Highly porous seeding-free boron-doped ultrananocrystalline diamond used as high-performance anode for electrochemical removal of carbaryl from water

New insight of electrogenerated H2O2 into oxychlorides inhibition and decontamination promotion: From radical to nonradical pathway during anodic oxidation of high Cl--laden wastewater process

Anodic oxidation (AO) has been extensively hailed as a robust and promising technology for pollutant degradation, but the parasitic formation of oxychlorides (ClOx-) would induce a seriously over-evaluated electrochemical COD removal performance and dramatical biotoxicity increasement of the AO-treated Cl--laden effluents. Herein, we shed new light on the roles of H2O2 high-efficiently electrogenerated in three-dimensional (3D) reactor in inhibiting ClOx- production and promoting pollutant degradation, which has been overlooked in previous literature. Total yield of ClOx- in phenol simulated wastewater containing 30 mM Cl- was dropped from 25 mM and 24.3 mM to only 0.26 mM and 0.23 mM within 120 min after treating by 3D H2O2-involing systems with Ti/Ru-IrO2 and BDD anode, respectively. Meanwhile, the COD removal of 3D Ti/Ru-IrO2-based system was increased by 57 % (85 % removal at 0.011 kWh g-1 COD), comparable to that of 3D BDD-based system (90 % removal at 0.008 kWh g-1 COD), the energy consumption of which were far less than those of conventional 2D and 3D electro-Fenton systems (0.08-0.2 kWh g-1 COD). During degradation process of Cl--bearing phenol by 3D AO-H2O2 systems, the anodically produced species (Cl•, Cl2•-, ClO-) were rapidly quenched by the in-situ electrogenerated H2O2 and then successfully transformed into 1O2. The radical pathway of reaction between H2O2 and Cl•/Cl2•- had a more obviously thermodynamical advantage (∆G = 11.5 kJ mol-1) than nonradical pathway between H2O2 and ClO- (∆G = 171 kJ mol-1) based on DFT analysis. And the steady-state concentration of 1O2 was 8.8 × 10-9 M and 4.2 × 10-10 M in 3D Ti/Ru-IrO2 and BDD-based system, respectively, which collectively took responsibility for the termination of ClOx- production and promotion of organic pollutant degradation. This work provides a technical feasibility in the practical utilization of AO technology to wastewater treatment without toxic oxychloride by-products.

Innovative approaches to electrochemical oxidation of Bisphenol B in synthetic and complex water environments

The substitution of Bisphenol A (BPA) with Bisphenol B (BPB) has raised concerns due to BPB's increased environmental presence and its potential hazards. Despite the frequent detection in water environments, effective removal methods for BPB are still limited. This study hypothesizes that electrochemical oxidation (EO) can effectively degrade BPB and its by-products. To test this, EO was applied under various conditions, analyzing the role of anode material, current density, pH, and BPB concentration. The results revealed that BPB degradation followed pseudo-first-order kinetics, with boron-doped diamond (BDD) anode showing a rate constant 27 times higher than iridium oxide electrodes. After 180 min, BDD achieved 81.8 % mineralization of BPB. The remaining organic load was associated to easily biodegradable short-chain carboxylic acids. Additionally, the EO process was evaluated in different matrices, including drinking water, tap water, simulated municipal wastewater, and synthetic urine, to assess the impact of matrix complexity. Electrogenerated oxidants, such as hydroxyl radicals, sulfate radicals, and active chlorine, significantly enhanced BPB degradation rates in real water matrices. Energy consumption varied from 5.32 kWh m-3 in drinking water to 2.28 kWh m-3 in synthetic urine, demonstrating the role of matrix composition in EO efficiency. These findings show that EO is a promising technology for removing BPB and similar chemicals in real-world water matrices.

Improved photocatalytic decomposition of carbaryl pesticide in wastewater using ZnO nanorods

This study explores the enhanced photocatalytic performance of ZnO nanorods (ZnO-R) for degrading the carbaryl pesticide (CB) in wastewater. For comparison, commercial ZnO (ZnO-C) was used to evaluate the differences in the photocatalytic decomposition of CB between ZnO-R and ZnO-C. The results regarding the material properties demonstrated that ZnO-R enhances CB removal performance due to its unique rod shape, which extends light absorption and improves electron-hole separation. The removal rates of the carbaryl pesticide from the aqueous solution using ZnO-R and ZnO-C were 98.2% and 87.3%, respectively. Besides, the presence of other pesticides had a more negative impact on the performance of CB than inorganic contaminants. The degradation rates of CB using ZnO-R in wastewater were 99.8%, 68.2%, and 21.7% under UV, solar, and visible light, respectively. In addition, the degradation mechanism of CB using ZnO-R under UV light was proposed based on the n-type photocatalysis process. This work provides a method for selecting a suitable type of ZnO photocatalyst to control pesticide residue pollutants that are commonly found in agricultural activities.

Construction of GO/Pr/PbO2 ternary electrode for highly efficient degradation of organic pollutants

A series of GO/Pr/PbO2 electrodes were prepared by electrodeposition method, and used for the electrocatalytic degradation of methylene blue (MB) in an electrolytic cell. The characteristic properties of the as-prepared electrodes were systematically characterized via different techniques such as XRD, SEM, and XPS. Interestingly, the ternary electrode was found to possess higher catalytic performance compared with PbO2 and Pr/PbO2 electrodes, suggested that synergistic catalytic activity existed between ternary electrode. Moreover, 94.64% of MB degradation was detected over the ternary electrode under the optimized conditions (50 mg/L, pH = 3, 180 min). Besides, when rhodamine B, methyl orange, acid red and tetracycline were respectively used as the model pollutants, with no less than 60% degradation being found. Additionally, hydroxyl radical (•OH) as reactive oxygen species was confirmed according to the free radical capture experiment, whereas, a proper degradation pathway was proposed. According to the DFT calculations, the electrons of the organic pollutants can be transferred to the anode via C-O-Pr bond and Pr-O-Pb bond between the dual interfaces.

Efficient electrochemical removal of ammoniacal nitrogen from livestock wastewater: The role of the electrode material

Wastewater from livestock farms contains high concentrations of suspended solids, organic contaminants, and nitrogen compounds, such as ammoniacal nitrogen. Discharging livestock effluents into water bodies without appropriate treatment leads to severe environmental pollution. Compared to conventional treatment methods, electrochemical oxidation exhibits higher nitrogen removal efficiencies. In the present work, the electrochemical removal of ammoniacal nitrogen from real livestock wastewater was investigated through a lab-scale reactor. Preliminary experiments were carried out to investigate the effects of different anode materials, including boron-doped diamond and iridium/ruthenium-coated titanium, on the total nitrogen removal efficiency using synthetic wastewater. Boron-doped diamond, a well-known non-active electrode, allowed to obtain 63.7 ± 1.21 % of total nitrogen degradation efficiency. However, the iridium/ruthenium-coated titanium electrode, belonging to the class of active anodes, showed a higher performance, achieving 78.8 ± 0.76 % contaminant degradation. Coupling iridium/ruthenium-coated titanium anode with a stainless-steel cathode improved the performance of the system, achieving even 96.2 ± 2.73 % of total nitrogen removal. The optimized cell configuration was used to treat livestock wastewater, resulting in the degradation of 67.0 ± 2.25 % of total nitrogen and 37.3 ± 0.68 % of total organic carbon when sodium chloride was added. At the end of the process, the ammonium content was completely removed, and only 17.7 ± 0.51 % of the initial nitrogen turned into nitrate. The results show that the proposed system is a promising approach to treating livestock wastewater by coupling high contaminant removal efficiencies with low operational costs. Anyway, further studies on process optimization with an emphasis on power requirements and electrode costs need to be carried out.

Electrochemical oxidation of surfactants as an essential step to enable greywater reuse

The practical application of electrochemical oxidation technology for the removal of surfactants from greywater was evaluated using sodium dodecyl sulfate (SDS) as a model surfactant. Careful selection of electrocatalysts and optimization of operational parameters demonstrated effective SDS removal in treating a complex greywater matrix with energy consumption below 1 kWh g-1 COD (Chemical Oxygen Demand), paving the way for a more sustainable approach to achieving surfactant removal in greywater treatment when aiming for decentralized water reuse. Chromatographic techniques identified carboxylic acids as key byproducts prior to complete mineralization. These innovative approaches represent a novel pathway for harnessing electrochemical technologies within decentralized compact devices, offering a promising avenue for further advancements in this field.

Treatment of Organics in Wastewater Using Electrogenerated Gaseous Oxidants

This work focuses on the comparison of the performance of direct electrochemical oxidation with indirect electrolysis mediated by gaseous oxidants in the treatment of diluted wastewater. To do this, energy consumptions of the electrolysis using mixed metal oxide (MMO) electrodes are compared with those required for the production and use of chlorine dioxide in the degradation of methomyl contained in aqueous solutions. Results demonstrate the feasibility of the mediated oxidation process and that this process is competitive with direct oxidation. The oxidants are produced under optimized conditions using the same anodic material applied for the direct degradation of organics, thus avoiding efficiency losses associated with mass transfer limitations in the degradation of dilute organic solutions. Thus, using the ClO2 gaseous oxidant, a concentration of 0.1 mM of methomyl from a solution containing 500 mL is completely removed with an energy consumption as low as 50 Wh. The application of the same energy to a direct electrolytic process for treating the same wastewater can only reach less than half of this removal. These findings may have a very important application in the use of electrochemical technology to achieve the remediation of persistent pollutants in wastewater, where their low concentrations typically make direct processes very inefficient.

Electrochemical oxidation of refractory compounds from hydrothermal carbonization process waters

Hydrothermal carbonization (HTC) is an emerging technology for treating sewage sludge. However, the resulting HTC process water is heavily contaminated with various carbonaceous and nitrogenous components, some of them being non-biodegradable. To implement HTC as a full-scale treatment alternative for sewage sludge, effective concepts for treating process water are crucial. This study focuses on the electrochemical oxidation (EO) using a boron-doped diamond electrode to treat one HTC process waters with different pretreatments: (i) without pretreatment, (ii) biologically pretreated with chemical oxygen demand (COD) removal, (iii) biologically pretreated with nitrification and denitrification. The EO removed COD of all HTC process waters by over 97%, but as COD concentrations decreased, the instantaneous current efficiency (ICE) dropped below 5% and energy consumption increased. The organically bound and refractory nitrogen was completely mineralized and converted to mainly NO3-N. After EO of process waters without nitrification/denitrification, nitrogen was present as NO3-N with up to 730 mg/L and NH4-N with up to 1813 mg/L. Such high ammonium concentrations treatment could be interesting for nitrogen recovery. In addition, the toxicity towards Vibrio fischeri could be reduced to a large extent. The findings suggest that EO after a biological step with COD removal is a viable solution for HTC process water treatment.