The effect of microwave and ultrasound activation on the characteristics of biochar produced from tea waste in the presence of H3PO4 and KOH

Abstract

The mechanism of chemical and physical activation during biomass pyrolysis is crucial for the more effective use of biomass and the preparation of functional porous biochemical materials. In this study, raw tea waste (RTW) was used to prepare the biochar samples by pyrolysis at 300 and 500 °C following phosphoric acid (H3PO4) and potassium hydroxide (KOH) activation along with microwave (MW) and ultrasound irradiation (US). Textural and chemical changes of biochar samples were designed by activation conditions and characterized in detail. SEM and N2 adsorption–desorption results showed that US and MW activation increased dramatically the surface area of the biochar from 942 m2/g to 1984 m2/g in the presence of H3PO4 and from 87 m2/g to 806 m2/g in the presence of KOH. While homogenous cylindrical macropores biochar was produced by acid-assisted US activation, microporous spherical nano-sized particles could be produced by acid-assisted US-MW hybrid activation. Above an optimum US activation time, the pores and channels in the biochar were destroyed and sintering of the particles occurred. Pyrolysis at 300 °C of the RTW activated with KOH led to the hydroxylation and reorganization of oxygen-containing functional groups and C-H groups on the surface. Before pyrolysis, the hybrid activation of biomass by physical and chemical methods can obtain the production of efficient engineered materials with preferable surface area and porosity, composition, and availability of functional groups in many applications, such as catalyst, energy storage and conversion, wastewater treatment, and soil remediation.