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Corrosive effects from the deposition of gaseous pollutants on surfaces of cultural and artistic value

Laboratory and field experiments have been carried out on deposition rates of acidic and other pollutants on calcium carbonate surfaces of various types.

Kinetic laboratory experiments, at various temperatures, using reversed flow gas chromatography (RF-GC), permitted the simultaneous determination of physico chemical parameters pertaining to pollutant deposition velocities and reaction probabilities on solids under nonsteady state conditions (eg propane and hydrogen sulphide on silica, aluminium, marble or silver foil).
A flat diffusion stripper has been built for studying the reactivity of sulphur dioxide and of mixtures of various gases. The heterogeneous oxidation of sulphur dioxide in the presence of nitrogen dioxide was found to depend on the nature of the sample chosen and on relative humidity and thus it was impossible to formulate a general statement regarding the behaviour of carbonaceous materials of different origin (eg charcoal, soot from propane, acetylene flames).
Acid fog is contributing together to other pollutants to destroy the historic buildings in Venice. The concentrations of anions present in fog was found to be higher than in rain. The high sulphate concentration found in fog water confirmed that urban fog conditions enhance the rate of sulphur dioxide oxidation and consequently the damage on stone monuments.
The deposition of sulphur dioxide on marble as a function of time was studied. The sulphur dioxide uptake on clean marble has a strong time dependence. However, towards the end of a 20 hour experiment the samples reach a steady state with respect to sulphur dioxide deposition. The acceleration of sulphur dioxide deposition by humidity was also obvious. The presence of sodium chloride on the stone surface increased the deposition of sulphur dioxide.
Sulphur dioxide was found to be rapidly adsorbed on the surface of calcite forming at least 2 different species, one being present in both humid and dry conditions, while the other is only detect ed in the presence of water vapour. The study also showed that water vapour is necessary for the formation of surface sulphite. The surface sulphite on calcite is rather stable, no evidence for sulphate formation being visible in the infrared (IR) spectra after 20 hours exposure to humid, sodium oxide containing air.

Additional experiments, using other gases as aliphatic and aromatic hydrocarbons, dimethyl sulphide, etc, have been performed with the RF-GC method. The theoretical model was improved to include interactions between two gaseous pollutants and the synergistic effect of these interactions on the deposition and corrosive parametewrs on cultural surfaces.

Reported by

University of Patras
Physical Chemistry Laboratory
26500 Patras
Greece
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