Integrating anodic sulfate activation with cathodic H2O2 production/activation to generate the sulfate and hydroxyl radicals for the degradation of emerging organic contaminants

Defects-rich Ru-doped black TiO2 nanotube arrays for photoelectrochemical levofloxacin degradation coupled with simultaneous cathodic H2 production

As an emerging and promising technology, the bifunctional photoelectrocatalytic (PEC) systems have shown remarkable potential in treating wastewater and producing energy. A central critical challenge in this field is the development of high-performance electrode materials that exhibit superior PEC properties. In this work, the defect-rich Ru-doped black TiO2 nanotube arrays (Ru-BTNAs) bifunctional electrodes were engineered and utilized in a PEC system, aiming to achieve efficient antibiotics levofloxacin degradation and hydrogen production simultaneously. In-depth characterization characterizations and the Density functional theory (DFT) calculations reveal that the synergistic effect between Ti3+-oxygen vacancies (Ovs) defects and Ru doping significantly improves light absorption, accelerates the separation and transmission of photoexcited e--h+ pairs, and optimizes PEC performance. The coupled photocatalytic and electrocatalytic processes enhance the generation of h+, 1O2, HO•, and SO4•- radicals, which effectively degrade levofloxacin. The abundant Ovs facilitate electron transfer from BTNAs to Ru, accelerating hydrogen evolution reaction (HER) on electron-rich Ru at a low overpotential. This work provides a theoretical framework for designing bifunctional electrode to achieve the energy-efficient hydrogen production from antibiotics-contaminated wastewater.

Predicting the Fate of Bisphenol A During Electrochemical Oxidation: A Simple Semiempirical Method Based on the Concentration Profile of Hydroxyl Radicals

The efficiency of electrochemical advanced oxidation processes (EAOPs) is fundamentally governed by hydroxyl-radical (•OH) generation. While direct experimental measurements of these transient species remain complex and impractical, robust computational methods for predicting their temporal profiles are notably scarce. This work presents a semi-empirical methodology based on H2O2 measuring experiments that enables indirect •OH quantification. We employed a recently developed carbon-based electrode and the priority pollutant bisphenol A (BPA) as the model system. The system achieved 92.3% BPA degradation with 84% mineralization efficiency during 5-h electrooxidation at 15 mA/cm2. Gas chromatography/mass spectrometry (GC/MS) was used for tracking BPA and detection of intermediates. On this basis, we developed a computational model that successfully predicts temporal concentration profiles of all reactive species interacting with •OH, along with degradation kinetics across current densities (10-20 mA/cm2). By incorporating predictions from the Toxicity Estimation Software Tool (T.E.S.T.), the developed model accurately simulates time-dependent evolution of relative toxicity throughout the treatment process. The presented approach has a general character and requires rather simple experimental input to predict and optimize degradation outcome in terms of input concentration, degradation time, current density, and final toxicity. Further modifications of the model would enable widening to other EAOPs systems.

Per-and polyfluoroalkyl degradation in a hybrid dielectric barrier discharge plasma and electrooxidation system through involving more reactive species by air and water circulation

The presence of PFAS in water matrices has become a global environmental issue in the last half-century. Dielectric barrier discharge (DBD) and electrooxidation (EO) showed potential for PFAS degradation but have yet to find practical application due to relatively high energy consumption. In this study, a hybrid DBD-EO system for efficient degradation of PFAS was developed by involving more reactive oxygen, sulfate radicals (SO4•-) and nitrogen species (RONS). The results showed that using the hybrid DBD-EO system under optimal conditions (applied voltage = 6 kV and current density = 7.5 mA/cm2) could increase PFOA degradation efficiency from 65.0 % (DBD) and 62.5 % (EO) to 89.14 %. While the EE/O decreased from 67.0 kWh/m3 (DBD) and 47.82 kWh/m3 (EO) to 21.61 kWh/m3. In addition, the effect of operational parameters and water matrices revealed that the hybrid DBD-EO system had high potential for PFOA removal from water under various conditions. According to the EPR and DFT calculation results, integration of reactive species in EO (SO4•-, •OH, O2•-) and ONOOH) and DBD (•OH, O2•-, NO2•-, 1O2 and ONOOH) processes in the DBD-EO system led to efficient degradation of PFOA through a mechanism of decarboxylation/defluorination cycle. Our findings suggested the combination of DBD and EO is a promising approach for complete degradation of PFAS from water with low energy consumption and minimal environmental side effects.

Regulating the Electronegativity Difference and Piezoelectric Strain of the S-Mo-S Structure via Introducing Mo Vacancies for Boosting Piezo-Photoelectric Activity

Recently, piezoelectric and photocatalytic processes have shown excellent synergistic effect addressing environmental remediation challenges. Herein, a nanoflower-like Mo vacancy-modulated MoS2 (VMo-MoS2) piezo-photocatalyst with different VMo densities has been successfully synthesized using a one-step hydrothermal method. The high VMo density (12%) facilitates the enhancement of the photocatalytic activity but compromises its structural stability, resulting in unsatisfactory piezoelectric activity. Among all VMo-MoS2 piezo-photocatalysts, VMo-MoS2 with 6% VMo density exhibits the highest piezo-photocurrent density (15.50 μA cm-2), the largest potential difference (0.188 V), and the best carbamazepine (CBZ) degradation efficiency (95.81%) in only 10 min under light-ultrasound action, exhibiting a remarkable synergistic effect of the piezoelectric and photocatalytic processes. The synergistic performance originates from the simultaneous modulation of the charge distribution and the self-polarization capability of the S-Mo-S structure by VMo, as confirmed by the molecular theory calculations and finite-element simulation results. This work provides a defect engineering strategy for achieving the synergistic effect of the piezoelectric and photocatalytic processes, which opens a new research avenue for the design and application of the piezo-photocatalyst.

High H2O2 production in membrane-free electrolyzer via anodic bubble shielding towards robust rural disinfection

Hydrogen peroxide (H2O2) can be sustainably synthesized through the electrochemical oxygen reduction reaction in a dual-chamber water electrolyzer separated by expensive ion exchange (IX) membranes. The development of an IX membrane-free electrolyzer has been limited by direct anodic degradation of the produced H2O2. Here, we devise a bubble shielding strategy by using a low-cost polytetrafluoroethylene hydrophobic porous layer (HPL) on the anode that enables numerous sites for anodically generated oxygen bubbles and significantly suppresses H2O2 degradation in the electrolyte. The H2O2 production increases by ~600% compared to that using non-bubble shielded anode. A high H2O2 concentration of 10.05 ± 0.05 g L-1 at 40 mA cm-2 can be obtained with both HPL-coated anode and cathode. A solar-driven disinfection device equipped with HPL-coated electrodes achieves >99.9% E. coli inactivation within 60 min. This innovative approach for achieving high electrochemical H2O2 concentrations in IX membrane-free electrolyzers more generally provides insights for fine tuning three-phase interfaces and could be applicable to other reactions pathways in electrochemical applications.

Principles, challenges and prospects for electro-oxidation treatment of oilfield produced water

The oil industry is facing substantial environmental challenges, especially in managing waste streams such as Oilfield Produced Water (OPW), which represents a significant component of the industrial ecological footprint. Conventional treatment methods often fail to effectively remove dissolved oils and grease compounds, leading to operational difficulties and incomplete remediation. Electrochemical oxidation (EO) has emerged as a promising alternative due to its operational simplicity and ability to degrade pollutants directly and indirectly, which has already been applied in treating several effluents containing organic compounds. The application of EO treatment for OPW is still in an initial stage, due to the intricate nature of this matrix and scattered information about it. This study provides a technological overview of EO technology for OPW treatment, from laboratory scale to the development of large-scale prototypes, identifying design and process parameters that can potentially permit high efficiency, applicability, and commercial deployment. Research in this domain has demonstrated notable rates of removal of recalcitrant pollutants (>90%), utilizing active and non-active electrodes. Electro-generated active species, primarily from chloride, play a pivotal role in the oxidation of organic compounds. However, the highly saline conditions in OPW hinder the complete mineralization of these organics, which can be improved by using non-active anodes and lower salinity levels. The performance of electrodes greatly influences the efficiency and effectiveness of OPW treatment. Various factors must be considered when selecting the electrode material, such as its conductivity, stability, surface area, corrosion resistance, and cost. Additionally, the specific contaminants present in the OPW, and their electrochemical reactivity must be considered to ensure optimal treatment outcomes. Balancing these considerations can be challenging, but it is crucial for achieving successful OPW treatment. Active electrode materials exhibit a high affinity for chloride molecules, generating more active species than non-active materials, which exhibit more significant degradation potential due to the production of hydroxyl radicals. Regarding scale-up, key challenges include low current efficiency, the formation of by-products, electrode deactivation, and limitations in mass transfer. To address these issues, enhanced mass transfer rates and appropriate residence times can be achieved using flow-through mesh anodes and moderate current densities, which have proven to be the optimal configuration for this process.

Electrochemical degradation of diclofenac generates unexpected thyroidogenic transformation products: Implications for environmental risk assessment

Diclofenac (DCF) is an environmentally persistent, nonsteroidal anti-inflammatory drug (NSAID) with thyroid disrupting properties. Electrochemical advanced oxidation processes (eAOPs) can efficiently remove NSAIDs from wastewater. However, eAOPs can generate transformation products (TPs) with unknown chemical and biological characteristics. In this study, DCF was electrochemically degraded using a boron-doped diamond anode. Ultra-high performance liquid chromatography coupled with high-resolution mass spectrometry was used to analyze the TPs of DCF and elucidate its potential degradation pathways. The biological impact of DCF and its TPs was evaluated using the Xenopus Eleutheroembryo Thyroid Assay, employing a transgenic amphibian model to assess thyroid axis activity. As DCF degradation progressed, in vivo thyroid activity transitioned from anti-thyroid in non-treated samples to pro-thyroid in intermediately treated samples, implying the emergence of thyroid-active TPs with distinct modes of action compared to DCF. Molecular docking analysis revealed that certain TPs bind to the thyroid receptor, potentially triggering thyroid hormone-like responses. Moreover, acute toxicity occurred in intermediately degraded samples, indicating the generation of TPs exhibiting higher toxicity than DCF. Both acute toxicity and thyroid effects were mitigated with a prolonged degradation time. This study highlights the importance of integrating in vivo bioassays in the environmental risk assessment of novel degradation processes.

Investigation of singlet oxygen and superoxide radical produced from vortex-based hydrodynamic cavitation: Mechanism and its relation to cavitation intensity

Presently, the hydroxyl radical oxidation mechanism is widely acknowledged for the degradation of organic pollutants based on hydrodynamic cavitation technology. The presence and production mechanism of other potential reactive oxygen species (ROS) in the cavitation systems are still unclear. In this paper, singlet oxygen (1O2) and superoxide radical (·O2-) were selected as the target ROS, and their generation rules and mechanism in vortex-based hydrodynamic cavitation (VBHC) were analyzed. Computational fluid dynamics (CFD) were used to simulate and analyze the intensity characteristics of VBHC, and the relationship between the generation of ROS and cavitation intensity was thoroughly revealed. The results show that the operating conditions of the device have a significant and complicated influence on the generation of 1O2 and ·O2-. When the inlet pressure reaches to 4.5 bar, it is more favorable for the generation of 1O2 and ·O2- comparing with those lower pressure. However, higher temperature (45 °C) and aeration rate (15 (L/min)/L) do not always have positive effect on the 1O2 and ·O2- productions, and their optimal parameters need to be analyzed in combination with the inlet pressure. Through quenching experiments, it is found that 1O2 is completely transformed from ·O2-, and ·O2- comes from the transformation of hydroxyl radicals and dissolved oxygen. Higher cavitation intensity is captured and shown more disperse in the vortex cavitation region, which is consistent with the larger production and stronger diffusion of 1O2 and ·O2-. This paper shed light to the generation mechanism of 1O2 and ·O2- in VBHC reactors and the relationship with cavitation intensity. The conclusion provides new ideas for the research of effective ROS in hydrodynamic cavitation process.

A review of electrooxidation systems treatment of poly-fluoroalkyl substances (PFAS): electrooxidation degradation mechanisms and electrode materials

The prevalence of polyfluoroalkyls and perfluoroalkyls (PFAS) represents a significant challenge, and various treatment techniques have been employed with considerable success to eliminate PFAS from water, with the ultimate goal of ensuring safe disposal of wastewater. This paper first describes the most promising electrochemical oxidation (EO) technology and then analyses its basic principles. In addition, this paper reviews and discusses the current state of research and development in the field of electrode materials and electrochemical reactors. Furthermore, the influence of electrode materials and electrolyte types on the deterioration process is also investigated. The importance of electrode materials in ethylene oxide has been widely recognised, and therefore, the focus of current research is mainly on the development of innovative electrode materials, the design of superior electrode structures, and the improvement of efficient electrode preparation methods. In order to improve the degradation efficiency of PFOS in electrochemical systems, it is essential to study the oxidation mechanism of PFOS in the presence of ethylene oxide. Furthermore, the factors influencing the efficacy of PFAS treatment, including current density, energy consumption, initial concentration, and other parameters, are clearly delineated. In conclusion, this study offers a comprehensive overview of the potential for integrating EO technology with other water treatment technologies. The continuous development of electrode materials and the integration of other water treatment processes present a promising future for the widespread application of ethylene oxide technology.

Low energy consumption pathway to improve sulfamethoxazole degradation by carbon fiber@Fe3O4-CuO: Electrocatalysis activity, mechanism and toxicity

Catalysts play a pivotal role in advanced oxidation processes for the remediation of organic wastewater. In this study, a 3D carbon fiber@Fe3O4-CuO catalyst was fabricated, and its efficacy for persulfate activation to remove sulfamethoxazole (SMX) was investigated at extremely low current density. The results of characterization revealed that the catalyst was uniformly distributed on the carbon fiber, and the loaded catalyst was Fe3O4-CuO nanoparticles with a diameter range of 20-50 nm. The SMX removal rate was significantly enhanced at extremely low current density by the metallic oxide catalyst loaded on carbon fiber. Approximately 90 % of SMX was degraded within 90 min when the electric current density was set at 0.1 mA cm-2. This modification process not only improved the persulfate activation efficiency but also enhanced the generation of hydrogen peroxide. Both radical and non-radical pathways were involved in the degradation of SMX. The degradation pathway mainly included hydroxylation, carboxylation, aniline cleavage, and desulfonation reactions. The quantitative structure-activity relationship model indicated that the potential risk of intermediate products to fish, daphnia, and green algae significantly decreased during the electrocatalytic oxidation process. This study provides a novel strategy for persulfate activation, which can significantly enhance the degradation efficiency, toxicity abatement, and energy usage effectiveness of electrocatalytic technology.

Scientometric analysis of electrocatalysis in wastewater treatment: today and tomorrow

Electrocatalytic methods are valuable tools for addressing water pollution and scarcity, offering effective pollutant removal and resource recovery. To investigate the current status and future trends of electrocatalysis in wastewater treatment, a detailed analysis of 9417 papers and 4061 patents was conducted using scientometric methods. China emerged as the leading contributor to publications, and collaborations between China and the USA have emerged as the most frequent partnerships. Primary article co-citation clusters focused on oxygen evolution reaction and electrochemical oxidation, transitioning towards advanced oxidation processes ("persulfate activation"), and electrocatalytic reduction processes ("nitrate reduction"). Bifunctional catalysts, theoretical calculations, electrocatalytic combination technologies, and emerging contaminants were identified as current research hotspots. Patent analysis revealed seven types of electrochemical technologies, which were compared using SWOT analysis, highlighting electrochemical oxidation as prominent. The technological evolution presented the pathway of electro-Fenton to combined electrocatalytic technologies with biochemical processes, and finally to coupling with electrocoagulation. Standardized evaluation systems, waste resource utilization, and energy conservation were important directions of innovation in electrocatalytic technologies. Overall, this study provided a reference for researchers to understand the framework of electrocatalysis in wastewater treatment and also shed light on potential avenues for further innovation in the field.

o-Semiquinone Radical and o-Benzoquinone Selectively Degrade Aniline Contaminants in the Periodate-Mediated Advanced Oxidation Process

Advanced oxidation processes (AOPs) often employ strong oxidizing inorganic radicals (e.g., hydroxyl and sulfate radicals) to oxidize contaminants in water treatment. However, the water matrix could scavenge the strong oxidizing radicals, significantly deteriorating the treatment efficiency. Here, we report a periodate/catechol process in which reactive quinone species (RQS) including the o-semiquinone radical (o-SQ•-) and o-benzoquinone (o-Q) were dominant to effectively degrade anilines within 60 s. The second-order reaction rate constants of o-SQ•- and o-Q with aniline were determined to be 1.0 × 108 and 4.0 × 103 M-1 s-1, respectively, at pH 7.0, which accounted for 21% and 79% of the degradation of aniline with a periodate-to-catechol molar ratio of 1:1. The major byproducts were generated via addition or polymerization. The RQS-based process exhibited excellent anti-interference performance in the degradation of aniline-containing contaminants in real water samples in the presence of diverse inorganic ions and organics. Subsequently, we extended the RQS-based process by employing tea extract and dissolved organic matter as catechol replacements as well as metal ions [e.g., Fe(III) or Cu(II)] as periodate replacements, which also exhibited good performance in aniline degradation. This study provides a novel strategy to develop RQS-based AOPs for the highly selective degradation of aniline-containing emerging contaminants.

A state-of-art-review on emerging contaminants: Environmental chemistry, health effect, and modern treatment methods

Pollution problems are increasingly becoming e a priority issue from both scientific and technological points of view. The dispersion and frequency of pollutants in the environment are on the rise, leading to the emergence have been increasing, including of a new class of contaminants that not only impact the environment but also pose risks to people's health. Therefore, developing new methods for identifying and quantifying these pollutants classified as emerging contaminants is imperative. These methods enable regulatory actions that effectively minimize their adverse effects to take steps to regulate and reduce their impact. On the other hand, these new contaminants represent a challenge for current technologies to be adapted to control and remove emerging contaminants and involve innovative, eco-friendly, and sustainable remediation technologies. There is a vast amount of information collected in this review on emerging pollutants, comparing the identification and quantification methods, the technologies applied for their control and remediation, and the policies and regulations necessary for their operation and application. In addition, This review will deal with different aspects of emerging contaminants, their origin, nature, detection, and treatment concerning water and wastewater.