Synthesis of magnesium ferrite/poly(cysteine methacrylate) nanocomposites for pH-tunable selective adsorption of indigo carmine and methylene blue

Abstract

This study focuses on the preparation of magnesium ferrite-based nanoparticles /poly(cysteine methacrylate) nanocomposites (i.e., MgFe2O4/PCysMA and MgFe2O4@SiO2/PCysMA) and utilization of the synthesized nanocomposites as anionic indigo carmine (IC) and cationic methylene blue (MB) nanoadsorbents. This work is divided into three parts: syntheses of MgFe2O4/PCysMA and MgFe2O4@SiO2/PCysMA nanocomposites, characterization of the as-synthesized nanocomposites, and investigation of pH-selective adsorption behavior for IC and MB by the nanocomposites. The preparation of the nanocomposites comprised magnesium ferrite-based nanoparticles and PCysMA was performed via a radical polymerization reaction. The characteristics of the MgFe2O4/PCysMA and MgFe2O4@SiO2/PCysMA nanocomposites were investigated by several characterization techniques: XRD, FTIR, XPS, TGA, FE-SEM, TEM, VSM, BET, and zeta potential measurement. After fully characterization of the synthesized nanocomposites, the pH-selective adsorption of IC and MB by the nanocomposites was performed. At the optimal pH condition, the MgFe2O4/PCysMA and MgFe2O4@SiO2/PCysMA nanoadsorbents could selectively adsorb either IC (pH ~2) or MB (pH ~10). The MgFe2O4/PCysMA and MgFe2O4@SiO2/PCysMA nanoadsorbents exhibited highly pH-selective adsorption for IC and MB dyes from either single-component or binary dye systems, especially MgFe2O4@SiO2/PCysMA nanoadsorbents. The selective adsorption of these dyes was proposed by the electrostatic interaction between the nanoadsorbents and the dyes. Adsorption isotherms also showed enhanced maximum capacities (qm) of MgFe2O4 and MgFe2O4@SiO2 NPs for the adsorption of IC and MB after modified with PCysMA. Interestingly, the MgFe2O4@SiO2/PCysMA nanoadsorbent provided higher pH-selective adsorption and the qm for adsorption of IC and MB compared to those of MgFe2O4/PCysMA nanoadsorbent, which were attributed to the higher amounts of PCysMA grafted onto surfaces of the MgFe2O4@SiO2 NPs. Moreover, regeneration tests showed that high removal efficiencies for IC and MB adsorption by the PCysMA-modified nanoadsorbents were obtained after five adsorption-desorption cycles. This study shows that the synthesized MgFe2O4/PCysMA and MgFe2O4@SiO2/PCysMA nanocomposites exhibit excellent pH-tunable adsorption of anionic IC and cationic MB dyes, easy magnetic separation by induction with an external magnet, good reusability, and high stability, demonstrating promising and alternative nanoadsorbents for applications in wastewater treatment.