Caffeine adsorption on biochar and subsequent desorption using solvents and supercritical CO2

Caffeine is an emerging contaminant frequently detected in aquatic environments, and its effective removal and adsorbent regeneration remain key challenges for sustainable water treatment. In this study, caffeine adsorption and subsequent adsorbent regeneration were investigated using two activated biochars derived from birchwood pellets and miscanthus, with a commercial mineral-derived activated carbon as a reference. Adsorption performance was evaluated through batch experiments by optimizing contact time and pH conditions. The miscanthus-derived activated biochar exhibited the highest caffeine adsorption capacity (up to 176 mg g−1) at optimal pH condition, outperforming both birchwood-derived biochar (97 mg g−1) and commercial activated carbon (172 mg g−1), despite having a lower specific surface area. This behavior was attributed to differences in pore structure and surface functional groups, highlighting the importance of surface chemistry beyond surface area alone. Regeneration of caffeine-loaded adsorbents was examined using solvent desorption with various solutions, as well as supercritical CO2 (scCO2) extraction. Polar organic solvents, particularly ethanol and methanol, achieved the highest desorption efficiencies, reaching 78% for ASMB, while acidic, alkaline, and aqueous media exhibited markedly lower efficiencies (<27%). Supercritical CO2 enabled solvent-free regeneration, with desorption efficiencies ranging from 20 to 35% strongly dependent on adsorbent composition and surface polarity. Carbon-rich adsorbents exhibited higher scCO2 compatibility, while oxygen-rich biochars showed reduced desorption. In conclusion, by directly comparing solvent-based desorption and scCO2 extraction within a single framework, this study provides new insights into the regeneration potential of caffeine-loaded biochars and underscores the role of adsorbent properties in selecting sustainable recovery strategies.