Enhanced Adsorption of Methylene Blue Triggered by the Phase Transition of Thermoresponsive Polymers in Hybrid Interpenetrating Polymer Network Hydrogels

The enhanced adsorption of methylene blue in aqueous solutions is triggered by the phase transition of the thermoresponsive copolymer poly(di(ethylene glycol) methyl ether methacrylate-co-poly(ethylene glycol) methyl ether methacrylate) (P(MEO2MA-co-OEGMA300)) in alginate-Ca2+ hydrogels containing the photocatalyst graphitic carbon nitride (g-C3N4). The obtained hybrid alginate-Ca2+/P(MEO2MA-co-OEGMA300)/g-C3N4 hydrogels possess an interpenetrating polymer network (IPN) structure. The embedded g-C3N4 induces a porous structure, which not only significantly enhances the hydration ability but also improves the adsorption capability of methylene blue. The collapse of P(MEO2MA-co-OEGMA300) causes the hybrid IPN hydrogels to become more porous when the temperature is increased to its transition temperature (TT, 45 °C). Thereby, the adsorption capability is further enhanced. Absorption of 100% methylene blue in the aqueous solution can be achieved in 2 h, which is three times faster than that at 40 °C (below TT). As the TT of acrylate-based thermoresponsive polymers is correlated to the number of ethoxy groups in the side chains, the collapse temperature of thermoresponsive hydrogels can be easily adjusted to the desired temperature by simply changing the types and molar ratios of the monomers. Due to the embedded g-C3N4, all adsorbed methylene blue can be degraded after placing in ambient conditions and exposure to visible light for 12 h. Thus, the hybrid IPN hydrogels are well recyclable. Based on the capability of enhanced adsorption and reutilization, the hybrid alginate-Ca2+/P(MEO2MA-co-OEGMA300)/g-C3N4 IPN hydrogels can be used for the removal of dye pollutants from wastewater with high efficiency and good recyclability.