Enhanced methylene blue capture by silanol-rich silica-modified biomass composites via low-temperature liquid-phase synthesis

A sodium silicate-biomass composite (Na2SiO3-RSB120) was fabricated via low-temperature liquid-phase deposition using hydrothermally treated rapeseed straw as a scaffold. Acidification-induced formation of amorphous SiO2 created a conformal coat rich in Si-OH active sites, despite decreasing the specific surface area from 1.994 to 1.569 m2·g−1. The point of zero charge shifted from 6.6 to 4.0, enhancing electrostatic attraction toward cationic methylene blue (MB) at neutral pH. Kinetic data followed the pseudo-second-order model (R2=0.999), indicating chemisorption control, while the Freundlich isotherm provided the best fit (R2=0.994). The Langmuir maximum adsorption capacity reached 462.92 mg·g−1, representing a marked improvement over pristine RSB120 (212.00 mg·g−1) and showing competitive performance relative to other alkali- or acid-activated biomass adsorbents reported in the literature. Thermodynamic analysis confirmed spontaneous, exothermic adsorption (ΔG < 0, ΔH < 0). The material retained over 85% removal efficiency after five cycles. The siloxane network enables synergistic contributions from electrostatic attraction, hydrogen bonding, and π-π stacking, with possible additional contributions from Lewis acid-base coordination, demonstrating that surface functional group density outweighs porosity in cationic dye capture, thereby offering a low-energy route to high-performance biomass-based adsorbents.