Nitrification of exchangeable ammonium as a source of nitrate in coal mine rock
摘要
Waste rock from coal mining in the Elk Valley, Canada primarily consists of mudstones and siltstones, and contains two natural nitrogen (N) reservoirs: organic N and exchangeable ammonium in clay minerals (NH4+-ex). Although the mean concentration of organic N is greater than that of NH4+-ex (713 ± 615 vs. 31.0 ± 24 mg N/kg), laboratory experiments show NH4+-ex undergoes rapid (< 50 d) nitrification to nitrate (NO3−) while organic N is nonreactive over the time frame of the experiments (393 days). A conceptual model was developed, based on the results of batch experiments to describe the release of NH4+-ex within individual mine-rock particles and its diffusion to particle surfaces where it is nitrified to NO3−. Oxic experiments with various parent rock particle sizes (< 0.71 to 75 mm), along with N form analyses on aged mine-rock particles (< 2 to 1000 mm; estimated to be 22 years old at the time of sampling in 2022), show NO3− release occurs rapidly (< 1 year) in particles less than several tens of millimetres in diameter. However, larger particles exhibit slower NO3− release due to delayed migration of NH4+ to particle surfaces. The results show nitrification of NH4+-ex can be a major contributor to the NO3− load to surface waters from mine-rock piles and potentially lead to prolonged NO3− release. Such a process may affect aquatic systems associated with other mine-rock piles. Residual ammonium nitrate used as blasting agents provide an additional source of NO3− and NH4+ in the mined rock. Based on the current results, it is not possible to distinguish the relative contributions of NO3− from blasting agents and NH4+-ex.