A hydrophilic/hydrophobic switch on polymer surface triggered by calcite towards separation of hazardous PVC from plastic mixtures

Hydrophilic modification of polymer surfaces is crucial for the emerging flotation separation of plastic waste towards resources recycling. In this study, we investigated a novel hydrophilic regulation induced by calcite to modify the surface wettability of PVC, ABS, PS, PC, and PET. The interactions between calcite and plastic molecules contributed to the selective formation of hydrophilic calcite shells on plastic surfaces. Due to the strong polarity of C-Cl bonds on PVC surfaces, calcium ions tended to act as bridges between calcite particles and PVC, and a non-covalent interaction leaded to strong physical adsorption of calcite on PVC surfaces. However, other plastics adsorbed fewer calcite than PVCs owing to weaker interactions. Like a hydrophilic/hydrophobic switch, ultrasonic cleaning erased hydrophilic calcite shells to restore the hydrophobicity of hydrophilic plastics without destroying the chemical structure of polymers. The recovery and purity of PVC were 100 % and 96.3 %, under the conditions of calcite dosage 1.5 g L^-1, coating time 15 min, coating temperature 50 °C, pH 10, and rotational speed 900 r min^-1. Its attractive applicability and reversibility were further demonstrated by the flotation separation of actual plastic mixtures and the eversible floatability of sinking plastics.