TiO₂/LDPE composites: A new floating photocatalyst for solar degradation of organic contaminants

Abstract

In this work, floating photocatalysts based on composites of low density polyethylene (LDPE) containing 30, 68 and 82 wt.% of TiO₂ P25 were prepared and characterized by Scanning Electron Microscopy, Powder XRD data, surface area (B.E.T. method) and Thermogravimetric analyses. Degradation of the model dye methylene blue with UV (Hg 245 nm) and solar irradiation under constrained conditions, i.e. non-stirring and no-oxygenation showed excellent results. Under the same conditions pure TiO₂ showed very low activities. The floating photocatalyst can be reused for at least three consecutive times without any significant decrease on the discoloration and Total Organic Carbon removal after each reuse.

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.