Rapid oxidative fragmentation of polypropylene with pH control in seawater for preparation of realistic reference microplastics

Abstract

Various tiny plastic particles were retrieved from the sea and studied using scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX) analysis to prepare realistic reference microplastics (MP). Most of the MP exhibited a diameter of < 20 × 10−6 m and 0.1–0.2 molar ratios of oxygen to carbon atoms (O/C), indicating that they primarily comprised polyethylene (PE), polypropylene (PP), and polystyrene (PS). It took a long time to reproduce such O/C ratios in standard laboratory weathering methods. For example, degrading of 30 × 30 × 0.060 mm PP film required 75 days for the 0.1 ratio, even with an advanced oxidation process (AOP) using a sulfate radical anion (SO4·−) initiator in distilled water at 65 °C. However, seawater drastically improved the PP degradation performance of AOP under a weak acid condition to achieve the 0.1 ratio of PP film in only 15 days. The combination of seawater and the SO4·− initiator accelerated the degradation process and showed that the MP’s size could be controlled according to the degradation time.

Verification of the existence of copper-based antifouling paint particles in the sea and their implications for marine microplastic degradation

“…This color change is due to the valence change of copper ions. We have previously reported that seawater drastically improved PP degradation performance during the AOP under pH control [15,25]. Cl − reacts with OH• generated by degradation actions such as solar irradiation and AOP in seawater and yields Cl 2 .…”

“…Because the pH of seawater is around 8, the equilibrium is biased toward the less reactive ClO − , suppressing PP degradation in seawater [27]. We, therefore, adjusted the pH to shift the equilibrium toward ClOH [15] in Eq. (1).…”

“…where reaction ( 3) is reversible but kinetically slower, and reaction ( 4) is reversible and faster [19,30] [15]. This procedure ensures a bias towards the ClOH-rich equilibrium for a period before exchanging with a fresh K 2 S 2 O 8 seawater solution.…”

“…MP mainly comprises polypropylene (PP), polyethylene (PE), and polystyrene (PS) products and is produced by sunshine exposure [12][13][14]. In our previous study [15], PP degradation was performed in seawater via an advanced oxidation process (AOP) using SO 4 • − [15], where SO 4 • − functions as a highly e cient initiator for degrading the plastic. The combination of seawater and the SO 4 • − initiator resulted in the excellent acceleration of the degradation process under pH control.…”

“…1452 cm − 1 )[31][32][33]. In the case of the AOP in seawater[15], a reaction between Cl − and OH• occurs and produces Cl•; then, two Cl• atoms couple to produce Cl 2 , which then reacts with H 2 O and forms ClOH, with the equilibria depending on pH. The pH value of K 2 S 2 O 8 in the seawater solution (normal salinity concentration) decreased from ca.…”