Alkali-Assisted Sol-Gel Synthesis of Amorphous Silica from Coconut Shell Charcoal Ash: Effects of NaOH Concentration and Extraction Temperature

Abstract

The utilization of coconut shell charcoal ash as an alternative silica source offers a potential route for biomass waste valorization. However, the effects of NaOH concentration and extraction temperature on product yield and silica characteristics remain insufficiently understood. This study investigated amorphous silica synthesis through acid leaching, alkaline extraction, and sol-gel precipitation. NaOH concentrations of 2.5, 3.5, and 5 N were applied at 60 and 100 °C. The sodium silicate filtrate was acidified with HCl to pH 9, followed by aging, washing, and drying. Product yield responded non-monotonically to the extraction conditions, with the highest observed value of 6.50% at 3.5 N NaOH and 100 °C. SEM analysis showed that higher NaOH concentration generally produced finer and more uniformly distributed morphological features, although the 5 N conditions yielded only 3.67–4.00%. FTIR confirmed characteristic Si–OH and Si–O–Si functional groups, while XRD showed broad diffraction halos centered at approximately 2θ = 26°, indicating predominantly amorphous silica. These results show that extraction conditions affect product recovery and material characteristics differently, highlighting the need to balance yield and physicochemical properties when selecting synthesis conditions.