Experimental study on the catalytic action of heavy metals in the sulphation process and formation of black crusts by accelerated ageing tests

Research conducted on the degradation of cultural and architectural heritage caused by the deposition of air pollutants indicates that the deterioration of carbonate materials is due primarily to the interaction of the substrate with sulphur dioxide (SO2) and particulate matter (PM) deriving from the combustion of fossil fuels. The main chemical degradation process is the sulphation of the substrate, which consists in the initial conversion of SO2 into sulphuric acid (H2SO4) and the subsequent reaction of this product with calcium carbonate (CaCO3). This leads to the formation of black crusts composed mainly of gypsum (CaSO4⋅2 H2O), inside which PM is embedded. These carbonaceous particles also contain heavy metals that can act as catalysts in both stages of the sulphation process, favouring the degradation of the substrate. However, the degree to which each specific metal is able to affect sulphation is still unclear. This research aims to evaluate which heavy metals activate the sulphation process by carrying out targeted exposure mock-up tests in special climatic chambers. The selected cations were the following: Fe3+, Cu2+, Mn2+, Pb2+, Cr3+, V5+ (deposited concentrations were calculated based on data from urban PM). In addition, mixtures involving three or more metal cations were also used to evaluate possible synergistic effects. Finally, PM2.5 extracted from quartz-fibre filters sampled in the city of Milan was also included in the experimentation. The physicochemical characterization of the different mock-up samples was performed both in the pre-exposure and post-exposure phases using different analytical techniques such as: colorimetric analysis, stereomicroscopic observations, SEM-EDX (Scanning Electron Microscopy coupled to Energy Dispersive X-Ray Spectroscopy), IC (Ion Chromatography), and XRPD (X-ray powder diffraction). Results show that some metal cations (Pb, Cu, Cr) are able to activate the catalytic process faster than others (Fe, Mn). Also, samples treated with metal mixtures and PM2.5 exhibited the greatest catalytic action, highlighting a synergistic effect of more heavy metals acting together.