This study presents a sustainable method for synthesizing high-purity amorphous silica from geothermal slag using citric acid as a green catalyst and microwave-assisted sol–gel processing. Silica was extracted from the slag of PT Geo Dipa Dieng, Central Java, and precipitated at controlled pH levels (2, 4, 6, and 12). Characterization revealed that acidic conditions (pH 2 and 6) yielded the highest SiO₂ purity (97.59% and 97.26%, respectively), with optimal recovery at pH 6 (68.69%). Fourier Transform Infrared (FTIR) spectroscopy confirmed the formation of Si–OH and Si–O–Si networks, while X-ray Diffraction (XRD) verified the amorphous nature of the silica. Field-Emission Scanning Electron Microscopy (FESEM) revealed that the synthesized particles were spherical and nanosized (50–80 nm) at low pH, whereas at alkaline conditions they exhibited irregular morphologies. The results demonstrate that citric acid combined with microwave irradiation provides a practical, eco-friendly route to produce high-quality amorphous silica from industrial geothermal slag. © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2026.