BIOAUGMENTATION WITH PSEUDOMONAS STUTZERI KC FOR CARBON TETRACHLORIDE REMEDIATION - Bioaugmentation for Groundwater Remediation - page 255

O

O O

O

O

C O CO

C S CCl 3

CCl 4

C S CS CCl 3

OH - Cl -

CCl 4 CCl 3

Cu Cu

N Cu N N

N Cu Cu

N

C S C S C O

C S

C S

Cl -

Cl 3 CSH

O O O

O O

HCl

S
C

Cl Cl

OH -

HCl, Cl -

COS

OH -

HS -

CO 2

Figure 9.2. Pathway for Cu:PDTC-promoted decomposition of CT. All intermediates have been
detected through direct or indirect analytical procedures.

to give thiophosgene, and (5) hydrolysis of thiophosgene via carbonyl sulfide to give CO 2 .
Intermediates of all steps except (2) were identified directly or indirectly in that earlier work
with bacterial cultures or synthetic PDTC (Lewis et al., 2001 ; Lewis and Crawford, 1995 ).
Indirect evidence for the trichloromethyl thioester, or at least an electrophilic PDTC
derivative, was subsequently obtained using ethylamine as a competing nucleophile, because
an ethylamido pyridine derivative was detected rather than the carboxylate derivative expected
by hydrolytic attack (Figure 9.3 ). These data could explain the nonvolatile fraction observed in
the initial mass balance experiments as adducts produced from thiophosgene condensation with
any of a variety of nucleophiles present in the bacterial cultures. These results also indicate that
dechlorination by this process will be quantitative because carbon-chlorine bonds are labilized
through this pathway such that nucleophiles present in any aqueous environment will lead to
quantitative release of chloride.
The data also indicate that the process is stoichiometric rather than catalytic. Without
additional electron donor, the stoichiometry of CT transformation per PDTC approached 2:1,
corresponding to both thiocarboxylate sulfur atoms. Though synthetic PDTC might be consid-
ered for direct application in a remediation scenario, its oxidative and hydrolytic lability may
reduce its cost-effectiveness relative to in situ bacterial production, unless other microflora are
used for regeneration of synthesized PDTC activity (see Section 9.5 ).

9.3.2 Transition Metal Chelation of PDTC

The fact that the Cu:PDTC complex is required for CT transformation suggests that other
transition metals also might be effective, or could be inhibitory through their ability to form
competing PDTC complexes. Accordingly, experiments were undertaken to understand the role
of PDTC in potential metal sequestration and detoxification reactions. PDTC complexes of iron
(III) (Fe[III]), nickel (II) (Ni[II]), and cobalt (II) (Co[II]) were tested for CT transformation, but
none of these showed any significant activity in the absence of added reducing agents (Lewis
et al., 2001 ). Even when iron (III) was added in a 50-fold molar excess to Cu(II) it did not
effectively inhibit CT dechlorination (Lewis et al., 2001 ). The stability constant for the Fe(III):
PDTC complex was later determined as 10 33 (Stolworthy et al., 2001 ). The lack of effective
competition by Fe(III) suggests that the stability constant for the Cu(II):PDTC complex may be