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

Efficient Photocatalytic Removal of Aqueous Ammonia Nitrogen by g-C3N4/CoP Heterojunctions Under Visible Light Illumination

With the development of industry, agriculture, and aquaculture, excessive ammonia nitrogen mainly involving ionic ammonia (NH4+) and molecular ammonia (NH3) has inevitable access to the aquatic environment, posing a severe threat to water safety. Photocatalytic technology shows great advantages for ammonia nitrogen removal, such as its efficiency, reusability, low cost, and environmental friendliness. In this study, CP (g-C3N4/CoP) composite materials, which exhibited high-efficiency ammonia nitrogen removal, were synthesized through a simple self-assembly method. For the optimal CP-10 (10% CoP) samples, the removal rate of ammonia nitrogen reached up to 94.8% within 80 min under visible light illumination. In addition, the nitrogen selectivity S(N2) is about 60% for all oxidative products. The high performance of the CP-10 photocatalysts can be ascribed to the effective separation and transmission of electron-hole pairs caused by their heterogeneous structure. This research has significance for the application of photocatalysis for the remediation of ammonia nitrogen wastewater.

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.

Construction of a Dual-Function Mo-ZIS@Ti for Photocatalytic Benzyl Alcohol Oxidation and Hydrogen Evolution Performance

The photocatalytic oxidation of benzyl alcohol and the simultaneous evolution of hydrogen from water are efficient dual-optimal routes. It is important to develop composite catalysts that combine redox properties and facilitate electron-hole separation and transport. Herein, the bimetallic-doped Mo-ZIS@Ti photocatalyst was designed and synthesized, and the selective oxidation of benzyl alcohol and hydrogen evolution by water splitting was realized at the same time. Under visible light irradiation, benzyl alcohol was completely converted with more than 99% selectivity for benzaldehyde, and the H2 production rate was 5.6 times higher than the initial ZIS. The exceptional catalytic performance was ascribed to utilizing Ti-MIL-125 as a precursor, wherein slowly releasing-doped Ti formed robust Ti-S bonds that quickly transfer electrons and reduce sites. Meanwhile, doping Mo effectively captures photogenerated holes and acts as active sites for oxidation reactions. Both experimental characterization and work function calculations demonstrate that the bimetallic synergism effectively modulates the electronic structure of ZIS, promotes the directional separation of electrons and holes, and significantly improves the photoactivity and stability of ZIS. This work contributes a route to obtain benzaldehyde and green hydrogen at the same time and also gives new insights for the construction and mechanism study of bimetallic-doping catalysts.

An update on sustainabilities and challenges on the removal of ammonia from aqueous solutions: A state-of-the-art review

An insightful attempt has been made in this review and the primary objective was to meticulously provide an update on the sustainabilities, advances and challenges pertaining the removal of ammonia from water and wastewater. Specifically, ammonia is a versatile compound that prevails in various spheres of the environment, and if not properly managed, this chemical species could pose severe ecological pressure and toxicity to different receiving environments and its biota. The notorious footprints of ammonia could be traced to anoxic conditions, an infestation of aquatic ecosystems, hyperactivity, convulsion, and methaemoglobin, popularly known as the "blue baby syndrome". In this review, latest updates regarding the sustainabilities, advancements and challenges for the removal of ammonia from aqueous solutions, i.e., river and waste waters, are briefly elucidated in light of future perspectives. Viable routes and ideal hotspots, i.e., wastewater and drinking water, for ammonia removal under the cost-effective options have been unpacked. Key mechanisms for the removal of ammonia were grossly bioremediation, oxidation, adsorption, filtration, precipitation, and ion exchange. Finally, this review denoted biological nutrient removal, struvite precipitation, and breakpoint chlorination as the most effective and promising technologies for the removal of ammonia from aquatic environments, although at the expense of energy and operational cost. Lastly, the future perspective, avenues of exploitation, and technical facets that deserve in-depth exploration are duly underscored.

Self-assembly construction of 1D carbon nitride nanotubes and cobalt-modified for superior photocatalytic degradation of sulfonamide antibiotics

In the present work, a cobalt-doped carbon nitride nanotubes (Co-CNt) was synthesized via self-assembly process. Contributed to the narrow band gap, enlarged specific surface area and abundant active sites, Co-CNt has excellent photoelectric properties and superior performance than pristine CN in sulfisoxazole (SIZ) degradation under blue light irradiation, which achieved 100% removal within 40 min. Meanwhile, the system not only exhibited practical applicability by efficiently degrading SIZ, but also generating high levels of H2O2. Moreover, the Co-CNt/visible light system shows superior operability over a wide pH range, micro-concentration contaminants, various anions, water matrices and other sulfonamides with promising catalytic stability and applicability. The contribution of RSs in the degradation process were elucidated based on radical scavenging and spin-trapped tests, clarifying that O2·- and h+ majorly dominated the process. In addition, 4 probable degradation pathways of SIZ were provided and the generated intermediates' toxicity were evaluated. Overall, this study successfully synthesized a self-assembled 1D tubular photocatalyst with Co-doped and demonstrated the potential Co-CNt/visible light system for environmental remediation, providing a promising approach for the development of photocatalysis.

Photocatalytic performance of biochar-modified TiO2 (C/TiO2) for ammonia-nitrogen removal

Biochar-modified TiO2 (C/TiO2) was prepared by a sol-gel method in this study to improve the photocatalytic capacity for ammonia-nitrogen (NH3-N) removal from aqueous solutions. The results showed that biochar was successfully modified on TiO2 and helped improve its photocatalytic performance for pollutant degradation. The removal capacity of ammonia-nitrogen on the synthesized photocatalyst performed well at pH 10 with 1 g L-1 C/TiO2 under both 60 (12.25 mg g-1) and 120 min (16.31 mg g-1) irradiation (xenon lamp, AM1.5, 25 A). Characterization of C/TiO2 through scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectrometry (FT-IR) analyses showed the successful introduction of biochar on TiO2. SEM-EDS and BET analyses displayed that C/TiO2 had a larger surface area and more pores than the raw materials. XRD spectroscopy illustrated that C/TiO2 had typical characteristic peaks of anatase-TiO2 and presented a good photocatalytic degradation performance. It was confirmed from XPS and FT-IR analyses that -COOH groups were present in C/TiO2 and originated from biochar modification, and these enhanced the photocatalytic performance. Through radical quenching experiments, it was found that superoxide radicals (˙O2-) played a dominant role in NH3-N photocatalytic reactions with hydroxyl radicals (˙OH) and valence band holes (h+) playing a synergistic role. N2 was the main degradation product after 6 h NH3-N photocatalytic degradation, which was much larger than NO3-/NO2- (both almost undetected) and NH3 (ca. 2 times lower than N2). The new composite C/TiO2 has potential for ammonia-nitrogen degradation in wastewater treatment and favorable for treating sewage sludge.

Emerging high-ammonia‑nitrogen wastewater remediation by biological treatment and photocatalysis techniques

The bacterial and photocatalysis techniques have been widely applied into the remediation of ammonia nitrogen wastewater. Although traditional microbial methods had been verified useful; more efficient, energy-saving and controllable candidate treatment methods are still urgently needed to cover the increasingly diverse ammonia nitrogen pollution cases. The bacterial treatment technique for ammonia nitrogen mainly depends on the ammonia nitrogen oxidation-reduction (e.g. nitrification, denitrification) by nitrifying bacteria and denitrifying bacteria, but these reactions suffer from slow denitrifying kinetic process and uncontrolled disproportionation reaction. In comparison, the photocatalysis technique based on photoelectrons is more efficient and has some advantages, such as low temperature reaction and long life, while the photocatalysis technique can not perform multiple complex biochemical reactions. Despite much scientific knowledge obtained about this issue recently, such research has yet not been widely adopted in the industry because of many concerns about subsequent catalyst stability and economic feasibility. This review summarized and discussed the very recent achievements and key problems on remediation of high-ammonia‑nitrogen wastewater and oxidation driven by bacterial treatment and photocatalysis techniques, as well as the most promising future directions for these two techniques, especially the potential of jointly bacterial-photocatalysis techniques.

Nitrogen-containing wastewater fuel cells for total nitrogen removal and energy recovery based on Cl•/ClO• oxidation of ammonia nitrogen

The excess nitrogen discharge into water bodies has resulted in extensive water pollution and human health risks, which has become a critical global issue. Moreover, nitrogenous wastewater contains considerable chemical energy contributed by organic pollutants and nitrogenous compounds. Therefore, the treatment of various kinds of nitrogen-containing wastewater for nitrogen removal and energy recovery is of significance. Biological methode and advanced oxidation processes (AOPs) are the main methods for nitrogen removal. However, biological treatment is easily inhibited by high-salinity, high ammonia nitrogen (NH₃-N/NH₄⁺-N), nitrite and toxic organics in wastewater, which limits its application. AOPs mainly induce in situ generation of highly reactive species, such as hydroxyl radical (HO•), sulfate radical (SO₄•^-) and chlorine radicals (Cl•, ClO•, Cl₂•^-), for nitrogen removal. Nevertheless, HO• shows low reactivity and N₂ selectivity towards NH₃-N/NH₄⁺-N oxidation, and SO₄•^- also demonstrates unsatisfactory NH₃-N/NH₄⁺-N removal. It has been shown that Cl•/ClO• can efficiently remove NH₃-N/NH₄⁺-N with high N₂ selectivity. The generation of Cl•/ClO• can be triggered by various techniques, among which the PEC technique shows great potential due to its higher efficiency for Cl•/ClO• generation and eco-friendly approach for pollutants degradation and energy recovery by utilizing solar energy. Cl•/ClO• oxidation of NH₃-N/NH₄⁺-N and nitrate nitrogen (NO₃^--N) reduction can be strengthened through the design of photoanode and cathode materials, respectively. Coupling with this two pathways, an exhaustive total nitrogen (TN) removal system is designed for complete TN removal. When introducing the mechanism into photocatalytic fuel cells (PFCs), the concept of nitrogen-containing wastewater fuel cells (NFCs) is proposed to treat several typical types of nitrogen-containing wastewater, achieving high-efficiency TN removal, organics degradation, toxic chlorate control, and energy recovery simultaneously. Recent research progress in this field is reviewed, summarized and discussed, and in-depth perspectives are proposed, providing new ideas for the resource treatment of nitrogen-containing wastewater.

Improved photodegradation of antibiotics pollutants in wastewaters by advanced oxidation process based on Ni-doped TiO2

The number of antibiotic compounds in wastewaters has been growing globally due to the covid-19 problem. Using antibiotics to treat the patients would produce larger amounts of these compounds into the environment with negative impacts. Hence, finding out the method for the elimination of toxic organic pollutants as well as antibiotics in water is urgent (In this study, the treatment of antibiotic pollutants including cefalexin (CF) and tetracycline (TC) was investigated by applying the advanced oxidation process based on Ni-doped TiO₂ (Ni-TiO₂). The characterizations technologies such as XRD, XPS, UV-vis, PL, and PC indicated that Ni doping would improve the photocatalytic performance of TiO₂. In the photodegradation experiments, the Ni-TiO₂ possessed high photocatalytic degradation efficiencies with 93.6% for CF and 82.5% for TC. Besides, the removal rates of antibiotics after five cycles are higher than 75%, implying excellent stability of Ni-TiO₂ photocatalyst. The result from the treatment of wastewater samples revealed that the Ni-TiO₂ photocatalytic had good performance for removal of CF and TC at a high level of 88.6 and 80.2%, respectively.

Meta-analysis of the prevalence of dissolved organic nitrogen (DON) in water and wastewater and review of DON removal and recovery strategies

Most wastewater removal and recovery processes primarily target dissolved inorganic nitrogen (DIN) species, leaving the untreated non-reactive dissolved organic nitrogen (DON) in the effluent. This DON fraction can account for a substantial part of the total nitrogen (N) load. We analyzed large datasets of N species and concentrations (with a focus on quantifying the fraction of DON) in surface water, ground water, and wastewater effluent across the United States. We then reviewed strategies to remove and recover DON based on results of a range of treatment technologies reported in the literature, including laboratory-scale up to full-scale operation in wastewater treatment plants. Our meta-analysis showed that DON concentrations are greatest in wastewater effluent followed by surface water and groundwater. The concentration of DON in wastewater effluent varied from 0.01 to 10.9 mg N/L (number of data points, n = 163), where the range in surface water was 0.002 to 14.3 mg N/L (n = 11,803). Organic N accounted for the majority of total N in 12.3% of wastewater effluent samples and 49.1% of surface waters. Our literature review showed that currently available wastewater treatment processes do not efficiently target DON removal nor recovery of the DON as a valuable product. One potential DON removal and recovery strategy is transforming DON into DIN, which is generally more easily removed and recovered. Transformation strategies reported in the literature include ozonation, UV/H₂O₂, and electrooxidation. However, as advanced oxidation processes are often energy- and cost-intensive, further research is needed to improve DON removal and recovery.

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.

Kinetic Study and Modeling of the Degradation of Aqueous Ammonium/Ammonia Solutions by Heterogeneous Photocatalysis with TiO2 in a UV-C Pilot Photoreactor

The degradation mechanism of NH4+/NH3 in aqueous solutions by heterogeneous photocatalysis (TiO2/SiO2) and photolysis in UV-C pilot photoreactor has been studied. Under the conditions used, NH4+/NH3 can be decomposed both by photolytically and photocatalytically, without disregarding stripping processes. The greatest degradation is achieved at the highest pH studied (pH 11.0) and at higher lamp irradiation power used (25 W) with degradation performances of 44.1% (photolysis) and 59.7% (photocatalysis). The experimental kinetic data fit well with a two parallel reactions mechanism. A low affinity of ammonia for adsorption and surface reaction on the photocatalytic fiber was observed (coverage not higher than 10%), indicating a low influence of surface phenomena on the reaction rate, the homogeneous phase being predominant over the heterogeneous phase. The proposed reaction mechanism was validated, confirming that it is consistent with the photocatalytic and photolytic formation of nitrogen gas, on the one hand, and the formation of nitrate, on the other hand. At the optimal conditions, the rate constants were k3 = 0.154 h−1 for the disappearance of ammonia and k1 = 3.3 ± 0.2 10−5 h−1 and k2 = 1.54 ± 0.07 10−1 h−1 for the appearance of nitrate and nitrogen gas, respectively.

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.

Removal of N,N-dimethylformamide by dielectric barrier discharge plasma combine with manganese activated carbon

Manganese activated carbon (Mn-AC) was successfully prepared by the incipient wetness method and characterized by SEM, XRD, and FTIR. This study chose N,N-dimethylformamide (DMF) as the target pollutant, and the removal rate of DMF and removal mechanism were systematically studied by dielectric barrier discharge (DBD) plasma combined with Mn-AC. This study indicated that DBD plasma combined with Mn-AC could effectively remove DMF. With the addition of Mn-AC, the removal rate and mineralization rate of DMF within 40 min increased from 51.5% and 36.0% to 82.2% and 58.2%, respectively. The discharge power, initial concentration of DMF, initial pH of the solution, and dosage of Mn-AC affect the removal of DMF. The optimal discharge power is 16.19 W, and energy efficiency is 20.79 mg·kJ-1; low concentration DMF could be removed more effectively. Neutral and alkaline conditions showed better removal effect of DMF than acid conditions; Mn-AC optimal dosage is 1.0 g L-1. The concentration variations of O3, H2O2, and ·OH manifested that Mn-AC could effectively convert O3 and H2O2 to ·OH, thereby increasing the DMF removal rate. Quenching experiments showed that ·OH is the main active species in the reaction. Based on reaction products of DMF such as N-methylformamide, methanol, formaldehyde, and formic acid, possible degradation pathways were proposed. Prospect analysis demonstrated combining plasma systems with catalysts is promising.

Nickel hydroxide as a non-noble metal co-catalyst decorated on Cd0.5Zn0.5S solid solution for enhanced hydrogen evolution

The study of non-noble metal photocatalysts provides practical significance for hydrogen evolution applications. Herein, new Cd_0.5Zn_0.5S/Ni(OH)₂ catalysts were fabricated through simple hydrothermal and precipitation methods. The photocatalytic performance of the Cd_0.5Zn_0.5S/Ni(OH)₂ composites under visible light was significantly improved, which was attributed to the wider visible light absorption range and less recombination of electron–hole pairs. The composite with a Ni(OH)₂ content of 10% showed the best hydrogen evolution rate of 46.6 mmol g⁻¹ h⁻¹, which was almost 9 times higher than that of pristine Cd_0.5Zn_0.5S. The severe photo-corrosion of Cd_0.5Zn_0.5S was greatly improved, and the Cd_0.5Zn_0.5S/Ni(OH)₂ composite exhibited a very high hydrogen evolution rate after three repeated tests. The excellent photocatalytic performance was due to the non-noble metal Ni(OH)₂ co-catalyst. The excited electrons were transferred to the co-catalyst, which reduced electron–hole recombination. Moreover, the co-catalyst offered more sites for photocatalytic reactions. This study researched the mechanism of a co-catalyst composite, providing new possibilities for non-noble metal photocatalysts.

This study researched the mechanism of a co-catalyst composite, providing new possibilities for non-noble metal photocatalysts.