Sm³⁺-doped CaCO₃/PEG nanoparticles with concentrations of 0.02, 0.04, 0.06, and 0.08 mol were synthesized from dolomite using a biomimetic method. XRD confirmed the rhombohedral calcite structure, while dislocation density, microstrain, and crystallite sizes (29–35 nm) were measured. FTIR analysis verified the calcite phase and Sm–O bonding near 529 cm⁻¹. Doping effects were examined using XPS, FE-SEM with EDX mapping, and UV-visible DRS spectra. The 0.02 mol sample showed a red shift and blue-region emissions, with an aggregated spherical morphology. Photocatalytic degradation under UV light was tested against methylene blue (MB), crystal violet (CV), and Congo red (CR), achieving degradation efficiencies of 95%, 92%, and 89%, respectively. Mechanisms, scavenger effects, recyclability, and catalyst stability were analyzed. After two reuse cycles, MB degradation remained high at 92%. Thus, 0.02 mol Sm³⁺-doped CaCO₃/PEG nanoparticles demonstrate strong potential for wastewater dye degradation. © 2025 Elsevier B.V., All rights reserved.