Mode of occurrence of arsenic in subbituminous coals

The mode of occurrence of arsenic in a number of low-arsenic (<5 ppm) coals of subbituminous rank from western regions of the U.S. and Canada has been examined by XAFS spectroscopy, It is found that the mode of occurrence is quite different from that (substitutional replacement for S in pyrite) observed earlier for arsenic in bituminous coals from the eastern U.S. The spectroscopic evidence indicates that the arsenic in low-rank coals is present as As3+ in oxygen coordination, consistent with being held at maceral surfaces by polar oxygen functional groups, During storage after exposure to air, the As3+ in low-rank coals is oxidized to As5+ in the form of the arsenate oxoanion, but less quickly than was observed for arsenical pyrite in an Upper Freeport bituminous coal.