Electrosynthesis of hydrogen peroxide in acidic solutions by mediated oxygen reduction in a three-phase (aqueous/organic/gaseous) system Part II: Experiments in flow-by fixed-bed electrochemical cells with three-phase flow

The second part of the work concerned with mediated electrosynthesis of H₂O₂ in acidic solutions (pH 3) deals with investigations using divided flow-by fixed bed electrochemical cells operated with co-current three-phase flow (aqueous/organic emulsion and O₂ gas at 0.1 MPa). Graphite felt (GF) and reticulated vitreous carbon (RVC) were evaluated as cathodes at superficial current densities up to 3000 A m^–2. Typically, at current densities above 600 A m^–2 graphite felt yielded higher peroxide concentrations per pass and current efficiencies, most likely due to the almost an order of magnitude higher organic liquid to solid mass transfer capacity for 2-ethyl-9,10-anthraquinone (EtAQ) mediator, that is, 0.13 s^–1 in the case of GF vs 0.015 s^–1 for RVC with 39 ppc (pores per cm). Factorial experiments revealed a positive interaction effect between superficial current density and emulsion load with respect to the current efficiency for H₂O₂ electrosynthesis. Thus, at the highest investigated superficial current density of 3000 A m^–2, the current efficiency was 84% when the emulsion load was at the highest explored level of 11.7 kg m^–2 s^–1, whilest for the lowest level of emulsion load, 2.8 kg m^–2 s^–1, the current efficiency for H₂O₂ was 18%. Furthermore, the presence of 1 mM cationic surfactant, tricaprylmethylammonium chloride (CH₃(C₈H₁₇)₃N⁺Cl^–, A336), had a positive main effect of about 12% on H₂O₂ current efficiency and there was also a positive synergistic effect between surfactant and emulsion load, estimated at about 7%. The aqueous to organic phase volume ratio, in the range of 0.9/1 and 3/1, had a statistically insignificant effect on the current efficiency for H₂O₂ generation. A decrease of the aqueous to organic phase volume ratio from 3 to 0.9 increased the cell voltage from about 6.5 to 7.3 V.     

Electrosynthesis of hydrogen peroxide by partial reduction of oxygen in alkaline media. Part II: Wall-jet ring disc electrode for electroreduction of dissolved oxygen on graphite and glassy carbon

A wall-jet ring disc electrode was constructed by adapting a wall-jet flow through electrochemical cell. Commercially available spectral graphite and glassy carbon were used as working disc electrodes and the ring electrode was made of stainless steel. The efficiency and rate constants, measured in a planar parallel flow hydrodynamic regime, indicated the partial electroreduction of dissolved oxygen as a quasi-reversible two-electron process for both electrode materials tested.     

Effect of the oxidation of activated carbon by hydrogen peroxide on its catalytic activity in the regeneration of Co(II)TETA

Cobalt(II) triethylenetetramine (Co(II)TETA) formed by soluble cobalt(II) salt combining with triethylenetetramine will be used as a wet denitration technique since it can interact with nitric oxide to accomplish quick absorption of NO from gas phase. However, the oxygen in the flue gas will oxidize Co(II)TETA to Co(III) TETA, resulting in the reduction of denitrification efficiency. Activated carbon has been used to promote the regeneration of Co(II)TETA due to its unique surface characteristics. Hydrogen peroxide solution is utilized as a modifier in the carbon modification to improve the catalytic performance of activated carbon. The experiments demonstrate that the best regeneration efficiency of Co(II)TETA is gained by the modified carbon impregnated in 0.05 mol L⁻¹ H₂O₂ solution at 70°C for 12 h with a solid/liquid ratio of 1/50 (g/mL) followed being activated at 400°C for 2 h in N₂. After being treated with hydrogen oxide solution, the surface area and acidity of the carbon is increased. Continuous experiments reveal that the NO removal efficiency gained by modified activated carbon is about 8.36% higher than that gained by the original carbon.     

Electrochemical reduction of 2-ethyl-9,10-anthraquinone (EAQ) and mediated formation of hydrogen peroxide in a two-phase medium Part I: Electrochemical behaviour of EAQ on a vitreous carbon rotating disc electrode (RDE) in the two-phase medium

The electrochemical reduction of 2-ethyl-9,10-anthraquinone (EAQ) has been examined by voltammetry on a vitreous carbon rotating disk electrode (RDE) in a two-phase medium. The medium consisted of an organic phase consisting of a mixture of tributylphosphate (TBP) and diethylbenzene (DEB) (15/85,vol/vol) and an aqueous phase, 2m NaOH. The ratio between the two phases was 40% for the organic phase and 60% for the aqueous phase. The electrochemical behaviour of EAQ in this medium was examined on a vitreous carbon RDE in the presence and absence of oxygen. The formation of hydrogen peroxide mediated by the electrochemical reduction of EAQ was studied analytically. The voltammetric results indicate that hydrogen peroxide formation mediated by the electrochemical reduction of EAQ is feasible.     

The removal of cephalexin antibiotic in aqueous solutions by ultrasonic waves/hydrogen peroxide/nickel oxide nanoparticles (US/H2O2/NiO) hybrid process

ABSTRACT

Antibiotics are non-biodegradable and can remain for a long time at aquatic environments and they have a big potential bio-accumulation in the environment. The antibiotics are broadly metabolized by humans, animals and plants and they or their metabolites, after metabolization, are entered into the aquatic environment. This study aimed to optimize the operational parameters by Taguchi design and to carry out the kinetic studies for removal of cephalexin antibiotic from aqueous solutions by US/H₂O₂/NiO hybrid process. This experimental study was performed on a laboratory scale in a 500 mL pyrex-made reactor. The main operational parameters to influence the US/H₂O₂/NiO process were identified as the initial concentration of CEX (20–80 mg/L), hydrogen peroxide (H₂O₂) (10–40 mL/L), NiO nanoparticle (2.5–10 mg/L) and reaction time (15–90 min) and therefore, the influence of these factors were studied. Under optimum conditions (pH = 3, reaction time = 90 min, CEX = 40 mg/L, NiO = 7.5 mg/L and H₂O₂ = 30 mL/L) and using the US/H₂O₂/NiO process, the removal efficiencies of CEX, COD and TOC were 93.86%, 72.46% and 54.55%, respectively. The percentage contribution of each factor was also determined. Results introduced the solution pH as the most powerful factor, and its percentage contribution value was up to 94% in the studied process. It was also identified that the removal of CEX antibiotic using the hybrid process obeys the pseudo-first-order kinetics.