Lignite degradation and mineralization in lignite-containing mine sediment as revealed by 14C activity measurements and molecular analysis

The influence of sediment pH and redox conditions on the oxidation of lignite and sediment organic carbon was studied using a series of laboratory microcosms. The experiment was conducted over 450 d in reducing sediment suspensions maintained at four redox potentials (−170, 0, +350 and +500 mV) and two pH values (3.2 and 5.3). Carbon and lignite mineralization were determined over time using 14C analysis, DOC carbon production, solid state 13C NMR (nuclear magnetic resonance spectroscopy) and pyrolysis–gas chromatography/mass spectrometry (GC/MS). More than 50% of measured carbon mineralization occurred during the first 200 d. Maximum mineralization occurred under the more oxidizing conditions. The amount of CO2 produced was greater at pH 5.3 than at pH 3.2, indicating that low pH reduces organic matter mineralization regardless of sediment redox potential. The 14C analysis showed that lignite carbon was mineralized at each redox potential level. Under the lowest redox level (−170 mV) mineralization was minimal as compared to the other redox levels. Pyrolysis–GC/MS showed that the dissolved aromatic fraction accumulated in solution. Solid state 13C NMR spectroscopy analysis showed that lignite, although generally recalcitrant in nature, was structurally diverse, including containing easily decomposable components. The 14C measurements of plant material sampled in the field provided evidence that lignite carbon mineralization occurs under field conditions. From the data, it can be concluded that the lignite in mine sediments is susceptible to oxidation or degradation and therefore must be considered at least as a semi-reactive compartment in the sediment carbon cycle.