Unexpected arsenic compounds in low-rank coals

Nine low-rank coal samples from three different coal basins (Velenje and Trbovlje basins, Slovenia, and Sokolov basin, Czech Republic) were analyzed for their elemental composition and the presence of arsenic compounds. Total arsenic concentrations in the samples were 1.59-5.77 mu g g(-1) with one exception, 142 mu g g(-1) for a sample from the Sokolov basin. A methanol/water mixture (1:1) extracted 15.1-38.7% of the total arsenic from Velenje basin samples but only 2.2-7.1% from Sokolov and Trbovlje basin samples. Extracts from the Velenje basin samples contained mainly the tetra methylarsonium ion (0.14-0.92,mu g g(-1)) with considerable amounts of arsenate (0.15-0.85 mu g g(-1)) and monomethyl arsonic acid (0.04-0.27 mu g g(-1)). In methanol/water extracts from samples from the Sokolov and Trbovlje basins inorganic arsenic (arsenate) prevailed (0.26-37.1 mu g g(-1)), but at least trace amounts of organic arsenic compounds were found as well. It is likely that biogeochemical degradation of organic material is related to the unexpected organoarsenic compounds found in low-rank coals. However, it should also not be ruled out that abiogenic synthesis could have taken place in the carbon-rich environment under the harsh conditions present (elevated temperature and pressure).