Use of climatic chambers to analyse the catalytic action of heavy metals in sulphation process

Research conducted on the degradation of cultural and architectural heritage caused by the deposition of air pollutants indicates that both sulphur dioxide and particulate matter produced by the combustion of fossil fuels are the main agents responsible for the deterioration of carbonate materials. The main chemical degradation process is the sulphation of the substrate and consists in the initial conversion of sulphur dioxide (SO2) into sulphuric acid (H2SO4) and the subsequent reaction of sulphuric acid with calcium carbonate which forms the black crusts. The sulphation process takes place only thanks to the presence of heavy metals which catalyze the reaction. Despite the presence of numerous studies related to black crusts formation [1,2], a specific role of the single metals is still unclear. Aiming to unveil the catalytic action of different metals, a series of experimental tests was carried out in specific climatic chambers for accelerated aging. This research work concerns with a second exposure test in accelerated aging chambers following the first experimental phase [3] which did not allow to discriminate the catalytic action of the metals. More specifically, two climatic chambers were employed: a “corrosion chamber” providing a source of SO2 and humidity at determined concentration and temperature, and a “light-irradiation chamber” to simulate the sunlight irradiation naturally falling on monumental stone during the day. This second monitoring campaign was carried out at different experimental conditions inside the “corrosion chamber” with respect to the first campaign. Accordingly, upon lowering the concentration of SO2 by 20 times, the reactions took place more slowly thus decelerating the phenomenon and, hence, better highlighting the catalytic role of individual metals. After the preparation and exposure of the marble samples inside the climatic chambers (carried out as in the first campaign), the physico-chemical characterization was carried out by means of a multi-analytical approach involving different techniques. Colorimetric analysis was used to define the variation of the chromatic coordinate L (brightness), which is related to the formation of gypsum on the surface. SEM-EDX was employed to study the elemental composition and morphology of the surface and degradation products. Moreover, XRPD was used to identify the mineralogical composition of the degradation layer and IC was performed to identify and quantify soluble salts (in particular sulphate ions). All the experimental data collected will be processed in the next step of the project to create a predictive mathematical model. This will help predict the formation of black crusts on carbonate surfaces based on the outdoor pollution present in a given site. In fact, this research work is part of the interdepartmental SEED 2019 project of the University of Milan entitled SciCult (mathematical modelling and Scientific analysis for Cultural heritage: prediction and prevention of chemical and mechanical degradation of monumental stones in outdoor environments).