Converting Agroforestry Biowaste into Stable Near-Natural Chars via Hydrothermal Humification and Pyrolysis for Immobilizing Plasticizer

To ensure agricultural safety and ecological security, it is crucial to effectively immobilize emerging organic pollutants, such as plasticizers, to prevent their migration in various environmental matrices. However, the ideal immobilization agent with the advantages of being environmentally friendly is very rare. In this study, low-cost and stable near-natural immobilization agents, char-derived artificial humic acids, CHAs, were proposed and prepared via hydrothermal humification (180 °C) and pyrolysis (300, 500, or 700 °C) of agroforestry biowaste. The resulting CHAs exhibit high purity (composed primarily of C (67.28–81.35%), O (6.65–21.64%), H (1.40–5.28%), and N (0.36–0.58%)) with remarkably low ash content (5.43–10.02%). Characterization revealed a compact structure with a limited porosity with small surface area (0.27–0.32 m2 g−1) and pore volume (2.99–3.43 × 10−4 cm3 g−1). Notably, high-temperature pyrolysis induced consumption of oxygen-containing functional groups while promoting aromatic structure formation. The sorption behavior of diethyl phthalate, a representative plasticizer, on CHAs was well described by both Langmuir isotherm and pseudo-second-order kinetic models. The CHAs exhibited remarkable sorption performance for diethyl phthalate, with a maximum sorption capacity reaching 3345 mg kg−1 as determined by the Langmuir model. The sorption of diethyl phthalate onto CHAs is mainly multi-layer sorption dominated by physical processes, mainly including pore filling, partitioning, hydrogen bonding, and π–π stacking. Mean sorption energies ranging from 2.56 to 4.99 × 10−3 kJ mol−1 indicate the predominance of physical sorption mechanisms. This study developed a method to convert the liquid by-product produced during hydrothermal humification of biowaste into stable near-natural and carbon-rich char materials, and the proposed materials show great promise in immobilizing pollutants from various environmental matrices.