Ammonia nitrogen adsorption performance and mechanisms of biochar and lignite modified composite functional materials

The efficient removal of ammonia nitrogen (NH₄⁺) from wastewater is of significant importance in agricultural, industrial, and environmental contexts. In this study, a composite functional material (Biochar-Lignite composite) was synthesized from agricultural biomass-derived biochar and low-rank lignite through low-temperature oxidative pyrolysis, with the aim of developing a high-performance adsorbent for NH₄⁺. The material was systematically characterized in terms of its microstructure, surface morphology, functional group composition, and chemical bonding properties. Adsorption performance and mechanisms for NH₄⁺ were also investigated. Results showed that the composite possesses a rough surface, a well-developed porous structure, and an abundance of oxygen-containing functional groups such as carbonyl and hydroxyl. Under the optimal preparation conditions obtained through a single-factor experiment (biochar-to-lignite ratio of 1:1, pyrolysis temperature of 225 °C, and duration of 2 h), the composite achieved an NH₄⁺ adsorption capacity of 3.14 mg/g with a removal efficiency of 20.92%. The composite exhibited a removal efficiency that was 3 and 5 times that of the unmodified biochar and raw lignite, respectively. Based on qualitative analysis of surface functional groups, adsorption kinetics, isotherm modeling, and multiple characterization methods, the adsorption process was found to be synergistic, dominated by chemisorption via ion exchange and complexation, with supplementary physisorption through electrostatic interaction, van der Waals forces, and pore filling. This work provides a theoretical basis and key data for designing efficient NH₄⁺ adsorbents, while also proposing a novel strategy for the resource utilization of agricultural biomass waste and low-rank lignite.