Lignin-based Hydrogel for Water Retention in Silt Loam Soil

Superabsorbent polymers (hydrogels) have been proposed as soil amendments to increase plant available water in soil. Synthetic hydrogels have been widely investigated for use in agriculture. Due to increasing environmental concerns related to synthetic hydrogels, hydrogels from natural sources which are more degradable and biocompatible compared to synthetic hydrogels are being developed. Here, a lignin-based hydrogel was synthesized. The swelling properties of the hydrogel were determined in different aqueous solutions and in soil. Fourier Transform Infrared (FTIR) spectroscopy was used to characterize the hydrogel. Using the hanging water column and the pressure plate method, the soil water retention curve was measured from a soil water pressure head range of -3 cm to the permanent wilting point i.e - 15,000 cm for a silt loam soil. For this purpose, the soil was amended with the lignin-based hydrogel at rates of 0, 0.1 and 0.3% (w/w) concentration. Results of the swelling properties of the lignin-based hydrogel show a maximum swelling ratio of 2030% of the hydrogel's original mass in deionized water, 1092% in tap water, and 825% in the 0.9% NaCl solution. FTIR spectra of the hydrogel show the presence of O-H bonds which come from the lignin structure and render the hydrogel reactive to water molecules causing swelling as a result. Lignin hydrogel treatment significantly increased water retention near saturation compared to a control treatment of soil with no added lignin hydrogel.