The impact of cyclic freezing-thawing on the physicochemical properties of superabsorbent polyacrylic acid (PAA) hydrogel used as a soil conditioner

Superabsorbent polymers (SAPs) such as polyacrylic acid (PAA) are increasingly used as soil conditioners to enhance water retention under drought, yet their long-term stability and transformation under environmental stress remain poorly understood. In particular, the effects of cyclic freezing-thawing on their physicochemical properties and behavior in soils have not been systematically assessed. Here, we examined the structural and physicochemical evolution of PAA hydrogel, both freely incubated and embedded in contrasting soil matrices (sand, loam), under static and cyclic freezing-thawing conditions. A multi-method approach combining swelling index (SI), ¹ H-NMR relaxometry, rheometry, FTIR spectroscopy, and ESEM imaging was applied to assess hydration dynamics, network rigidity, and polymer-mineral interactions. Freely incubated PAA remained highly hydrated, flexible, and chemically stable, confirming that freezing-thawing alone does not alter its physicochemical structure. In sand, moderate confinement and cation-mediated crosslinking produced a thin, reversible organomineral shell that increased stiffness while preserving an elastic core. In loam, finer texture, higher cation exchange capacity, and organic ligands promoted deeper polymer-mineral integration and compaction, yielding dense, solid-like composites. Across all treatments, soil matrix effects clearly dominated over freezing-thawing. Overall, PAA followed a mechanistic transformation pattern from a hydrated flexible hydrogel to a confined organomineral composite, overall driven by soil-induced mechanical and ionic interactions. These findings highlight how SAPs may gradually persist as plastic-like residues in soil, emphasizing the need to evaluate soil-specific polymer-mineral interactions in future SAP applications.