Impact of coal depository and slag disposal from the Plomin thermal power plant on soil composition: insights from geochemical, mineralogical, and organic petrological analyses, Istria, Croatia

Abstract

The potential impact of the open coal depository and the slag disposal of the Plomin thermal power plant (PTPP) on the composition of the surrounding soils was investigated. A comprehensive approach taken included analyses of mineral composition, potentially toxic element content, their bonding properties, distribution to specific geochemical fractions in the soil and investigation of the soil organic petrology and geochemistry. The Cambisols at two sampling sites exhibited a coal dust-covered upper horizon, which led us to consider these soil profiles as Technosol over Cambisol. For practical reasons, we have used the terms Technosol and Cambisol for the upper and lower parts of these profiles respectively. These formations were developed by the long-term deposition of coal dust and slag particles originating from the open coal depository and slag disposal of the PTPP, and widespread wind dispersion. The Technosols, characterized by higher concentrations of lignite and lignite-subbituminous coal, showed elevated levels of C and S. Analysis of the slag sample revealed a mineral composition of mullite, tridymite, graphite, haematite and an amorphous phase, indicating material formation by high temperature coal combustion. The PTPP was identified as the main contributor to the elevated concentrations of Hg, Mo, Se, Sb, U and Cd. The Technosols showed a significant enrichment in Hg, Sb, Se, Mo and U compared to the Cambisols. In the Technosols, elements such as As, Cr, Cu, Fe, Ni and Zn were primarily associated with the organic and Fe-Mn fractions, whereas in the Cambisols these elements, (with the exception of Cu), are mainly associated with the organic and residual fractions. Cadmium, Mn and Pb were predominantly bound to the carbonate and Fe-Mn fractions in all the analyzed samples. The non-residual fraction has been proven to be the predominant repository for As, Zn and Ni in the Technosols and slag, and for Mn, Zn and Cd in the Cambisols. The total content of C and S in the soils and their ratio (C/S) confirmed the higher contamination of soils in the vicinity of the PTPP with coal material. Detection of the coke and soot in the slag sample substantiated its composition as bottom coal ash. In addition, the presence of the bottom coal ash particles in the Cambisol underlined the airborne dispersal of the slag and its integration into the soil composition.