Synthesis and characterization of CMC–HEC natural polymer hydrogels reinforced with rice husk ash–derived SiO2: swelling kinetics, mechanical performance, and soil water retention

Abstract

Natural polymer hydrogels derived from renewable biopolymers represent a sustainable alternative to synthetic soil conditioners. This study synthesized amorphous silica (SiO₂) from rice husk ash (RHA) via alkaline dissolution–acid precipitation under four preparation conditions, and incorporated it as a particulate filler into carboxymethyl cellulose–hydroxyethyl cellulose (CMC–HEC) composite hydrogels ionically crosslinked with calcium nitrate. Silica samples were characterized by FTIR, XRD, SEM, and UV–Vis diffuse reflectance spectroscopy. Although all samples were amorphous, sample S1 was selected as the filler for its narrowest, most homogeneous particle-size distribution and preserved surface silanol density, which favors dispersion and interfacial bonding within the polymer matrix. Hydrogels were fabricated at filler loadings of 0, 1, 3, and 5 wt% and characterized by FTIR, SEM, and thermogravimetric–differential thermal analysis. Swelling in distilled water, KOH, NPK + Cu-EDTA fertilizer, and KH₂PO₄ media followed pseudo-second-order kinetics (R² ≥ 0.960); response-surface analysis confirmed that increasing filler content significantly reduced equilibrium swelling (p < 0.001), attributed to densified crosslinking via Si–OH···COO⁻ hydrogen bonding. Tensile stress increased significantly only at loadings ≥ 3 wt% (one-way ANOVA, p < 0.001), with the 1 wt% sample statistically indistinguishable from the unfilled control. Hydrogel H2 incorporating 1 wt% SiO₂ demonstrated an optimal balance of homogeneous filler dispersion, swelling performance, thermal stability, and soil water retention, confirming its potential as a biodegradable soil conditioner for sustainable agricultural applications.