Investigation of physicochemical and chemical properties of biochar activated with carbonate, nitrate, and borohydride

Abstract

Activation of biomass before pyrolysis with various chemicals significantly affects the surface area and porosity, chemical composition, and formation and distribution of functional groups on the surface of the biochar produced. For this purpose, raw tea waste (RTW) was mixed with potassium nitrate (KNO3), potassium sodium carbonate (NaKCO3), and sodium borohydride (NaBH4) in solid form and pyrolyzed at 500 °C for 1 h. The effects of the chemical activators on biomass char formation were investigated using DTA-TGA and DSC. Compared to conventional pyrolysis, carbonate, nitrate, and hydrides increase the gasification of biochar by catalyzing the decomposition of cellulose and lignin. The effect of NaBH4 on graphitization and deoxidation of carbon is higher than that of carbonates and nitrides. In addition, all prepared biochar samples were characterized by XRD, SEM, FT-IR, elemental analysis, and N2 adsorption–desorption. While treatment of RTW with KNO3 and NaKCO3 increased the hydroxylation of the biochar, treatment with NaBH4 decreased hydroxylation by increasing dehydrogenation and dehydroxylation. Increasing boron content led to hydroxylation of the material with hydratation of NaBO2. The surface area and pore distribution results showed that nitrates and carbonates have insignificant effect on the surface area of biochar, while NaBH4 almost doubles the surface area and total pore volume of biochar by forming hydrogen.