Surface treatment through polymeric ferric sulfate for progressive flotation separation of waste plastics

Plastics offer considerable convenience in daily life and human health. However, a substantial amount of waste plastics is released into the environment without proper treatment, posing significant environmental challenges. Recycling and separating waste plastics is difficult due to their similar chemical and physical properties. This study introduces a novel method combining polymeric ferric sulfate (PFS) modification with flotation for efficient plastic separation. In order to achieve the optimal flotation conditions, single parameter experiments were used to confirm the key variables that have a close correlation with the flotation performance, namely PFS dose, temperature, time and pH value. PFS has different hydrophilic effects on polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS) and polyvinyl chloride (PVC) surfaces, whereas polyethylene terephthalate (PET) remains unaffected. For purpose of achieving the individual separation of the plastics, a stepwise flotation method was used. After the primary separation, PET and PC were discharged, while all plastics were individually separated and retained at high purity through the secondary separation process, resulting in 100% purity for both PET and PVC. PC and ABS can achieve 97.4% and 94.5% purity in triplicate validation tests, respectively. Surface modification mechanisms were investigated using X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FT-IR), and Scanning Electron Microscopy (SEM). Based on varying functional groups and surface potentials, PFS can adhere to different plastic surfaces for extended durations, creating the hydrophilicity difference. Nonhomogeneous adsorption of PFS on plastics surfaces was described by characterization and adsorption kinetics. The hydrophilization order and PFS adsorption capacity were consistent: PVC > ABS > PC > PET. The interaction between PFS and plastics was explained by density functional theory (DFT) simulations, the long-chain molecules and numerous hydroxyl groups primarily interact with the plastic surface through electrostatic forces and hydrogen bonds. Through optimization of the PFS treatment process, the consumption of the modification reagent was successfully reduced to 56.51 g/ton of waste plastic with the reuse of PFS. Our process optimization assessment highlights that PFS treatment is a highly effective method for recycling waste plastics.