Photo-induced degradation of single-use polyethylene terephthalate microplastics under laboratory and outdoor environmental conditions

There is a lack of knowledge regarding the mechanisms that induce microplastic fragmentation and degradation within the environment. This research aimed to quantify the combined degradative effects that mechanical abrasion, in conjunction with photo-oxidation and hydrolysis, have on polyethylene terephthalate (PET) microplastics. To accomplish this, common routes of degradation were evaluated. Degradation was assessed using three indices indicative of polymer degradation: the carbonyl index, carbon-to-oxygen index, and hydroxyl index. This study assessed the effects that mechanical abrasion (MA), photo-oxidation, and various simulated environmental conditions: aqueous (Aq), Aq + UV, and UV only within two distinct settings (laboratory vs. outdoor) have on PET microplastic degradation. Photo-oxidation exposure across a 60-day period induced significant degradation on PET microplastics, resulting in a 1%-22% increase in carbonyl groups across all treatments except UV and Aq + UV Chamber (MA). A 6-214% increase in hydroxyl groups across all treatments. A 1-10% decrease in carbon-to-oxygen groups in all treatments except the Chamber Aqueous and Outdoor UV (MA). Mechanical abrasion seemed to accelerate this degradation in combination with both UV and aqueous treatments. Using simulated environmental conditions to induce degradation on PET microplastics in both laboratory and simulated environmentally relevant settings revealed that the combined effects of hydrolysis and photo-oxidation can accelerate the process, especially in conjunction with mechanical abrasion. The novel findings presented here provide insight into the complex relationship between various polymer degradation pathways and the effects that mechanical abrasion can have on them while also providing additional data for an understudied yet prevalent plastic polymer.