In
the biotechnological desulfurization process under haloalkaline
conditions, dihydrogen sulfide (H2S) is removed from sour
gas and oxidized to elemental sulfur (S8) by sulfide-oxidizing
bacteria. Besides S8, the byproducts sulfate (SO42–) and thiosulfate (S2O32–) are formed, which consume caustic and form
a waste stream. The aim of this study was to increase selectivity
toward S8 by a new process line-up for biological gas desulfurization,
applying two bioreactors with different substrate conditions (i.e.,
sulfidic and microaerophilic), instead of one (i.e., microaerophilic).
A 111-day continuous test, mimicking full scale operation, demonstrated
that S8 formation was 96.6% on a molar H2S supply
basis; selectivity for SO42– and S2O32– were 1.4 and 2.0% respectively.
The selectivity for S8 formation in a control experiment
with the conventional 1-bioreactor line-up was 75.6 mol %. At start-up,
the new process line-up immediately achieved lower SO42– and S2O32– formations compared to the 1-bioreactor line-up. When the microbial
community adapted over time, it was observed that SO42– formation further decreased. In addition, chemical
formation of S2O32– was reduced
due to biologically mediated removal of sulfide from the process solution
in the anaerobic bioreactor. The increased selectivity for S8 formation will result in 90% reduction in caustic consumption and
waste stream formation compared to the 1-bioreactor line-up.
Biological
desulfurization under haloalkaliphilic conditions is
a widely applied process, in which haloalkalophilic sulfide-oxidizing
bacteria (SOB) oxidize dissolved sulfide with oxygen as the final
electron acceptor. We show that these SOB can shuttle electrons from
sulfide to an electrode, producing electricity. Reactor solutions
from two different biodesulfurization installations were used, containing
different SOB communities; 0.2 mM sulfide was added to the reactor
solutions with SOB in absence of oxygen, and sulfide was removed from
the solution. Subsequently, the reactor solutions with SOB, and the
centrifuged reactor solutions without SOB, were transferred to an
electrochemical cell, where they were contacted with an anode. Charge
recovery was studied at different anode potentials. At an anode potential
of +0.1 V versus Ag/AgCl, average current densities of 0.48 and 0.24
A/m2 were measured for the two reactor solutions with SOB.
Current was negligible for reactor solutions without SOB. We postulate
that these differences in current are related to differences in microbial
community composition. Potential mechanisms for charge storage in
SOB are proposed. The ability of SOB to shuttle electrons from sulfide
to an electrode offers new opportunities for developing a more sustainable
desulfurization process.
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