Effect of the pyrolysis conditions and type of feedstock on nanobiochars obtained as a result of ball milling

Currently, the production of nanobiochar (n-BC) using ball milling is a promising and cost-effective method. Studies concerning the physicochemical properties of bulk biochar (b-BC) in relation to pyrolysis temperature, various feedstocks or types of carrier gas have been thoroughly investigated. However, there is a lack of information on how the mentioned conditions determine the properties of the obtained n-BCs. In this study, b-BCs produced from different feedstocks (willow (WL), sewage sludge (SSL), rice husk (RH) and oilseed rape (OSR)), temperatures (450-750 °C) and carrier gases (N2 or CO2) were ball-milled to produce n-BCs. The specific surface area (SBET), elemental composition (C, H, N, O), morphology (SEM), functional groups (FT-IR), and crystalline and amorphous nature of the carbon (RAMAN) were determined. The size of n-BCs depended on the feedstock used (particles ranged from 30 to 87.9 nm). The type of feedstock mostly determined the properties of n-BCs. The most visible differences between n-BCs and b-BCs were observed for surface properties and C content. For all n-BCs, an SBET increase was observed after ball milling, which may enhance the adsorption potential of biochar. The most intense SBET changes were noted for WL-derived n-BCs obtained at 450 °C (increase from 7.2 to 146.3 m2/g) and at 750 °C (increase from 3.9 to 160.8 m2/g) and for RH-derived n-BCs produced at 550 °C (increase from 3.7 to 158.6 m2/g). The C content decreased for n-BCs produced from plant biomass (WL, RH and OSR) and increased (from 20 to 28%) for n-BCs derived from sewage sludge (SSL). Other physicochemical properties mainly depended on the feedstock used for biochar production.