Nitrate-Nitrogen Adsorption Characteristics and Mechanisms of Various Garden Waste Biochars

Nitrate-nitrogen (NO₃^--N) removal and garden waste disposal are critical concerns in urban environmental protection. In this study, biochars were produced by pyrolyzing various garden waste materials, including grass clippings (GC), Rosa chinensis Jacq. branches (RC), Prunus persica branches (PP), Armeniaca vulgaris Lam. branches (AV), Morus alba Linn. sp. branches (MA), Platycladus orientalis (L.) Franco branches (PO), Pinus tabuliformis Carrière branches (PT), and Sophorajaponica Linn. branches (SL) at three different temperatures (300 °C, 500 °C, and 700 °C). These biochars, labeled as GC300, GC500, GC700, and so on., were then used to adsorb NO₃^--N under various conditions, such as initial pH value, contact time, initial NO₃^--N concentration, and biochar dosage. Kinetic data were analyzed by pseudo-first-order and pseudo-second-order kinetic models. The equilibrium adsorption data were evaluated by Langmuir, Freundlich, Temkin and Dubinin-Radushkevich models. The results revealed that the biochar yields varied between 14.43% (PT700) and 47.09% (AV300) and were significantly influenced by the type of garden waste and decreased with increasing pyrolysis temperature, while the pH and ash content showed an opposite trend (p < 0.05). The efficiency of NO₃^--N removal was significantly influenced by the type of feedstock, preparation process, and adsorption conditions. Higher pH values had a negative influence on NO₃^--N adsorption, while longer contact time, higher initial concentration of NO₃^--N, and increased biochar dosage positively affected NO₃^--N adsorption. Most of the kinetic data were better fitted to the pseudo-second-order kinetic model (0.998 > R² > 0.927). Positive b values obtained from the Temkin model indicated an exothermic process of NO₃^--N adsorption. The Langmuir model provided better fits for more equilibrium adsorption data than the Freundlich model, with the maximum NO₃^--N removal efficiency (62.11%) and adsorption capacity (1.339 mg·g^-1) in PO700 under the conditions of pH = 2, biochar dosage = 50 mg·L^-1, and a reaction time of 24 h. The outcomes of this study contribute valuable insights into garden waste disposal and NO₃^--N removal from wastewater, providing a theoretical basis for sustainable environmental management practices.