Freezing-induced microplastic degradation in an anoxic Fe(ii)-containing solution: the key role of Fe(iv) and ·OH

Abstract

The enrichment of microplastics in polar ice is a typical phenomenon discovered in recent years. Thus, it is worthwhile to explore the degradation behavior of microplastics in frozen environments, which is crucial to evaluate their impact on polar ecosystems. Here, taking polystyrene (PS) as an example, the degradation behaviors of MPs in frozen anoxic Fe(II)-containing solutions were investigated. The results revealed that PS degradation rate in frozen solution was 14.6-fold higher than that in water. Mechanism study showed that freezing-induced aggregation and extrusion of PS enhanced the generation of environmentally persistent free radicals (EPFRs), which in turn contributed to the formation of H₂O₂. Moreover, freezing-induced enrichment of Fe(II) further interreacted with H₂O₂ to form Fe(IV) and OH free radicals at a pH of ∼6.5, and thus accelerated the degradation of PS. Such reaction was accompanied by the formation of Fe(III)-containing products, which would further react with aged PS to produce new EPFRs and Fe(II). Such cycles of aged PS/EPFRs and Fe(III)/Fe(II) continued to accelerate the degradation of PS in the frozen solution. This study provides an overlooked pathway for microplastic degradation in frozen Fe(II)-containing solutions, which would be of key importance to understand the degradation behaviors of microplastics in frozen environments.