| Summary: | Carbonaceous materials are a highly appealing class of adsorbents, owing to their exceptional properties, such as high surface area and thermal and chemical stability. These materials have found successful applications in water purification. Sweet chestnut (<i>Castanea sativa</i>) cupules are disposed of as waste. Valorization of these residues is a step forward in terms of circular economy and sustainability. Meanwhile, per- and poly-fluoroalkyl substances (PFASs) pose significant concerns due to their persistence, bioaccumulation, and toxicity, emerging as contaminants of concern for human health and the environment. This study focuses on preparing carbonaceous material by hydrothermal carbonization from chestnut cupules, followed by their use as adsorbents for PFAS removal from polluted water. The cupule waste material was crushed, ground, sieved, and subjected to hydrothermal treatment at temperatures ranging from 180–200 °C to produce hydrothermal carbons. The adsorbents obtained were characterized by various techniques such as nitrogen adsorption isotherm, porosimetry, point of zero charge, Fourier-transform infrared, scanning electron microscopy, and thermal, elemental, and energy dispersive X-ray analyses. Surface area (S<sub>BET</sub>) values of 42.3–53.2 m<sup>2</sup>·g<sup>−1</sup> were obtained; pH<sub>PZC</sub> ranged from 3.8 to 4.8. This study also determined the adsorption kinetics and isotherms for removing perfluorooctanoate-contaminated water. The equilibrium was established at 72 h and <i>q<sub>e</sub></i> = 1029.47 mg·g<sup>−1</sup>. To summarize, this research successfully valorized a biomass residue by transforming it into hydrothermal carbon, which was then utilized as an adsorbent for water decontamination.
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