Rationally designed tetra (4-carboxyphenyl) porphyrin/graphene quantum dots/bismuth molybdate Z-scheme heterojunction for tetracycline degradation and Cr(VI) reduction: Performance, mechanism, intermediate toxicity appraisement

Constructing novel Z-scheme heterojunctions is an effective strategy for obtaining high-performance photocatalysts. Herein, tetra (4-carboxyphenyl) porphyrin (TCPP) and graphene quantum dots (GQDs) were loaded on the surface of Bi2MoO6 (BMO) to fabricate novel Z-scheme heterojunctions of TCPP/G/BMO. Especially, TCPP/G/BMO-2 showed an exceptional visible-light photoactivity for tetracycline (TC) degradation (81.0%, 40 min) and Cr(VI) reduction (90.7%, 60 min), respectively by 2.38 folds and 2.96 folds enhancement compared to sole Bi2MoO6. The substantial enhancement of activity is attributed to the synergy effect of the Z-scheme charge transfer mechanism and the improved light absorption. The degradation pathways of TC were inferred by determining the generated intermediates using high performance liquid chromatography-mass spectrometry (HPLC-MS), and the toxicity of the transformation products was assessed by Toxicity Estimation Software Tool (T.E.S.T). Overall, on the basis of trapping experiments and electron spin resonance spectra (ESR) analysis, the photocatalytic mechanisms of Cr(VI) reduction and TC degradation by the TCPP/G/BMO Z-scheme heterojunction was proposed. This work indicates TCPP/G/BMO could be a promising photocatalyst for wastewater treatment.