Valorization of Biomass-Derived Lignin into Hydrogels for Soil Hydration and Slow-Release Fertilization
Tarun Kumar Gayen, Sudhir G Warkar, Mohammad Amdad Ali
Abstract
In this study, lignin was extracted and isolated from agricultural waste mustard stalks by using an alkaline method. Subsequently, the isolated lignin polymers were chemically crosslinked using epichlorohydrin to produce hydrogels. The viscosity-average molecular weight of the extracted lignin was determined to be 27,725 g/mol using the Mark–Houwink–Sakurada equation. The lignin hydrogel with 6 g of lignin and 1 mL of ECH exhibited a swelling index of 253% and a gel fraction of 57%. Rheological analysis also confirmed the formation of lignin hydrogel with a larger storage modulus than loss modulus. The Young modulus, tensile strength, and breakpoint were observed to be ∼1005 Pa, ∼141 Pa, and ∼10%, respectively. When incorporated into soil, this hydrogel extended water retention by up to 10 days compared to the control. As a proof of concept, potassium was loaded into the lignin hydrogel via an in situ method, and its release behavior was evaluated using various kinetic models. The Peppas–Sahlin model provided the best fit, with a correlation coefficient ( R 2 ) of 0.99, indicating that potassium release followed Fickian diffusion as the dominant mechanism. Biodegradation studies revealed a residual dry mass of 82.74% after 60 days of being buried in soil, confirming partial degradability. Furthermore, phytotoxicity tests demonstrated that the hydrogel was nontoxic to plants. Therefore, lignin-based hydrogels present a sustainable solution for agriculture, combining efficient water retention and controlled nutrient release to enhance crop growth, particularly in arid regions. This approach not only adds value to biomass but also contributes to circular bioeconomy efforts aimed at reducing the environmental impact and enhancing resource efficiency in modern agriculture.