Abstract
Biogenic hydroxyapatite (HAp)–TiO 2 composites were synthesized from waste eggshells and seashells as calcium precursors and evaluated as sustainable photocatalysts for organic pollutant degradation. The resulting HAp-derived seashell–TiO 2 (HSST) and eggshell–TiO 2 (HEST) catalysts exhibited comparable crystalline phases and porous structures, indicating minimal structural differences. Under UVA irradiation, both catalysts achieved high decomposition efficiencies (> 90%) for BPA, BTA, and MB; however, CR degradation with HEST was comparatively lower, at approximately 80%. Distinct differences were observed in mineralization behavior: HEST showed superior total organic carbon removal for BPA, BTA, and MB, whereas HSST exhibited enhanced mineralization of CR, primarily due to its higher dye adsorption capacity. Reactive oxygen species (ROS) scavenging experiments identified 1 O 2 and O 2 • ⁻ as the dominant oxidizing species, with OH• playing a secondary role. Both composites maintained stable degradation performance across a wide pH range, while mineralization was strongly influenced by solution pH and pollutant type through surface charge effects, adsorption–degradation coupling, and reactive species dynamics. Overall, HEST favored mineralization of non-dye and selected dye contaminants, whereas HSST more effectively removed anionic dyes via enhanced adsorption. This study demonstrates a scalable approach for converting shell waste into tailored photocatalysts for sustainable water purification.