Biodegradable Kraft Lignin-Based Agro Hydrogels for Soil Hydration and Slow Release of Urea
Nataliia Fihurka, Tetyana M. Budnyak
Abstract
High Resolution Image Download MS PowerPoint Slide Natural polymer-based hydrogels hold significant potential for sustainable agriculture by enhancing soil hydration and enabling the controlled release of nutrients. This study shows an easy synthetic approach to substitute up to 75% of commonly used polyacrylamide (PAAm) for commercial agro hydrogels by kraft lignin, keeping swelling performance on the same level. A new semisynthetic hydrogel has been synthesized via cross-linking of kraft lignin produced via the LignoBoost process (LB) with PAAm. The ratio of LB and PAAm in the hydrogel composition and cross-linking density were optimized to obtain hydrogels with suitable performance properties. Hydrogel samples were characterized by thermogravimetric analysis, differential scanning calorimetry, and Fourier transform infrared (FTIR) spectroscopy to evaluate the main characteristics and suggest possible reaction mechanisms and by scanning electron microscopy to study features of the hydrogel network. The investigation of functional properties demonstrated that optimized hydrogel samples with 60% lignin in the composition possess water swelling capacity up to 235 g g –1 and can significantly increase the water retention of the soil when 2.5% hydrogel is added to the substrate. Moreover, obtained lignin-derived hydrogels have the ability to retain and slow-release urea fertilizer. Thus, lignin-derived urea formulations ensure the sustained cumulative release of urea within 8 days under laboratory conditions, while unencapsulated urea possesses a cumulative release of 97% within 1 h. Investigation of biodegradation using a soil burial test showed that weight loss ranges from 9.5% to 14.5% after 40 days for different hydrogel samples. The proposed method of synthesis of lignin-based hydrogels opens new frontiers in advancing sustainable agricultural technologies.