Enhancing Silica Extraction from Rice Husk Ash Using Acid Solvent and Polyethylene Glycol-400: Efficiency and Mechanism

Abstract

Silica derived from agricultural waste offers a sustainable pathway for producing advanced biomaterials, however, precise control over purity, structure, and morphology remains a challenge for dental applications. This study systematically investigates the effects of acidic solvent and polyethylene glycol-400 (PEG-400) concentration on the physicochemical properties of silica synthesized from rice husk ash (RHA). Silica was prepared via sol–gel route using NaOH, followed by acid neutralization with hydrochloric acid, sulfuric acid, or nitric acid. To tailor structural and morphological features, PEG-400 was introduced as a structure-directing agent at silica-to-PEG mass ratios of 1:1, 1:3, 1:5, and 1:7. Among the investigated acids, HCl treatment produced high-purity (99.4 wt%) amorphous silica, characterized by silanol (Si–OH) and siloxane (Si–O–Si) functional groups as confirmed by FTIR. Incorporation of PEG-400 at a 1:1 ratio induced the formation of a hybrid amorphous–crystalline phase, including cristobalite and tridymite while maintaining a high silica content (99.06 wt%). Morphological analysis revealed that the optimized PEG-400 concentration promoted uniform block-like agglomerates with enhanced structural homogeneity, attributed to controlled polymer–silica interactions during gelation. These findings demonstrate that synergistic HCl extraction and PEG-400 modification enable tunable control over silica phase composition and morphology, demonstrating RHA-derived silica as a promising candidate for next-generation dental biomaterials.