Oil-phase cyclic magnetic adsorption to synthesize Fe3O4@C@TiO2-nanotube composites for simultaneous removal of Pb(II) and Rhodamine B
Graphical abstract
Introduction
With the quick development of chemical industries including printing, oil refining, electroplating, mining and battery manufacturing, wastewater treatment has been a hot topic in recent years [1], [2]. Many kinds of pollutants exist in wastewater. Among these impurities, elimination of heavy metals and organic dyes has been extensively studied in recent years because they are very long-lived in ecosphere and pose threats to the environment [3], [4]. Except for traditional treatment methods, such as advanced oxidization, extraction etc, nanotechnology and nanomaterials have been developed and applied to remove heavy metals or degrade dyes [5], [6], [7], [8], [9], [10].
Benefiting from its low cost and high chemical stability, nano titanium dioxide has been widely used in pollutants decontamination [11]. When it is illuminated with UV light, holes (h+) and electrons (e−) are excited and migrate in their respective band with potentials of 3.1 V and −0.1 V vs. SHE (Standard Hydrogen Electrode) approximately [12], [13], [14], [15]. As the potentials of OH/H2O and OH/OH− are below 3.1 V, OH will be produced from H2O and OH− to degrade organic dyes [16], [17]. Besides, the reduction potentials of most hazardous metal ions such as Cu2+, Ag+, Hg2+ are above −0.1 V vs. SHE and they can be removed by photocatalytic induced reduction and deposition [18], [19], [20]. Therefore, over the past three decades, great efforts have been made to synthesize TiO2 to degrade organics and reduce heavy metal ions simultaneously [8], [18]. Nevertheless, it was found that pure TiO2 cannot reduce toxic metal ions whose reduction potentials are below −0.1 V, such as Pb(II). Pb(II) is a common heavy-metal pollutant with low reduction potentials. Two removal strategies have been developed for elimination of Pb(II) in TiO2 photocatalytic system. One strategy is to introduce electron donors such as methanol, ethanol [21]. The second strategy is to couple with ozone bubbling [22]. However, both of these two manners will increase the cost of wastewater treatment since the introducing of other chemicals. Thus, simultaneous removal of Pb(II) and organic dyes remains an urgent task and new nanocomposites of TiO2 with other economical materials need to be designed [23], [24], [25].
TiO2 nanotubes (TNTs) have been considered as a better photocatalyst compared with other morphologies due to its superior charge-transfer properties, high surface area, and spatially ordered distributions [26], [27]. The hollow structure of TiO2 nanotubes enables direct embedding of features into the tube walls [28]. On the other hand, it has been widely reported that Fe3O4 nanoparticle is an efficient and cost-effective adsorbent for most of the heavy metal ions including Pb(II) [29], [30]. Moreover, TiO2 with moderate Fe3O4 deposition displays enhanced photocatalytic activity than pure TiO2 due to effective separation of h+ and e− resulted from the reduction of iron ions into Fe2+ or Fe0 [31]. The generated Fe0 can also enhance removal of adsorbed Pb(II) by reduction. Therefore, Fe3O4@TNT will be a promising composite for simultaneous decontamination of Pb(II) and organic dyes due to the outstanding adsorptive and magnetic performance of Fe3O4 and the photocatalytic properties of TiO2 nanotubes [28], [32], [33]. However, the combination of Fe3O4 and TiO2 is considered as an included band alignment (Type III) and photo-induced charge carriers will migrate towards Fe3O4 [34], [35], [36]. In this process, e− can be captured by Fe(III) ions to produce Fe(II) (Fe3O4/Fe2+, 1.23 V vs. SHE). The photodissolution of Fe3O4 will destroy the catalyst structure and reduce photocatalytic activity. Methods such as inserting barrier layer or introducing electron transporter have been developed to solve this problem [36], [37]. As a good electron acceptor and effective electron transporter, carbon materials can protect the composites from electron interaction or photodissolution of Fe3O4 [36].
To date, there is little report on efficient preparation and application of Fe3O4@C@TNT composites with moderate and uniform Fe3O4 deposition, which might be attributed to different surface conditions between highly dispersed Fe3O4 nanoparticles and hydrous TiO2 nanotubes [32], [33]. Furthermore, the simultaneous removal of Pb(II) and dyes hasn’t been investigated yet in Fe3O4@C@TiO2. During the photocatalytic process, although the degradation mechanism of organics has been well explained by the radical decontamination theory [11], the systematic elimination mechanism of Pb(II) remains unsolved till now due to the complex process including adsorption, reduction and reaction with photo-induced carriers [21], [22].
In this contribution, to develop new materials which can simultaneously remove organic dyes and inorganic impurities such as Pb(II), TNT-supported Fe3O4-decorated composites with carbon as electron transporter were designed for the first time. One novel oil-phase cyclic magnetic adsorption (OCMA) method was developed to synthesize the designed Fe3O4@C@TNT composites. The obtained Fe3O4@C@TNT composites were characterized and analyzed systematically from four aspects: structural and elemental characterizations, surface characterizations, photo-chemical characterizations and magnetic characterizations. The performance of the obtained Fe3O4@C@TNT composites was evaluated by simultaneous decontamination of Rhodamine B (RhB) and Pb(II). More importantly, the Pb(II) removal mechanisms were established by monitoring Pb(II) concentrations in solutions and immobilized Pb(II) on catalysts. The composites developed in this work have the potential to be used to treat wastewater from industries, such as printing industry and dye industry.
Section snippets
Reagents
Titanium foil (99.95% purity) was obtained from Beijing Qianshuo Non-ferrous Metal Co., Ltd. (China). FeCl3·6H2O (99% purity), FeSO4·7H2O (99% purity), Pb(NO3)2 (99.5% purity) and NH4F (99.5% purity) were obtained from Aladdin Co., Ltd. (China). Analytical grade hexane, ethanol, ethylene glycerol, oleic acid, RhB and ammonia water (25 wt%) were purchased from Guangfu Co., Ltd. (China).
Fabrication of Fe3O4@oleic acid
Fe3O4@oleic acid (OA) nanoparticles were synthesized according to a reported co-precipitation method [38].
Structural and elemental characterizations
The morphologies and microstructures of hydrous TNT and synthesized catalysts 0FeCTi, 1FeCTi and 3FeCTi were characterized by SEM and TEM, as displayed in Fig. 1. Fig. 1A shows the SEM image of hydrous TiO2 nanotubes with an outer diameter of 140 nm on average. The clean surface and ordered macropores are beneficial for the deposition of Fe3O4@oleic acid. After OCMA method, the nanotubes loaded with different amount of deposition are annealed at 450 °C for 2 h and the morphologies of the
Conclusions
In this work, a series of magnetic Fe3O4@C@TiO2-nanotube composites were designed and a facile oil-phase cyclic magnetic adsorption (OCMA) method was developed to synthesize the newly designed composites. Uniform dispersion of Fe3O4 in 1FeCTi was verified by TEM and moderate deposition of C was detected by XPS. The uniform loading of Fe3O4@C contributes to the increment of BET surface area and average pore size, which is beneficial for the photocatalytic process. Better performance, originating
Acknowledgments
This research was financially supported by National Key Research and Development Program of China (no. 2016YFB0600504).
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