Nitrogen-Doped Carbon Nanotubes—Bucky Papers and Magnetic Nanotubes as Innovative Electrodes Assisting Advanced Oxidation Processes to Improve Reclaimed Water Quality

Inspired by the ONU sixth sustainable development goal, clean water and sanitation, this paper presents two types of electro-Fenton, EE, advanced oxidation process, AOP, to improve reclaimed water quality produced in a wastewater treatment plant in Tijuana, B.C. Mexico. Fenton-type processes are among the known AOPs that require the supply of H₂O₂ and Fe(II) to create a high oxidant medium. Two cases were studied to address this issue when removing recalcitrant organic pollutants (ROPs) from reclaimed water. Physicochemical characterization of the electrode materials, as well as of the treated water, allows the evaluation of the electro-Fenton efficiency.<br/>For the first case, H₂O₂ is formed through nitrogen-doped carbon nanotubes, N-CNTs, bucky paper, and BP, serving as a membrane and electrocatalyst at the same time for the oxygen reduction reaction (ORR) to form H₂O₂, the Fe(II) was ensured from a BP of MWCNTs@Fe₃O₄ as a composite anode. In this case, the formation of hydroxy radicals, OH°, may depend on the flow through the membrane reactor, since on the cathode, H₂O₂ was generated and went through the anode to find the iron species from Fe₃O₄ on their path. The proof of concept to evaluate the electrocatalyst and reactor performance showed an efficient remotion of recalcitrant pollutants in reclaimed wastewater (obtained from Tijuana’s city wastewater treatment plant). Fluorescence spectroscopy, chemical oxygen demand, and total organic carbon were used to evaluate water quality parameters. The success of this concept test is mainly due to the design of BP-N-CNTs in a pyrolytic reactor at a specific temperature to promote the formation of graphitic sites to catalyze ORR into two electron mechanisms.<br/>The second case started with a dispersion of N-CNTs with Fe₃O₄ deposited by electrophoretic deposition, EPD, onto Ti sheets to obtain all-in-one nanocomposite renewable cathodes joining the electrocatalyst for H₂O₂ and Fe(II) supply in the same electrode. The nanocomposite cathode N-CNTs@Fe₃O₄ has inserted the magnetite nanoparticles, ensuring the Fe(II) species supply in the same physical space. In this case, the electro-Fenton reactor was a simple mixed tank. The concept test used a saturated oxygen solution of sulfamethoxazole, a common pharmaceutical found in reclaimed water due to its persistence after traditional wastewater treatment processes.<br/>The new N-doped nanocomposite materials improved water quality in both the membrane reactor and the mixed tank with an all-in-one electro-Fenton cathode. The possibilities are open to design desired nanocomposites to drive a reaction mechanism caring for the environmental and social impact.