Biocatalysis - Green Chemistry and Technology - page 202

the first step in the biodegradation of the former class
of compounds (Fig. 9.26) [90]. This process has been
examined systematically as a means for the removal
of organic solvents such as benzene, toluene and
xylenes from aqueous effluent streams [91]; het-
eroaromatic products such as the quinolines, typi-
cally released in coal gasification and fossil fuel
processing activities, also are subject to biodegrada-
tion by this and other pathways [92].
An alternative application for the nitrile-
hydrolysing enzymes of Rhodococcus , other than the
production of acrylamide (Fig. 9.1), lies in the ability
of several strains of Rhodococcus bacteria to hydrolyse
both nitriles and amides to carboxylic acids at very
low substrate concentrations. This makes them ideal
biocatalysts for the removal of materials such as
acrylonitrile and acrylamide from aqueous waste
streams, in addition to their application for the
removal of impurities in the manufacturing pro-
cesses involving these compounds [93].

action—can take place at ambient temperature and
pressure with loss of calorific value of the product
[94]. One of the major sulfur-containing contami-
nants of middle-distillate petroleum fractions is
dibenzothiophene, and a process for its removal by
treatment with a Rhodococcus bacterium (strain IGTS8)
is based on the degradation of dibenzothiophene to 2-
hydroxybiphenyl, as illustrated in Fig. 9.27.
The sulfur atom of the precursor is released as
sulfite ion, which can be converted readily to
sulfate under the conditions of the transformation
and removed as a water-soluble or precipitated salt
[95]. The enzyme system of Rhodococcus also is able
to transform substituted dibenzothiophenes by an
analogous pathway [96,97], and the process has
been developed by Energy BioSystems Corporation
(now Enchira Biotechnology Corporation) as a prac-
tical alternative and supplement to chemical
hydrodesulfurisation of middle-distillate petroleum
[94-96]. Other sulfur-containing contaminants, par-
ticularly the dihydrobenzothiophenes formed during
chemical hydrodesulfurisation, can be oxidised by
other microbial biocatalysts such as Pseudomonas , but
without removal of the sulfur atom [98].

3.3 Biodesulfurisation

The removal of sulfur from fossil fuels, particularly
crude oil, is a priority problem for the energy indus-
try. Conventional desulfurisation involves reaction of
petroleum fractions with an inorganic catalyst and
hydrogen under high temperature and pressure con-
ditions (hydrodesulfurisation or hydrotreating). This
is an energy-consuming process that may involve
some degradation of the fuel value of the product,
whereas biodesulfurisation—a process for the
removal of organically bound sulfur by microbial

4 Conclusions

Although replacement of chemical processes by
biological catalysts usually results in a more sustain-
able process, the costs associated with the use of a
biocatalyst, arising for example from short catalyst
lifetimes and the need to invest in specialised equip-
ment, often can exceed those of the traditional

Fig. 9.27 Conversion of
dibenzothiophene to 2-
hydroxybiphenyl by Rhodococcus
IGTS 8 .