Methylparaben was degraded in the presence of ABO₃ perovskite catalysts (A: La, Bi, and B: Ti-Fe, Fe) synthesized on a monolithic structure using the sol–gel method. The catalysts were characterized by BET, SEM, XRD, and FTIR. The highest degradation was observed using BiFeO₃ on the monolithic structure. The methylparaben degradation and iron leaching were 82.8% and 0.06 ppm, respectively, under the optimum conditions, which were pH=7, [H₂O₂]₀=0.5 mM, and 0.1 g/L BiFeO₃ loading. The toxicity tests were carried out by cress seeds and 1.09% inhibition was observed. Methylparaben degradation followed the second-order reaction kinetics, whereas the activation energy was 54 kJ/mol.

This study explores the synthesis and application of carbon quantum dots (CQDs)-based composite photocatalysts, including CQDs, CdS@CQDs, and Mn-doped CdS@CQDs, for the degradation of methylene blue (MB) and Reactive Black 5 (RB5) through photocatalysis. The synthesis of the photocatalysts involved a meticulous procedure utilizing olive oil as a precursor. Characterization studies employing transmission electron microscopy (TEM), X-ray diffractometer (XRD), and Brunauer-Emmett-Teller (BET) surface area confirmed the successful synthesis of the composite photocatalysts with well-dispersed nanoparticles and varying surface areas. Photocatalytic degradation experiments revealed that Mn-doped CdS@CQDs exhibited the highest degradation efficiency for both MB and RB5 under optimized reaction conditions, with pH identified as the most significant parameter, and statistical analyses supported the validity of the experimental data. Based on the results, the highest MB degradation efficiency (99.87%) was achieved at the following reaction conditions: pH = 9, catalyst amount = 0.55 g/L and initial hydrogen peroxide concentration (HPC) = 1 mM while the highest RB5 degradation efficiency (98.15%) was obtained at the following reaction conditions: pH = 3, catalyst amount = 1 g/L and HPC = 0.55 mM. Comparison with the literature showcased the competitive performance of the synthesized photocatalysts, achieving higher efficiencies with lower amounts of photocatalysts and hydrogen peroxide. Kinetic studies revealed that the first-order reaction kinetic was observed in both MB and RB5 degradation. This comprehensive investigation underscores the potential of Mn-doped CdS@CQDs as efficient photocatalysts for wastewater treatment, offering insights for future research and application in environmental remediation efforts.