Sustainable production of high-purity amorphous silica from rice husk: A comparative study on the impact of fresh and reused hydrochloric acid leaching

Rice husk (RH), an abundant agricultural by-product, offers a sustainable resource for silica production. However, achieving high purity while reducing costs and environmental impacts remains challenging. This study proposes a sustainable pretreatment approach using reused hydrochloric acid (HCl) compared to fresh HCl, with varying leaching durations, to produce high-purity amorphous RH silica. RH was either unleached or leached with a 3 % fresh or reused HCl solution (first and second reuse) for 1, 2, or 3 h, followed by calcination at 700°C for 6 h. HCl leaching reduced RH mass by ∼50 %, conserving energy during calcination. Silica from leached RH had a significantly higher purity (96.35 - 96.80 % SiO 2 ) than silica from unleached RH (81.76 % SiO 2 ), and commercial silica (93.40 % SiO 2 ), with no significant difference between fresh and reused HCl treatments. Leached RH silica had a higher amorphous content (50.30 - 53.70 %) than commercial silica (46.05 %). Silanol and siloxane were identified as the primary functional groups in leached RH silica. RH silica had a specific surface area exceeding 200 m²/g, higher than that of commercial silica (130.57 m²/g). Leached RH silica was classified as mesoporous, with pore diameters ranging from 3.39 to 53.43 nm. Reusing HCl for up to two cycles and limiting leaching to 1 h effectively removed impurities and consistently produced high-purity silica, while reducing HCl consumption, energy use, and environmental impact. These findings demonstrate a sustainable, cost-efficient approach for producing high-purity mesoporous silica from RH.