Insights into Mechanism of Hypochlorite-Induced Functionalization of Polymers toward Separating BFR-Containing Components from Microplastics

Surface functionalization of polymers is significant for an emerging flotation technique for separation of microplastics toward the recycling of plastic wastes. In this study, the hypochlorite-induced functionalization of polymers, including ABS, PMMA, PS, and PVC polymers, was intensively investigated. Afterward, its emerging application in flotation separation of microplastic mixtures was assessed based on a Box-Behnken design of the response surface methodology. The functionalization favorably induced decreases in the contact angle and zeta potential of polymers, suggesting hydrophilic and negatively charged surfaces. Particularly, the functionalization of ABS polymers was the most effective, leading to the obviously decreased contact angle (from 92.5° to 67.8°) and zeta potential (from -26.4 mV to -41.7 mV) at neutral condition. The major mechanism for these variations was the oxidation of the sp³-C and butenyl group by hydroxyl radical and the hydrolysis of cyano group, which introduced the hydroxyl, carboxyl, and amide groups and rough topographies on the surface of ABS polymers. Oxygen functionalities introduced on the surfaces of other polymers were far less than those of ABS polymers. This selectivity inspired us to apply the functionalization in flotation separation of ABS microplastics from microplastic mixtures. After functionalization, ABS microplastics showed a significantly decreased floatability in flotation tests since the hydrophilic surface was repulsive to the adhesion of air bubbles. An empirical model was built to optimize the separation efficiency using the overall desirability function. Under optimum conditions, ABS microplastics were efficiently separated, and their removal rate, recovery, and purity were 99.8%, 99.8%, and >99.9%, respectively. These findings provide significant insights into the mechanism of the functionalization of polymers and show a promising prospect for pollution control of plastic wastes.