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

Abstract

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 TiO 2 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 TiO 2 was deposited on PSZ-HGM surfaces using the sol–gel method to develop photocatalytic composites (TiO 2 /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 TiO 2 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 TiO 2 /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. • Newly designed tri-layer floating TiO 2 /pure silica MFI-type zeolite/hollow glass microsphere (TiO 2 /PSZ/HGMs) was developed. • The effects of HGM size distribution, precursor composition, and synthesis time on PSZ layer formation on HGM were examined. • Anatase/rutile-TiO 2 were homogeneously embedded onto PSZ/HGMs without clogging the zeolite pores by using the sol–gel method. • Optimized TiO 2 /PSZ/HGMs could remove 98% of methylene blue under UV-C irradiation for 2 h.