EFFECT OF ZERO-VALENT IRON ON GROUNDWATER CONTAINING CHLORINATED SOLVENTS, METHANOGENS AND SULFATE-REDUCERS IN  MICROCOSMS.

 

Margaret Findlay, Samuel Fogel (Bioremediation Consulting Inc, Watertown MA)

Brad Droy (Toxicological & Environmental Associates, Baton Rouge LA)

 

 

Summary and Conclusion

 

The purpose of this work was to determine whether the addition of iron filings to the saturated subsurface at Canaveral, FL, would inhibit microbial populations.

 

The groundwater initially contained 300 mg/L TCE and 290 mg/L sulfate.  Microcosms were constructed with soil and groundwater using anaerobic technique, amended with organic acids, and maintained under anaerobic conditions. Microcosms were monitored until they had demonstrated methanogenic, sulfate-reducing and dechlorinating microbial activity prior to the addition of iron filings.

 

The particle size of the iron filings was such that 90% passed through a US #16 standard sieve.    Addition of iron filings resulted in the conversion of TCE directly to ethene, ethane,  and small amounts of other hydrocarbons.  For 3 grams iron in a 100 ml microcosm, the initial rate of TCE destruction was 200 µM /day.

 

After the addition of iron filings, methanogens growing with formic acid as an electron donor were not inhibited by the presence of iron filings, while methanogens in a microcosm previously depleted of electron donor were able to use the hydrogen produced by iron filings to support methane production.

 

Sulfate-reducing bacteria growing with acetate as electron donor were also not inhibited by the addition of iron filings In fact, a slight stimulation of sulfate reduction  was observed. The effect on dechlorinating bacteria was not studied because of the rapid abiotic degradation of chlorinated compounds caused by ZVI.

 

Our results show that one type of ZVI, when placed in direct contact with anaerobic bacteria, is not inhibitory, and may even stimulate growth. The results support the idea that all types of ZVI produce molecular hydrogen that can be used by most major microbial groups involved in biological reductive dechlorination.

 

Introduction

 

Biological methods may be beneficially combined with physical or chemical methods to provide integrated treatment of groundwater contaminated with chlorinated compounds, if there is no significant inhibition by physical/chemical treatment on the microbial community.    In the case of treatment with zero valent iron, the generation of molecular hydrogen as an electron donor may confer a direct benefit on the entire microbial community. 

 

A healthy anaerobic microbial community is important for the successful degradation of many chlorinated compounds found in groundwater.  Key microbial groups that function directly to degrade chlorinated compounds include methanogens, sulfate reducers, acetogens and dehalorespirers. In addition to dechlorination, certain members of the community are known to produce essential nutrients and co-factors which benefit the dehalorespirers such as Dehalococcoides ethenogenes. 

 

Objective

 

The objective of this study was to evaluate the effect of one type of zero valent iron on the function of two groups of anaerobes, methanogens and sulfate reducing bacteria.  (The effect on dechlorinating bacteria could not be directly determined because of the rapid abiotic degradation of chlorinated compounds caused by ZVI).  Prior to studying the effects on bacteria, studies were conducted on the abiotic dechlorinating action of varying concentrations of ZVI on site groundwater.

 

Groundwater Characteristics. 

 

The groundwater initially contained about 300 ppm TCE, with much smaller amounts of cDCE, VC and Freon.  The addition of soil and iron filings resulted in adsorption of the contaminant, such that the initial dissolved concentration of TCE was 100 ppm.  The groundwater also contained 3,000 ppm chloride and 290 ppm sulfate.

 

 

Materials and Methods

 

Microcosms for dechlorination tests were constructed in 160 ml serum bottles using anaerobic technique.  Each contained anaerobic groundwater (65 ml) and soil (40 grams)..  Then  different amounts of iron filings + sand (totalling 30 grams) were added to the microcosm, and monitoring was continued to record destruction of chlorinated compounds and production of gasses. 

Microcosms for microbial tests contained 100 ml groundwater and no soil. Microcosms were monitored until methanogen and sulfate reducing activity were demonstrated, then 1 or 10 grams of iron flings were added.

 

Iron Filings, from Connelly-GPM (Chicago, IL), had a particle size that allowed 90 % to pass through US Screen Size 16,  and 45 % to pass through Screen Size 30.      

 

Analytical Methods: Chlorinated compounds and hydrocarbons were analyzed by injecting 100 uL of microcosm headspace into a HP5890 GC/FID.  Standards were prepared in microcosm bottles having the same ratio of headspace to aqueous portion. Quantitation was conducted using ChemStation software. Dissolved H2 was also determined using 100 uL headspace samples, injecting into a Trace Analytical RG-5 reduction gas analyzer.   Amounts are reported as though all of each compound were in the aqueous portion.  Sulfate was determined using capillary ion electrophoresis, EPA Method 6500.

 

 

RESULTS

 

Hydrogen Gas Generation:

 

The reaction of iron filings with water under anaerobic conditions is known to generate H2 gas and Fe++ (Reardon 1996 ES&T 29:2936)

                             Feo  +  2H+  ® Fe++ + H2 + 2 OH-

 

Figure 1 shows the generation of H2 from site groundwater with different concentrations of iron over a period of 39 days.  The data show that 30 grams of iron in 65 ml groundwater generated 400 µmoles H2 over a 39 day period.

 

TCE destruction by Iron Filings: 

 

The destruction of TCE by iron filings is thought to be catalyzed by Fe++ and to involve removal of Cl atoms and replacing with H atoms (Orth and Gilham 1996 ES&T 30:66).

 

Figure 2 shows the dechlorinating action of 6 concentrations of ZVI on total chlorinated ethenes in site groundwater over seven days. The extent of degradation of chlorinated ethenes increased with increasing iron concentration.  These results are summarized in Table 1

 

Figure 3 shows the data for individual chlorinated ethenes for one iron concentration.  ZVI reacted most rapidly with TCE, slowly with cis-DCE, and very slowly with vinyl chloride.

 

 

Table 1  Destruction of

900 µM  Chlorinated Ethenes  by Iron Filings

in 65 ml anaerobic ground water and 40 g soil.

Iron Filings

Loss in 1 day

Loss in 7 days

  0    g

  0 %

  0 %

  0.3 g

15 %

23 %

  1    g

25 %

43 %

  3    g

31 %

63 %

10    g

59 %

92 %

30    g

70 %

98 %

 

C1 to C4 Hydrocarbons Generated from TCE: 

 

Figure 4 shows the generation of C1 to C4 hydrocarbon products by the reaction of ZVI with chlorinated ethenes.   The amounts of hydrocarbons generated in 7 days was proportional to the extent of destruction of chlorinated ethenes during that time. 

 

Figure 5 shows the generation of individual hydrocarbons for the microcosm having 30 grams of iron.  The major products of TCE destruction were ethene and ethane, representing 50% and 39% respectively.  Minor components were propane and propene, butane and butene and methane, at 6%, 3% and 2% respectively. Although the total C1-C4 hydrocarbons analyzable by headspace sampling represented only 22 % of the TCE destroyed, additional amounts were probably adsorbed to the soil and/or iron filings.

 

 

 

Effect of Iron Filings on Sulfate Reducing Bacteria:

 

Procedure:  Each microcosm contained 200 ppm sulfate, as well as 300 ppm acetate as electron donor for the sulfate-reducers.  The experimental bottle received 10 gram of iron filings and the control bottle received the same volume of sand. 

 

Figure 6 A&B.  For both well 2-D and well 5-D, sulfate was completely reduced in 10 days in both the control and the iron filings-containing microcosm.

 

The data for well 2-D are presented in Table 2.  The control bottle removed 176 ppm of sulfate, while the bottle with 10 grams iron filings removed all 200 ppm of sulfate, suggesting a possible beneficial effect of  H2. 

 

Table 2

Effect of Iron Filings on Sulfate Reducing Bacteria Growing on Acetate

 

iron filing

Sulfate ppm

Sulfate decrease

 

 

Day 0

Day 10

ppm

Experiment

10 gram

200

0

200

Control

0

200

24

176

 

Effect of Iron Filings on Methanogens in the Presence of Electron Donor:

 

Methanogen Cultures: Two microcosms were flushed to remove excess methane produced by on-going methanogenic activity, then fed formate which is quickly converted to H2 , the electron donor used by the methanogens.

Addition of Iron Filings.  The following day, the experimental bottle was given 1 gram of iron filings, but not the control.  The microbes in the two bottles were allowed to produce methane for 14 days.

 

Figure 7-A.   Methane increased at the same rate in the microcosms with and without iron filings, showing that iron filings did not have an inhibitory effect.  The data is also summarized in Table 3.

 

Table 3

Effect of 1 g of Iron Filings on Methanogens in Presence of Electron Donor

 

Day 0  µM CH4

Day 14 µM  CH4

µM  CH4  increase

 Iron added

1190

2590

1400

No Iron

1010

2240

1230

 

 

Effect of Iron Filings on Methanogens in the Absence of Electron Donor: 

 

Procedure:  After the previous experiment, the excess methane were flushed out of the headspace of each bottle, but no electron donor was added.  Then 9 gram of iron filings were added to the experimental bottle, and the same volume (5 grams) of sand was added to the control bottle. 

 

Figure 7-B.   Methane was generated in the bottle having iron filings, but not in the control bottle which had no H2.  This data indicates that H2 produced by addition of zero valent iron can serve as electron donor for methanogens.  The results are also summarized in Table 4. 

 

Table 4

Effect of 1 g of Iron Filings on Methanogens in Absence of Electron Donor

 

Day 0  µM CH4

Day 14 µM  CH4

µM  CH4  increase

9 g  Iron added

950

2590

1640

No Iron

580

680

100

 

 

Conclusions

 

·        After the addition of iron filings, methanogens growing with formic acid as an electron donor were not inhibited by the presence of iron filings.

 

·        Methanogens in a microcosm previously depleted of electron donor were able to use the hydrogen produced by iron filings to stimulate methane production.

 

·        Sulfate-reducing bacteria growing with acetate as electron donor were also not inhibited by the addition of iron filings.   In fact, a slight stimulation of sulfate reduction  was observed. 

 

·        Although the effect on dechlorinating bacteria was not studied because of the rapid abiotic degradation of chlorinated compounds caused by ZVI, it may be inferred that dehalorespirer’s response will be similar to that observed with methanogens.

 

 

 

 

 

References

 

Orth, W. and R. Gilham. 1996. Dechlorination of Trichloroethene in aqueous Solution Using Fe0.  Enviromental Science & Technology 30:66-71