Rice Husk Wastes as a Precursor for Synthesis and Adsorption Modeling of Silicon Oxide Nanoparticles for Methylene Blue and Textile Wastewater Treatment

Abstract

Silicon oxide nanoparticles (SiONPs) were successfully synthesized from rice husk waste using a low-temperature, bio-derived route, and their structural, optical, surface, and environmental remediation properties were comprehensively investigated. UV–visible spectroscopy revealed a strong absorption maximum at ~452–453 nm, corresponding to a reduced optical band gap of 2.73 eV, calculated using the Planck relation ​. This band gap is significantly lower than that of stoichiometric SiO₂ (≈5.0–9.0 eV), indicating a pronounced red shift attributed to non-stoichiometry (SiO), oxygen vacancies, surface defects, and heteroatom incorporation. FTIR analysis confirmed the presence of Si–O–Si and Si–O bonds, abundant surface hydroxyl (Si–OH) groups, and minor residual organic functionalities, supporting a defect-rich and surface-functionalized structure. XRF and SEM–EDX analyses showed that the nanoparticles are silicon-rich, with Si contents of ~90.9 wt%, alongside minor K (2.52 wt%), Ca (1.63 wt%), Mg (0.47 wt%), Al (0.20 wt%), P (0.15 wt%), and S (0.15 wt%), and no detectable transition-metal impurities, confirming that the visible-light activity is intrinsic to the SiONPs. Zeta potential measurements indicated a well-defined point of zero charge (PZC) at pH ≈ 5.8, with positive surface charge under acidic conditions and increasingly negative charge at neutral to alkaline pH, favoring adsorption of cationic pollutants. X-ray diffraction showed a silica-dominated, semi-crystalline system composed mainly of nanocrystalline quartz with minor magnesium and aluminium silicates and a significant amorphous fraction (≈17–25 wt%). Rietveld refinement yielded acceptable agreement factors (R_p = 6.21%, R_wp = 8.94%, χ² = 2.20) and crystallite sizes of 25–45 nm, lattice strain of (3.2–6.8) × 10⁻³, and dislocation densities of (0.5–1.6) × 10¹⁵ m⁻², indicating a defect-rich nanostructure. Dynamic light scattering confirmed a relatively narrow particle size distribution consistent with the nanocrystalline dimensions. The functional performance of the SiONPs was demonstrated through adsorption studies using methylene blue and real textile wastewater. Adsorption kinetics followed a pseudo-second-order model (R² = 0.987), while equilibrium data fitted the Langmuir isotherm best (q_max = 92.5 mg g⁻¹, R² = 0.994), indicating monolayer adsorption on a homogeneous surface. When applied to textile effluent, the SiONPs achieved removal efficiencies of 91.2% for color, 77.2% for COD, 74.4% for BOD₅, and 74.4% for TSS. Overall, the combination of visible-light activity, tunable surface charge, high adsorption capacity, and strong agreement across multiple characterization techniques highlights rice-husk-derived SiONPs as low-cost, sustainable, and highly effective materials for wastewater treatment, photocatalysis, and related environmental applications