| Summary: | Nitrate-nitrogen (NO<sub>3</sub><sup>−</sup>–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), <i>Rosa chinensis Jacq</i>. branches (RC), <i>Prunus persica</i> branches (PP), <i>Armeniaca vulgaris Lam.</i> branches (AV), <i>Morus alba Linn.</i> sp. branches (MA), <i>Platycladus orientalis</i> (L.) Franco branches (PO), <i>Pinus tabuliformis Carrière</i> branches (PT), and <i>Sophorajaponica Linn</i>. 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 NO<sub>3</sub><sup>−</sup>–N under various conditions, such as initial pH value, contact time, initial NO<sub>3</sub><sup>−</sup>–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 (<i>p</i> < 0.05). The efficiency of NO<sub>3</sub><sup>−</sup>–N removal was significantly influenced by the type of feedstock, preparation process, and adsorption conditions. Higher pH values had a negative influence on NO<sub>3</sub><sup>−</sup>–N adsorption, while longer contact time, higher initial concentration of NO<sub>3</sub><sup>−</sup>–N, and increased biochar dosage positively affected NO<sub>3</sub><sup>−</sup>–N adsorption. Most of the kinetic data were better fitted to the pseudo-second-order kinetic model (0.998 > <i>R</i><sup>2</sup> > 0.927). Positive b values obtained from the Temkin model indicated an exothermic process of NO<sub>3</sub><sup>−</sup>–N adsorption. The Langmuir model provided better fits for more equilibrium adsorption data than the Freundlich model, with the maximum NO<sub>3</sub><sup>−</sup>–N removal efficiency (62.11%) and adsorption capacity (1.339 mg·g<sup>−1</sup>) in PO700 under the conditions of pH = 2, biochar dosage = 50 mg·L<sup>−1</sup>, and a reaction time of 24 h. The outcomes of this study contribute valuable insights into garden waste disposal and NO<sub>3</sub><sup>−</sup>–N removal from wastewater, providing a theoretical basis for sustainable environmental management practices.
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