Hydrothermal synthesis of Fe3O4 nanoparticles at different pHs and its effect on discoloration of methylene blue: Evaluation of alternatives by TOPSIS method

In this study, hydrothermal synthesis was employed to produce Fe3O4 nanoparticles for the methylene blue removal from an aqueous medium. The production of magnetite nanoparticles involved the utilization of Fe2+ and Fe3+ salts, and their respective solutions were created at pH levels of 10, 11, and 12, subsequently subjected to autoclave treatment. The evaluation of pH-induced variations in nanoparticle characteristics was conducted through X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), vibrating-sample magnetometer (VSM), and Dynamic light scattering (DLS) analyses. pH levels within the synthetic medium yielded diminutive Fe3O4 nanoparticles characterized by augmented saturation magnetization, heightened zeta potential, and a narrower size distribution. Following nanoparticle synthesis under varied pH conditions, their efficacy in methylene blue (MB) adsorption was investigated. Notably, nanoparticles synthesized at pH 11 demonstrated the highest adsorption capacity, registering maximum MB adsorption capacities of 188.68 mg/g, while pH 10 and 12 counterparts exhibited capacities of 169.49 mg/g and 175.44 mg/g, respectively. The adsorption processes conformed to the Langmuir–Freundlich isotherm model, with the pseudo-second-order model offering superior fitting to elucidate MB adsorption kinetics, thereby indicating the involvement of chemical adsorption mechanisms. It was discerned that MB adsorption encompassed both physical and chemical mechanisms, encompassing electrostatic attraction between the negatively charged surface of the nanoparticles and the cationic dye, as well as electron exchange interactions between MB and the functional groups present on the adsorbent surface. Thermodynamically speaking, this adsorption process exhibited an exothermic nature, with a preference for ambient temperatures. Magnetic separation post-adsorption was conveniently executed, and all nanoparticles demonstrated potential for reuse for a minimum of five cycles. Moreover, the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) method was employed to assess and select the most favorable alternative from the available options. Based on the evaluation of 27 distinct criteria, it was determined that Fe3O4 synthesized in an environment with pH 11 emerged as the optimal choice, aligning with the initial findings from the adsorption assessments.