Extraction of high-purity silica from palm oil mill bottom ash via citric acid leaching coupled with ultrasonic irradiation

Bottom ash (BA) is a solid industrial waste generated in boiler units for energy production. Its high silica content makes BA a suitable precursor for silica extraction. This study aims to develop a BA silica extraction method by combining citric acid leaching and ultrasonic irradiation. In this study, the effects of sonication time and aging time were systematically optimized using Response Surface Methodology. Optimum conditions were achieved with a sonication time of 4 h and an aging time of 36 h, yielding a silica yield of 33.63% and extraction efficiency of 72.35%. The developed mathematical model showed good agreement between predictions and experimental data, with aging time being the most significant factor. The presence of siloxane groups (Si-O-Si) was detected in the silica based on the resulting infrared spectrum. XRD characterization showed that the resulting silica has an amorphous structure. The combination of citric acid leaching and ultrasonic irradiation achieved very high purity (99.40%) while improving the meso- and microporous structure of silica, making it an alternative for managing BA waste from industry.