Lanthanum-modified biochar for the selective and efficient adsorption of ammonium nitrogen in wastewater

Biochar has been widely applied in wastewater treatment and agricultural applications; however, its practical utilization is limited by poor selectivity and insufficient adsorption capacity. In this study, La₂O₃-modified biochars were prepared from rapeseed straw (RS), corn straw (BC), rice husk (RH), and cotton stalk (CS) via a sol-gel modification method followed by pyrolysis at 500 °C under oxygen-limited conditions. The adsorption performance, selectivity, and agricultural applicability of the prepared materials for NH₄ ⁺ recovery were systematically investigated. The modified biochars exhibited increased surface area, pore volume, and microporosity compared with the pristine biochars. RS-La and BC-La exhibited the highest NH₄ ⁺ adsorption capacities, reaching 43.37 and 42.23 mg g^-1 under optimal pH conditions, respectively. Langmuir fitting showed excellent agreement with experimental data (R ² > 0.99), with maximum monolayer adsorption capacities of 46.11, 47.93, 45.42, and 43.32 mg g^-1 for RS-La, BC-La, RH-La, and CS-La, respectively. All La-modified biochars demonstrated strong NH₄ ⁺ selectivity, whereas Cl^- adsorption remained negligible (0.3-0.6 mg g^-1). This selectivity was attributed to electrostatic attraction and ion exchange between NH₄ ⁺ and La-OH groups, hydrogen bonding with surface functional groups, and weak Cl^- interaction with La sites. The adsorption kinetics were satisfactorily described by both PFO and PSO models. Preliminary pot experiments indicated the potential of NH₄ ⁺-loaded biochar for nitrogen recovery and agricultural reuse.