Comparative synthesis of zeolite A from agricultural residue ashes: Influence of silica source on water softening and fixed-bed column performance

ABSTRACT Agricultural residues such as rice husk, rice straw, and sugarcane bagasse generate ashes rich in silica that can serve as sustainable precursors for zeolite synthesis. However, systematic comparisons of zeolite crystallization and ion-exchange performance derived from different agricultural residue silicas remain limited. This study presents a comparative synthesis of zeolite A from silica extracted from rice husk, rice straw, and bagasse ashes and evaluates how silica source influences zeolite properties and water softening performance. The precursor ashes and extracted silicas were first characterized to determine their physicochemical properties. Zeolite A was subsequently synthesized via hydrothermal crystallization under controlled conditions. Well-defined cubic zeolite A crystals were obtained at crystallization temperatures of 80–100°C, while extended crystallization time reduced crystal size without altering morphology. The synthesized materials were evaluated for Ca²⁺ and Mg²⁺ removal through ion exchange using both batch experiments and fixed-bed column systems. Zeolite synthesized at 80°C for 16 h achieved hardness removal efficiencies of 86.98%, 87.50%, and 97.92% for rice husk-, rice straw-, and bagasse-derived materials, respectively. The superior performance of the bagasse-derived zeolite was attributed to its favorable Al/Na ratio and smaller crystal size, which enhance ion-exchange capacity. In fixed-bed column operation, this material exhibited the highest ion-exchange capacity (528.6 mg g⁻¹) and the longest breakthrough time, outperforming commercial zeolite. These findings demonstrate that silica source strongly influences zeolite crystallization and ion-exchange performance and highlight agricultural residue-derived zeolite A as a promising sustainable material for water softening.