Using scanning electron microscopy and Raman spectroscopy to characterize heavy metal-containing compounds in soils around metal facilities from the Bronze Age to the present day

Since the Bronze Age, heavy metals have been accumulating in soils at non-ferrous metal production sites, allowing these accumulations to be used in modern geoarchaeological research as evidence of metallurgical production and metalworking at archaeological settlements. Soils in cities where modern metallurgical plants are located have been actively studied to assess the accumulation and transformation of heavy metal compounds, evaluate soil health, and determine the impact on urban and surrounding native ecosystems. Based on the example of Bronze Age archaeological sites, where metallurgical enterprises were small and had been inactive for several thousand years, especially if the settlement ceased to exist after that, it can be assumed that concentrations decrease, especially when this is facilitated by hydromorphic conditions with acidic pH, when many heavy metals remain mobile. However, the duration of heavy metal persistence in soils and their chemical forms remains unclear. Scanning electron microscopy (SEM) allows, in some cases, to identify individual areas of heavy metal accumulation (Cu, Zn, Ni, Pb, Co) at the submicron level. In the case of soils exposed to the influence of a modern large copper-nickel metallurgical plant in the city of Monchegorsk (Murmansk region, Russia, Kola Arctic), individual metal and slag particles ranging in size from 1 to several tens of micrometers of mono- and poly-element composition were identified at the submicron level. Raman spectroscopy has revealed a variety of chemical compounds that contain nickel, copper, and zinc. High concentrations of copper and zinc, similar to those found in medieval cultural layers with archaeological traces of metallurgical production in the nearest large city of Rostov Veliky, were found in the soils of a Bronze Age settlement (Pesochnoe-1 settlement, Textile ceramic archaeological culture, 14C age – 2100–1800 cal BC, Yaroslavl’ region, central part of European Russia). Using SEM, it was possible to identify anomalous zones of Cu and Zn accumulation in burnt, finely dispersed animal bones (within the first micrometers) used as fuel for metalworking. Even in humid conditions, high concentrations were preserved in these soils with cultural layers due to the abundance of calcium phosphate (small fragments of animal bones, including burnt ones, more than 10% of the total mass).