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Description:
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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 . |