Reactivity enhancement of Nanoscale Zero Valent Iron particles for groundwater remediation
F. Lussier*, S. Chandrasekar#, S. Ghoshal#
*Department of Biochemistry - #Department of Civil Engineering - McGill University
Results
NZVI Synthesis
The NZVI particles used for these experiments were synthesized in
anaerobic conditions following Liu et al.’s method [4].
Conclusions
1.2
1.2
1.0
Sulfide (55.4 mg/L) in DI water
Sulfide (55.4 mg/L) in AGW
1.0
0.8
C/C0
Zero valent iron (ZVI or Fe0) can be actively used to transform
various pollutants like chlorinated chemicals such as Trichloroethylene
(TCE) into harmless chemicals.
Materials and Methods
0.8
5.5 mg/L
27.7 mg/L
55.4 mg/L
110.8 mg/L
277.0 mg/L
554.0 mg/L
2216.2 mg/L
4432.4 mg/L
0.6
0.4
0.2
C/Co
Introduction
•The addition of sulfide increased the reactivity of the NZVI by a factor of 30.
0.6
•The optimal dose of Na2S was found to be 0.0639 mmol (55.4 mg/L).
0.4
0.2
0.0
0
20
40
60
80
100
120
140
160
0.0
0
180
20
40
60
80
100 120 140 160 180
Time (hours)
Time (hours)
Figure 8: Effect of Sulfide added
on TCE dechlorination
Figure 9: Effect of AGW on
addition of Na2S
Figure 8 shows the existence of an optimal concentration of
sulfide. The rate constant associated with this sulfide concentration was
0.0527 hr-1, which was 30 times higher than the rate constant of 0.00163 hr1 associated with NZVI particles alone [Table 1, Figure 10].
Table 1: Sulfide concentrations tested
Figure 3: NZVI synthesis protocol
0.06
Figure 5: TCE control vial (Left) and Bare-NZVI control vial (Right)
However, reactivity of NZVI particles are hindered by the oxide shell
formed around the Fe0 core during synthesis, storage and handling [2]. The
role of sulfur in iron redox reactions represents a promising method of
increasing the efficiency of NZVI [1].
Sulfide-doped NZVI and 35 mg/L of TCE in deionized water were
placed in a sealed reaction bottle to measure the rate of TCE degradation.
The sulfide-doped NZVI samples were also tested using artificial
groundwater, which was similar to groundwater composition from a TCE
contaminated site [5]. Reaction bottles were prepared in triplicates.
The dissolved sulfide concentration of the solution was determined
using the colorimetric, methylene blue reaction assay [6]. The sulfide
concentration was obtained by measuring the absorption at 625nm using a
spectrophotometer.
16
0.04
•The optimal sulfide concentration converted TCE mainly into ethylene,
while higher sulfide concentrations reduced TCE to acetylene. This change
in product formation may indicate a modification in the reduction mechanism
of the particles.
GC-FID Results
0.03
0.02
0.01
10
Literature
8
6
4
2
0.0
0.5
1.0
1.5
2.0
2.5
Sulfide concentration (mM)
Figure 10: : Rate Constant
for
different
sulfide
dosage
3.0
Figure 7: Picture from LM14C
NTA
A Nano-particles Tracking Analysis (NTA) was made using an
LM14C NTA to measure the size of the NZVI particles.
Figure 13: GC-FID results for
Na2S-NZVI [4432mg/L]
12
0
0
20
40
60
80
100
120
140
Sulfide added (mg)
Figure 11: Sulfide reacted
by the NZVI versus sulfide
added
The amount of sulfide reacted by the fixed amount of 40mg of NZVI
particles used in each experiment increased with an increasing amount of
sodium sulfide, indicating that the particles were not saturated with sulfide.
A size analysis was performed using nanoparticles tracking
analyses to determine if the increase in reducing capacity observed from
the addition of sulfide was due to a variation in the hydrodynamic diameter
of the NZVI particles, for example by disaggregation.
Table 2: Hydrodynamic Diameter of particles from the NTA results
The degradation products of TCE changed depending on the
amount of sulfide. In fact, TCE was transformed into different ratios of
Ethane, Ethylene and Acetylene at lower concentrations.
Figure 6: LM10-HS NTA
•There was no change in size or aggregation when NZVI was doped with
sulfide.
14
The values were very similar for the particles treated with sulfide and the
ones that were not treated indicating that they were the same size.
Objectives
The central goal of this research was to enhance the reactivity of NZVI to
provide more cost-effective remediation. The specific objective of this
research was to assess the optimal dose of sulfide in synthesizing
nanoparticles with Fe0 cores and S-doped shells.
Sulfide-doped NZVI particles were shown to be significantly more reactive
than bare-NZVI. This improvement in reactivity would increase the
remediation cost by reducing the amount of NZVI needed to be injected at
contaminated sites.
•The artificial groundwater reduced the efficiency of the NZVI particles.
Solutes such as Cl-, NH4+, Ca2+ in artificial ground water may have acted
as a competitive inhibitor.
Figure 12: GC-FID results for
Na2S-NZVI [55.4mg/L]
18
0.05
0.00
A Gas Chromatograph [Variant CP 3800 FID Detector] was used to
measure the concentration of TCE and its degradation products (ethane,
ethylene and acetylene) over time.
Figure 2: Products of TCE upon reduction with NZVI particles [4]
The final products are a mix of harmless chemicals like ethane,
ethylene and acetylene [4].
•This sudden increase in efficiency may be explained by the sulfide-doped
shell acting as a reactive and conductive surface and the bare-NZVI serving
as an electron reservoir [3].
Artificial groundwater seemed to decrease the reducing power of
NZVI particles. This is likely due to adsorption of ions at various reaction
sites. However, the enhancement in reaction of sulfide-doped NZVI
particles compared to the NZVI particles are still significant, going from
0.0120 hr-1 to 0.0639 hr-1.
Sulfide sorbed (mg)
A promising technology for the clean-up of such sites is the injection of
nanoparticles of zero valent iron (NZVI) into the TCE-contaminated zones
deep in the aquifers to enable rapid, in situ destruction of those
compounds. The reactivity and high surface area to volume ratios of the
nanoparticles can provide orders of magnitude higher rates of pollutant
degradation for NZVI compared to its traditional use in larger granular
forms.
Reactivity Experiments
The reactivity of solid phase FeS to chlorinated hydrocarbons is
attributed to many factors including (i) increased conductivity of FeS surface
layer, (ii) the higher hydrophobicity of FeS compared to iron oxides, and (iii)
the reductive potential of adsorbed Fe2+ ions on the FeS surface [10-13].
Rate constant (1/hr)
Figure 1: NZVI injection into the groundwater
TCE is a widely used industrial solvent, and accidental spills and
past improper disposal practices have lead to widespread contamination.
Approximately 1.8 million pounds of chlorinated solvents have been
improperly discharged into groundwater [7]. Several hundred chlorinated
solvent-contaminated sites have been identified in the U.S. and Canada.
[8-9]. Drinking water standards of chlorinated solvent compounds are 5
ug/L level, and thus even relatively small spills have the potential to
contaminate large volumes of groundwater. Successful remediation of
chlorinated solvent sites will protect critical and renewable resources of
land and water, and human health.
Figure 4: TEM picture of
Bare-NZVI and its oxide shell
Sulfide-doped NZVI Synthesis
Modifications of the particles were performed by aqueous phase
reactions with variable quantities of Na2S and 1 g/L of NZVI [Table 1].
•The bare-NZVI showed moderate reducing capability, demonstrated by its
slow TCE degradation in Figure 8.
The results from GC-FID [Figure 12-13] confirmed the hypothesis
that the main products of the reaction were Ethane, Ethylene and
Acetylene. However, the curve representing 55.41 mg/L of sulfide seemed
to produce more Ethylene, while the higher concentration generated mainly
Acetylene.
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Acknowledgement
•Summer Undergraduate Research in Engineering (SURE)
•Prof. A. Moores – Department of Chemistry
For further information please contact
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