Leachability Results

There are three key analyses from this study, total concentration analyses by EPA Method 8015 as modified by California DHS of the untreated and treated samples and the cumulative total of the amount of TPH released over the six pore volumes in the column leaching. As shown in Table 3, the Siallon encapsulation of the diesel-contaminated soils is so confining or tight that even the aggressive solvent extraction of Method 8015 Modified cannot extract the contaminant in the treated samples. However, as is also shown in Table 3, the total amount of contaminant leached out of the treated samples is much less than is shown as available for leaching or as unencapsulated by the TCA method. This held for all of the samples, with the total amount leached ranging from only 10.2% to 67.6% of the available TPH as determined by TCA. Obviously, when the TCA type of analysis is used on Siallon-treated samples, it will detect not

Table 3 Comparison of TCA and Total Leachate Release for Sandy Soil Samples

EPA Method 8015 Modified

Total mg leached

Contaminant

Before After treatment mg. treatment (mg)

Before After treatment (mg) treatment (mg)

Diesel

2600 3.35

2605 1.9

only the unencapsulated hydrocarbon but also the partially or poorly encapsulated hydrocarbon. In essence, the TCA methods overstate the impact on the environment.

As was mentioned previously, all of the samples were subjected to the column leach procedure in triplicate, resulting in more than 75 samples being analyzed for each pore volume of leachate. A portion of the mass of data generated is summarized and averaged for the ambient pressure study in Table 4.

As can be seen in Table 4, regardless of soil type, contaminant type, compacted or un-compacted, the decrease in TPH leached over the six pore volumes follows the same rapid decline for both treated and untreated samples. When the cumulative release of TPH is plotted against the number of pore volumes (and each pore volume has a time and flow rate factor), the

Table 4 Column Leachate Study Results—Ambient Pressure

Total petroleum hydrocarbon (mg/L) Pore volume

Soil type

Contaminant

Sample

Compaction condition

1

2

3

4

5

6

Sandy

Gasoline

Untreated

Uncompacted

2350

840

60

5

ND

ND

Sandy

Gasoline

Treated

Uncompacted

1.77

0.37

ND

ND

ND

ND

Sandy

Gasoline

Untreated

Compacted

1450

360

110

40

10

5

Sandy

Gasoline

Treated

Compacted

1.25

0.13

ND

ND

ND

ND

Sandy silt

Gasoline

Untreated

Uncompacted

2100

900

650

120

80

20

Sandy silt

Gasoline

Treated

Uncompacted

7.1

1.3

0.4

ND

ND

ND

Sandy silt

Gasoline

Untreated

Compacted

1650

1040

450

150

60

20

Sandy silt

Gasoline

Treated

Compacted

2.6

0.65

0.2

ND

ND

ND

Clay

Gasoline

Untreated

Uncompacted

1250

1100

950

600

400

250

Clay

Gasoline

Treated

Uncompacted

9.9

5.3

3.6

1.9

0.65

0.2

Clay

Gasoline

Untreated

Compacted

875

950

700

650

300

120

Clay

Gasoline

Treated

Compacted

7.9

4.8

3.4

2.2

1

0.4

Sandy

Diesel

Untreated

Uncompacted

5600

3100

1150

420

100

50

Sandy

Diesel

Treated

Uncompacted

6.2

1.3

0.3

ND

ND

ND

Sandy

Diesel

Untreated

Compacted

4400

2300

600

300

80

20

Sandy

Diesel

Treated

Compacted

3.2

0.6

0.1

ND

ND

ND

Sandy silt

Diesel

Untreated

Uncompacted

4200

1800

750

120

50

10

Sandy silt

Diesel

Treated

Uncompacted

27.8

8.9

4.3

1.3

0.5

0.2

Sandy silt

Diesel

Untreated

Compacted

3600

1400

600

100

80

20

Sandy silt

Diesel

Treated

Compacted

17.3

6.3

1.5

0.6

ND

ND

Clay

Diesel

Untreated

Uncompacted

3350

1150

420

60

5

1

Clay

Diesel

Treated

Uncompacted

148.6

68

33.4

19

6.1

2.1

Clay

Diesel

Untreated

Compacted

2400

850

450

100

20

5

Clay

Diesel

Treated

Compacted

73.3

27.1

9.2

4

1.3

0.6

INCREASING PORE VOLUME

Figure 1 Cumulative release of TPH gasoline vs. increasing pore volume.(■) Sandy treated; (□) sandy untreated; (♦) silty sand treated; (o) silty sand untreated; (A) clay treated; (A) clay untreated.

INCREASING PORE VOLUME

Figure 1 Cumulative release of TPH gasoline vs. increasing pore volume.(■) Sandy treated; (□) sandy untreated; (♦) silty sand treated; (o) silty sand untreated; (A) clay treated; (A) clay untreated.

resultant curves shown as Figure 1 have identical shapes for both treated and untreated samples. The release of TPH from the untreated samples is due to simple washoff or desorption from the soil surface. There obviously has been no treatment of these samples, since diffusion and ad-vection do not play a part in contaminant release. As the Siallon-treated samples show identically shaped release versus pore volume curves, the method of contaminant removal must be very similar to that in the untreated samples—a simple washoff of the unencapsulated TPH. The differences lie in the amounts of TPH released.

One of the uses of leach tests is to determine the actual kinetics of contaminant leaching from within treated wastes. There are several mechanisms for leaching contaminants from encapsulated or solidified materials.

At the particle-leachate interface, both dissolution and desorption can occur. The contaminant can diffuse out of the encapsulated or solidified matrix. Dissolution of the encapsulant or solidified material will lead to steadily increasing cumulative release results.

Chemical reaction can occur between the leachant used and either the encapsulation or solidifying material or between the leachant and the contaminant.

According to the Godbee and Joy [18] model, a linear relationship between the cumulative fraction of TPH leached and the square root of time for the leaching is a consequence of diffusion control. When the results from Table 4 are plotted this way, the resulting curve shows a rapid rise followed by a straight line of zero slope. Since the slope is equivalent to De' (the apparent diffusion coefficient), it holds that following the rapid washoff of the unencapsulated TPH, there is no detectable diffusion from within the Siallon silica cell. This was to be expected because the Siallon silica cell is resistant to the extraction effects of even an aggressive solvent such as methylene chloride. This resistance to extraction means that the solvent cannot penetrate the silica cell to extract the hydrocarbon; similarly, the hydrocarbon cannot diffuse out of the cell in any leaching protocol. This is clearly shown in Figure 2 and is especially evident in the clay soil sample, where there is a rapid washoff of the unencapsulated TPH followed by a straight line with zero slope, indicating no diffusion from within the Siallon silica cell.

Square Root - Time

Figure 2 Cumulative fraction leached vs. square root of time—Siallon-treated diesel-contaminated soil. (■) Sandy; (A) silty sand; (♦) clay.

Square Root - Time

Figure 2 Cumulative fraction leached vs. square root of time—Siallon-treated diesel-contaminated soil. (■) Sandy; (A) silty sand; (♦) clay.

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