Permanent free radicals in bamboo biochar-based flow bed reactor: a sustainable solution for dye degradation via adsorption and radical oxidation

This study investigates the generation of hydroxyl radicals (˙OH) using the biochar-packed column flow system for methylene blue (MB) degradation in the presence of oxygen. Electron paramagnetic resonance (EPR) was employed to quantify and analyse the stability of carbon-centred permanent free radicals (PFRs) in the solid state (1017–1019 spins per g). In water, PFRs act as catalysts for the generation of hydroxyl radicals (˙OH), which are responsible for the advanced oxidation process (AOP) of organic molecules. Bamboo-based activated carbons (BACs) were produced through fast pyrolysis at various temperatures ranging from 300 to 800 °C, with the gas environment switched from N2 to CO2. The BAC treated at 400 °C (B400), which balanced physicochemical properties (lower surface area, larger micropore volume, and higher formation of reactive oxygen species in water), demonstrated the highest performance in removing MB compared to B500 and B600. The efficiency of MB removal depended on the presence of ˙OH in aerated or purged solutions, and the crucial role of dissolved oxygen in the formation of ˙OH was established. Additionally, the study explores the kinetics of MB removal, emphasising the predominance of chemical mechanisms such as electron transfer reactions and PFR-mediated oxidative degradation. This work provides valuable insights into the potential application of BACs for environmental remediation, particularly in treating dye-contaminated wastewater, eliminating the need to add H2O2 as a chemical source of reactive oxygen species (ROS) in solution.