The effect of trace element content on the rate of biotic and abiotic pyrite oxidation
EMSL Project ID
49595
Abstract
The oxidation of pyrite is a common source of toxic trace elements and acid generation in many contaminated systems. A key component in understanding the potential threat to ecosystems and possible remediation measures is the rate at which pyrite is oxidized and the release rate of toxic trace elements. In this study we investigate how trace elements affect the oxidation rate of pyrite and how these trace elements affect biologically mediated sulfide mineral oxidation rates. Lattice-bound Co and Ni have been proposed to create a net positive charge on the surface of pyrite, resulting in suppressed pyrite oxidation. Conversely, lattice-bound As has been suggested to destabilize the pyrite lattice, enhancing the rate of pyrite oxidation. Not all trace elements are lattice bound. Often they are held within micro-inclusions. These can also affect the rate of pyrite oxidation because when different sulfide minerals are in contact with one another they form galvanic cells. The sulfide mineral with the higher Eh will act as the cathode and oxidation is suppressed until the other sulfide mineral is consumed. Pyrite has higher Eh than most other sulfide minerals, theoretically resulting in initial high oxidation rates of the inclusions. The opposite is expected when pyrite has a lower Eh (e.g., with CuS inclusions at neutral pH). The effect of the oxidation rate due to trace element content of pyrite has not been quantitatively determined. In this study we will perform a series of batch pyrite oxidation experiments with pyrite of different trace element content to determine oxidation rates vary. In order to understand the oxidation mechanism, we must also study how the trace elements are held within the pyrite. Currently little is known about how trace elements are held within pyrite (with the exception of As and Au). We will use nano-SIMs to identify large micro-inclusions and regions of homogenous trace element enrichment. Zones of micro-inclusions will be analyzed by micro-XRD to determine the mineralogy of those inclusions because it will affect whether the pyrite or inclusion is the cathode in the galvanic cell and oxidizes last. Zones of homogenous trace element enrichment will be further analyzed with APT to determine whether the trace elements are evenly distributed (lattice held) or clustered together (nano-inclusions). The micro-XRD and APT results will be confirmed using TEM. The oxidation state of the trace elements in pyrite will be determined using XPS. This will help to understand the reaction pathway of the pyrite oxidation that is affected by the presence of different trace element concentrations. This will be supplemented by the Mossbauer spectroscopy to determine the oxidation state of Fe in the micro-inclusions. The final step of the project will investigate how trace element content of pyrite affects biologically mediated pyrite oxidation. Previous studies have shown that pyrite oxidation rates can be increased an order of magnitude by microbial activity. However it is not known how trace elements may affect this. Either the rates will be similarly increased or the release of toxic trace elements will limit the biologic activity suppress the oxidation rate. To test this we will establish a series of parallel microbial cultures with pyrite of different trace element contents and compare the oxidation rates of the different pyrite.
Project Details
Project type
Exploratory Research
Start Date
2016-11-01
End Date
2017-09-30
Status
Closed
Released Data Link
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