Solar thermoplasmonic convection for sustainable removal of microplastics

Abstract

Vast quantities of plastic waste are deposited into marine environments annually, posing significant risks to aquatic organisms and biodiversity. This paper proposes a novel concept for the removal of microplastics from aquatic systems using solar thermoplasmonic convection. This method uses plano-convex lens to concentrate sunlight into an intense beam at a focal point. Alignment of this beam with the oscillation frequency of free electrons in a gold nanocone array induces surface plasmon resonance, which enhances the local electric field, catalyzes hot electron production, and producing intense heating to create a localized hotspot. The hotspot temperature reaches 353 K upon one sun flux. This hotspot induces large-scale thermal convections within the fluid, with a maximum velocity of 2.97 mm/s, efficiently drawing dispersed microplastics toward these thermal traps. Experimental results reveal that microplastics could coalesce into larger aggregates at the hotspot within about 120 s, even for microparticles as large size as 50 μm. Both the experiment and numerical analysis support that the convection current is crucial in the aggregation process. Pool and flow-through reactors with hotspot arrays are further explored for practical applicability. It was found that the optimizing reactor design and the spatial arrangement of hotspots can improve the collection efficiency. This study suggests a valuable method for microplastic remediation in aquatic settings, providing new insights into the multiphase flow dynamics.