Materials Transactions, Vol. 55, No. 4 (2014) pp. 708 to 712
© 2014 The Japan Institute of Metals and Materials
Degradation of Chlorinated Organic Compounds by Mixed Particles
of Iron/Iron Sulfide or Iron/Iron Disulfide
Masahiro Shiba1,+1, Md. Azhar Uddin1, Yoshiei Kato1,+2 and Tomoshige Ono2
1
Department of Material and Energy Science, Graduate School of Environmental and Life Science,
Okayama University, Okayama 700-8530, Japan
2
Steel Research Laboratory, JFE Steel Corporation, Chiba 260-0835, Japan
Kinetic study was carried out in order to clear a mixing effect of Fe­FeS or Fe­FeS2 particles on increase in degradation rate of chlorinated
organic compounds. Trichloroethylene (TCE) contained solution was used for dechlorination in 30 mL of vial bottle and mixed by rotary device.
Estimating that TCE concentration in solution was proportional to TCE gas concentration in head space of vial bottle, gas was obtained from the
head space with syringe and measured by GC-FID. TCE degradation occurred on Fe, FeS, Fe­FeS and Fe­FeS2 particles except FeS2. TCE
degradation of mixed particles of Fe­FeS or Fe­FeS2 was explained by anode (Fe)­cathode (FeS or FeS2) reaction. TCE degradation rates of Fe­
FeS and Fe­FeS2 caused by anode/cathode reaction were 2.81 © 10¹6 and 1.37 © 10¹5 (m/h), respectively, which were larger than those of pure
Fe and FeS. There was no difference in TCE degradation rate of mixed particles of Fe­FeS or Fe­FeS2 between aerobic and anaerobic solutions.
TCE degradation rate of mixed particles of Fe­FeS or Fe­FeS2 increased in decrease in size of FeS or FeS2. TCE degradation rate of mixed
particles of Fe­FeS or Fe­FeS2 increased with increase in decomposition temperature. [doi:10.2320/matertrans.M2013412]
(Received November 11, 2013; Accepted February 7, 2014; Published March 14, 2014)
Keywords: iron, zero-valent iron, iron sulfide, iron disulfide, chlorinated organic compounds, volatile organic compounds, trichloroethylene,
dechlorination, cathode, anode, contaminated soil, groundwater
1.
Introduction
Soil or groundwater remediation with zero-valent iron is
one of the most effective techniques for chlorinated organic
compounds contamination in terms of shorter processing
period and lower treatment cost. There are two main types
of methods, namely permeable granular iron walls1,2) for
groundwater and mixture of soil and granular iron.3)
Reductive degradation of chlorinated organic compounds
with zero-valent iron was firstly studied by Senzaki et al.4­6)
They showed that degradation of tetrachloroethylene or
trichloroethylene (TCE) was controlled by many factors
such as electric conductivity, dissolved oxygen content, iron
surface condition, etc., and obtained that its degradation
efficiency became higher when part of iron surface was
nickel- or copper-plated. It was estimated to be caused by
local cell reaction between iron (anode) and plated layer
(cathode).
Expanding upon the previous studies by Senzaki et al.,4­6)
Nakamaru et al.7) showed TCE degradation rate increased
with sulfur-contained iron of which surface iron sulfide of
electric conducting property was precipitated as the second
layer (cathode). Schematic view of the TCE degradation
mechanism is shown in Fig. 1. Electron of Fe anode is
transferred to FeS cathode and RCl is decomposed on the FeS
cathode. Overall TCE degradation, Fe dissolution at anode
and dechlorination of TCE at cathode are expressed by
eqs. (1), (2) and (3), respectively.
2þ
Fe þ RCl þ H2 O ! Fe þ RH þ OH þ Cl
Fe ! Fe2þ þ 2e
RCl þ H2 O þ 2e ! RH þ OH þ Cl
+1
ð1Þ
ð2Þ
ð3Þ
Graduate Student, Okayama University, Present address: Miura Co., Ltd.,
Matsuyama 799-2696, Japan
+2
Corresponding author, E-mail: [email protected]
RH
2+
OH- Fe
ClH2O
RCl
Fe
Anode
FeS
Cathode
Fig. 1 Schematic view of TCE degradation by sulfur containing iron
particle.
However, highly sulfur-contained iron which they experimented as ½S ; 0:1 mass% is generally difficult to
produce by normal steelmaking process. In this study,
mixture of Fe­FeS Fe­FeS2 was used instead of high
sulfur-contained iron because it enabled us to change sulfur
content widely, and effect of its mixing ratio on TCE
degradation rate was made clear in batch experiment.
2.
Experimental Procedure
Figure 2 shows schematic view of experimental apparatus.
Rotator (AS ONE, VMRC-5) was used to stir 30 mL of vial
bottle horizontally. 100 mg of mixed powder of iron (Fe)/iron
sulfide (FeS) or iron/iron disulfide (FeS2) was prepared and
its mixing ratio was changed. Anaerobic electrolyte solution
which consisted of 40 mg/L of CaCO3, 80 mg/L of Na2SO3
and ion exchange water was usually used as a standard,
although ion-exchange water was used for aerobic condition.
1 mg/L of TCE, 10 mL of electrolyte solution and 100 mg of
Top view A
Cross-sectional view (A-A’)
252 mm
Vial bottle
65 mm
Vial
bottle
Normalized TCE concentration,
C(t)/C(0)
Degradation of Chlorinated Organic Compounds by Mixed Particles of Iron/Iron Sulfide or Iron/Iron Disulfide
TCE
solution
32.2 mm
31 mm
Rotary
device
Iron+Iron
sulfide powder
32.2 mm
Rotary device
1
0.9
Fe
FeS
FeS2
70Fe-30FeS
0.6
0.5
Particle size and specific surface areas of Fe, FeS and FeS2.
Particle size
(µm)
Specific surface area
(m2/g)
100­150
0.17
53­100
1.25
100­150
0.54
53­100
0.21
100­150
0.12
the mixed powder were added into 30 mL of vial bottle and
rotated with 100 rpm. The above reagents were made by
Wako Pure Chemical Industries, Ltd. Standard experimental
temperature was 298 K and it was changed to 308 and 323 K.
Particle size range and specific surface area of Fe, FeS and
FeS2 powder measured by nitrogen adsorption method (BET)
are shown in Table 1. The particle size range was 100­
150 µm as a standard and that of 53­100 µm was added for
FeS and FeS2. It was found that the specific surface areas of
FeS and FeS2 are 3 and 0.7 times larger than that of Fe,
respectively.
Estimating that TCE concentration in liquid was proportional to TCE gas concentration in head space of vial bottle,7)
gas was obtained from the head space with syringe and
measured by GC-FID (Shimadzu, G-8A). The multiple
samples were rotated under the same condition and drawn
one by one at a given measurement time.
Results and Discussions
3.1 Degradation behavior of TCE
Figure 3 shows some typical examples of temporal change
in TCE concentration in liquid. 90Fe­10FeS in the figure
means 90 mass% Fe and 10 mass% FeS. It was carried out at
temperature of 298 K, anaerobic condition and particle size of
100­150 µm. Semilog plots of TCE concentration decreased
linearly and that was also recognized under the other
conditions. Therefore, TCE degradation rate is expressed
by the following first-order reaction equation.7)
dCðtÞ=dt ¼ KCðtÞ
ð4Þ
where C(t) is TCE concentration in liquid (mass%), t is time
(h) and K is rate constant (h¹1). K is obtained from a linear
curve based on temporal change of measured C(t).
20
40
60
Time, t / h
80
100
Fig. 3 Typical examples of time dependence of TCE degradation in water.
1
0.8
TCE (100Fe)
TCE
(90Fe-10FeS)
0.6
Ethylene
(90Fe-10FeS)
0.4
Ethylene (100Fe)
0.2
Ethane (100Fe)
Ethane
(90Fe-10FeS)
0
0
3.
90Fe-10FeS
0.7
0
Volume fraction of gas (-)
Table 1
100Fe
0.8
A’
Fig. 2 Schematic view of experimental apparatus for TCE degradation in
water.
709
50
100
150
200
Time, t /h
Fig. 4 Typical examples of temporal change in TCE and its degradation
products in gas chromatograph.
Gas by-product was identified by GC-FID during TCE
degradation practice with 100Fe or 90Fe­10FeS. Temporal
change in product and TCE concentrations in head space
was shown in Fig. 4. Ethylene was mainly detected and the
other was a small amount of ethane. They increased with a
decrease in TCE concentration.
Effect of mixing ratio of Fe­FeS or Fe­FeS2,
anaerobic or anaerobic treatment, and specific
interfacial area on TCE degradation rate
Figure 5 shows relation between rate constant of TCE
degradation, K and mass% of FeS or FeS2. Every particle size
was 100­150 µm and experimental temperature was 298 K. It
was found that TCE degradation rate increased with mixing
of FeS or FeS2 to Fe and it had a peak value at 30Fe/70FeS
or 70Fe/30FeS2. The peak sulfur ratio, ms (mass%) (= 0.26)
in 30Fe/70FeS approached to ms (mass%) (= 0.16) in 70Fe/
30FeS2, which was not known exactly why. Although
TCE degradation of Fe or FeS occurred slightly, FeS2 did
not decomposed TCE. TCE degradation rate of Fe­FeS
was larger than that of Fe­FeS2, which will be discussed
in 3.3. Compared between aerobic and anaerobic conditions,
anaerobic TCE degradation rate with 100Fe was larger than
aerobic one, although the difference of TCE degradation rate
between aerobic and anaerobic solutions was not recognized
for mixed powder and 100FeS. According to an increase
in hydroxyl ion under an aerobic liquid atmosphere, Fe2+
changes to Fe(OH)2 and adheres to the surface of iron,8,9)
whereas sulfur in FeS or FeS2 consumes oxygen in aerobic
3.2
710
M. Shiba, M. A. Uddin, Y. Kato and T. Ono
0.025
Particle size : 100-150μm
Rate constant, K/h-1
Rate constant , K/ h-1
0.01
0.008
Fe-FeS
0.006
Fe-FeS2
0.004
anaerobic
0.002
aerobic
FeS i size (μm)
53-100
0.02
100-150
Fe-FeS
Fe-FeS 2
0.015
Anaerobic
0.01
0.005
Fe-FeS
0
Fe-FeS 2
0
0
0
20
40
60
80
20
40
60
80
100
Ratio of FeS or FeS2 (%)
100
Ratio of FeS or FeS2 (%)
Fig. 5 Relation between TCE degradation rate and mass% of iron sulfide
or iron disulfide.
Fig. 6 Relation between TCE degradation rate and mass% of iron sulfide
or iron disulfide.
(a)Excess Fe
(b) Excess FeS(FeS2)
Fe2+
Fe 2+
e
Fe
Fe
FeS
(FeS2)
Cathode
Anode
Anode
RCl
e
FeS
(FeS2)
Cathode
RCl
Fe2+
H2 O
Fe
Anode
e
H2 O
Fe2+
RH
OHCl-
RH
OHCl-
Fe2+
RH
OHCl-
H2O
FeS
(FeS2)
Cathode
e
Fe2+
Fe
Anode
e
H2 O
RCl
H2 O
RH
OHCl-
RH
OHCl-
RCl
Fig. 7
Schematic view of TCE degradation by mixture of Fe­FeS or Fe­FeS2.
water.10) As mixed powder or FeS changed aerobic liquid
atmosphere to anaerobic one rapidly, the TCE degradation
rate remained constant.
Figure 6 shows relation between K and mass% of FeS or
FeS2. Particle size was varied to 100­150 and 53­100 µm
and experimental temperature was kept to 298 K under
anaerobic condition. TCE degradation rate of particle size
of 53­100 µm was larger than that of 100­150 µm for Fe­
FeS and Fe­FeS2 system. A smaller particle size led to an
increase in specific surface area and interfacial area, and it
resulted in larger TCE degradation rate. However, mixing
ratio of FeS or FeS2 which showed the peak TCE degradation
rate was kept to the same values in spite of the different
particle size.
According to Fig. 1,7) TCE degradation occurs by the local
cell action between anode (Fe) and cathode (FeS or FeS2).
Thus, the peak values of TCE degradation rate in Fe­FeS or
Fe­FeS2 system shown in Figs. 5 and 6 were obtained from
the most effective anode/cathode reaction between Fe­FeSi
(i = FeS or FeS2) particles, which means no excess Fe or
FeSi particle existed. Schematic view of TCE degradation
mechanism in Fe­FeS or Fe­FeS2 is shown in Fig. 7. When
Fe is excess as shown in Fig. 7(a), TCE decomposes weakly
on the surface of the surplus Fe in addition to the anode/
cathode reaction between Fe and FeS(FeS2) between Fe­FeSi
particles. When excess FeS or FeS2 exists, the surplus FeS or
FeS2 does not contribute to the anode/cathode reaction as
Fig. 7(b). The most effective TCE degradation occurs in case
of no excess Fe nor FeS(FeS2), which corresponds to Fe/
FeS = 30/70 and Fe/FeS2 = 70/30 in Fig. 5. TCE degradation rates for Figs. 7(a) and 7(b) will be discussed in the
following chapter. Furthermore, Cl¹ and OH¹ in solution
were identified during the practice and both of them increased
with an increase in TCE degradation.
3.3
Effect of excess Fe or FeS(FeS2) on TCE degradation
rate
In case of more than 30Fe in Fe­FeS and 70Fe in Fe­FeS2
system, TCE degradation caused by only Fe occurs in addition to anode/cathode reaction between Fe­FeSi particles.
Then, TCE degradation rate presented by eq. (4) is given by
eq. (5).
W 0 Fe ¼ WFe ½ðmass% FeÞ=ðmass% FeSi Þpeak WFeSi
ð8Þ
where complete anode/cathode reaction between Fe­FeSi
particles occurs at [(mass% Fe)/(mass% FeSi)]peak = 30/70
in Fe­FeS and 70/30 in Fe­FeS2 system.
On the other hand, in case of less than 30Fe in Fe­FeS and
70Fe in Fe­FeS2 system, TCE degradation caused by only
FeS or FeS2 occurs in addition to anode/cathode reaction
between Fe­FeSi particles and its rate is given by eq. (9).
dCðtÞ=dt ¼ KCðtÞ
¼ ½ðkFe­FeSi a00 FeSi þ kFeSi a000 FeSi Þ=V CðtÞ
ð9Þ
a00 FeSi ¼ WFe ½ðmass% FeSi Þ=ðmass% FeÞpeak £ FeSi ð10Þ
ð11Þ
a000 FeSi ¼ W 000 FeSi £ FeSi
where aAAFe is interfacial area of FeSi contributing to anode/
cathode reaction between Fe­FeSi particles (m2), kFeSi is rate
constant of TCE degradation by FeS or FeS2 with no
involvement in anode/cathode reaction between Fe­FeSi
particles (m/h), aAAAFeSi is surface area of FeSi with no
involvement in anode/cathode reaction between Fe­FeSi
particles (m2), WAAAFeSi is mass of FeSi with no involvement
in anode/cathode reaction between Fe­FeSi particles (kg),
[(mass% FeSi)/(mass% Fe)]peak is mixing ratio of FeSi to Fe
when complete anode/cathode reaction between Fe­FeSi
particles occurs. W AAAFeSi is given by subtracting mass of FeSi
contributing to anode/cathode reaction between Fe­FeSi
particles from WFeSi as well as eq. (8).
W 000 FeSi ¼ WFeSi ½ðmass% FeSi =mass% FeÞpeak WFe ð12Þ
By substituting K obtained from a single Fe or FeS in
Fig. 6 into eq. (5) or (9), respectively, the following values
were achieved.
kFe ¼ KV =ðWFe £ Fe Þ
¼ ð0:0034Þð10 106 Þ=½ð0:1Þð0:1705Þ
¼ 1:99 106
ð13Þ
kFeS ¼ KV =ðWFeS £ FeS Þ
¼ ð0:00077Þð10 106 Þ=½ð0:1Þð0:5337Þ
¼ 1:44 107
ð14Þ
711
2
Fe-FeS2
1.6
1.2
FeSi size (μm)
53-100
0.8
100-150
Fe-FeS
Fe-FeS
0.4
Fe-FeS2
FeS
Fe
0
0
20
40
60
80
Ratio of FeS or FeS2 (%)
100
Fig. 8 Relation between kFe­FeS or kFe­FeS2 and mass% of iron sulfide or
iron disulfide.
8
Rate constant, K/ 10-3h-1
dCðtÞ=dt ¼ KCðtÞ
ð5Þ
¼ ½kFe­FeSi aFeSi þ kFe a0 Fe Þ=V CðtÞ
ð6Þ
aFeSi ¼ WFeSi £ FeSi
ð7Þ
a0 Fe ¼ W 0 Fe £ Fe
where kFe­FeSi is rate constant of TCE degradation by anode/
cathode reaction between Fe­FeSi particles (m/h), kFe is rate
constant of TCE degradation by Fe with no involvement in
anode/cathode reaction between Fe­FeSi particles (m/h),
aFeSi is interfacial area of FeSi contributing to anode/cathode
reaction between Fe­FeSi particles (m2), aAFe is surface
area of Fe with no involvement in anode/cathode reaction
between Fe­FeSi particles (m2), £FeSi and £Fe are specific
surface areas (m2/kg) of FeS(FeS2) and Fe, respectively,
WFeSi and WAFe are masses (kg) of FeS(FeS2) and Fe with no
involvement in anode/cathode reaction between Fe­FeSi
particles, respectively, V is water volume (m3). WAFe is given
by subtracting mass of Fe contributing to anode/cathode
reaction between Fe­FeSi particles from total amount of Fe,
WFe (kg) as shown in eq. (8).
Rate constant based on unit surface,
k Fe-FeS or k Fe-FeS2/10 -5m.h-1
Degradation of Chlorinated Organic Compounds by Mixed Particles of Iron/Iron Sulfide or Iron/Iron Disulfide
γ Fe=0.17 (m2/g)
7
γ FeS=0.34 (m2/g)
6
5
γ FeS=0.17 (m2/g)
4
3
2
γ FeS=0.085(m2/g)
1
0
0
20
40
60
Ratio of FeS (%)
80
100
Fig. 9 Relation between TCE degradation rate and specific surface area
of FeS.
Subsequently, kFe­FeS in Fe­FeS and kFe­FeS2 in Fe­FeS2
were calculated from eqs. (5)­(14). Relation between kFe­FeS
or kFe­FeS2 and mass% of FeS or FeS2 is shown in Fig. 8.The
values of kFe in eq. (13) and kFeS in eq. (14) were added in
Fig. 8. Although there was variability in FeS2 ratio = 10, 30
and 50 of FeS2 particle size in 53­100 µm, kFe­FeS or kFe­FeS2
was kept almost constant in the other conditions. It means
that TCE decomposes through the reaction mechanism of
Fig. 7. The average kFe­FeS was 2.81 © 10¹6 (m/h) which is
1.4 times larger than kFe, whereas the average kFe­FeS2 was
1.37 © 10¹5 (m/h) which is 6.9 times larger than kFe.
Although K values of Fe­FeS was larger than that of Fe­
FeS2 as shown in Figs. 5 and 6, kFe­FeS values became smaller
than kFe­FeS2, which is due to about 4 times larger £FeS than
£FeS. TCE degradation rate of Fe­FeS2 is more efficient than
that of Fe­FeS.
Calculated K vs. ratio of FeS or FeS2 is shown in Fig. 9
or 10, respectively, when £FeSi is varied from 0.085 to 0.34
(m2/g), sum of WFe and WFeSi is fixed to 100 (mg), and £Fe
kept to 0.17 (m2/g). Because there is no significant difference
between kFe = 1.99 © 10¹6 (m/h) and kFe­FeS = 2.81 © 10¹6
(m/h), increase in TCE degradation rate was not recognized
for £Fe = £FeS = 0.17 m2/g and less than ratio of FeS of 70%
in Fe­FeS system of Fig. 9. However, as known in Fig. 10,
TCE degradation rate in Fe­FeS2 system was increased for
£Fe = £FeS = 0.17 m2/g and less than ratio of FeS of 30%
because of about 6.9 times larger kFe­FeS than kFe.
712
M. Shiba, M. A. Uddin, Y. Kato and T. Ono
-8
-9
-10
-11
-12
-13
-14
-15
-16
-17
0.003
γ
14
Fe=0.17
(m2/g)
12
ln ki
Rate constant, K/10-3h-1
16
γ FeS2=0.34 (m2/g)
10
γ FeS2=0.17 (m2/g)
8
6
4
γ
2
FeS2=0.085
kFe-FeS2 (50Fe-50FeS2)
kFe-FeS2 (50Fe-50FeS)
kFe (100Fe)
kFeS (100FeS)
0.0031
0.0032
0.0033
0.0034
1/T
(m2/g)
0
Fig. 12 Relation between ln ki and 1/T.
0
50
100
Ratio of FeS2 (%)
Fig. 10 Relation between TCE degradation rate and specific surface area of
FeS2.
Table 2 Activation energy of 100Fe, 50Fe­50FeS, 50Fe­50FeS2 and
100FeS.
(kJ/mol)
Rate constant, K/h-1
0.02
50Fe­50FeS
50Fe­50FeS2
100FeS
33.8
29.4
25.1
7.1
0.018
50Fe-50FeS
0.016
0.014
0.012
50Fe-50FeS2
0.01
0.008
0.006
100Fe
0.004
100FeS
0.002
0
290
300
310
320
330
Temperature, T/K
Fig. 11
Relation between TCE degradation rate and temperature.
3.4 Effect of temperature on TCE degradation rate
Figure 11 shows relation between K and decomposition
temperature, T(K) under anaerobic electrolyte solution and
particle size of 100­150 µm. TCE degradation rate increased
with increase in temperature although no noted tendency was
seen in 100FeS.
As known from the previous sections, 50Fe­50FeS system
had the excess Fe which did not contribute to the anode/
cathode reaction, whereas 50Fe­50FeS2 system had the
excess FeS2 which did not decompose TCE. Calculating ki
(i = Fe, Fe­FeS, Fe­FeS2, FeS) in a manner similar to 3.3,
relation between ki and 1/T are shown in Fig. 12. Each
straight line was obtained by linear approximation. It was
found that linearity came into effect approximately. Activation energy calculated from each slope of Fig. 12 is shown
in Table 2. The approximately same activation energy was
obtained for anode/cathode reaction in Fe­FeS and FeFeS2.
4.
100Fe
Conclusion
Batch test of TCE degradation with mixed particles of Fe­
FeS or Fe­FeS2 was carried out and effect of its mass% on
TCE degradation rate was made clear.
(1) TCE degradation occurred on Fe, FeS, Fe­FeS and Fe­
FeS2 particles except FeS2.
(2) TCE degradation of mixed particles of Fe­FeS or Fe­
FeS2 was explained by anode (Fe)­cathode (FeS or
FeS2) reaction.
(3) TCE degradation rate of Fe­FeS and Fe­FeS2 caused by
anode/cathode reaction were 2.81 © 10¹6 and 1.37 ©
10¹5 (m/h), respectively, which are larger than those of
pure Fe and FeS.
(4) There was no difference in TCE degradation rate of
mixed particles of Fe­FeS or Fe­FeS2 between aerobic
and anaerobic solutions.
(5) TCE degradation rate of mixed particles of Fe­FeS or
Fe­FeS2 increased in decrease in size of FeS or FeS2.
(6) TCE degradation rate of mixed particles of Fe­FeS
or Fe­FeS2 increased with increase in decomposition
temperature and activation energy was 33.8 for Fe, 29.4
for Fe­FeS, 25.1 for Fe­FeS2 and 7.1 kJ/mol for FeS.
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