Rate enhancement by cations in supercritical water oxidation of 2- chlorophenol

Article abstract of DOI:10.1021/es981316y

Interactions between cations and Cl species in the supercritical water oxidation (SCWO) of 2.-chlorophenol (2CP) has been investigated in a high- pressure quartz-lined reactor at 673-773 K. Experimentally, we found that the S/D ratio (defined as the amount of 2CP converted to CO₂ and H₂O/disappearance of 2CP) for SCWO of 2CP was enhanced by cations (such as Li⁺, Na⁺, K⁺, Ca²⁺, Fe²⁺, or Fe³⁺). The S/D ratio increased as the charge density of cations increased (Fe³⁺> Fe²⁺ > Li⁺ > Ca²⁺ > Na⁺ > K⁺). Due to the extremely low solubility of metal salts in the supercritical water, abstraction of Cl in 2CP via an intermediate ((OH)PhCl(δ^-)---- M(δ⁺) was postulated. Formation of these metal chloride (such as KCl, CaCl₂, and FeCl₃) precipitates in the SCWO of 2CP were identified by X-ray diffraction (XRD) spectroscopy. Since the formation of toxic highly chlorinated phenols and heavy polycyclic aromatic hydrocarbons (PAHs) was notably reduced, abstraction of Cl of 2CP by cations may occur in the early stage of the SCWO process. Interactions between cations and Cl species in the supercritical water oxidation (SCWO) of 2-chlorophenol (2CP) has been investigated in a high-pressure quartz-lined reactor at 673-773 K. Experimentally, we found that the S/D ratio (defined as the amount of 2CP converted to CO₂ and H₂O/disappearance of 2CP) for SCWO of 2CP was enhanced by cations (such as Li⁺, Na⁺, K⁺, Ca²⁺, Fe²⁺, or Fe³⁺). The S/D ratio increased as the charge density of cations increased (Fe³⁺>Fe²⁺>Li⁺>Ca²⁺ >Na⁺>K⁺). Due to the extremely low solubility of metal salts in the supercritical water, abstraction of Cl in 2CP via an intermediate ((OH)PhCl^δ-- - - - -M^δ+) was postulated. Formation of these metal chloride (such as KCl, CaCl₂, and FeCl₃) precipitates in the SCWO of 2CP were identified by X-ray diffraction (XRD) spectroscopy. Since the formation of toxic highly chlorinated phenols and heavy polycyclic aromatic hydrocarbons (PAHs) was notably reduced, abstraction of Cl of 2CP by cations may occur in the early stage of the SCWO process.

Full text of DOI:10.1021/es981316y

Research Communications
Maximum DRE as well as minimum formation of un-
desired byproducts in the disposal process of toxic com-
pounds are also of increasing interest. It is also very important
from an environmental viewpoint, especially when one
considers that incineration of wastes can produce carcino-
genic byproducts such as PAHs and chlorinated dioxins or
furans (15, 16). In a separate experiment, we have found that
a highly enhanced oxidation of 2CP was effected in the
presence of Na or Li in the supercritical water (17).
Therefore, the main objectives of the present work were to
investigate the abstraction of Cl of 2CP by metal cations with
different charge densities in the SCWO of 2CP. In addition,
the reduction of trace toxic byproducts (higher chlorinated
phenols and PAHs) formed in the SCWO of 2CP with cations
was also studied.
Rate Enhancement by Cations in
Supercritical Water Oxidation of
2
-Chlorophenol
K U E N - S O N G L I N A N D H . P A U L W A N G *
+
+
Department of Environmental Engineering, National Cheng
Kung University, Tainan, 70101 Taiwan, Republic of China
Interactions between cations and Cl species in the
supercritical water oxidation (SCWO) of 2-chlorophenol
(
2CP) has been investigated in a high-pressure quartz-lined
Experimental Section
reactor at 673-773 K. Experimentally, we found that the
S/D ratio (defined as the amount of 2CP converted to CO2
and H2O/disappearance of 2CP) for SCWO of 2CP was
The SCWO experiments of 2CP (Merck, purity >98%) were
conducted in a high-pressure quartz-lined batch reactor at
6
73-773 K with residence times of 0.5-5 min. The system
pressure was controlled by a back-pressure regulator (Tescom,
max ) 408 atm) and a pressure regulator (Tescom, Pin ) 238
enhanced by cations (such as Li , Na , K , Ca , Fe2+, or
+
+
+
2+
3+
Fe ). The S/D ratio increased as the charge density of
P
3+
2+
+
2+
+
+
cations increased (Fe > Fe > Li > Ca > Na > K ).
Due to the extremely low solubility of metal salts in the
supercritical water, abstraction of Cl in 2CP via an intermediate
atm; Pout ) 7 atm). A safety rupture disk rated at 400 atm was
installed. Concentrations of 2CP in the SCWO experiments
were between 189 and 1500 mg/ L. Hydrogen peroxide (Merck,
30 wt %) was used as the oxidant (O/ C ratio ) 1.05-1.95) in
the SCWO experiments.
δ-
δ+
((OH)PhCl - - - - -M ) was postulated. Formation of
these metal chloride (such as KCl, CaCl2, and FeCl3) precipitates
in the SCWO of 2CP were identified by X-ray diffraction
High purity oxalates (Merck, purity >99%) were used in
the SCWO of 2CP as cations sources (equivalent mole ratios
(XRD) spectroscopy. Since the formation of toxic highly
(
[M]/ [Cl]) were 1.2-1.3) for the abstraction of Cl of 2CP in
chlorinated phenols and heavy polycyclic aromatic
hydrocarbons (PAHs) was notably reduced, abstraction of
Cl of 2CP by cations may occur in the early stage of the
SCWO process.
2
-
the SCWO process. The counteranions (C
2
O
4
) were de-
composed in the SCWO process. Salt precipitates formed in
the SCWO process were determined by X-ray powder
diffraction spectroscopy (RIGAKU model D/ MAX III-V).
Trace PAHs (extracted with a dichloromethane (Merck,
purity >99%) solvent) formed in the SCWO of 2CP were
determined quantitatively by GC/ MSD (HP 5890A and 5972).
A HP Ultra 2 capillary column was heated programmably up
to 563 K to obtain a resolvable separation of PAH species.
Masses of primary and secondary ions of PAHs were
determined using the scan mode for PAH standards (Mix
Introduction
2
-Chlorophenol is a very toxic and poorly biodegradable
pollutant (1-3). Oxidation of 2CP in supercritical water is of
practical interest since a wastewater stream containing 2CP
over 200 ppm may not be treated effectively by direct
biological methods (2-5). Water above its critical point (647.3
K and 217.6 atm) becomes completely miscible with oxygen
and most nonpolar organic compounds, and the oxidation
is allowed to proceed rapidly by an elimination of the potential
interface mass transport limitations (3, 6, 7). Thus, high
carbon conversion levels can be obtained with a very short
residence time in the SCWO process. The desired destruction
and removal efficiency (DRE) may be achieved in seconds
or minutes (8-10). In addition, with increased reaction
temperatures, solubility of inorganic salts is decreased
dramatically in supercritical water (11, 12).
Solubility of metal chlorides has been reported to decrease
from its ambient solubility (298 K, 1 atm) of about 10% to
3
of 10 -fold (13, 14). Supercritical water, as compared to liquid
water, exhibits an abrupt decrease in the static dielectric
constant (ꢀ) from 79.11 (liquid water, 298 K) to 1.83
6
10-M (Supelco, purity >99%) and PNA-550JM (Chem
Service, purity >99%)) and samples. Analyses of trace
byproducts of the SCWO of 2CP were also conducted by
HPLC (spectra system, SP) with a 3-D UV detector (model
UV-3000). PAHs were separated by a Spherisorb S5 PAH 5
µm column (150 mm × 4.6 mm) with a mixed acetonitrile/
water mobile phase. Chlorinated phenols were analyzed by
an Envirosep-PP column (125 mm × 3.2 mm) with a mixed
methanol/ water (both with 1% acetic acid) mobile phase. A
variable wavelength program was used in the system software
(
model PC-1000) to optimize detector sensitivity and selec-
tivity.
Results and Discussion
Figure 1a shows that oxidation of 2CP (represented by S/ D
-120 ppm in supercritical water (773 K, 250 atm), a decrease
4
+
+
+
ratio) is highly enhanced by cations (such as Li , Na , K ,
2
+
2+
3+
Ca , Fe , or Fe ) in supercritical water at 673 K with a
residence time of 1 min. The observed high S/ D ratios for the
SCWO of 2CP with cations would suggest perturbation of the
(
supercritical region, 673 K) (3, 6, 7). Ionic dissociation
-
22
mol/ kg2
constant (K
at 773 K and 247 atm (6, 7).
w
) of supercritical water is less than 10
2
CP ring by cations that enhanced the oxidation of 2CP in
supercritical water. Possibly the interaction of Cl of 2CP with
cations was the major distinguishing factor in increases of
S/ D ratios for the SCWO of 2CP with cations. The S/ D ratio
*
Corresponding author fax: 011-8866-275-2790; e-mail: wanghp@
mail.ncku.edu.tw.
3
2 7 8
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 18, 1999
10.1021/es981316y CCC: $18.00

1999 Am erican Chem ical Society
Published on Web 08/13/1999

FIGURE 1. Effect of charge-density of cations on (a) the S/D ratio
and the relative concentrations (RC) of (b) PAHs and (c) chlorophenols
formed in the SCWO of 2CP at 673 K for 1 min. S/D ) (amount of
2
CP converted to CO
2
and H
2
O)/(disappearance of 2CP); RCPAHs
)
)
concnPAHs (with cations)/concnPAHs (without cation); RCchlorophenols
concnchlorophenols (with cations)/concnchlorophenols (without cation).
for the SCWO of 2CP is shown as a function of charge-density
of cations in Figure 1a. The reaction rates for the SCWO of
+
2+
3+
2
CP with K , Ca , and Fe cations can be expressed as
0
.79
0.41
0.44
follows: 109 exp(-11.4/ RT)[2CP] [O
2
]
[H
O] , and 82 exp(-9.0/ RT)-
O] , respectively (rate ) A exp(-E
2
O] , 94.5 exp-
0
.85
0.52
0.48
(
[
-10.8/ RT)[2CP] [O
2
0.58
]
[H
2
1
.05
0.55
2CP] [O
2
]
[H
2
a
/
a
]b[H
c
RT)[2CP] [O
2
2 a
O] and E is in kcal/ mol). Because of the
rapid decrease in the static dielectric constant of water and
the solubility of inorganic salts at high temperatures and
pressures in supercritical water, one may expect an increase
in the attractive forces between anions and cations (17-19).
Most interestingly, for the higher charge density cation (e.g.,
3
+
Fe ), the S/ D ratio was very high, whereas for the low charge
density cations a reduced S/ D ratio was found, assuming
approximately equal and low solubility in the SCWO process.
Due to the extremely low solubility of metal salts in the
supercritical water, the metal salt species may be precipitated
in the early stage of the SCWO process. The precipitated
2 3
metal salts identified as, for instance, KCl, CaCl , and FeCl
species by X-ray diffraction spectroscopy are evidently shown
in Figure 2.
3 2
FIGURE 2. X-ray diffraction patterns of (a) FeCl , (b) CaCl , and (c)
To obtain detectable quantities of incomplete combustion
compounds or byproducts in the SCWO experiments, it is
necessary to operate at conditions that do not provide a good
DRE. We intentionally conducted the SCWO experiments at
temperatures somewhat lower than 773 K so that the
incomplete combustion products or byproducts in the SCWO
of 2CP could be detected. As expected in Table 1, formation
of trace byproducts (higher chlorinated phenols) in the SCWO
of 2CP was substantially decreased in the presence of cations
KCl precipitates in the SCWO of 2CP with cations. An asterisk (*)
denotes the XRD characteristic peaks of each metal chloride.
and pentachlorophenol (PCP) were not formed in the
presence of metal cations in the SCWO of 2CP. Although the
mechanism of the formation of higher chlorinated phenols
during SCWO of 2CP is not well-known, the fact that an
extraction of Cl species from 2CP is significantly effected in
supercritical water oxidation condition with higher charge
density cations (see Figure 1). Most importantly, due to the
suppressed formation of higher chlorinated phenol byprod-
ucts (Table 1), it is apparent that abstraction of Cl of 2CP by
cations may occur in the early stage of oxidation of 2CP in
supercritical water.
+
2+
3+
(
for instance, K , Ca , or Fe ). Abstraction of chlorine species
from 2CP by a metal cation may be an important step
regarding the formation of the trace byproducts. Table 1
indicates that increasing the charge density of the cations in
SCWO of 2CP may suppress the reinsertion of Cl species into
2
CP that forms highly chlorinated phenols. 2,4-Dichlorophe-
The reduction of highly chlorinated phenols byproducts
as well as the enhancement of the S/ D ratio for SCWO of 2CP
nol (2,4-DCP), 2,4,5-trichlorophenol (2,4,5-TCP), and 2,4,6-
trichlorophenol (2,4,6-TCP) were the major high chlorinated
phenols formed in the SCWO of 2CP at 673 K. These trace
byproducts may be formed directly from Cl reinsertion. Large
molecules such as 2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP)
+
2+
3+
with K , Ca , or Fe cations may be due to at least two
possibilities: (a) The solubility of KCl, CaCl , and FeCl
2
3
abruptly decreased, and extremely fast formation of metal
chloride precipitates occurred in supercritical water. (b) The
VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 2 7 9

XRD spectroscopy. Since the formation of toxic highly
chlorinated phenols and heavy PAHs was notably reduced,
abstraction of Cl of 2CP by cations may occur in the early
stage of the SCWO process.
TABLE 1. Formation of Trace Byproducts in the SCWO of 2CP
with Metal Cations (0.02 N) at 673 K for 1 min
concentration of byproducts
(µg/g 2CP)
none
K+ Ca2+
Fe3+
Acknowledgments
Chlorophenols
2590 239.1 134.2 48.9
2
2
2
2
,4-dichlorophenol
,4.5-trichlorophenol
,4.6-trichlorophenol
The financial support of the National Science Council,
Taiwan, R.O.C. (NSC 87-TCP-E-006-005, NSC-86-2113-M-
006-020, and NSC-85-2621-P-006-003) is gratefully acknowl-
edged.
5740
7060
184.7
202.6
47.3
ND
73.5 27.5
95.5 36.7
5.4
NDa
,3,4,6-tetrachlorophenol 2100
pentachlorophenol
160
PAHs
25.6
ND
ND
naphthalene
10.4
5.1
1.9
1.0
1.2
1.4
0.2
ND
0.1
0.1
0.1
ND
ND
ND
ND
ND
7.9
2.6
1.0
1.2
0.7
0.2
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
3.6
0.9
0.4
acenaphthylene
acenaphthene
fluorene
phenanthrene
anthracene
fluoranthene
pyrene
benz[a]anthracene
chrysene
benzo[b]fluoranthene
benzo[k]fluoranthene
benzo[a]pyrene
indeno[1,2,3-cd]pyrene
dibenz[a,h]anthracene
benzo[ghi]perylene
6.7
2.5
3.3
4.0
3.2
1.2
0.8
1.8
0.3
0.2
ND
0.5
0.2
ND
ND
Literature Cited
0.1
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
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(
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(
(
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a
ND denotes not detectable.
SCHEME 1a
7) Shaw, R. W.; Brill, T. B.; Clifford, A. A.; Eckert, C. A.; Franck, E.
U. Chem. Eng. News 1991, 69, 26.
(
(
8) Koo, M.; Lee, W. K.; Lee, C. H. Chem. Eng. Sci. 1997, 52, 1201.
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1
995, 29, 2748.
a
Mn+ denotes K , Ca2+, or Fe3+ cation. Note that >99% of Cl was
+
abstracted by cations and form ed m etal chloride precipitates in SCWO
of 2CP.
(
(
10) Martino, C. J.; Savage, P. E. Ind. Eng. Chem. Res. 1997, 31, 1385.
11) Cui, S. T.; Harris, J. G. J. Phys. Chem. 1995, 99, 2900.
electron-withdrawing effect of cations was involved in which
an intermediate species (I) was postulated. A possible reaction
pathway for SCWO of 2CP with K , Ca , or Fe cations at
(12) Armellini, F. J.; Tester, J. W. Fluid Phase Equilib. 1993, 84, 123.
+
2+
3+
(
(
13) Jasmund, K. Heidelb. Beitr. Mineral. Petrogr. 1952, 3, 380.
6
73 K is shown in Scheme 1.
Frenklach indicated that the formation of PAHs increases
14) Martynova, O. I. In High Temperature, High-Pressure Electro-
chemistry in Aqueous Solutions; Jones, D. de G.; Staehle, R. W.,
Eds.; National Association of Corrosion Engineers: Houston,
TX, 1976.
with an increase of Cl/ H ratio in the combustion of
chlorinated hydrocarbons (20). Chlorine atoms may acceler-
ate the abstraction of aromatic H from PAH molecules that
are activated for further mass growth of PAHs to form
eventually soot (20). Experimentally, because of the abstrac-
tion of Cl by cations that reduced the availability of free Cl
species in the SCWO of 2CP, the formation of PAHs was
considerably reduced (see Figure 1b). In Table 1, only trace
low molecular weight PAHs such as naphthalene (Nap),
fluorene (Flu), and anthracene (Ant) were observed in the
SCWO of 2CP with cations. In the presence of high charge-
(15) Bjørseth, A. Handbook of Polycyclic Aromatic Hydrocarbons, 2nd
ed.; Marcel Dekker: New York, 1983.
(
(
16) Katritzky, A. R.; Ignatchenko, E. S.; Allin, S. M.; Siskin, M.; Hudson,
C. W. Energy Fuels 1997, 11, 160.
17) (a) Lin, K.-S.; Wang, H. P.; Li, M. C. Chemosphere 1998, 36, 2075.
(
1
b) Lin, K.-S.; Wang, H. P.; Yang, Y. W. Chem. Lett. 1998, 12,
203. (c) Lin, K.-S.; Wang, H. P.; Yang, Y. W. Chemosphere (in
2
+
3+
density cations such as Ca and Fe , carcinogenic PAHs
including benzo[a]pyrene (BaP), indeno[1,2,3-cd]pyrene
press). (d) Lin, K.-S.; Wang, H. P. Langmuir (in press). (e) Lin,
K.-S.; Wang, H. Paul Appl. Catal. B: Environ (in press).
(
(
IND), dibenz[a,h]anthracene (DBA), and benzo[ghi]perylene
BghiP) were essentially not found in the SCWO of 2CP. Note
(
(
18) Ikushima, Y.; Hatakeda, K.; Arai, M. J. Phys. Chem. B 1998, 102,
that reactions of the ring opening of 2CP in which products
were ultimately oxidized to CO and H O (see Scheme 1)
3
029.
2
2
were predominant in the overall reaction network.
In conclusion, we found that the S/ D ratio for SCWO of
19) Biswas, R.; Bagchi, B. Chem. Phys. Lett. 1998, 290, 223.
2
CP was enhanced by cations experimentally. The S/ D ratio
(20) Frenklach, M. Combust. Sci. Technol. 1990, 74, 283.
increased as the charge density of cations increased. Due to
the extremely low solubility of metal salts in the supercritical
water, abstraction of Cl in 2CP via an intermediate ((OH)-
PhCl - - - - -M ) was postulated. Formation of these metal
chloride precipitates in the SCWO of 2CP was identified by
Received for review December 17, 1998. Revised manuscript
received July 7, 1999. Accepted July 20, 1999.
δ-
δ+
ES981316Y
3
2 8 0
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 18, 1999