This work reports the synthesis of nanosilica-coated magnetic carbonaceous adsorbents (MCA@SiO₂) using low-temperature hydrothermal carbonization technique (HCT) and the feasibility to utilize it for methylene blue (MB) adsorption. Initially, a carbon precursor (CP) was synthesized from corn starch under saline conditions at 453 K via HCT followed by the magnetization of CP again via HCT at 453 K. Subsequently, MCA was coated with silica nanoparticles. MCA and MCA@SiO₂ were characterized using X-ray diffraction, Fourier transform infrared, scanning electron microscopy/energy-dispersive spectroscopy, transmission electron microscopy, and Brunauer-Emmett-Teller (BET) N₂ adsorption-desorption isotherms. The BET surface area of MCA and MCA@SiO₂ were found to be 118 and 276 m² g^-1, respectively. Adsorption of MB onto MCA@SiO₂ was performed using batch adsorption studies and in the optimum condition, MCA@SiO₂ showed 99% adsorption efficiency with 0.5 g L^-1 of MCA@SiO₂ at pH 7. Adsorption isotherm studies predicted that MB adsorption onto MCA@SiO₂ was homogeneous monolayer adsorption, which was best described using a Langmuir model with the maximum adsorption capacity of 516.9 mg g^-1 at 25 °C. During adsorption kinetics, a rapid dye removal was observed which followed pseudo-first- as well as pseudo-second-order models, which suggested that MB dye molecules were adsorbed onto MCA@SiO₂ via both ion exchange as well as the chemisorption process. The endothermic and spontaneous nature of the adsorption of MB onto MCA@SiO₂ was established by thermodynamics studies. Mechanism of dye diffusion was collectively governed by intraparticle diffusion and film diffusion processes. Furthermore, MB was also selectively adsorbed from its mixture with an anionic dye, that is, methyl orange. Column adsorption studies showed that approximately 500 mL of MB having 50 mg L^-1 concentration can be treated with 0.5 g L^-1 of MCA@SiO₂. Furthermore, MCA@SiO₂ was repeatedly used for 20 cycles of adsorption-desorption of MB. Therefore, MCA@SiO₂ can be effectively utilized in cationic dye-contaminated wastewater remediation applications.

Reshma Babu

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