Removal of Hydrogen Sulfide by Chlorobium thiosulfatophilum in Immobilized‐Cell and Sulfur‐Settling Free‐Cell Recycle Reactors

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Abstract
Bioconversion of hydrogen sulfide to elementary sulfur by the photosynthetic bacterium Chlorobium thiosulfatophilum was studied in immobilized-cell and sulfur-settling free-cell recycle reactors. The cells immobilized in strontium alginate beads excreted elementary sulfur and accumulated it as crystal in the bead matrices, which made it possible that the reactor broth remained clear and the light penetrated the reactor deeper than with the free cells. In comparison with the free cells, the immobilized cells required 30% less light energy at a H2S removal rate of 2 mM/(L.h) and showed an activity of 2.4 times that of the free cells. However, in 40 h after the reaction the deterioration of the H2S removal efficiency became significant due to the accumulation of sulfur in the beads. The scanning electron micrograph (SEM) and energy-dispersive X-ray spectrometer (EDS) studies showed that the sulfur in the beads existed within a layer of 0.4 mm from the bead surface. In the sulfur-settling free-cell recycle reactor, about 80% of the sulfur excreted by the free cells could be removed in a settler. The 4-L fed batch reactor with the settler improved the light transmission to result in a H2S removal rate of 3 mumol/(mg of protein.h), 50% higher than that without it. The settling recycle reactor was much better in the removal of H2S than the immobilized-cell reactor because the former was a continuous system with the constant removal of sulfur particles by settling and of spent medium by supplying fresh medium at the same rate as the filtering rate of the reactor broth, while the latter was essentially a batch system where toxic metabolites and produced sulfur could not be removed.