Insights on the hydrogel-forming ability and post-gelling mechanical properties of structural extracellular polymeric substances (sEPS) from aerobic granular sludge (AGS): A comparison with model biopolymers

The hydrogel-forming ability and post-gelling mechanical properties of structural extracellular polymeric substances (sEPS) extracted from aerobic granular sludge (AGS) were studied in comparison to well-known biopolymers (i.e., alginate and κ-/ι-carrageenan) taking advantage of material-saving, reproducible and robust experimental protocols. With respect to alginate and κ-carrageenan, sEPS and ι-carrageenan hydrogels formed in presence of divalent metal ions M2+ behaved similarly once subjected to consecutive compression-decompression cycles, deforming elastically in all the applied range of deformations. While the overall mechanical response remained almost unchanged varying polymer concentration and ionic cross-linker concentration and nature, the Young's modulus E appeared significantly affected by the applied gelling conditions (E ≃ 4–20 kPa). As a result of the higher complexity of the extracellular biopolymeric matrix, higher driving forces (sEPS and M2+ concentrations) were needed to form stable and stiff hydrogels with respect to the studied model biopolymers: the establishment of an extended 3D network started for sEPS concentrations around 2.5 wt% (Ca2+ ≥ 0.1 M). Oscillatory shear experiments confirmed that sEPS were able to form hydrogels with solid-like mechanical properties at 1–10 wt% sEPS concentrations. Overall, the optimization of the gelling methods performed might help to overcome many bottlenecks characterizing this research area. The feasibility of forming sEPS hydrogels with mechanical properties comparable to other biopolymer-based systems currently applied for commercial purposes led to an awareness of the potential application and might open new valorisation scenarios able to contribute to a more bio-based and circular economy