Preparation of stimuli-responsive hydrogel vitrimers as recyclable dye adsorbent for water purification

The concept of water purification by recyclable vitrimers through topology control has been introduced new class of polymeric materials called covalent adaptable networks (CAN), with great interest of production. Meantime, for applying the feature of stimuli-responsivity and recyclability, we synthesized poly(glycidyl methacrylate-comethacrylic acid) copolymers (P(GMA-co-MAA)) with different molar ratios of GMA to MAA. Synthesis of copolymers and their compositions were confirmed by proton nuclear magnetic resonance 1HNMR analysis. Vitrimer hydrogels were prepared by using citric acid as crosslinker through solution casting method. Successful formation of vitrimer hydrogels was confirmed by Fourier transform infrared (FT-IR) analysis. Physical and thermal characteristics of vitrimers were investigated by differential scanning calorimetry (DSC), thermal gravimetric (TGA) analysis, and field emission scanning electron microscopy (FE-SEM). The swelling ratio behaviors of hydrogels was investigated, and the results showed that V(GMA5-co-MAA95) had maximum swelling ratio. Finally, methylene blue (MB) adsorption and desorption behavior of vitrimers and their recycled samples was investigated. The results showed that samples with the highest molar ratio of MAA had the most methylene blue adsorption in all pH values (2, 4, 7.4, and 10). The as-prepared and recycled V(GMA-co-MAA) vitrimers exhibited maximum adsorption efficiency (99.9%) at pH= 10 and initial MB concentration of 5 mg/L. Also, minimum adsorption efficiency was dedicated to pH= 2 and initial MB concentration of 2 mg/L. For both conditions adsorbent dosage and aqueous solution volume was 0.7 g and 100 mL, respectively. Also, the kinetics of adsorption was investigated by pseudo-first-order, pseudo-second-order, and intraparticle diffusion models and kinetic isotherm was evaluated by Langmuir and Freundlich models. Results showed that unlike intraparticle diffusion, pseudo-second-order and Langmuir models correlated experimental data better than on other models, suggesting these two models as the depiction for our adsorption process. Synopsis: Prepared dye adsorbents are based on recyclable vitrimers which can be applied in water purification without extra plastic pollution.