K ₂ CO ₃ -activated carbon derived from industrial hemp waste: characterization and methylene blue adsorption studies

This study investigated the effects of the potassium carbonate impregnation ratio of industrial hemp waste (stems with leaves) and pyrolysis temperature on the properties of carbon adsorbents and the adsorption of methylene blue (MB). Detailed characterization of the materials was performed using low-temperature nitrogen adsorption/desorption, Raman spectroscopy, thermal analysis, point of zero charge (pHPZC) determination, scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FT-IR). An increase in the impregnation ratio led to an increase in surface area, enhanced porosity, and a decrease in the number of functional groups according to the FT-IR spectra. The mean adsorption energy values (E > 8 kJ/mol) indicate that the adsorption process follows an ion-exchange mechanism. Among the four adsorption isotherms studied, the R2 values were highest for the Langmuir and Temkin models (0.999 and 0.993, respectively). The maximum experimental MB adsorption capacity was 432.2 mg/g, which is in close agreement with the Langmuir isotherm model. The heat of adsorption values, estimated from the bT constant of the Temkin equation, exceed 20 J/mol, indicating chemical interactions. Kinetic data are best described by the pseudo-second-order model (R2 > 0.999), suggesting that electrostatic interactions are the rate-limiting step. The AC800-15 sample demonstrated higher kinetic efficiency with an initial adsorption rate (h) of 159.42 mg/(g·h), which is 20% higher than that of AC800-10. These parameters correlate with its improved structural characteristics: a higher specific surface area (SBET = 1483.1 m2/g) and total pore volume (Vtot = 0.710 cm3/g). Intraparticle diffusion model (IDM) analysis revealed a multi-stage mechanism in which pore diffusion follows the initial boundary layer effect. Thermodynamic analysis confirmed the spontaneous nature of the process (Gibbs free energy ΔG° = −3.7, −3.6 kJ/mol). Adsorption is driven by the synergistic effect of electrostatic interactions, π–π stacking, hydrogen bonding, and micropore filling.