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

Introduction Zero valent iron (ZVI or Fe0) can be actively used to transform various pollutants like chlorinated chemicals such as Trichloroethylene (TCE) into harmless chemicals. 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. 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. 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]. 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]. 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.

Results

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 hr-

1 associated with NZVI particles alone [Table 1, Figure 10]. 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]. 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.

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 values were very similar for the particles treated with sulfide and the ones that were not treated indicating that they were the same size. 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.

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.

Materials and Methods NZVI Synthesis

The NZVI particles used for these experiments were synthesized in anaerobic conditions following Liu et al.’s method [4].

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]. Reactivity Experiments

Figure 5: TCE control vial (Left) and Bare-NZVI control vial (Right) 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. 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. 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. A Nano-particles Tracking Analysis (NTA) was made using an LM14C NTA to measure the size of the NZVI particles.

Conclusions

•The bare-NZVI showed moderate reducing capability, demonstrated by its slow TCE degradation in Figure 8.

•The addition of sulfide increased the reactivity of the NZVI by a factor of 30.

•The optimal dose of Na2S was found to be 0.0639 mmol (55.4 mg/L).

•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].

•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.

•There was no change in size or aggregation when NZVI was doped with sulfide.

•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.

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4.Liu, Y., et al.. Environ Sci Technol, 2005. 39(5): p. 1338-45.

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110

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edition, p. 503, Method 428C (1975).

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Volume 3, Issue 5.

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Contaminants in the Subsurface, 2004.Contaminants in the subsurface:

Source zone assessment and remediation. National Academia Press.

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Quality Guidelines Trichloroethylene Environmental and Human Health

Effects, Soil Quality Guidelines, SSD.

10.Elsner, M.; Schwarzenbach, R. P.; Haderlein, S. B., Environmental

Science & Technology 2003, 38 (3), 799-807

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2010, 74 (7), 2025-2039

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Figure 3: NZVI synthesis protocol Figure 4: TEM picture of

Bare-NZVI and its oxide shell

Table 1: Sulfide concentrations tested

Figure 8: Effect of Sulfide added

on TCE dechlorination

Figure 9: Effect of AGW on

addition of Na2S

Figure 10: : Rate Constant

for different sulfide

dosage

Figure 11: Sulfide reacted

by the NZVI versus sulfide

added

Acknowledgement

•Summer Undergraduate Research in Engineering (SURE)

•Prof. A. Moores – Department of Chemistry

For further information please contact

subhasis.ghoshal@mcgill.ca

GC-FID Results

Figure 12: GC-FID results for

Na2S-NZVI [55.4mg/L]

Figure 13: GC-FID results for

Na2S-NZVI [4432mg/L]

Figure 6: LM10-HS NTA Figure 7: Picture from LM14C

NTA

Time (hours)0 20 40 60 80 100 120 140 160 180

C/C

0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

5.5 mg/L27.7 mg/L55.4 mg/L110.8 mg/L277.0 mg/L554.0 mg/L2216.2 mg/L4432.4 mg/L

Sulfide added (mg)0 20 40 60 80 100 120 140

Sul

fide

sorb

ed (m

g)

0

2

4

6

8

10

12

14

16

18

Sulfide concentration (mM)0.0 0.5 1.0 1.5 2.0 2.5 3.0

Rat

e co

nsta

nt (1

/hr)

0.00

0.01

0.02

0.03

0.04

0.05

0.06

Time (hours)0 20 40 60 80 100 120 140 160 180

C/Co

0.0

0.2

0.4

0.6

0.8

1.0

1.2Sulfide (55.4 mg/L) in DI waterSulfide (55.4 mg/L) in AGW