Source identification of sulphate forming salts on

Authors: Nina Schleicher Clemente Recio Hernández

Publish Date: 2009/06/07

Volume: 17, Issue: 3, Pages: 770-778

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Abstract

Salt efflorescences markedly contribute to the alteration and deterioration of building material in this case the Villamayor Sandstone of the facades in the Old Town of Salamanca Spain United Nations Educational Scientific and Cultural Organization world cultural heritage site A better understanding of the mechanisms of salt formation and the involved elements would allow more precise measures in monument conservation The magnesium which is required for the salt precipitation originates from selective processes of hydrolysis The source of sulphate however is presently not as clear Identifying the source of the sulphur was the main goal of this research Isotope ratio measurement of δ34S and δ18O was used to clarify the origins of Mg sulphate saltsA total of 56 Mg sulphate samples were collected in two different seasons July and November 2005 from monuments of the Old Town of Salamanca These sampled salt efflorescences were analysed for δ34S and δ18O by mass spectrometry A ‘dualinlet’ type by VG Isotech was used for δ34S and continuous flow type Isoprime by GV Instruments for δ18O Samples were measured in triplicates and standard material was analysed for quality controlδ34S values range between 36‰ and 154‰ with a median value of 102‰ for the July samples and of 101‰ for November samples The results of the sulphur ratios hint towards a bimodal distribution with modes at δ34S = 6‰ and 12‰ for winter samples which is less obvious during summer δ18O values range from 71‰ to 411‰ However most values range from 71‰ to 208‰ whereas only few summer samples show outliers towards higher δ18O values The median δ18O value for July samples is 155‰ and for November samples 146‰The isotopic ratios of the analysed sulphate samples were compared with values of possible source materials Sulphur sources in the case of Salamanca are barites from the Villamayor Sandstone itself sea spray sulphides from regional rocks biogenic sulphur soil avian excreta as well as sulphur from anthropogenic sources such as building materials especially mortar or traffic exhaust Salamanca is a representative site for nonindustrial cities with no heavy industry and thus there are no significant SO2 emissions from industryBased on the measured isotopic ratios it was ascertained that more than one sole sulphur source is present However based on additional information about the source material and possible transport ways some sources could be excluded whereas others only played a minor role Finally there is strong indication that the main sulphur source is atmospheric pollution and the exhaust emissions from vehicles in particular while mortar as building material also contributes to a minor extent The δ18O values support this hypothesis Moreover the reported δ18O values are a strong indicator of the secondary nature of the Mg sulphates Isotope ratio measurement and especially the combined use of δ34S and δ18O values have proven to be a good instrument in clarifying the origin of salt efflorescences on buildingsFurther studies should investigate more closely the isotopic composition of atmospheric aerosols in Salamanca in order to get a more detailed knowledge about the main sulphur sources as well as to quantify the relation between the isotopic values and the amount and mineralogical form of the saltsThe authors thank Meggie Sotelo Martínez Félix García García and Juan Manuel Rodríguez Salvador Bartolo for their laboratory assistance and general support The authors also gratefully thank Dr Stefan Norra from the Institute of Mineralogy and Geochemistry Universität Karlsruhe TH Germany for his helpful comments on the manuscript

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