Effect of Earth-Abundant Minerals and Metal Salts on Hydrothermal Recycling of Plastic Wastes: A Study of Polystyrene and Polycarbonate

Hydrothermal treatment (HTT) has emerged as an alternative approach for plastic waste management, operating at relatively low reaction temperatures (<350 °C) in water without the need for organic solvents or strong acids or bases. This study investigates how Earth-abundant minerals and dissolved metal salts influence the hydrothermal conversion of two structurally distinct plastics, polystyrene (PS) and polycarbonate (PC). PS exhibited limited degradation, reaching ∼12% conversion after 168 h at 275 °C, with aromatic alkenes and alkanes as the dominant products. In contrast, PC degraded much more rapidly, achieving ∼76% conversion after 6 h at 250 °C and producing bisphenol A and phenolic compounds as major products. Mineral additives had a minimal effect on PS conversion but increased PC conversion (from 28% in DI water to 33−63% depending on the mineral), while dissolved metal salts further enhanced PC conversion to ∼78−88% under the same conditions. These results demonstrate that polymer backbone chemistry strongly governs hydrothermal reactivity and that Earth-abundant minerals and metals can be used selectively for plastic waste recycling.