| To reduce the detrimental impact of nitrogen oxide (NOx) emissions on ambient air quality and human health, NOx emissions from small sources must be treated in a cost-effective manner. Development of a fungal vapor phase bioreactor system for NOx removal may provide an effective alternative technology. To better understand the response of fungi to NO, a series of batch experiments were conducted to elucidate the effect of nitric oxide (NO), nitrite (NO 2-), nitrate (NO3-), and ammonium (NH4+) on fungal growth in the presence of NO. Batch results indicate that Exophiala lecanii-corni can utilize a certain level of NO as an external nitrogen source for growth under aerobic conditions even though high NO concentrations inhibit fungal metabolism. In addition, the fungus oxidizes nitrite species ([NO2 -]+[HNO2]) to nitrate under acidic conditions perhaps as a neutralizing response to protect itself from toxic nitrous acid species. NH4+ concentration was found to have a significant influence on fungal growth under acidic conditions; the inhibitory effect of NO was alleviated at low NH4+ levels, and enhanced at high NH4+ levels. Based on the batch study results, a fungal vapor-phase bioreactor system was developed for simultaneous removal of NOx and toluene. Average NH4+ levels in the bioreactors were maintained at less than 100 mug NH4+-N/g dry pellet using the periodic aerosol transfer method. Greater than 99% removal of a 45 g/m3/hr toluene loading rate was maintained at a 30 sec empty bed contact time. Simultaneously, a maximum elimination capacity of 31 g NOx/m3/hr was achieved at the same contact time, a 89% improvement over the highest NOx removal reported in previous studies. At a 14.9 g NOx/m3/hr loading rate, 69 to 85% NOx removal efficiencies were maintained over 200 hours of extended operation when the bioreactor was operated in a 12 hr on - 12 hr off mode. In addition, exposure to NOx increased the respiration rate of the fungi and lowered the pressure drop in the fungal bioreactors. This study has elucidated the response of fungi such as E. lecanii-corni to NO, and has laid the foundation for biological NOx treatment. |
