A sustainable approach to waste treatment: Activated hydrochars for catalytic wet air oxidation of hydrothermal carbonisation effluent

ABSTRACT Hydrothermal carbonisation (HTC) of refinery-sludge produces a valuable hydrochar and a highly polluted aqueous effluent whose treatment remains a critical engineering challenge. Although the application of hydrochar from lignocellulosic biomass has been extensively investigated, little is known about the activation and catalytic potential of hydrochars derived from mineral-rich refinery residues. This study evaluates activated refinery-sludge hydrochars (HCR) as heterogeneous catalysts for the Catalytic Wet Air Oxidation (CWAO) of the HTC effluent, and it compares their performance with orange peel-derived hydrochars (HCO), a lignocellulosic precursor material with contrasting composition. Both hydrochars were activated by chemical (KOH, CuCl 2 ) and physical (steam, CO₂) routes. KOH activation generated highly porous catalysts, reaching 924 m²/g (HCR-KOH) and 1837 m²/g (HCO-KOH), whereas CuCl 2 activation introduced redox-active Cu species. Comprehensive characterisation (CHNS, ICPOES, XRD, XPS, N₂ isotherms, SEM-EDS) revealed strong precursor-dependent differences in mineral content, oxygen functionalities, and porosity development. CWAO experiments demonstrated that HCR-derived catalysts exhibited the highest oxidation performance. Among the activated materials, HCR–CuCl 2 displayed the highest TOC removal (>90%). However, this activity was largely driven by copper leaching into the liquid phase, leading to predominantly homogeneous oxidation. In contrast, the KOH-activated refinery hydrochar (HCR–KOH) provided high catalytic activity without detectable metal leaching, confirming its role as a truly stable heterogeneous catalyst. HCO-based catalysts also improved TOC and COD removal but exhibited lower mineralisation, driven primarily by microporosity rather than by redox sites. The higher performance of HCR-KOH is due to the presence of redox-active minerals and its moderate mesoporosity, which ensures accessibility. This produces a synergistic effect in the HCR-KOH, whereas the HCO-KOH, despite its extremely high surface area, lacks such mineral contributions, and its microporosity alone does not improve the performance of the CWAO. GC-MS analysis confirmed a reliable shift toward low-molecular-weight oxygenated compounds, consistent with an improved average oxidation state. These results demonstrate the technical feasibility of KOH-activated refinery-sludge hydrochars as effective CWAO catalysts for HTC effluent treatment, supporting a circular approach to refinery wastewater management through activation process that avoid metal leaching.