Modifications to microplastics by potassium ferrate(VI): impacts on sorption and sinking capability in water treatment

Abstract

Pre-treatment (oxidation) may induce potential modifications to microplastics (MPs), further affecting their behaviors and removal efficiency in drinking water treatment plants. Herein, potassium ferrate(VI) oxidation was tested as a pre-treatment for MPs with four polymer types and three sizes each. Surface oxidation occurred with morphology destruction and oxidized bond generation, which were prosperous under low acid conditions (pH 3). As pH increased, the generation and attachment of nascent state ferric oxides (Fe_xO_x) gradually became dominant, making MP-Fe_xO_x complexes. These Fe_xO_x were identified as Fe(III) compounds, including Fe₂O₃ and FeOOH, firmly attaching to the MP surface. Using ciprofloxacin as the targeted organic contaminant, the presence of Fe_xO_x enhanced MP sorption dramatically, e.g., the kinetic constant K_f of ciprofloxacin raised from 0.206 (6.5 μm polystyrene) to 1.062 L g^-1 (polystyrene-Fe_xO_x) after oxidation at pH 6. The sinking performance of MPs was enhanced, especially for small MPs (< 10 μm), which could be attributed to the increasing density and hydrophilicity. For instance, the sinking ratio of 6.5 μm polystyrene increased by 70% after pH 6 oxidation. In general, ferrate pre-oxidation possesses multiple enhanced removals of MPs and organic contaminants through adsorption and sinking, reducing the potential risk of MPs.

Keywords: Advanced oxidation processes; K2FeO4; Nanoplastics; Plastic debris; Pollution control.