Mussel-inspired polydopamine functionalized with ionic liquid as a novel, eco-efficient adsorbent for the selective removal of anionic pollutants from aqueous solutions

Abstract

In this work, novel mussel-inspired polydopamine nanoparticles functionalized with ionic liquid (PDA-IL) were synthesized via one step oxidative polymerization and Schiff-base addition reaction using dopamine and ionic liquid (3-aminopropylammonium acetate). The synthesized polydopamine-ionic liquid (PDA-IL) was utilized as an efficient adsorbent in the removal of anionic dyes from aqueous solutions, using Alizarin Red S (ARS) as a model pollutant. Multiple analytical tools, including scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analyzer (TGA), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy and Zeta potential were applied in the characterizations of PDA-IL. A parametric study was conducted to examine the effects of pH, contact time, initial concentration, adsorbent dose, and temperature on PDA-IL adsorption performance. Characterization results confirmed the formation of nano-sized PDA-IL particles with polycrystalline structure. Additionally, the porous structure, surface charge, surface moieties and thermal stability were progressively tuned along with the introduction of the custom-synthesized IL within the matrix of PDA. Results showed a significant maximum ARS adsorption capacity of PDA-IL, up to 234.2 mg g−1 at 25 °C. Adsorption kinetics data revealed a pseudo-second-order kinetics, governed by chemisorption. Equilibrium data for ARS removal were in agreement with Freundlich isotherm, indicating multilayer adsorption on the heterogeneous PDA-IL surface. Furthermore, regeneration studies demonstrated an efficient re-use of PDA-IL adsorbent in at least 4 adsorption/desorption cycles. A selectivity study was also conducted, which showed that PDA-IL selectivity towards anionic dyes can be tuned via pH control. Finally, an adsorption mechanism was proposed, based on FTIR and electron energy loss spectroscopy (EELS) mapping results, among others, for an understanding of the underlying adsorption phenomena on PDA-IL. Overall, the developed PDA-IL adsorbent had a great removal capacity of anionic ARS dye, fast adsorption kinetics, excellent selectivity towards anionic dyes and high regeneration efficiency.