Biofabrication of novel ZnO nanoparticles for efficient photodegradation of industrial dyes

The removal of persistent dye pollutants from wastewater streams has drawn a lot of interest by the scientific community and photocatalysis is the most widely accepted practical strategy for addressing environmental contamination. Transition metal oxides are proved to be the most prospective catalysts for efficient and environment-friendly wastewater treatment because of their significant photocatalytic activity, excellent solubility and durability. Here, we describe a simple, eco-sustainable and cost-effective strategy for the synthesis of ZnO nanoparticles using Morinda umbellata leaf extract. The phytochemicals such as polyphenols, flavonoids and tannins present in the leaf extract act as reducing and stabilising agents. We investigated the photocatalytic activity of synthesised ZnO nanoparticles to break down organic dyes like Congo red (CR) and Malachite green (MG) in aqueous media. In aqueous solution, at ambient temperature, ZnO nanoparticles showed outstanding photocatalytic degradation efficiency of 98.9% (20 ppm) for the MG dye and 92.8% (10 ppm) for the CR dye at pH values of 10 and 6 respectively, in a short period of time. The optimum catalyst dosage was observed to be 0.1 and 0.125 g for MG and CR respectively. From the scavenger studies using different scavengers, it is confirmed that the superoxide radical is the main reactive species involved in the photocatalytic degradation of MG and CR. The kinetics of photodegradation was also investigated and followed a pseudo-first-order mechanism, with rate constants of 0.0204 min −1 for MG and 0.0123 min −1 for CR in accordance with the Langmuir–Hinshelwood model. The ZnO nanoparticles displayed excellent recycling capability for both dyes and the combined effects of their high adsorption capacity and photodegradation ability of organic dyes make them versatile choice for future applications. Graphical Abstract