TiO2/LDPE composites: A new floating photocatalyst for solar degradation of organic contaminants
Graphical abstract
Research Highlights
► Floating photocatalysts based on TiO2/polyethylene were prepared and characterized. ► Highly active and efficient floating photocatalyst for degradation of dyes. ► Potential applications, as in the treatment of wastewater located in remote areas. ► Photocatalyst can be recycled and reused.
Introduction
Titanium dioxide is the most promising photocatalyst for the oxidation of organic contaminants in wastewaters. Several efforts have been directed towards the improvement of the photocatalytic activity of TiO2 in order to increase its efficiency for the treatment of wastewaters [1], [2], [3]. Some of these approaches are: doping with transition metals, surface modification by noble metals (e.g. Pd, Au, Ag, Pt) and coupling with different semiconductors [1], [2], [3], [4], [5], [6], [7], [8]. Different reactor designs, e.g. thin TiO2 films in rotating disk [9] and solar reactors [10], [11], [12], have been used to increase the photocatalytic activity by improving the illumination and oxygenation processes.
Also, floating photocatalysts have also been investigated in different applications. These floatable photocatalysts are specially interesting for solar remediation of non-stirred and non-oxygenated reservoirs since the process maximizes the: (i) illumination/light utilization (due their ability to float), (ii) oxygenation of the photocatalyst by the proximity with the air/water interface. The optimization of illumination and oxygenation should result in higher rates of radical formation and oxidation efficiencies.
Floating photocatalysts can be applied for solar remediation in loco, i.e. directly in the contaminated wastewater reservoirs located in remote areas without any special equipment or installation. Also, they can be used for the more efficient destruction of suspended insoluble organic contaminants, e.g. in oil-spill accidents.
In the literature only few works have been published on floating photocatalysts. For example, TiO2 hollow microspheres [13], TiO2 supported on SiO2 hollow glass microbeads [14], expanded graphite [15], natural porous pumice [16], expanded perlite [17], exfoliated graphite [18], expanded vermiculite [19], and grafted on expanded polystyrene beads [20]. However, in all these works the catalyst preparation is very complex and expensive using titanium alcoxides as precursors producing TiO2 phase mixtures with low photocatalytic activity.
In this work, it is described the preparation of a floatable photocatalyst by a very simple process using highly active TiO2 P25 Degussa catalyst and low density polyethylene (LDPE) as a floating substrate. LDPE is a suitable support for photocatalytic applications since it is an innocuous material with good chemical, mechanical and thermal stability. Moreover, LDPE is a hydrophobic material and can pre-concentrate organics from water on its surface improving the removal and oxidation efficiency.
Section snippets
TiO2/LDPE photocatalyst preparation and characterization
The floating photocatalysts TiO2/LDPE were prepared from a suspension of TiO2 P25 (Degussa) in a LDPE dissolved in xylene at 80 0C. Different TiO2 contents of 30, 68 and 82 wt.% were used. Preliminary work showed that TiO2 contents higher than 82% did not produce any increase on the photocatalysis and TiO2 lower than 30% showed very low catalytic activity. The solvent was evaporated and the obtained material ground to 2–4 mm beads and dried under vacuum at 80 °C for 5 h. Powder XRD data were
TiO2/LDPE photocatalyst characterization
The floating photocatalysts TiO2/LDPE were prepared with different amounts of TiO2 P25 (Degussa) and LDPE. TG analyses (Fig. 3) of the composites showed that pure LDPE completely decomposes at 222–490 °C. The weight losses for the different prepared photocatalysts showed TiO2 contents of c.a. 30, 68 and 82 wt.% (Fig. 1). These results suggest that all the TiO2 added in the preparation was incorporated in the polymer beads.
XRD analyses of pure TiO2 P25 and the composites TiO2/LDPE showed very
Conclusions
Results obtained in this work, showed that a highly active and efficient floating photocatalyst can be prepared from TiO2 P25 and low density polyethylene. The floating catalyst can be prepared with a wide range of TiO2 contents. In special constraining conditions, i.e. no stirring and no oxygenation, these catalysts show higher activity than pure TiO2 P25 for the discoloration and TOC removal of the model dye methylene blue under UV (254 nm) and solar irradiation. This higher activity is likely
Acknowledgments
The authors are grateful to CNPq, FAPEMIG, CAPES and UFMG for their financial support.
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