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

https://doi.org/10.1016/j.ultsonch.2020.104966Get rights and content

Highlights

  • FHAP was synthesized by coprecipitation method sonochemically assisted.

  • Turning kitchen trash into effective Hg2+ adsorbent with considerable capacity.

  • Ultrasonic-assisted adsorption for Hg2+ was done at a very short equilibrium time.

  • Langmuir isotherm with extremely high adsorption capacity for Hg2+ was obtained.

  • The pseudo-second order model gives the better description of adsorption phenomena.

  • This work shows low process cost and short process time.

Abstract

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.

Introduction

Nanotechnology has applications in all divisions of science and technology with diverse substances being prepared and used for the elimination of water pollutants and numerous attempts being made to discuss various aspects of water treatment by adsorption using nano-adsorbent materials [1]. The nanomaterial-based sorbent is an excellent solid phase due to its demonstrated ability to elevate, to a great extent, the sorption capacities of the target analytes. This is because of a special feature of nano-scaled materials, i.e., high surface-area-to-volume ratio, making it a promising solid sorbent for adsorption procedures and homogeneous distribution in solution. Fish is consumed worldwide on a day-to-day basis contributing toward a huge amount of fish biowaste, comprising of fish scales. This may leave residues in the environment leading to health problems and pollution if not properly treated or utilized. Fish scales have a potential for reuse as raw material or could be converted into valuable products, which could be an effective measure toward waste management of fish biowaste, resulting in a lower environmental impact. Over the past several years, fish scales which are rich in calcium and phosphorus, have been used for the synthesis of calcium hydroxide phosphate (IUPAC name) or hydroxyapatite (HAP, Ca10 (OH)2 (PO4)6) by several methods. HAP has been extensively studied due to its practical applications and natural occurrence and is used as an artificial bone substitute in orthopedic, neuro, dental, and plastic surgeries [2]. In addition, HAP and its composite can also be utilized as an alternative adsorbent [3], [4]. HAP is prepared by precipitation methods using calcium nitrate (Ca (NO3)2, potassium dihydrogen phosphate (KH2PO4), and ammonia (NH3) [5] or by using phosphoric acid (H3PO4) solution with calcium hydroxide (Ca(OH)2) [6]. The development of alternative methods toward eco-friendly experimentation has greatly progressed. For example, the preparation of HAP from Labeo rohita fish scales was performed by Modal et al. [7]. They used an alkaline heat treatment method with very high-temperature thermal treatment at 700–800 °C for calcination and a temperature of 1200 °C for sintering to get the HAP. Paul et al. [8] also synthesized nano-crystalline HAP from Catla catla fish scales using calcination at 200 °C, 400 °C, 800 °C, 1000 °C, and 1200 °C to produce biogenic HAP powder. Pon-On et al. [9] produced HAP material from fresh Probarbus jullieni scales by alkaline hydrolysis using the magnetic stirrer method, while Kongsri et al. [10] employed the alkaline heat treatment method using a magnetic stirrer device to extract the HAP from Tilapia nilotica fish scales. Similarly, Sathiskumar et al. [11] used the fish scales of Labeo rohita to synthesize HAP by alkaline-heat method followed by calcination. However, HAP production using the magnetic stirrer assisted method requires a long period for completion. Huang et al. [12] employed the process of enzymatic hydrolysis with tilapia fish scales for obtaining HAP. However, it is quite expensive due to the chemicals used for enzymatic hydrolysis and the process is also difficult to scale up. It is widely known that ultrasound waves are powerful in accelerating various steps such as homogenization and mass transfer followed by acoustic cavitation. This emerges from the growth and collapse of micro-bubbles and generation of final acoustic streaming induced by sound waves creating microscopic turbulence through the solid particles. It also increases the dispersion of the material leading to lower consumption of the adsorbent, which could result in high removal efficiency and equilibrium in a very short time [1], [13], [14]. Water pollution and water impurities along with the depletion of water resources are serious environmental threats faced by us today. Heavy metals are the basic and most important pollutants which are widely distributed in air, water, soil and foods released via different operations [15]. Mercury is one of the most toxic heavy metals due to its bioaccumulation and is highly toxic even at very low dosages causing a serious threat to the ecological system as well as human health [16]. For example, Minamata and amyotrophic lateral sclerosis (ALS) diseases are neurological syndromes involving the neurons responsible for controlling muscle movement [17]. Mercury ions tend to accumulate in living organisms and are known to be carcinogenic [18]. Therefore, the removal of mercury species and controlling these compounds is an essential task.

To the best of our knowledge, although an extensive amount of information is at hand concerning the synthesis/preparation of HAP from fish scales (FHAP), no ultrasonic method has yet been applied for this purpose. Among a variety of approaches, the utilization of ultrasound for material synthesis has been extensively examined over many years and is now positioned as one of the most powerful tools in nanostructured material synthesis [19]. For these reasons, alternatively, we present a simple, fast, green and efficient way to synthesize FHAP using ultrasonic technology. Elements like calcium and phosphorus present in fish scales have an important role in FHAP production. Hence, firstly the concentration of elements in Nile tilapia fish scale derived FHAP was increased and the ultrasonic-assisted extraction (UAE) conditions of Ca and P in the sample, such as extraction time, acid concentration, temperature, and ultrasonic power were optimized. The UAE was carried out under the synergistic effect of ultrasound action, vibration as well as heating, which significantly improved extraction efficiency, enhanced sample throughput, and accelerated the process of extraction [20]. Then the FHAP powders synthesized at different pH with a suitable ultrasonic power were characterized by XRD, SEM, EDS, TEM, FTIR, and BET while ICP-AES was used for the determination of Ca and P in the extracted solutions. The next step in our work is to use FHAP as an adsorbent for Hg2+ adsorption in ultrasonic-assisted batch experiments. Since FHAP material is low-cost, found in natural source, and does not produce toxic hazardous waste, and the Hg2+ is highly toxic and its removal from water is of the utmost importance. So, we believe that this study involves a good adsorbent used for Hg2+ adsorption, and because thus far there have only been a limited number of studies, it will make a valuable and original contribution to the literature. Different conditions (initial pH, adsorbent dosage, and ultrasonic power) were optimized to obtain the best performance of FHAP for Hg2+ removal from aqueous solution and the adsorption isotherm and kinetics of Hg2+ were analyzed.

Section snippets

Materials and reagents

All the chemicals were of analytical reagent grade. Calcium nitrate, ammonium dihydrogen phosphate, sodium acetate, sodium bicarbonate, monosodium phosphate, sodium phosphate dibasic, sodium carbonate and sodium hydroxide were purchased from QRec™ (New Zealand). Mercury nitrate, diammonium hydrogen phosphate and calcium nitrate tetrahydrate were bought from Sigma-Aldrich (USA). Hydrochloric acid (UNILAB, Australia), acetic acid (UNILAB, Australia) and sodium chloride (APS, Australia) were used.

UAE optimization of Ca and P from fish scales toward hydroxyapatite synthesis

The simultaneous determination of Ca and P by ICP-AES (Perkin–Elmer OPTIMA 2100 DV ICP-AES, Wellesley, Massachusetts, USA) was performed. The operational system was controlled using the PE Winlab software. Table S1 shows all the conditions used for ICP-AES.

For the extraction of Ca and P, 5 g of dried fish scales and 100 mL of acid solution were fixed to study the extraction performance of the UAE under different experimental conditions. All the experiments were performed in three replicates.

Conclusion

We demonstrated that the FHAP prepared from cheap and biowaste sources through a green method exhibit the ability to remove Hg2+ from aqueous solutions. Ca and P elements from Nile tilapia fish scales were ultrasonically extracted in acid solution and the FHAP phase was formed upon mixing the extraction phase in alkaline medium using the homogenous precipitation assisted with the ultrasound energy. The characterization of the FHAP synthesized through the ultrasonic process suggested the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was supported by the Post-Doctoral Program from Research Affairs and Graduate School, Khon Kaen University (60162). The authors also thank Department of Chemistry and Center of Excellence for Innovation in Chemistry, Khon Kaen University, Thailand for all the support.

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