Migration and transformation regulation of organic and inorganic nitrogen in a western coal mine groundwater reservoir

Based on the occurrence conditions of shallow buried depth,thin bedrock and thick coal seam in western coal mine areas,the contents of organic and inorganic nitrogen (NO- -N,NO- -N or NH+ -N) in mine water are often high. If it is directly discharged without treatment,the ecological environment will be damaged seriously. As one of the water-preserved mining and ecological protection measures,groundwater reservoir technology has been implemented in many ecologically fragile coal mining areas in Western China. However,there are few reports on water quality and safe operation during the water storage in a groundwater reservoir. Column experiments were carried out to study the migra- tion and transformation regulation of organic and inorganic nitrogen in the simulated groundwater reservoir packed with coal gangue in Bulianta Coal Mine,combining with the test of total nitrogen (TN),NO- -N,NO- -N,NH+ -N and other physicochemical parameters. The results showed that the mineralization of organic nitrogen and denitrification of nitrite and nitrate occurred simultaneously in the simulated anoxic water-rock environment at an average seepage flow rate of 0. 51 mL / min during the test duration of 1 016 h (12. 54 pore volume). During 1. 19 -2. 47 PV at the beginning of experiments,the concentration of organic nitrogen in the effluent decreased rapidly while the concentration of ammonia nitrogen increased rapidly,indicating the strengthened ammonification of organic nitrogen at the initial stage of the ex- periment. Thereafter,the mineralization of organic nitrogen got gradually weakened. Thus the content of ammonia nitro- gen in the effluent decreased gradually and tended to be stable. With the ration of organic carbon to nitrogen ( C / N) ranging from 2. 32 to 3. 08 in the studied mine water,the removal efficiencies of nitrate and nitrite were 74% -90%and > 99. 9% individually,suggesting the strong reduction action. Because of the low nitrate content in the input solu- tion,the removal of TN with the efficiency ranging from 57% to 71% was mainly related to the reduction of nitrite. The formation of organic acids with low molecular weight during the degradation of organic matter,such as ammonification of organic nitrogen,induced the increase of H+ in the mine water. Consequently,the pH value in the eluted solution was always lower than that in the input mine water during the column experiment. The findings could provide a theoret- ical support for the effective implementation of groundwater reservoir technology in terms of water quality assurance and safe operation.