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12. - 14. 10. 2010, Olomouc, Czech Republic, EU
STUDY OF CHLORINATED ETHYLENES REMEDIATION BY NON-STABILIZED AND SI-
STABILIZED NANOIRON
Petra JANOUŠKOVCOVÁ, Lenka HONETSCHLÄGEROVÁ, Petr BENEŠ
INSTITUTE OF CHEMICAL TECHNOLOGY IN PRAGUE, Department of Environmental Chemistry,
166 28 Prague 6, Technická 5, e-mail: [email protected]
Abstract
This study deals with the ability of nanoscale zero-valent iron particles (NZVI) to decompose chlorinated
ethylenes. Chlorinated ethylenes are common contaminants found on many sites in the Czech Republic.
The application of NZVI suspension is a very useful technique for cleaning up these sites. In the study, batch
experiments were performed to simulate an in situ chemical reduction remediation technology. We studied
an interaction of an aqueous suspension of nanoiron particles RNIP-10APS (TODA Kogyo Corp.)
with tetrachlorethylene (PCE) and trichloroethylene (TCE).
In the original NZVI suspension, the particles agglomerate quickly after preparation. Therefore, we proposed
an environmental friendly and cost effective method of NZVI stabilization by silicate stabilizing agent.
This method prevents the particles from aging and agglomerating.
The main goal was to compare the ability of the original NZVI and the Si-stabilized NZVI to decompose
selected contaminants. The used stabilizing Si-agent forms protective barrier around the NZVI particles so
they stay dispersed and in submicron size. This fact was confirmed by SEM analysis. The conditions
of the batch experiments were set up to simulate a full-scale in situ NZVI application. The NZVI concentration
was in experiments approximately 2.5 g/L. During the experiments, we obtained kinetic parameters
of the decomposition which we used to evaluate the ability of both NZVI suspensions to decompose
the studied contaminants. In the case of the original NZVI suspension, the degradation rate of TCE was
higher than the degradation rate of PCE. In the case of stabilized NZVI, limited degradation ability was
observed in some cases.
1. INTRODUCTION
Soils and groundwater contaminated by chlorinated ethylenes represent a worldwide problem.
The chlorinated ethylenes are widespread because of their extended industrial use as a solvent. They are
harmful for both the environment and human health because of their high toxicity. The nanoscale zero valent
iron particles (NZVI) could provide a cost effective, fast and efficient solution to the contamination
of chlorinated ethylenes. With contribution of their large specific reactivity, the NZVI particles chemically
reduce the chlorinated ethylenes to less or non toxic carbon compounds. The NZVI particles tend to
form aggregates in the aqueous suspension used for remediation. The agglomeration reduces the reactivity
of the particles and causes technical difficulties in an in-situ application. In our previous work [1],
the stabilization of the NZVI particles by a silica agent was investigated. The silica agent provides a suitable
stabilization for the NZVI particles.
In this study, we investigated the degradation ability of both the original and silica stabilized NZVI particles.
We studied a commercially available aqueous suspension of reactive nanoscale iron particles RNIP-10APS
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12. - 14. 10. 2010, Olomouc, Czech Republic, EU
Fig.1: NZVI structure and surface reaction[3].
(TODA Kogyo Corp.) We chose tetrachlorethylene (PCE) and trichloroethylene (TCE) as model
contaminants.
2. THEORETICAL SECTION
The NZVI particles have a great reactivity because of a larger
specific surface area when compared to a classic iron powder.
The large specific surface area supports the mass transfer
on the particle surface and increases the adsorption
and reduction capacity for the degradation of the contaminant.
The NZVI reactivity also depends on the structure
and composition of the particle. The typical NZVI particle
consists of a zerovalent iron core and a ferrous
oxide/hydroxide shell (the so called core-shell structure) [2].
Because of the highly reactive specific surface area, NZVI particles adsorb to the surrounding soil matrix,
tend to agglomerate and oxidize. Surface coatings are applied to the NZVI particles in order to overcome
these effects and to get a stabilized dispersion of NZVI. Suitable coatings enhance subsurface mobility
and have no negative impact on the reactivity. Silica species could be used as a useful stabilized coating
agent. Because of its high affinity towards ferric oxides, the silica can sorb on the Fe2O3 on the surface of the
NZVI particles. The adsorbed silica can influence the surface chemistry, particle mobilization, coagulation
and iron corrosion. The silica coating is environmental friendly and cost effective [4].
A reductive dechlorination of chlorinated ethylenes by NZVI in a water solution is an electrochemical
corrosion process. The oxidation of NZVI provides electrons for the reduction of the chlorinated ethylenes.
The reductive dechlorination is mainly a direct reduction during which the contaminant is adsorbed
on the metal surface to form a chemisorption complex. Because of their high reactivity, NZVI particles also
react with dissolved oxygen and water under aerobic conditions. The reduction of water produces gaseous
hydrogen and hydroxide anions. The hydroxide anions increase pH of the solution which contributes
to a long-term particle stability [5].
The degradation process of chlorinated ethylenes is described by a kinetics of a pseudo-first order. This
kinetics was chosen because the NZVI is often in excess and the concentration of NZVI almost does not
change during the reaction. The observed pseudo first order kinetics constant is used to assess
the degradation rate [6].In general, the degradation rate increases with increasing number of chlorine atoms
in the molecule of the chlorinated ethylene [7]. The carbon atoms in TCE are in a lower oxidizing state than
in PCE. That is why the TCE is degraded more slowly than the PCE.
3. EXPERIMENTAL SECTION
3.1 MaterialsA commercially available suspension of nanoiron particles RNIP-10APS (TODA Cogyo
Corp.) was used for the experiments. The concentration of the total iron in the NZVI stock suspension was
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12. - 14. 10. 2010, Olomouc, Czech Republic, EU
approximately 168.8 g.L-1 (deviation 7.5%). RNIP are prepared by a gas phase reduction of an iron oxide in a
H2 atmosphere at a high temperature. The prepared particles have α-Fe0 core and a protective Fe2O3 shell
[2]. Tetrachloroethylene and trichloroethylene (p.a., Penta) were used as contaminants. Degradation
experiments were performed in a batch system which was constituted by set of glass bottles with PTFE caps
(Fischer, total volume 310 mL).
3.2 Stabilization of NZVI particles suspension
For the experiments with the stabilized suspension Si-RNIP, the stabilized suspension of the NZVI particles
was prepared at once for the whole experimental set of two contaminants. Nine bottles were prepared
for the PCE experiment and eight bottles were for the TCE experiment. The silicate agent was prepared by
slow mixing of water glass (Na2O.nSiO2, 34-38 %, KMplus) and distillated water in the volume ratio 1:8.
During the mixing, sulphuric acid (p.a., Lachema) was added dropwise to the solution. A gel with a pH
of approximately 10.35 was formed which was afterwards used as the silicate agent. The stock suspension
of NZVI was added to the silicate agent in the volume ratio 1:5 (the silicate gel considered in liquid state).
This mixture was mixed for 5 minutes at 13,000 rpm. After the mixing, the mixture of silica gel and NZVI was
diluted by distilled water in volume ratio 1:2. This mixture was mixed for 30 minutes at 10,000 rpm.
The mixture was then diluted by distilled water to achieve 3,400 mL. The mixing for 20 minutes at 360 rpm
followed out. The succession of mixing helped to disintegrate the agglomerates and to support
the stabilization process. The pH of the final prepared suspension was 10.16. SEM micrographs (Quanta200
FEG) of the NZVI particles of both suspensions were obtained.
3.3 Degradation experiments
During the experiments with the original suspension five milliliters of the original RNIP stock suspension was
added into each bottle. In the experiments with stabilized suspension, 200 ml of the prepared stabilized
suspension was added into each bottle. The bottles were filled completely with distillated water to achieve
the concentration of approximately 2.5 g.L-1 of total iron. The contaminant dissolved in methanol was added
to the required number of bottles to get the concentration approximately 20 mg.L-1. It means that the molar
concentration was 152.22 µmol.L-1 for TCE and 120.6 µmol.L-1 for PCE. The bottles were capped without
headspace immediately and shaken in a rotate shaker. Experiments contained a control series of solutions
without NZVI to verify the leak of the contaminant. Control and degradation samples were sampled
after different incubation periods with respect to the rate of the degradation of the contaminant.
3.4 Analysis
The samples were extracted by hexane (n-hexane 95+, p.a., Penta) in a volume ratio 1:1 (vortex shaker)
for 20 minutes. The hexanes extracts were analyzed by GC/ECD. In every bottle, the pH was measured
by GHM 3530 measuring instrument. Total amount of iron was analyzed by the method of flame AAS
after sulfuric acid mineralization. From the experimental data both reaction order and kinetic constant was
evaluated using ERA3.0 program.
4. RESULTS AND DISCUSSION
Both the original (RNIP) and stabilized (Si-RNIP) suspensions were scanned by the SEM microscopy.
Some samples of the stabilized particles were covered by carbon instead of platinum (figure. 2 B)) because
the stabilized NZVI particles can be easily mistaken for artifacts of platinum. In the figure 2 A),
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12. - 14. 10. 2010, Olomouc, Czech Republic, EU
Fig. 3: Measured concentrations of PCE and model
curves for both the original suspension RNIP
and silica stabilized suspension Si-RNIP.
Fig. 4: Measured concentrations of TCE for both
the original suspension RNIP and silica stabilized
suspension Si-RNIP. The model curves fit only RNIP.
the micrographs showed aggregates of the NZVI particles in the original suspension. The size of aggregates
ranged from several nanometers to micrometers. They were formed in the clusters of an irregular sharp
shape. In the Fig. 2 B), individual particles with regular shape and bigger clusters formed from these particles
are depicted. In the Figure 2 B), the particles have a spherical shape with size of approximately several
hundreds of nanometers.
Fig. 2: SEM micrographs of NZVI particles. A) Aggregates in the original suspension RNIP. B) Particles
of the stabilized suspension of NZVI by the silica agent (Si-RNIP).
Figure 3 shows the degradation of TCE by the original suspension RNIP (full triangle) and the silica stabilized
suspension Si-RNIP (full diamond). In the Figure 4, the results of the degradation TCE by the original
suspension RNIP (full circle) and the silica stabilized suspension Si-RNIP (full square) are depicted.
The measured concentrations of both the PCE and TCE after the degradation by the original suspension
RNIP have an exponential form which is characteristic for the first-order reaction.
0
20
40
60
80
100
120
0 2 4 6 8 10 12 14
c P
CE
[µ
mo
l.L
-1]
t [day]
RNIP control RNIP 1-order (RNIP) 1.29-order (RNIP)
Si-RNIP control Si-RNIP 0-order (Si-RNIP) 0.07-order (Si-RNIP)
0
40
80
120
160
200
0 2 4 6 8
c T
CE
[u
mo
l.L
-1]
t [day]
Si-RNIP control Si-RNIP RNIP control RNIP 1-order (RNIP) 0.82-order (RNIP)
The concentration data were fitted by the kinetics of n-order to evaluate a reaction order and an observed
kinetic constant kobs using ERA program [8]. The evaluated kinetic models of n-order are indicated by the full
lines. The dash lines represent the assumed first-order kinetics from the literature sources [6]. The kinetic
parameters for both contaminants are summarized in Table I. On the basis of small differences between
the evaluated and the first-order kinetics, we can consider the first order reaction for the degradation of TCE
and PCE by the original suspension RNIP. Moreover, we used the large excess of total iron (2.5 g.L-1).
In order to compare the reaction rates, we calculated the values of half-lives T1/2 (Table I). The calculated
half-lives of PCE and TCE degradation by the original suspension RNIP showed that PCE is decomposed
A) B)
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12. - 14. 10. 2010, Olomouc, Czech Republic, EU
Tab. I: Evaluated kinetic parameters from degradation PCE and TCE by both the original RNIP and silica
stabilized suspension Si-RNIP. Experimental conditions presented by pH and total final concentration of iron
(average values from all bottles).
slower than TCE. One of the possible explanations is that the agglomerates of the original suspension could
partially influence the reaction properties of the iron particles. A greater part of PCE was then probably
adsorbed on the nonreactive sites of the agglomerated particles and thus did not take part in the degradation
reactions. Because TCE is not as hydrophobic as PCE (log KowPCE = 2.88, log KowTCE = 2.42) TCE sorption
could have been less significant [9]. The degradation of both contaminants with stabilized suspension
Si-RNIP exhibited a different character. The PCE concentrations decrease linearly which indicates a zero-
order reaction (Figure 3). These PCE concentrations were fitted by an n-order and the presumed zero-order
kinetic models. The obtained kinetic parameters are shown in Table I. On the basis of similar trends of PCE
fitting by both evaluated kinetic models, the zero-order reaction for the degradation by the stabilized
suspension Si-RNIP is proposed. The presence of the silica agent could have caused a change
of the reaction order. According to the half-life values T1/2, the reaction rate of the degradation of PCE
by Si-RNIP is approximately four times lower that with the original RNIP. The TCE concentration almost did
not change in the first 24 hours. The exponential decrease was then observed. The usage of the silica agent
probably extended the time necessary for the total decomposition of TCE up to 1-2 days, as seen
in the Figure 4. These experimental data were not evaluated by a kinetic model because of a complicated
character of the concentration trend.
The presence of the silica layer on the surface of the particles could have limited the transport of PCE
or TCE molecules to the surface of the iron particle and thus change the kinetic parameters of the reaction.
Another possible reason could be found in limited available surface of particle for reaction [10] Thus,
the measured TCE concentration (Figure 4, full diamond) has an exponential character following a first-order
reaction with an initial time delay. The change of the reaction order and reaction rate for the PCE (Figure 3,
full circle) could be explained by the adsorption ability described previously and the already presented
reasons.
During the experiments, the initial molar concentration of both the contaminants was not same because
the mass concentration (20 mg.L-1) was assumed. According to the literature [11], for TCE water concentration
below 0.46 mmol, the reaction rate should be the same. This condition was realized in our experiments thus
we could compare our evaluated kinetic parameters of the TCE and PCE measurements.
All of the monitored experimental conditions before and/or during the tests are summarized in Table I.
The average values of Fetotal in degradation suspensions slightly varied from the intended 2.5 g.L-1.
The differences were probably caused by the heterogenenity of the agglomerated stock suspension. The pH
values were in the alkaline range during the experiments (Table I). The alkaline pH can be convenient
contaminant/ suspension
pH susp. initial
pH susp. samples
rate order kobs T1/2 [hour] Fetotal [g.L-1
]
PCE/RNIP 11,11±0,05 10,93±0,10 1.29 0,342 (mol.L-1)-0.29/h 32.03
2.6±0.6 1 0.020 h-1 34.96
PCE/Si-RNIP 10,07±0,01 10,17±0,06 0.07 7.21E-7 (mol.L-1)0.93/h 141.55
2.1±0.2 0 3.24E-7 (mol.L-1)/h 148.77
TCE/RNIP 11.05±0,03 10,94±0,07 0.82 0.007 (mol.L-1)0.18/h 19.56
2.3±0.3 1 0.038 h-1 18.33
TCE/Si-RNIP 10,07±0,01 10,17±0,05 - - - 2.2±0.1
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12. - 14. 10. 2010, Olomouc, Czech Republic, EU
to keep the stability of the silica agent. During the degradation process with RNIP and Si-RNIP suspensions
no significant change of pH was observed.
5. CONCLUSION
The results of the batch experiments confirmed the degradation ability of the original suspension RNIP
to decompose both of the chosen contaminants according to assumed first-order reaction. The reaction rate
of TCE degradation is higher in comparison with PCE (T1/2,PCE ≈ 32.03 h, T1/2,TCE ≈ 19.56 h). Our experiments
with the silica stabilized suspension Si-RNIP revealed that the presence of the silica agent partially limited
the degradation of both contaminants. In the case of the TCE degradation, we observed a delay
in the concentration trend of 1-2 days. On the other hand, the PCE degradation exhibited a change
from the first to the zero-order reaction. According to the value of the half-lives, the reaction rate
of the degradation of PCE by the original suspension was four times higher than for the stabilized
suspension. The influence of the silica agent on the NZVI degradation ability will be further studied.
Moreover, the effect of the adsorption of the contaminants on the surface of the particles and the effect
of this adsorption on the degradation ability of NZVI has to be properly investigated.
ACKNOWLEDGMENTS
„Financial support from specific university research MSMT no. 21/2010“
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