Calcium–Magnesium oxide by the ball-milling method using eggshell as calcium source: its study for photodegradation of methylene blue

Abstract

In this study, the synthesis of Calcium–Magnesium oxide through the ball milling method using eggshell as a CaO source, developing an ecological process synthesis, is presented. After ball milling, an annealing process was carried out in order to increase the crystallinity of the material to enhance its optoelectronic properties by varying the temperature from 450 to 900 °C. The samples were analyzed by X-ray diffraction, Raman and UV–Vis spectroscopy, FE-SEM, photoluminescence, and BET analysis. The ball milling process itself promoted a reduction in particle size, which increased slightly during the annealing process, achieving a more homogeneous size distribution and improving its crystallinity, according to XRD and Raman spectroscopy. The samples were tested for the photodegradation of methylene blue (100 mL of a 10 ppm solution), using LED lamps (200 W). The best photodegradation performance was recorded with the sample annealed at 750 °C, which is explained by the more homogeneous size distribution and the greater surface area, a smaller band gap, and fewer surface defects, which causes greater absorption of light, a better transport of the charge carriers and consequently a better performance in the photodegradation of MB. The complete removal of MB was achieved in 20 min under sunlight stimulation, which can be reduced to 12 min using 400 mg of the catalyst. On the other hand, the trapping experiment showed that superoxide radicals (·O2−) and holes (h+) are the key reactive species in the photodegradation process of MB. The kinetic study showed to follow a pseudo-first-order for all cases. These photodegradation results reported for the first time are better than those reported in other Calcium–Magnesium oxide synthesized by combustion or ultrasound-assisted methods.