straction of a structural hydrogen atom followed by in-
tramolecular rearrangement (eq 4 (11)).
in the acidity of water molecules coordinated with nonex-
changeable Na+. Our results, however, showed that the
reduction of structural Fe led to an increase in the degradation
rate of RCl via Brønsted and Lewis-base promoted mech-
anisms. Consequently, the transformation of RCl may not be
attributed to their interaction with Na+ and/ or coordinating-
water molecules, nor with structural hydroxyl groups, but to
their association with the clay surface and/ or surface-bound
water.
The dechlorination of RCl either via reduction or elimi-
nation in the presence of Fe(II)-bearing clays could be an
important abiotic transformation pathway in soils and thus
have an impact on transport and bioavailability of these
compounds in soils. For these reasons, conducting funda-
mental research on the structural factors that control the
surface chemistry of redox-manipulated smectites is neces-
sary in order to assess more fully their potential implemen-
tation as an in situ remediation technology for the removal
of RCl from contaminated soils and groundwater.
CCl3CHCl2 f CCl2 ) CCl2 + H+ + Cl-
(4)
The susceptibility of PCA, TeCA, and TCA to adsorb and
undergo dehydrochlorination in the presence of SWa-R was
similar to their trend in reactivity to undergo basic hydrolysis
in pure water (kb ) 7.86 × 10-3 h-1, 5.82 × 10-7, and 0.00,
respectively; kb ) base-catalyzed hydrolysis rate constant
(26, 27)), and which is consistent with the inductive con-
tributions from the substituents to the â carbon atom, ΣσI
(ΣσI ) 1.43, 1.26, and 0.51 (28, 29)). Likewise, the slow
adsorption observed for TCE and PCE in the presence of
SWa-R was consistent with their tendency to undergo slow
basic hydrolysis (kb ) 6.42 × 10-11, 8.22 × 10-14 h-1 (26, 27)).
On the other hand, the reduction of structural Fe accelerated
the adsorption of TCA (kb ) 0; kn ) 7.44 × 10-5 h-1, where
kn ) neutral-catalyzed hydrolysis rate constant, and khyd
)
kb + kn, where khyd ) hydrolysis rate constant). These results
suggest that the interactions of RCl with redox-manipulated
smectites are not limited by the hydrolysis mechanism of
RCl in bulk water but on the ability of RCl to hydrate.
Reduction. Trichloroacetonitrile (TCAN) and trichloro-
nitromethane (CP) underwent single- and double-dechlo-
rination (Figures 1 and 2 (12)). The dechlorination of both
TCAN and CP was fast to produce DCAN (62 ( 3.3%), DCNM
(57 ( 1.9%), and CNM (24.8 ( 4.3%), respectively. The loss
of TCAN is coupled with the formation of DCAN, which reacts
much more slowly.
Acknowledgments
The authors thank Karen Marley and Jie Rong for valuable
technical support, Dr. James Amonette (Pacific Northwest
National Laboratory), and Dr. Alan Stone (Johns Hopkins
University) for helpful discussions, and the comments of
two anonymous reviewers. This work was supported in part
by the U.S. Department of Agriculture (Grant Numbers NRI-
CRGP 93-37102 and 98-35107-6313) and the National Science
Foundation (Grant Numbers EAR 95-23902).
The reaction of CP with SWa-R led to the simultaneous
formation of DCNM and CNM (Figure 2), indicating that the
SWa surface served as a bulk reductant and participated in
two independent electron transfer pathways with CP. The
dechlorinations of TCAN and CP are presumably initiated
by the transfer of an electron from the SWa surface, with the
odd electron delocalized to the C-Cl bond to form a three-
electron bond prior to homolysis (eq 5). The dechlorination
of TCAN (Figure 1) and CP (Figure 2) but not of PCM shows
that the simultaneous resonance and inductive contributions
from the substituents to the carbon center (captodative effect
(28, 30)) facilitate charge delocalization during clay-organic
interactions.
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-
•CCl2NO2 + Cl- (5)
Quantum mechanical calculations (31) of the isomorphic
substitution of Fe(III) by Fe(II) in smectites reveal that
increasing the amount of Fe(II) in the clay causes a decrease
in the energy gap existing between the conduction and
valence band energies, ∆ꢀ. The shift in ∆ꢀ (from 4.4 to 3.5
eV) was interpreted to mean that Fe(II) center(s) donate
electron density to the apical oxygen groups and form a new
energy level(s) below the conduction band, which facilitates
electron transfer to the conduction band. Furthermore, recent
EXAFS (13, 20) studies show that the reduction of structural
Fe in Fe-rich smectites generates electronic disorder in the
crystal structure in the form of octahedral holes and channels
and, as a result, an excess of negative charge. This electronic
density surplus in turn is likely to provoke a shift in the
electronic distribution (single electron shift (32)) of sorbed
RCl molecules that leads to their degradation via polar (eq
4) or radical (eq 5) pathways.
The oxidation state of the structural Fe has a strong effect
on the basicity of the smectite surface. Early studies (33)
reported that reducing structural Fe in SWa results in a
decrease of water content (mw/mc) and an increase in
nonexchangeable Na+ and, as a consequence, to an increase
9
8 0 8 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 4, 2001