Calcium oxide addition and ultrasonic pretreatment-assisted hydrothermal carbonization of granatum for adsorption of lead

Abstract Hydrochar is a promising adsorbent for the removal of heavy metals, but the low surface area limits the removal efficiency and practical application. Therefore, improving the surface area of the hydrochar is critical to increasing the adsorbent removal. In this study, the ultrasonic pretreatment of biomass (10‒30 min) and CaO addition (5‒15%) were adopted to assist the hydrothermal carbonization (HTC) of granatum at 220°C. The properties of the modified hydrochar and the removal efficiency for Pb 2+ in the aqueous solution were investigated. Results showed that the porosity of hydrochar was obviously improved by the CaO addition, and the largest surface area of 21.86 m 2 ·g −1 was obtained during HTC with 15% CaO addition. Meanwhile, the functional groups of ‒OH and C═O increased and the pH of the hydrochar increased from weakly acidic to alkaline by CaO addition. The Pb 2+ adsorption capacity of raw hydrochar was 10.03 mg·g −1 , and it was enhanced by 80.76‒171.58% after CaO addition. The ultrasonic pretreatment of granatum had little effect on the characteristics of hydrochar except to improve the surface area from 8.27 to 9.06 m 2 ·g −1 , resulting in a 1.30‒6.78% increase in the adsorption capacity.