Carbon\nNanotube Membrane Stack for Flow-through Sequential\nRegenerative Electro-Fenton

Abstract

Electro-Fenton is a promising advanced\noxidation process for water\ntreatment consisting a series redox reactions. Here, we design and\nexamine an electrochemical filter for sequential electro-Fenton reactions\nto optimize the treatment process. The carbon nanotube (CNT) membrane\nstack (thickness ∼200 μm) used here consisted of 1) a\nCNT network cathode for O₂ reduction to H₂O₂, 2) a CNT-COOFe²⁺ cathode to chemical reduction\nH₂O₂ to ^•OH and HO^– and to regenerate Fe²⁺ <i>in situ</i>, 3) a\nporous PVDF or PTFE insulating separator, and 4) a CNT filter anode\nfor remaining intermediate oxidation intermediates. The sequential\nelectro-Fenton was compared to individual electrochemical and Fenton\nprocess using oxalate, a persistent organic, as a target molecule.\nSynergism is observed during the sequential electro-Fenton process.\nFor example, when [DO]_in = 38 ± 1 mg L^–1, <i>J</i> = 1.6 mL min^–1, neutral pH,\nand <i>E</i>_cell = 2.89 V, the sequential electro-Fenton\noxidation rate was 206.8 ± 6.3 mgC m^–2 h^–1, which is 4-fold greater than the sum of the individual\nelectrochemistry (16.4 ± 3.2 mgC m^–2 h^–1) and Fenton (33.3 ± 1.3 mgC m^–2 h^–1) reaction fluxes, and the energy consumption\nwas 45.8 kWh kgTOC^–1. The sequential electro-Fenton\nwas also challenged with the refractory trifluoroacetic acid (TFA)\nand trichloroacetic acid (TCA), and they can be transferred at a removal\nrate of 11.3 ± 1.2 and 21.8 ± 1.9 mmol m^–2 h^–1, respectively, with different transformation\nmechanisms.