Mechanism Study on a Combined Denitrification Approach for Nitrate-Contaminated Groundwater Remediation

Nitrate in groundwater has become a worldwide environmental and public health issue. Nitrate can cause methaemoglobinaemia, septicemia, hepatopathy, and even cancers. Currently, single chemical reduction (CR), autotrophic denitrification (AD) and heterotrophic denitrification (HD) have been reported worldwide, however their combined denitrification approaches are rarely studied. In this paper the denitrification capacities, by products, pathways and mechanisms of a combined denitrification approach via batch experiments are explored, which are supported by zero valent iron, methanol and mixed bacteria. The results indicate that the elimination rates of 5.79%, 14.30% and 63.03% were achieved by single zero valent iron-based CR, AD and HD respectively after 5 days, whereas close to 100% was attained by the combined approach. The combined approach is superior to single CR, AD or HD. Nitrite accumulation did not occur in the single CR or AD, but did take place in the single HD. Ammonium variations of <0.6 mg/L were respectively found in CR, AD and HD. Nitrate was reduced to nitrogenous gas by CR, AD and HD. The combined system, in which HD played a dominant role, included three denitrification pathways, those of CR, AD and HD. Anaerobic zero valent iron corrosion provided cathodic hydrogen and ferrous iron for autotrophic denitrifiers. HD provided carbon dioxide for autotrophic denitrifiers. The combined system improved the elimination rate, which was not achieved by single AD and HD system. This combined approach was a potential, feasible and effective approach for groundwater in situ remediation.