The rhizosphere as a bioprocess environment for the bioconversion of hard coal

Fundamental processes involved in the microbial degradation of coal and its derivatives have been well investigated and documented over the past two decades. However, limited
progress in industrial application has been identified as bottleneck in further active development of the field. The sporadic and unanticipated growth of Cynodon dactylon
(Bermuda grass) has been observed on the surface of some coal dumps in the Witbank coal mining area of South Africa. Preliminary investigations showed the formation of a
humic soil-like material from the breakdown of hard coal in the root zone of these plants.
The potential of this system to contribute to industrial scale bioprocessing of hard coal was investigated.
This study involved an investigation of the C. dactylon/coal rhizosphere environment and demonstrated the presence of fungal species with known coal bioconversion capability.
Amongst these Neosartorya fischeri was identified and its activity in coal bioconversion was described for the first time. Cynodon dactylon plant roots were also shown to be
colonized by mycorrhizal fungi including Glomus, Paraglomus and Gigaspora species.
The role of plant photosynthate translocation into the root zone, providing organic carbon supplementation of fungal coal bioconversion was investigated in deep liquid culture with the N. fischeri isolate used as the biocatalyst. Organic acids, sugars and complex organic carbon sources were investigated and it was shown that glutamate provided significant enhancement of bioconversion activity in this system.
The performance of N. fischeri in coal bioconversion was compared with Phanaerochaete chrysosporium and Trametes versicolor, both previously described fungal species in the coal bioconversion application. Fourier transform infrared
spectroscopy indicated more pronounced oxidation and introduction of nitro groups in the matrix of the humic acid product of coal bioconversion in N. fischeri and P.
chrysosporium than for T. versicolor. Macro-elemental analysis of biomass-bound humic acid obtained from the N. fischeri catalyzed reaction showed an increase in the oxygen
and nitrogen components and coupled with a reduction in carbon and hydrogen. Pyrolysis gas chromatography mass spectroscopy further supported the proposal that the
mechanism of bioconversion involves oxygen and nitrogen insertion into the coal structure.
The C. dactylon bituminous hard coal dump environment was simulated in a fixed-bed perfusion column bioreactor in which the contribution of organic supplement by the
plant/mycorrhizal component of the system was investigated. The results enabled the proposal of a descriptive model accounting for the performance of the system in which
the plant/mycorrhizal component introduces organic substances into the root zone. The non-mycorrhizal fungi utilize the organic carbon supplement in its attack on the coal substrate, breaking it down, and releasing plant nutrients and a soil-like substrate which
in turn enables the growth of C. dactylon in this hostile environment.
Based on these results, the Stacked Heap Coal Bioreactor concept was developed as a large-scale industrial bioprocess application based on heap-leach mineral processing technology. Field studies have confirmed that bituminous hard coal can be converted to a humic acid rich substrate in a stacked heap system inoculated with mycorrhizal and N. fischeri cultures and planted with C. dactylon.