The Removal of Tetracycline from Aqueous Solutions Using Peanut Shell Biochars Prepared at Different Pyrolysis Temperatures

Abstract

The pyrolysis temperature strongly affects the properties of the peanut shell biochar, and influences its adsorption behavior and mechanisms for contaminant removal in aqueous solutions. In this study, peanut shells were pyrolyzed at 400 °C and 700 °C to prepare two biochars (PSBC400 and PSBC700), which were then characterized using scanning electron microscopy/X-ray energy spectrum analysis, Brunauer–Emmett–Teller, elemental analysis, X-ray fluorescence, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The adsorption behavior of typical tetracycline (TC) onto the biochars was investigated, and the potential adsorption mechanisms explored. The results show that compared with PSBC400, PSBC700 has a larger specific surface area and pore volume and contains higher levels of carbon and ash, but shows lower O, N, and H content. The hydrophilicity and polarity of PSBC700 is lower, but its aromaticity is higher. Furthermore, the mineral content of PSBC400 is higher than for PSBC700. The functional groups differ between PSBC400 and PSBC700, especially those containing C and O. The Elovich and two-compartment adsorption kinetic models are a good fit to the TC adsorption processes on both biochars, but the Langmuir adsorption isotherm model provides better results. The theoretical maximum adsorption capacities of TC onto PSBC700 and PSBC400 are 33.4346 mg·g−1 and 26.4185 mg·g−1, respectively. The main adsorption mechanisms of TC onto PSBC400 are hydrogen bonding and complexation, and are closely related to the functional groups and minerals found in PSBC400. In contrast, the main adsorption mechanisms of TC onto PSBC700 are pore filling and the π–π interaction, and are mainly determined by the surface area and graphited carbon structure of PSBC700. In summary, effective biochar can be manufactured from peanut shell biomass and can be used to remove TC from aqueous solutions.