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 sub-stances (sEPS) extracted from aerobic granular sludge (AGS) were studied in comparison to well-known bio-polymers (i.e., alginate and kappa/iota-carrageenan) taking advantage of material-saving, reproducible and robust experimental protocols. With respect to alginate and kappa-carrageenan, sEPS and iota-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 ex-periments 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 prop-erties 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.