Sorption of reactive red by biochars ball milled in different atmospheres: Co-effect of surface morphology and functional groups

In this study, biochars were ball milled in a planetary mill to investigate the effect of atmosphere (air, N2, and vacuum) on physicochemical properties and pollutant removal capacities of resulted ball-milled biochars. Ball milling turned granular biochars into nanosized particles and increased their surface area (SA) from 2.60-343 m2 g−1 to 300–452 m2 g−1. Oxygen-limited atmosphere (N2 and vacuum) was more favorable for reducing biochar size than air atmosphere. Decrease of particle size increased external surface area of biochar produced at 450 °C (HC450), so SA of ball-milled biochars produced in N2 (BMHC450-N) and vacuum (BMHC450-V) was higher than that produced in air (BMHC450-A). However, for biochar produced at 600 °C (HC600), smaller particle size also caused severe agglomeration, thus BMHC600-N and BMHC600-V had smaller SA than BMHC600-A. Apart from morphology, atmosphere also affected the formation of surface functional groups. N2 and vacuum inhibited the introduction of O-moieties to HC450 during milling process with the decrease of C O to 2.78–3.02%. Ball milling increased the removal capacity of biochars to reactive red from 1.70–3.60 mg g−1 to 9.20–34.8 mg g−1. Compared to air atmosphere, N2 and vacuum were superior for the enhanced removal due to higher external SA and less negative charge, facilitating the external diffusion of reactive red. Overall, both morphology and surface functional groups of ball-milled biochars were strongly affected by milling atmosphere, influencing their ability to remove contaminants.