Electrochemical degradation of nanoplastics in water: Analysis of the role of reactive oxygen species

Abstract

Microplastics and nanoplastics (NPs) are emerging water contaminants which have recently gained lots of attention because of their effects on the aquatic systems and human life. Most of the previous works on the treatment of plastic pollution in water have been focused on microplastics and a very limited study has been performed on the NPs treatment. In this work, the role of main reactive oxygen species (ROSs) in the electrooxidation (EO) and electro-peroxidation (EO-H₂O₂) of NPs in water is investigated. In-situ generation of hydroxyl radicals (^•OH), persulfates (S₂O₈^2-), and hydrogen peroxide (H₂O₂) were performed using boron-doped diamond (BDD) as the anode, whereas titanium (in EO process) and carbon felt (CF, in EO-H₂O₂ process) were used as cathode. In the EO process, NPs were mainly oxidized by two types of ROSs on the BDD surface: (i) ^•OH from water discharge and (ii) SO₄^•- via S₂O₈^2- reaction with ^•OH. In EO-H₂O₂ process, NPs were additionally degraded by ^•OH formed from H₂O₂ decomposition as well as SO₄^•- generated from direct or indirect reactions with H₂O₂. Analysis of the degradation of NPs showed that EO-H₂O₂ process was around 2.6 times more effective than EO process. The optimum amount of NPs degradation efficiency of 86.8% was obtained using EO-H₂O₂ process at the current density of 36 mA·cm^-2, 0.03 M Na₂SO₄, pH of 2, and 40 min reaction time. In addition, 3D EEM fluorescence analysis confirmed the degradation of NPs. Finally, the economic analysis showed the treatment of NPs using EO-H₂O₂ process had an operating cost of 2.3 $US.m^-3, which was around 10 times less than the EO process. This study demonstrated that the in-situ generation of ROSs can significantly enhance the degradation of NPs in water.

Keywords: Degradation; Electrooxidation; H(2)O(2) electrogeneration; Nanoplastic; Polystyrene; Water treatment.