Ultrasonic-assisted recycling of Nile tilapia fish scale biowaste into low-cost nano-hydroxyapatite: Ultrasonic-assisted adsorption for Hg2+ removal from aqueous solution followed by "turn-off" fluorescent sensor based on Hg2+-graphene quantum dots.

This study was planned to recycle calcium and the phosphorus-rich Nile tilapia fish scale biowaste into nano-hydroxyapatite (FHAP), using ultrasonic-assisted extraction of calcium and phosphorus from fish scales, which was optimized in term of extraction time, acid concentration, extraction temperature, and ultrasonic power. These two elements were determined simultaneously by inductively coupled plasma atomic emission spectrometry and the FHAP phase was formed upon addition of the extracted element solution in alkaline medium using homogenous precipitation assisted with ultrasound energy. The FHAP adsorbent was characterized by x-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and Brunauer-Emmett-Teller. A combination of FHAP and the ultrasonic method was then used to remove Hg2+ from aqueous solution. Four significant variables affecting Hg2+ removal, namely, adsorbent dosage, pH, ultrasonic power, and adsorption time, were studied. The results exhibited that the optimal conditions for maximizing the removal of Hg2+ were 0.02 g adsorbent dosage, pH 8, 0.4 kW ultrasonic power, 20 min adsorption time, and 30 °C adsorption temperature. The sorption mechanism of Hg2+ was revealed by isotherm modeling, indicating that FHAP adsorbent has a potential for Hg2+ removal in aqueous media with the maximum adsorption capacity being 227.27 mg g-1. This adsorption behavior is in agreement with the Langmuir model as reflected by a satisfactory R2 value of 0.9967, when the kinetics data were fitted with pseudo-second-order. Therefore, the FHAP could be an alternative adsorbent for the ultrasonic-assisted removal of Hg2+ at very high efficiency and within a very short period of time.