Developing a circular process for the removal of direct and reactive dyes from wastewater using three-dimensional electrochemical oxidation: are poly(ionic liquid)s the answer?

Water pollution caused by dyes from the textile industry poses a serious environmental challenge, driving the development of efficient adsorbents and strategies for their reuse in water purification processes. In this work, a polar hydrophobic poly(ionic liquid), poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (Poly(DADMA)NTf₂), was synthesized, characterized, and used as adsorbent for the removal of two anionic textile dyes, Direct Red 80 (DR80) and Reactive Blue 5 (RB5), from water. Characterization results confirmed the successful synthesis and revealed high thermal stability (Tonset ≈ 421.6 °C) and a semi-crystalline nature. The aim is to develop a polar adsorbent, capable of exhibiting high extraction efficiencies in the removal of charged dyes from the textile industry, and electrochemically stable so that it can be recycled through electrochemical processes, which degrade the dyes with minimal adsorbent loss. Two different particle sizes, less than 0.071 mm and between 0.45 and 1.00 mm, were studied. The kinetic studies indicate that the adsorption equilibrium was achieved within 20 min for direct red 80 and 5 min for reactive blue 5, with maximum adsorption efficiency of 99.2% and 99.3%, respectively, using the smaller particles size. Generally, the adsorption mechanisms followed the Langmuir isotherm model, except for RB5 with larger Poly(DADMA)NTf₂ particles, where the Freundlich model provided a better fit. The poly(DADMA)NTf₂ was reused in eight successive adsorption cycles without intermediate regeneration. Regeneration was performed using three-dimensional (3D) electrochemical oxidation with a mixed metal oxide anode and a stainless-steel cathode. Under optimal conditions (35 mL solution, 1 mg/mL polymer concentration, 5 V potential, 0.04 M NaCl as the electrolyte), over 85.0% polymer adsorbent recovery was achieved after two regeneration cycles. However, the polymer's adsorption capacity slightly declined with each cycle, likely due to degradation during electrochemical oxidation. Although no clear trend was observed regarding dye order during reuse, Reactive Blue 5 consistently showed higher adsorption than Direct Red 80, across all cycles.