Engineered Polystyrene-Based Microplastics of High Environmental Relevance

Microplastic (MP) pollution—an emerging environmental challenge of the 21st century—refers to accumulation of environmentally weathered polymer-based particles with potential environmental and health risks. Because of technical and practical challenges when using environmental MPs for risk assessment, most available data are generated using plastic models of limited environmental relevancy (i.e., with physicochemical characteristics inherently different from those of environmental MPs). In this study, we assess the effect of dominant weathering conditions—including thermal, photo-, and mechanical degradation—on surface and bulk characteristics of polystyrene (PS)-based single-use products. Further, we augment the environmental relevance of model-enabled risk assessment through the design of engineered MPs. A set of optimized laboratory-based weathering conditions demonstrated a synergetic effect on the PS-based plastic, which was fragmented into millions of 1–3 μm MP particles in under 16 h. The physicochemical properties of these engineered MPs were compared to those of their environmental counterpart and PS microbeads often used as MP models. The engineered MPs exhibit high environmental relevance with rough and oxidized surfaces and a heterogeneous fragmented morphology. Our results suggest that this top-down synthesis protocol combining major weathering mechanisms can fabricate improved, realistic, and reproducible PS-based plastic models with high levels of control over the particles’ properties. Through increased environmental relevancy, our plastic model bolsters the field of risk assessment, enabling more reliable estimations of risk associated with an emerging pollutant of global concern.

. HR-SEM images of raw plastic and their corresponding particle size distributions calculated using the ImageJ postprocessing software. show the generation of carbonyl bond peak at ~1700 cm -1 .

Calculation of energy requirements of single degradation procedures and combined Protocol 3:
Probe sonication technique was performed in order to imitate mechanical degradation. Energy consumption by probe sonication (Ep) can be estimated using equation S1: where Wp is the sonicator wattage (125 W), A% is the amplitude percentage being used in the accelerated procedure (70%), t is the sonication time in seconds (600 sec), and V is the volume of the solution in liters (0.05L). The calculated energy consumption was divided by the concentration of 1µm particles yielded during sonication (i.e., particle concentration following subtraction of background particles in raw plastic and particle contamination from probe erosion), to report on energy required to create a single 1-µm particle. The energy consumption by probe sonication at the applied conditions was found to be 1.05 × 10 6 J/L and energy required to create a single 1-µm particle was 1.66 J/L per particle.
Similarly, we evaluated the energy consumption from heat treatment using equation S2: where m is the heated plastic mass in kg (10 -4 kg), c is the specific heat capacity in j/kg C° (1200 j/kg C° for polystyrene), 9 ΔT is the temperature differences between oven temperature and room temperature in C° (45°C), and V is the volume of 100 mg of the particles in liters (ρPS= 1.04-1.08 gr/cm 2 and equal to 0.0001 L) 10 . Finally, the calculated energy needed to create a single 1-µm particle was 540 J/L per particle, much higher than that of probe sonication. The reason for this large difference between energy consumption for heat treatment and probe sonication is the fact that heat resulted in negligible particle formation compared to probe sonication.
UV-O treatment was used to imitate photo-chemical degradation. The evaluated energy consumption to irradiated area covered with layer of PS particles (Euv) under the suggested setup was calculate in joule (J) per particle (1-µm size) in litter (L) using equation S3 and S4: where W is the instrument power in milliwatts per cm 2 (4.6 mW/cm 2 ), A is the irradiated area in cm 2 (21 cm 2 ), t is the oxidation time in seconds (10800 sec), and V is the volume of 100 mg particles in liters (0.0001L). The calculated energy needed to create a single 1-µm particle is 76.05 J/L per particle.
In order to evaluate potential synergistic effect of the combined accelerated oxidation (3h of dry UVozone treatment), thermal (12h at 70 °C), and mechanical (10 min of 7 sec on/3 sec off cycles of S18 probe sonication) procedures, the energetic efficiency of the overall protocol was compared to that of single approaches. The energetic efficiency of protocol 3 was calculated using equation S5: where n is the protocol number, ΣE is the energy summary of accelerated mechanical, thermal, and photo-oxidation procedures, and P(n) is the 1-µm particle count following the degradation protocol.
Interestingly, the energy consumption calculated for Protocol 3 was 0.37 J/L per particle, lower than each individual degradation procedures.