The complete dechlorination of DDT by magnesium/palladium bimetallic particles

Article abstract of DOI:10.1016/S0045-6535(00)00163-6

The complete dechlorination of 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT) by a magnesium/palladium bimetallic system has been accomplished. The reaction takes place under ambient temperature and pressure and mild reaction conditions requiring only 0.25 g of magnesium and 0.3% palladium (wt/wt) to drive the dechlorination of 100 μg DDT (50 ppm in 2 ml). The process is both rapid and complete requiring less than 10 min to attain total dechlorination within the detection limit (～10 pg for DDT) of electron capture detection gas chromatography (GC-ECD). The major product formed, as deduced from mass spectrometry (GC-MS) is the hydrocarbon skeleton, 1,1-diphenylethane. This technology may allow for the development of an economic and environmentally benign method of DDT remediation.

Full text of DOI:10.1016/S0045-6535(00)00163-6

Chemosphere 43 -2001) 195±198
The complete dechlorination of DDT by magnesium/palladium
bimetallic particles
*
Mark D. Engelmann, John G. Doyle, I. Francis Cheng
Department of Chemistry, College of Letters and Sciences, University of Idaho, Moscow, ID 83844, Russian Federation
Received 25 January 2000; received in revised form 17 April 2000; accepted 28 April 2000
Abstract
The complete dechlorination of 1,1-bis-4-chlorophenyl)-2,2,2-trichloroethane -DDT) by a magnesium/palladium
bimetallic system has been accomplished. The reaction takes place under ambient temperature and pressure and mild
reaction conditions requiring only 0.25 g of magnesium and 0.3% palladium -wt/wt) to drive the dechlorination of 100
lg DDT -50 ppm in 2 ml). The process is both rapid and complete requiring less than 10 min to attain total dechlo-
rination within the detection limit -ꢀ10 pg for DDT) of electron capture detection gas chromatography -GC-ECD).
The major product formed, as deduced from mass spectrometry -GC±MS) is the hydrocarbon skeleton, 1,1-diphenyl-
ethane. This technology may allow for the development of an economic and environmentally benign method of DDT
remediation. Ó 2001 Published by Elsevier Science Ltd.
Keywords: Pesticide; Zero-valent; Hydrodehalogenation; Diphenylethane
1. Introduction
chlorination. It is generally accepted that by removing
the chlorine from DDT, the hydrocarbon backbone
that results is far less toxic. In contrast with the
present method of disposal, chemical dechlorination
may prove to be a low energy alternative to high
temperature incineration.
Over the past two decades there has been a strong
movement to discontinue the use of numerous pesti-
cides, and the search for remediation methods ad-
dressing existing pesticide accumulation has been
pursued with fervor -Hitchman et al., 1995; Renner,
1998). DDT use in USA began in 1939 and continued
extensively until its ban in 1972. Today, due to its
e€ectiveness and low cost, it remains the chemical of
choice for combating mosquitoes in Malaria infested
areas of the world. DDT has proven to be especially
recalcitrant because of its chemical stability and poor
soil mobility. Currently, there is no single method that
can satisfactorily address the remediation of DDT.
One approach that holds promise, however, is de-
Recent literature has not indicated that rapid and
complete dechlorination of DDT has been achieved.
Furthermore, many methods involve complicated
procedures such as anaerobic conditions, high energy
input, or several hours or even days to reach com-
pletion -Sherman et al., 1971; Otero and Rodenas,
1986; Hall et al., 1996; Sayles et al., 1997; Wei, 1999).
The method described in this communication is rapid,
simple, and complete under room temperature and
pressure conditions and thus represents a promising
step towards an ideal mechanism for DDT destruc-
tion.
The method is based on the reduction potential of
magnesium metal coupled with the catalytic activity of
small amounts of palladium. This system reduces the
* Corresponding author. Tel.: 208-885-6387.
E-mail address: ifcheng@uidaho.edu -I.F. Cheng).
0045-6535/01/$ - see front matter Ó 2001 Published by Elsevier Science Ltd.
PII: S 0 0 4 5 - 6 5 3 5 - 0 0 ) 0 0 1 6 3 - 6

196
M.D. Engelmann et al. / Chemosphere 43 *2001) 195±198
chlorinated organic compound to its hydrocarbon skel-
eton
mixed solvents, the addition of small amounts of a weak
acid increases proton availability for hydrogenation.
R±Cl ‡ H ‡ 2eÀ ! R±H ‡ ClÀ; E0 ˆ 0:5±1:5 V S:H:E:
‡
ꢁ1†
2. Experimental
This reductive process has been studied in a similar
system consisting of iron and palladium and has been
demonstrated by Grittini et al. -1995) to e€ectively de-
chlorinate polychlorinated biphenyls -PCBs), and by
Muftikian et al. -1995) to dechlorinate trichloroethene
-TCE) and other chlorinated organic compounds. Like
iron, magnesium ions are environmentally benign, but
magnesium was chosen for this work for several rea-
sons. It has a greater thermodynamic driving force,
which is illustrated by a comparison of reduction po-
tential:
2.1. Dechlorination solutions
Dechlorination solutions began with the addition of
100 lg DDT -isomer mix, purchased from Chem Ser-
vice, Westchester, PA) to a 5 ml vial containing 0.25 g of
20 mesh magnesium metal -Aldrich, Milwaukee, WI)
and 2.5 mg of K2PdCl6 -Alfa, Ward Hill, MA) -0.27%
wt/wt Pd/Mg). The magnesium was washed with acetone
prior to use in order to remove any adsorbed organic
compounds, then stored in an airtight container. This
mix was then diluted to 2 ml such that the ®nal solution
was 50 ppm DDT in 50/50 percent -v/v) acetone/water.
Finally, 1 drop of glacial acetic acid was added. The vial
was then shaken vigorously for several seconds and
vented to allow for the escape of hydrogen gas. Depo-
sition of the palladium onto the magnesium ensued
spontaneously after addition of the aqueous component.
After 10 min, the dechlorination products were extracted
into 1 ml of hexane and immediately analysed by GC-
FID, GC-ECD and GC±MS.
Mgꢁ0† ! Mg2‡ ‡ 2eÀ; E0 ˆ À2:2 V vs S:H:E:
ꢁ2†
Feꢁ0† ! Fe2‡ ‡ 2eÀ; E0 ˆ À0:44 V vs S:H:E:
ꢁ3†
However, the key advantage palladized magnesium
holds over iron, and palladized iron, is that dechlori-
nation by Mg/Pd takes place in the presence of oxygen
and does not require any surface pretreatment. In all
instances of successful dechlorination by zero-valent or
palladized iron, reactions require anaerobic conditions
and careful surface pretreatment with acid -Grittini et
al., 1995; Muftikian et al., 1995; Sayles et al., 1997;
Doyle et al., 1998; Fennelly and Roberts, 1998). Such
advantage of Mg-0) over Fe-0) may be due the relative
2.2. Sample analysis
FID analyses were performed on a Hewlett Packard
5890 gas chromatograph equipped with an Alltech EC-5
capillary column of 0.320 mm ID, 0.25 lm ®lm thickness
and 30 m length. ECD analyses were performed on a
separate Hewlett Packard 5890 gas chromatograph
equipped with a Alltech AT-5 capillary column also of
0.320 mm ID, 0.25 lm ®lm thickness and 30 m length.
Injection was made in splitless mode with a 30 s purge
delay for both instruments. Carrier gas and make-up gas
were helium for FID, and nitrogen for ECD at ¯ow rates
of 3.0 and 30 ml/min -carrier/make-up) for both. The
following temperature program was used for the chro-
matograms in Figs. 1-a)±2-b): the initial temperature of
100°Cwas held for 3 min, then ramped at 10 °C/min up
to the ®nal temperature of 275°Cfor 9.5 min. Mass
spectrometry was performed on a Hewlett Packard 6890
solubility of its hydroxide -pKsp MgꢁOH† ˆ 11:15) vs
2
that of ironÕs -pKsp FeꢁOH† ˆ 15:1). The increased
2
hydroxide solubility may decrease the tendency towards
passivation thus increasing metal reactivity.
Uncatalyzed zero-valent iron alone has been dem-
onstrated to dechlorinate DDT, however, palladized
magnesium is vastly superior with regard to reaction
time requiring 10 min compared to several hours or days
for the former -Sayles et al., 1997). On a semipreparative
scale, both iron and magnesium are relatively inexpen-
sive and comparable in cost. Furthermore, ®eld-scale
handling of magnesium will prove more economical
since aerobic degradation of Mg/Pd is not an issue as it
is with Fe-0) or Fe/Pd.
gas chromatograph equipped with
a JOEL JMS-
In a previous investigation, Cheng et al. -1997) pro-
posed a detailed mechanism to explain the role of the
palladium in the hydrodehalogenation process. It is
based on the ability of palladium to intercalate hydrogen,
which in turn acts as a powerful reducing agent at the
palladium/zero-valent metal interface, where chlorinated
organic compounds are likely adsorbed. This model in-
cludes the presence of protons in solution and therefore
requires aqueous or mixed organic/aqueous solvents. In
AX505HA mass spectrometer and a 30 m HP-5 capillary
column -320 lm ID, 0.25 lm ®lm). The helium carrier
gas ¯ow rate was set at 3.0 ml/min. The injection port
was utilized in splitless mode with a 45 s purge delay and
maintained at 270°C. The temperature program ramped
from 125±275°Cat a rate of 5 °C/min and was held for 5
min at the initial and ®nal temperatures. 70 eV electron
impact ionization was used with a source temperature of
200°C.

M.D. Engelmann et al. / Chemosphere 43 *2001) 195±198
197
Fig. 1. -a) GC-FID of 330 ng DDT with 160 ng internal standard -area 48,750). 4-chlorobiphenyl -BZ#3) was used as the internal
standard for all chromatograms; -b) GC-FID of 330 ng DDT after dechlorination by the Mg/Pd bimetallic system with 160 ng I.S.
-area 48,372). Peaks A±Cwere identi®ed by GC±MS as diphenyl methane -A); 1,1-diphenylethane -B); and tetraphenylbutane
-C, speculative).
3. Results and discussion
identi®ed by mass spectrometry. All three of the major
products detected display base peaks at m/z 167 of the
electron impact mass spectra. These correspond to
diphenylmethane, which for peak A -Fig. 1-b)) is also
the molecular ion. Comparison of the peak A mass
spectrum with that of diphenylmethane from the NIST
database bears a near identical resemblance. The pres-
ence of the molecular ion at m/z 182 in the peak B mass
spectrum led to its identi®cation as diphenylethane, the
hydrocarbon backbone of DDT. This is the major
product of DDT dechlorination. It is speculated that
peak Cis tetraphenylbutane due in part to strong sim-
ilarities between its mass spectrum and that of diph-
enylmethane, and also to the longer chromatographic
retention time. Although a corresponding molecular ion
peak at m/z 362, the molecular weight of tetra-
Figs. 1-a)±2-b) illustrate the eciency of this system.
A 100 lg sample -2 ml at 50 ppm) of DDT is completely
dechlorinated by the Mg/Pd bimetalic within the detec-
tion limit of GC-ECD -ꢀ10 pg). The ECD chromato-
gram in Fig. 2-b) clearly shows not only the complete
destruction of DDT but also the absence of any partially
dechlorinated by-products. Multiple samples were ana-
lysed with similar results. A control solution consisting
of the magnesium metal alone proved to be ine€ective ±
no decrease in DDT nor appearance of product was
observed over several hours. A detailed mass balance
was not possible without the existence of dechlorination
product standards. The dechlorination products are,
however, easily detected by GC-FID -Fig. 1-b)), and

198
M.D. Engelmann et al. / Chemosphere 43 *2001) 195±198
strated case in which dechlorination does not yield toxic
intermediates, i.e. DDD -1,1-dichloro±2,20-bis--p-chlor-
ophenyl)ethane), and DDE -2,20-bis-p-chlorophenyl)±
1,10-dichloroethylene) under such mild conditions. Fur-
thermore, the process is able to complete the dechlori-
nation reaction under room temperature and pressure
conditions without attention to aerobic contamination.
The Pd/Mg bimetallic system may form the basis of an
economical alternative to present methods of DDT de-
struction.
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Fig. 2. -a) GC-ECD of 3.3 ng DDT with 1.6 ng I.S. -tR 9.6 min,
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