Abstract Developing versatile and energy-efficient processes to synthesize functional nanomaterials is of significant in response to economic concerns, enviroment, and technological challenges. This study presents a synergistic route for the facile, green, and low-cost synthesis of nitrogen-doped nanobiochars (NNBs) from an agriculture waste without any chemical supplements, promoting environmental sustainability. Specifically, rice husk is treated at 800 °C for 5 min in an enclosed reactor, followed by quenching in water and ultrasonic vibration in a water/ethanol mixed solvent. Surface morphology, specific surface area, crystalline structure, phase component, and chemical composition of the NNBs are characterized by electron microscopy, Brunauer–Emmett–Teller, x-ray diffraction, Raman, Fourier transform infrared spectroscopy, and x-ray photoelectron spectroscopy measurements, respectively. The results indicate that the NNBs possess porous structures with a high specific surface area of 303.4 m 2 /g and a large pore volume of 1.23 cm 3 g −1 . Moreover, the porous nature and functional groups, including C=NH (55.0%) and N-H (34.35%), in NNBs are harnessed for removing Ciprofloxacin, a common antibiotic pollutant in water, via hydrogen bonding and other interactions. As expected, NNBs demonstrate a high removal efficiency of 72.73% and and adsorption capacity of 7.27 mg g −1 at a pH of 5 and contact time of 150 min. These findings therefore opens new possibilities for scalable production of value-added materials from agriculture wastes for water treatment, enhancing public health and environmental protection.