Efficient ammonia removal and toxic chlorate control by using BiVO4/WO3 heterojunction photoanode in a self-driven PEC-chlorine system

Bias-free driven ion assisted photoelectrochemical system for sustainable wastewater treatment

Photoelectrochemical (PEC) systems have emerged as a prominent renewable energy-based technology for wastewater treatment, offering sustainable advantages such as eliminating dependence on fossil fuels or grid electricity compared to traditional electrochemical treatment methods. However, previous PEC systems often overlook the potential of ions present in wastewater as an alternative to externally applied bias voltage for enhancing carrier separation efficiency. Here we report a bias-free driven ion assisted photoelectrochemical (IAPEC) system by integration of an electron-ion acceptor cathode, which leverages its fast ion-electron coupling capability to significantly enhance the separation of electrons and holes at the photoanode. We demonstrate that Prussian blue analogues (PBAs) can serve as robust and reversible electron-ion acceptors that provide reaction sites for photoelectron coupling cations, thus driving the hole oxidation to produce strong oxidant free radicals at photoanode. Our IAPEC system exhibits superior degradation performance in wastewater containing chloride medium. This indicates that, in addition to the cations (e.g., Na+) accelerating the electron transfer rate, the presence of Cl- ions further enhance efficient and sustainable wastewater treatment. This work highlights the potential of utilizing abundant sodium chloride in seawater as a cost-effective additive for wastewater treatment, offering crucial insights into the use of local materials for effective, low-carbon, and sustainable treatment processes.

Efficient purification of a nitrate and chlorate mixture in water via photoredox activated intermediate coupling-decoupling pathway

Nitrate (NO3-) is a widespread contaminant that threatens human health and ecological safety. Meanwhile, the disinfection byproducts chlorate (ClO3-) is generated inevitably in conventional wastewater treatment. Therefore, the contaminants mixture of NO3- and ClO3- are universal in common emission units. Photocatalysis technology is a feasible approach for the synergistic abatement of contaminant mixture, where matching suitable oxidation reactions is a potential strategy to improve the photocatalytic reduction reactions. Herein, formate (HCOOH) oxidation is introduced to facilitate the photocatalytic reduction of the NO3- and ClO3- mixture. As a result, high purification efficiency of NO3- and ClO3- mixture are achieved, evidenced by 84.6% e--dependent removal of the mixture at a reaction time of 30 min, with 94.5% N2 selectivity and 100% Cl- selectivity, respectively. Specifically, by the close combination of in-situ characterizations and theoretical calculations, the detailed reaction mechanism is revealed, in which the intermediate coupling-decoupling route from NO3- reduction and HCOOH oxidation is established by the chlorate-induced photoredox activation, leading to the significantly enhanced efficiency for the wastewater mixture purification. The practical application of this pathway is established for simulated wastewater to show its wide applicability. This work provides new insights into photoredox catalysis technology for its environmental application.

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.

Efficient electrochemical generation of active chlorine to mediate urea and ammonia oxidation in a hierarchically porous-Ru/RuO2-based flow reactor

The electrochemical chlorination of urea to CO₂ and N₂ end-products, via active-chlorine-mediated oxidation under nearly neutral conditions, is an effective treatment for medium-concentrated urea-containing wastewater. Herein, we design a novel flow reactor integrated with three-dimensional hierarchically porous Ru/RuO₂ architectures anchored on a Ti mesh. The hierarchically macroporous electrode can create sufficient exposure of catalytically active sites and facilitate the microscopic mass transport and diffusion inside the active layer, thereby contributing to the increased removal efficiency of urea-N and ammonia-N. The combined results of electrochemical measurements, UV-visible spectrometry and in situ Raman spectrometry, show that the OCl^- species produced by chlorine evolution reaction (CER) are the main active constituents for removing urea-N. Theoretical calculations reveal thLTWAat the Ru/RuO₂ possesses a moderate Cl binding strength, lower theoretical overpotentials of CER and a higher conductivity, compared with pure RuO₂. On this basis, we assemble a circular flow reactor with the hierarchically porous electrodes in a two-electrode system to obtain an enhanced microfluidic process, which during 9 days of uninterrupted operation, at a high electrolysis current of 500 mA, achieve a total nitrogen removal of 92.6% and an energy consumption of 7.94 kWh kg^-1 N, demonstrating the promising application of the novel process.

Activation of chloride by oxygen vacancies-enriched TiO2 photoanode for efficient photoelectrochemical treatment of persistent organic pollutants and simultaneous H2 generation

Photoelectrochemical (PEC) activation of chloride ions (Cl^-) to degrade persistent organic pollutants (POPs) is a promising strategy for the treatment of industrial saline organic wastewater. However, the wide application of this technology is greatly restricted due to the general photoanode activation of Cl^- with poor capability, the propensity to produce toxic by-products chlorates, and the narrow pH range. Herein, oxygen vacancies-enriched titanium dioxide (O_v-TiO₂) photoanode is explored to strongly activate Cl^- to drive the deep mineralization of POPs wastewater in a wide pH range (2-12) with simultaneous production of H₂. More importantly, nearly no toxic by-product of chlorates was produced during such PEC-Cl system. The degradation efficiency of 4-CP and H₂ generation rate by O_v-TiO₂ were 99.9% within 60 min and 198.2 μmol h^-1 cm^-2, respectively, which are far superior to that on the TiO₂ (33.1% within 60 min, 27.5 μmol h^-1 cm^-2) working electrode. DFT calculation and capture experiments revealed that O_v-TiO₂ with abundant oxygen vacancies is conducive to the activation of Cl^- to produce more reactive chlorine species, evidenced by its high production of free chlorine (48.7 mg L^-1 vs 7.5 mg L^-1 of TiO₂). The as-designed PEC-Cl system in this work is expected to realize the purification of industrial saline organic wastewater coupling with green energy H₂ evolution.

Boosting generation of reactive oxygen and chlorine species on TNT photoanode and Ni/graphite fiber cathode towards efficient oxidation of ammonia wastewater

Photoelectrocatalytic (PEC) process combining the merits of photocatalysis and electrocatalysis is considered as a promising ammonia oxidation technology for water treatment. However, some key issues, such as the limited in situ generation of oxidants on photoanode, slow mass transfer problem and generation of nitrate/nitrite by-products hinder the further application of PEC process in the treatment of ammonia pollutant. In this study, the graphite felt (GF) cathodes modified by different transition metals (Ni, Fe, Mn, Co, Cu) were screened by physicochemical and photoelectrochemical characterizations. The results show that the Ni-GF cathode with more Ni⁰ uniformly distributed on the GF surface had the best electrocatalytic activity to generate H₂O₂. The PEC system composed of 10.0 wt% Ni-GF cathode and optimized titania nanotubes (TNTs) photoanode selectively converted about 96.1% ammonia to N₂ within 90 min. Compared with the single TNTs photoanode system, the ammonia oxidation reaction rate constant of the synergistic PEC oxidation system was increased by about two times, which demonstrated the role of the oxidants simultaneously generated on both anode and cathode. The in situ generated reactive oxygen-based oxidants and chlorine-based oxidants interacted together, and ClO• acted a leading role in the ammonia oxidation which were confirmed by quenching and probe experiments. In addition, the contributions of •OH and ClO• were significantly improved in the synergistic PEC oxidation system, compared with the single TNTs photoanode system. Furthermore, the nitrate by-products generated by the ammonia oxidation were further reduced on the Ni-GF cathode. The large amount of active chlorine and active oxygen generated on the electrode diffused into the bulk, effectively overcoming the mass transfer limitation of direct oxidation. Therefore, the developed TNTs photoanode/Ni-GF cathode system can continuously and efficiently convert ammonia to N₂ without the formation of nitrate/nitrite by-products.

Integrating Reactive Chlorine Species Generation with H2 Evolution in a Multifunctional Photoelectrochemical System for Low Operational Carbon Emissions Saline Sewage Treatment

Conventional wastewater treatment plants (WWTPs) suffer from high carbon emissions and are inefficient in removing emerging organic pollutants (EOPs). Consequently, we have developed a low operational carbon emissions multifunctional photoelectrochemical (PEC) system for saline sewage treatment to simultaneously remove organic pollutants, ammonia, and bacteria, coupled with H₂ evolution. A reduced BiVO₄ (r-BiVO₄) photoanode with enhanced PEC properties, ascribed to constructing sufficient oxygen vacancies and V⁴⁺ species, was synthesized for the aforementioned technique. The PEC/r-BiVO₄ process could treat saline sewage to meet local WWTPs' discharge standard in 40 min at 2.0 V vs Ag/AgCl and completely degrade carbamazepine (one of EOPs), coupled with 633 μmol of H₂ production; 93.29% reduction in operational carbon emissions and 77.82% decrease in direct emissions were achieved by the PEC/r-BiVO₄ process compared with large-scale WWTPs, attributed to the restrained generation of CH₄ and N₂O. The PEC system activated chloride ions in sewage to generate numerous reactive chlorine species and facilitate ^•OH production, promoting contaminants removal. The PEC system exhibited operational feasibility at varying pH and total suspended solids concentrations and has outstanding reusability and stability, confirming its promising practical potential. This study proposed a novel PEC reaction for reducing operational carbon emissions from saline sewage treatment.

Optimized titania nanotubes photoanode mediated photoelectrochemical oxidation of ammonia in highly chlorinated wastewater via Cl-based radicals

Efficient removal of low-concentration ammonia from chlorinated wastewater is a challenge for decentralized wastewater treatment due to its notorious environmental effect and lethal influence on aquaculture. Photoelectrocatalytic (PEC) oxidation process is considered as an efficient and environment-friendly approach, whereas a low-cost and stable photoanode is crucial. In this study, TiO₂ nanotubes (TNTs) photoanode (Ar-TNT-500 °C) with excellent physicochemical and photoelectrochemical properties was prepared by optimizing the parameters of anodization, including the voltage/times of anodization and the atmosphere/temperature of heat treatment. During the synthesis, the electrochemical and heat treatment processes promoted the formation of oxygen vacancies (O_V) on the TNTs surface and enhanced its electrocatalytic activity. The optimized Ar-TNT-500 °C photoanode could selectively convert ammonia to N₂ (86%) and a small amount of nitrate (14%). Radical quenching and probe experiments confirmed that the ClO produced by rapid quenching of OH and Cl by free chlorine dominated the selective degradation of ammonia in the synergistic process of photocatalysis and electrocatalysis. The cycle of chlorine-based radicals (ClO and Cl) and Cl^- provided a continuous and efficient ammonia oxidation system, because chlorine-based radicals could efficiently and selectively oxidize ammonia and reduce the production of toxic (per) chlorate.

Photoelectrochemical Sensors with Near-Infrared-Responsive Reduced Graphene Oxide and MoS2 for Quantification of Escherichia Coli O157:H7

The photoelectric response is crucial for photocatalysis, having applications in solar cells and photoelectrochemical (PEC) sensors. In this study, we demonstrate improvements in the near-infrared (NIR)-light-driven PEC response via synergism between reduced graphene oxide (rGO) and MoS₂. Intriguingly, rGO modulates the morphology of MoS₂, facilitating carrier generation and migration, improving the PEC performance of the resultant rGO-MoS₂ sheets (GMS), and yielding an approximately 8-fold increase in the photocurrent compared to that of the pure MoS₂. Based on these findings, a NIR-responsive PEC immunosensing platform for the "turn-on" analysis of Escherichia coli O157:H7 on 980 nm light irradiation is reported. Specifically, the device is a three-dimensional magnetic screen-printed paper-based electrode assembled on a home-made PEC cell, and it enables integrated separation and detection. Using a sandwich-type immunocomplex bridged by E. coli O157:H7 and a GMS PEC probe, the immunosensing platform detected E. coli O157:H7 between 5.0 and 5.0 × 10⁶ CFU mL^-1, having an extremely low detection limit of 2.0 CFU mL^-1. Further, the assay enables the direct analysis of E. coli O157:H7 in milk without the need for pretreatment. Our findings suggest directions for the development of NIR-responsive paper-based PEC materials for portable biomolecule sensing.

Ion exchange membrane optimized light-driven photoelectrochemical unit for efficiency simultaneous organic degradation and metal recovery from the mine wastewater

Resource recovery from wastewater is a promising and challenging topic. Herein, a well-designed ion exchange membrane optimized light-driven photoelectrochemical unit (MPECS) was constructed to reduce the effect of inorganic salt on the photoelectrochemical performance of the photoanode. TiO₂/carbon dots/WO₃ (TCDW) photoanode with the indirect Z-scheme heterojunction structure was successfully fabricated, achieving a strong light harvest performance (10.82%) and a high photocurrent density (5.39 mA/cm²). For the simulated solution (0.01 M phenol and 0.01 M CuSO₄), the phenol degradation and Cu recovery efficiencies reached 99.67% and 62.20% in 60 min, respectively, and the corresponding photoelectric conversion efficiency (PECE) reached 4.64% in the TCDW/Pt-based MPECS. For the actual Cu-laden mine wastewater, over 98% of inorganic salt was removed. Compared to the traditional photoelectrochemical system (PECS), the COD removal and Cu recovery efficiencies were further improved by 23.77% and 49.41% in MPECS, respectively. The results exhibited a promising light-driven mine wastewater treatment technology.

Iron Phosphide Precatalyst for Electrocatalytic Degradation of Rhodamine B Dye and Removal of Escherichia coli from Simulated Wastewater

Electrocatalysis using low-cost materials is a promising, economical strategy for remediation of water contaminated with organic chemicals and microorganisms. Here, we report the use of iron phosphide (Fe2P) precatalyst for electrocatalytic water oxidation; degradation of a representative aromatic hydrocarbon, the dye rhodamine B (RhB); and inactivation of Escherichia coli (E. coli) bacteria. It was found that during anodic oxidation, the Fe2P phase was converted to iron phosphate phase (Fe2P-iron phosphate). This is the first report that Fe2P precatalyst can efficiently catalyze electrooxidation of an organic molecule and inactivate microorganisms in aqueous media. Using a thin film of Fe2P precatalyst, we achieved 98% RhB degradation efficiency and 100% E. coli inactivation under an applied bias of 2.0 V vs. reversible hydrogen electrode in the presence of in situ generated reactive chlorine species. Recycling test revealed that Fe2P precatalyst exhibits excellent activity and reproducibility during degradation of RhB. High-performance liquid chromatography with UV-Vis detection further confirmed the electrocatalytic (EC) degradation of the dye. Finally, in tests using Lepidium sativum L., EC-treated RhB solutions showed significantly diminished phytotoxicity when compared to untreated RhB. These findings suggest that Fe2P-iron phosphate electrocatalyst could be an effective water remediation agent.

Nitrate Removal in an Electrically Charged Granular-Activated Carbon Column

Nitrate removal from groundwater remains a challenge. Here, we report on the development of a flow-through, electrically charged, granular-activated carbon (GAC)-filled column, which effectively removes nitrate. In this system, the GAC functioned as an anode, while a titanium sheet acted as a cathode. The high removal rate of nitrate was achieved through a combination of electrosorption and electrochemical transformation to N₂. The column could be readily regenerated in situ by reversing the polarity of the applied potential. We demonstrate that in the presence of chloride, the mechanism responsible for the observed nitrate removal involves a combination of electroadsorption of nitrate to the anodically charged GAC, electroreduction of nitrate to ammonium, and the oxidation of ammonium to N₂ gas by reactive chlorine and other oxidative radicals (with nearly 100% N₂ selectivity). Given the ubiquitous presence of chloride in groundwater, this method represents a ready, green, and sustainable treatment process with significant potential for the remediation of contaminated groundwater.

Recent Progress and Prospects in Catalytic Water Treatment

Presently, conventional technologies in water treatment are not efficient enough to completely mineralize refractory water contaminants. In this context, the implementation of catalytic processes could be an alternative. Despite the advantages provided in terms of kinetics of transformation, selectivity, and energy saving, numerous attempts have not yet led to implementation at an industrial scale. This review examines investigations at different scales for which controversies and limitations must be solved to bridge the gap between fundamentals and practical developments. Particular attention has been paid to the development of solar-driven catalytic technologies and some other emerging processes, such as microwave assisted catalysis, plasma-catalytic processes, or biocatalytic remediation, taking into account their specific advantages and the drawbacks. Challenges for which a better understanding related to the complexity of the systems and the coexistence of various solid-liquid-gas interfaces have been identified.

Targeted degradation of refractory organic compounds in wastewaters based on molecular imprinting catalysts

Efficient removal of low-concentration refractory pollutants is a crucial problem to ensuring water safety. The use of heterogeneous catalysis of molecular imprinting technology combined with traditional catalysts is a promising method to improve removal efficiency. Presently, the research into molecular imprinting targeting catalysts focuses mainly on material preparation and performance optimization. However, more researchers are investigating other applications of imprinting materials. This review provides recent progress in photocatalyst preparation, electrocatalyst, and Fenton-like catalysts synthesized by molecular imprinting. The principle and control points of target catalysts prepared by precipitation polymerization (PP) and surface molecular imprinting (S-MIP) are introduced. Also, the application of imprinted catalysts in targeted degradation of drugs, pesticides, environmental hormones, and other refractory pollutants is summarized. In addition, the reusability and stability of imprinted catalyst in water treatment are discussed, and the possible ecotoxicity risk is analyzed. Finally, we appraised the prospects, challenges, and opportunities of imprinted catalysts in the advanced oxidation process. This paper provides a reference for the targeted degradation of refractory pollutants and the preparation of targeted catalysts.

Competitive near-infrared PEC immunosorbent assay for monitoring okadaic acid based on a disposable flower-like WO3-Modified screen-printed electrode

The long-term toxic effects of okadaic acid (OA) in shellfish pose a serious threat to public health, negatively impacting the development of the shellfish aquaculture industry. In this study, a novel competitive near-infrared-mediated photoelectrochemical immunosorbent assay (cNIR-PECIA) was developed for ultrasensitive and highly selective detection of OA based on NaYF₄:Yb, Tm upconversion nanophosphors (UCNPs) and a flower-like WO₃-modified screen-printed electrode (FL-WO₃ SPE). The UCNPs function as a self-powder to convert NIR excitation into visible emissions. FL-WO₃ fully utilizes the visible illumination and induces the separation of electron-hole pairs, thus generating a photocurrent. After conjugating monoclonal antibodies against OA on UCNPs (UCNPs-Ab), the bright PEC immunoprobe selectively captured OA molecules, which were then determined by a competitive indirect immunosorbent assay. Under optimal conditions, the 50% inhibitory concentration of the immunosensor was 0.09 ng mL^-1. The OA concentration had a linear relationship with the antibody binding rate in the range of 0.01-60 ng mL^-1 with an extremely low detection limit of 0.007 ng mL^-1. Finally, the proposed cNIR-PECIA was successfully utilized to analyze OA content in mussel samples. This study affords new ideas for constructing NIR PEC sensors by using upconversion luminescent materials to match semiconductors. The superior sensing properties indicate their potential applicability in food safety analysis.