Environmental Engineering Reference
In-Depth Information
substituents, and with substituents farther from the S atom, are generally desulfurized more
slowly [10]. However, these enzymes have little activity toward the smaller thiophenes and
benzothiophenes, with the result that new catalysts must be developed for efficient
desulfurization of gasoline. Development of catalysts with broad specificities, especially ones
that can accommodate compounds with sterically obstructed S atoms, will be essential for
commercial applications [1, 7].
2.2.4. Regulation
. The oxidized sulfur, released as sulfite, is used as a nutrient by the cell,
and indeed the cell will not carry out desulfurization unless it is experiencing sulfur
deprivation (2). The nutritional needs of the cells thus impose an additional limit on the rate
of desulfurization. The HBP, all carbon atoms intact, then leaves the cell by an unknown
mechanism to rejoin the nonaqueous phase (1). methionine (2) by means of a promoter
responsive to sulfur-containing amino acids (1). DszD, in contrast, is encoded
chromosomally. The genes for these enzymes from a variety of organisms have been cloned,
sequenced, and engineered [8].
The specificity, potential to proceed nearly to completion without loss of valuable carbon,
and low capital and operating costs in comparison to HDS are highly attractive features of
microbial desulfurization [7], while the rate at which whole microbial cells can remove sulfur
remains the greatest challenge to commercialization [8]. The throughput of substrates in this
pathway may be hindered at several steps, including substrate acquisition, the supply of
reducing equivalents, and enzyme turnover rates for specific substrates (Figure 22) [8].
Figure 22. Conceptual diagram of biodesulfurization in R. erythropus [1].
