The contribution of soil extract composition and cyclic moisture dynamics to the physicochemical aging of superabsorbent polyacrylic acid and polyacrylamide hydrogels

Abstract Polyacrylic acid (PAA) and polyacrylamide (PAM), two synthetic superabsorbent polymers (SAPs) commonly used in agriculture, can form three‑dimensional hydrogels that enhance soil water retention and soil structural stability. Yet, their potential aging and transformation under natural drying–rewetting dynamics and in contact with soil solutes remains unclear.In this study, we examined the effect of soil extracts from sand, loam, and clay soil in both a 72 h free swelling experiment (FSE) and in an incubation experiment (IE), where PAA and PAM hydrogels underwent ten successive drying-rewetting cycles. Samples were taken after cycles 0, 3, 5, and 10 and investigated for their swelling index (SI), water entrapment ( 1 H proton nuclear magnetic resonance relaxometry), structural stability (rheometry), morphology (scanning electron microscopy), and surface chemistry (fourier transform infrared spectroscopy).In the FSE, PAA swelling in all soil extracts reduced SI, shortened T₂ relaxation, and increased mechanical rigidity, whereas PAM properties remained stable except when swollen in sand extract. During the IE, PAA exhibited progressive hydrogel network densification, further SI loss, T 2WL shortening, band intensity shifts, and microstructural compaction, whereas PAM remained largely inert. Multivariate analysis confirmed that polymer identity and its interaction with the conducted drying-rewetting cycles and soil extract composition drove SAP aging. Overall, drying–rewetting cycles seem to induce irreversible chemo-structural alterations in anionic PAA, thereby diminishing its rehydration potential and triggering the potential formation of persistent, solid-like residues, whereas neutral PAM seems more resilient under dynamic environmental conditions.