Rice Straw-Derived Silica Nanoparticles via a Combustion-Free Route: Physicochemical Properties and Dye Removal Potential

Abstract

Amorphous silica nanoparticles were sustainably synthesized from rice straw waste via a simple non-combustive route involving alkaline extraction, acid precipitation, and calcination. The process yielded up to 18.9% silica recovery, closely matching the 20% ash produced by combustion at 1000 °C. Two variants were prepared: plain SiO₂ (without additives) and ethylene glycol-assisted SiO₂-G, which represents a distinctive strategy to tailor silica’s physicochemical properties. Characterization revealed semi-spherical, mesoporous nanoparticles, with SiO₂-G (2–19 nm) exhibiting higher surface area (402 m²/g) and more uniform morphology than plain SiO₂ (5–29 nm, 174 m²/g), which showed a more fragile and heterogeneous structure. Over 95% of dye equilibrium capacity was reached within 10 min, indicating rapid uptake. Despite its lower surface area and less uniformity, plain SiO₂ showed superior adsorption capacities, 75 mg/g for methylene blue and 285 mg/g for astrazon blue, compared to SiO₂-G (49 mg/g and 136 mg/g, respectively), attributed to its higher density of free silanol groups. Compared to literature values, both silica variants exhibited competitive performance, surpassing several conventional and silica-based adsorbents, including those derived from rice residues. This work introduces a low-cost, eco-friendly route for dye remediation using agricultural waste, with ethylene glycol modification offering a promising avenue for future material design.