Study On Nitrate Removal From Groundwater By Pyrite-based Autotrophic Denitrification

Nitrate which is regarded as a typical ubiquitous nonpoint-source(NPS) contaminant in groundwater, has some negative influence on the application of groundwater for the drinking water supplement. Nowadays, some human activities such as the excessive fertilization and the discharge of the untreated wastewater into the river or soil all made contribution to the elevated nitrate concentration of groundwater. The nitrate contamination has not only developed seriously and widely, but increased the risk for public health. Hence a number of techniques in terms of the nitrate-contaminated groundwater remediation have been developed. At present, the autotrophic denitrification especially the sulfur-based autotrophic denitrification has gradually become a hotspot due to its steady performance, high efficiency and cheap materials utilized in denitrification.In the present study, batch experiments and long-term experiments were both designed with pyrite as the electron donor for nitrate removal from groundwater. In order to determine the alternative for elemental sulfur and the ability of pyrite for autotrophic denitrification, pyrite as well as elemental sulfur was used with shell powders for the batch experiments. In addition, the mechanism of pyrite-based autotrophic denitrification was illustrated by means of the measurement of the effluent, the analysis of the microbial community and the detection of the immediate products. Moreover, the bioreactors filled with pyrite and other fillers(e.g. zeolite and Medical Stone) were designed for the long-term experiments, so the feasibility of pyrite for the remediation of the nitrate-contaminated groundwater could be demonstrated. Several key factors such as the external temperature and the nitrate concentration of the influent were discussed to optimize the performance of denitrification for the bioreactors.The results showed that the nitrate removal was efficient as well as a lower sulfate production and a stable p H in the flasks with pyrite compared with the elemental sulfur. Therefore, pyrite could be used as the electron donor for autotrophic denitrification. Based on the analysis of microbial community and the intermediate products, Sulfurimonas denitrificans could connect with pyrite and participate in the denitrification in vitro, when pyrite was oxidized to sulfate gradually. For the long-term experiments, the bioreactor filled with pyrite, zeolite and Medical Stone showed the best denitrification performance including the highest nitrate removal efficiency of 99% and the lowest production of nitrite and ammonium. Furthermore, when the bioreactor was operated in the environment of 20 ℃, the nitrate removal efficiency declined by 50%. The nitrate removal efficiency also decreased with the increase of the nitrate concentration in influent. For example, just 50% of nitrate was reduced with the production of nitrite when the nitrate was 100 mg-N/L in the influent in Day 52.This study supported the pyrite-based autotrophic denitrification in the remediation techniques for nitrate removal from groundwater theoretically, which could contribute to the development of the techniques in terms of the nitrate-contaminated groundwater.