Conferenciant: Christopher Tournassat, BRGM Orléans, França

26 d'abril de 2006, Sala d'Actes de l'Institut de Ciencies de la Terra "Jaume Almera", a les 12h.

The sorption of 57Fe(II) onto an Fe-free, mineralogically pure, and Ca- saturated synthetic montmorillonite sample (structural formula: Ca0.15(Al1.4Mg0.6)(Si4)O10(OH,F)2), was studied as a function of pH under strictly anoxic conditions (N2 glove-box atmosphere, O2 content < 1 ppm), using wet chemistry and cryogenic (T = 77 K) 57Fe Mössbauer spectroscopy. No Fe(III) was detected in solution at any pH. However, in pH conditions where Fe(II) is removed from solution, a significant amount of surface-bound Fe(III) was produced, that increased with pH from 0-3% of total Fe in a pre-sorption edge region (i.e. at pH < 7.5 where about 15 % of total Fe is sorbed) to 7% of total Fe when all Fe is sorbed. At low pH where the pre-sorption edge plateau occurs (2 < pH < 7.5), the total sorbed-Fe amount remains constant but, within this sorbed-Fe pool, the Fe(III)/Fe(II) ratio increases with pH, from 0.14 at pH 2 up to 0.74 at pH 7. The pre-sorption edge plateau is interpreted as a sorpt.
In a parallel study, the redox transformations of Callovian-Oxfordian (COX) claystone samples under oxidizing perturbation have been analysed using Mössbauer spectrometry. The samples had different oxidation histories ranging from no oxidation (transport from field to laboratory under liquid N2, experiments carried out in a glove box) to massive air oxidation. Pyrite and Fe(II) carbonates are found to oxidise very slowly or not at all when the samples are brought into contact with the atmosphere. In contrast, iron associated with clay minerals exhibits very fast oxidation kinetics, which is in full agreement with the high redox reactivity of Fe(II) sorbed on synthetic smectite. An important consequence of the present results concerns sorption and transport experiments involving redox-sensitive species in natural clayey systems where the reduced capacity of these systems are probably underestimated both in quantity and efficiency.