Toward the Utilization of Char from the Hydrothermal Liquefaction of Polyethylene Plastic Waste: Studies of the Adsorption of Rhodamine B in an Aqueous Environment

ABSTRACT Plastic waste management remains a major global challenge due to the widespread use of plastics such as polyethylene in wraps, films, and bags. As part of the PLASTICE project, we are exploring hydrothermal liquefaction (HTL) for the chemical recycling of polyethylene. While liquid hydrocarbons are the primary product of HTL, char is generated as a by‐product, and its utilization is essential to improve process sustainability and economics. In this study, HTL‐derived char was evaluated as an adsorbent for the removal of rhodamine B (RhB) from aqueous solution. An equilibrium contact time of 40 min was selected to investigate the influence of adsorption parameters. Results showed that the char effectively adsorbed RhB, with nearly complete dye removal achieved using 14 mg of char under optimum conditions. RhB uptake increased significantly at pH 6–8. Adsorption behavior was best described by the Langmuir isotherm and pseudo‐first‐order kinetic models, with a higher correlation coefficient for the pseudo‐first‐order model (R 2 = 0.96) compared to the pseudo‐second‐order model (R 2 = 0.90). Thermodynamic analysis indicated that the adsorption process is spontaneous and endothermic. These findings demonstrate the potential of HTL‐derived char for sustainable plastic waste valorization and wastewater treatment.