Utilization of Indonesian coal fly ash as a silica precursor for mesoporous silica based slow-release fertilizer

This study explores the utilization of coal fly ash (FA) as a silica source for the synthesis of mesoporous silica (MS) for slow-release fertilizers (SRF). Initially, the FA was washed with concentrated hydrochloric acid to produce silica with 77.95% purity. The extracted silica was treated with sodium hydroxide at 80 °C to form sodium silicates. The synthesis of MS utilized 2 w/w% cetyltrimethylammonium bromide (CTAB) as a template at various pH values (6, 8, and 10) and a hydrothermal method at 100 °C for 24 h, resulting in uniform particle sizes and improved surface areas. MS synthesized with CTAB at pH 10 (MSFA_CTAB10) shows the largest surface area of 1351 m² g^-1 with a particle size of 138.97 nm ± 24.87 nm and a purity of 93-97% silica. MS can adsorb up to 565.83 m g^-1 of urea and release 88% within 96 h. Reusability tests showed that the material retained adsorption-release functionality over three cycles. When applied to tomato plants, the urea-loaded formulations improved several biomass, leaf-number, and chlorophyll-related parameters compared with conventional urea under the tested conditions. These results indicate that coal fly ash-derived MS is a promising waste-derived carrier for the further development of slow-release fertilizer applications.