Development of Novel Fe3O4/AC@SiO2@1,4-DAAQ Magnetic Nanoparticles with Outstanding VOC Removal Capacity: Characterization, Optimization, Reusability, Kinetics, and Equilibrium Studies

The adsorption of pollutants to the surface of adsorbents plays a critical role in the effectiveness of adsorption technology for air purification applications. Herein, novel magnetic nanoparticles functionalized with 1,4-diaminoanthraquinone (1,4-DAAQ), namely, Fe3O4/activated carbon (AC)@SiO2@1,4-DAAQ, were innovatively synthesized via co-precipitation and sol–gel techniques. After that, these nanoparticles were used for high-efficiency removal of volatile organic compounds (VOCs) (i.e., benzene and toluene). The synthesized nanoparticles were characterized by various techniques such as Fourier transform IR spectroscopy, thermogravimetric analysis/differential thermal analysis, scanning electron microscopy, and Brunauer–Emmett–Teller analysis. The dynamic adsorption process of VOCs was optimized based on operating parameters. The adsorption experiments revealed that Fe3O4/AC@SiO2@1,4-DAAQ showed exceptional performance for the removal of VOCs. It was observed that for benzene, Fe3O4, AC, Fe3O4/AC, Fe3O4/AC@SiO2, and Fe3O4/AC@SiO2@1,4-DAAQ exhibited dynamic adsorption capacities of 180.25, 228.87, 295.84, 382.10, and 1232.77 mg/g, respectively. Additionally, for toluene, they exhibited dynamic adsorption capacities of 191.08, 274.53, 310.26, 421.30, and 1352.16 mg/g, respectively. This indicated that the modification of 1,4-DAAQ could greatly enhance the dynamic adsorption capacity of Fe3O4/AC@SiO2@1,4-DAAQ for VOCs. In addition to the apparent adsorptive behavior in removing VOCs, Fe3O4/AC@SiO2@1,4-DAAQ exhibited high repeatability. After ten consecutive adsorption/desorption cycles, for benzene and toluene, Fe3O4/AC@SiO2@1,4-DAAQ retained 79.36 and 78.24% of its initial adsorption capacity, respectively. According to the characterization results, the average pore diameter for Fe3O4/AC@SiO2@1,4-DAAQ was determined to be 24.46 nm, indicating that they were in the mesopore range. The adsorption mechanism of the VOCs on Fe3O4/AC@SiO2@1,4-DAAQ was clarified by investigating the isotherm and kinetic criteria in detail. Isotherm models suggested that the adsorption process of VOCs is physical. Moreover, from the analysis of diffusion-based rate-limiting kinetic models, the findings reveal a combination of intraparticle diffusion as well as film diffusion throughout the adsorption process of VOCs. In addition, it was concluded from the analysis of the mass transfer model factors that global mass transfer and internal diffusion are more effective than film diffu