Preparation of cheap nano-silica from rice straw for environmental applications

Abstract

Nano-silica was successfully synthesized from sodium silicate by precipitation (SiO_P) or sol-gel (SiO_SG), from tetraethyl orthosilicate by surfactant-assisted sol-gel (SiO_CTAB), and from rice straw (RS) by precipitation (SiO_RS) or combustion (SiO_C) methods. These samples were utilized as low-cost and methylene blue (MB) adsorbents from aqueous solutions. XRD, FT-IR, HR-TEM, SEM, and BET measurements were carried out to identify the physicochemical properties of the prepared adsorbents. HR-TEM images of all nanomaterials were observed to consist of irregular nano-spherical particles with agglomeration. Nanomaterials' mean particle sizes can be arranged in the following descending order: SiO_P (20.7 nm) > SiO_CTAB (12.8 nm) > SiO_SG (10.8 nm) > SiO_RS (8.8 nm) > SiO_C (8.6 nm). The order of BET surface areas for the different SiO₂ nanomaterials was SiO_C (226.9 m²/g) > SiO_RS (212.2 m²/g) > SiO_SG (115.8 m²/g) > SiO_CTAB (96.2 m²/g) > SiO_p (90.6 m²/g). Different experimental parameters' impact, including adsorbent amount, temperature, contact time, pH, and initial dye concentration, were examined. The results revealed high removal of MB onto most Nano-silica samples. SiO_P, SiO_CTAB, SiO_SG, SiO_RS, and SiO_C adsorbents exhibited rapid adsorption rates of around 20.3%, 64.1%, 64.5%, 83.8%, and 96.4% of the MB dye, respectively, during the first 5-minute period of contact time. At 90 minutes, SiOC adsorbent demonstrated a high efficiency of 97.6%. Adsorption is affected by both intraparticle diffusion and pseudo-second-order kinetic models. Adsorption equilibrium follows the Langmuir isotherm. For each Nano-silica adsorbent, the thermodynamic parameters ΔS^o, ΔH^o, and ΔG^o were analyzed.