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Open research questions in Advanced oxidation water treatment

32 unresolved questions extracted from the limitations and future-work sections of 475 Advanced oxidation water treatment papers in our library. Each links back to the study that raised it.

What the literature leaves open

  • Future work should focus on scaling up the process to pilot-scale applications and evaluating its performance in real wastewater matrices, where varying conditions may influence efficiency; investi- gating catalyst stability and reusability will also be essential to assess long-term applicability.

    Eco-friendly AOP for Amoxicillin removal: hybrid optimization, by-products identification via QUECHERS-HRMS technique, and toxicity assessment · 2026 · DOI

  • In order to develop a green and environmentally friendly catalyst for the persulfate system, this study used sewage treatment sludge containing iron and manganese as a raw material to prepare activated carbon containing Fe and Mn by pyrolysis, and used it to activate the persulfate system to degrade BaP in sediments. ■ Biochar with catalytic properties was prepared by pyrolysis of iron-manganese sludge from a sewage treatment plant. Different pyrolysis temperatures and times were used in the experiment, and the optimal preparation conditions were determined by compari- son: 800 °C for 2 h. ■ The processed sludge biochar was used to activate persulfate for the degradation of benzo[a]pyrene (BaP) in sediments. The experiment examined the impact of variables like catalyst quantity, persulfate concentration, and reaction pH on degradation ef- ficiency. The findings indicated that, under ideal cir- cumstances, the BaP removal rate was 99.85%. ■ The research, using material characterization and quenching tests, identified active species including •SO₄⁻, •OH, and ¹O₂ in this environment, which to- gether facilitated the degradation of BaP via both radical and non-radical mechanisms. In addition, the presence of iron and manganese oxides and func- tional groups on the material surface significantly enhanced the catalytic activity. This study successfully prepared a biochar catalyst that can efficiently activate PMS to degrade BaP using sew- age treatment sludge containing iron and manganese as raw materials. This provides a new method for controlling polycyclic aromatic hydrocarbon pollution in sediments and developing catalysts for the persulfate system. At the same time, there are also shortcomings in our experiment: First, the experiment was carried out under laboratory conditions, and environmental variables such as temperature, humidity, and actual pollutant concentrations may be different in the real environment, which leads to the need for further verification of the practical applicabil- ity of the experimental results. Second, there is insufficient research on the long-term application effects. Although the system has shown high degradation efficiency in a short period of time, the stability and regeneration ability of the catalyst and its potential impact on the environment have not yet been explored. Finally, there is a lack of research on complex pollutants. This study focused on the degra- dation of BaP, but pollutants in the real environment are often a mixture of various organic compounds. In the fu- ture, research should be conducted on the performance of this catalytic system when multiple organic pollutants are present at the same time. Based on the above three con- siderations, we believe that future research can be further extended to experiments in real environments to verify the performance of the catalyst under complex environmen- tal conditions and optimize the operating conditions for practical applications. In order to improve economic and environmental sustainability, future research could investi- gate the regeneration and reuse of the catalyst to reduce costs and minimise waste. Future research could explore the synergistic removal of different PAHs and other organic pollutants by the activated sludge biochar persulfate sys- tem, thereby expanding its scope of application and im- proving its practical treatment capabilities.

    Sludge-based catalysts for the remediation of PAH-contaminated sediments · 2026 · DOI

  • Electrochemical oxidation employing lead dioxide (PbO2) electrodes is a promising approach for degrading refractory pollutants; however, its application for Azithromycin removal remains unexplored.

    Enhanced Electrocatalytic Degradation of Azithromycin using an SDS-Modified PbO₂ Anode: Performance, Mechanism, and Intermediate Analysis · 2026 · DOI

  • Utilising AOPs to create ZNPs has great potential for treating wastewater. By using naturally occurring, non-toxic, and recyclable zeolites, this technique incorporates green chemistry concepts and supports sustainability objectives. Nanotechnology developments will enable ZNPs to have customised characteristics, increasing their effectiveness in removing a variety of contaminants. Zeolite nanoparticles and AOPs combined in hybrid water treatment systems may improve efficiency and lower energy usage. The S. Palanisamy et al.: A review of zeolite nanoparticles in wastewater treatment 299 developments in real-time monitoring and intelligent, responsive nanomaterials. Even if cost and scalability are still issues, more study indicates that ZNPs could become a crucial component of sustainable wastewater treatment, treating a wide variety of contaminants.

    A review of zeolite nanoparticles in wastewater treatment for sustainable water management in the advanced oxidation process application · 2026 · DOI

  • Despite five decades of work, several practical issues continue to limit the deployment of inorganic-chemistry methods for dye treatment. Real textile effluents contain salt loads, surfactants, sizing agents and natural organic matter that compete for adsorbent sites and scavenge radicals; performance figures obtained on a single dye in deionised water rarely survive translation to a real effluent matrix without recalibration. Catalyst recovery from small particle sizes, and the prevention of secondary metal leaching from Fe-, Cu- and Co-based catalysts into the treated water, remain demanding outside structuredsupport formats. Cost is a barrier for MOFs and for BDDanode electrochemistry at full scale; energy demand limits the deployment of electrocatalytic oxidation in regions with expensive electricity. The toxicity of degradation intermediates particularly the aromatic amines produced by anaerobic or reductive cleavage of azo dyes is well documented but routinely under-monitored in laboratory studies than mineralisation. that report only decolorisation rather Several themes are likely to dominate the next decade. Solar-driven photocatalysts with truly visible-light response and low cost (g-C₃N₄, Bi-based oxides, Fe- MOFs) are an active frontier. Sulfate-radical AOPs are expanding rapidly because they extend the operating pH range of Fenton chemistry. Hybrid systems adsorbents that double as Fenton supports, photocatalysts that double as adsorbents, electrocoagulation followed by photocatalytic polishing are increasingly the norm. And the integration of through in-situ monitoring the nanomaterial-sensing approaches discussed in our companion reviews is, in our view, the most likely route by which laboratory performance will translate into deployed technology. XI. CONCLUSION drive electro-Fenton electrocoagulation, semiconductors and photocatalytic Inorganic chemistry supplies the central toolkit for the treatment of dye-polluted water. Metal oxides, layered double hydroxides, clays and mesoporous silicas drive their adsorption; metal-oxide degradation; heterojunctions transition-metal redox couples drive Fenton and sulfateradical advanced oxidation processes; metal nodes in coordination polymers drive selective adsorption and multifunctional catalysis; and electrochemical interfaces and drive electrocatalytic oxidation. The five methods are complementary rather than competing: each handles a different combination of dye class, concentration, matrix and budget, and the most effective modern treatment trains combine two or three of them on a single platform. The trajectory is unambiguous.

    Inorganic-Chemistry Approaches to the Treatment of Dye-Polluted Water: A Comparative Review of Five Methods · 2026 · DOI

  • Abstract Transformations in metal coordination environments, particularly their impact on spin-state transitions, remain underexplored in peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs).

    Spin-selection-favored peroxymonosulfate activation on reaction-induced Co–N1O2 sites enables 1O2-biased yet radical-parallel oxidation · 2026 · DOI

  • ABSTRACT The practical application of the Fenton reaction is limited by the excessive generation of iron sludge, which results from the inefficient regeneration of ferrous ions during the Fe III /Fe II redox cycle.

    Pd Location–Dependent Confinement Effects in UiO‐66(Zr) Toward Catalytic Degradation of Carbamazepine · 2026 · DOI

  • Future studies should focus on (i) continuous flow pilot trials, (ii) characterization and dewatering of Fe rich sludge; (iii) reusability of the sludge as a coagulant or Fenton catalyst; and (iv) multivariate optimization using RSM to capture interaction effects.

    Mechanistic Optimization of FeSO4 for Synthetic Petroleum Wastewater Treatment · 2026 · DOI

  • Synergistic catalysis for the co-removal of NOx and VOCs has evolved from a fundamental curiosity to a technology nearing commercial reality, driven by increasingly stringent air quality policies and catalyt- ic innovations. In this mini review, we have dis- cussed the synergistic catalytic elimination of NOx and VOCs, critically analyzing how engineered acid-redox bifunctionality and dynamic active site cooperation can overcome inherent challenges such as temperature-window mismatch and competitive adsorption in complex flue gases. However, several critical aspects require further elucidation to advance the rational design and performance of these catalysts. These include the precise engineering of reactive sites, optimal tuning of acid-redox proper- ties, enhancement of poisoning resistance, and resolving the cost-performance dilemma. Novel catalyst design strategies are urgently needed to address the core challenges of tempera- ture-window mismatch and competitive adsorption. A primary focus is the rational design and construc- tion of multifunctional active sites to minimize com- petitive interactions between different pollutant molecules. Here, ML shows immense potential for accelerating the discovery of optimal bimetallic pairs tailored for specific NOx-VOCs combinations. Integrating ML with high-throughput experimental screening will create a powerful, efficient pipeline for next-generation catalyst discovery. Furthermore, dynamic active site regulation is emerging as a promising frontier. The goal is to develop smart, self- -adapting catalysts capable of switching their redox states in response to fluctuating flue-gas composi- tions, thereby dynamically mitigating competitive interference. Developing advanced poison-resistance strategies is paramount to ensure the long-term dura- bility and robustness of synergistic catalysts under harsh, realistic industrial flue gas conditions. A key insight from the reviewed studies is the power of fully utilizing catalyst interfaces to skillfully con- struct dual or multiple active sites on a single cat- alyst. This strategy enables the specific and simulta- neous conversion of different pollutant molecules within the same spatiotemporal domain, achieving true synergistic multipollutant control. Importantly, this paradigm can be extended beyond traditional pollutants. It holds promise for the synergistic cataly- sis of traditional and emerging pollutants (for exam- ple, NOx with dichloromethane, CH2Cl2) or even greenhouse gases such as CO2. Looking further ahead, the rational design of synergistic sites could pave the way for the hierarchical and intelligent conversion of small molecules, such as CO2, to val- ue-added products. In conclusion, the synergistic cat- alytic strategy represents a transformative approach with the potential to fundamentally address energy and environmental challenges. Chen et al. Greenverse Sci. 2026, 1, 7 Page 17 of 21 DECLARATIONS Authors’ contributions Conceptualization: Chen, A. Writing - original draft preparation: Chen, A. Writing - review & editing: Chen, A.; Gao, M.; Wang, F.; Jiang, X.; Qu, W.; Shen, Y.; Palacio, R. Supervision: Chen, A. Project administration: Chen, A. Funding acquisition: Chen, A.; Wang, F. Investigation: Gao, M.; Wang, F.; Jiang, X.; Shen, Y.; Palacio, R. Data curation: Gao, M.; Jiang, X. Visualization: Gao, M. Validation: Wang, F.; Jiang, X. Methodology: Qu, W.

    Synergistic catalytic strategies for multipollutant control · 2026 · DOI

  • The primary objective of this study was to evaluate the effectiveness of the electro- chemical-Fenton (ECF) process for the removal of Cr(VI) and COD from real tannery wastewater under optimized operational conditions. The experimental results dem- onstrate that the optimized ECF system achieved substantial removal efficiencies of 91.25% for Cr(VI) and 96.2% for COD, confirming the capability of the process to sig- nificantly improve effluent quality. The strong agreement between experimental and model-predicted values further validates the reliability of the RSM-based optimiza- tion approach, indicating that the electrochemical-Fenton process is a promising and scalable treatment technology for chromium-rich industrial wastewater. However, the present study was conducted at laboratory scale, and variations in wastewater composition under real industrial conditions may influence treatment performance. In addition, long-term electrode stability, operational costs, and energy consumption were not comprehensively evaluated, which may affect the economic feasibility of full-scale implementation. Therefore, future studies should focus on pilot-scale validation, long-term performance assessment, and techno-economic anal- ysis to better understand the process viability. Further research should also emphasize optimizing operating factors to enhance removal efficiencies while minimizing energy requirements. Integrating the ECF process with complementary treatment technolo- gies and exploring renewable energy-driven or green oxidant-based systems may fur- ther improve treatment sustainability and operational feasibility.

    Optimization of the electrochemical Fenton process for hexavalent chromium removal from tannery wastewater · 2026 · DOI

  • Ceramic electrodes based on Sb-doped SnO2 for removal of emerging compounds in wastewater need comparative studies across different pollutant types and concentrations.

    Treatment of petroleum refinery wastewater by photo-anodic oxidation process with Gr /Bi-Ni-Sb-SnO2 rotating photo-anode: Performance and kinetic study · 2026 · DOI

  • Enhanced photoelectrocatalytic degradation using complex bifunctional hybrid platforms combining TiO2 nanotubes with Ni-Sb-SnO2 electrocatalysts needs further validation in industrial applications.

    Treatment of petroleum refinery wastewater by photo-anodic oxidation process with Gr /Bi-Ni-Sb-SnO2 rotating photo-anode: Performance and kinetic study · 2026 · DOI

  • BiVO4-based photoanodes for photoelectrocatalytic removal of trace organic pollutants from water need recent developments and optimization beyond current literature.

    Treatment of petroleum refinery wastewater by photo-anodic oxidation process with Gr /Bi-Ni-Sb-SnO2 rotating photo-anode: Performance and kinetic study · 2026 · DOI

  • Heterogeneous high-valent cobalt-oxo [≡Co(IV)=O] is a widely focused reactive species in oxidant activation; however, the relationship between the catalyst interfacial defects and ≡Co(IV)=O formation remains poorly understood.

    Utilizing the oxygen-atom trapping effect of Co <sub>3</sub> O <sub>4</sub> with oxygen vacancies to promote chlorite activation for water decontamination · 2024 · DOI

  • The studies on the origin of versatile oxidation pathways toward targeted pollutants in the single-atom catalysts (SACs)/peroxymonosulfate (PMS) systems were always associated with the coordination structures rather than the perspective of pollutant characteristics, and the analysis of mechanism commonality is lacking.

    Fenton-like activity and pathway modulation via single-atom sites and pollutants comediates the electron transfer process · 2024 · DOI

  • Coordination engineering of high-valent Fe(IV)-oxo may break the activity-selectivity trade-off of traditional reactive oxygen species but the controllable generation of heterogeneous FeIV=O remains unexplored.

    Coordination engineering of heterogeneous high-valent Fe(IV)-oxo for safe removal of pollutants via powerful Fenton-like reactions · 2024 · DOI

  • Coordination engineering of high-valent Fe(IV)-oxo (FeIV=O) is expected to break the activity-selectivity trade-off of traditional reactive oxygen species, while attempts to regulate the oxidation behaviors of heterogeneous FeIV=O remain unexplored.

    Coordination engineering of heterogeneous high-valent Fe(IV)-oxo for safe removal of pollutants via powerful Fenton-like reactions · 2024 · DOI

  • Despite extensive research on iron-based Fenton systems, the application of manganese oxides, particularly α-Mn2O3, in chlorpyrifos degradation remains insufficiently explored.

    Fenton and Photo-Fenton Degradation of Chlorpyrifos Using α-Mn2O3 Heterogeneous Catalysis · 2026 · DOI

  • Future research will focus on enhancing electrode stability through surface modifica- tion or protective coatings to improve long-term durabil- ity during repeated electrochemical operation.

    Electrochemical Degradation of Synthetic Wastewater Containing Reactive Red 4 Using Mixed Metal Oxide Composite Electrodes · 2026 · DOI

  • Literature shows optimization of operational parameters for various pollutants, but comprehensive kinetic studies comparing Iron and Cerium-loaded activated carbon catalysts for petroleum refinery wastewater are limited.

    Petroleum refinery wastewater degradation by heterogeneous-electro-fenton process using activated carbon loaded with Iron and Cerium: A kinetic study · 2026 · DOI

  • Studies on heterogeneous electro-Fenton degradation have primarily focused on individual pollutants (tetracycline, sulfamethoxazole, acetaminophen, rhodamine B) rather than complex petroleum refinery wastewater mixtures.

    Petroleum refinery wastewater degradation by heterogeneous-electro-fenton process using activated carbon loaded with Iron and Cerium: A kinetic study · 2026 · DOI

  • Recent advances in heterogeneous electro-Fenton processes show trends toward bifunctional and multi-component catalysts, suggesting gaps in understanding optimal catalyst compositions for petroleum refinery wastewater specifically.

    Petroleum refinery wastewater degradation by heterogeneous-electro-fenton process using activated carbon loaded with Iron and Cerium: A kinetic study · 2026 · DOI

  • A review of electro-Fenton and ultrasound processes suggests a need for integration of these technologies, indicating that combined novel integrated technology for wastewater treatment remains underdeveloped.

    Petroleum refinery wastewater degradation by heterogeneous-electro-fenton process using activated carbon loaded with Iron and Cerium: A kinetic study · 2026 · DOI

  • The photoelectrocatalytic decomplexation of metal-chelate complexes and simultaneous metal recovery using bifunctional photoanodes requires optimization for practical wastewater treatment.

    Treatment of petroleum refinery wastewater by photo-anodic oxidation process with Gr /Bi-Ni-Sb-SnO2 rotating photo-anode: Performance and kinetic study · 2026 · DOI

  • Redirecting the pathways of carbon evolution from molecular fragmentation to polymerization is critical for energy harvesting during chemical oxidation, yet the regulation means remain to be exploited.

    Carbon redirection via tunable Fenton-like reactions under nanoconfinement toward sustainable water treatment · 2024 · DOI

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32 open questions have been extracted from the limitations and future-work passages of 475 Advanced oxidation water treatment papers in our library. Each one below links back to the study that raised it, so you can read the original claim in context.

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