Preparation and application of microwave radiation-modified lignin-grafted polyaspartic acid hydrogels

Microwave-assisted HBr modification produced porous lignin with more phenolic hydroxyl active sites. • Mannich reaction covalently grafted modified lignin onto polyaspartic acid to form a stable network. • The optimal hydrogel achieved 93.29 g/g water absorption, 38% higher than pure PASP hydrogel. • It showed 50 h water retention and maintained 70% capacity after five repeated use cycles. • Lignin grafting lowers the cost and boosts properties of PASP hydrogel, serving as an excellent eco-friendly water retainer for green agriculture. Lignin-based hydrogels have attracted considerable attention due to their low cost and renewability; however, their limited water absorption capacity restricts practical applications. In the present work, an environmentally friendly composite hydrogel was developed using renewable alkaline lignin (AL) and polyaspartic acid (PASP) as raw materials. The preparation involved three steps: first, modified alkaline lignin (HAL) was obtained via microwave irradiation; second, it was grafted onto PASP through the Mannich reaction; finally, a crosslinking agent was introduced to form the HAL-PASP composite hydrogel. Fourier transform infrared (FT-IR), Proton Nuclear Magnetic Resonance (¹HNMR), and EDS analyses confirmed the successful grafting of modified lignin onto the PASP backbone. Scanning electron microscopy (SEM) observations indicated that HAL was transformed from a spherical structure into a porous, irregular sheet-like form. Performance tests showed that the water absorption and retention properties of the hydrogel were significantly affected by the content of HAL, the optimal water absorption performance was achieved at a HAL content of 4.17 wt%, yielding a maximum water absorption of 93.29 g/g and a retention duration of 50 h. Compared to conventional PASP hydrogels, the composite showed 38% higher water absorption and 50% longer water retention, retained 70% of its initial retention after five cycles of reuse, and lost water absorption only after seven cycles. Furthermore, the incorporation of PASP increased the active sites of the lignin matrix, endowing it with superior hydrophilicity and stability. This work not only opens up new avenues for the high-value utilization of lignin but also provides innovative insights into the development of high-performance hydrogels. Download: Download high-res image (180KB) Download: Download full-size image