Sorption of Mercury in Batch and Fixed-Bed Column System on Hydrochar Obtained from Apple Pomace

Abstract

This paper presents the methodology for the preparation of hydrochar obtained from waste materials of natural origin and investigates its applicability for removing mercury ions from aqueous systems. The sorption properties of the obtained hydrochar were investigated in a batch and in a flow-through column system. The hydrochar material was obtained from apple pomace, which was hydrothermally carbonized in 230 °C for 5 h in a hydrothermal reactor. The hydrochar formed in the process was thermally activated with an inert gas flow—CO2. Obtained materials were characterised with XRD, FTIR-ATR, SEM-EDS and nitrogen sorption (BET) analyses, which confirmed the obtaining of a highly porous carbon material with a specific surface area of 145.72 m2/g and an average pore diameter of 1.93 nm. The obtained hydrochar was analysed for sorption of mercury ions from aqueous solutions. Equilibrium isotherms (Langmuir, Freundlich, Dubinin–Radushkevich, Temkin, Hill, Redlich-Peterson, Sips and Toth) and kinetic models (pseudo-first order, pseudo-second order, Elovich and intraparticle diffusion) were determined. The sorption process of mercury on the obtained material is best described using the Freundlich isotherm and a pseudo-second-order kinetic model. This indicates that the process is chemical in nature The sorption of mercury ions from an aqueous solution with a concentration of C0 = 100 mg Hg/dm3 has been also carried out in a flow-through column system. The data obtained from adsorption were fitted to mathematical dynamic models (Bohart–Adams, Thomas, Yoon–Nelson, Clark, BDST and Yan) to illustrate the bed breakthrough curves and to determine the characteristic column parameters. The Yan model has the best fit across the study area, although the Thomas model better predicts the maximum capacity of the bed, which is qmax = 111.5 mg/g.