel, in accordance with other recent results. The implications of these results, in environmental applications, in particular in biouptake, and for the development of in situ sensors for environmental monitoring, are discussed, as well as the optimum conditions of voltammetric measurements with gel integrated microelectrode (GIME) in natural media containing colloidal complexes. In particular it is shown that, provided the gel is correctly preequilibrated with the test solution, and its thickness is larger than 150μm, diffusion/reaction in the test solution do not influence the voltammetric flux at microelectrodes in the gel. The same conclusion is applicable to microorganisms inside biofilms. [Copyright &y& Elsevier]

In order to predict metal bioavailability by microorganisms in natural waters, analytical speciation techniques such as the permeation liquid membrane (PLM) are required. A planar sheet PLM has been characterized by measuring Pb fluxes in the absence and presence of tiron and nitrilotriacetic, iminodiacetic, malonic, citric, polyacrylic and fulvic (Suwannee River fulvic, SRFA) acids. Important parameters such as the diffusion coefficient in the membrane phase and the effective distribution coefficient between the solution and membrane were evaluated in order to determine limiting conditions for the overall transport flux through the membrane. Subsequently, the PLM was tested for its ability to predict bioavailability by the freshwater alga, Chlorella kesslerii by comparing Pb PLM fluxes (JPLM) to Pb biouptake fluxes (Jint) in the absence and presence of the synthetic ligands and SRFA. The capability of the PLM to mimic transport across biological membranes was demonstrated, in particular, from the similarity between the accumulated (PLM, algal) Pb vs. time plots under the different conditions. Under membrane transport limiting conditions, fluxes across both the PLM and biological membranes were proportional to the free metal ion and directly correlated to each other in the zone below saturation of the biological metal uptake sites. The correlation between the different fluxes may be used to predict Pb uptake by C. kesslerii in the presence of the synthetic ligands. However, in the presence of SRFA, the observed Jint was much higher than predicted by results obtained either in the absence or presence of the synthetic ligands. [Copyright &y& Elsevier]

Abstract: Monte Carlo simulations, experimental titrations and fluorescence correlation spectroscopy experiments were used to investigate the conformational and electrical properties of polyacrylic acids (PAA). On the one hand, titration curves were calculated to get an insight into the role of pH on the degree of ionization and conformation of PAA chains. On the other hand, experimental potentiometric titrations of PAA were also achieved for different PAA molecular weights and compared to the calculated titration curves obtained by Monte Carlo coarse grained simulations. It was found that for a large range at intermediate PAA ionizations, a good correlation is obtained between experimental and simulations data thanks to the prominence of electrostatic interactions in this domain. The effect of ionic concentration and PAA molecular weight on the titration curves was also investigated. In order to get a better understanding of PAA conformational behavior, we also investigated PAA diffusion properties in aqueous solutions as a function of pH and ionic strength by fluorescence correlation spectroscopy (FCS), thanks to its high sensitivity to measure diffusion coefficients of tracer solutes. Good qualitative agreements were observed between experimental diffusivities and polymer properties calculated from MC simulations. It was shown that the high molecular weight PAA chains display more significant changes in diffusivity in agreement with the ionization degrees and conformational changes observed in the simulations. [Copyright &y& Elsevier]