Magnesium-doped hydroxyapatite for application in the photocatalytic degradation of different dye mixtures simultaneously under sunlight irradiation

The search for efficient materials for cleaning water is still being explored today. With this objective, pure hydroxyapatite (HAp) was prepared in this work as a ceramic matrix doped with 1, 3 and 5% magnesium by a microwave-assisted hydrothermal method (MAH) aimed at photocatalytic application under sunlight irradiation. XRD and FTIR analyses showed that only HAp was obtained from all the samples, and Rietveld refinement indicated a reduced crystallite size and increased microstrain after Mg doping. Nanorod-like morphologies were observed after analysis by SEM-FEG, with a decrease in the length and width of the particles after inserting the dopant. The UV‒Vis results revealed that the HAp bandgap decreased to 3.75 eV after doping. The photoluminescence spectra showed that oxygen vacancies were generated in the presence of Mg, which also caused an increase in the specific surface area of HAp. Photocatalytic tests were conducted with mixtures of methylene blue, crystal violet and malachite green dyes. After 120 min of sunlight irradiation, the sample with 3% Mg obtained the best results, achieving up to 94% degradation of the dyes. Furthermore, reusability tests showed that there was a small reduction in photocatalytic efficiency after five cycles, without structural changes in the photocatalyst. Inhibition tests showed that superoxides and holes are the active radicals, and tests with inorganic anions revealed that carbonates, nitrates and sulphates promote degradation, while phosphates reduce photocatalytic performance. This work highlights the promising alternative application of hydroxyapatite for simultaneous efficient photodegradation of different pollutants using sustainable energy.

Graphical abstract