Separation of microplastics from water using superhydrophobic silane-coupling-agent-modified geopolymer foam

Microplastics are a topical environmental problem that requires urgent solutions. They are ubiquitously present in various wastewaters and are discharged into aquatic environments because of difficulties in their removal. In this study, a novel filtration medium, superhydrophobic geopolymer foam, was prepared and investigated for the separation of microplastics from water. The foam was prepared using metakaolin, sodium silicate, sodium hydroxide, hydrogen peroxide, and Triton X-100 surfactant as raw materials and superhydrophobized with a silane coupling agent, triethoxy(octyl)silane. The purpose of the superhydrophobization was to improve the attachment of hydrophobic microplastic particles to the foam surface via chemical interactions. The modified geopolymer foam exhibited a water contact angle of 152°, and the presence of octyl chains on its surface was confirmed using Fourier transform infrared and X-ray photoelectron spectroscopies. When applied as a filter, the modified foam separated 53-63-µm sized polyethylene microspheres with ∼99 % removal efficiency, and no change in its separation efficiency was observed for ∼200 bed volumes of treated water. A comparison with an unmodified foam filter confirmed that the removal mechanism was not based on physical separation at higher flow rates, because the performance of the unmodified foam began to degrade after treating ∼5 bed volumes of wastewater. The performance of the modified foam was also validated with laundry washing effluents (particle size of microplastics varied roughly within 2–2000 μm), achieving ∼84 % separation efficiency for ∼50 bed volumes of wastewater. This study provides proof of concept of using superhydrophobic geopolymers as efficient, easy-to-prepare, and potentially low-cost separation media for microplastics from water effluents.