Fabrication of di-selective adsorption platform based on deep eutectic solvent stabilized magnetic polydopamine: Achieving di-selectivity conversion through adding CaCl2

Seeking effective synthesis strategy of adsorbents with di-selective adsorption ability to remove dyes in wastewater is of great significance for solving the increasing environmental problems. Hence, three deep eutectic solvents (DESs) were synthesized and polymerized on the surface of magnetic (Fe3O4) polydopamine (MPDA), in which DES acted as both stabilizer and surface modifier for PDA coating. The fabricated nanoparticles (named as MPDA-PDES) were used as adsorbents for selective adsorption of binary dyes solution. Langmuir and Dubinin-Radushkevich isotherm models were suitable to describe adsorption process of malachite green (MG) and sunset yellow FCF (SY) on MPDA-PDES1, respectively, indicated that the adsorption of MG was monolayer uptake, and that of SY was physical adsorption. The effect of solution pH and ionic strength suggested that electrostatic interaction was mainly adsorption driving force. Interestingly, the addition of CaCl2 in adsorption system realized dynamic transformation from selective adsorption of MG by MPDA-PDES1 to selective adsorption of SY. And selectivity factor reduced from 37.57 to 0.30. MPDA-PDES1 adsorbent possessed excellent anti-interference ability, regeneration and reusability, can be continuously reused six times and adsorption capacity remained basically unchanged. The adsorption of MG-SY from synthetic wastewater manifested that MPDA-PDES1 had unlimited potential for selective removal of target dyes in actual wastewater. Mechanism exploration revealed that MPDA stabilized by DES containing long-chain alkyl groups exhibited better selective adsorption capacity for cationic dyes in binary dyes solutions. This was due to the synergistic effect of electrostatic interaction, hydrophobic interaction, hydrogen bonding and π-π stacking between MPDA-PDES1 and dyes. Therefore, proposed DES-based PDA modification strategy improved the stability and reusability of PDA materials, and achieved outstanding selective adsorption ability because of tailorability of DES structure, which provided new ideas for design and synthesis of high-efficiency adsorbents.