Particulate Pyrite Autotrophic Denitrification (PPAD) for Remediation of Nitrate-contaminated Groundwater

Abstract:

The rapid movement of human civilization towards urbanization, industrialization, and increased agricultural activities has introduced a large amount of nitrate into groundwater. Nitrate is a toxic substance discharged from groundwater to rivers and leads to decreased dissolved oxygen and eutrophication. For this experiment, an electron donor is needed to convert nitrate into non-toxic nitrogen gas. Pyrite is one of the most abundant minerals in the earth’s crust making it an ideal candidate as an electron donor. The overall goal of this research was to investigate the potential for pyrite to be utilized as an electron donor for autotrophic denitrification of nitrate-contaminated groundwater. Batch studies of particulate pyrite autotrophic denitrification (PPAD) of synthetic groundwater (100 mg NO₃^--N L^-1) were set up with varying biomass concentration, pyrite dose, and pyrite particle size. Reactors were seeded with mixed liquor volatile suspended solids (VSS) from a biological nitrogen removal wastewater treatment facility. PPAD using small pyrite particles (<0.45mm) resulted in a favorable nitrate removal. The nitrate removal rate increased from 0.26 to 0.34 mg L^-1h^-1 and then to 0.86 mg L^-1h^-1, approaching that of the sulfur oxidizing denitrification (SOD) rate of 1.19 mg L^-1h^-1. Based on Box-Behnken design (BBD) and response surface methodology (RSM), the optimal amount of biomass concentration, pyrite dose, and pyrite particle size were 1,250 mg VSS L^-1, 125 g L^-1, and 0.815-1.015 mm, respectively. PPAD exhibited substantial nitrate removal rate, lower sulfate accumulation (5.46 mg SO₄^2-/mg NO₃^--N) and lower alkalinity consumption (1.70 mg CaCO₃/mg NO₃^--N) when compared to SOD (7.54 mg SO₄^2-/mg NO₃^--N, 4.57 mg CaCO₃/mg NO₃^--N based on stoichiometric calculation). This research revealed that the PPAD process is a promising technique for nitrate-contaminated groundwater treatment and promoted the utilization of pyrite in the field of environmental remediation.