One-Step Synthesis of Amino-Functionalized MnFe2O4 Magnetic Nanoparticles for Single-Solute Removal Adsorption of Cr(VI) and Congo Red from Water

To enable removal of heavy metals and anionic dyes from industrial wastewater, amino-functionalized manganese ferrite magnetic nanoparticles (MnFe2O4–NH2, denoted as PMNx) were synthesized by a facile one-pot polyol process using ethanolamine as the amination agent. XRD, FT-IR, SEM, BET and VSM confirmed a cubic spinel MnFe2O4 phase with successful surface introduction of –NH2 groups, a porous cluster morphology (BET surface area 134.69 m2·g−1) and superparamagnetic behavior (Ms = 30.79 emu·g−1) enabling rapid magnetic recovery. The amount of ethanolamine used had a significant effect on adsorption performance, with PMN20 (20 mL of ethanolamine) exhibiting the best adsorption performance for both pollutants. Under acidic conditions (pH = 2), PMN20 achieved high removal efficiencies for both hexavalent chromium (Cr(VI)) and Congo red (CR). The adsorption kinetics of both pollutants followed the pseudo-second-order model. The adsorption of Cr(VI) is more accurately described by the Freundlich model, whereas CR follows the Langmuir model. In the temperature range of 298–318 K, the adsorption of Cr(VI) is endothermic and spontaneous, while the adsorption of CR is exothermic and spontaneous across the entire temperature range, although the adsorption efficiency decreases at higher temperatures. PMN20 exhibits good reusability; after five adsorption–desorption cycles, the removal efficiencies for Cr(VI) and CR remained at ≥85% and ≥90%, respectively. These results confirm the potential of MnFe2O4–NH2 as a low-cost, magnetically recoverable adsorbent for treating complex industrial wastewater.