Ms. Anderson et al., Pyrite oxidation in unsaturated aquifer sediments. Reaction stoichiometry and rate of oxidation, ENV SCI TEC, 35(20), 2001, pp. 4074-4079
Citations number
26
Categorie Soggetti
Environment/Ecology,"Environmental Engineering & Energy
The oxidation of pyrite (FeS2) contained in unsaturated aquifer sediment wa
s studied by sediment incubation in gas impermeable polymer laminate bags.
Reaction progress was followed over a period of nearly 2 months by monitori
ng the gas composition within the laminate bag. The gas phase in the incuba
tion bags became depleted in Q(2) and enriched in CO2 and N-2 and was inter
preted as due to pyrite oxidation in combination with calcite dissolution.
Sediment incubation provides a new method to estimate low rates of pyrite o
xidation in unsaturated zone aquifer sediments. Oxidation rates of up to 9.
4.10(-10) mol FeS2/g.s are measured, and the rates are only weakly correlat
ed with the sediment pyrite content. The reactivity of pyrite, including th
e inhibition by FeOOH layers formed on its surface, apparently has a major
effect on the rate of oxidation. The code PHREEQC 2.0 was used to calculate
the reaction stoichiometry and partitioning of gases between the solution
and the gas phase. Pyrite oxidation with concurrent calcite dissolution was
found to be consistent with the experimental data while organic carbon oxi
dation was not. The reaction involves changes in the total volume of the ga
s phase. The reaction scheme predicts the volume Of O-2 gas consumed to be
larger than Of CO2 produced. In addition the solubility Of CO2 in water is
about 30 times larger than Of O-2 causing a further decrease in total gas v
olume. The change in total gas volume therefore also depends on the gas/wat
er volume ratio and the lower the ratio the more pronounced the loss of vol
ume will be. Under field conditions the change in total volume may amount u
p to 20% in the absence of calcite and over 10% in the presence of calcite.
Such changes in gas volume during the oxidation of pyrite are expected to
result in pressure gradients causing advective transport of gaseous oxygen.