Reusable dialdehyde nanocellulose cryogels for desalination of multi-layered porous structures

Designing bio-based porous materials that combine high water uptake, mechanical resilience, and reusability without requiring additional components or crosslinkers remains a central challenge in sustainable materials science. Cellulose nanocrystal (CNC)-based porous materials offer low density, high surface area, hydrophilicity, and tunable surface chemistry. However, combining high water uptake with structural stability during repeated hydration-dehydration cycles remains challenging in additive-free CNC structures, as their stability when wet typically requires crosslinking, surface modification, or secondary components. Periodate oxidation of CNCs to dialdehyde nanocellulose (DAC) introduces reactive aldehyde groups that are proposed to facilitate network stabilization through hemiacetal-mediated interactions. Here, we assess their ability to retain porous architecture and water uptake over repeated hydration-dehydration cycles for reusable desalination of multi-layered cultural heritage porous substrates. Compared with pristine CNC cryogels, periodate oxidation resulted in reduced crystalline order, increased pore network accessibility to water, while providing structural stabilization of the resulting cryogels. Multiscale physicochemical characterization revealed an interconnected porous architecture with preferential pore orientation arising from freeze-templating. The hydrated DAC cryogels contain 95.8 wt% free water and reach ~2780% swelling. They retain an elastic gel-like response and full recovery under cyclic compression, unlike reference foams. When applied to chloride-impregnated fresco painting mock-ups, the cryogels achieved 98.5% chloride removal over three 15-minute application cycles, exceeding the performance of reference foams despite a higher initial salt load. These results demonstrate that chemical modification of nanocellulose provides a bio-based, high-capacity, and short-term reusable desalination platform for complex, multi-layered porous substrates relevant to cultural heritage conservation.