Radium in Bulgarian coals

The radium content of 14 Bulgarian coal deposits has been determined and its geochemical behavior is discussed and compared with that of uranium and thorium.The radium content varies greatly, from 0.1 to 3.1 parts per trillion (ppt) in individual samples. In several deposits it is 1.2 to 4.4 times higher than the supposed coal Clarke value, which is 0.7 ppt (Yudovich and Ketris, 2006). The deposits comprise lignites, subbituminous, bituminous, and anthracite coals. No relation of the Ra content to rank was established. The radium content usually varies significantly between the mines and seams of individual deposits. In some vertical seam sections Ra and U are enriched in the bottom part of the seam. The accumulation and distribution of Ra were found to be dependent on factors such as: (a) source provinces comprising granites and metagranitoids that favor the accumulation of radium, (b) location of the deposits in uranium-bearing formations, (c) hydrological conditions of the area and the mines; water may act both as a carrier of the elements and as a dissolving agent, and (d) tectonic structure of the deposit and processes of sedimentation. Complicated tectonic regions favor the input of Ra, or by contrast Ra leaching and redistribution. The Ra/U ratio for most deposits is lower than the supposed values for brown (2.4 × 10− 7), and hard (3.7.10− 7) coals. Radium in the coals is of mixed organic and inorganic association, based on the correlations between radium and ash yield, thorium and uranium.

► The concentrations of Ra, Th and U are described for a range of Bulgarian coal deposits.
► The Ra content varies considerably between the mines and seams of individual deposits.
► Ra in the coals is inferred to be both organically and inorganically associated.
► Geological factors favoring accumulation of Ra in the coals are identified.