Influence of organic matter in the prediction of iodine migration in natural environment
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Influence of organic matter in the prediction of iodine migration in natural environment Pascal Reiller1 and Valérie Moulin CEA, CE Saclay, Nuclear Energy Division. 1 DPC/SECR/LSRM Bâtiment 391, pièce 40 F-91191 Gif-sur-Yvette Cedex, France ABSTRACT Halogenation of phenolic moieties is one of the most important reactions between humic substances (HS) and iodine or chlorine. These reactions were studied in order to assess the importance of these interactions with reduced (I–) and oxidised (I2) form of iodine. I– was fixed by the addition of Na2S2O3. After separation of HS and I– by ultrafiltration, no significant uptake was evidenced in the time framework of this study. The consumption of I2(aq) by HS was – followed by the decay of I3 absorbance (351 nm). It comes out that (i) when I– is fixed by Na2S2O3, no interaction with HS could be quantified; (ii) I2(aq) does react with HS but the kinetics cannot be linearalised. This study confirms the significance of iodine-organic matter interactions as a governing process for iodine migration in surface environment. More studies are on going to develop predictive models of iodination kinetics. INTRODUCTION Radioisotopes of iodine (125I, 129I, 131I) are important radionuclides in nuclear safety as they represent one of the most significant release following a reactor accident, and for their toxicological impact [1, 2]. Halogens are known to be associated to natural organic matter in natural waters up to 10 % in mass [3-6]. It has been shown that the mechanism of this association is an electrophilic substitution of molecular halogens (X2) catalysed by hypohalogenous acid (HOX) on phenolic compounds [7, 8]. The interaction of iodide with humic substances (HS) was also postulated to interpret an apparent reversibility of the electrophilic substitution catalysed by peroxydase enzymes [9].* The structure of HS can now be seen as a supra-molecular aggregate of phenolic molecules, held together by weak interactions [10-13]. In the case of HS, the mechanisms involved in the iodination reaction are not clear yet, as it involves more than one step [14]. Nevertheless, the formation of iodo derivatives of phenolic structures in HS induces the formation of a covalent bonding between carbon and iodine [15, 16]. The kinetic analysis of the iodination reaction of HS has only been done on a narrow pH range (from 4.75 to 5.75) [17], and with rough approximations about the use of Hammet’s coefficient [18]. This study proposes to assess some key points: the reactivity between I– and HS; the influence of pH on a large pH scale; the influence of the physico-chemical parameters of the solution. EXPERIMENTAL Ultrafiltration Purified Aldrich HA and aquatic FA (Mol, Belgium) are used [13, 19]. 125I was obtained from Amersham (Amersham). Ultrafiltration experiments were conducted with Millipore® Ultrafree-4 centrifugation units with a polyethersulfone membrane of a molecular weight cut-off *
Lactoperoxydase and chloroperoxydase
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of 5 kDa. The membranes were washed thrice with Mil
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