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Structural changes of polystyrene particles in subcritical and supercritical water revealed by in situ small-angle neutron scattering
Summary
Using in situ small-angle neutron scattering, researchers tracked how polystyrene microplastic particles swell, undergo chain cleavage, and ultimately decompose into oligomers and monomers under subcritical and supercritical water conditions. Understanding the structural breakdown pathway of polystyrene in supercritical water is important for developing thermochemical degradation technologies that could destroy persistent microplastics without generating secondary toxic byproducts.
Marine ecosystem degradation due to microplastic pollution is a significant environmental problem, as acknowledged by Sustainable Development Goal 14. Decomposition of plastics using near critical or supercritical water is a promising method to remove microplastics. To optimize this method for realizing environmental benefits, it is necessary to clarify the structural change of materials during the process. Thus, we investigated the decomposition processes of polystyrene particles dispersed in deuterated water (D2O) during heating under near critical or supercritical conditions by using in situ small-angle neutron scattering. Under subcritical conditions, the PS particles were swollen by D2O due to increased compatibility with temperature. In subcritical conditions near the critical point, cleavage of PS chains in the particles occurred, so that the swollen ratio was enhanced despite the PS particles keeping their shapes. Under supercritical conditions, the PS particles were degraded into oil, including oligomers or monomers and phase-separated structures with styrene-rich and D2O-rich regions. The decomposition process of polystyrene particles dispersed in D2O was analyzed by in situ small-angle neutron scattering under near critical and supercritical conditions. Upon heating in the subcritical state, the particles were swollen by D2O because of enhanced miscibility between polystyrene and D2O. In the supercritical state, the particles were completely degraded and formed monomer- or oligomer-rich domains due to phase separation. The findings and utilized techniques provide essential knowledge about the ways to elucidate the structural change of plastics in sub- and supercritical fluids.