Field Application of a Lactic Acid Ester for PCE Bioremediation.

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ABSTRACT: As part of a strategy to enhance reductive dechlorination with a timed release of hydrogen, an application of glycerol polylactate (GPL) was made to a dry cleaner site contaminated with perchloroethylene (PCE). GPL is one of several proprietary lactic acid esters sold commercially as Hydrogen Release Compound (HRCTM). When HRC contacts the saturated contaminated zone, the lactic acid moiety is released slowly by hydrolysis and is eventually degraded through pyruvic acid to acetic acid producing hydrogen at each step. Under the conditions of this test the GPL-HRC lasted between four and five months; other formulations in development could last considerably longer.

PCE degradation rates in the HRC injected zone were 11.5 times faster than background rates at Day 70 and 4.9 times faster at Day 120. Overall, 80% of the mass of PCE was removed in 253 days with good mass balance relative to TCE and DCE. HRC injection technology is conceptually applicable to enhancing the reductive dechlorination of any of the chlorinated aliphatic hydrocarbons (CAHs) and possibly any of the other chlorinated compounds that are anaerobically degradable.

INTRODUCTION

Chlorinated hydrocarbon contamination in the environment has been a difficult and persistent problem requiring and stimulating innovative solutions. Monitored Natural Attenuation (MNA), offered as a remedy in other less complicated petroleum hydrocarbon venues, is not immediately practical for many sites impacted with chlorinated hydrocarbons. Consequently, the enhancement of Monitored Natural Attenuation is a valid and important focus. To this end, we have successfully demonstrated the field application of Hydrogen Release Compound (HRCTM); specifically glycerol polylactate ester (GPL). GPL is one of a number of proprietary HRC compounds that provide a time-release source of hydrogen which, under the right conditions, can promote enhanced rates of reductive dechlorination.

MATERIALS AND METHODS

A dry cleaning site impacted with PCE was chosen for the field study, because in exhibited the recognized biogeochemical signs of natural attenuation. A zone about 60 square feet was chosen for the study. It was injected by Geoprobeâ with 240 pounds of a slightly flowable formulation of GPL-HRC; at 20 pounds per hole in 12 holes. Figure 1 shows the combination of existing monitoring wells and newly installed Geoprobe wells that were used to monitor upgradient, downgradient and within the injection area at intervals of two to eight weeks for eight months; it also shows some hydrogen data to be discussed later.

Samples were analyzed for the parameters deemed important in a number of standard manuals on MNA. The monitoring of hydrogen was made once on Day 149. The monitoring well network was constructed of 1-in. diameter PVC. The geology consists of a saturated sand layer of 1.8 ft. to 5 feet thick. A thick clay layer is present below the sand. Weathered silt and clay is present above the sand layer. Groundwater flow is almost due South and traveled at 0.01 – 0.1 ft./day.

FIGURE 1. HRC injection diagram with hydrogen concentration at Day 149

RESULTS

The first effect of the HRC on the aquifer was to further depress the redox potential. Nitrate, sulfate and iron levels responded accordingly in the classical patterns and there was an immediate reduction of PCE accompanied by the expected increases in the daughter products - trichloroethylene (TCE) and dichloroethylene (DCE). In latter points in time over the 253 days test period, the degradation of daughter products began and vinyl chloride began to appear. Laboratory studies on saturated soil from the site, spiked with TCE, displayed this pattern in an accelerated fashion as shown in Table 1.

TABLE 1. TCE test tube experiments data summary.

Days

TCE

DCE

VC

TCE

DCE

VC

0

10

0

0

25

0

0

9

5.27

2.19

0

8.06

1.98

0

15

2.32

0.29

0

3.6

0.22

0

21

2.13

0.32

0.49

2.24

0.25

0.17

29

0.9

0

0

1.5

0.08

0.06

The remainder of the Tables and Figures give a comprehensive picture of the enhanced degradation dynamics in this aquifer. Figure 2 presents a series of contour plots and total mass calculations for PCE, TCE and DCE at key points in time as indicated. Also, returning to Figure 1, the injection array map, we see the results of the single measurement of hydrogen made on Day 149. It shows that a zone of hydrogen exists in the range that is associated with reductive dechlorination; considered to be 2-10 nM. The individual wells and their hydrogen measurements are as follows: TW01 (.8), TW02 (6.73), TW03 (5.42), TW04 (.8), TW05 (.8), TW06 (5.56), TW07 (5.86) and TW08 (.8). If these data are considered in conjunction with degradation rate data as presented in Tables 2 and 3, it is clear that hydrogen had an immediate impact where it was applied and then migrated readily out of the injection area to impact CAHs downgradient.

Table 2 summarizes changes in total mass in the system over different points in time showing both the percent change in mass for each CAH constituent and a mass balance between parent and daughter products. Mass balances in excess of 25% are considered to be evidence for biodegradation as a dominant mechanism in PCE loss; a mass balance of 60% would be considered ideal in studies of this nature.

Table 3 enables one to provide some quantification of the term enhanced natural attenuation. PCE data from Table 2 is abstracted and used to calculate various decay rates at different points in time for different elements in the system. The elements chosen are 1) total system mass as derived from the contouring effort in Figure 2, 2) the mass in TW08 - the most reactive well with clear evidence of HRC impact based on loss of PCE and the concomitant appearance of daughter products and 3) the mass in TW01 - the least reactive well in which there was minimal if any impact of HRC based on minimal loss of PCE and an actual decline of daughter products.

After the half lives are calculated by the first order rate equation, one can look at various ratios between the elements and quantify natural attenuation. Higher degradation rates are not occurring uniformly throughout the entire system so the total system mass loss is lower than for active wells like TW08. Rates in the HRC injected zone were 11.5 times faster than background at Day 70 and 4.9 times faster at Day 120; which was near the practical extinction point of the HRC. If one compares the total system rate to the least impacted point (TW01) the enhanced natural attenuation rates are about half; 5.11 times faster at Day 70 and 2.66 times faster at Day 120. These data also provide evidence of residual HRC effects that are still apparent at Day 253.

Figure 3 provides some other general information including the full spectrum of mass changes over time for all the CAHs, lactic acid and its derivatives and key electron acceptors. On the matter of vinyl chloride (VC) - it was never detected at the site before the HRC application, however, one can see it begins to emerge between Day 191 and Day 253. Its ability to form was also accelerated in the microcosm study (Table 1) which demonstrates microbially degradation competency in the aquifer. Lactic acid is readily released and is converted to both propionic acid and pyruvic acid, the latter being quite ephemeral leading to a more immediate registration of acetic acid. Propionic acid will essentially revert to back lactic acid and eventually move through to acetic acid. This creates a fortuitous 'reserve mechanism' as propionic acid is slower to degrade. The practial endpoint for the dissolution of GPL-HRC is 4 months, but through propionic acid one sees persistence through 6 months and signs of CAH degradation through 8 months.

CONCLUSION

Injection of HRC at the site has resulted in a more rapid and significant degradation rate of PCE throughout and downgradient of the injection zone and has begun to impact the daughter products. Ultimately plume size for the daughter products will be reduced relative to their potential spread under conditions of monitored natural attenuation. An enhanced rate of natural attenuation was clearly demonstrated and shown to be persistent based on a simple cost effective application of HRC.

FIGURE 2. PCE, TCE, and cis-1,2-DCE concentration contour maps at days 0, 120, and 253.

TABLE 2. PCE, TCE, cis-1,2-DCE, and VC mass summary.

Mass Summary in Grams - TW01-TW08 (Days 0-70)

 

PCE

TCE

DCE

VC

Day 0

158

0.56

0.81

ND

Day 70

75.9

1.38

13.94

ND

Grams Loss/(Gain)

82.1

(0.82)

(13.13)

-

% Decrease/(Increase)

52%

(246%)

(1,721%)

-

Moles Loss/(Gain)

0.501

(0.006)

(0.139)

-

Mass Balance (DP/PP)*

29%

 

Mass Summary in Grams - TW01-TW08 (Days 0-120)

 

PCE

TCE

DCE

VC

Day 0

158

0.56

0.81

ND

Day 120

55.4

1.76

27.09

ND

Grams Loss/(Gain)

102.6

(1.2)

(26.28)

-

% Decrease/(Increase)

65%

(314%)

(3,344%)

-

Moles Loss/(Gain)

0.626

(0.009)

(0.279)

-

Mass Balance (DP/PP)*

46%

 

Mass Summary in Grams - TW01-TW08 (Days 0-149)

 

PCE

TCE

DCE

VC

Day 0

158

0.56

0.81

ND

Day 149

47.0

1.10

17.66

ND

Grams Loss/(Gain)

111.0

(0.54)

(16.85)

-

% Decrease/(Increase)

70%

(196%)

(2,180%)

-

Moles Loss/(Gain)

0.677

(0.004)

(0.179)

-

Mass Balance (DP/PP)*

27%

 

Mass Summary in Grams - TW01-TW08 (Days 0-191)

 

PCE

TCE

DCE

VC

Day 0

158

0.56

0.81

ND

Day 191

56.2

1.41

17.82

ND

Grams Loss/(Gain)

101.8

(0.85)

(17.01)

-

% Decrease/(Increase)

64%

(252%)

(2,200%)

-

Moles Loss/(Gain)

0.606

(0.006)

(0.174)

-

Mass Balance (DP/PP)*

30%

 

Mass Summary in Grams - TW01-TW08 (Days 0-253)

 

PCE

TCE

DCE

VC

Day 0

158

0.56

0.81

ND

Day 253

31.8

0.79

28.59

0.021

Grams Loss/(Gain)

126.2

(0.23)

(27.78)

(0.021)

% Decrease/(Increase)

80%

(141%)

(3,530%)

(210%)

Moles Loss/(Gain)

0.751

(0.002)

(0.283)

(0.00033)

Mass Balance (DP/PP)*

38%

*Mass Balance = Moles Daughter Products (DP) = Moles TCE+DCE+VC

Moles Parent Products (PP) Moles PCE

TABLE 3. PCE degradation rates.

Days

Total Mass (grams)

Half Life (d) Total Mass

Half Life (d) Well TW01

Half Life (d) Well TW08

Ratio

TW01/Total Mass

Ratio TW01/TW08

70

158

66

338

29

5.11

11.50

120

75.9

79

211

43

2.66

4.94

149

55.4

85

256

53

3.00

4.84

191

47.0

128

309

45

2.41

6.80

253

56.2

109

205

38

1.87

5.41

FIGURE 3. Mass changes in chlorinateds, iron, sulfate, nitrate and organic acids.

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