Aqueous ammonia abatement on Pt- and Ru-modified TiO 2 : Selectivity effects of the metal nanoparticles deposition method

Photocatalytic removal of ammonia nitrogen from water: investigations and challenges for enhanced activity

Ammonia nitrogen (NH3-N/NH4+-N) serves as a crucial chemical in biochemistry and fertilizer synthesis. However, it is also a toxic compound, posing risks from eutrophication to direct threats to human health. Ammonia nitrogen pollution pervades water sources, presenting a significant challenge. While several water treatment technologies exist, biological treatment, though widely used, has its limitations. Hence, green and efficient photocatalytic technology emerges as a promising solution. However, current monolithic semiconductor photocatalysts prove inadequate in controlling ammonia nitrogen pollution. Therefore, this review focuses on enhancing semiconductor photocatalysts' efficiency through modification, discussing four mechanisms: (1) mono-ionic modification; (2) metallic and non-metallic modification; (3) construct heterojunctions; and (4) enhancement of synergistic effects of multiple technologies. The influencing factors of photocatalytic ammonia nitrogen removal efficiency are also explored. Moreover, the review outlines the limitations of current photocatalytic pollution treatment and discusses future development trends and research challenges. Currently, the main products of ammonia nitrogen removal include NO3-, NO2-, and N2. To mitigate secondary pollution, the green process of converting ammonia nitrogen to N2 using photocatalysis emerges as a fundamental approach for future treatment. Overall, this review aims to deepen understanding of photocatalysis in ammonia nitrogen treatment and guide researchers toward widespread implementation of this endeavor.

Ammonia-nitrogen removal from water with gC3N4-rGO-TiO2 Z-scheme system via photocatalytic nitrification-denitrification process

Photocatalytic removal of NH₃-N is expected to be an alternative to the biological method that accompanied with high energy consumption and secondary pollution. However, NH₃-N is always oxidized into nitrate and nitrite during the photocatalytic processes, which also need to be removed from the water. Herein, the g-C₃N₄/rGO/TiO₂ Z-scheme photocatalytic system was prepared and used for the NH₃-N removal. The results showed the rate constant of NH₃-N conversion on it was 0.705 h^-1, 1.7 times as high as that on g-C₃N₄/TiO₂, and most of the NH₃-N were converted into gaseous products. And the experiment result indicated NH₃-N and NO₃^- in water could enhance the removal of each other. According to the results, the main reaction mechanism is speculated as: ·OH radicals and ·O₂^- radicals were generated on TiO₂ and oxidized the NH₃-N into NO₃^-, and the latter was reduced into non-toxic N₂ on the conduction band of g-C₃N₄. Finally, NH₃-N removal performance for actual coking wastewater was investigated, and the stability of the photocatalyst was tested. This work provides some theoretical basis for the two-step degradation of pollutants by Z-scheme photocatalytic system.

Coupling photocatalytic and electrocatalytic oxidation towards simultaneous removal of humic acid and ammonia-nitrogen in landscape water

Aiming to bring photocatalytic and electrocatalytic oxidation processes into solving practical issue of organics and ammonia-nitrogen pollution in landscape water that resulting in algae bloom and eutrophication, this work firstly investigates photocatalytic oxidation of humic acid and electrochemical oxidation of ammonia upon optimization of each process parameters, respectively. The platinum modified titania (Pt/TiO₂) exhibits improved activity than pure titania and CuO_x, MnO_x and NiO_x modified titania for decomposition of humic acid. As an application-oriented study, this work has developed a simple and effective brushing and annealing method for immobilization of TiO₂ and Pt/TiO₂ onto ceramic foam for further application. In addition, the RuO₂-IrO₂/Ti electrode presents the best electrocatalytic activity compared with RuO₂/Ti and IrO₂/Ti electrodes in terms of ammonia oxidation, and the ammonia conversion pathways have been studied. Lastly, an integrated and enlarged reactor system employing optimized photocatalytic ceramic foam and stable electrodes has been developed for simultaneous oxidation of humic acid and ammonia-nitrogen in water circulated flow condition, based on cooperative production of reactive oxidant species between photocatalysis and electrocatalysis. The results show that coupled photocatalytic and electrocatalytic oxidation is a promising approach for treatment of organic matter and inorganic ammonia nitrogen in landscape water.

Improved photocatalytic conversion of high-concentration ammonia in water by low-cost Cu/TiO2 and its mechanism study

Platinized TiO₂ (Pt/TiO₂) as a benchmark photocatalyst shows superior photocatalytic performance in environmental remediation. In order to reduce the cost of photocatalyst for practical use, a series of cooper loaded TiO₂ (Cu/TiO₂) photocatalysts were prepared by photoreduction method and compared with pure TiO₂ and Pt/TiO₂ in terms of overall ammonia conversion efficiency and selective oxidation. The as-prepared Cu/TiO₂ samples were characterized and analyzed by physicochemical instrumental measurements. The results show that about 60% Cu²⁺ ions in suspension can be photodeposited onto the surface of TiO₂ under UV light irradiation, and is mainly composed by a mixture of Cu/Cu⁺. The Cu/P25 (0.3 wt% Cu) sample was screened out as the optimal photocatalyst, via photoilluminance spectra analysis and photocatalytic oxidation of ammonia. It shows even better performance compared to Pt/TiO₂ in the oxidation of high concentration of ammonia, due to the strong coordination effect by Cu(NH₃)_n complex formation. Through Electron Spin Resonance (EPR) analysis, and free radical suppression experiments, the active oxidative species account for ammonia oxidation and selective product generation were analyzed, and the possible reaction mechanisms involving photocatalytic ammonia conversion were proposed. ^●OH has been identified as the main oxidant that affects the removal efficiency of ammonia nitrogen, whereas O₂^●- mainly affects the production of N₂ and h⁺ is mainly responsible for the production of NO₃^-. These results indicate that Cu/TiO₂ could be used as a low-cost and efficient photocatalyst in pretreatment process for conversion of high concentration of ammonia in wastewater.

Characterization and Effect of Ag(0) vs. Ag(I) Species and Their Localized Plasmon Resonance on Photochemically Inactive TiO2

Commercial TiO2 (anatase) was successfully modified with Ag nanoparticles at different nominal loadings (1%–4%) using a liquid impregnation method. The prepared materials retained the anatase structure and contained a mixture of Ag0 and AgI species. Samples exhibited extended light absorption to the visible region. The effect of Ag loading on TiO2 is studied for the photocatalytic reduction of CO2 to CH4 in a gas–solid process under high-purity conditions. It is remarkable that the reference TiO2 used in this work is entirely inactive in this reaction, but it allows for studying the effect of Ag on the photocatalytic process in more detail. Only in the case of 2% Ag/TiO2 was the formation of CH4 from CO2 observed. Using different light sources, an influence of the localized surface plasmon resonance (LSPR) effect of Ag is verified. A sample in which all Ag has been reduced to the metallic state was less active than the respective sample containing both Ag0 and Ag+, indicating that a mixed oxidation state is beneficial for photocatalytic performance. These results contribute to a better understanding of the effect of metal modification of TiO2 in photocatalytic CO2 reduction.