Performance of FeII/FeIII-LDH@CF cathode for degradation of xenobiotic 4-hydroxy-TEMPO by heterogeneous electro-Fenton: Kinetics and degradation pathways

Advances in technology, coupled with the need to preserve the environment, necessitate the development of novel industrial procedures that rely on chemicals whose effects on living organisms remain insufficiently studied. 2,2,6,6-tetramethylpiperidin-1-oxyl, also known as 4-hydroxy TEMPO (TMP) or Tempol, is a widely used aminoxyl compound in chemistry and medicine. However, no studies have reported on the removal of TMP from aqueous environments, despite its extensive industrial applications. This study investigates the performance of both classical electro-Fenton (CEF) and heterogeneous electro-Fenton (HEF) processes for TMP elimination from contaminated water. To this end, a FeII/FeIII-layered double hydroxide (LDH)-modified carbon felt cathode (synthesized carbon felt, SCF), capable of operating across a wide pH range, was synthesized and applied in the HEF process. The effects of operating parameters such as initial pH, cathode formulation, and applied current were systematically evaluated. Oxidation kinetics and the mineralization pathway of TMP were established, with the second-order rate constant for hydroxyl radical oxidation determined as 1.45 x 109 M- 1 s- 1. Chromatographic analyses identified intermediate products, carboxylic acids, and inorganic end-products, enabling the proposal of degradation pathways. The FeII/FeIII-LDH@CF cathode achieved complete degradation of 0.1 mM TMP within 60 min at 300 mA. Nearly complete mineralization (98.0 %) was reached after 6 h at 500 mA, demonstrating the superior efficiency of the HEF process. Overall, these findings highlight the potential of the FeII/FeIII-LDH@CF cathode as a highly effective electrode material for the removal of emerging organic pollutants from contaminated water.