Effects of Stirring Speed and Impregnation Duration on Phosphate Impregnation and Physicochemical Properties of Rice Husk Biochar

Efficient phosphate fertilization remains challenging due to the limited bioavailability of phosphorus in soil and losses from conventional phosphate fertilizers. This study investigated the effects of stirring speed (600 and 1200 rpm) and impregnation duration (6–30 h) on phosphate impregnation and mass transfer into rice husk-derived biochar using monoammonium phosphate (MAP). Proximate composition and acid- and water-soluble P content were analyzed to assess nutrient retention and diffusion behavior. Impregnation duration exerted the strongest influence, producing progressive reductions in moisture and volatile matter and increased ash content across all treatments. Water- and acid-soluble P increased consistently with longer contact time, with maximum values of 14.350 ± 0.450% and 15.787 ± 0.186% (mean ± SD), respectively, at 1200 rpm and 30 h. Mass transfer rates were highest during the initial 6 h, followed by a sharp decline as adsorption sites became saturated and diffusion resistance increased. While higher stirring speed moderately improved early mass transfer, its effect diminished at extended durations. These findings indicate that phosphate impregnation in rice husk biochar is primarily governed by impregnation duration, with stirring speed contributing only a modest enhancement. Further release kinetics studies are required to determine whether the resulting phosphate-impregnated biochar exhibits slow-release fertilizer behavior.