Nationwide Mass Inventory and Degradation Assessment of Plastic Contact Lenses in US Wastewater.

Plastics pose ecological and human health risks, with disposable contact lenses constituting a potential high-volume pollution source. Using sales data and an online survey of lens users (n = 416) alongside laboratory and field experiments at a conventional sewage treatment plant, we determined the environmental fate and mass inventories of contact lenses in the United States. The survey results revealed that 21 ± 0.8% of lens users flush their used lenses down the drain, a loading equivalent to 44 000 ± 1700 kg y-1 of lens dry mass discharged into US wastewater. Biological treatment of wastewater did not result in a measurable loss of plastic mass (p = 0.001) and caused only very limited changes in the polymer structure, as determined by µ-Raman spectroscopy. During sewage treatment, the lenses were found to accumulate as fragments in sewage sludge, resulting in an estimated accumulation of 24 000 ± 940 kg y-1 of microplastics destined for application on US agricultural soils contained in sewage sludge. Recycling of the contact lenses and their packaging amounted to only 0.04% of the total waste volume associated with contact lens use. This is the first study to identify contact lenses and more specifically silicone hydrogels, as a previously overlooked source of plastic and microplastic pollution.

Chemical and physical changes of microplastics during sterilization by chlorination.

Wastewater treatment plants are known to release microplastics that have been detected in aquatic and terrestrial organisms constituting part of the human diet. Chlorination of wastewater-borne microplastics was hypothesized to induce chemical and physical changes detectable by Raman spectroscopy and differential scanning calorimetry (DSC). In the laboratory, virgin plastics (∼0.05 × 2 × 2 mm) were exposed to differing sterilization conditions representative of dosages used in the disinfection of drinking water, wastewater, and heavily contaminated surfaces. Polypropylene (PP) was most resistant to chlorination, followed by high density polyethylene (HDPE) and polystyrene (PS). Polystyrene showed degradation, indicated by changes in Raman peak widths, at concentration-time regimes (CT values) as low as 75 mg min/L, whereas HDPE and PP remained unaltered even at chlorine doses characteristic of wastewater disinfection (150 mg min/L). However, HDPE and PS were not completely resistant to oxidative attack by chlorination. Under extremely harsh conditions, shifts in Raman peaks and the formation of new bonds were observed. These results show that plastics commonly used in consumer products can be chemically altered, some even under conditions prevailing during wastewater treatment. Changes in polymer properties, observed for HDPE and PP under extreme exposure conditions only, are predicted to alter the risk microplastics pose to aquatic and terrestrial biota, since an increase in carbon-chlorine (C-Cl) bonds is known to increase toxicity, rendering the polymers more hydrophobic and thus more prone to adsorb, accumulate, and transport harmful persistent pollutants to biota in both aquatic and terrestrial environments.