Characterization of superhigh-organic-sulfur Raša coal, Istria, Croatia, and its environmental implication

The Paleocene high volatile B bituminous Raša coal, mined on the Istrian Peninsula, Croatia, for nearly 400 years up to 1999, is notable in having a superhigh organic sulfur (SHOS) content, exceeding 11% in some cases. The latter feature contributed to interest in the coal, far outweighing its status as a locally-used coal. Investigation by organic petrology, X-ray diffraction mineralogy, X-ray fluorescence and ICP-MS chemistry, SEM-EDS and TEM-EDS-SAED, micro-FTIR spectroscopy, and Raman spectroscopy showed that the Raša coal is dominated by vitrinite-group macerals, CaO and SO₃, carbonate minerals with contributions by Sr- and Ba-sulfates at the sub-micron scale, and the contribution of sulfur to the organic structure. Compared to Chinese SHOS coals and world ash, a majority of analyzed trace elements in Raša coal and ash, respectively, were depleted (up to 0.8 times), with the exception of V, Se, Sr, and Ba, which were increased 1.7 to 26 times. This fact, supported by elevated trace element concentrations in dripstone, are evidence for weathering of Raša coal by underground water, which is contaminating the local environment with trace elements, selenium in particular. Results of this study warrant further research.