New insight into electrochemical denitrification using a self-organized nanoporous VO-Co3O4/Co cathode: Plasma-assistant oxygen vacancies catalyzed efficient nitrate reduction.

A novel self-organized nanoporous VO-Co3O4/Co cathode was prepared via anodization and plasma treatment and obtained a significant nitrate reduction efficiency. In the anodization, an oxide layer with the nano-sized pore structure initially grew in-situ on the Co substrate and showed a better surface area. Subsequently, He-plasma increased surface oxygen vacancies (VO) from 24 % to 57 %. Electrons in vacancies were charged into empty eg orbital of low-spin Co3+(Oh, octahedral) and firstly generated high-spin Co2+(Oh) with the configuration of t2g6eg1, accounting for 71.7 % of cobalt species. Accordingly, two original mechanisms (Vo-catalyzed and Co2+(Oh)-catalyzed) were concluded in this study. Oxygen vacancies increased the charge intensity and served as absorption sites in nitrate reduction. Meanwhile, massive Co2+(Oh) provided electrons in the eg orbital with a higher energy state and mediated the faster electron transfer through a Co2+-Co3+-Co2+ redox cycle, compared with Co2+ (Td, tetrahedral). Ultimately, a faster reaction kinetic of 0.0220 min-1 was achieved by VO-Co3O4 than other cathodes e.g., Co3O4 (0.0150 min-1). Such VO-Co3O4/Co cathode-based denitrification strategy displayed great advantages in engineering application and completely removed 90 % of TN from actual wastewater.

New insight into degradation of chloramphenicol using a nanoporous Pd/Co3O4cathode: characterization and pathways analysis.

The growing chloramphenicol (CAP) in wastewater brought a serious threat to the activity of activated sludge and the spread of antibiotics resistance bacteria. In this study, a highly ordered nanoporous Co3O4layer on Co foil through anodization was prepared as cathode for nitro-group reduction and electrodeposited with Pd particles for dechlorination to reduce CAP completely. After 3 h treatment, almost 100% of CAP was reduced. Co2+ions in Co3O4served as catalytic sites for electrons transfer to CAP through a redox circle Co2+-Co3+-Co2+, which triggered nitro-group reduction at first. With the presence of Pd particles, more atomic H* were generated for dechlorination, which increased 22% of reduction efficiency after 3 h treatment. Therefore, a better capacity was achieved by Pd/Co3O4cathode (K = 0.0245 min-1,Kis reaction constant) than by other cathodes such as Fe/Co3O4(K = 0.0182 min-1), Cu/Co3O4(K = 0.0164 min-1), and pure Co3O4(K = 0.0106 min-1). From the proposed reaction pathway, the ultimate product was carbonyl-reduced AM (dechlorinated aromatic amine product of CAP) without antibacterial activity, which demonstrated this cathodic technology was a feasible way for wastewater pre-treatment.