Quantification Of Pyrrhotiye O 2 Consumption By Using Pyrite Oxidation Kinetic Data
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Quantification Of Pyrrhotiye O2 Consumption By Using Pyrite Oxidation Kinetic Data Rojo, I.1*, Clarens, F. 1, de Pablo, J. 1, Domènech, C2, Duro, L. 2, Grivé, M. 2, Arcos, D. 2 1 Fundació CTM Centre Tecnològic, Plaça de la Ciència 2, 08243 Manresa (E) 2 Amphos 21 Consulting, S.L. (E) * Corresponding author: [email protected] ABSTRACT Experiments on the dissolution kinetics of natural pyrrhotite (FeS1-x-) and pyrite (FeS2) under imposed redox conditions to evaluate the oxygen uptake capacity of both minerals were carried out at 25ºC and 1 bar. Experimental data indicate that in both cases, Fe(II) released from dissolution of these Fe-bearing sulphides is kinetically oxidized to Fe(III) to precipitate as Fe(III)oxyhydroxides. While the system is pH controlled by the extent of the sulphide oxidation, Eh is controlled by the redox pair Fe2+/Fe(III)-oxyhydroxides. Pyrrhotite dissolution is faster than that of pyrite but generates less acidity. Consequently, the achieved redox value is more reducing. Experimental data show that the oxidation rates of both minerals (in mol·g-1·s-1) are equivalent under the studied conditions. This fact gives a new opportunity to quantify the reductive buffering capacity of pyrrhotite, for which no kinetic rate law has been still established. INTRODUCTION The maintenance of reducing conditions in and in the vicinity of a repository for high level nuclear waste is one of the main issues of concern in any safety assessment exercise. Mobility of Uranium and other redox sensitive actinides is importantly decreased under reducing conditions. The study of processes contributing to the maintenance of the systems under reducing redox potentials has received special attention and has generated a numerous publications in the open scientific literature, as well as in specifically focused national programmes. It is well accepted that in most deep groundwaters the redox state is governed by electron transfer between Fe(II) and Fe(III) species and/or by sulphate/sulphide reactions. The occurrence of Fe(II) in nature is dominated by minerals such as Fe(II) sulphides, pyrite and pyrrhotite being the most common natural Fe-bearing minerals (Belzile et al. 2004). Due to the difficulty to obtain stable redox potentials from field data, Scott and Morgan (1990) proposed the use of intensive magnitudes to define the redox state of geological systems. They used the concepts of OXidising and ReDucing Capacities (OXC and RDC) defined similarly to magnitudes such as acidity or alkalinity in the case of acid/base systems. The major contribution to the ReDucing Capacity of a system will be given by those species able to react with oxidants and therefore buffer an increase in oxidant concentration.
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Despite the intensive magnitudes presented above can give us an idea of the maximum redox buffering capacity of a system, it is also important to consider the rates at which the redox buffer reacts. Iron sulphides are solid phases whose oxidative dissolution is kinetically controlled. The oxidation of sulphide to
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