Efficient degradation of rhodamine B using modified graphite felt gas diffusion electrode by electro-Fenton process

Electrochemical regeneration of granular activated carbon using an AQS (9,10- anthraquinone-2-sulfonic acid)/PPy modified graphite plate cathode

In the present study, we investigate the regeneration efficiency of Rhodamine B (RhB)-saturated granular activated carbon (GAC) in an electrochemical regeneration system by using a 9,10-anthraquinone-2-sulfonic acid/polypyrrole modified graphite plate (AQS/PPy-GP) cathode. The response surface methodology based on the Box-Behnken design (RSM-BBD) approach was used to optimize regeneration parameters, whereby the optimum condition of the independent variables was as follows: applied current = 155 mA, concentration of supporting electrolyte = 0.13 M, and regeneration time = 7 h. The electrochemical regeneration system with the AQS/PPy-GP electrode achieved high regeneration efficiency and significantly reduced energy consumption. H₂O₂ concentration generated in the electrolysis system was notably increased, and the time of complete degradation of organics was shortened by 25% compared to the electrode without modification. The mechanism for RhB degradation was proposed as AQS acting as a catalyst to promote the formation of H₂O₂. The regeneration study showed that AQS/PPy-GP cathode had appreciable reusability for GAC regeneration with a regeneration efficiency of 76.6% after 8 regeneration cycles. In summary, the electrochemical regeneration based on AQS/PPy-GP cathode would have practical industrial applications in treating spent activated carbons.

Efficient degradation of semi-coking wastewater in three-dimensional electro-Fenton by CuFe2O4 heterocatalyst

Semi-coking wastewater contains a rich source of toxic and refractory compounds. Three-dimensional electro-Fenton (3D/EF) process used CuFe₂O₄ as heterocatalyst and activated carbon (AC) as particle electrode was constructed for degrading semi-coking wastewater greenly and efficiently. CuFe₂O₄ nanoparticles were prepared by coprecipitation method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy disperse spectroscopy (EDS). Factors like dosage of CuFe₂O₄, applied voltage, dosage of AC and pH, which effect COD removal rate of semi-coking waste water were studied. The results showed that COD removal rate reached to 80.9% by 3D/EF process at the optimum condition: 4 V, 0.3 g of CuFe₂O₄, 1 g of AC and pH = 3. Trapping experiment suggesting that hydroxyl radical (•OH) is the main active radical. The surface composition and chemical states of the fresh and used CuFe₂O₄ were analyzed by XPS indicating that Fe, Cu, and O species are involved into the 3D/EF process. Additionally, anode oxidation and the adsorption and catalysis of AC are also contributed to the bleaching of semi-coking waste water. The possible mechanisms of 3D/EF for degrading semi-coking waste water by CuFe₂O₄ heterocatalyst were proposed.

Degradation of refractory organics in dual-cathode electro-Fenton using air-cathode for H2O2 electrogeneration and microbial fuel cell cathode for Fe2+ regeneration

The electrogeneration of H₂O₂ and electro-regeneration of ferrous are conflicting matters in electro-Fenton system. In this research, the degradation of Rhodamine B, methyl orange (MO) and 4-chlorophenol (4-CP) was investigated using a novel dual-cathode microbial fuel cell (MFC) electro-Fenton (EF) hybrid system. An air-cathode of an EF system was used for H₂O₂ electrogeneration and a carbon felt cathode of a MFC was used to accelerate Fe²⁺ regeneration. Synergistic improvement of MFC power generation and the degradation of the above refractory organics through EF reaction was achieved. The EF air-cathode was fabricated by adopting activated carbon/graphite powder mixture and PVDF binder, which showed higher H₂O₂ generation but slower Fe³⁺ reduction rate than MFC carbon felt cathode. The Rhodamine B removal rate constant and mineralization current efficiency of the MFC coupled EF were 64% and 42% higher than that of uncoupled EF, respectively. The MFC-EF coupled system also exhibited significantly higher removal efficiency for MO and 4-CP than that of un-coupled EF system. Moreover, the power density of MFC was greatly enhanced by coupling EF due to higher Fe³⁺/Fe²⁺ redox potential than oxygen reduction.

Degreasing cotton used as pore-creating agent to prepare hydrophobic and porous carbon cathode for the electro-Fenton system: enhanced H2O2 generation and RhB degradation

A porous carbon cathode was prepared using graphite, polytetrafluoroethylene (PTFE), and degreasing cotton (DC) through sintering treatment. The carbonization of DC by heat treatment played an ideal role in pore-creating, which weakened the mass transfer resistance of O2, and as a result, the adoption of degreasing cotton significantly improved the performance of H2O2 electro-generation. The optimized cathode was able to generate 567 mg L-1 H2O2 with a current efficiency (CE) of 86.7% by the electrochemical reaction of 60 min in a divided reactor. Furthermore, the degradation of rhodamine B (RhB) was carried out by an electro-Fenton system using the optimal cathode selected. The developed electro-Fenton system exhibited an excellent RhB degradation performance. The RhB solution of 50 mg L-1 was decolorized completely by the treatment of 10 min. Moreover, the degradation of 50~90 mg L-1 RhB solution presented over 90% TOC removal by the treatment of 120 min, indicating the ideal mineralization of organic pollutants. In addition, it was found that •OH was the major oxidizing specie responsible for the organics degradation. Finally, the possible pathway of RhB degradation in the electro-Fenton system was proposed by GC-MS analysis. The adoption of natural fibers for pore-creating provides an innovative and low-cost method to prepare porous cathode, which may promote the application of electro-Fenton oxidation in wastewater treatment.

pH-controlled mechanism of photocatalytic RhB degradation over g-C3N4 under sunlight irradiation

Photocatalysis of dye degradation is one of green and cheap technologies for solving environmental pollution. Whereas it is rarely concerned that the degradation process varied with the change of solution condition, this work studied the influence of hydrion in the solution on the photodegradation process of Rhodamine B (RhB) over g-C₃N₄. The photocatalytic activity of RhB degradation was enhanced gradually with increased hydrion content in the system. The efficiency for RhB degradation over g-C₃N₄ in weak acidic system with interference of multiple metal-ions still reached near 95% after 30 min of natural sunlight irradiation. A large amount of oxidation species and the hydroxylation mineralization process were induced by increasing the hydrion concentration. Two degradation processes for deethylation of four ethyl groups and the direct chromophoric degradation were discovered and proved by multifarious intermediates in different systems using the ESR technique, LC/MS and GC/MS analysis. In addition, the photosensitization played a critical role in the RhB degradation. A feasible degradation mechanism was proposed for the RhB degradation based on the experimental results.

Degradation of antibiotic ciprofloxacin by different AOP systems using electrochemically generated hydrogen peroxide

The present work reports the degradation of the antibiotic ciprofloxacin (CIP) by different advanced oxidative process systems (UV; Anodic Oxidation; H₂O₂; H₂O₂/UV; H₂O₂/Fe²⁺ and H₂O₂/UV/Fe²⁺) in an electrochemical cell using gas diffusion electrode (GDE) for the synthesis of hydrogen peroxide. CIP degradation and mineralization were evaluated by high efficiency liquid chromatography (HPLC) and total organic carbon (TOC) techniques. Of all the systems investigated, the photoelectro-Fenton system presented the best degradation efficiency; this system promoted highly significant mineralization percentages of 54.8% and 84.6% in 90 and 360 min, and relatively lower energy consumption rates of 4110.0 and 9808.2 kWh kg^-1 TOC, respectively. In 6 h period of experiment, the main degradation products of ciprofloxacin were identified, and the aliphatic acids obtained helped confirm the rupture of the aromatic ring. The application of the photoelectro-Fenton process with in situ eletroctrogeneration of H₂O₂ using GDE has proved to be suitably promising for the treatment of organic pollutants.

Fabrication of multi-walled carbon nanotubes and carbon black co-modified graphite felt cathode for amoxicillin removal by electrochemical advanced oxidation processes under mild pH condition

Hydrogen peroxide (H₂O₂) electrogenerated via two-electron oxygen reduction reaction at cathode plays an important role in electrochemical advanced oxidation processes for organic pollutants removal from wastewater. Herein, multi-walled carbon nanotubes and carbon black co-modified graphite felt electrode (MWCNTs-CB/GF) was prepared as an efficient cathode for H₂O₂ electrogeneration and amoxicillin removal by anodic oxidation with hydrogen peroxide (AO-H₂O₂) and electro-Fenton (EF) under mild pH condition. Besides, the physicochemical and electrochemical properties of MWCNTs-CB/GF were characterized by scanning electron microscopy, N₂ adsorption and desorption experiment, contact angle measurement, X-ray photoelectron spectroscopy, and linear sweep voltammetry. Compared with GF, MWCNTs-CB/GF showed a higher H₂O₂ generation of 309.0 mg L^-1 with a current efficiency of 60.9% (after 120 min) and more effective amoxicillin removal efficiencies of 97.5% (after 120 min) and 98.7% (after 30 min) in AO-H₂O₂ and EF (with 0.5 mM Fe²⁺) processes, under the condition of current density 12 mA cm^-2 and initial pH 5.5. Meanwhile, the TOC removal efficiency was 45.2% during EF process after 120 min. Anodic oxidation, H₂O₂ oxidation, and methanol capture indicated that ∙OH generated via electro-activation reaction at MWCNTs-CB/GF and Fenton reaction in solution played the dominant role in amoxicillin removal. Moreover, the TOC removal was associated with ∙OH generated during Fenton reaction in the solution. The major intermediates of AMX degradation by EF process were identified using LC-MS and the possible degradation pathways were proposed containing of β-lactam ring opening, hydroxylation reaction, decarboxylation reaction, methyl groups in the thiazolidine ring oxidation reaction, bond cleavage, and rearrangement processes. All of the above results proved that MWCNTs-CB/GF was an excellent cathode for AMX degradation under mild pH condition.

Degradation of folic acid wastewater by electro-Fenton with three-dimensional electrode and its kinetic study

The three-dimensional electro-Fenton method was used in the folic acid wastewater pretreatment process. In this study, we researched the degradation of folic acid and the effects of different parameters such as the air sparging rate, current density, pH and reaction time on chemical oxygen demand (COD) removal in folic acid wastewater. A four-level and four-factor orthogonal test was designed and optimal reaction conditions to pretreat folic acid wastewater by three-dimensional electrode were determined: air sparge rate 0.75 l min^-1, current density 10.26 mA cm^-2, pH 5 and reaction time 90 min. Under these conditions, the removal of COD reached 94.87%. LC-MS results showed that the electro-Fenton method led to an initial folic acid decomposition into p-aminobenzoyl-glutamic acid (PGA) and xanthopterin (XA); then part of the XA was oxidized to pterine-6-carboxylic acid (PCA) and the remaining part of XA was converted to pterin and carbon dioxide. The kinetics analysis of the folic acid degradation process during pretreatment was carried out by using simulated folic acid wastewater, and it could be proved that the degradation of folic acid by using the three-dimensional electro-Fenton method was a second-order reaction process. This study provided a reference for industrial folic acid treatment.

Effective degradation of rhodamine B by electro-Fenton process, using ferromagnetic nanoparticles loaded on modified graphite felt electrode as reusable catalyst: in neutral pH condition and without external aeration

Polytetrafluoroethylene/ferromagnetic nanoparticle/carbon black (PTFE/MNP/CB)-modified graphite felt (GF) was successfully applied as cathode for the mineralization of rhodamine B (RhB) in electro-Fenton (EF) process. The modified cathode showed high decolorization efficiency for RhB solution even in neutral pH condition and without external aeration, achieving nearly complete decolorization and 89.52 % total organic carbon (TOC) removal after 270-min oxidation with the MNP load 1.2 g at 50 A/m(2). Moreover, the operational parameters (current density, MNP load, initial pH, and airflow rate) were optimized. After that, adsorption isotherm was also conducted to compare the absorption quantity of CB and carbon nanotube (CNT). Then, the surface morphologies of MNPs were characterized by transmission electron microscope (TEM), energy-dispersive X-ray detector (EDX), and Fourier transform infrared spectroscopy (FTIR); and the modified cathode was characterized by SEM and contact angle. Finally, the stability and reusability of modified cathode were tested. Result uncovered that the PTFE/MNP/CB-modified cathode has the potential for industrial application and the solution after treatment was easily biodegradable.