Mechanism of ultrasonic impregnation on porosity of activated carbons in non-cavitation and cavitation regimes

Ultrasonic impregnation has proven to be an effective method to improve surface area and pore volume during preparation of activated carbons. However, the mechanism by which the promotion effect of ultrasonic impregnation is still ambiguous. Fundamental wave pressure (FWP) and broadband integrated pressure (BIP) were used to estimate the non-cavitation (vibration) energy and cavitation energy, respectively. The effects of FWP and BIP on the pore volume, surface area, surface functional groups, and microcosmic morphology were investigated in non-cavitation and cavitation regimes. Ultrasonic vibration promoted the surface enlargement and pore development of activated carbons, and it mainly affected the development of mesopore volume (Vmes) in both the pore volume and the mesopore-size-distribution range. The Vmes was enhanced by 60%–100% in the non-cavitation regime. Ultrasonic cavitation also facilitated porosity development of activated carbons, and it mainly affected the development of specific surface area (SBET) and micropore volume (Vmic). The excessive cavitation led to a decrease of the porosity of activated carbons, so the BIP should be optimized during impregnation. The highest SBET, Vmic, and Vmes for activated carbons were obtained by in the presence of both FWP and BIP, which were enhanced by 29.05%, 30.23%, and 113.33%, respectively, compared with the corresponding value for the activated carbon prepared without using ultrasonic impregnation. This work provided new insight into the role of the acoustic energy present during impregnation in tuning properties of activated carbons.