This research investigated the long-term performance of
zero-valent iron for mediating the reductive dechlorination
of trichloroethylene (TCE). Over a 2-year period, rates of
TCE dechlorination in columns packed with iron filings were
measured in simulated groundwaters containing either 3
mM CaSO4, 5 mM CaCl2, or 5 mM Ca(NO3)2. At early elapsed
times, TCE reaction rates were pseudo-first-order in TCE
concentration and were independent of the solution pH. With
increasing elapsed time, reaction rates deviated from
pseudo-first-order behavior due to reactive site saturation
and increased iron surface passivation toward the
influent end of each column. The extent of passivation
was dependent on both the TCE concentration and the
background electrolyte solution. For most of the investigation,
TCE reaction rates in 3 mM CaSO4 and 5 mM CaCl2
solutions were statistically identical at the 0.05 confidence
level. However, TCE reaction rates in 5 mM Ca(NO3)2
were slower. In columns operated using chloride- and sulfate-containing waters, the effective half-life for TCE dechlorination increased from approximately 400 min after 10 days
elapsed to approximately 2500 min after 667 days. The
effective TCE half-life in the nitrate-containing water increased
from approximately 1500 min after 10 days to approximately
3500 min after 667 days. Measurements of iron corrosion
rates in nitrate and chloride solutions showed that nitrate
contributed to increased iron surface passivation and
decreased rates of iron corrosion. Corrosion current
measurements indicated that halocarbon reduction on
fresh iron surfaces was cathodically controlled, whereas
on aged iron surfaces, iron corrosion was anodically
controlled. Anodic control of iron corrosion contributed to
the development of reactive site saturation with time
and to similar reaction rates for TCE and perchloroethylene.
Passivation of the iron surfaces was found to be dependent
on the adhering tendency of the corrosion products
and not on the overall mass of corrosion products in the
columns. The decrease in TCE reaction rates over time can
be attributed to anodic control of iron corrosion and not
to increasing reactant mass transfer limitations associated
with diffusion through porous corrosion products.
The results indicate that UV irradiation induces superactivity of K+ channels in the membrane is an early event mediating signaling transduction and resulting in corneal epithelial cell death in response to UV irradiation.
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