In search of the active chlorine species on Ti/ZrO2-RuO2-Sb2O3 anodes using DEMS and XPS

An alternative approach to the kinetic modeling of pharmaceuticals degradation in high saline water by electrogenerated active chlorine species

A semiempirical approach considering the rate of reactive chlorine species-RCS- production (Φ_E) as a function of current and Cl^- concentration for the modeling of acetaminophen (ACE) degradation is presented. A filter-press reactor having a Ti/RuO₂-ZrO₂ (Sb₂O₃ doped) anode, NaCl (0.04-0.1 mol L^-1) as supporting electrolyte, and operated in continuous mode, was considered. A current of 100 mA and a flow of 11 mL min^-1 favored the electrogeneration of RCS and ACE degradation. Hydraulic retention time and Φ_E were the most relevant parameters for the RCS production. These two parameters, plus the pollutant concentration, were very determinant for the ACE degradation. The model successfully reproduced the ACE removal in distilled water at different concentrations (10, 20, 40, and 60 mg L^-1). The electrochemical system achieved removals between 80 and 100% of ACE in distilled water. The ACE treatment in actual seawater (a chloride-rich matrix, 0.539 mol L^-1 of Cl^-) was assessed, and the degradation was simulated using the developed model. The competing role toward electrogenerated RCS by intrinsic organic matter (3.2 mg L^-1) in the seawater was a critical point, and the simulated values fitted well with the experimental data. Finally, the action of the electrochemical system on ciprofloxacin (CIP) in real seawater and its antimicrobial activity was tested. CIP removal (100% at 120 s) was faster than that observed for ACE (100% of degradation after 180 s) due to CIP has amine groups that are more reactive toward RCS than phenol moiety on ACE. Moreover, the system removed 100% of the antimicrobial activity associated with CIP, indicating a positive environmental effect of the treatment.

Active-chlorine-mediated oxidation of 5-fluorouracil on a hierarchically ordered macroporous RuO2 electrode

A hierarchically ordered macroporous RuO₂ electrode (HOM-RuO₂) was fabricated to enhance in situ active chlorine production in an electrochemical system intended for treatment of pharmaceutical active compounds (PhACs). The unique structure of HOM-RuO₂ resulted in a decrease of the chlorine evolution potential, a large electro-active area available for in situ conversion of Cl^- to active chlorine, and hence improved the active chlorine production by 40%. 5-Fluorouracil (5-FU) was used as a target pollutant to explore the performance of the HOM-RuO₂ for PhACs degradation based on the in situ generated active chlorine. The results showed that the reaction rate of active-chlorine-mediated oxidation of 5-FU produced using the HOM-RuO₂ was 18.4 times higher than that in the case of hydroxyl radicals (OH)-initiated oxidation using a PbO₂ electrode at 30 mA cm^-2. The effects of current density and initial solution pH on the 5-FU removal were investigated. The mechanism of 5-FU degradation was proposed taking into accounts both active chlorine production, and change of the speciation of 5-FU caused by pH variations. The dominant degradation products observed for the degradation of 5-FU using the HOM-RuO₂ were lactic acid, propanol, acetic acid, urea and other small molecules, but no chlorinated products were detected. These study demonstrates the promise of the HOM-RuO₂-based electrochemical systems for the active-chlorine-mediated treatment of recalcitrant pharmaceuticals found in wastewater.

Modeling the sulfamethoxazole degradation by active chlorine in a flow electrochemical reactor

The aim of this study is to propose a continuous physicochemical model accounting for the active chlorine production used to degrade recalcitrant sulfamethoxazole (SMX) in an electrochemical flow reactor. The computational model describes the fluid mechanics and mass transfer occurring in the re/actor, along with the electrode kinetics of hydrogen evolution reaction arising on a stainless steel cathode, and the chloride oxidation on a DSA. Specifically, the anodic contributions assume the heterogeneous nature of the adsorbed chlorine species formed on this surface, which are a model requirement to correctly define the experimental reactor performance and degradation efficiency of the contaminant. The experimental validation conducted at different applied current densities, volumetric flows, and chloride concentrations is adequately explained by the model, thus evidencing some of the phenomena controlling the electrocatalytic chlorine production for environmental applications. The best conditions to eliminate the SMX are proposed based on the theoretical analysis of the current efficiency calculated with the model, and experimentally confirmed. The use of the Ti/RuO₂-ZrO₂-Sb₂O₃ anode at the bench scale improves the SMX removal by using electro-generated chlorine species adsorbed on its surface, which remarkably increases the oxidation potential of the system along with chlorine desorbed from the electrode. This is a technological innovation concerning other mediated oxidation methods entirely using oxidants in solution.

Improved Surface-Enhanced Raman Scattering Properties of ZrO2 Nanoparticles by Zn Doping

In this study, ZrO₂ and Zn–ZrO₂ nanoparticles (NPs) with a series of Zn ion doping amounts were synthesized by the sol-gel process and utilized as substrates for surface-enhanced Raman scattering (SERS). After absorbing the probing molecule 4–mercaptobenzoic acid, the SERS signal intensities of Zn–ZrO₂ NPs were all greater than that of the pure ZrO₂. The 1% Zn doping concentration ZrO₂ NPs exhibited the highest SERS enhancement, with an enhancement factor (EF) value of up to 10⁴. X-ray diffraction, X-ray photoelectron spectroscopy, Ultraviolet (UV) photoelectron spectrometer, UV–vis spectroscopy, Transmission Electron Microscope (TEM), and Raman spectroscopy were used to characterize the properties of Zn–ZrO₂ NPs and explore the mechanisms behind the SERS phenomenon. The charge transfer (CT) process is considered to be responsible for the SERS performance of 4–MBA adsorbed on Zn–ZrO₂. The results of this study demonstrate that an appropriate doping ratio of Zn ions can promote the charge transfer process between ZrO₂ NPs and probe molecules and significantly improve the SERS properties of ZrO₂ substrates.

Efficient cephalexin degradation using active chlorine produced on ruthenium and iridium oxide anodes: Role of bath composition, analysis of degradation pathways and degradation extent

The elimination of cephalexin (CPX) using electro-generated Cl₂-active on Ti/RuO₂-IrO₂ anode was assessed in different effluents: deionized water (DW), municipal wastewater (MWW) and urine. Single Ti/RuO₂ and Ti/IrO₂ catalysts were prepared to compare their morphologies and electrochemical behavior against the binary DSA. XRD and profile refinement suggest that Ti/RuO₂-IrO₂ forms a solid solution, where RuO₂ and IrO₂ growths are oriented by the TiO₂ substrate through substitution of Ir by Ru atoms within its rutile-type structure. SEM reveals mud-cracked structures with flat areas for all catalysts, while EDS analysis indicates atomic ratios in the range of the oxide stoichiometries in the nominal concentrations used during synthesis. A considerably higher CPX degradation is achieved in the presence of NaCl than in Na₂SO₄ or Na₃PO₄ media due to the active chlorine generation. A faster CPX degradation is reached when the current density is increased or the pH value is lowered. This last behavior may be ascribed to an acid-catalyzed reaction between HClO and CPX. Degradation rates of 22.5, 3.96, and 0.576 μmol L^-1 min^-1 were observed for DW, MWW and urine, respectively. The lower efficiency measured in these last two effluents was related to the presence of organic matter and urea in the matrix. A degradation pathway is proposed based on HPLC-DAD and HPLC-MS analysis, indicating the fast formation (5 min) of CPX-(S)-sulfoxide and CPX-(R)-sulfoxide, generated due the Cl₂-active attack at the CPX thioether. Furthermore, antimicrobial activity elimination of the treated solution is reached once CPX, and the initial by-products are considerably eliminated. Finally, even if only 16% of initial TOC is removed, BOD₅ tests prove the ability of electro-generated Cl₂-active to transform the antibiotic into biodegradable compounds. A similar strategy can be used for the abatement of other recalcitrant compounds contained in real water matrices such as urine and municipal wastewaters.