Utilization of agricultural waste to herbicide removal: Magnetic BEA zeolite adsorbents prepared by dry-gel conversion using rice husk ash–derived SiO2 for paraquat removal

This study presents a method that will allow for the sustainable utilization of rice husk ash-SiO2 (RHAS), an agricultural-waste–derived material, as a bulky solid SiO2 source in the zeolite beta (BEA)/Fe3O4 composite (magnetic RHAS-BEA) synthesis via a dry-gel conversion method for the removal of paraquat, a typical herbicide, from aqueous solutions. Ultrasonic waves were used to facilitate the formation of uniform Fe3O4-contained dry precursor gel before converting to the magnetic BEA zeolite composite for the first time. Magnetic RHAS-BEA was then characterized and compared with a non-magnetic sample (RHAS-BEA) using X-ray diffraction and field-emission scanning electron microscopy. In terms of adsorption, we found that paraquat had a high adsorption rate on the samples and conformed to a pseudo-second-order kinetic model. The adsorption behavior of paraquat on the synthesized RHAS-BEA and magnetic RHAS-BEA obeyed the Langmuir adsorption isotherm model. The synthesized adsorbents were reusable for at least four cycles, via thermal regeneration at 450 °C. The inclusion of Fe3O4 particles in magnetic RHAS-BEA disrupted neither the crystallization of the BEA zeolite nor the adsorption of paraquat and enabled easy collection via an external magnet.