Valorization of shoe sole waste into high-performance cationic dye sorbents via sulfonation

Abstract

Ethylene-vinyl acetate (EVA)-based copolymers are widely employed in various applications such as packaging, adhesives, and shoe soles due to their relatively low cost and versatile properties, which are controlled by their chemical composition. Noteworthily, EVA in shoe soles is commonly crosslinked, which is necessary to increase their melt strength and other material properties for durable use. However, this crosslinked nature precludes traditional melt reprocessing to address the end-of-life of crosslinked EVA materials, posing a critical sustainability challenge for waste management which necessitates new recycling methods. This work develops a simple sulfonation-based method to valorize virgin EVA and shoe soles from real-world waste, imparting sulfonic acid groups on to the polymer backbones, while partially retaining the macroscopic foam structures. The reaction kinetics of sulfonation-based functionalization and their impact on the development of pore structures of EVA and their derived shoe wastes are systematically investigated. Importantly, the presence of acid groups from the upcycled shoe wastes leads to strong interactions with cationic micropollutants, enabling their high performance as sorbent materials for water remediation. The resulting materials can efficiently remove methylene blue and crystal violet from aqueous solution, exhibiting high sorption capacity and fast kinetics. Collectively, this work demonstrates a simple method to convert real-world shoe sole waste into value-enhanced sorbent products with high potential to address emerging micropollutant threats.