This study compared properties of silica (SiO2) from rice husk (RH) and rice husk ash (RHA) extracted by acid- and heat-treatment. The SiO2 from RH was in amorphous phase with nearly 100% purity while that from RHA was in crystalline phase with 97.56% purity. Both extracted SiO2 were used in the synthesis of zeolite NaY but that from RH was better due to the efficiency in product recovery and simplicity of extraction. After the NaY was exchanged to NH4Y and calcined to convert to HY, the product did not carry over the textural properties of the parent NaY and NH4Y.

The nature of paraquat adsorption is compared between zeolite NaX and NaY which have the same faujasite structure but different Si/Al ratio, namely 1.2 and 2.2, respectively. The adsorption was proposed to occur via ion exchange and expected to increase with Al content. However, NaX had a lower paraquat adsorption capacity than NaY. The bare and paraquat-containing zeolites (PQX and PQY) were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, N₂ adsorption–desorption analysis, magic-angle spinning nuclear magnetic resonance (MAS) NMR, and X-ray photoelectron spectroscopy. The presence of adsorbed paraquat was confirmed by Fourier transform infrared spectroscopy. Paraquat adsorbed in supercages of the zeolites resulting in a decrease of surface area and displacement of sodium cations. Results from ²³Na MAS NMR and X-ray photoelectron spectroscopy indicated that interaction of sodium ions in the cavity of NaX was stronger than that in NaY, making it less exchangeable with paraquat.

The goal of this work was to utilize rice husk silica (RHS) and porous materials synthesized with RHS, including mesoporous material (MCM-41) and microporous materials (zeolite NaY and NaBEA), for adsorption of herbicide paraquat. The adsorption occurred although cation exchange and the capacity decreased in the following order: NaY > NaBEA > MCM-41 > RHS, consistent with the amount of Al. The adsorption on all adsorbents fitted well with the Langmuir model and the maximum adsorption capacity of 185.2 mg/g-adsorbent was obtained on NaY. In addition, blue dye in commercial grade paraquat did not interfere with paraquat adsorption. Although MCM-41 was the most efficient adsorbent for the blue dye, RHS was favorable in terms of production cost. A mixture of NaY and RHS is recommended for simultaneous adsorption of paraquat and blue dye.