Combining Oxidative Torrefaction and Pyrolysis of Phragmites australis: Improvement of the Adsorption Capacity of Biochar for Tetracycline

PAS biochar production and TC adsorption. The objective of this study was to evaluate the effect of oxidative torrefaction on the biochar characteristics of Phragmites australis (PAS) and its tetracycline (TC) adsorption capacity. Oxidative torrefaction combined with pyrolysis of PAS was performed, and the physicochemical properties of the biochar were characterized. Subsequently, the effects of adsorbent dosage, initial TC concentration, salinity, and temperature on the TC adsorption capacity of PAS biochar were evaluated; the kinetic, equilibrium, and thermodynamic results were used to assess the adsorption mechanism. The results showed that the biochar derived from oxidatively torrefied PAS pyrolysis (TPBC) had higher specific surface area and lower ash content than the biochar derived from raw PAS pyrolysis (PBC). TPBC showed a higher TC adsorption capacity than PBC. The adsorption kinetics were more in agreement with pseudo-second-order model than pseudo-first-order and intraparticle diffusion models. The rate of adsorption by PAS biochar was controlled by external mass transfer and intraparticle diffusion, and the adsorption process was favorable and irreversible. Moreover, the dominant mode of adsorption is physical, and the organic functional groups of PAS biochar participate in the adsorption process. In summary, oxidative torrefaction could be an effective approach for improving the TC adsorption capacity of PAS biochar.