Pepsin-Assisted Synthesis of Nano-Spinel (CoAl2O4) for Photodegradation of Congo Red, Methylene Blue and Methyl Orange

Abstract The spinel-type of blue-coloured cobalt aluminate (CoAl 2 O 4 ) was synthesized with a cubic lattice structure, exhibiting enhanced photocatalytic properties by using a pepsin-assisted co-precipitation procedure. The optimum sample prepared at 1200 ºC and 3 h, following an ultrasound treatment with pepsin enzyme, exhibited nanoparticle formation in the 25–40 nm range. The highest removal percentages were found as 87.41% for Congo Red (CR), 74.49% for Methylene Blue (MB), and 68.35% for Methyl Orange (MO). The photodegradation kinetics were found to be best described by the first-order Langmuir–Hinshelwood model. The photocatalytic half-life (t 0.5 ) values were found as 63.59, 192.54 and 216.61 min for the CR, MB and MO, respectively. Comprehensive radical trapping experiments and mechanism analysis revealed that while photogenerated holes (h + ) are the primary reactive species for CR, hydroxyl radicals ( . OH) play a dominant role in the degradation of the cationic MB dye. FT-IR analysis of the recycled catalyst confirmed the structural integrity and stability of the CoAl 2 O 4 nano-spinels after three cycles. These findings suggest that pepsin-assisted CoAl 2 O 4 is a robust and versatile photocatalyst for the effective degradation of diverse organic pollutants in wastewater treatment.