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 Canad a has been examined by XAFS spectroscopy, It is found that the mode of occurrence is quite different from that (substitutional replacement f or 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, consiste nt with being held at maceral surfaces by polar oxygen functional grou ps, During storage after exposure to air, the As3+ in low-rank coals i s oxidized to As5+ in the form of the arsenate oxoanion, but less quic kly than was observed for arsenical pyrite in an Upper Freeport bitumi nous coal.