Development of slow-release urea material based on nanohydroxyapatite and activated carbon composite via ball milling synthesis

Abstract Background Urea fertilizer is the primary nitrogen source in agriculture, but its conventional use often leads to significant nitrogen loss through leaching and volatilization. To address this issue, a slow-release fertilizer system is required to regulate nutrient delivery. Nanohydroxyapatite (nHA), a biocompatible inorganic material, can bind nutrients, while activated carbon (AC) offers high surface area and porosity, supporting controlled urea release. This study investigated the effect of mechanochemical ball milling on a previously developed nHA-AC composite for slow-release urea applications. Results The composites were synthesized by varying the milling speeds (100, 150, and 200 rpm) for 30 min to examine the effect of speed on the composite properties. The optimum milling speed speed was subsequently applied while varying the milling duration (10, 20, and 30 min). The resulting composites were characterized and evaluated for urea adsorption and release performance. The composite synthesized at 200 rpm for 10 min exhibited a relatively high specific surface area (148.94 m 2 /g), the most negative zeta potential (-38.0 mV), the highest adsorption capacity (Qt = 0.8932 g urea/g composite), and the slowest observed urea release among the evaluated composites (399.7 µg urea/min, corresponding to 10.7% release within 240 min). Conclusions This study investigates the effect of mechanochemical ball milling conditions on the physicochemical properties, urea adsorption, and release behavior of the nHA-AC composites. Comparison with the previously reported unmilled nHA–AC composite prepared from the same precursor provides contextual evidence of improved adsorption performance following ball milling; however, this comparison does not constitute a direct experimental control because the unmilled material was not evaluated in parallel in the present study. The composite prepared at 200 rpm for 10 min showed the most favorable urea adsorption and release characteristics among the composites evaluated, supporting its potential for further investigation as a carrier for slow-release fertilizer applications.