Sulfide minerals have been suggested to play an important role in regulating
dissolved metal concentrations in anoxic environments. Pyrite is the most common
sulfide mineral and it has shown an affinity for arsenic, but little is known about the
arsenic retention mechanisms of pyrite. In this study, interactions of arsenic with pyrite
were investigated in an anoxic environment to understand geochemical cycling of
arsenic better and to predict arsenic fate and transport in the environment better. A
procedure using microwaves was studied to develop a fast and reliable method for
synthesizing pyrite. Arsenic-pyrite interactions were investigated using macroscopic
(solution phase experiments) and microscopic (X-ray photoelectron spectroscopic
investigation) approaches.
Pyrite was successfully synthesized within a few minutes via reaction of ferric
iron and hydrogen sulfide under the influence of irradiation by a conventional
microwave oven. The SEM-EDX study revealed that the nucleation and growth of pyrite
occurred on the surface of elemental sulfur, where polysulfides are available. Compared to conventional heating, microwave energy results in rapid (< 1 minute) formation of
smaller particulates of pyrite. Higher levels of microwave power can form pyrite even
faster, but faster reaction can lead to the formation of pyrite with defects.
Arsenic removal by pyrite was strongly dependent on pH and arsenic species.
Both arsenite (As(III)) and arsenate (As(V)) had a strong affinity for the pyrite surface
under acidic conditions, but As(III) was removed more effectively than As(V). Under
acidic conditions, arsenic removal continued to occur almost linearly with time until
complete removal was achieved. However, under neutral to alkaline conditions, fast
removal was followed by slow removal and complete removal was not achieved in our
experimental conditions. A BET isotherm equation provided the best fit to arsenic
removal data, suggesting that surface precipitation occurred at high arsenic/pyrite ratio.
The addition of competing ions did not substantially affect the ultimate distribution of
arsenic between the pyrite surface and the solution, but changing pH affected arsenic
stability on pyrite.
X-ray photoelectron spectroscopy revealed that under acidic conditions, arsenic
was removed and formed solid phases similar to As2S3 and As4S4 by reaction with
pyrite. However, under neutral to alkaline conditions, arsenic was removed and formed
As(III)-O and As(V)-O surface complexes, as well as As2S3/As4S4-like precipitates. As
pH increases, the amount of arsenic that formed As2S3/As4S4-like precipitates decreased,
while the amount that formed As(III)-O and As(V)-O surface complexes increased.
Under alkaline conditions, a FeAsS-like phase was also detected.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-3138 |
| Date | 15 May 2009 |
| Creators | Kim, Eun Jung |
| Contributors | Batchelor, Bill |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | electronic, application/pdf, born digital |
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