A Review on Electrochemical Degradation and Biopolymer Adsorption Treatments for Toxic Compounds in Pharmaceutical Effluents

Reaction Mechanism of Oxytetracycline Degradation by Electrogenerated Reactive Chlorine: The Influence of Current Density and pH

A binary dimensionally stable anode Ti/TiO2-RuO2 electrode was used to abate the antibiotic oxytetracycline (OTC) (C22H24N2O9) in chloride water. The anode was prepared using the Pechini method and subsequently characterized by X-ray diffraction, scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS), and cyclic voltammetry (CV). The optimum values of the operational parameters affecting removal efficiency were determined using a 2 × 3 factorial design by screening j (6.0, 10, and 20 A m-2) and pH (3, 6.5, and 10). The textural analysis revealed the formation of active oxides (RuO2 and TiO2 coating rutile-type P42/mnm, space group 136), with a cracked surface and good dispersion of metal components. A contour graph verified that the most suitable condition for contaminant degradation was 20 A m-2 at a circumneutral pH of 6.5, resulting in approximately 97% degradation after 20 min of electrolysis according to pseudo-first-order reaction kinetics and the loss of the antibiotic activity of OTC. In addition, the results of oxidant formation and CV indicate that the best electrochemical activation of the anode to form Cl2-active mainly depended on pH. Liquid chromatography-mass spectrometry (LC-MS) and density functional theory were employed to propose a reaction pathway for OTC degradation. Three byproducts with m/z 426, 256, and 226 were identified corresponding to the removal of amide and amine groups, which are susceptible sites to electrophilic attack by active chlorine species. The findings from this work stand out for prospective applications of anodic electrochemical oxidation to efficiently eliminate antibiotics with similar chemical structures in wastewater containing chlorides.

Robust and Self-Cleaning Electrochemical Production of Periodate

Periodate, a platform oxidizer, can be electrochemically recycled in a self-cleaning process. Electrosynthesis of periodate is well established at boron-doped diamond (BDD) anodes. However, recovered iodate and other iodo species for recycling can contain traces of organic impurities from previous applications. For the first time, it was shown that the organic impurities do not hamper the electrochemical re-oxidation of used periodate. In a hydroxyl-mediated environment, the organic compounds form CO2 and H2 O during the degradation process. This process is often referred to as "cold combustion" and provides orthogonal conditions to periodate synthesis. To demonstrate the strategy, different dyes, pharmaceutically active ingredients, and iodine compounds were added as model contaminations into the process of electrochemical periodate production. UV/Vis spectroscopy, NMR spectroscopy, and mass spectrometry (MS) were used to monitor the degradation of organic molecules, and liquid chromatography-MS was used to control the purity of periodate. As a representative example, dimethyl 5-iodoisophthalate (2 mm), was degraded in 90, 95, and 99 % while generating 0.042, 0.054, and 0.082 kilo equiv. of periodate, respectively. In addition, various organic iodo compounds could be fed into the periodate generation for upcycling such iodo-containing waste, for example, contrast media.

Electrochemical removal of 4-chlorophenol in water using a porous Magnéli-phase (Ti4O7) electrode

Electro-oxidation is a promising technology for removal of refractory organic pollutants. While the appeal of this technology lies in its chemical-free nature, commercially scale-up application may be limited by the availability of electrode materials and mass transport. Here we report the development of a flow-through electro-oxidation system for removal of chlorophenols in water using Magnéli-phase (Ti₄O₇) tubular anode and a 304 stainless steel (SS) tubular cathode. The key to this system was the porous and conductive Magnéli-phase Ti₄O₇ anode, the structure and composition of which was confirmed by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. System efficacy was evaluated by using 4-chlorophenol (4-CP) as a typical refractory contaminant and model chlorophenol. Under optimized conditions, a complete removal of 4-CP could be obtained within 120 min in 0.04 mol L^-1 Na₂SO₄ solution. Electro-produced HO^• and direct electron transfer were both shown to contribute to the 4-CP electro-oxidation process due to the high selectivity and oxygen evolution potential of the Ti₄O₇ anode. The intermediates of 4-CP degradation were identified and a pathway for its electro-oxidation was proposed. When challenged with industrial wastewater containing 4-CP, chemical oxygen demand (COD) and total organic carbon removal efficiencies of 67.5% and 63.1% respectively could be obtained, accounting for energy consumption of 85.1 kWh·kg COD^-1 for degradation of 1 kg of COD in industrial wastewater. This study provides an effective and robust solution for the removal of refractory emerging contaminants from industrial wastewaters using a continuous-flow electro-oxidation system.