Photodegradation of macroplastics to microplastics : A laboratory study on common litter found in urban areas

Abstract: During the last 60 years the plastic production has increased more than 190 times and plasticpollution both at sea and land is a growing issue. Every year millions of tons of plastic waste fromland reaches the oceans, but the land-based sources are diffuse. One possible source of plasticwaste and microplastics are from plastic litter in urban areas which is common all over the world.The aim with this laboratory study was to study the photodegradation patterns of macroplasticsthat is usually found as litter in urban areas to contribute with knowledge and to theunderstanding of how macroplastics degrade to microplastics. The laboratory study wasstructured around the use of ultraviolet light exposure from UVA 340 nm lamps to acceleratephotodegradations of plastics in air. The test was divided into four different time intervals: stage7 days, stage 14 days, stage 28 days, and stage 56 days to study the evolution of plasticfragmentation over time. Effect of the UV radiation and test duration were combined to derivethe equivalent real time duration. Using Luleå as a benchmark the computed equivalence were0.27 years for every seven days of UV exposure. For stage 7d, a test with different mediums(water and air) were performed to compare the degradation processes between differentenvironments. However, for the longer time intervals air was the only tested environment. Newplastic products were bought which were among the most produced types of plastic or mostcommon plastic litter. The plastics were the following: polystyrene (PS) as plastic coffee cup lid,polypropylene (PP) as chocolate wrapping, polyethylene terephthalate (PET) as plastic bottle,low- density polyethylene (PE-LD) as plastic grocery bag and cellulose acetate (CA) as cigarettefilter or butts. The analytical techniques used were a particle size and number counter, with theselected particle size interval between 4-120 μm, and a camera mounted microscope to studyshapes of microplastic particles. Before photographing the particles, the samples were filtered ona 10 μm aluminium filter. The results showed that photodegradation with UV light did in factaccelerate the degradation process even for short time intervals. Potential for fragmentation ofparticles in air was larger, due to air being a more oxidizing environment and weakening theplastics. The results implied that the degradation processes for PS is slower in water compared tothe other plastics in the same environment. In PS there was a larger amount of particles for theUV- exposed samples compared to the other plastics. This is interpreted as it has a slowerdegradation processes due to the fact when looking on the other plastics in stage w.7 (in water),the control samples have a higher particle count than for the UV exposed samples. It can beinterpreted as PS does not become as effected by the UV light while in water compared to theother plastics. Therefore, the conclusion is that the particles degraded and became smaller thanthe analysed size range (4 μm) and were therefore not detected, consequently, showing a lowerparticle count. After 56 days of UV radiation the largest amount of detected particle mass wasproduced by PP (chocolate wrapping) with 0.0143 mg/cm2 material and the least amount ofdetected particle mass in stage 56d was of PE-LD (plastic bag) with 0.00042 mg/cm2 material.Based on the comparison of the water stage and air stages together with conclusions from earlierstudies, the potential for a substantial destructive breaking of large particle are considered higherin air than in water, because the oxidation weakens the material making it less resilient tomechanical stress.