Development and characterization of a carboxymethyl cellulose-alginate hybrid superabsorbent hydrogel designed for water management in agriculture

Water scarcity and inefficient irrigation methods continue to be major issues in agriculture, which consumes 70 % of freshwater. In this context, creating effective water management strategies is crucial, and superabsorbent polymers (SAPs) offer a promising new solution. However, traditional synthetic SAPs and hybrid SAPs with a lot of synthetic material contribute to microplastic pollution, making it urgent to find eco-friendly alternatives that deliver good performance while protecting the environment and supporting sustainability. To address this, a novel carboxymethyl cellulose (CMC)/sodium alginate (Na-Alg) hybrid hydrogel was prepared via free-radical graft copolymerization of acrylic acid (AA) and acrylamide (AM) onto a polysaccharide backbone in aqueous solution, designed for agricultural water management. Design-Expert software was employed to furnish 11 hydrogel formulations to minimize acrylic monomer content while maximizing swelling capacity. The optimal formulation CMC-6 (AA/CMC = 3.25, Na-Alg/CMC = 0.6) revealed a maximum water absorption capacity (1636.69 g/g). FTIR and XRD analysis confirmed successful grafting and crosslinking reactions, while SEM analysis revealed a highly porous morphology, confirming the high swelling capacities reached. TGA and rheological analysis demonstrated that CMC-6 exhibited good thermal and mechanical stability. The swelling behavior was evaluated under varying pH, salinity, and temperature conditions. The SAP maintained a good swelling capacity across a wide pH range, reaching the maximum at pH 8. Due to lower osmotic pressure, the swelling capacity decreased as the concentration of NaCl solution increased. It ranged from 307 g/g to 164.75 g/g as the concentration went from 0.3 wt% to 1.2 wt%. The effect of cation charge followed the order Na > Ca > Al, demonstrating that multivalent cations reduce swelling through additional cross-linking. Smaller cations (Na and Mg) facilitated water penetration compared to large ones. CMC-6 showed a thermoresponsive behavior, and the water retention test highlighted its ability to retain water for five days. Additionally, it demonstrated good reusability through reswelling and water re-retention over multiple cycles. These responsive and durable properties make it a promising candidate for sustainable agricultural practices.