Investigation of the speciation of arsenic and selenium in pyrites and implications for element mobilization during oxidation of arsenic- and selenium-bearing pyrites

Arsenic and selenium can be harmful to human, animal, and plant health so it is important to understand how these elements accumulate in the environment. A process known to release these elements is the oxidation of pyrites with substituted arsenic and/or selenium. Previous studies on arsenic and selenium focused on their mobility once released; however, there has been less research on their mobilization during pyrite oxidation. This study examines the occurrence of arsenic and selenium in pyrite and their mobilization through pyrite oxidation. Samples containing pyrites with arsenic and/or selenium were characterized through geochemical and mineralogical (XRD, SEM, synchrotron XANES and micro-XRF) analyses. Laboratory-scale oxidation experiments were conducted on the pyritic samples to determine arsenic and selenium mobilization rates from pyrite oxidation. Reaction path models were also run to better understand the processes in the experiments. The characterization study corroborates previous findings that arsenic and selenium occur in the -1 oxidation state substituting for sulfur in pyrite. Furthermore, the study demonstrates that XANES of arsenic- and selenium-pyrites have intrinsic features which differentiate the arsenic or selenium substituting for sulfur from oxidized arsenic and selenium species. The experimental oxidation study finds that arsenic is initially mobilized at a faster rate than sulfate when pyrite is oxidized. Arsenic mobilization rates appear to slow over time, but this decrease in rate may be because arsenic is depleted at the pyrite surface. Selenium may be mobilized at a similar rate to sulfate but findings from this study are inconclusive and further work is needed for confirmation. The modeling study shows that model results for arsenic mobilization from oxidation of arsenic-pyrite are clearly different than experimental results. Model results for selenium are inconclusive because of the limited experimental results. The difference between model and experimental results for arsenic are because the models do not account for faster arsenic mobilization rates relative to sulfate. More experimental data on arsenic- and selenium-pyrite oxidation is clearly required to develop more comprehensive models. Moreover, more experimental data on arsenic and selenium mobilization rates during pyrite oxidation will facilitate the development of procedures for long-term monitoring and remediation.