PNNL-13501 - Pacific Northwest National Laboratory

TCE + 3H + + 3Fe 2+ Fe)=> acetylene + 3Fe 3+ + 3Cl-
Laboratory experiments with partially and fully reduced
sediment showed a highly nonlinear relationship between
the fraction of reduced iron and trichloroethylene
reactivity (Szecsody et al. 2000a), where more than 40%
of the iron needs to be reduced for trichloroethylene
dechlorination to proceed. Field sites that contain a large
fraction of iron may not be able to be economically
reduced, so an alternative approach may be to add an
electron donor or catalyst.
Approach
In this study, electron donors or reaction catalysts were
added to partially reduced sediment in order to determine
the efficiency of the amendments to dechlorinate
trichloroethylene. Naturally occurring electron donors
added included Fe(II), Mn(II), 2:1 iron-bearing clays, and
humic acid. Engineered electron donors added included
Ti(III) and Ni(II). The addition of Ti(III)EDTA should
result in the slow ligand-promoted dissolution of iron
oxides (Heron et al. 1994), removing Fe(III). Ni(0) is
currently being used with Fe(0) at the field scale (Su and
Puls 1999). Anthroquinone-2-6-disulfonic acid and
humic acid are biotic and abiotic electron shuttles (Curtis
and Reinhard 1994). Palladium catalyst is currently being
used for trichloroethylene dechlorination at the field scale
with Fe(0) (Li and Farrell 2000).
Results and Accomplishments
The addition of Fe(II) to partially reduced sediment could
result in the increase in the trichloroethylene degradation
rate, but only under specific geochemical conditions.
Trichloroethylene degradation rate for the partially
reduced sediment [no addition of Fe(II)] of 17.5 hours
indicated that the sediment was about 40% reduced,
relative to 100% reduced sediment which would result in
a 1.2-hour half-life. The addition of a mass of Fe(II)
(Figure 1) showed that the trichloroethylene reduction
rate with the Fe(II) addition had a 6.5-hour half-life, or
about the 8- to 10-hour rate predicted based on the mass
of Fe(II) added. However, the Fe(II) addition was
successful at high pH (10.5) when added with dithionite,
but was not successful at lower pH (6.8 to 9.9) or if added
after the dithionite reduced the sediment. This result may
have been caused by the formation of Fe(OH)2, which
precipitated and blocked the reduced sediment. The
addition of Mn(II) to partially reduced sediment
(Figure 2) also resulted in an increase in the
trichloroethylene degradation rate. Because Mn(II)
252 FY 2000 Laboratory Directed Research and Development Annual Report
conc (µmol/L)
10
8
6
4
2
initial: 1.1 ppm TCE
TCE
acetylene
Fe(II) addition: 6.5 h
control: half life 17.3 h
0
0 5 10 time (h) 15 20 25
Figure 1. Trichloroethylene dechlorination to acetylene in
the presence of reduced natural sediment and with the
addition of Fe(II)
conc (µmol/L)
10
8
6
4
2
initial: 1.1 ppm TCE in DI water
TCE
acetylene
Mn(II) addition: 9.9 h
control: half life 17.3 h
0
0 5 10 time (h) 15 20 25
Figure 2. Trichloroethylene dechlorination to acetylene in
the presence of reduced natural sediment and with the
addition of Mn(II)
oxidizes to Mn(IV) (donates two electrons), the Mn(II)
addition experiment should have resulted in a 3- to 4-hour
trichloroethylene degradation half-life and not the10-hour
half-life observed.
Three smectite clays were added that had differing
amounts of iron in the structure: hectorite (0% Fe),
montmorillonite (2.3% Fe), and nontronite (22% Fe). The
addition of oxic clays, as predicted, had a large negative
impact on dechlorination (Figure 3), correlated with the
fraction of iron in the clay. When reduced smectite clays
(Stucki et al. 1984) were added, trichloroethylene
dechlorination decreased (17- to 22-hour versus 10-hour
for the control), even though the separate sediment and
clay could dechlorinate trichloroethylene faster. This may
have been caused by the clay coating reactive sediment
surfaces.
conc (µmol/L)
10
8
6
initial: 1.1 ppm TCE in DI water
TCE
control: half life 12.6 h
4
2
0
acetylene
hectorite, montmorillonite: 15 h
nontronite: 92 h
0 5 10 time (h) 15 20 25
Figure 3. Trichloroethylene dechlorination to acetylene in
the presence of reduced natural sediment and with the
addition of oxic smectite clays showing that the iron content
in the clay correlated with the impact on the
trichloroethylene dechlorination rate