Laminar gypsum crust on lede stone: Microspatial characterization and laboratory acid weathering

The Historic Built Environment As a Long-Term Geochemical Archive: Telling the Time on the Urban "Pollution Clock"

This study introduces a novel methodology for utilizing historic built environments as reliable long-term geochemical archives, addressing a gap in the reconstruction of past anthropogenic pollution levels in urban settings. For the first time, we employ high-resolution laser ablation mass spectrometry for lead isotope (206Pb/207Pb and 208Pb/206Pb) analysis on 350-year-old black crust stratigraphies found on historic built structures, providing insights into past air pollution signatures. Our findings reveal a gradual shift in the crust stratigraphy toward lower 206Pb/207Pb and higher 208Pb/206Pb isotope ratios from the older to the younger layers, indicating changes in lead sources over time. Mass balance analysis of the isotope data shows black crust layers formed since 1669 primarily contain over 90% Pb from coal burning, while other lead sources from a set of modern pollution including but not limited to leaded gasoline (introduced after 1920) become dominant (up to 60%) from 1875 onward. In contrast to global archives such as ice cores that provide integrated signals of long-distance pollution, our study contributes to a deeper understanding of localized pollution levels, specifically in urban settings. Our approach complements multiple sources of evidence, enhancing our understanding of air pollution dynamics and trends, and the impact of human activities on urban environments.

Identification of Intermediates in the Reaction Pathway of SO2 on the CaO Surface: From Physisorption to Sulfite to Sulfate

The interaction of CaO and Ca(OH)₂ with solvated or gaseous SO₂ plays a crucial role in the corrosion of urban infrastructure by acid rain or in the removal of SO₂ from flue gas. We carried out a combined spectroscopic and theoretical investigation on the interaction of SO₂ with a CaO(001) single crystal. First, the surface chemistry of SO₂ was investigated at different temperatures using polarization-resolved IR reflection absorption spectroscopy. Three species were identified, and an in-depth density functional theory study was carried out, which allowed deriving a consistent picture. Unexpectedly, low temperature exposure to SO₂ solely yields a physisorbed species. Only above 100 K, the transformation of this weakly bound adsorbate first to a chemisorbed sulfite and then to a sulfate occurs, effectively passivatating the surface. Our results provide the basis for more efficient strategies in corrosion protection of urban infrastructure and in lime-based desulfurization of flue gas.

Characterization of black crusts developed on historic stones with diverse mineralogy under different air quality environments

Black crusts (BCs) are one of the most critical alteration forms found on stones belonging to architectural heritage. Since they could be considered as passive samplers of atmospheric pollution, it would be plausible to establish relations between the air contamination and the BCs. With this aim, we have characterized BCs collected on historic buildings from two Spanish cities (Granada and Vigo) with different polluted atmospheres, as well as formed on stone substrates of varied mineralogy and texture. Likewise, in order to assess the impact of the atmospheric pollutants on the growth of BCs, quartz fiber filters were used as surrogate substrates and placed nearby the studied buildings to collect and analyze the aerosol particulate matter (PM). To this end, an array of complementary analytical techniques was used to evaluate the mineralogy, chemical composition, and texture of the BCs and to establish the correlation with the ions, OC (organic carbon), and EC (elemental carbon) detected in the PM on the quartz fiber filters. As result, BCs developed on carbonate substrates from Granada show more complex structure than those from Vigo, which are thinner because of frequent rain episodes. In both cities, NaCl, Pb-Cl, and Ca-Cl-rich particles, Ca-phosphate particles and clusters of Ba-sulfate-rich particles were detected. However, metal-rich rounded particles were more abundant in Granada's BCs, including soot particles. BCs from Granada were richer in carbonaceous components (OC and EC) than the Vigo's BCs. Although in the filters PM did not show EC-mainly due to traffic-, in the BCs from both locations OC and EC were detected. Therefore, this different composition was related to the mineralogy of the stones and the higher pollution of Granada in contrast to the industrial and sea-exposed city of Vigo.

Differential colonization of microbial communities inhabiting Lede stone in the urban and rural environment

Air pollution is one of the main actors of stone deterioration. It influences not only the material itself but also prokaryotes colonizing rocks. Prokaryotes can affect rock substrates and biological colonization will most likely become relatively more important during the course of the 21st century. Therefore, it is necessary to understand the effects of air pollution on biological colonization and on the impact of this colonization on rock weathering. For this reason, we studied the prokaryotic community of Lede stone from two deteriorated monuments in Belgium: one in the urban and one in the rural environment. This research conducts 16S rRNA gene Next Generation Sequencing combined with an isolation campaign. It revealed diverse and complex prokaryotic communities with more specialized bacteria present in the urban environment, while archaea were barely detected. Some genera could cause biodeterioration but the isolates did not produce a significant amount of acid. Soluble salts analysis revealed an important effect of salts on the prokaryotic community. Colour measurements at least indicate that a main effect of prokaryotes might be on the aesthetics: In the countryside prokaryotic communities seemed to discolour Lede stone, while pollution most likely blackened building stones in the urban environment.

Rock Features and Alteration of Stone Materials Used for the Built Environment: A Review of Recent Publications on Ageing Tests

This work presents a review of recent publications, with publication date between 2017 and 2019, with information on the relation between rock characteristics and the effects of diverse agents associated with alteration of stone materials in the built environment. It considers information obtained from ageing tests performed under laboratory conditions and by exposure to outdoor agents. Several lithological groups were considered, with sedimentary carbonate rocks being the most frequently studied lithotypes and silicate metamorphic rocks being the group with scarcer information. In terms of ageing tests, salt weathering was the most frequent one while there was a noticeable lesser amount of information from tests with biological colonization. The collected data showed the influence of diverse features, from specific minerals to whole-rock properties and the presence of heterogeneities. These information are discussed in the context of formulating a general framework for stone decay.

Application of micro X-ray fluorescence and micro computed tomography to the study of laser cleaning efficiency on limestone monuments covered by black crusts

Laser cleaning is widely used to remove black crusts from weathered limestone monuments. The cleaning efficiency is commonly tested using conventional analytical techniques, which do not allow to analyze the same sample before and after the treatment. In this paper, micro computed tomography (μ-CT) and micro X-ray fluorescence spectroscopy (μ-XRF) techniques were used for the first time to evaluate the laser cleaning efficiency on two different encrusted quoins collected from a limestone monument. Analyses were carried out non-destructively on the same portion of the two lithotypes before and after the treatment. μ-XRF confirmed the presence of gypsum in the black crust, and showed a marked decrease of S and other typical elements after laser cleaning of both samples. μ-CT clearly showed the different structure of limestone before and after cleaning and the crust portion removed by the laser. The combination of the two techniques allowed to assess that, even if the two samples had a similar chemical composition, their response to laser cleaning was different on dependence of their different fabric/structure. In fact, in one sample calcium sulphate was still partially retained also after the black crust removal, whereas in the other sample the sulphate layer was almost completely ablated due to its more compact structure. In both cases, laser cleaning operation was shown not to cause any structural modification or mechanical damage of the original stone material. In conclusion, the use of these novel techniques appears very promising for studying the effects of laser ablation on rock samples in order to set the best working conditions for their cleaning.