Lignin from mustard stalks as a reinforcing scaffold for multifunctional bio-hydrogels with enhanced water retention...

Biopolymers are increasingly investigated for hydrogel synthesis owing to their renewability and environmental compatibility. Carboxymethyl tamarind kernel gum (CMTKG)-based hydrogels exhibit exceptionally high swelling ratios, reaching up to 12,291%, making them highly effective for water retention. However, their application in soil is limited by rapid degradation and poor mechanical integrity under field conditions. To address this limitation, a composite hydrogel was prepared by incorporating lignin into CMTKG. This reinforcement markedly improved stability in soil while maintaining a comparable swelling capacity. After 30 days of soil burial, pure CMTKG hydrogels lost ~98% of their original weight, whereas CMTKG-lignin hydrogels retained ~50%. Thermal analysis revealed an increase in the onset degradation temperature from 200 °C to 250 °C upon lignin incorporation. Rheological analysis confirmed good compatibility between lignin and CMTKG, demonstrating elastic-dominant, solid-like behavior with enhanced network rigidity and an extended linear viscoelastic region. The composite hydrogel exhibited a swelling capacity of 7885% and a gel fraction of 54%, and it significantly prolonged soil water retention by ~40 days compared to untreated soil. Furthermore, as a proof of concept, phosphorus was successfully loaded into the hydrogel via an in-situ method, and its release in both water and soil followed Peppas-Sahlin kinetics, predominantly governed by diffusion.