Mine water and seepage from waste rock dumps often contain elevated concentrations of nitrogen compounds originating from the use of explosives such as ammonium nitrate fuel oil (ANFO).
A reduction in concentrations of nitrogen compounds from these mine effluents can be achieved using natural biological processes. The nitrogen cycle is well documented and briefly summarized in TABLE 1.
Aerobic Nitrogen Removal
Nitrification is a three-stage process whereby ammonia is converted to nitrate. Each stage is carried out by different bacteria. First, ammonia is oxidized to hydroxylamine in the presence of Nitrosococcus bacteria. Second, the hydroxylamine is oxidized further to nitrous acid and water in the presence of Nitrosomonas bacteria. And third, the nitrous acid is oxidized to nitrate in the presence of Nitrobacter. These processes require oxygen, which is supplied by wetland plants through their root systems hence the importance of wetlands in the remediation of mine sites.
Anaerobic Nitrogen Removal
Dissimilatory Nitrate Reduction and denitrification occur under anaerobic conditions. Nitrate is reduced, by dissimilatory reduction, to nitrite which can be converted to nitrogen oxides or nitrogen gas. The reactions are carried out by a wide variety of bacteria which require organic matter as a source of carbon and energy. Dissimilatory Nitrate Reduction and Denitrification generate alkalinity.
Nitrite may also be reduced to ammonia. This process generates alkalinity and eliminates all the acidity generated by nitrification. Ammonia, when present in the sediment, becomes available for uptake by plants or is reconverted to nitrate under aerobic conditions.
When conditions favor denitrification, nitrogen is lost from the ecosystem as gases (nitrogen and nitrous oxide). Sediments can break down nitrogen at a rate of 1 kg.ha-1.day-1 or 100 mg.L-1 of sediment.day-1 through both denitrification and dissimilatory. Test work carried out by Boojum on high-nitrogen effluents has shown a reduction in NH3 -N from about 100 mg.L-1 to 34 mg.L-1 in 22 days.
Ammonification
This is the process in which organic forms of nitrogen (e.g., nitrogen present in dead plant material compounds) are converted to ammonium (NH4+) by decomposers (heterotrophic bacteria). This process can occur in aerobic or anaerobic conditions. The nitrogen from organisms is thus released to the environment and can either be assimilated (taken up by organisms) or converted to nitrate via nitrification.
Floating cattail covers assist in the management of nitrogen removal from mine effluents. Denitrification, in addition to removing excess nitrogen, generates alkalinity. The root zone of floating cattail covers supports nitrification, dissimilatory nitrate reduction, and denitrification. The release of oxygen by the roots favors the former processes, and the contribution of carbon, by way of organic matter that falls to the underlying sediments, where reducing conditions prevail, enhances the latter.