<p>The present research reports the fabrication of a sustainable and economically viable adsorbent for efficient fluoride removal from aqueous media. Biochar derived from <i>Brassica juncea</i> (Indian mustard) crop residue, an abundant agricultural by-product which was further functionalized with biogenically synthesised nano-zirconium oxide (ZrO₂) using Brassica juncea leaf extract as a natural reducing and capping agent. The resulting composite, designated ZrO@ABC, was characterised using SEM, EDX, XRD, BET, and PZC analyses. The material exhibited an enhanced surface area of 103 m²&#xa0;g⁻¹, a total pore volume of 0.032 cm³&#xa0;g⁻¹, and an average pore diameter of 12&#xa0;nm, confirming the formation of a mesoporous structure favourable for adsorption. Batch experiments investigated the influence of pH, initial fluoride concentration, contact time, and adsorbent dosage on removal efficiency. Under optimised conditions (pH 7.0, 10&#xa0;mg L⁻¹ fluoride, 2&#xa0;g L⁻¹ dosage, 120&#xa0;min contact), ZrO@ABC achieved 99.2% of fluoride removal, outperforming unmodified biochar. The adsorption followed the Langmuir isotherm with a maximum capacity of 74.2&#xa0;mg&#xa0;g⁻¹ and was best described by the pseudo-second-order kinetic model, indicating chemisorption. The presence of common co-existing anions such as chloride, sulphate, nitrate, and bicarbonate exhibited minimal interference, confirming the selectivity of ZrO@ABC towards fluoride ions. Reusability tests further revealed over 90% retention of adsorption capacity after four cycles while the economic feasibility analysis demonstrated the low-cost nature of production. The adsorbent maintained over higher efficiency after five reuse cycles and demonstrated more than 90% fluoride stripping activity from real water samples collected from various districts of Uttar Pradesh. The eco-friendly synthesis, and practical potential of ZrO@ABC supports it as a sustainable adsorbent for water defluoridation.</p>