Effects of KMnO4 pre- and post-treatments on biochar properties and its adsorption of tetracycline

The point at which a modification treatment is applied may affect the structure and properties of biochars (BCs). Therefore, it is important to investigate the interfacial chemical behaviors of BCs with pre- or post-modification. In this study, wheat straw biochars were modified by KMnO4 via pre-treatment of the biomass and post-treatment of the biochar (Mn-BC and BC-Mn, respectively). Pristine BC was also prepared as a control. The biochars were characterized by scanning electron microscopy, elemental analysis, Brunauer-Emmett-Teller measurement, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffractometry, and X-ray photoelectron spectroscopy. The BCs were used to remove tetracycline (TC) from aqueous solution, and the influences of environment temperature, pH, and ionic types and strength were systematically investigated. The potential mechanism associated with TC adsorption by BCs was further explored. The results showed that compared to post-treatment, pre-treatment with KMnO4 greatly increased the surface area and pore volume, changed the surface functional groups, produced more carbon structure defects, and altered the element and crystalline mineral components in BC. The pH of the solution had little influence on TC removal, but the cations and anions in the solution slightly inhibited the adsorption of TC by BCs. Increasing the temperature spontaneously enhanced the adsorption behavior, which improved the TC adsorption by BCs. The pseudo-second-order and Freundlich models best fitted the adsorption of TC by Mn-BC. The Mn-BC adsorption capacity for TC reached 107.77 mg·g−1, which was 2.7 and 3.4 times higher than that of BC-Mn and BC, respectively. Tetracycline adsorption by Mn-BC was a spontaneous (ΔG < 0), endothermic (ΔH > 0), and randomness increased process (ΔS > 0). The mechanisms associated with TC adsorption by Mn-BC included pore-filling, hydrogen binding, π-π interactions, and electrostatic interactions. Mn-BC showed a good regeneration performance by using NaOH as the desorption agent. Therefore, pre-treatment with KMnO4 efficiently improved the properties of BC and promoted its adsorption performance toward TC.