Nitrous oxide (N 2 O) emissions from wastewater treatment plants vary substantially between plants, ranging from negligible to substantial (a few per cent of the total nitrogen load), probably because of different designs and operational conditions. In general, plants that achieve high levels of nitrogen removal emit less N 2 O, indicating that no compromise is required between high water quality and lower N 2 O emissions. N 2 O emissions primarily occur in aerated zones/compartments/periods owing to active stripping, and ammonia-oxidizing bacteria, rather than heterotrophic denitrifiers, are the main contributors. However, the detailed mechanisms remain to be fully elucidated, despite strong evidence suggesting that both nitrifier denitrification and the chemical breakdown of intermediates of hydroxylamine oxidation are probably involved. With increased understanding of the fundamental reactions responsible for N 2 O production in wastewater treatment systems and the conditions that stimulate their occurrence, reduction of N 2 O emissions from wastewater treatment systems through improved plant design and operation will be achieved in the near future.

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Competition for electrons among different steps of denitrification has previously been shown to occur, and to play an important role in the accumulation and emission of N2O in wastewater treatment. However, this electron competition is not recognized in the current denitrification models, limiting their ability to predict N2O accumulation during denitrification. In this work, a new denitrification model is developed for wastewater treatment processes. It describes electron competition among the four steps of denitrification, through modeling the carbon oxidation and nitrogen reduction processes separately, in contrast to the existing models that directly couple these two types of processes. Electron carriers are introduced to link carbon oxidation, which donates electrons to carriers, and nitrogen oxides reduction, which receives electrons from these carriers. The relative ability of each denitrification step to compete for electrons is modeled through the use of different affinity constants with reduced carriers. Model calibration and validation results demonstrate that the developed model is able to reasonably describe the nitrate, nitrite, and N2O reduction rates of a methanol-utilizing denitrifying culture under various carbon and nitrogen oxides supplying conditions. The model proposed, while subject to further validation, is expected to enhance our ability to predict N2O accumulation in denitrification.

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Photodynamic therapy (PDT) based on photosensitizers (PSs) constructed with nanomaterials has become popular in cancer treatment, especially oral carcinoma cell. This therapy is characterized by improved PS accumulation in tumor regions and generation of reactive oxygen species (ROS) for PDT under specific excitation. In the selection of near-infrared (NIR) window, 808 nm NIR light because it can avoid the absorption of water is particularly suitable for the application in PDT. Hence, multiband emissions under a single 808 nm near-infrared excitation of Nd -sensitized upconversion nanoparticles (808 nm UCNPs) have been applied for the PDT effect. 808 nm UCNPs serve as light converter to emit UV light to excite inorganic PS, graphitic carbon nitride quantum dots (CNQDs), thereby generating ROS. In this study, a nanocomposite consisting UCNPs conjugated with poly-l-lysine (PLL) to improve binding with CNQDs is fabricated. According to the research results, NIR-triggered nanocomposites of 808 nm UCNP-PLL@CNs have been verified by significant improvement in ROS generation. Consequently, 808 nm UCNP-PLL@CNs exhibit high capability for ROS production and efficient PDT in vitro and in vivo. Moreover, the mechanism of PDT treatment by 808 nm UCNP-PLL@CNs is evaluated using the cell apoptosis pathway.