A tri-layer floating photocatalyst/adsorbent for the removal of organic compounds from wastewater: Layer-by-layer deposition of silicalite-1 and titania on hollow glass microspheres

Removal of organic compounds from wastewater is necessary to ensure that aquatic organisms are not adversely affected. In the past, it had been shown that zeolite-based systems provide good opportunities for wastewater treatment; however, most of the zeolite systems reported thus far face the disadvantages of sinking in water and saturated adsorption site. Therefore, in the current study, we attempted to develop zeolite-based composite systems with good floatability, adsorption capacity, and photocatalytic activity. In addition, our aim was to ensure that these composites could be easily separated from the treated water. Thus, a tri-layer floating photocatalyst/adsorbent was prepared via the layer-by-layer deposition of silicalite-1, a pure silica MFI-type zeolite (PSZ), and TiO₂ on hollow glass microspheres (HGMs). A thin layer of silicalite-1 was crystallized on the dense walls of HGMs to fabricate floating adsorbents (PSZ-HGMs). Subsequently, a layer of TiO₂ was deposited on PSZ-HGM surfaces using the sol–gel method to develop photocatalytic composites (TiO₂/PSZ-HGMs). A good PSZ coverage and attachment were achieved on HGM surfaces by adding HGMs to an alkali solution containing tetrapropylammonium hydroxide and tetraethyl orthosilicate, followed by hydrothermal synthesis at 180 °C for 40 h. Subsequent deposition of TiO₂ was effective in homogeneously covering the PSZ surface without clogging the zeolite pores. The wastewater treatment capacity of the synthesized materials was verified using methylene blue (MB). It was found that TiO₂/PSZ-HGMs, which exhibited both photocatalytic and adsorption activities, acted synergistically with UV-C irradiation to drastically degrade MB concentration by ∼98% within 2 h. Therefore, these materials may be considered to have high potential in wastewater-treatment processes.