Biochar Derived from Chinese Herb Medicine Residues for Rhodamine B Dye Adsorption

In this study, one well-known CHM residue (Atropa belladonna L., ABL) was used to prepare biochar capable of adsorbing rhodamine B (RhB) with an ultrahigh surface area for the first time. Three micropore-rich ABL biochars including ABL@ZnCl2 (1866 m2/g), ABL@H3PO4 (1488 m2/g), and ABL@KOH (590 m2/g) were obtained using the one-step carbonization method with activation agents (ZnCl2, H3PO4, and KOH) via chemical activation and carbonization at 500 °C, and their adsorption performance for RhB was systematically studied with adsorption kinetics, isotherms, and thermodynamics. Through pore diffusion, π–π interaction, and hydrogen bonding, ABL biochar had excellent adsorption performance for RhB. Moreover, when C0 was 200 mg/L, biochar dosage was 1 g/L, and the contact time was 120 min; the maximum RhB adsorption capacity and removal efficiency on ABL@ZnCl2 and ABL@H3PO4 were 190.63 mg/g, 95% and 184.70 mg/g, 92%, respectively, indicating that it was feasible to prepare biochar from the ABL residue for RhB adsorption. The theoretical maximum adsorption capacities of ABL@ZnCl2 and ABL@H3PO4 for RhB were 263.19 mg/g and 309.11 mg/g at 25 °C, respectively. Furthermore, the prepared biochar showed good economic applicability, with pay back of USD 972/t (ABL@ZnCl2) and USD 987/t (ABL@H3PO4), respectively. More importantly, even after five cycles, ABL@H3PO4 biochar still showed great RhB removal efficiency, suggesting that it had a good application prospect and provided a new method for the resource utilization of traditional CHM residues. Additionally, pore diffusion, π–π interactions, and hydrogen bonding all play roles in the physical adsorption of RhB on ABL biochar. π–π interactions dominated in the early stage of RhB adsorption on ABL@H3PO4, while pore diffusion played a crucial role in the whole adsorption process on both adsorbents.