28th International Meeting on Organic Geochemistry

17 – 22 September 2017, Florence, Italy

REAL-TIME OBSERVATION OF SULFUR SPECIATION AND PARTITIONING

BETWEEN AQUEOUS AND HYDROCARBON PHASES DURING TSR EXPERIMENTS

R. Michels1, G. Barré1, L. Truche2, V. Vitzthum3, R. Bounaceur3, C. Lorgeoux1

1GeoRessources CNRS-UMR 7359, Université de Lorraine, France 2ISTerre-UMR 5275, France 3LRGP CNRS-UMR 7274, ENSIC, Université de Lorraine, France Introduction In organic geochemistry, the vast majority of thermal reactions experiments are conducted in batch reactor conditions. In some cases, reactants may be injected or products retrieved during the experiments. Yet most commonly, reaction products are analysed in post experiment conditions (after dismantlement of the reactor or retrieval during the run). Mass balance is therefore calculated accordingly to sets of experiments at various degrees of transformation ratios, while end products of reaction are characterized at each step. Usually, reaction pathways are proposed as to conceal reactants, mass balance and nature of reaction products. Yet, intermediate reactants appearing during the run of the experiment disappear once temperature is set back to ambient. Their nature, abundance and partitioning between phases may change with experimental conditions. Overlooking them may have consequences on our understanding on reaction mechanisms and hence alter the quality of modelling. Also, rarely do experiments account for phase state changes or chemical species partitioning during runs. It is only when specific formalisms can be used that intermediate reaction species can be proposed and reaction networks tested by simulation. It is for instance the case when radical chemistry formalism is applied (REF). The radical chemistry formalism illustrates well the progress in chemical modelling that can be achieved when intermediate reactional species are considered. The formalism explicitly takes into account radical reaction species which have extremely short life time and can therefore not be easily observed during experiments. Yet, radicals have been identified and described in specific conditions, then introduced within the radical chemistry formalism as true reaction intermediates. The derived computer models allow then to relate reactants to products with details on the nature, distribution, quantity of compounds as well as thermodynamic or kinetic parameters. The recent description of the S3

- radical ion and its role in the Thermochemical Sulfate Reduction (TSR, the reduction of sulfate into sulphides by reaction with hydrocarbons; Orr, 1974; Goldstein and Aizenshtat, 1994; Worden et al., 1995; Amrani, 2014) reactions is another example of the importance to consider intermediate reaction species (Pokrovski and Dubessy, 2015; Truche et al., 2014). TSR is a reaction that involves the transfer of 8 electrons to reduce sulfates (+6) into sulfides (-2), that implied necessary intermediate valence sulfur species to occur. S3

- is the major intermediate sulfur species in this process due to its stability and its high reactivity. But it is a species only stable at temperature >100°C, so it is necessary to studying it at relevant temperature. A key aspect in the identification of intermediate reaction species at temperature is the possibility to monitor them in-situ the reaction medium. In case of TSR, at least two if not three reaction media are to be monitored: the water phase, the hydrocarbons phase and if present the gas phase. In our work, we are presenting results of the in-situ monitoring of the fate of sulfur species within each phase.

28th International Meeting on Organic Geochemistry

17 – 22 September 2017, Florence, Italy

We loaded a two phase solution (hydrocarbons and an aqueous phase) in fused silica capillary capsule (FSCC) purchased from Polymicro Technologies, LLC. It consists of round cross section silica-fused capillary tubing of 360 µm external and 100 µm internal diameters. These capillary reactors enables in situ measurements up to ~500°C and ~2 kbar (Chou et al., 2008; Truche et al., 2014; Pokrovski and Dubessy, 2015). After sealing at both ends with a micro-torch, the cell was heated step-by-step up to 300°C on a heating-stage (®CAP-500 Linkam) which ensure a very good thermal stability (± 1°C) and negligible temperature gradients (<1°C over the cell length of 10-15 mm). This technique allows performing both Raman and Fourier Transform Infrared spectroscopy (FTIR) measurements independently on each phase in presence. Reaction species as well as end-products were identified and quantified. Additional post-run experiments were performed as to complete the chemical information. To our knowledge, our experimental and in-situ monitoring designs provide a very unique insight a three phase reaction medium in which TSR is taking place. Important aspects of our results concern 1) which reactive species are important to consider in each of the phases 2) which species may partition between the phases 3) which formalism should be applied to account for the reaction This latter point is crucial since we are dealing with a dual solvent system i.e. water vs hydrocarbons. Usually the thermodynamic formalism is being used to describe reactions in the water phase (REF), while radical reaction formalism is applied to the hydrocarbon phase (Nguyen et al., 2013, 2014). In such context, how should the radical ion S3- be considered? How can we conceal both formalisms to describe the complete reactional system and eventually access kinetic modelling?

Figure 1 View of a fused silica capillary reactor showing aqueous, octane and gas phases. At experimental temperatures the transparent reactor allows determination of phase state and proportions as well as to apply Raman and FTIR micro-spectroscopy for in-situ chemical species quantitation. References Amrani A. (2014). Annu. Rev. Earth Planet. Sci. 2014. 42, 733-68 Chou I-M., Song Y., Burruss R.C. (2008). Geochim. Cosmochim. Acta 72, 5217-5231 Goldstein TP, Aizenshtat Z. (1994). J. Therm. Anal. 42, 241-290. Nguyen V.P., Burkle-Vitzthum V., Marquaire P.M., Michels R. (2013). Jour. An. Appl. Pyr., 103, 307–319 Nguyen V.P., Burkle-Vitzthum V., Marquaire P.M., Michels R. (2014). J. An. and Appl. Pyr. 113, 46-56 Orr, W.L., 1974. AAPG Bulletin 58, 2295-2318. Pokrovski G., Dubessy, J. (2015). Earth Planet. Sci. Lett. 411, 298-309 Truche L., Bazarkina E.F., Barré G., Thomassot E., Berger G., Dubessy J., Robert P. (2014).. Earth Planet. Sci. Lett. 396, 190-200 Worden, R. H., P. C. Smalley, and N. H. Oxtoby, 1995, AAPG Bulletin, v. 79, p. 854–863.