AN EXPERIMENTAL INVESTIGATION OF MULTIPLE SULFUR ISOTOPE FRACTIONATIONS DURING HETEROGENEOUS REACTIONS BETWEEN SO2 AND ACTIVATED CARBON

Open Access
Author:
Hamasaki, Hiroshi
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
October 21, 2011
Committee Members:
  • Hiroshi Ohmoto, Thesis Advisor
  • Dr Yumiko Watanabe, Thesis Advisor
Keywords:
  • Sulfur isotopes
  • adsorption
  • activated carbon
  • sulfur dioxide
Abstract:
Recent theoretical and experimental investigations on multiple sulfur isotope fractionations (Lasaga et al., 2008; Watanabe et al., 2009) have suggested the possible importance of reactions between solid organic compounds and oxidized sulfur species in the creation of anomalous isotope fractionation of sulfur (AIF-S) in nature. In order to understand the details of chemical and isotopic fractionation processes involving chemisorption and redox reactions, we have conducted laboratory experiments on reactions between SO2(g) and powdered activated carbon (AC; BET surface area = ~460 m2/g) at 200° and 250 °C in a specially constructed closed system. During each of the three series of experiments, which lasted for up to 480 hours, we monitored the changes with time in pSO2(g) due to adsorption/desorption of SO2(g) on/from the AC, and periodically sampled aliquots of SO2(g) for S isotopic analyses. The SO2-reacted AC was investigated for the chemical compositions using an X-ray photoelectron spectrometer (XPS), an elemental analyzer (EA), and sequential sulfur extraction by H2O, HCl, Cr-solution, and Kiba solution, and the different forms of S-bearing compounds were analyzed for 32S, 33S, and 34S abundance ratios. Results of experiments and analyses indicate that three kinds of S species were continuous incorporated in the AC during reaction with SO2(g) at 200-250 °C: (A) weakly adsorbed SO2 (i.e., SO2(w. ads)) which was in chemical equilibrium with SO2(g); (B) strongly adsorbed SO2 (i.e., SO2(w. ads)), which was degassed at 300-400 °C from the AC at the end of each series of adsorption/desorption experiments; and (C) non-degassable S compounds (i.e., S(NDG)). After reacting with a total of 10.97 mmoles of SO2(g), the AC obtained 0.31 mmoles of SO2(s. ads) and 1.20 mmoles of S(NDG). Approximately 60% of the S(NDG) occur as oxidized-S compounds (i.e., S-O-C compounds) that were extracted by water and recovered as BaSO3 (and/or BaSO4), ~20% as reduced-S compounds (i.e., S-C compounds) that were extracted by Cr-reducing solution, and the remaining ~20% as unidentified (but probably reduced) S-bearing) residual S-compounds that were extracted by Kiba solution. The sequence of reactions among these S-bearing clusters was estimated to be: SO2(g) ⇒ SO2(w. ads) ⇒ SO2(s. ads) ⇒ (S-O-C)AC ⇒ (S-C)AC, representing the trends of increasing S/O and S/C ratios of the AC caused by continuous oxidation of C to CO2. The bonding energy for the SO2(w. ads) was estimated to be ~17 kJ/mol. Large kinetic isotope fractionations of sulfur isotopes occurred during the adsorption/reduction processes. Compared to the δ34S of co-existing SO2(g), the δ34S values of the oxidized S-bearing species increased to +4±1‰ for SO2(w. ads), +7.3±0.2‰ for SO2(s. ads) and +11.6±0.2‰ for S-O-C species, but the reduced-S-C species (i.e., Cr-reducible S, and the residual S) are enriched in the lighter isotopes with the δ 34S values of -8.3±0.2‰. The δ33S - δ34S relationships of the S species incorporated in the AC follow the normal (i.e., mass-dependent) isotopic fractionation, but their Δ33S values are slightly positive (Δ33S = 0 to +0.14‰). These small Δ33S values are consistent with the prediction from the relatively large bonding energy for the SO2(w, ads). The preferential enrichments of heavier isotopes in the adsorbed S-O bearing species, and the preferential enrichments of lighter isotopes in the reduced S species during the reduction of oxidized-S species, agree with the theoretical predictions. Based on comparisons of our results with those of other experimental studies on TSR and the S isotopic characteristics of H2S (and other S-bearing compounds) in petroleum, natural gas, and some ore-forming solutions, we suggest the following: (1) The natural TSR probably occurred by solid C-bearing compounds (e.g., dead bodies of (micro)organisms and kerogen), rather than by gaseous, aqueous, or liquid C-bearing compounds. (2) The small isotopic fractionations between H2S and their source SO42- (i.e., Δ34S = δ34SH2S – δ34SSO4 = -10 to 0‰) in petroleum and natural gas have been previously interpreted as a result of nearly-complete thermochemical reduction of SO42- in closed systems. However, the results of this study suggest that the small fractionations are the results of TSR involving solid C-bearing compounds. The results of our study also suggest that the large AIF-S signatures (Δ33S = -1.37 to +1.84‰) observed for SO42- in some air pollutants (Ding et al., 2006) were probably created by chemisorption isotope effects between coal and the SO2 generated by the burning of pyrite-rich coal. Because the adsorption energy for the surface S-O compounds varies depending on the physical and chemical properties of solid C-bearing compounds, as well as the S-O speciation, some kerogen in Archean sediments, at a particular maturation stage, may have had small adsorption energy to produce AIF-S signatures during TSR.