A new green method to NiO/MnO₂ nanocomposite synthesis for efficient and sustainable dye removal from aqueous media

The bimetallic nickel oxide/manganese oxide nanocomposite (NiO/MnO₂ NC) was prepared for the first time using a one-pot plant-mediated route and applied for the adsorption of malachite green (MG) dye from aqueous media compared with the monometallic nickel oxide nanoparticles (NiO NPs). The materials were characterized by TGA, FTIR, XRD, EDX, SEM, TEM, and BET analyses. The adsorption parameters, including pH, MG initial concentration, agitation time, adsorbent mass, and temperature, were systematically studied. The results indicated a significant effect of all parameters on the MG removal percent, attaining 90.4% by NiO NPs and 99.64% by NiO/MnO₂ NC within 60 min. The experimental data are best fitted by the Freundlich model for NiO NPs and the Langmuir model for NiO/MnO₂ NC, having maximum adsorption capacities of 39.84 and 208.3 mg/g, respectively. This can be attributed to the higher surface area of NiO/MnO₂ NC (143.65 m²/g) than NiO NPs (3.75 m²/g), as well as the presence of different metal atoms in the nanocomposite, which form unique adsorption sites with variable energies, thereby improving adsorption. Although the pseudo-second-order kinetic model more accurately describes kinetics, isotherm models, particularly the D-R isotherm model, show that physisorption is the primary mechanism. The combined data point to a multi-mechanistic adsorption process, with physisorption being the primary mechanism.The thermodynamic findings displayed that MG adsorption process is spontaneous, feasible, and endothermic. Both adsorbents are stable after five adsoption cycles, and the MG adsorption is mainly due to electrostatic attraction, hydrogen bonds, and π-π stacking. These results conclude that NiO/MnO₂ NC is a better alternative to NiO NPs for removing MG, providing insights into designing more efficient adsorbents for dye removal in the water treatment systems.