Chelator-free nanocellulose hydrogels as a biobased alternative for patina removal in bronze heritage objects

The removal of active corrosion layers from bronze heritage objects is a critical conservation challenge, requiring methods that balance cleaning efficiency with surface preservation and environmental sustainability. Traditional chemical treatments often rely on chelating agents loaded into hydrogels, which can pose risks of uncontrolled diffusion and residue retention. This study presents a biobased, chelator-free alternative using cellulose nanofibril (CNF) hydrogels cross-linked with glycine whose use in conservation of cultural heritage is new. Unlike conventional systems, these physically cross-linked supramolecular hydrogels utilize the amphoteric properties of natural amino acids to sequester copper corrosion products via a coordination-driven mechanism, eliminating the need for adding complexing agents. We synthesized and characterized a series of hydrogel formulations. Mechanical compressive testing and water retention allowed to identify the right formulation offering a high-water content (96.3%), superior elastic recovery, and excellent conformability to complex 3D surfaces compared to traditional agar-EDTA hydrogels. The cleaning performance was evaluated against the traditional agar-EDTA alternative on artificially patinated bronze coupons, as well as a complex 3D model sculpture. A non-invasive multimodal imaging approach combining Raman spectroscopy, Hyperspectral Imaging in the SWIR, and quantitative micro Energy Dispersive X-Ray Fluorescence spectrometry revealed that the chelator-free nanocellulose hydrogels achieved a more uniform and effective removal of sulfated and chlorinated corrosion products than the agar-EDTA hydrogel. Infrared spectroscopy further confirmed the coordination of extracted Cu (II) ions within the porous nanostructure of the new hydrogel. Finally, a life cycle assessment (LCA) was conducted to quantify the environmental impacts, identifying production hotspots, with sensitivity analyses pointing toward strategies for future improvement.