study of chlorinated ethylenes remediation by non-stabilized...

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


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


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),


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)


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


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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