Multilayer adsorption of reactive orange 16 dye onto Fe2O3/ZnO hybrid nanoadsorbent: mechanistic insights from kinetics, isotherms and dynamic light scattering studies

BACKGROUND

Fe₂O₃/ZnO hybrid nanoadsorbent was synthesized by the coprecipitation method and characterized. The hydrodynamic size and stability of the nanoadsorbent were investigated in batch adsorption of toxic Reactive orange (RO)16 dye at increasing concentrations. The mechanism of monolayer and multilayer dye adsorption was elucidated for the first time together through dynamic light scattering (DLS), isotherm, kinetic and thermodynamic studies. A tomato seed germination assay was performed to confirm the material and treated water toxicity.

RESULTS

The ferromagnetic nanoadsorbent exhibited 85% RO16 dye removal. Nonlinear fitting of isotherm and kinetic models suggest the chemisorption (monolayer) and physisorption (multilayer) of dye over the heterogeneous surface of the nanoadsorbent, respectively, at lower and higher concentration of dye. In line with these mechanistic insights, DLS studies demonstrate that monolayer adsorption increased hydrodynamic size up to 100 mg L⁻¹ by face-off binding of dye molecules, whereas multilayer adsorption was in the 100–500 mg L⁻¹ concentration range; besides increase in adsorption capacity, did not magnify hydrodynamic size owing to face-on binding with multiple dye molecule stacking. Zeta potential data confirmed greater stability of the nanoadsorbent at solution pH with large hydrodynamic size. Thermodynamic studies suggested that endothermic and spontaneous adsorption process primarily controlled physical adsorption at higher dye concentrations. A toxicity assay proved that the nanoadsorbent and treated water are environmentally safe.

CONCLUSION

DLS, isotherm and kinetic studies elucidated the complex adsorption mechanism over a heterogenous surface of a hybrid nanoadsorbent. It was found that monolayer chemisorptive face-off binding of RO16 molecules increased hydrodynamic size whereas multilayer physisorptive face-on binding did not affect hydrodynamic size; rather, it remained stable with increasing RO16 concentration. © 2024 Society of Chemical Industry (SCI).