Aluminium (Al) substituted nickel ferrite (NiAl_xFe₂-_xO₄ with x = 0.0, 0.5 and 1.0) nanoparticles were synthesized by the technique of co-precipitation. The microstructural and optical properties of nickel ferrite nanoparticles were meticulously evaluated from X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis and UV–vis spectroscopy. The X-ray diffraction pattern of nanoparticles confirmed the single phase of cubic spinel structure. The particle size of nickel ferrite nanoparticles was in the range of 25–35 nm, while the lattice parameter “a” decreased with respect to Al content. The presence of Ni, Fe, Al, and O elemental peaks in the energy dispersive X-ray spectrum confirmed the formation of nickel ferrite nanoparticles. The functional groups of Al–Ni spinel ferrite phase were further confirmed from Fourier transform infrared spectroscopy. Two major absorption bands in the IR spectrum were observed at 600 cm⁻¹ (the high frequency band, ν₁) and 450 cm⁻¹ (the low frequency band, ν₂) from tetrahedral (A) and octahedral (B) interstitial sites in the spinel lattice. The magnetic hysteresis curves were recorded with respect to Al concentration at room temperature using vibrating sample magnetometry. The magnetization and coercivity of nickel ferrite were decreased as the Al contents increased.

Chromium (Cr) is a toxic heavy metal present in industrial effluent which could cause degenerative diseases on ingestion. Therefore, elimination of Cr from contaminated water becomes essential to preserve and sustain the quality of life. In this study, superparamagnetic iron oxide nanoparticles (SPIONs) were impregnated on activated carbon and checked for enhanced removal of Cr(VI). Thus, granular activated carbon (N-GAC) of mesh size was impregnated with SPIONs to produce SPIONs impregnated activated carbon (Sp-GAC) and characterized using UV-VIS spectrophotometer, FT-IR, SEM-EDX, XRD, and VSM. Sp-GAC was found to be possessing superparamagnetic behavior, and it was used in batch adsorption study where the adsorption parameters like pH, concentration of adsorbent, and adsorbent-adsorbate were optimized. It was found that Cr(VI) adsorption was predominant at acidic pH (pH 3) for both cases; moreover, N-GAC showed better adsorption comparatively. Both the N-GAC and Sp-GAC adsorption were found to be obeying to Langmuir.