Oleo-extraction of microplastics using flotation plus sol-gel technique to confine small particles in silicon dioxide gel

Extracting microplastics from natural sources is challenging, especially microplastics with sizes smaller than 100 μm. The flotation method is the most common microplastic extraction, but it struggles with fine particles due to the difficulty in collecting floating plastic particles from the liquid during the separation process. This study proposes a new floating media, tetraethyl orthosilicate (TEOS), that could separate microplastics using its hydrophobic-oleophilic properties. Most interestingly, TEOS transformed from a liquid state to a solid state (gel) by hydrolysis and condensation reactions, thus safely capturing the separated microplastic particles after flotation and mitigating particle loss from scooping since its gel acted as a particle holder. The average recovery rates obtained were in the range of 95–100% for polyethylene terephthalate, polyethylene, polypropylene, polyvinyl chloride, polystyrene, polytetrafluoroethylene, and aged tyre rubber. The recovery rates were slightly reduced for finer particles (sizes down to 40 μm) at 82–98%. TEOS-based extraction provided a higher recovery rate for non-polar plastics than for polar or hydrophilic plastics. The separated microplastics maintained their characteristics for polymer identification, as proven by spectroscopic and thermal analysis techniques. Therefore, TEOS-based extraction could be a new approach to microplastic extraction, especially for preventing fine particle loss. Graphical abstract: (Figure presented.)