Rectorite–TiO2–Fe3O4 composites: Assembly, characterization, adsorption and photodegradation
Graphical abstract
Introduction
Rectorite (REC) is a regularly interstratified clay mineral composed of alternating pairs of a nonexpansible dioctahedral mica-like layer and an expansible dioctahedral smectite-like layer in a 1:1 ratio [1]. The REC structure cleaves easily between the smectite-like interlayers, forming monolithic REC layers (2 nm thick). The interlayer cations Na+, K+, and Ca2+ in the smectite-like layers can be easily exchanged with either organic or inorganic cations, and therefore rectorite can adsorb pollutants including cationic ions such as Cu(II), Pb(II), Cd(II) [2], Cr(VI), Sr(II) [3] and NH4+ [4], and cationic organic dyes such as acid red [5], neutral red and methylene blue (MB) [6].
REC has been proven to be a good adsorbent though modification is required to improve its adsorption capacity. REC layers can be intercalated or exfoliated by large volumes of exchange groups which usually resulting in large specific surfaces and better adsorption capacity of the modified REC. Huang et al. [1] proved that cation exchange reaction between REC and the surfactants (dodecyl benzyl dimethyl ammonium chloride, hexadecyl trimethyl ammonium bromide and octadecyl trimethyl ammonium bromide, respectively) resulting in RECs that were intercalated with molecules of organic cations. The d-spacing of the REC layers increased when the surfactant chain length increased, and the adsorption capacity for Cr(VI) also improved. The adsorption of phenol was also studied in a similar manner and the adsorption capacity order was consistent with the chain length of the surfactants used for REC modification [7].
Many cationic polymers have also been used to intercalate the interlayer of REC. Two chitosan derivates, i.e. quaternized chitosan-N-(2-hydroxy) propyl-3-trimethylammonium chitosan chloride (HTCC) and quaternized carboxymethyl chitosan (QCMC) were used to intercalate the REC interlayers [8]. Since HTCC had a larger free volume and more free chains due to weaker intermolecular and intramolecular hydrogen bonds in the aqueous solution than QCMC, HTCC easily intercalated the REC interlayers. In other research [9], cetyltrimethyl ammonium bromide-modified REC was intercalated during graft polymerization of guar gum-g-poly(sodium acrylate), while HCl-modified REC were exfoliated. REC participated in the polymerization reaction through active Al–OH groups on the surface. The obtained composites exhibited the improved swelling properties, used as the superabsorbent.
In inorganic modification of REC, metal oxide catalyst particles formed in the REC interlayer to prepare pillared-REC for the removal of pollutants. A heterogeneous catalyst, Fe2O3-pillared REC, was employed to study photodegradation of an azo-dye rhodamine B and nitrophenol in the presence of hydrogen peroxide under visible light irradiation [10]. Zhang et al. [11] developed TiO2-pillared REC with photocatalytic activity using a facile low-cost method at low temperature (70 °C) to remove acid red G (a dye) and nitrophenol. Pillared RECs combine the strong adsorption capacity of REC with the photocatalytic degradation of the catalysts to effectively remove pollutants. Fe3O4 nanoparticles were synthesized on the REC to obtain magnetic REC for the removal of the dyes [6], which can easily separate from the aqueous solution with the magnetic rather than the filtration or centrifugation.
The organic pollutants are often considered to be toxic or carcinogenic [12]; their appearance in waterways will inevitably put our already fragile ecosystem in danger, and threaten public health, fisheries, wildlife habitats, recreation opportunities, and ultimately our quality of life. It is imperative to appropriately treat organic pollutant-containing wastewater/effluent before it is disposed of in the environment.
In this work, TiO2 was incorporated in REC to prepare REC–TiO2 (RT) composites, and Fe3O4 was introduced to obtain REC–TiO2–Fe3O4 (RTF) composites. RT and RTF were characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). TiO2 particles partially exfoliated the REC sheets, and the Fe3O4 particles completely exfoliated them. A novel method was provided to prepare REC monolayer. RT and RTF were used to remove cationic methylene blue (MB) dye and the persistent organic pollutant 4-nitrophenol (4-NP) from water. In RTF composites, REC acts as a good adsorbent (or the supporter of TiO2) and TiO2 degrades organic pollutants on REC under UV-light irradiation. Magnetic Fe3O4 makes separation of RTF from the aqueous solution easy using a magnet.
Section snippets
Materials
Sodium rectorite was provided by Hubei Zhongxiang Rectorite Mine (Wuhan, China). MB dye was provided by Tianjin Benchmark Chemical Reagent Co., Ltd. All other reagents were commercially available and of analytical grade.
Preparation of REC–TiO2 composite (RT)
Tetrabutyl titanate, Ti(OC4H9)4 (5 mL), was added to 6 M HCl solution (10 mL) and stored at room temperature for 6 h. 0.5 g REC was dispersed in 125 mL distilled water with ultrasonication for 15 min, and the suspension was stirred for 6 h at room temperature. Tetrabutyl titanate
Characterization of RT and RTF
As seen in Fig. 1a, REC had platelets with a thickness of 30–100 nm. In Fig. 1b aggregates of TiO2 nanoparticles, which may have formed during precipitation followed by a step-like aggregation process, are shown attached to the REC surface. The REC was thinner and the REC platelets were transparent when TiO2 nanoparticles formed at the REC surface. At this point some REC layers were exfoliated, but not completely; and some REC stacks were still composed of several layers. As seen in Fig. 1c, RTF
Conclusions
TiO2 and Fe3O4 nanoparticles were synthesized on REC to obtain magnetic REC–TiO2–Fe3O4 composites by hydrolyzing tetrabutyl titanate and depositing Fe3O4 particles. Since the adsorption of Fe3+ or Fe2+ occurred on REC surfaces, Fe3O4 particles could form on REC surface rather than on the TiO2 particles. The contents of TiO2 and Fe3O4 particles in RTF were about 65 wt.% and 5 wt.%, respectively. The introduction of TiO2 particles partially exfoliated the REC sheets, and the Fe3O4 particles
Acknowledgements
This research was supported by the Science and Technology Project of Jiangxi Provincial Office of Education (KJLD12082 and Innovation Platform “project 311”) and Nature Science Foundation of Jiangxi Province (20132BAB 206006) and the National Nature Science Foundation of China (51162011).
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