Insight into the KOH/KMnO4 activation mechanism of oxygen-enriched hierarchical porous biochar derived from biomass waste by in-situ pyrolysis for methylene blue enhanced adsorption

Abstract

Abstract A high-performance hierarchical porous carbon enriched in oxygen-containing functional groups was prepared by using pine as the raw material, KOH and KMnO4 as a green activator via cost-effective one-step or two-step in-situ pyrolysis processes. The adsorption performance, kinetics and thermodynamics of activated biochar were evaluated using methylene blue (MB) as a model pollutant. Biochar structure was investigated by X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), N2 adsorption/desorption isotherms (BET) and scanning electron microscopy (SEM) to explore its morphological, physical, and chemical properties. A possible adsorption mechanism and pathway were also proposed. The results showed that a two-step process, higher activation (KOH and KMnO4) to biomass ratio led to excellent adsorption performance. AC-KOH-1-1 and AC-KMnO4-1-3 presented the maximum equilibrium adsorption capacities of 637.5 mg/g and 439.5 mg/g, respectively. The adsorption isotherm fit well with the Freundlich isotherm model and pseudo-second-order kinetic model, which exhibited better monolayer adsorption and chemical adsorption control. In addition, the adsorption was dominated by pore diffusion and π-π stacking interactions for KOH activation due to the well-developed pore volume (0.04990 mg/L), pore channel (18.65 nm), aromatized structure (66.03 %) and surface defective structure. However, the adsorption capacity was controlled by hydrogen bonding, electrostatic adsorption interactions, cation exchange and the donor-receptor-effect for AC-KMnO4 due to it having the highest proportion of O- and N-containing functional groups. Therefore, this cost-efficient biochar preparation technology for the efficient removal of organic pollutants has great potential for practical application.