Removal of congo red dye from aqueous solution using zinc oxide nanoparticles synthesised from Ocimum sanctum (Tulsi leaf): a green approach

Abstract

ABSTRACT In this study, we synthesised ZnO (zinc oxide) NPs from Ocimum sanctum (Indian Tulsi leaves) through green approach using zinc nitrate and sodium hydroxide as main materials and Ocimum sanctum leaf extract as a precursor. The resulting product was characterised by UV-visible, Fourier transform infrared (FT-IR), X-ray diffraction (XRD), Field emission scanning electron microscope (FE-SEM), Energy-dispersive X-ray (EDX), Transmission electron microscopy (TEM), High-resolution Transmission electron microscopy (HR-TEM) BET isotherm and Thermo gravimetric analysis. Our investigation on adsorption of Congo red (CR) dye from dye contaminated water by green synthesised ZnO NPs was found to be more efficient (97%) than the conventional commercially available zinc oxide (78%). The maximum CR dye removal (97% by ZnO-T and 78% by ZnO-C) was obtained with initial pH 4.0, contact time 30 min, adsorbent dosage 0.20 g, initial CR concentration 40 mg/L, shaking speed 300 rpm and the adsorption experiment was carried out in room temperature. To understand deep into the mechanism of CR dye adsorption on the adsorbent, we have investigated adsorption kinetics both by pseudo-first order and pseudo-second order. Due to high correlation coefficient value (R2 = 0.99) of the pseudo-second order kinetics, the adsorption process followed chemisorptions where the experimental data for the isotherm followed the Langmuir isotherm model. Reusability efficiency of the NPs was explored using ethanol and based on the results it can be inferred that ZnO-T NPs can be reused for dye removal. The outcome of this research suggested that the green synthesised zinc oxide NPs could be a potential adsorbent material for removal of CR dye from aqueous solution.