Magnesium Ferrite/Poly(cysteine methacrylate) Nanocomposites for pH-Tunable Selective Removal and Enhanced Adsorption of Indigo Carmine and Methylene Blue

Magnesium ferrite (MgFe2O4) and silica-coated MgFe2O4 nanoparticles were grafted with poly(cysteine methacrylate) (i.e., MgFe2O4/PCysMA and MgFe2O4@SiO2/PCysMA nanocomposites) to study pH-tunable adsorption and enhanced capacities for the adsorption of anionic indigo carmine (IC) and cationic methylene blue (MB) dyes. Several characterization techniques (i.e., XRD, FTIR, TGA, ζ potential analysis, VSM, FE-SEM, TEM, N2 adsorption–desorption isotherm, and XPS) indicated successful syntheses of these nanocomposites. The adsorption behaviors of the dyes demonstrated that the PCysMA-modified nanoadsorbents could selectively adsorb either IC or MB from either single-component or binary dye systems if the initial pH of the dye solution was tailored appropriately (i.e., pH ∼2 for IC and pH ∼10 for MB). The selective adsorption of these dyes was proposed by the electrostatic attractions of the nanoadsorbents and the dyes. Adsorption isotherms also showed enhanced capacities of MgFe2O4 and MgFe2O4@SiO2 NPs for the adsorption of IC and MB after grafting with PCysMA. Interestingly, the MgFe2O4@SiO2/PCysMA nanoadsorbent provided highly pH-selective adsorption and large increases in the capacities for the adsorption of IC and MB, which were attributed to the amounts of PCysMA grafted onto different magnetic substrates. The coating of silica on the surfaces of magnetic nanoparticles provided a higher amount of 3-methacryloxypropyltrimethoxysilane, promoting the polymerization of CysMA monomer. Recycling tests indicated that high efficiencies (∼80%) for the adsorption of IC and MB by the PCysMA-modified nanoadsorbents were obtained after five adsorption–desorption cycles. These key findings showed that the MgFe2O4/PCysMA and MgFe2O4@SiO2/PCysMA nanocomposites exhibited excellent pH-tunable adsorption of anionic and cationic dyes, easy magnetic separation, good reusability, and high stability. Specifically, the MgFe2O4@SiO2/PCysMA nanocomposite offers highly pH-selective adsorption and high adsorption capacities for dyes, demonstrating promising and alternative nanoadsorbents for applications in wastewater treatment and sensors.