Nitrit in Versatzprodukten

This present Master's thesis focuses on identifying the origin of elevated nitrite concentrations in the eluates of the backfill products from the Webing mine and on deriving measures that can reduce or control these nitrite concentrations in order to comply with the guideline value agreed with the mining authority. The backfill produced in the mine consists of a mixture of biomass combustion ashes, cement, and mine water. By sampling and analyses of the ashes used, it was possible to identify the ashes as the source of the nitrite concentrations, although the concentrations vary widely. Total nitrogen analyses in both the eluate and solids of the ashes show that the nitrogen is not completely leached from the solids and that the nitrogen present in the eluate is not exclusively in the form of nitrite. Loss on ignition tests on two ashes indicate a relationship between incomplete combustion and increased nitrite content. In experiments aimed at reducing the nitrite concentration in the backfill, ferrous sulfate heptahydrate (FeSO4*7H2O), which is used in cement as a ¿chromate reducer¿, was employed. According to the electrochemical series, it is capable of reducing the nitrite. Increasing the cement content in the backfill led only to a slight decrease in nitrite concentration. However, the targeted addition of a calculated amount of FeSO¿*7H¿O directly to the ashes before elution resulted in partial, and in some cases in complete reduction of nitrite. The only partially effective reduction is attributed to competing ions present in solution since the reducing agent does not react selectively. In addition, zero-valent iron was tested as an alternative reducing agent, but in this case, it did not show sufficient effectiveness. It is assumed that the zero-valent iron had at least partially undergone oxidation. In final laboratory experiments, the complete effectiveness of FeSO4*7H2O as a reducing agent for nitrite in the eluate was clearly demonstrated through stoichiometrically excess dosing (based on the dissolved nitrite content).