On the design of a jet-aerated microfluidic flow-through reactor for wastewater treatment by electro-Fenton

A Critical Review of Deep Oxidation of Gaseous Volatile Organic Compounds via Aqueous Advanced Oxidation Processes

Volatile organic compounds (VOCs) are considered to be the most recalcitrant gaseous pollutants due to their high toxicity, diversity, complexity, and stability. Gas-solid catalytic oxidation methods have been intensively studied for VOC treatment while being greatly hampered by energy consumption, catalyst deactivation, and byproduct formation. Recently, aqueous advanced oxidation processes (AOPs) have attracted increasing interest for the deep oxidation of VOCs at room temperature, owing to the generation of abundant reactive oxygen species (ROS). However, current reviews mainly focus on VOC degradation performance and have not clarified the specific reaction process, degradation products, and paths of VOCs in different AOPs. This study systematically reviews recent advances in the application of aqueous AOPs for gaseous VOC removal. First, the VOC gas-liquid mass transfer and chemical oxidation processes are presented. Second, the latest research progress of VOC removal by various ROS is reviewed to study their degradation performances, pathways, and mechanisms. Finally, the current challenges and future strategies are discussed from the perspectives of synergistic oxidation of VOC mixtures, accurate oxidation, and resource utilization of target VOCs via aqueous AOPs. This perspective provides the latest information and research inspiration for the future industrial application of aqueous AOPs for VOC waste gas treatment.

Oxidation of organics in water by active chlorine performed in microfluidic electrochemical reactors: a new way to improve the performances of the process

Wastewater polluted by organics can be treated by using electro-generated active chlorine, even if this promising route presents some important drawbacks such as the production of chlorinated by-products. Here, for the first time, this process was studied in a microfluidic electrochemical reactor with a very small inter-electrode distance (145 μm) using a water solution of NaCl and phenol and a BDD anode. The potential production of chloroacetic acids, chlorophenols, carboxylic acids, chlorate and perchlorate was carefully evaluated. It was shown, for the first time, up to our knowledge, that the use of the microfluidic device allows to perform the treatment under a continuous mode and to achieve higher current efficiencies and a lower generation of some important by-products such as chlorate and perchlorate. As an example, the use of the microfluidic apparatus equipped with an Ag cathode allowed to achieve a high removal of total organic carbon (about 76%) coupled with a current efficiency of 17% and the production of a small amount of chlorate (about 30 ppm) and no perchlorate. The effect of many parameters (namely, flow rate, current density and nature of cathode) was also investigated.

Treatment of gaseous streams polluted with H2S: Comparison of electrolytic and electro-Fenton assisted absorption processes

H₂S is a gaseous compound that contributes to air pollution. In this work, the electrochemical oxidation treatment of gaseous streams polluted with H₂S is evaluated using a jet mixer and electrochemical cell device, in which the performance of electrolytic and electro-Fenton assisted absorption processes are compared. Results demonstrate the feasibility of both processes to remove H₂S, reaching coulombic efficiencies of nearly 100% in the electrolytic assisted absorption, and 70-80% in the electro-Fenton assisted absorption. Aqueous solutions containing phosphate salts as electrolyte were found to be suitable as absorbents for the process. Efficiency in the cathodic production of H₂O₂ in these solutions using the experimental device was found to be as high as 32.8% (1.184 mgH₂O₂/min) at 12 °C and atmospheric pressure. Sequential formation of SO₂ and SO₃ is obtained by the oxidation of H₂S contained in the gas. These species are hydrolysed, and a part remained in the absorbent as SO₃^2- and SO₄^2-, while the rest is dragged in the outlet gas. SO₃ production is promoted by electrolytic assisted absorption and polysulphides by the electro-Fenton technology. Low concentrations of elemental sulphur are detected in the solid suspensions formed during the process.

Investigating the electrochemical advanced oxidation mechanism of N-doped graphene aerogel: Molecular dynamics simulation combined with DFT method

Nitrogen-doped graphene as a perfectly-efficient and environmentally compatible electrocatalyst won widespread attention in electrochemical advanced oxidation processes (EAOP). However, the relationship between surface structure regulation and activity of catalysts is still lacking in systematic scientific guidance. Herein, nitrogen-doped graphene aerogel (NGA) was conveniently prepared through hydrothermal treatment, and then utilized to fabricate the gas diffusion electrode (GDE) as the cathode for tetracycline (TC) removal. High free radical yield (81.2 μM) and fast reaction rate (0.1469 min^-1) were found in NGA system. The molecular dynamics simulation (MD) results showed that the interaction energy of NGA was greater than the raw graphene aerogel (GA). The adsorption activation of H₂O₂ and the degradation of TC occurred in the first adsorption layer of catalysts, and both processes turned more orderly after nitrogen doping. Moreover, the van der Waals interaction was stronger than the electrostatic interaction. Density function theory (DFT) revealed that the adsorption energy of H₂O₂ at graphitic N, pyridinic N, and pyrrolic N sites was -0.03 eV, -0.39 eV, and -0.30 eV, respectively. Pyridinic N sites were inferred as the main functional regions of in-situ activation •OH, there were more likely to occur ectopic reaction in pyrrolic N, and graphitic N were responsible for improving H₂O₂ production. By revealing the microstructure and activation characteristics of NGA, an experiment-simulation complementary strategy is provided in the EAOP to discover or to optimize new catalysts.

State-of-the-art review and bibliometric analysis on electro-Fenton process

The electro-Fenton (EF) process was first proposed in 1996 and, since then, considerable development has been achieved for its application in wastewater treatment, especially at lab and pilot scale. After more than 25 years, the high efficiency, versatility and environmental compatibility of EF process has been demonstrated. In this review, bibliometrics has been adopted as a tool that allows quantifying the development of EF as well as introducing some useful correlations. As a result, information is summarized in a more visual manner that can be easily analyzed and interpreted as compared to conventional reviewing. During the recent decades under review, 83 countries have contributed to the dramatic growth of EF publications, with China, Spain and France leading the publication output. The top 12 most cited articles, along with the top 32 most productive authors in the EF field, have been screened. Four stages have been identified as main descriptors of the development of EF throughout these years, being each stage characterized by relevant breakthroughs. To conclude, a general cognitive model for the EF process is proposed, including atomic, microscopic and macroscopic views, and future perspectives are discussed.

Homogeneous Photo-Fenton Degradation and Mineralization of Model and Simulated Pesticide Wastewaters in Lab- and Pilot-Scale Reactors

The homogeneous photocatalytic degradation of model pesticide clopyralid (CLPR) has been investigated under various experimental setups. Lab-scale experiments under UV-A radiation in an acidic environment showed that the degradation rate generally increased when increasing either Fe3+ or H2O2 concentration up to a point beyond which (i.e., 100 mg L−1 for peroxide or 7 mg L−1 for ferric ions) Fenton reagents had little or even detrimental effect on degradation. Thus, there is an optimum concentration of Fenton reagents for maximizing treatment performance, beyond which degradation rates are not enhanced. Excessive concentrations of peroxide and/or catalyst may (i) introduce unnecessary treatment costs, (ii) reduce performance due to scavenging effects, and (iii) raise environmental concerns associated with the disposal of, e.g., high concentrations of iron in the receiving water courses. Switching from UV-A to visible light led to similar rates of degradation, i.e., 86% and 82.2%, respectively, after 90 min of reaction, highlighting the potential of using renewable energy, i.e., natural sunlight, to drive the process. Treatment for 120 min also led to 90% mineralization and quantitative release of nitrogen originally present in the pesticide; this was also accompanied by complete elimination of eco-toxicity to Vibrio fischeri. Pilot-scale experiments were performed in a fountain-type reactor using a commercial pesticide formulation containing CLPR. Both the degradation and mineralization rates increased with increasing the intensity of the incident UV-A radiation from 1.88 to 4.03 mW cm−2. Experiments were also conducted with different liquid volumes, i.e., from 3 to 8 L. Illumination of 5 L wastewater resulted in 80% mineralization after 60 min and this only slightly decreased to 73% at 8 L. Overall, the findings underline the promising perspectives of the application of the treatment method in upgrading the quality of water and liquid waste containing pesticides.

Electro-Fenton-Based Technologies for Selectively Degrading Antibiotics in Aqueous Media

The viability of the Electro-Fenton (EF) process in the selective degradation of penicillin G (PenG) in complex solutions has been studied. The role of the anode material (boron-doped diamond (BDD) or mixed metal oxide (MMO)) and the cathode 3D support (foam or mesh), as well as the synergistic effect of UVC light irradiation (photoelectron-Fenton, PEF), have been evaluated. The results show that Pen G can be efficiently and selectively removed by EF, obtaining higher PenG removal rates when using the BDD anode (100%) than when using the MMO anode (75.5%). Additionally, mineralization is not favored under the experimental conditions tested (pH 3, 5 mA cm−2), since both aromatic and carboxylic acids accumulate in the reaction system as final products. In this regard, the EF-treated solution presents a high biological oxygen demand and a low percentage of Vibrio fischeri inhibition, which leads to high biodegradability and low toxicity of this final effluent. Furthermore, the combination with UVC radiation in the PEF process shows a clear synergistic effect on the degradation of penicillin G: 166.67% and 83.18% using MMO and BBD anodes, respectively. The specific energy required to attain the complete removal of PenG and high inhibition of the antibiotic effect is less than 0.05 Ah dm−3. This confirms that PEF can be efficiently used as a pretreatment of conventional wastewater treatment plants to decrease the chemical risk of complex solutions polluted with antibiotics.

Enhanced degradation of 2,4-dichlorophenoxyacetic acid by electro-fenton in flow-through system using B, Co-TNT anode

A flow-through system was constructed for 2,4-dichlorophenoxyacetic acid (2,4-D) degradation for the first time using efficient boron and cobalt co-doped TiO₂ nanotubes (B, Co-TNT) as the anode and carbon black doped carbon felt (CB-CF) that had a high H₂O₂ yield as the cathode. Compared with dimensionally stable anode (DSA), whether in anodic oxidation (AO) or AO-electro-Fenton (EF) system, 2,4-D degradation in B, Co-TNT anode system was more efficient accompanying with a lower energy consumption (Ec). Different operating parameters including applied current density, initial pH and flow rate were explored, supporting that the optimal Fe²⁺ dosage was 0.5 mM while decreasing the initial pH and increasing the current intensity and flow rate were beneficial to 2,4-D removal. In this AO-EF system, the involved mechanisms for 2,4-D degradation were anodization and Fenton oxidation, possessing the comprehensive effect of ^•OH and SO₄^•- with their contribution of 92.7% and 4.8%, respectively. This flow-through AO-EF system performed a stable performance, and an efficient degradation performance with low Ec (5.8-29.5 kWh (kg TOC)^-1) was obtained for different kinds of contaminants (methylene blue, phenol, p-nitrophenol and sulfamethazine). Therefore, B, Co-TNT anode coupled with CB-CF cathode in flow-through system was effective for contaminants degradation.

Advanced oxidation processes in microreactors for water and wastewater treatment: Development, challenges, and opportunities

The miniaturization of reaction processes by microreactors offers many significant advantages over the use of larger, conventional reactors. Microreactors' interior structures exhibit comparatively higher surface area-to-volume ratios, which reduce reactant diffusion distances, enable faster and more efficient heat and mass transfer, and better control over process conditions. These advantages can be exploited to significantly enhance the performance of advanced oxidation processes (AOPs) commonly used for the removal of water pollutants. This comprehensive review of the rapidly emerging area of environmental microfluidics describes recent advances in the development and application of microreactors to AOPs for water and wastewater treatment. Consideration is given to the hydrodynamic properties, construction materials, fabrication techniques, designs, process features, and upscaling of microreactors used for AOPs. The use of microreactors for various AOP types, including photocatalytic, electrochemical, Fenton, ozonation, and plasma-phase processes, showcases how microfluidic technology enhances mass transfer, improves treatment efficiency, and decreases the consumption of energy and chemicals. Despite significant advancements of microreactor technology, organic pollutant degradation mechanisms that operate during microscale AOPs remain poorly understood. Moreover, limited throughput capacity of microreactor systems significantly restrains their industrial-scale applicability. Since large microreactor-inspired AOP systems are needed to meet the high-throughput requirements of the water treatment sector, scale-up strategies and recommendations are suggested as priority research opportunities. While microstructured reactor technology remains in an early stage of development, this work offers valuable insight for future research and development of AOPs in microreactors for environmental purposes.

Photo-assisted electrochemical advanced oxidation processes for the disinfection of aqueous solutions: A review

Disinfection is usually the final step in water treatment and its effectiveness is of paramount importance in ensuring public health. Chlorination, ultraviolet (UV) irradiation and ozone (O3) are currently the most common methods for water disinfection; however, the generation of toxic by-products and the non-remnant effect of UV and O3 still constitute major drawbacks. Photo-assisted electrochemical advanced oxidation processes (EAOPs) on the other hand, appear as a potentially effective option for water disinfection. In these processes, the synergism between electrochemically produced active species and photo-generated radicals, improve their performance when compared with the corresponding separate processes and with other physical or chemical approaches. In photo-assisted EAOPs the inactivation of pathogens takes place by means of mechanisms that occur at different distances from the anode, that is: (i) directly at the electrode's surface (direct oxidation), (ii) at the anode's vicinity by means of electrochemically generated hydroxyl radical species (quasi-direct), (iii) or at the bulk solution (away from the electrode surface) by photo-electrogenerated active species (indirect oxidation). This review addresses state of the art reports concerning the inactivation of pathogens in water by means of photo-assisted EAOPs such as photo-electrocatalytic process, photo-assisted electrochemical oxidation, photo-electrocoagulation and cathodic processes. By focusing on the oxidation mechanism, it was found that while quasi-direct oxidation is the preponderant inactivation mechanism, the photo-electrocatalytic process using semiconductor materials is the most studied method as revealed by numerous reports in the literature. Advantages, disadvantages, trends and perspectives for water disinfection in photo-assisted EAOPs are also analyzed in this work.

Gas diffusion electrodes for H2O2 production and their applications for electrochemical degradation of organic pollutants in water: A review

Nowadays, it is a great challenge to minimize the negative impact of hazardous organic compounds in the environment. Highly efficient hydrogen peroxide (H₂O₂) production through electrochemical methods with gas diffusion electrodes (GDEs) is greatly demand for degradation of organic pollutants that present in drinking water and industrial wastewater. The GDEs as cathodic electrocatalyst manifest more cost-effective, lower energy consumption and higher oxygen utilization efficiency for H₂O₂ production as compared to other carbonaceous cathodes due to its worthy chemical and physical characteristics. In recent years, the crucial research and practical application of GDE for degradation of organic pollutants have been well developed. In this review, we focus on the novel design, fundamental aspects, influence factors, and electrochemical properties of GDEs. Furthermore, we investigate the generation of H₂O₂ through electrocatalytic processes and degradation mechanisms of refractory organic pollutants on GDEs. We describe the advanced methodologies towards electrochemical kinetics, which include the enhancement of GDEs electrochemical catalytic activity and mass transfer process. More importantly, we also highlight the other technologies assisted electrochemical process with GDEs to enlarge the practical application for water treatment. In addition, the developmental prospective and the existing research challenges of GDE-based electrocatalytic materials for real applications in H₂O₂ production and wastewater treatment are forecasted. According to our best knowledge, only few review articles have discussed GDEs in detail for H₂O₂ production and their applications for degradation of organic pollutants in water.

Physico-chemical processes

By summarizing 187 relevant research articles published in 2019, the review is focused on the research progress of physicochemical processes for wastewater treatment. This review divides into two sections, physical processes and chemical processes. The physical processes section includes three sub-sections, that is, adsorption, granular filtration, and dissolved air flotation, whereas the chemical processes section has five sub-sections, that is, coagulation/flocculation, advanced oxidation processes, electrochemical, capacitive deionization, and ion exchange. PRACTITIONER POINTS: Totally 187 research articles on wastewater treatment have been reviewed and discussed. The review has two major sections with eight sub-topics.

Storage of energy using a gas-liquid H2/Cl2 fuel cell: A first approach to electrochemically-assisted absorbers

In this work, the use in fuel cell mode of three electro-absorbers is evaluated for the chloralkaline process and performance is compared with that of a conventional PEMFC operated at the same operation conditions (room temperature). To do this, four cells have been in-house manufactured and compared, in order to determine which electrolyte (solution containing the active species or the membrane) is the best and which is the influence of the absorption stage on the operation of the cell. Because of the high solubility of chlorine, only the hydrogen absorption has been considered in order to evaluate relevant differences in the performance. Results demonstrate that design of the cell has a superb significance on the performances obtained. Cells with membrane-electrode assemblies are more efficient than those in which the membrane is used only as an electrodic compartment separator and utilization of devices which produce tiny bubbles of gas into the electrolyte is also very advantageous in order to obtain higher efficiencies. Results are of a great significance for the design of electro-absorbers and this paper is a first approach to face the design of reversible electrochemical cells for the chloralkaline process.