This study investigates the citric acid-assisted sol-gel synthesis of silica nanoparticles from geothermal slag under microwave heating, with a focus on the effect of pH (1, 3, 5, 7, 13) on product purity, recovery, and morphology. Characterisation via XRF, XRD, FTIR, and FESEM revealed that pH critically determines the synthesis outcome. The highest silica purity (97.494% SiO₂) was achieved at pH 3, whereas maximum recovery (68.94%) occurred at pH 7. Alkaline conditions (pH 13) reduced purity to 84.257% due to metal contamination. FTIR analysis indicated enhanced silanol (Si–OH) formation at pH 3–7, while pH 13 promoted a polymerised Si–O–Si network. Morphological analysis showed homogeneous nanoparticles with a narrower size distribution (80–120 nm) at neutral pH, compared to larger aggregates (150–300 nm) under acidic conditions. These findings demonstrate that pH can be strategically tuned to prioritise either high purity or high yield, providing a scalable and sustainable pathway for valorising geothermal waste into high-value nanomaterials and supporting circular economy objectives. © 2026, Rasayan Journal of Chemistry, c/o Dr. Pratima Sharma. All rights reserved.