Feedstock-Dependent Water Sorption and Retention of Citric-Acid-Modified Lignocellulosic Biogels Under Simulated Climatic Conditions

Increasing drought frequency and irregular precipitation patterns highlight the need for sustainable soil water management. In this study, lignocellulosic biogels were prepared from Norway spruce and silver birch sawdust and from rapeseed and poppy straw using a 10 wt.% citric acid solution, with a fixed addition of 5 mL of 80% lactic acid as a processing modifier. Two formulations differing in the volume of citric acid solution (180 or 250 mL per 12 g of dry feedstock), and consequently in both the liquid-to-solid ratio and absolute citric acid dose, were evaluated. Biomass chemical composition and density were determined, the amount of water absorbed by soil–biogel mixtures containing approximately 3.1% dry biogel relative to dry soil mass was measured gravimetrically after saturation and free drainage and water-loss kinetics of the separately saturated biogels were monitored under cyclic climatic conditions ranging from 12 to 35 °C and 25 to 75% relative humidity. FTIR spectroscopy was used to compare cellulose isolated from each feedstock with the corresponding biogels prepared using both the 180 and 250 mL formulations. Biogel performance depended on feedstock composition and physical characteristics. Among the tested formulations, the poppy-derived 180 mL biogel produced the highest water absorption in the soil–biogel system, whereas spruce-derived biogels exhibited more stable residual water content during prolonged desorption. The poppy-derived 180 mL formulation increased water absorption in the soil–biogel system from 88.46 g in untreated soil to 122.86 g, corresponding to an increase of 38.9%. FTIR analysis indicated an increased contribution of carbonyl-containing structures consistent with incorporation of the acid modifiers and possible ester formation; however, the spectra did not provide direct evidence of crosslinking density. Increasing the citric acid solution volume did not consistently improve water absorption by the soil–biogel system or water-loss behavior of the isolated biogels. These laboratory results demonstrate a feedstock-dependent interaction with formulation and provide a basis for further evaluation of lignocellulosic residues as renewable water-retaining soil amendments under soil–plant and field conditions.