Removal of polystyrene nanoplastics from urban treated wastewater by electrochemical oxidation

Abstract

This work investigates for the first time the removal of polystyrene nanoplastics (NPs) from synthetic urban treated wastewater by electrochemical oxidation with Boron-Doped Diamond (BDD). The influence of current density (10–100 mA cm⁻²) and initial pollutant concentration (20–100 mg L^-1) was evaluated. Additionally, the effect of the anode material on NPs removal was studied, employing Mixed Metal Oxide (MMO) anodes. The process efficiency was assessed through Total Organic Carbon (TOC) reduction and acute toxicity tests using the Aliivibrio fischeri luminescent bacterium. Results demonstrate that BDD anodes achieve mineralisation rates exceeding 90 % at current densities above 50 mA cm⁻², whereas MMO anodes exhibit significantly lower degradation. Due to competing reactions, such as oxygen evolution, the process efficiency decreases at higher current densities with BDD anodes. NPs removal occurs primarily by a mediated oxidation process involving electrochemically generated oxidants. After treatment, transmission electron microscopy (TEM) analysis reveals a progressive reduction in NPs size from 150 nm to 68 nm. Regarding toxicity analysis, acute toxicity can be eliminated by applying current densities above 50 mA cm⁻², indicating that the generated by-products are non-toxic. These findings highlight the feasibility of electrochemical oxidation with BDD anodes for removing NPs from urban treated wastewater since the experimental conditions used in the previous literature often differ significantly from those in real wastewater. The study underscores its potential as a sustainable technology to mitigate the spread of these emerging pollutants into the environment.