Lignin-based hydrogels have recently garnered attention for use in a variety of aqueous separations as lignin is a sustainable, naturally abundant biopolymer with a high concentration of hydroxyl groups, which are potential sites for chemical functionalization. However, to date, the use of these materials in separations technologies has been hindered by our limited understanding of how the addition of lignin, both as a crosslinker and passive filler, affects the network (pore) structure of the crosslinked composite hydrogel. In this study, lignin-poly(vinyl alcohol) (PVA) composites were synthesized using lignin of prescribed molecular weights (MWs) with low dispersity, allowing for the fabrication of membranes with, in theory, more homogeneous network structures. The permeability of different pollutants (e.g., methylene blue (MB)) through the hydrated composites was measured via ultraviolet-visible spectroscopy, where the penetrant permeability was found to depend on both the MW of the lignin, as well as the initial functionality of the starting lignin (e.g., a portion of the hydroxyl (OH) groups are replaced with vinyl groups prior to hydrogel formation). Nuclear magnetic resonance indicated a higher degree of functionalization of OH groups for low MW fractions of lignin (MW < 1300 Da).
In addition, poroelastic relaxation indentation was used to characterize both the mechanical (elastic modulus) and transport properties (diffusivity, effective pore size) of the composites. While increasing the lignin content created stiffer membranes, the water diffusivities and pore sizes were constant across all hydrogels. In contrast, the introduction of lignin into the hydrogel, both unfunctionalized and functionalized, significantly reduced the permeation of MB through the composite hydrogels. Furthermore, an initial lag time was observed in all membranes containing lignin, whereby no MB could be detected as having been permeated across the membrane. The time-scale of this initial lag was seen to be a function of the lignin content of the membrane, indicating that the reduction in MB permeability was almost entirely governed by the interaction between the MB and the lignin.