Abstract
Abstract A Co/Ti electrode for nitrate reduction was prepared by electrode-deposition. In the single-compartment electrolysis cell, nitrate (100 mg/L) removal reached nearly 100% after 3 h electrolysis under the current density of 20 mA cm–2 by using the Co/Ti electrode as cathode, and the main reduction products were ammonium nitrogen (66.5%) and nitrogen gas (33.5%). This performance on nitrate removal was comparable to a Co3O4/Ti electrode, and the electroactivity of the Co/Ti electrode towards nitrite reduction was higher than that of a Co3O4/Ti electrode. The Co/Ti electrode exhibited an improved stability with 18.7% of mass loss and 25.5% of Co dissolution compared with the Co3O4/Ti electrode after ultrasonic interference. The presence of chlorine ion (1,000 mg/L) could promote the total nitrogen (TN) removal to approximately 100% after 3 h electrolysis because of the ammonium oxidation by the free chlorine produced from the anode. In the presence of calcium (50 mg/L) and phosphate (0.5 mg/L), the nitrate removal decreased from 85.4 ± 1.5 to 57.7 ± 3.5% after ten reuse cycles. This result suggests that Ca and P should be pre-removed before the electro-reduction of nitrate.
Highlights
Excess nitrate in water has caused a serious problem due to its threats to both human health and ecological safety
It was found that the removal of total nitrogen (TN) increased from 0.6 to 8.3%, which could be explained by the direct oxidation of the increased ammonium on anode (Figure 1(b))
X-ray diffraction (XRD) was used for the electrode characterization, and the result indicated that Co crystals were generated on the surface of Ti substrate (Figure 2)
Summary
Excess nitrate in water has caused a serious problem due to its threats to both human health and ecological safety. Nitrate can produce nitrite and nitrosamines that pose direct harm to proteins or cells, causing diseases such as gastrointestinal cancers. China has limited the maximum level of 15 mg L–1 for total nitrogen for wastewater discharge which was issued in 2002 (NEPA 2002; Ye et al 2019). The World Health Organization and many countries including the United States have set the limited maximum contaminant level of 10 mg L–1 for nitrate-nitrogen or 50 mg L–1 for nitrate in drinking water (NHCPRC 2006; USEPA 2010; WHO 2010)
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