EFFECT OF ARSENIC SUBSTITUTION ON PYRITE OXIDATION IN BITUMINOUS COAL SAMPLES: EXPERIMENTS AND SPECTROSCOPIC MONITORS

Trace-constituent substitution in pyrite is an important factor that may affect its stability during oxidation of coal-bearing strata and waste from coal washing operations. Arsenic, the most abundant trace/minor constituent in pyrite, is thought to have a de-stabilizing effect, as indicated by previous XAFS studies showing oxidation of pyritic arsenic to arsenate in advance of significant iron oxidation. In this study, we exposed coal samples with differing amounts of pyrite and differing extents of arsenic substitution in pyrite, to controlled experimental conditions. Samples investigated include a Springfield coal from Indiana (whole coal pyritic S=2.13 wt. %; As in pyrite=detection limit to 0.06 wt. %), two Pittsburgh coal samples from West Virginia (pyritic S=1.32 to 1.58 wt. %; As in pyrite=d.l. to 0.34 wt. %), and two samples from the Warrior Basin, Alabama (pyritic S=0.26 to 0.27 wt. %; As in pyrite=d.l. to 2.72 wt. %). Samples were collected from active mine faces and selected for expected differences in pyritic arsenic, confirmed by electron microprobe analysis. Splits of each sample are being exposed to four room-temperature experimental conditions over a period of 1-2 years, including: 1) dry argon atmosphere; 2) dry oxygen atmosphere; 3) room atmosphere (relative humidity ~20 to 60 %), and 4) room atmosphere with samples wetted periodically with double-distilled water. The extent of arsenic and iron oxidation is monitored by arsenic XAFS spectroscopy and iron Mössbauer spectroscopy.

Arsenic XANES spectra for each coal sample, obtained after the first month of study, show major peaks for pyritic arsenic and minor peaks for arsenate forms. Least-squares fitting of the spectra shows little difference among equivalent samples stored under different atmospheres. However, the wetted samples showed significant formation of arsenate. Mössbauer spectroscopy also showed significant differences between samples stored under argon and those periodically wetted, but much less change with the other environments. The initial results show that arsenate formation can be induced and monitored, allowing comparison of oxidation progress in the test coal samples.