Unveiling the pivotal role of pH in the development of a mechanically strong and adsorptive nanocomposite double-network hydrogel

Recently, due to outstanding mechanical performance, the double-network (DN) strategy has drawn considerable interest for the synthesis of hydrogels used in environmental engineering. Among these materials, polyacrylic acid (PAA)-based hydrogels have strong potential in environmental applications, especially for water pollutant removal. Given their pH sensitivity, the formation of a DN structure in PAA-based hybrid hydrogels is expected to depend on pH. However, its impact on mechanics and microstructure has not been fully clarified. In this study, we investigated the role of pH in the development of a DN structure within the κ-carrageenan/PAA/graphene oxide hybrid hydrogels. Hybrid hydrogels prepared at pH 3 exhibited higher strength, toughness, and flexibility (σ f = 0.370 vs. 0.075 MPa, ε f = 1361 vs. 602%, E = 0.069 vs. 0.015 MPa, and W = 2.58 vs. 0.21 MJ/m 3 ), as well as thermo-reversibility, than those prepared at pH 7. This improvement was attributed to strong synergistic effects between κ-carrageenan and PAA networks, along with the thermo-reversible κ-carrageenan network. At neutral pH, however, deprotonation of COOH groups and reduced potassium ion availability restricted the aggregation of κ-carrageenan double-helices, weakening the DN structure formation and drastically lowering mechanical performance. Moreover, results from adsorption isotherm models demonstrated that the acidic hydrogels efficiently removed both cationic and anionic dyes (maximum adsorption capacity: methylene blue = 135.87 mg/g, Congo red = 61.88 mg/g). We hope that this knowledge can offer guidance for designing durable PAA-based adsorbent hydrogels for wastewater purification, by highlighting pH as a key factor controlling structure and function. • pH levels regulate the formation of the double-network structure in polyacrylic acid-based hybrid hydrogels. • A κ-Car/PAA/GO adsorbent hydrogel with a double-network structure was synthesized under acidic conditions for water treatment. • The adsorbent hydrogel demonstrated enhanced mechanical properties and synergistic effects between κ-carrageenan and polyacrylic acid. • The adsorbent hydrogel exhibited thermo-reversible behavior and a distinctive porous microstructure. • The adsorbent hydrogel also showed good adsorption efficiency, particularly for the cationic dye methylene blue.