Non-invasive combined FT-IR and NMR protocol to assess the cleaning action of a lignin-based hydrogel on stones

Lignocellulosic biomass is found on top of agricultural waste and includes cellulose, lignin, and hemicellulose. Of the three polymers, only lignin is considered inexpensive and 98% is burned to generate energy (Vásquez-Garay et al., 2021). Lignin is also an important by-product of paper industry. Due to the growing interest in converting waste into high-value-added products and thanks to the key properties of lignin, such as biodegradability, biocompatibility, thermal stability, antioxidant and antimicrobial properties, several procedures have been developed to regenerate lignin into advanced materials, including hydrogels, nanotubes, films, nanofibers, and nanoparticles (Akhtar et al., 2016). These are suitable for a variety of applications, such as drug delivery systems for agriculture or medicine, in water remediation applications, and in sensors since they can serve as absorbents for heavy metal ions, controlled release agent for controlled delivery and water retention, smart materials for stimuli response, and biosensors and electrodes (Rico-Garcìa et al., 2020). In this scenario, a novel emerging application of lignin-based hydrogels could be in the cleaning practice of cultural heritage stones. This work aimed to evaluate the cleaning potential of a PVA-lignin hydrogel to remove metal ions from stone surfaces using a non-invasive protocol based on the combination of single-sided NMR and portable FT-IR (Stagno et al., 2021). Different stones were artificially stained with copper corrosion products and then cleaned using the lignin-based hydrogel. The spin-spin relaxation time (T2) and the IR spectra were collected on the stone surface before and after the artificial staining, as well as after the application of the lignin-based gel. Moreover, FT-IR spectroscopy was also used to study the changes in bonds within the PVAlignin system before and after being used to clean the stone.