Enhancing insight into sorption-degradation interplay: A comparative study on the removal of organics by biochar through experiments and theoretical calculations

Sorption and degradation occur simultaneously during the removal of aromatic organics by biochar. However, the unclear association between sorption and degradation limited our understanding of the underlying mechanisms of organic removal by biochars. Herein, the removal of monocyclic 2,6-dimethylphenol (DMP) and bicyclic bisphenol A (BPA) by raw and potassium hydroxide modified biochars was comparatively investigated by batch removal experiments and theoretical calculations. Under identical initial concentrations, biochar displayed lower sorption but higher degradation for DMP compared to BPA, whilst higher sorption corresponded to greater degradation for the same chemical. Sorption mechanisms revealed by molecular dynamic simulations include π-π, hydrophobic, and hydrogen bonding interactions. Density functional theory calculations identified the preferred degradation sites on biochar for adsorbed organics are graphitic nitrogen, defects, carboxyl, and hydroxyl functional groups. Analysis of free radicals and degradation intermediates concluded that the degradation pathways involve both free radical pathways and electron transfer processes for DMP, while only free radicals for BPA. Experimental and computational results indicated an increase in ring quantity can lead to higher sorption affinity but lower degradability of aromatic organics by biochar. Findings of this research provide theoretical support for the advancement of biochar based aromatic contaminant mitigation strategies.