2D/2D Bi2WO6/Protonated g-C3N4 step-scheme heterojunctions for enhancing the photodegradation of 17β-estradiol: promotional role of electrostatic interaction

Abstract

17β-Estradiol (E2) is a type of endocrine-disrupting chemical (EDC) with the strongest estrogenic activity, which poses a huge threat to ecosystems and human health even at extremely low concentrations. In this work, a bismuth tungstate (Bi₂WO₆)/protonated graphite carbon nitride (pg-C₃N₄) 2D/2D step-scheme (S-scheme) heterojunction (BPG) with an intimate and stable interface was successfully constructed by a hydrothermal reaction for the photodegradation of E2 under visible light irradiation. The g-C₃N₄ surface changed from negative to positively charged after protonation treatment, and it could combine with negatively charged Bi₂WO₆ through strong electrostatic interactions to accelerate the electron transfer and photogenerated carrier separation. The B1PG1 heterojunction (mass ratio of Bi₂WO₆ and pg-C₃N₄ is 1 : 1) showed higher E2 removal efficiency than Bi₂WO₆, pg-C₃N₄ and Bi₂WO₆/g-C₃N₄, with a photodegradation rate constant about 13.0, 10.4 and 5.0 times higher, respectively. This could be attributed to the promoted photogenerated carrier separation because of the formation of the S-scheme heterojunction and its tight contact interface through the strong electrostatic attraction between negatively charged Bi₂WO₆ and positively charged pg-C₃N₄. Moreover, a strong electrostatic interaction existed between positive B1PG1 and anionic E2 at pH 6.3, which enhanced E2 adsorption, favoring photocatalysis. The possible photocatalytic degradation pathways of E2 were proposed according to the LC-MS/MS results, and it was preliminarily predicted that the activity of all the intermediates would also be lower than that of E2. This study provides new insight into the rational design and synthesis of highly active photocatalysts, which have great potential application in the treatment of wastewater containing EDCs.