Photocatalytic degradation of Microcystin-LR by visible light active and magnetic, ZnFe2O4-Ag/rGO nanocomposite and toxicity assessment of the intermediates

Abstract

In this work, we aimed to study photocatalytic degradation of Microcystin-LR (MC-LR), a cyanotoxin known to cause acute as well as chronic toxicity and even mortality. The nanocomposite (NC) based on zinc ferrite (ZnFe₂O₄) was modified with graphene oxide (GO) and Ag nanoparticles (NPs) to enhance its photocatalytic properties under visible light. The so-formed ZnFe₂O₄-Ag/rGO NC exhibited superior performance in visible light allowing complete degradation of MC-LR within 120 min of treatment with pseudo rate constant, k = 0.0515 min^-1, several times greater than other photocatalysts, TiO₂ (k = 0.0009 min^-1), ZnFe₂O₄ (k = 0.0021 min^-1), ZnFe₂O₄-Ag (k = 0.0046 min^-1) and ZnFe₂O₄/rGO (k = 0.007 min^-1) respectively. The total organic carbon analysis revealed that only 22% of MC-LR was mineralized on 120 min of treatment time indicating presence of different intermediate by-products. The intermediates formed during photocatalytic treatment were identified using liquid chromatography-mass spectrometry (LCMS) based on which probable degradation pathways were proposed. The attack from OH radicals formed during the photocatalytic process resulted to hydroxylation and subsequent cleavage of diene bond. The toxicity assessment with Daphnia magna revealed that the degradation process has alleviated toxicity of the MC-LR and no toxic intermediates were formed during the treatment which is very important from eco-toxicological view point. Therefore, ZnFe₂O₄-Ag/rGO has a good potential in the field of environmental applications as visible light active and magnetic photocatalyst with enhanced performance.

Keywords: Daphnia magna; Intermediates; Microcystin; Photocatalytic degradation; Toxicity.