Biomimetic Solubilization of a Low Rank Coal:  Implications for Its Use in Methane Production

There is substantial interest in processes that result in solubilization of low rank coal as a potential carbon source for anaerobic production of methane. It is known that certain fungi are able to solubilize selected low rank coals by secreting oxalate ions which chelate metal ions that are present. Apparently, metal ions form ionic bonds which link individual coal macromolecules. Once these metal ions are chelated and the ionic bonds are broken, the relatively polar coal macromolecules are soluble in water. Much research has focused on the coal-solubilizing ability of wood-rotting fungi. The wood-rotting fungi Trametes versicolor and Phanerochaete chrysosporium are known to be able to solubilize extensively an oxidized North Dakota lignite (leonardite). We have used a biomimetic approach to study oxalate-mediated solubilization of this low rank coal. A concentration of approximately 75 mM sodium oxalate was found to be near optimal for leonardite solubilization when the initial concentration of leonardite was 2 mg/mL. This is of importance because oxalate concentrations in liquid cultures of wood-rotting fungi are typically well below this concentration. Nevertheless, substantial leonardite solubilization was also observed at oxalate concentrations more typical of those found in these fungi. The effect of pH was also studied. It was found that oxalate-mediated leonardite solubilization increased with increasing pH and appeared to be a function of divalent oxalate ion concentration. Similar results were found for dihydrogen phosphate/hydrogen phosphate/phosphate and bicarbonate/carbonate ions. If the solubilized coal macromolecule from leonardite becomes a viable carbon source for use in anaerobic methane production, these studies suggest that chemical solubilization by common inexpensive Lewis bases would likely be more cost competitive than fungal solubilization processes.