Lead Dynamics in Black Crusts: Elemental Distribution and Mobility Analysis in Lede Stone from Antwerp, Belgium

Black crusts commonly form on historic buildings as a result of interactions between building materials and atmospheric pollutants. These crusts primarily consist of gypsum layers that develop on calcium-rich surfaces through sulfation processes, especially in urban environments with elevated atmospheric contaminants. Moreover, black crusts accumulate particulate matter, polyaromatic hydrocarbons, and heavy metals such as lead (Pb), largely originating from anthropogenic activities like vehicular emissions, coal combustion, and industrial operations. Acting as passive environmental samplers, these crusts offer valuable insight into urban pollution trends.

Although the water-soluble components of black crusts, such as Ca²⁺, Mg²⁺, Na⁺, and SO₄²⁻, have been extensively studied, the understanding of trace elements, particularly Pb, remains incomplete, especially regarding their behavior and mobility. This study aims to fill this gap by examining Pb distribution, availability, and interactions within black crusts and the underlying stone substrates. Samples collected from historical buildings in Antwerp were analyzed using a multi-technique approach. SEM-EDX was employed for initial chemical and morphological characterization, while LA-ICP-TOF-MS enabled the generation of high-resolution quantitative elemental distribution maps for major, minor, and trace elements. Depth-resolved analysis of Pb migration was further explored through portable LIBS, contributing to a deeper understanding of crust stratigraphy and pollutant dynamics.

Preliminary findings indicate that Pb is predominantly concentrated in the outermost layers of the black crust. Given the crust’s primary composition of gypsum, a sulfate mineral, it is hypothesized that Pb is sequestered as lead sulfates, contributing to its immobilization within the crust. However, this contrasts with existing literature, which highlights Pb’s stronger affinity for carbonate phases, suggesting a tendency for it to migrate into carbonate layers and potentially into the underlying stone substrate. The confinement of Pb within the crust deviates from expected behavior, raising important questions about its speciation. Understanding the conditions under which Pb could become mobile is crucial, with factors such as kinetic limitations, local pH variations, and environmental conditions like humidity and wet-dry cycles likely influencing its migration.

This research investigates the behavior of Pb within black crusts, aiming to advance the conservation of historic buildings while addressing the public health risks associated with urban lead exposure. By examining the factors influencing Pb mobility, the study seeks to inform the development of targeted mitigation strategies for lead contamination. The expected outcomes will not only contribute to the long-term preservation of cultural heritage but also enhance urban environmental safety, providing critical insights that bridge the fields of heritage conservation and public health.