Tuning the Adsorption of Cationic and Anionic Dyes Using NiFe 2 O 4 Nanoparticles: Role of pH Modulation

ABSTRACT The development of efficient and reusable adsorbents for wastewater treatment is crucial for mitigating environmental pollution caused by industrial dyes. In this study, nickel ferrite (NiFe 2 O 4 ) nanoparticles were synthesized via a citrate‐assisted auto‐combustion method and explored as a pH‐responsive dual adsorbent for the removal of both anionic and cationic dyes from aqueous solutions. The structural, morphological, and optical properties of the synthesized nanoparticles were confirmed using X‐ray diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR), Field Emission Scanning Electron Microscope (FE‐SEM), Transmission Electron Microscopy (TEM) and UV–Vis spectroscopy, verifying the formation of phase‐pure spinel NiFe 2 O 4 with nanoscale dimensions. Batch adsorption experiments revealed that NiFe 2 O 4 exhibited excellent ~92% removal efficiency for Fast Sulphon Black F (FSBF), an anionic sulfonated azo dye, at low pH due to favorable electrostatic interactions with the protonated surface, and most of the adsorption occurred within the first 15 min. Conversely, at high pH, the negatively charged surface of NiFe 2 O 4 facilitated the adsorption of the cationic dye Crystal Violet (CV), demonstrating dual adsorption behavior modulated by pH. The adsorption data of FSBF were analyzed using Langmuir, Freundlich, Temkin, Dubinin–Radushkevich, Redlich‐Peterson, and Hill isotherm models, confirming favorable adsorption characteristics, spontaneous (ΔG° = −ve) and exothermic nature (ΔH° ≈ –21.3 kJ mol −1 ), and strong surface affinity under optimized conditions. Leaching studies under extreme acidic and alkaline environments further confirmed the chemical stability of the adsorbent, with negligible metal ion release. These findings highlight the potential of NiFe 2 O 4 nanoparticles as a cost‐effective, magnetically separable, and pH‐tunable dual adsorbent for the efficient removal of diverse dye pollutants from water.