The inhibition of free nitrous acid (FNA) on denitrifying phosphorus removal has been widely reported for enhanced biological phosphorus removal; however, few studies focus on the nitrous oxide (N₂O) production involved in this process. In this study, the effects of FNA on N₂O production and anoxic phosphorus metabolism were investigated using phosphorus-accumulating organisms (PAOs) culture highly enriched (91±4%) inCandidatus Accumulibacter phosphatis. Results show that the FNA concentration notably inhibited anoxic phosphorus metabolism and phosphorus uptake. Poly-β-hydroxyalkanoate (PHA) degradation was completely inhibited when the FNA concentration was approximately 0.0923 mgHNO₂-N/L. Higher initial FNA concentrations (0.00035 to 0.0103 mgHNO₂-N/L) led to more PHA consumption/TN (0.444 to 0.916 mmol-C/(mmol-N·gVSS)). Moreover, it was found that FNA, rather than nitrite and pH, was likely the true inhibitor of N₂O production. The highest proportion of N₂O to TN was 78.42% at 0.0031 mgHNO₂-N/L (equivalent to 42.44 mgNO₂-N/L at pH 7.5), due to the simultaneous effects of FNA on the subsequent conversion of NO₂into N₂O and then into N₂. The traditional nitrite knee point can only indicate the exhaustion of nitrite, instead of the complete removal of TN.

To achieve high-value utilization of rare earth resources and meet the demands for nitrogen oxide (NO _x ) pollution control, this study focuses on the Ce-rich Bayan Obo rare earth mineral, aiming to develop efficient mineral-based denitration catalysts and elucidate the underlying mechanisms. Although this type of mineral is rich in Ce with catalytic potential, the associated Ca severely restricts the intrinsic activity of Ce and the overall denitration performance. In this work, acid leaching was applied to selectively remove Ca, and combined with multiple characterization techniques, we systematically clarified how Ca removal optimizes the speciation and surface properties of Ce, thereby significantly enhancing the selective catalytic reduction (NH ₃ -SCR) activity. The results indicate that the modified catalyst exhibits a notably increased specific surface area, an optimized Ce ³⁺ /Ce ⁴⁺ redox pair ratio, an increased oxygen vacancy concentration, and a predominant presence of Ce species in low-polymerization states. The NO _x conversion rate at 350 °C was dramatically improved from 18.9% for the raw ore to 82.1%, while maintaining excellent N ₂ selectivity. In situ diffuse reflectance infrared Fourier transform spectroscopy ( in situ DRIFTS) further confirmed that Ca removal exposes more Ce active sites and enhances the synergistic effect between Brønsted and Lewis acid sites. The established correlation system of ‘Ca removal–Ce speciation–denitration reaction mechanism’ in this work provides a solid theoretical foundation for the design of high-performance rare earth mineral-based catalysts.