Pivotal Contribute of EPR‐Characterized Persistent Free Radicals in the Methylene Blue Removal by a Bamboo‐Based Biochar‐Packed Column Flow System

Water remediation with biomass derivatives has attracted attention for its sustainable impact on the earth. Chemical procedures for new adsorbent and active biomaterials must be implemented to remove organic pollutants more efficiently. Herein, we obtained a sustainable, environmentally, and low- cost biochar from bamboo Dendrocalamus giganteus (BBC, B400) by thermal treatment at 400 °C without physical and chemical pre- and post-treatments. By electron paramagnetic resonance (EPR), we determined the quantification and ageing of persistent free radicals (PFRs, 1016–1019 spins/g) present in BBC over time. We demonstrated the removal efficiency (R %) of B400 against methylene blue (MB) aqueous solutions without and with different concentrations of H2O2 using a novel B400- packed column-flow system. Results demonstrated that the MB removal efficiency via physical adsorption and chemical degra- dation strongly depended on the concentration of carbon- centred PFRs and their stability over time. Collectively, a reduced concentration of PFRs and their full passivation by proper treatments led to a remarkable loss of MB removal efficiency, thus evidencing that a PFR-mediated radical degra- dation mechanism was predominant in the MB removal process. Experiments with t-butanol alcohol (TBA) on aged B400 as a radical scavenger of the hydroxyl radical (* OH) have evidenced the critical role of PFRs in generating reactive oxygens species (ROSs) in solution, which catalysed the oxidative degradation of MB. Kinetics studies carried out on removal efficiency data vs time by all experiments established that the MB removal profiles of fresh B400 were best described by a pseudo-second order (PSO) kinetic model, thus suggesting that chemical mechanisms such as electron transfer reactions and PFRs- mediated degradation were the main contributors to MB removal. On the contrary, MB removal profiles of aged B400 with reduced or insignificant content of PFRs best fitted kinetic models, which describe physical absorption and diffusional nor radical processes, which led to a lower removal efficiency. These findings will be helpful for potential pilot and large-scale experiments to remove a wide range of contaminants in water.