Adsorption and desorption of emerging pollutants with engineered biochar adsorbents

Emerging pollutants and personal care products have raised significant environmental concerns due to their persistence and potential adverse effects. Among these, caffeine, which is widely present in water systems, has become a prominent model compound for studying adsorption and desorption processes. Adsorption is a widely used method in water treatment due to its simplicity, cost-effectiveness, and adaptability. However, adsorbents can become saturated over time, requiring frequent replacement and generating significant amounts of solid waste. Additionally, adsorption retains pollutants without degradation, posing challenges for managing used adsorbents. Improper disposal of spent adsorbents, such as landfilling, can lead to environmental contamination. Thus, effective regeneration of biochar adsorbents is essential for long-term and environmentally sustainable applications. There are various regeneration techniques, including thermal, chemical, ultrasonic, microwave, electrochemical, and supercritical regeneration. Supercritical solvent extraction, particularly with supercritical carbon dioxide, is an emerging alternative due to its unique properties. Under supercritical conditions, CO₂ exhibits high diffusivity in the adsorbent’s micropores and low surface tension at mild conditions, enabling efficient desorption with minimal impact on the adsorbent structure. After desorption, CO₂ and the adsorbate can be rapidly separated through decompression and recovered as pure compounds of potential added value. In this study, biochar was produced through slow batch pyrolysis of birchwood (woody biomass) and miscanthus (herbaceous biomass) pellets, followed by physical activation to enhance surface properties. Adsorption experiments demonstrated that the physicochemical characteristics of the biochar, including surface area, porosity, and functional groups, significantly influenced its adsorption capacity. The effects of pH were also analyzed to elucidate the interactions between caffeine molecules and the biochar surface. The performances of two regeneration methods, e.g. solvent extraction and supercritical CO₂ desorption, were compared featuring the potential of the latter to recover the pure adsorbate without contaminations and the need of expensive downstream separation processes.