(381b) Optimization of Fe3O4 Nanoparticles Loading on Reduced Graphene Oxide Nanosheets for the Efficient Removal of Aqueous P-Nitroaniline and Cr(VI)

para-Nitroaniline (4-NA) is a stable nitroaromatic derivative of aniline primarily used as a precursor for the organic syntheses of azo dyes, antioxidants, fuel additives, corrosion inhibitors, pesticides and pharmaceuticals. Although sparingly soluble in water (0.8 g/L at 19°C), 4-NA is extremely haemotoxic and nephrotoxic to humans even in traces. The presence of a nitro group renders 4-NA resistant to chemical and biological oxidative degradation, and anaerobic degradation produces nitroso- and hydroxyl-amines which are genotoxic and carcinogenic in nature. Chromium (Cr) is a heavy metal, large quantities of which are discharged into environmental water bodies from a variety of industries including leather tanning, wood preservation, textile dyeing, electroplating and steel fabrication. Since India is one of the major global producers of chromite, chromium also leaches into the soil from natural processes like weathering and anthropogenic activities like accidental leakage and improper disposal at chromite mining sites. In its hexavalent form, Cr(VI) is a potential teratogen, mutagen and carcinogen and is readily absorbed into the bloodstream, posing significant health risks to human beings like liver cirrhosis, gastric damage, skin cancer and pulmonary congestion. Cr(VI) is particularly lethal since it is recalcitrant, soluble at all pH conditions and has high aqueous mobility. Therefore, removal of 4-NA and Cr(VI) from industrial effluents is essential before being discharged into aquatic systems to prevent heavy metal pollution and bioaccumulation higher up the food chain.

Herein, we demonstrate a facile hydrothermal route for laboratory synthesis of magnetite (Fe₃O₄) nanoparticles decorated on Reduced Graphene Oxide (RGO) with an optimized Fe₃O₄ loading of 27% for the adsorptive removal of 4-NA and Cr(VI) from wastewater. Physicochemical properties of the as-synthesized nanomaterial are evaluated by various analytical and spectroscopic techniques. X-Ray Diffraction and Raman analysis confirm the formation of inverse spinel Fe₃O₄ nanocrystals. FESEM and TEM imaging display quasi-homogeneous dispersion of Fe₃O₄ nanoparticles (average crystallite size=10.67 nm) anchored onto the surface of RGO nanosheets. BET analysis confirms the existence of microporous networks in the nanomaterial. Notably, the material possesses a high specific surface area (192.8 m²/g), which facilitates adsorption of toxic pollutants. Goniometry verifies that the nanocomposite is sufficiently hydrophilic for use in aqueous medium (left and right contact angles are determined to be 10.9° and 11.2° respectively). The adsorption kinetics are determined based on batch adsorption experiments, conducted with a fixed adsorbent dosage of 0.25 g/L at 30°C. Adsorption equilibrium for 4-NA and Cr(VI) systems are achieved in 4 h and 6.5 h respectively, following pseudo-second-order kinetics. The nanocomposite demonstrates an excellent maximum adsorption capacity of 264.21 mg/g for 4-NA and 324.56 mg/g for Cr(VI) at pH=2 and 303 K. These results are superior compared to most of the contemporary magnetic carbon-based adsorbents. HR-XPS analysis of the spent nanocomposite suggests that the adsorption of 4-NA is predominantly physical and is governed by: (1) Extensive electrostatic attraction between the positively charged adsorbent surface and negatively charged 4-NA molecules; and (2) Ï€-Ï€ interactions between aromatic 4-NA molecules and the Ï€-electron rich RGO moiety. Mechanistically, adsorption of Cr(VI) is primarily achieved through chemical routes, viz-a-viz chelation, and partial reduction to Cr(III) catalyzed by nanocrystalline Fe₃O₄. Room temperature magnetization plot (M-H curve) shows that the nanomaterial possesses a saturation magnetization of 18.4 emu/g, which facilitates rapid and efficient separation of the adsorbent from aqueous solution post adsorption using an external magnet. The optimized nanocomposite is found to be chemically and morphologically stable for reuse up to 4 cycles for both 4-NA and Cr(VI) without any significant loss of adsorption capacity or leaching of iron above its maximum permissible limit in potable water. The prepared nanocomposite 27% Fe₃O₄/RGO therefore has potential for future commercial exploitation for remediation of 4-NA and Cr(VI) from industrial effluents.