Probing the photocatalytic competency of hydroxyapatite synthesized by solid state and wet chemical precipitation method

Abstract

In this work, we have demonstrated a comparative investigation on the photocatalytic competency of hydroxyapatite (HAp) prepared by solid state (S-HAp) and wet chemical (W-HAp) precipitation method. Both synthesis protocols utilized eggshell and di-ammonium hydrogen phosphate as the Ca and P precursors respectively while the calcination temperature was fixed at 900 °C. In both cases, X-ray diffraction analysis exposed that the synthetic HAp is biphasic in nature where β-tricalcium phosphate (β-TCP) appeared as the 2nd phase but obviously, the % volume ratio of the later phase was different. Methylene blue (MB), a most accustomed model dye was chosen as the targeted pollutant for photodegradation study and the reaction was carried out using both indoor (simulated sunlight) and outdoor (direct sunlight) irradiation. The photodegradation efficacy of both types of HAp was appraised by monitoring the dye degradation rates. It was clearly noticed that HAp prepared by solid state method triggered the photo-process more actively. S-HAp showed an efficacy of 99.82% under sunlight irradiation and 98.53% under simulated sunlight irradiation (0.2 g S-HAp; 40 ml 10 × 10−6 M MB; pH-9; 120 min irradiation time). On the contrary, W-HAp exhibited only 23.15% efficacy under solar irradiation and 18.76% under simulated sunlight irradiation (0.2 g W-HAp; 40 ml 10 × 10−6 M MB; pH-7; 120 min irradiation time). The reason for greater activity of S-HAp could be the consolidated effect of its inferior crystallinity as well as the capability of generating more active species with the presence of higher percentage of β-TCP.