Preliminary investigation of a lignin-based hydrogel for the removal of metal corrosion products from stones

Lignin is a major constituent of the plant biomass, on top of agricultural waste, and a by-product of the paper industry. At present, most waste lignin is burned for energy recovery, whereas only a limited fraction is exploited in industrial recycling processes. In the context of circular economy strategies and sustainable waste valorisation, lignin can be converted into advanced materials, such as hydrogels. Lignin-based hydrogels can serve as absorbents for heavy metal ions due to the negatively charged oxygens of lignin functional groups. These materials are currently employed as drug delivery systems, in water remediation, and as sensors. In this study, lignin is valorised through its conversion into nanoparticles embedded in a PVA–borax matrix and proposed for an innovative application: the cleaning of metal-contaminated stone artworks. Three lithotypes (Candoglia marble, Carrara marble, and travertine) were artificially contaminated with metal corrosion products and subsequently cleaned using the lignin-based hydrogel. The cleaning performance was evaluated directly on the stone surface by using a non-invasive approach based on portable Fourier Transform Infrared (FTIR) spectroscopy, single-sided Nuclear Magnetic Resonance (NMR) relaxometry and colorimetric analyses. In addition, Attenuated Total Reflectance FTIR (ATR-FTIR) spectroscopy was employed to characterize the corrosion products and the lignin-based hydrogel and elucidate lignin–metal interactions. Preliminary results suggest that the cleaning performance depends on the petrophysical properties of the stone substrate and on the nature of the contaminants. A synergistic contribution of the hydrogel components to the cleaning process was also observed.