Adsorption of Phenolic Allelochemicals by P-Doped Distiller’s Grains Biochar: A Synergistic Mechanism to Reduce the Accumulation of Autotoxic Substances

Continuous cropping obstacles are a significant challenge to sustainable agricultural development, with the accumulation of phenolic allelopathic substances being one of the main causes of these obstacles. In this study, different modified biochars (10P-BC, 30P-BC, and 50P-BC) were prepared by modifying distiller's grains biochars with different proportions of H3PO4 (1:9, 3:7, and 1:1), and their ability to remove representative phenolic allelochemicals (p-hydroxybenzoic acid (p-HBA), vanillin (Van), and coumarin (Cou)) was investigated. The physicochemical properties were determined by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The adsorption characteristics and mechanism of the modified biochar on phenols were discussed by experiments on pH, adsorption cycle times, and other factors combined with adsorption kinetics, adsorption isotherm, and thermodynamic model fitting. It was found that the modified biochars with different H3PO4 ratios optimized their porous structure and introduced phosphorus-oxygen functional groups (PO, C-O-P, CO). At lower pH, the modified biochar showed a better adsorption effect, and after five adsorption-desorption cycles, it could still maintain about 40% of the initial adsorption capacity. The adsorption experiment conforms to the pseudo-second-order kinetic model and shows a good fit with the Freundlich model. The adsorption process is an exothermic and entropy-reducing physicochemical adsorption process involving various synergistic effects such as pore filling, hydrogen bonding, weak electrostatic interaction, and π-π interaction. Pot experiments showed that the addition of the 1% biochar increased the dry weight, fresh weight, and soil organic matter of asparagus lettuce. This study provides an efficient and sustainable solution for recycling agricultural waste and overcoming the barriers to continuous cultivation and has broad application prospects.