Environmentally superior cleaning of diatom frustules using sono-Fenton process: Facile fabrication of nanoporous silica with homogeneous morphology and controlled size
Introduction
Nature shows extraordinary complex architecture in different microorganisms and this is why material scientists derive innovation in designing novel biomaterials using these structures [1]. Diatoms, as one of the examples of such microorganisms, has attracted considerable attention owing to their unique architecture of cell walls (frustules), which is related to their excellent photosynthesis performance [2]. They are found in the waterways, oceans, soils and produce approximately 20% of the oxygen generated on the earth annually [3]. Diatoms are generally known as fast-growing and dominant species (about 50%) of phytoplankton community under unlimited condition, given enough silicate and nutrients content higher than approximately 2 μmol L−1 silicate [4], [5]. Based on differences in morphology and structure of the frustules, more than 100,000 various diatoms species are classified [5]. Gordon and Drum, for the first time, proposed the potential application of diatom frustules in nanotechnology in 1994 [6]. Since then, considerable attention has been paid to the capability of unicellular diatoms to synthesize three-dimensional silica with a specific structure [5], [7]. According to the literature, the diatoms frustules have been characterized by their biocompatibility, multiple pore surfaces, excellently ordered micro/nanostructures, unique optical characteristics, mechanical and thermal stability and have been considered as potential candidates for different applications [8], [9]. Particularly, frustules can act as photonic crystals owing to the shape and size of pore morphology bodies, which leads to unique optical characteristics [10]. Incorporation of some biomolecules such as antibodies, enzymes and proteins into the individual structure of frustule can result in the development of hybrid bioreactor and biosensor that provides new opportunities in nanomedicine and biotechnology fields [11], [12]. Several research works have been conducted to study the application of frustules in solar cells, instead of the high-cost sensitized TiO2 [13]. Because of biological compatibility, chemical inertness and porous structure, the frustules possesses the promising potential to be applied as a drug delivery carrier [14]. The other interesting applications of diatoms include catalysis, adsorption, efficient filtration, immunoprecipitation and nanofabrication without altering the 3D structure which can provide the novel strategies in nanotechnology [8], [9], [15], [16]. The cell wall of the diatom is covered by an organic matrix that protects the frustule from dissolution in the aqueous medium. For most applications, the organic materials embedding the cell wall should be eliminated [17], [18]. Sulfuric acid, hydrogen peroxide, nitric acid, or sodium dodecyl sulfate (SDS)/ethylenediaminetetraacetic acid (EDTA) oxidants are most frequently proposed for removal of the organic components [19], [20], [21]. In the conventional cleaning processes using above-mentioned reagents, the concentrated and hot acidic solution is used for a certain period of time. However, the obtained acidic solution could not be removed using filter cloths due to its corrosive and oxidizing behavior. On the other hand, the subsequent dilution or centrifugation may take several hours to remove acidic solution. For instance, collection of frustules from 1000 mL diatom suspension (cultivated for 14–21 days) requires almost 600 mL of 98% sulfuric acid and produces 2–4 L of waste liquid [21]. Moreover, the organic compounds are not completely eliminated due to agglomeration of the cells and insufficient contact between the organic matrix and oxidant [21]. High temperature (500–900 °C) baking is also conducted to eliminate the organic matrix from diatom frustules, but may damage the cribellum [22]. Therefore, the conventional frustule cleaning processes need to be improved to produce large quantities of high-quality frustules for different applications.
Fenton process as one of the advanced oxidation processes (AOPs) uses the reaction between ferrous ions and hydrogen peroxide to produce •OH radicals (Eq. (1)), which can non-selectively oxidize most organic compounds [23]. Sonolysis is another AOP method which can produce •OH radicals through water dissociation as a result of generation of local fields, known as hot spots, with the high temperature and pressure (Eq. (2)) [24]. Recently, great attentions have been paid on the combination of Fenton process and ultrasonic irradiation as a new AOP, which is known as sono-Fenton (SF) process. Promising studies have been performed in the removal of different organic contaminants including dyes, pharmaceutical compounds and pesticides using SF process [25], [26].Fe2+ + H2O2 + H+ → Fe3 + OH + H2OH2O + ))) → OH + H
SF process can provide sufficient potential for the removal of organic compounds from diatom cells through additional production of reactive radical species. Moreover, utilization of ultrasonic irradiation during the cleaning process can remarkably inhibit the agglomeration of the cells, which can lead to the production of high-quality frustules. To the best of our knowledge, this is the first study that reports the removal of organic materials covering diatom cells using SF process. The aims of this study are: (a) to examine the ability of sono-Fenton process in cleaning of diatom frustules, (b) to investigate the effect of operational parameters such as concentration of Fe2+ and H2O2, suspension pH, the ultrasound power, the cell density of diatom and temperature on removal of total organic carbon (TOC) and total nitrogen (TN) from diatom cells, (c) and to characterize the properties and structure of diatom frustules before and after performing sono-Fenton process using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FT-IR), N2 adsorption–desorption process and thermal gravimetry (TG) analyses. Finally, the organic materials released from the diatom cells were identified using gas chromatography-mass spectroscopy (GC–MS).
Section snippets
Materials and methods
Cyclotella sp. cells were obtained from the Iranian Biological Resource Center. H2O2 (30%), FeSO4·4H2O (99%), NaOH (≥98%), H2SO4 (≥98%), HCl (37%), diethyl ether (C4H10O, ≥99.7%), ammonium formate (HCO2NH4, 99.9%), N,O-bis-(trimethylsilyl)-acetamide (C8H21NOSi2, ≥95%), and Methylene Blue (C16H18N3SCl) were purchased from Merck (Germany).
Cultivation of Cyclotella sp.
Cultivation of single colonies of Cyclotella sp. was carried out in batch mode f/2-enriched seawater at 18 ± 1 °C and pH 7, under a light intensity of 70 µmol m
Comparison of different processes for the removal of organic materials from diatom frustules
Fig. 1(a) shows a comparative investigation on the cleaning of diatom frustules by means of total organic carbon (TOC) and total nitrogen (TN) removal efficiencies (RE%) using different processes. Low TOC (6.9%) and TN (4.3%) removal efficiencies were observed when diatom suspension was subjected to H2O2 oxidant. To investigate the cleaning ability of mechanical stirring, the diatom suspension was vigorously stirred (2000 rpm) without adding H2O2 and Fe2+. The observations demonstrated that the
Conclusions
A comparative investigation on the cleaning of diatom frustules using different processes including Fenton, ultrasound and sono-Fenton systems has been conducted by means of determination of total organic carbon and total nitrogen removal efficiencies. Combining Fenton process with ultrasonication exhibited a positive synergy, in terms of TOC and TN removal efficiencies, while the addition of hydrogen peroxide alone to the diatom suspension had no remarkable beneficial influence. The results
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
The authors acknowledge the support provided by the University of Tabriz. P. Gholami would like to thank Centre for International Mobility (CIMO), Finland for providing EDUFI fellowship (decision number TM-18-10895).
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