Green synthesis of zinc oxide nanoparticles from Morinda Lucida leaf extract for biosorption application, kinetics, and thermodynamic studies

The release of dye-contaminated effluents from industrial processes poses a serious threat to aquatic ecosystems and public health. In this study, zinc oxide nanoparticles (ZnO NPs) were synthesized using Morinda lucida leaf extract through an eco-friendly green synthesis route. The nanoparticles were characterized using UV–Vis, FTIR, XRD, TEM, SEM–EDS, and pHpzc analyses. XRD confirmed a hexagonal wurtzite structure with an average crystallite size of 16.5 nm, while TEM revealed quasi-spherical particles with a narrow size distribution. The ZnO NPs were applied for the adsorptive removal of methylene blue (MB) and indigo carmine (IC) dyes under varying pH, contact time, concentration, temperature, and adsorbent dosage. The adsorption process showed a maximum removal of 95% for IC at pH 5, while MB achieved 90% removal at pH 9. Maximum adsorption capacities were 26.67 mg g⁻¹ for IC and 25.12 mg g⁻¹ for MB. Kinetic modelling revealed pseudo-second-order behavior, while isotherm fitting showed that MB followed the Langmuir model and IC followed the Freundlich model. Thermodynamic analysis indicated endothermic adsorption with increasing spontaneity at higher temperatures. FTIR before/after adsorption and pHpzc data supported electrostatic attraction and surface complexation as dominant mechanisms. ZnO NPs also demonstrated good reusability over three cycles. The study demonstrates that M. lucida-mediated ZnO NPs are promising, sustainable adsorbents for dye-contaminated wastewater treatment.