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.