The Photocatalytic Reduction of Hexavalent Chromium by Controllable Mesoporous Anatase TiO2 Nanoparticles

Titania (TiO2) nanoparticles with periodical mesopore size (up to 150？？) have successfully been synthesized by sol-gel template method, using titanium(IV) tetraisopropoxide as a starting precursor and isopropanol as a solvent. Different quantities of activated carbon (0%, 5%, and 10% by weight) were used as templates to control the porosity and particle size of titania nanoparticles. The templates were completely removed during the calcination in air at 500°C for 3？hr. The results showed that the specific surface area of titania is increased with increasing activated carbon content. The optical bandgap of synthesized titania exhibits a blue shift by 0.3–0.6？eV when compared to the reported value for the bulk anatase and rutile phases. The photocatalytic activity of porous titania is determined with its reduction efficiency of hexavalent chromium (Cr6+). The reduction efficiency is optimized under ultraviolet illumination. 1. Introduction In general, chromium has two stable oxidation states, that is, Cr(VI) and Cr(III). The toxicity is mainly caused by hexavalent chromium, Cr(VI), which is virtually found in wastewater from industrial processes such as leather tanning, paint making, electroplating, and chromate production. This oxidation state has been classified as a carcinogen and mutagen not only to humans but also to other creatures. In many countries, as a consequence, the wastewater must be treated until the limit of Cr(VI) is below 0.05？mg/L before releasing to the water source [1]. One solution method is to convert it into Cr(III), which is considered as a nontoxic and essential trace metal in human nutrition. By chemical process, Cr(VI) is precipitated into Cr(III) as Cr(OH)3 in neutral or alkaline solutions and removed as a solid waste [2, 3]. However, this process requires reducing agents such as ferrous sulfate, sodium hydrogen sulfite, sodium pyrosulfite, hydrazine hydrate, and sulfur dioxide, which are expensive and dangerous with human skin and to release other unwanted chemicals [1]. Recently, there are many research works related to the use of semiconducting materials such as TiO2 and ZnO as photocatalysts for various applications. This has many benefits including low cost, high efficiency, reusable performance, cleanliness, natural safety, and nontoxicity. The photocatalytic mechanism begins when a semiconducting material absorbs light with energy larger than its bandgap. As a consequence, electrons from the valence band would be excited to the conduction band, leaving holes behind. These electron-hole pairs could react with oxygen and