Abstract
We report the study of nanoporous silica-iron oxide composite generated from diatom frustules as a highly active catalyst for the photodegradation of the dye Rhodamine-6G. The unique architecture and high surface area of diatoms were utilized to immobilize iron oxide on their surface to form the composite. Photodegradation was carried out under 365-nm radiation and was observed using the absorption spectrum of the dye. The reaction was found to follow pseudo-first-order kinetics. The results were compared with commercially available granular iron oxide. The rate constant K (min−1) for photodegradation by the diatom composite was found to be as high as 0.0584 min−1 for diatom-FeOx composites, which is 52% higher than 0.0273 min−1 for granular FeOx at a dye concentration of 0.02 mm. The unique structural morphology and the synthetic strategy have led to the composites showing superior activity in the degradation of the dye Rhodamine-6G.
1 Introduction
Among the numerous organic pollutants released into water [1], various dyes used in the manufacture of textiles are a major concern to public health [2]. These dyes have carcinogenic and mutagenic effects on both aquatic and terrestrial life forms even when present in small concentrations [3]. Their presence in various aquatic bodies decreases the penetration of light and thereby inhibits photosynthesis apart from chelating with other metal ions, which may be present in the ponds and streams and cause microtoxicity to fish and other organisms [4]. In addition, direct inhalation or ingestion of dyes can cause difficulty in breathing and can be fatal in some cases [5]. These hazards call for a rapid and active degradation of these dyes without further production of toxic agents. Unfortunately, many of these dyes are resistant to biodegradation and typical nitrogen-containing dyes undergo reductive anaerobic degradation, during which they release potentially carcinogenic aromatic amines [6]. Consequently, the treatment of effluent-containing dyes has been a challenge.
Besides conventional biological, chemical, and physical treatments, heterogeneous photocatalysis is evolving as an efficient and inexpensive treatment technique for removing dyes from water [7]. Metal oxide nanoparticles have been of great interest, and several nanoparticles such as Al2O3, ZnO, and α-Fe2O3 have been studied as potential candidates as absorbents and catalysts for dye photodegradation [2], [8]. Semi-conductor photocatalysts such as TiO2 have been studied extensively, although their large band gaps limit absorption mainly to the ultraviolet (UV) regions [9], [10], [11]. This has led to the investigation of materials with narrow band gap, which absorb in the visible (Vis) region of the solar spectrum. Among metal oxides that absorb light up to 600 nm are iron oxides, which are widely available in the earth’s crust [12], [13], [14], [15], and micro/nano structured iron oxides have been used in the photocatalysis of dyes [16].
Typical photocatalysts are supported on substrates such as silica [17], glass [18], [19], carbon [20], mixed metal oxides [21], stainless-steel, alumina, activated carbon, concrete surfaces, and silica gel [22]. There is much interest in using silica as support for heterogeneous catalysts. TiO2-SiO2 has been widely used as a heterogeneous catalyst, as it prevents the release of TiO2 from the surface to the environment [23]. Mesoporous catalysts improve the access to the active sites and consequently increase in their catalytic activity [24].
Diatoms are a class of unicellular microalgae that are found in both fresh and marine environments with size varying from 2 μm to 2 mm. The siliceous cell walls, referred to as frustules, have unique porous structures that have surface area as high as 200 m2 g−1 [25]. The silica layer also contains functional groups such as -COOH, -OH, -NH2, and -SiOH that facilitate the attachment of different molecules [25]. Recently, they have been used in applications such as water treatment, sensing, electroluminescent displays, batteries, solar cells, and drug delivery [26]. Even though there have been reports on metal oxides impregnated on the diatomite [27], [28], [29], [30], [31], [32], as referred usually, these composites were not made in the nanoform and have been synthesized after the frustules have been harvested, thereby leading to differential distribution of the metal oxides on the diatomite [33]. Even though these composites have been used to study Fenton and photo-Fenton reactions, they have not been tested for their prowess in the degradation of the dye Rhodamine-6G (Rh-6G). Nano- and micro-sized iron oxides have been used for its photodegradation [16].
In the current study, we present a simple yet scalable technique of synthesis of diatom-FeOx composites from live cultures of diatoms and their application in the catalytic photodegradation of Rh-6G.
2 Materials and methods
All reagents were of analytical grade and used as acquired. Milli-Q water was used throughout the experiment. Granular ferric oxide was acquired from Sigma-Aldrich, and 30% H2O2, NaH2PO4, and Na2HPO4 were obtained from Merck. Commercially available granular ferric oxide was ball milled for 4 h, and the sub-micron-sized powder (5–10 microns) was used as a control to compare the effectiveness of the nanostructured diatom-FeOx composite.
Synthesis and characterization of the diatom-FeOx composite was carried out as reported in our previous paper [34]. The diatom-FeOx composite was characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM; Leo 1530 VP) equipped with an energy dispersive X-ray spectrometer (EDX), Brunauer-Emmett-Teller (BET) surface area analyzer (Quantachrome Autosorb-I), thermogravimetric analyzer (TGA; Pyris 1; Perkin Elmer Inc.), and Fourier transform infrared (FTIR) spectroscopy (IRAffinity-1; Shimadzu). UV-Vis absorption was monitored with a Shimadzu UV-2450 double-beam spectrophotometer using quartz cells with a path length of 10 mm at room temperature. X-ray photoelectron spectroscopy (XPS) analysis was done using a Surface Science Instruments SSX-100 with operating pressure ~2×10−9 Torr. Monochromatic AlK-α X-rays (1486.6 eV) were used with beam diameter of 1 mm. Photoelectrons were collected at a 55° emission angle. A hemispherical analyzer determined electron kinetic energy using a pass energy of 150 V for wide/survey scans and 50 V for high-resolution scans. A flood gun was used for charge neutralization of non-conductive samples.
Briefly, FeCl3·8H2O was added to the diatom cultures to help the uptake of Fe onto the biosilica in their natural state. The composites were finally obtained by precipitating the iron oxide on the diatoms at alkaline pH followed by thermal treatment with hydrogen peroxide to remove the organics (Scheme 1) [35], [36], [37]. The photodegradation experiments were done by adding 3 ml of 0.02 mm solution of Rh-6G and 10 mg of the photocatalyst to a glass vial and sonicated for 10 min. This mixture was transferred to a quartz tube, and 0.225 ml of 30% H2O2 was added to it and the absorption spectrum was recorded at time zero. The mixture was irradiated under UV light (λ=365 nm, 6 W) at room temperature, and absorption spectra were recorded at 10, 20, 40, 60, 120, 140, 180, 210, and 240 min. The effect of concentration was studied at 0.05, 0.04, 0.03, 0.02, and 0.01 mm concentrations of the dye. pH effects were studied at pH 4.0, 5.0, 6.0, 7.0, and 8.0 using phosphate buffer of 0.2 mm concentration.
3 Results and discussion
The surface of the diatom-FeOx composite was visualized using TEM and SEM. The TEM images of the original diatom and composite are presented in Figure 1. The treatment made them form a cluster mass rather than remain as single particles. EDX mapping confirmed the presence of Fe on the diatom Si along with its uniform distribution on the surface (Figure 1D–F). The SEM and TEM images clearly show the presence of FeOx in the nano form with a uniform distribution across the surface of the diatom silica and in the pores of the silica forming a composite, highlighting the importance of the synthetic route.
Functional group analysis and the presence of iron oxide was also confirmed with the help of the FTIR spectrum (Figure 2). The peak observed at 422 cm−1 was attributed to Fe-O vibration [38], while that at 1624 cm−1 and those between 3300 and 3500 cm−1 were from SiO2 and free silanol groups [39], [40]. This is expected, as the Si has been derived from diatoms using H2O2, which is an oxidizing agent and might lead to the presence of silanol groups. The thermal stability of the diatom-FeOx composites was determined using TGA (Figure 3). The weight loss below 120°C was attributed to the loss of physisorbed water, and that between 120°C and 300°C was due to loss of chemisorbed water [41]. The weight loss in the 400–600°C range was attributed to the decomposition of organosilanes and dehydration of silanol groups. The weight loss between 400°C and 800°C may also be attributed to the dehydroxylation of the silica surface [42], [43]. The diatom-FeOx composites displayed good thermal stability. The BET surface area analyzer gave the surface area of the composites as 70 m2 g−1 as compared to the 200 m2 g−1 of the pure diatom silica. This is expected, as N2 does not adsorb onto the metal oxide surfaces and can be viewed as a confirmation of the presence of FeOx on the surface of the silica.
The analysis of the XPS spectra (Figure 4) clearly proved the presence Fe2+ in the sample. The peaks for Fe2p at 706 and 721 eV clearly showed the presence of Fe2+. The shift is expected due to the lack of charge neutralization of the non-conductive samples. That is, due to the evolution of electrons, the sample accumulates a positive charge on the surface, which is neutralized by the surrounding stray electrons in that area of the sample when a non-monochromatic X-ray source is used. However, the use of a monochromatic X-ray source releases controlled electrons, which does not neutralize the positive charge, causing the XPS peaks to shift to higher binding energies. The atomic percentage of the Fe2+ in the composite was 22.12% and that in the granular ferric oxide was 4.18%, which acts as a factor for the higher activity by the diatom FeOx composite compared to granular FeOx.
The various characterization techniques used have brought forth the fact that the composites are naturally functionalized due to their biological origin. SEM imaging has shown the porous nature of the sample, and EDX mapping has proved that the iron oxide is uniformly distributed on the Si substrate. The XPS data have proved that the Fe2+, the main species that catalyzes the reaction, is available abundantly in the composites that have nano-FeOx when compared to granular bulk FeOx. This uniform distribution and better availability of the active species in the composite is responsible for the better degradation activity by the composite material when compared to granular bulk Fe. It is also clear that the current synthetic procedure has given rise to composites that has nano-FeOx distributed uniformly on porous silica.
3.1 Photodegradation in the presence of diatom-FeOx
Rh-6G in aqueous medium has a yellow color and absorbs strongly at 525 nm. The comparison of the absorption spectra of Rh-6G with catalyst, with granular FeOx, and in the absence of catalyst are shown in Figure 5A. In each case, the absorption spectra were recorded after a 201-min irradiation. The color disappeared completely in case of the diatom-FeOx composite, while significantly less photodegradation was seen in the case of granular FeOx and none in the case of the diatom control.
Figure 5B shows not only decrease in the absorption intensity of the dye with increase of irradiation time but also a slight bathochromic shift of the λmax. This indicates a change in the chromophore of the dye pointing out toward complete destruction of the aromatic rings in the dye [44]. The spectra clearly demonstrate that the diatom-FeOx composites effectively catalyzed the photodegradation of the dye Rh-6G. This is attributed to the large surface area of the composites available for catalysis and the unique architecture of the diatom frustules, leading to the uniform distribution of the catalyst.
The kinetics of photodegradation is presented in Figure 5. The absorbance at λmax of 525 nm reduced gradually with increase in the irradiation time. The solution containing the diatom-FeOx composites as catalysts showed complete reduction of the dye at 210 min when compared to granular FeOx. Figure 5B and C show the kinetics of the photodegradation of the dye Rh-6G by diatom-FeOx composites and by granular FeOx, respectively. Figure 6A shows degradation as a function of time, and Figure 6B shows that the reaction followed pseudo-first-order kinetics with the rate constant (K) being 0.0584 and 0.0273 min−1, respectively, for diatom-FeOx composites and granular FeOx at a dye concentration of 0.02 mm.
3.1.1 Mechanism of photodegradation
The photodegradation of Rh-6G followed pseudo-first-order rate kinetics, which was in line with what has been reported [45]. The proposed mechanism of photodegradation in the presence of diatom-FeOx composites is as follows. The FeOx on the diatom composite absorbed the UV radiation and generated an electron-hole pair [16]. This electron was scavenged by H2O2 in a Fenton mechanism or was captured by the Fe3+ to generate Fe2+ back [46]. The Fe2+ reacted with H2O2, forming OH* radicals. This was confirmed by carrying out the reaction in the absence of H2O2, which showed negligible reduction in the absorbance spectrum of Rh-6G. These radicals reacted with the dye and oxidized it [47]. The OH− ions combined with the holes and further generated hydroxyl radicals to assist the reaction:
3.1.2 Effect of pH
It has been reported that pH plays an important role in the rate of dye photodegradation [48]. The photodegradation of Rh-6G was carried out at different pH for both diatom-FeOx composites and granular FeOx. The results are presented in Figure 7. It was seen that the catalysis was most effective at pH 5.0. The higher reaction rates can be associated to two reasons. The pKa of H2O2 is 11.7 and thus the dissociation of H2O2 was maximum at around pH 5.0, leading to higher OH* radical formation at this pH. Additionally, the higher surface area of the catalyst along with the pH content played a role in the adsorption of the dye on the surface of the catalyst [49]. The structure of the dye allows a more facile hydrogen-bond-based adsorption on the catalyst surface in acidic pH when compared to basic. The OH* radicals that are formed by reaction with positive holes acted as the main oxidizing agents at lower pH [50]. The observed trend could be explained by the scavenging of the OH* radical by the H+ ions at pH <4.0. At a slightly acidic pH of 4.0–5.0, the OH* radicals are at an optimum level and hence show better activity. At pH >5.0, near pH 6.0–7.0, the FeOOH precipitated out to block out the surface pores, leading to a decrease in catalytic activity [51], [52], [53]. However, as the pH increased, more OH* radicals were produced and a slight increase in the activity was observed [54]. It was also noted that the granular iron oxide showed a somewhat different trend because it had significantly lower Fe2+ concentration (4.18% compared to the composite, which had 22.12%), which led to the generation of relatively lower numbers of OH* radicals. Consequently, its activity was less compared to the diatom composite, also limiting its response to change in pH.
3.1.3 Effect of concentration
In order to investigate the effect of concentration of the dye on photodegradation, the concentration of the dye Rh-6G was varied, keeping a constant amount of catalyst. Figure 8 clearly shows that the rate of the degradation by diatom-FeOx composites was higher than that of granular FeOx at all concentrations, although both systems showed similar trends. This is in line with what has been reported before [2], [55]. In case of the granular FeOx, the rate reached its minimum at a concentration of 0.03 mm, as all the active sites were occupied at this concentration. However, in case of the diatom composite, the rate was somewhat stable even up to a concentration of 0.04 mm and thereby decreased at 0.05 mm concentration of the dye. This is explained by the fact that the composite material is porous, has uniform distribution of the catalyst on its surface leading to significantly more active sites available for the reaction, and hence is able to deal to with larger concentration of dyes. Three individual effects explain the general trend in decrease in the activity with increase in the dye concentration. The increase in the dye concentration leads to the increase in the amount of the dye adsorbed on the catalyst surface, leading to the decrease in number of active sites that produce the OH* for catalysis [55], [56]. In addition to this, the increase in the dye concentration decreases the path length of the photon entering the dye solution and also leads to higher absorption of light, thereby decreasing the photocatalytic activity by lowering the light absorbed by the catalyst to produce the active species [57]. Table 1 compares the performance of different materials with diatom – FeOx composite, in their ability to degrade the dye.
3.2 Recycling ability of the catalyst
The ability of the catalyst to catalyze the reaction multiple times was tested by using the same amount of the catalyst for multiple solutions, and the data are presented in Figure 9. The catalyst showed stable degradation rates up to five cycles and thereby decreased in the sixth cycle. This might be due to the adsorption of the products of degradation on the catalyst surface and thereby reduction in the availability of the reaction cites for the degradation to occur.
4 Conclusion
The diatom-FeOx metal composite showed high activity in catalyzing the photodegradation of Rh-6G, which is a well-known pollutant in the textile industry. This high activity could be attributed to the high surface area available from the composite, the nanostructuring of iron oxide on the frustule surface, the uniform distribution of the catalyst on the Si surface, and also the large amount of Fe2+ available for catalysis as compared to granular FeOx. The reaction mechanism followed pseudo-first-order kinetics. A pH of 5.0 and a dye concentration of 0.04 mm were found to be optimum for dye photodegradation. The catalyst was found to be active for five cycles before decrease in its activity occurred.
Acknowledgments
This work is dedicated to Bhagawan Sri Sathya Sai Baba, Founder Chancellor of Sri Sathya Sai Institute of Higher Learning (SSSIHL). The authors acknowledge the support of the central library of SSSIHL for providing access to SciFinder®. Thanks are also due to Sai Manohar, research scholar, SSSIHL, for his help during the preparation of the manuscript.
References
[1] Bauer C, Jacques P, Kalt A. Photooxidation of an azo dye induced by visible light incident on the surface of TiO2. J. Photochem. Photobiol. A Chem. 2001, 140, 87–92.10.1016/S1010-6030(01)00391-4Search in Google Scholar
[2] Yu D, Cai R, Liu Z. Studies on the photodegradation of Rhodamine dyes on nanometer-sized zinc oxide. Spectrochim. Acta Pt. A Mol. Biomol. Spectrosc. 2004, 60, 1617–1624.10.1016/j.saa.2003.09.003Search in Google Scholar PubMed
[3] Senturk HB, Ozdes D, Duran C. Biosorption of Rhodamine 6G from aqueous solutions onto almond shell (Prunus dulcis) as a low cost biosorbent. Desalination 2010, 252, 81–87.10.1016/j.desal.2009.10.021Search in Google Scholar
[4] Hamdaoui O. Dynamic sorption of methylene blue by cedar sawdust and crushed brick in fixed bed columns. J. Hazard. Mater. 2006, 138, 293–303.10.1016/j.jhazmat.2006.04.061Search in Google Scholar PubMed
[5] Hameed B, Din AM, Ahmad A. Adsorption of methylene blue onto bamboo-based activated carbon: kinetics and equilibrium studies. J. Hazard. Mater. 2007, 141, 819–825.10.1016/j.jhazmat.2006.07.049Search in Google Scholar PubMed
[6] Horikoshi S, Hojo F, Hidaka H, Serpone N. Environmental remediation by an integrated microwave/UV illumination technique. 8. Fate of carboxylic acids, aldehydes, alkoxycarbonyl and phenolic substrates in a microwave radiation field in the presence of TiO2 particles under UV irradiation. Environ. Sci. Technol. 2004, 38, 2198–2208.10.1021/es034823aSearch in Google Scholar PubMed
[7] Kyung H, Lee J, Choi W. Simultaneous and synergistic conversion of dyes and heavy metal ions in aqueous TiO2 suspensions under visible-light illumination. Environ. Sci. Technol. 2005, 39, 2376–2382.10.1021/es0492788Search in Google Scholar PubMed
[8] Donoso R, Cárdenas C, Fuentealba P. Ab initio molecular dynamics study of small alkali metal clusters. J. Phys. Chem. A 2014, 118, 1077–1083.10.1021/jp4079025Search in Google Scholar PubMed
[9] Rajeshwar K, Osugi M, Chanmanee W, Chenthamarakshan C, Zanoni MVB, Kajitvichyanukul P, Krishnan-Ayer R. Heterogeneous photocatalytic treatment of organic dyes in air and aqueous media. J. Photochem. Photobiol. C Photochem. Rev. 2008, 9, 171–192.10.1016/j.jphotochemrev.2008.09.001Search in Google Scholar
[10] Zhang H, Chen G, Bahnemann DW. Photoelectrocatalytic materials for environmental applications. J. Mater. Chem. 2009, 19, 5089–5121.10.1039/b821991eSearch in Google Scholar
[11] Fujishima A, Honda K. Photolysis-decomposition of water at the surface of an irradiated semiconductor. Nature 1972, 238, 37–38.10.1038/238037a0Search in Google Scholar PubMed
[12] Cornell RM, Schwertmann U. The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses. John Wiley & Sons: Weinheim, 2006.Search in Google Scholar
[13] Hochella MF, Lower SK, Maurice PA, Penn RL, Sahai N, Sparks DL, Twining BS. Nanominerals, mineral nanoparticles, and earth systems. Science 2008, 319, 1631–1635.10.1126/science.1141134Search in Google Scholar PubMed
[14] Navrotsky A, Mazeina L, Majzlan J. Size-driven structural and thermodynamic complexity in iron oxides. Science 2008, 319, 1635–1638.10.1126/science.1148614Search in Google Scholar PubMed
[15] Waychunas GA, Kim CS, Banfield JF. Nanoparticulate iron oxide minerals in soils and sediments: unique properties and contaminant scavenging mechanisms. J. Nanopart. Res. 2005, 7, 409–433.10.1007/s11051-005-6931-xSearch in Google Scholar
[16] Zhou X, Yang H, Wang C, Mao X, Wang Y, Yang Y, Liu G. Visible light induced photocatalytic degradation of Rhodamine B on one-dimensional iron oxide particles. J. Phys. Chem. C 2010, 114, 17051–17061.10.1021/jp103816eSearch in Google Scholar
[17] Li D, Zheng H, Wang Q, Wang X, Jiang W, Zhang Z, Yang Y. A novel double-cylindrical-shell photoreactor immobilized with monolayer TiO2-coated silica gel beads for photocatalytic degradation of Rhodamine B and Methyl Orange in aqueous solution. Sep. Purif. Technol. 2014, 123, 130–138.10.1016/j.seppur.2013.12.029Search in Google Scholar
[18] Khataee A, Pons M-N, Zahraa O. Photocatalytic degradation of three azo dyes using immobilized TiO2 nanoparticles on glass plates activated by UV light irradiation: influence of dye molecular structure. J. Hazard. Mater. 2009, 168, 451–457.10.1016/j.jhazmat.2009.02.052Search in Google Scholar PubMed
[19] Watanabe T, Nakajima A, Wang R, Minabe M, Koizumi S, Fujishima A, Hashimoto K. Photocatalytic activity and photoinduced hydrophilicity of titanium dioxide coated glass. Thin Solid Films 1999, 351, 260–263.10.1016/B978-044450247-6.50076-XSearch in Google Scholar
[20] Carneiro J, Azevedo S, Teixeira V, Fernandes F, Freitas E, Silva H, Oliveira J. Development of photocatalytic asphalt mixtures by the deposition and volumetric incorporation of TiO2 nanoparticles. Construct. Build. Mater. 2013, 38, 594–601.10.1016/j.conbuildmat.2012.09.005Search in Google Scholar
[21] Abdel-Messih M, Ahmed M, El-Sayed AS. Photocatalytic decolorization of Rhodamine B dye using novel mesoporous SnO2-TiO2 nano mixed oxides prepared by sol-gel method. J. Photochem. Photobiol. A Chem. 2013, 260, 1–8.10.1016/j.jphotochem.2013.03.011Search in Google Scholar
[22] Boxi SS, Paria S. Visible light induced enhanced photocatalytic degradation of organic pollutants in aqueous media using Ag doped hollow TiO2 nanospheres. RSC Adv. 2015, 5, 37657–37668.10.1039/C5RA03421CSearch in Google Scholar
[23] Fu X, Clark LA, Yang Q, Anderson MA. Enhanced photocatalytic performance of titania-based binary metal oxides: TiO2/SiO2 and TiO2/ZrO2. Environ. Sci. Technol. 1996, 30, 647–653.10.1021/es950391vSearch in Google Scholar
[24] Pinho L, Mosquera MJ. Photocatalytic activity of TiO2-SiO2 nanocomposites applied to buildings: influence of particle size and loading. Appl. Catal. B Environ. 2013, 134–135, 205–221.10.1016/j.apcatb.2013.01.021Search in Google Scholar
[25] Wang Y, Cai J, Jiang Y, Jiang X, Zhang D. Preparation of biosilica structures from frustules of diatoms and their applications: current state and perspectives. Appl. Microbiol. Biotechnol. 2013, 97, 453–460.10.1007/s00253-012-4568-0Search in Google Scholar PubMed
[26] Aw MS, Simovic S, Yu Y, Addai-Mensah J, Losic D. Porous silica microshells from diatoms as biocarrier for drug delivery applications. Powder Technol. 2012, 223, 52–58.10.1016/j.powtec.2011.04.023Search in Google Scholar
[27] Peng HH, Chen J, Jiang DY, Li M, Feng L, Losic D, Dong F, Zhang YX. Synergistic effect of manganese dioxide and diatomite for fast decolorization and high removal capacity of methyl orange. J. Colloid Interf. Sci. 2016, 484, 1–9.10.1016/j.jcis.2016.08.057Search in Google Scholar PubMed
[28] Son BHD, Mai VQ, Du DX, Phong NH, Cuong ND, Khieu DQ. Catalytic wet peroxide oxidation of phenol solution over Fe-Mn binary oxides diatomite composite. J. Porous Mater. 2017, 24, 601–611.10.1007/s10934-016-0296-7Search in Google Scholar
[29] Jia Y, Han W, Xiong G, Yang W. Diatomite as high performance and environmental friendly catalysts for phenol hydroxylation with H2O2. Sci. Technol. Adv. Mater. 2007, 8, 106–109.10.1016/j.stam.2006.10.003Search in Google Scholar
[30] Inchaurrondo N, Font J, Ramos CP, Haure P. Natural diatomites: efficient green catalyst for Fenton-like oxidation of Orange II. Appl. Catal. B Environ. 2016, 181, 481–494.10.1016/j.apcatb.2015.08.022Search in Google Scholar
[31] Dehestaniathar S, Khajelakzay M, Ramezani-Farani M, Ijadpanah-Saravi H. Modified diatomite-supported CuO-TiO2 composite: preparation, characterization and catalytic CO oxidation. J. Taiwan Inst. Chem. Eng. 2016, 58, 252–258.10.1016/j.jtice.2015.05.030Search in Google Scholar
[32] Inchaurrondo N, Ramos CP, Žerjav G, Font J, Pintar A, Haure P. Modified diatomites for Fenton-like oxidation of phenol. Micropor. Mesopor. Mater. 2017, 239, 396–408.10.1016/j.micromeso.2016.10.026Search in Google Scholar
[33] Hinckley DA, Seybold PG. A spectroscopic/thermodynamic study of the rhodamine B lactone⇌zwitterion equilibrium. Spectrochim. Acta Pt. A Mol. Spectrosc. 1988, 44, 1053–1059.10.1016/0584-8539(88)80227-7Search in Google Scholar
[34] Thakkar M, Randhawa V, Mitra S, Wei L. Synthesis of diatom-FeOx composite for removing trace arsenic to meet drinking water standards. J. Colloid Interf. Sci. 2015, 457, 169–173.10.1016/j.jcis.2015.07.003Search in Google Scholar
[35] Swedlund PJ, Webster JG. Adsorption and polymerisation of silicic acid on ferrihydrite, and its effect on arsenic adsorption. Water Res. 1999, 33, 3413–3422.10.1016/S0043-1354(99)00055-XSearch in Google Scholar
[36] Herbillon AJ, Vinh An JT. Heterogeneity in silicon-iron mixed hydroxides. J. Soil Sci. 1969, 20, 223–235.10.1111/j.1365-2389.1969.tb01569.xSearch in Google Scholar
[37] Zeng L. A method for preparing silica-containing iron(III) oxide adsorbents for arsenic removal. Water Res. 2003, 37, 4351–4358.10.1016/S0043-1354(03)00402-0Search in Google Scholar
[38] Addo Ntim S, Mitra S. Removal of trace arsenic to meet drinking water standards using iron oxide coated multiwall carbon nanotubes. J. Chem. Eng. Data 2011, 56, 2077–2083.10.1021/je1010664Search in Google Scholar PubMed PubMed Central
[39] Dalagan JQ, Enriquez EP. Interaction of diatom silica with graphene. Philippine Sci. Lett. 2013, 6, 119–127.Search in Google Scholar
[40] Ullah R, Deb BK, Mollah MYA. Synthesis and characterization of silica coated iron-oxide composites of different ratios. Int. J. Compos. Mater. 2014, 4, 135–145.Search in Google Scholar
[41] Guan M, Liu W, Shao Y, Huang H, Zhang H. Preparation, characterization and adsorption properties studies of 3-(methacryloyloxy)propyltrimethoxysilane modified and polymerized sol-gel mesoporous SBA-15 silica molecular sieves. Micropor. Mesopor. Mater. 2009, 123, 193–201.10.1016/j.micromeso.2009.04.001Search in Google Scholar
[42] Möller K, Kobler J, Bein T. Colloidal suspensions of nanometer-sized mesoporous silica. Adv. Funct. Mater. 2007, 17, 605–612.10.1002/adfm.200600578Search in Google Scholar
[43] Kao HM, Shen TY, Wu JD, Lee LP. Control of ordered structure and morphology of cubic mesoporous silica SBA-1 via direct synthesis of thiol-functionalization. Micropor. Mesopor. Mater. 2008, 110, 461–471.10.1016/j.micromeso.2007.06.035Search in Google Scholar
[44] Li X, Kikugawa N, Ye J. A comparison study of rhodamine B photodegradation over nitrogen-doped lamellar niobic acid and titanic acid under visible-light irradiation. Chem. A Eur. J. 2009, 15, 3538–3545.10.1002/chem.200801770Search in Google Scholar PubMed
[45] Watanabe T, Takizawa T, Honda K. Photocatalysis through excitation of adsorbates. 1. Highly efficient N-deethylation of rhodamine B adsorbed to cadmium sulfide. J. Phys. Chem. 1977, 81, 1845–1851.10.1021/j100534a012Search in Google Scholar
[46] Wang Y, Du W, Xu Y. Effect of sintering temperature on the photocatalytic activities and stabilities of hematite and silica-dispersed hematite particles for organic degradation in aqueous suspensions. Langmuir 2009, 25, 2895–2899.10.1021/la803714mSearch in Google Scholar PubMed
[47] Gomathi Devi L, Girish Kumar S, Mohan Reddy K, Munikrishnappa C. Photo degradation of methyl orange an azo dye by advanced Fenton process using zero valent metallic iron: influence of various reaction parameters and its degradation mechanism. J. Hazard. Mater. 2009, 164, 459–467.10.1016/j.jhazmat.2008.08.017Search in Google Scholar PubMed
[48] Akpan U, Hameed B. Parameters affecting the photocatalytic degradation of dyes using TiO2-based photocatalysts: a review. J. Hazard. Mater. 2009, 170, 520–529.10.1016/j.jhazmat.2009.05.039Search in Google Scholar PubMed
[49] Fox MA, Dulay MT. Heterogeneous photocatalysis. Chem. Rev. 1993, 93, 341–357.10.1021/cr00017a016Search in Google Scholar
[50] Tunesi S, Anderson M. Influence of chemisorption on the photodecomposition of salicylic acid and related compounds using suspended titania ceramic membranes. J. Phys. Chem. 1991, 95, 3399–3405.10.1021/j100161a078Search in Google Scholar
[51] Pradhan AC, Varadwaj GBB, Parida K. Facile fabrication of mesoporous iron modified Al2O3 nanoparticles pillared montmorillonite nanocomposite: a smart photo-Fenton catalyst for quick removal of organic dyes. Dalton Trans. 2013, 42, 15139–15149.10.1039/c3dt51952jSearch in Google Scholar PubMed
[52] Von Sonntag C. Advanced oxidation processes: mechanistic aspects. Water Sci. Technol. 2008, 58, 1015–1021.10.2166/wst.2008.467Search in Google Scholar PubMed
[53] Hug SJ, Leupin O. Iron-catalyzed oxidation of arsenic (III) by oxygen and by hydrogen peroxide: pH-dependent formation of oxidants in the Fenton reaction. Environ. Sci. Technol. 2003, 37, 2734–2742.10.1021/es026208xSearch in Google Scholar PubMed
[54] Miller CJ, Rose AL, Waite TD. Hydroxyl radical production by H2O2-mediated oxidation of Fe (II) complexed by Suwannee River fulvic acid under circumneutral freshwater conditions. Environ. Sci. Technol. 2012, 47, 829–835.10.1021/es303876hSearch in Google Scholar
[55] Daneshvar N, Salari D, Khataee A. Photocatalytic degradation of azo dye acid red 14 in water on ZnO as an alternative catalyst to TiO2. J. Photochem. Photobiol. A Chem. 2004, 162, 317–322.10.1016/S1010-6030(03)00378-2Search in Google Scholar
[56] So C, Cheng MY, Yu J, Wong P. Degradation of azo dye Procion Red MX-5B by photocatalytic oxidation. Chemosphere 2002, 46, 905–912.10.1016/S0045-6535(01)00153-9Search in Google Scholar
[57] You-ji L, Wei C. Photocatalytic degradation of Rhodamine B using nanocrystalline TiO2-zeolite surface composite catalysts: effects of photocatalytic condition on degradation efficiency. Catal. Sci. Technol. 2011, 1, 802–809.10.1039/c1cy00012hSearch in Google Scholar
[58] Wilhelm P, Stephan D., Photodegradation of rhodamine B in aqueous solution via SiO2@TiO2 nano-spheres. J. Photochem. Photobiol. A Chem. 2007, 185, 19–25.10.1016/j.jphotochem.2006.05.003Search in Google Scholar
[59] Wu J-M, Zhang T-W. Photodegradation of rhodamine B in water assisted by titania films prepared through a novel procedure. J. Photochem. Photobiol. A Chem. 2004, 162, 171–177.10.1016/S1010-6030(03)00345-9Search in Google Scholar
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