In-situ hydrogen peroxide formation and persulfate activation over banana peel-derived biochar cathode for electrochemical water treatment in a flow reactor

Electrochemical advanced oxidation processes (EAOPs) are effective for the removal of organic contaminants from groundwater. The choice of an affordable cathode material that can generate reactive oxygen species (ROS) such as hydrogen peroxide (H₂O₂) and hydroxyl radicals (•OH) will increase practicality and cost effectiveness of EAOPs. Carbon enriched biochar (BC), which is derived from pyrolysis of biomass, has emerged as an inexpensive and environmentally-friendly electrocatalyst for removing contaminants from groundwater. In this study, a banana peel-derived biochar (BP-BC) cathode packed in a stainless steel (SS) mesh was used in a continuous flow reactor to degrade the ibuprofen (IBP), as a model contaminant. The BP-BC cathodes generate H₂O₂ via a 2-electron oxygen reduction reaction, initiate the H₂O₂ decomposition to generate •OH, adsorb IBP from contaminated water, and oxidize IBP by formed •OH. Various reaction parameters such as pyrolysis temperature and time, BP mass, current, and flow rate, were optimized to maximize IBP removal. Initial experiments showed that H₂O₂ generation was limited (∼3.4 mg mL⁻¹), resulting in only ∼ 40% IBP degradation, due to insufficient surface functionalities on the BP-BC surface. The addition of persulfate (PS) into the continuous flow system significantly improves the IBP removal efficiency via PS activation. The in-situ H₂O₂ formation and PS activation over BP-BC cathode results in concurrent generation of •OH and sulfate anion radicals (SO₄^•−, a reactive oxidant), respectively, which collectively achieve ∼ 100% IBP degradation. Further experiments with methanol and tertiary butanol as potential scavengers for •OH and SO₄^•− confirm their combined role in complete IBP degradation.