Phenol Sorption by Highly Porous Carbon Materials Based on Vegetable Waste from Rapeseed Processing

This paper is devoted to studies of the liquid-phase adsorption of an aromatic organic compound, phenol, on biochars produced from plant residues of rapeseed processing. The sorbent materials were prepared by hydrothermal carbonization of rapeseed meal (R/HTC) (grown in the Tambov region) followed by subsequent carbonization (R/HTC/C) and alkaline activation (R/HTC/C/KOH). Post-treatment of the HTC-char enables opening of the porous space of the carbon framework due to removal of amorphous organics and the formation of defects in the material structure. The physicochemical and morphological properties were investigated by scanning electron microscopy (SEM), FT‑IR spectroscopy and Raman (vibrational) spectroscopy, and X‑ray diffractometry (XRD). Adsorption studies showed that the phenol contact time differs among the materials: for the activated material within 10 min; for R/HTC and R/HTC/C within 60 min. The experimental adsorption capacity for phenol was 298 mg/g for R/HTC, 1236 mg/g for R/HTC/C and 1309 mg/g for R/HTC/C/KOH. To determine the likely adsorption mechanism, the kinetic experimental data were analyzed using pseudo-first-order and pseudo-second-order models, the Elovich model and the intraparticle-diffusion model, while the isotherms were fitted to the Langmuir, Freundlich, Temkin and Dubinin–Radushkevich models. The kinetics for all materials are described by the pseudo-second-order equation with a contribution from diffusion-controlled phenol adsorption. The Elovich model confirms chemical heterogeneity of the R/HTC adsorbent surface. The maximum adsorption capacity from the Langmuir model was 434.8 mg/g for R/HTC, 1429 mg/g for R/HTC/C and 1667 mg/g for R/HTC/C/KOH. Parameters of the Dubinin–Radushkevich model indicate that the interaction between the adsorbent active sites and phenol is physical in nature.