Chemically activated hydrochars as catalysts for the treatment of HTC liquor by catalytic wet air oxidation

Abstract

Hydrothermal carbonization (HTC) is a highly efficient and valuable technology for treating wet solid wastes and producing solid carbon-based materials named hydrochar. In this work, a hydrochar coming from the HTC of an anaerobic digestion sludge of wastewater treatment plant was used to assess the influence of several activation agents, a base (KOH) and different chloride salts (FeCl3, ZnCl2, and CuCl2) with the exact molar quantities, to develop materials with enhanced surface area and potential inclusion of metal active species for application in wet air oxidation processes. The KOH as an activating agent increased the surface area of hydrochar up to ca. 1000 m2/g of BET surface area. The employment of CuCl2 and FeCl3 as activating agents allows Cu- and Fe-rich doped materials of remarkable surface areas with 49.1 and 42.5 wt% of each metal, respectively. Likewise, the catalytic behavior of the different synthesized carbon-based materials as metal-free and metal-doped catalysts was evaluated for the Catalytic Wet Air Oxidation (CWAO) of a HTC aqueous liquor from a HTC process of animal manure to produce a valuable stream of higher biochemical methane potential in anaerobic digestion. CWAO effluents increased the proportion of carboxylic acids as final by-products due to the oxidation of more complex organic compounds of the initial effluent (ketones, phenols, aromatics and olefins). The CWAO treatments improve the anaerobic digestion rate in biochemical methane potential tests, although the methane production was limited by the lower TOC concentration of the treated streams after CWAO. This research contributes to developing sustainable and efficient strategies for the HTC-liquor treatment, using its solid hydrochar as catalysts, closing the loop of a Circular Economy.