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

17 beta-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.

Automated summary

This study successfully constructed a Bi2WO6/pg-C3N4 2D/2D S-scheme heterojunction for the photodegradation of E2 under visible light. The formation of the heterojunction accelerated the separation of photogenerated carriers and improved the E2 removal efficiency. The research provides new insights for the rational design and synthesis of highly active photocatalysts, with potential applications in the treatment of EDC-containing wastewater.