Hydrometallurgical Recovery and Sorption Behavior of Rare Earth Elements from Terra Rossa Deposits: Evidence from the Seydişehir–Akseki Region (Türkiye)

Terra rossa (red soils) developed over carbonate terrains represent an unconventional yet potentially valuable secondary source of rare earth elements (REEs) due to enrichment in Fe–Al-rich weathering products. In this study, the hydrometallurgical recovery of REEs from terra rossa samples collected from the Seydişehir–Akseki region (Türkiye) was systematically investigated through acid leaching. Sulfuric acid (H2SO4), hydrochloric acid (HCl), and nitric acid (HNO3) were comparatively evaluated as lixiviants under optimized conditions. A Taguchi L16 orthogonal experimental design was used to assess the effects of acid concentration (1–4 M), temperature (25–95 °C), leaching duration (60–480 min), and solid-to-liquid ratio (1:10–1:40). All three acids achieved high REE recoveries under optimized conditions (4 M, 95 °C, 480 min, and 1:40 S/L). HCl exhibited the highest overall efficiency, yielding recoveries of 93–98%, followed by H2SO4 (89–96%) and HNO3 (88–97%). Statistical analyses indicated that acid type and concentration were the main factors controlling REE dissolution, followed by temperature and leaching duration. Overall, the results confirm that terra rossa deposits constitute an underexplored secondary REE resource and demonstrate that optimized acid leaching is an effective approach for the valorization of weathering-derived materials.