suspected to achieve biofractionation (Molokwane and Chirwa 2007). In the latter
study, Molokwane and Chirwa observed with a modest degree of certainty that
microbial cells previously isolated from a high radiation-exposed facility accumulated C-14 while growing on a C-14/C-12 carbon matrix from powdered nuclear
graphite. The experiment was conducted in a closed loop chemostat system equipped
with biofilters for collection of suspended matter for analysis. The observed metabolic activity in the cells indicated that the process was possible under very low
dissolved oxygen suggesting that microorganisms preferred inorganic forms of
carbon as the primary carbon source. Bacteria that utilize inorganic carbon sources
such as CO 2 and HCO 3
- as primary carbon sources – known as autotrophic organisms – favor anaerobic conditions for growth. However, in this preliminary study,
the amount of C-14 remaining in solution was not measured which could be required
to draw a mass balance on C-14 in the system. These preliminary results on C-12/C14 bioseparation hold promise for development of the decontamination and reuse
process for graphite in model HTGR nuclear reactors that produce large amounts of
radioactive nuclear graphite from expired fuel containment (pebbles). Success in the
above process is also important for the decontamination and recovery of nuclear
graphite decommissioned plants for reuse in new reactors.
9 Conclusion
Since the first Cr(VI)-reducing bacteria were isolated in the 1970s and U(VI)reducing bacteria were isolated in the late 1980s, a lot of progress has been made
in isolating and developing higher performing cultures adapted to various environments. New research using genetic tools has yielded new cultures and new understanding of the metal removal processes both at the molecular level [through genetic
studies] and at culture community level [through genomics and proteomics]. Pure
and mixed cultures of bacteria have been applied successfully in treating industrial
effluents containing Cr(VI), U(VI), As(III), and many other metals. However,
application of biological systems in the remediation of contaminated environments
still faces a challenge. Although culture performance under natural conditions has
been evaluated using laboratory microcosms, more research is still required to
elucidate the fate and possibility of recovery of artificial microbial barriers. The
question of the fate of reduced species and what to do about the foreseeable blockage
by metal hydroxides, metal sulfides, and other complexing agents remains unanswered. In order for the in situ bioremediation technology to work for Cr(VI), U(VI),
Se(VI), Tc(VII), and other toxic heavy metals, a solution must be found for feasible
recovery of the barrier zones involving remobilization of reduced Cr species. So far,
in the current studies, this was achieved by applying an electric potential across a
section of contaminated soil. More work is required in the above process to prevent
localized precipitation of metals around the electrodes which eventually stopped the
migration of metals from the aquatic phase.
44
E. M. Nkhalambayausi-Chirwa et al.
study, Molokwane and Chirwa observed with a modest degree of certainty that
microbial cells previously isolated from a high radiation-exposed facility accumulated C-14 while growing on a C-14/C-12 carbon matrix from powdered nuclear
graphite. The experiment was conducted in a closed loop chemostat system equipped
with biofilters for collection of suspended matter for analysis. The observed metabolic activity in the cells indicated that the process was possible under very low
dissolved oxygen suggesting that microorganisms preferred inorganic forms of
carbon as the primary carbon source. Bacteria that utilize inorganic carbon sources
such as CO 2 and HCO 3
- as primary carbon sources – known as autotrophic organisms – favor anaerobic conditions for growth. However, in this preliminary study,
the amount of C-14 remaining in solution was not measured which could be required
to draw a mass balance on C-14 in the system. These preliminary results on C-12/C14 bioseparation hold promise for development of the decontamination and reuse
process for graphite in model HTGR nuclear reactors that produce large amounts of
radioactive nuclear graphite from expired fuel containment (pebbles). Success in the
above process is also important for the decontamination and recovery of nuclear
graphite decommissioned plants for reuse in new reactors.
9 Conclusion
Since the first Cr(VI)-reducing bacteria were isolated in the 1970s and U(VI)reducing bacteria were isolated in the late 1980s, a lot of progress has been made
in isolating and developing higher performing cultures adapted to various environments. New research using genetic tools has yielded new cultures and new understanding of the metal removal processes both at the molecular level [through genetic
studies] and at culture community level [through genomics and proteomics]. Pure
and mixed cultures of bacteria have been applied successfully in treating industrial
effluents containing Cr(VI), U(VI), As(III), and many other metals. However,
application of biological systems in the remediation of contaminated environments
still faces a challenge. Although culture performance under natural conditions has
been evaluated using laboratory microcosms, more research is still required to
elucidate the fate and possibility of recovery of artificial microbial barriers. The
question of the fate of reduced species and what to do about the foreseeable blockage
by metal hydroxides, metal sulfides, and other complexing agents remains unanswered. In order for the in situ bioremediation technology to work for Cr(VI), U(VI),
Se(VI), Tc(VII), and other toxic heavy metals, a solution must be found for feasible
recovery of the barrier zones involving remobilization of reduced Cr species. So far,
in the current studies, this was achieved by applying an electric potential across a
section of contaminated soil. More work is required in the above process to prevent
localized precipitation of metals around the electrodes which eventually stopped the
migration of metals from the aquatic phase.
44
E. M. Nkhalambayausi-Chirwa et al.