Chitin hybrid hydrogels for salt efflorescence removal from historic brick masonry

Soluble salt crystallization seriously challenges the conservation of brickwork in historic buildings. Salt-induced stone weathering affects the structural and aesthetic features of buildings, requiring appropriate materials for its treatment. Polymers from renewable carbon feedstock, with their hydrophilic character, large absorption, salt dissociation ability, and ease of processing, are perfect candidates. Following the 'safe and sustainable by design' (SSbD) framework we demonstrate the potential of microstructured gels based on chitin with deep eutectic solvents and mesoporous carbon for salt efflorescence removal in historic brick masonry buildings. The porous structures enable large uptake capacities without leakage, avoiding salt accumulation problems common in cellulosic poultices used in the heritage conservation. The removal capacity for aqueous solutions of eight salts relevant in built heritage is tested (NaCl, Na₂SO₄, Na₂CO₃, NaNO₃, MgCl₂, K₂SO₄, K₂CO₃, MgSO₄), achieving a maximum removal capacity of 13.5 ± 0.2 g·g^-1 in 24 h. The suitability of the gels for salt removal in actual bricks, both superficially and throughout the brick, is demonstrated using ancient mudbricks containing a wide variety of salts. The materials can be reused several times, providing conservators with a potentially economical formulation for restoration. This work opens up new avenues for sustainable practices in the conservation and restoration of historic buildings and other cultural heritage artefacts.