Biosynthesis of Zinc Sulphide (ZnS) nanoparticles from endophytic fungus Aspergillus flavus for degradation of organic pollutants

Abstract

Wastewater treatment is adopted from ages but the efficiency, time constraint, environmental friendliness and cost-effectiveness are the key aspects for applicability of any technique. Globally, large amounts of wastewater containing organic pollutants from various industries are being discharged into the environment. These emerging contaminants are persistently released into the oceanic environment majorly from manufacturing industries, consumers utility and effluent treatment plants. In this aspect naophotocatalysis with the amalgamation of a biological source has been explored where ZnS nanoparticles have been synthesized by endophytic fungal isolate Aspergillus flavus via a medicinal plant Nothapodytes foetida. Significant findings from the characterization study include the formation of spherical particles with an average diameter of about 18 nm by TEM and hydrodynamic diameter of 58.9 nm. Optical properties confirm the quantum confinement effect and the functional groups present on the surface of the nanoparticles, further the stability of nanoparticles is accessed by UV–Vis spectrometer, zeta potential and cyclic voltammeter. XRD reveals the phase structure and the crystallinity indicating the hexagonal phase of ZnS and diffraction peaks at 28.45o, 47.54o, 57o (2θ). The degradation efficiency of ZnS nanoparticles for methyl violet/ 2,4- dichlorophenoxyacetic acid / paracetamol are 87 % in 2 h, 33 % in 4.5 h and 51 % in 4 h respectively and the impact of various parameters on the photocatalytic activity is also investigated. The experimental results of COD and TOC are 78 % and 74 % for methyl violet; 55.5 % and 57.2 % for 2,4- dichlorophenoxyacetic acid; 47.6% and 44.5% for paracetamol respectively. Moreover, the plausible mechanism on the radical generation from ZnS nanoparticles upon irradiation for the degradation process is hypothesized based on the mass spectrum. In addition, the interaction between organic pollutants and ZnS nanoparticles is also elucidated based on AFM and fluorescence spectrum. Investigations reveal that the mechanism involved is extracellular based on micrographs/chromatogram and the peaks at 149, 301 and 579 (m/z) corresponds to proteins such as metallothioneins and phytochelatins. The formation of nanoparticles production is obtained at optimum inoculum volume of 10 % (w/v) at working volume of 1 L and agitation speed 80 rpm. XRD and TEM analysis confirmed the hexagonal phase of nanoparticles with the average diameter of 10-15 nm at an optimum concentration of 30 mM for 72 h.