Dual-functional hydrochar via hydrothermal carbonization for norfloxacin removal: Fractal adsorption kinetics and mechanism elucidation.

Escalating concentrations of norfloxacin (NFX) in surface and wastewaters demand sustainable remediation strategies. In this study, dual-functional hydrochars were synthesized from argan nut shells (ArNS) via hydrothermal carbonization (HTC), with process conditions optimized by varying temperature (150-200 °C) and residence time (2-6 h). Among the materials, H1:5@150-4-prepared at 150 °C for 4 h with a biomass-to-water ratio of 1:5-exhibited the best performance, achieving a monolayer NFX adsorption capacity of 27.85 mg g at 298 K. To describe the adsorption process, equilibrium data were fitted using advanced isotherms, including Generalized Langmuir (GL), Multisite Langmuir (MSL), and the Model of Linear Adsorbates (MLA). The GL model provided the best fit (R > 0.97), reflecting heterogeneous surface binding and deviation from classical monolayer assumptions. For adsorption kinetics, the modified fractal multiexponential model (mod(f-mexp)) demonstrated superior performance, capturing a two-step mechanism with strong R values (0.996-0.997) across 298-318 K. At 298 K, a high rate constant (k = 0.53 min) and fractal exponent (h = 0.61) indicated rapid surface uptake followed by slower diffusion-limited adsorption. To elucidate interaction mechanisms, pH-edge, ethanol disruption, and ionic strength experiments were conducted and complemented by FTIR, XPS, UV-Vis spectroscopy, and Boehm titration. These analyses revealed multiple synergistic pathways-electrostatic attraction, hydrogen bonding, π-π electron donor-acceptor (EDA) interactions, and hydrophobic effects-driven primarily by surface chemistry over porosity. Finally, thermal and regeneration studies highlighted the hydrochar's stability and reusability. H1:5@150-4 retained 95 % of its initial capacity after one cycle and 77 % after five cycles, maintaining effectiveness in real water matrices and under ionic competition. These findings validate HTC-derived, surface-functionalized hydrochars as efficient, sustainable adsorbents for pharmaceutical removal, with broad implications for fractal kinetics and adsorption mechanism design.