(371o) Synthesis of Mussel-Inspired Polydopamine Mediated with Ionic Liquid As a Sustainable Adsorbent for the Selective Removal of Anionic Pollutants for Wastewater Applications

The extensive rise in industrial activities has led to the presence of various pollutants, including toxic synthetic dyes, in industrial wastewater effluents. From all the technologies available for treating the wastewater, adsorption is found to be the most economically feasible and easily operated technology. Different adsorbents such as activated carbon, different types of zeolites, bio-polymers, clays and various nano-composites have been used in removing impurities from water. However, there are several challenges associated with the use of many of these adsorbents. Toxicity, recyclability, scaling up and selectivity are some key challenges yet to be addressed. Therefore, development of sustainable and efficient adsorbents with minimized cross-contamination of the feed water, which have high selectivity towards certain contaminants is crucial. Recently, novel mussel-inspired material known as polydopamine (PDA) has emerged as a promising adsorbent for wastewater treatment by adsorption technology. PDA is derived from dopamine (DA) and consists of catechol, amine and imine functional moieties. Its similar structure with mussel adhesive proteins is responsible for high binding ability with different substances even on wet surfaces. However, the adsorption capacity of PDA alone is low compared to other adsorbents which meet indusial demands. Therefore, it is crucial to increase the adsorptive capacity of PDA to be useful on industrial scale and help to utilize all of its functional groups without deterioration in adsorption performance. Increasing the surface-active sites for adsorption can be achieved by modification of its surface area and charge. One possible route for developing PDA with high adsorption capacity is to modify the adsorbent via Schiff-based addition reaction with sustainable green solvent such as ionic liquids (ILs). ILs are widely used in different applications, including separation and extraction processes, as they are capable of multiple types of interactions (π–π, n–π, and hydrogen bonding). The usage of sustainable ILs is gaining more emphasis because of their high thermal and chemical stability, low volatility, environmental-friendly behavior and very high ability to dissolve a wide range of compounds.

The emerging technology of combining the unique properties of ILs along with an efficient adsorbent is still relatively not investigated, regardless of the increasing efforts in recent years to find optimal adsorbents. Adsorbents with IL functionalities have the advantages of tunability and tailor-ability according to the desired application. Due to their designability, IL-based functional adsorbents can be applied in a variety of fields. With almost endless possibilities of IL cation and anion combinations, the applications for IL-based adsorbents become vastly unlimited. In addition, the strong adhesive abilities of PDA as well as its unique functional moieties are of paramount importance in endowing the final adsorbent its chemical properties and behavior in surrounding media. Furthermore, the introduction of functional groups within the IL components may not only broaden the type of adsorbent and scope of application but may also increase the stability of the overall compound. Overall, the advent of advanced functional materials is of utmost importance due to the need for specificity and effectiveness towards a plethora of fields, whether industrial or technological. Increasing interest in IL integration with other materials finds tremendous applications in adsorption and extraction technologies for water and wastewater treatment, electrochemistry, carbon capture, desulfurization, drug delivery, material synthesis and many others.

In this work, Polydopamine-Ionic Liquid (PDA-IL) adsorbent was synthesized by a facile oxidative polymerization method and used as an efficient adsorbent for the removal of anionic dyes (Alizarin Red S, ARS) and anionic nutrients (Phosphates) from aqueous solutions. Characterizations of the synthesized PDA-IL particles were performed using multiple techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, thermogravimetric analysis (TGA), Zeta potential, and X-ray diffraction (XRD). Moreover, the effects of initial pH of the solution, initial concentration, contact time, adsorbent dose and temperature on the removal of ARS and phosphates using PDA-IL were investigated. Results showed that the maximum adsorption capacity at 25 °C could reach up to 234.19 mg/g for ARS removal and 93.08 mg/g for phosphate. Additionally, analysis of adsorption kinetics showed that the adsorption behavior followed the pseudo-second-order kinetic model, which indicates that adsorption is governed by chemisorption. The equilibrium adsorption data were in agreement with the Freundlich isotherm, indicating multilayer adsorption on heterogeneous surfaces. Thermodynamic analysis confirmed that the adsorption process using PDA-IL was exothermic and spontaneous, with a tendency towards increase in disorder. Furthermore, regeneration of the loaded PDA-IL adsorbent was successfully performed using methanol as an eluting agent. PDA-IL was used for 4 consecutive adsorption-desorption cycles without any deterioration in its morphology or performance. A selectivity test was also conducted to investigate the adsorption behavior of PDA-IL in dye mixtures. Lastly, a thorough understanding of the possible adsorption mechanism was proposed based on FTIR and electron-energy loss (EELS) mapping results. The high adsorption capacities obtained can be attributed to the pi–pi stacking, electrostatic interactions and the unique structural properties of both PDA and the custom-synthesized IL. Hence, it can be concluded from the results of this study that PDA-IL adsorbents could be of great potential for wastewater applications as a novel, efficient and sustainable adsorbent for removal of anionic pollutants.