Soil Washing: Practical Considerations and Pitfalls

Dennis Dove Alok Bhandari John Novak

Dennis Dove is a senior researcb associate in tbe end- engineering section of tbe Cbarks E Via, Jr. Department of Civil Engineering at Virginia Polytecbntc Institute @TI) and State University in Blacksburg, Virginia. ACok Bbandari is a graduate student in environmental engineering, and Jobn Nova4 Pb.A, P.E, is tbe Nick Prtllaman Professor of Environmental Engineering, botb at VPI.

The use of soil washing to removepehleum hydrvcarbon contamina- tion from the soil matrix is becoming more widely used. when viewed as a volume reduction tool, this technology shows some promise. However, ongoing research and treatability studies indicate that without further treatment, even larger-sized soil fractions (sands and cobbles) may retain hydrocarbon contamination at levels that require further cleaningprior to permanent disposal or m e . 7beperception has been that by removing the sandfrom thesoil matrix, thusachieoing a30percent to dopercent volume reduction, expensive post-washing treatment or approved disposal of the finer materials (silts and clays) would be cost-effective. There exists evi- dence to the contrary, however. Hydrocarbon retention after soil washing may be influenced by a number of factors unrelated to particle sire. Soil chamctmtics that may play a role include soil humic aciak, metal oxide coatings, geologic origin of the soilparticles, and clay type. In this article theauthots describe a laboratory study designed to evaluate the “cleanability ” of two soils.

Soil washing is a promising innovative technology and one of the alternative treatment methods specified by the US. Environmental Protec- tion Agency for use at Superfund sites to reduce the quantity and impact of pollutants in soil. In fiscal year 1989, soil washing or flushing was prescribed by EPA in seven source control records of decision for waste site remediation (Daley, 1989). Nash and Traver (1988) define soil washing as the use of mechanical or chemical means to disperse contaminated soil and isolate the contaminants into as little soil as possible. The process separates the bulk soil into different fractions and then directs appropriate treatment toward each of the fractions.

The relationship between soil particle size and the contaminant depends on the manner of waste application or disposal, the soil matrix characteristics, the soil cation-exchange capacity, and the soil organic matter. Contamination is generally greater for clay and silt than for coarser particles because fine particles have a higher ratio of surface area to mass. Clay also has a high cation-exchange capacity, which encourages contami- nant association. Silt and clay may also physically adhere to sand and gravel; washing the soil under high shear helps separate silt and clay from the coarser materials.

Soil washing may be used on a very wide range of contaminants, such as heavy metals and semivolatile organics such as polychlorinated

REMEDIATION/WINTER 1992/93 55

D m s DOVE ACOK BHANDARX JOHN NOVAK

Figure 1. Particle Analysis for Uncontaminated Eagle Point and New River Soils.

Silt

Clay 27%

Clay 26%

36%

Eagle Point Soil

Silt

34%

New River Soil

biphenyls (PCBs) and pesticides. The system has the potential for removing organic and inorganic contaminants simultaneously and encour- ages recycle and reuse of decontaminated material. Experience in Europe shows that soil washing can be conducted on a large scale at relatively low costs (Pheiffer, 19!30), especially if the concentration of fines is less than 20 percent.

The major drawbacks to soil washing are that (1) it may generate a large quantity of wastewater and (2) the washing solution may carry away a large quantity of soil. In addition, there are instances in which soils that appear to be good candidates for the soil washing process fail to attain the desired degree of "cleanliness." Differences in the cleaning ability of soils using the soil washing process are the focus of this study. In this article we describe an investigation of two petroleum-contaminated soils that were physically similar but differed dramatically in contaminant retention. One of these soils was taken from a location adjacent to a refinery where soil washing is being considered as one of the options for remediation; the other was from a site near an industrial complex.

The information presented should help those considering soil washing as a potential remediation technology avoid using a technology that will fail to provide the desired treatment goals.

METHODS Two different soils were used in this study. These soils were obtained

from field locations adjacent to contaminated areas in river flood plains from areas of similar landscape. The New River soil was collected from a location within the flood plain of the New River in Montgomery County, Virginia. A number of contaminated sites exist in this general area. Eagle Point soil was collected at the floodplain of the Delaware River south of Cherry Hill, New Jersey, near a refinery. Soil washing was being considered as a possible remediation method at the Eagle Point site. Particle size

56 REMEDIATION/WLNTER 1992/93

SOIL WASHING: PRACTICAL CONSIDERATIONS AND PITFALLS

analysis for the soils (Figure 1) indicates that the soils were similar and can be classified as silty clay loam.

Soil Preparation To allow for direct comparison of soils and removal efficiencies, clean

soil was contaminated in the lab rather than collecting contaminated soils at the si'es. Bulk samples of each soil were sieved to pass a 2-mm screen and airdried prior to contamination. Subsamples of 500 g were placed in glass bottles and contaminated with 8 percent hydrocarbon by weight. (The hydrocarbon was a petroleum distillate obtained from a fuel depot in California.) The oil was allowed to seep into the soil, which was then mixed for four hours to insure a thorough distribution of oil on the soil.

soil washing Pilot Plant Soil washing.. . fiactionatea contaminated soil into different particle size f?actions . . . in order to isolate the contaminant onto the small-size fractions and leave behind a Clean Sand-

Soil washing is a physicochemical process that fractionates contami- nated soil into different particle size fractions (sand, silt, and clay) in order to isolate the contaminant onto the small-size fractions and leave behind a clean sand. The process also removes hydrocarbons from particles of all sizes by dissolution into the washing medium, air stripping, and transfer- ring contaminants into the froth fraction.

In this study, contaminated soil was washed with water using a pilot- scale soil washing system that hydraulically separated the soil into fractions based on particle size and weight. A diagram of the system is shown in Figure 2. The system included a high-shear mixer, an air flotation unit, a low-shear mixer, an upflow column, and a sedimentation tank.

High-shear mixing was carried out for forty-five minutes in order to break up soil clumps and to knock the fines off the coarser particles. A Wemco Agitair laboratory flotation unit was used to induce air into the system. The flotation unit suspended the soil particles and at the same time injected air into the slurry, resulting in some of the hydrocarbons being carried to the surface with the air bubbles. The surface froth that formed was skimmed off.

After removal of the froth, the slurry was transferred to a low-shear mixing cell that was hydraulically connected to an upflow column followed by a sedimentation tank. Hydraulic connections were made between the units so that the slurry would pass through each, then be recycled back to the upflow column. The flows at the lower end of the upflow column were adjusted so that the upflow velocity was greater than the settling velocity of the silt and clay but less than that of sand. The settled sand was removed from the bottom of the column.

After the sand was removed, the silt- and clay-laden water flowed from the top of the upflow column into the sedimentation tank. The sedimen- tation tank was designed so the silt-sized particles would settle but the clay would not. The effluent from the sedimentation tank was then recycled into the lower end of the upflow column to provide a more efficient separation of silts and clays.

Each soil was washed in the pilot system for three hours. After the washing was complete, the fractions were air-dried and the petroleum

REMEDUTION/~I"ER 1992/93 57

DENNLS DOVE ALOR BHANDARI JOHN N o v a

Figure 2. Schematic Diagram of Soil Washing System.

[CONTAM~NATED sory

i High Shear Mixing

I

i

Sedimentation Tan [ UpfIow Column

[rn) i

hydrocarbon content of each fraction determined by extraction in meth- ylene chloride followed by analysis using capillav column gas chromatog- raphy. The distribution of n-alkanes following various treatments was also studied.

RESULTS AND DISCUSSION The pilot-scale soil washing process was very efficient in separating

particles by size, especially for the sand-sized (> 45p) fraction. The distribution of petroleum products between the various soil fractions and in the wash water was measured for the two soil types. The results of the pilot tests indicated many similarities between the two soils, but also spotlighted some differences.

TPH Distriiiiution on Washed Soils The soil size distributions and associated total petroleum hydrocarbons

CPH) for each particle size fraction are shown in Figures 3 and 4. It can be seen that for both soils, the sand fraction contained only a minor portion of the T’PH and the clay fraction was the most contaminated.

The specific TPH concentration associated with each fraction (Figure 5) indicated that both the clay and froth fractions were highly contami- nated. The solids contained in the froth are comprised primarily of clays

58 REMF.DIATION/WINTER 1992/93

Son. WASHING: PRACTIGU CONSIDERATIONS AND l?rr~m

Figure 3. Particle Size Fractionation of Eagle Point and New River Soil after Soil Washing.

(36 -0%)

Sand (32.8%)

NEW RIVER SOIL

Sand (60.4%)

EAGLE POINT SOIL

Froth (6.9%)

y (24.3%)

that have been trapped in the bubbles, and it is reasonable to expect that these materials would contain high TPH concentrations. The wash water was relatively low in TPH. The reasons for this are that the volume of water was large relative to the amount of soil and that because of the limited solubility of the TPH, contaminants that were transferred to the water phase

RF.MF.DIATION/WINTER 1992/93 59

DENNIS DOVE ALQK BHANDAXI JOHN N o v a

Figure 4. TPH Distribution by Fraction for Eagle Point and New River Soil after Soil Washing.

A Y c (7

New River Soil Eagle Point Soil

probably redeposited onto soil surfaces. The most important difference between the two soils is that the sand

fraction of the New River soil retained a TPH concentration of approxi- mately 1,000 ppm, whereas the TPH concentration in the Eagle Point sand was less than 200 ppm. Although target levels of TPH vary among regulatory bodies, a level of 100 ppm is often used as a decontamination level. Obviously the New River sand would not be acceptable for disposal without further treatment.

Distribution of Sped& Hydrocarbons The distribution of specific hydrocarbons on the Eagle Point and New

River soils are shown in Figures 6 and 7. Most of the n-alkanes of C,, (dodecane) or lighter were eliminated during washing, probably by volatilization. The clay fraction retained the largest amount of hydrocar- bons, but other than the loss of the C,, hydrocarbons, there was little difference in the distribution of specific hydrocarbons. It was expected that during high-shear mixing, the lower molecular weight organics might be transferred preferentially to the clay fraction. However, the distribution of contaminants on each soil fraction and in the froth generally reflected the distribution in the original soil samples.

The data shown in Figure 5 suggest that the degree of cleanliness of the two soils varied considerably, especially for the sands. Apparently something other than particle size was responsible for the TPH retention in New River soils. The prime candidate for the cause of enhanced TPH retention in New River soil was thought to be humic acids associated with the soil surfaces, which tightly bound the hydrocarbons. To study this factor, an experiment was conducted in which humic acids were selec- tively removed from each of the soil fmctions. Samples of New River and Eagle Point soils were hydraulically separated into three size fractions: sand, silt, and clay. The sand and silt from each soil were then split into two subsamples. One subsample from each size fraction was treated to

60 REMEDIATION/WINTER 1992/93

Son, WASHING: h m w CONSIDERATIONS AND PITFALLS

Figure 5. Hydrocarbon Concentration by Fraction for Eagle Point and New River Soils after Soil Washing.

0- I

0

0 c 0

Lc

0

E Q Q

0- I

0

0

cc

c 0 0

4000

3000

2000

1000

0 INITIAL FROTH CLAY SILT SAND WATER

16000

14000

12000

10000

4000

3000

2000

1000

0 INITIAL FROTH CLAY SILT SAND WATER

remove surface organic compounds that were present as humic acids. Humic acids were removed by adjusting the pH to greater than 12 by adding sodium hydroxide, then rinsing with distilled water. All subsamples were then contaminated with the petroleum distillate and allowed to equilibrate for fourteen days; they were then measured for TPH. Several cleaning methods were then applied to each soil subsample in a wash study:

Additional mixing with water Additional mixing with a surfactant/water solution Adjustment of the wash solution to an alkaline pH with and without surfactant

61

DENNIS DOVE ALOK BHANDARI JOHN N o v s

Figure 6. Distribution of Specific Hydrocarbons by Fraction for Eagle Point Soil.

t 0

60 t I I

.- U 0

z 7 20 [r I t - - 15 c 0 c .- 0 l o

50

c t 5 t 0

0

cr I

0-

1 c12

L INITIAL FROTH ClAY SILT SAND

Removal of humic acids before contamination of the sand fraction strongly affected TPH removal efficiency when the wash solution was neutral water. For both sand types the amount of TPH removed increased by approximately 10 percent (Table 1). The use of a surfactant wash solution at neutral pH dramatically improved removal efficiencies for Eagle Point sands, but had no effect on the New River sand. Removal of humic acids before washing with pH 7 surfactant solutions had no effect on TPH removal from Eagle Point sands; however, a slight increase was observed for New River sands. When alkaline wash solutions (- pH = 12) were used, no improvement in TPH removal was observed for New River and Eagle Point sand for which humic acids were intact before washing. Overall removal efficiencies were approximately 30 percent greater for Eagle Point sands when surfactant and alkaline pH adjustment was used. These results suggest that:

Humic acids contribute to TPH retention. Surfactant wash solutions are able to enhance removal in some sand fractions. An additional factor inherent in the soil matrix of New River sand was responsible for TPH retention.

62 REMEDIATION/WINTER 1992/93

Son. WASHING: PRA~~ICAL CONSIDERATIONS AND PITFALLS

Figure 7. Distribution of Specific Hydrocarbons by Fraction for New River Soil.

90 I I I i I 1

.- 35 U 0

30 7 E 25 CT

- U

c 20 0 c .- E 15

0 10 0

5

0

U

c U

m C l 6 c20

m C 2 4 m C32 P 1

i INITIAL FROTH CLAY SILT SAND

Results from extended wash experiments of the silt fraction from Eagle Point and New River soils indicated that a more complex set of interactions occurred (Table 2). Overall removal efficiencies were low. The use of neu- tral surfactant wash solutions improved TPH removal somewhat; however, the interaction with humic acid was not clear. The presence or absence of humic acids did not consistently alter TPH removal in Eagle Point silts. In New River silts only a slight improvement was observed when humic acids were removed prior to washing. Overall, washing with an alkaline surfactant solution was very effective in improving TPH removal efficiency.

The specific levels of residual TPH following soil washing remained high for both soils because of the extreme levels of applied contamination. In general, the TPH levels for the New River sand exceeded those for the Eagle Point sand by factors of greater than ten following most washing processes. This suggests that not only will soil washing not be successful for the New River soils, but also that other remediation methods may also be inadequate because of the strong affinity of contamination for these surfaces.

Iron Oxide Removal Throughout the experiments to examine the “cleanabdity” of soil

REMEDIATION/W~R 1992/93 63

DENNIS DOVE ALOK BHANDARI JOHN I%VAK

Table 1. TPH Removal from Sand-Size Fractions of Eagle Point and New River Soils with Several Wash Solutions.

Treatment Eagle Pointsand New River Sand

Initial TPH Contamination

H,O Wash

Humics Present

H,O Wash

Humics Present

Surfactant Wash

Humics Removed

Surfactant Wash

Humics Present

H,O Wash pH 12 Humics Intact

Surfactant pH 12 Humics Intact

PH 7

PH 7

PH 7

PH 7

1835 84.7

73.2

98.7

98.7

98.0

5125

5670

5557

5452

68.1

3200

152

152

240 5055

350 97.0 4380 72.7

64.7

65.4

66.0

68.5

fractions separated from New River and Eagle Point soils, residual TPH retention had been higher for New River sands. This appeared to be due to differences in geologic origins of the material. Several authors (Murphy et al., 1991; Jardine et al., 1390; Zielke et al., 1989) have suggested that the chemistry of mineral surfaces may play an important role in binding contaminants in stream and aquifer sediment. Based on the literature, it was thought that differences in the iron oxide coatings on the surfaces of the sand could account for some of the differences in TPH binding. Hydrous iron oxide coatings were removed from the sand fractions of

64 REMEDIATION/~INTER 1992/93

Son. WASHING: PRACTICAL CONSIDEIUTIONS AND Pmurs

Table 2. TPH Removal from Sand-Size Fractions of Eagle Point and New River Soils with Several Wash Solutions.

Eagle Pointsand New River Sand

TPH TPH TPH TPH

(O/O) ("/.)

Treatment

( p p d Removed (ppm) Removed

Initial TPH Concentration

H,O Wash

Humics Removed

H,O Wash

Humics Present

Surfactant Wash PH 7 Humics Removed

Surfactant Wash PH 7 Humics Intact

H,O Wash pH 12 Humics Intact

Surfactant pH 12 Humics Intact

PH 7

PH 7

14259 21916 - -

7082

9961

8387

51 14

8838 59.6 5618 60.6

1432 93.4 1077 92.4

67.6 4673 67.2

54.5

61.7

76.6

5405 62.1

2720 80.9

3540 75.2

Eagle Point and New River soils that had undergone the soil washing process. A two-step procedure was used to remove iron and also to characterize and quantlfy iron distribution in the sands. The first step removed only the amorphous oxides and the second removed any remaining iron oxides (crystalline). The iron content of the sands was calculated from the iron content in the extract; it is estimated that the iron removal process is 80 percent to 85 percent effective. The results of these experiments are presented in Table 3.

Both sands contained a similar total iron content, but the distribution

REMEDIATION/~INTER 1992/93 65

DENNIS DOVE ALOK BHANDARI JOHN NOVAK

Table 3. TPH Retention by Sand-Sized Fractions of Eagle Point and New River Soils before and after Iron Oxide Removal.

TPH TPH After Soil After Soil Washing Iron Content

soil washing and Iron Removal Amorphous Crystalline Total (PP@ (PPd (Ppm) (PPd (PPm)

Eagle Point 143

New River 948

86 7392 (SY% 880 (11Yo) 8272

370 4884 (66%) 2508 (34%) 7392

differed considerably. Nearly 90 percent of Eagle Point sand iron was in the amorphous form and the TPH retention was much less than for the New River sand. For the New River sand, 35 percent of its iron was in the crystalline form and TPH binding was substantially greater than for the Eagle Point sand. Removal of surface iron from the New River sand resulted in a 61 percent drop in the retained TPH. These data suggest that much of the petroleum is associated with the crystalline iron fraction in sands and this association is strong enough to make their removal by soil washing difficult.

In nature, the division between amorphous and crystalline iron oxides is not very sharp. In actuality, iron oxides probably exist as a continuum from strongly amorphous to strongly crystalline. Also, it should be recognized that the minerals represented in Eagle Point and New River sands probably differed and so comparisons based only on the iron content may not be entirely valid. However, the data suggest strongly that hydrous iron oxides play a role in the retention of TPH on the mineral surface of sand-sized particles, and crystalline iron appears to be associated with the higher and more difficult to remove TPH compared to sands with a lower crystalline iron content.

CONCLUSIONS In this study, two soils contaminated by a petroleum distillate were

processed through a pilot-scale soil washing process, and the residual TPH concentrations of each fraction were compared. The goal was to see if these soils were amenable to treatment by soil washing. The results showed that the Eagle Point soil was a good candidate for the soil washing process, but the New River soil was not. Both soils were easy to separate into various size fractions and the clays retained the majority of contami- nation. However, the New River sand fraction could not be cleaned to levels of TPH that would achieve regulatory limits. The Eagle Point sand was much easier to clean, and depending on the specific level of contamination encountered at the site, should yield a satisfactory TPH level following soil washing.

It appears that characterization of the soil by conventional particle size

Son, WASHING: Pmcrrc~~ CONSIDERATIONS AND P~~FALLS

analysis may not be adequate to determine the probability of success due to complicating factors related to soil components not normally considered to be important in other remediation technologies. For the soils in this investigation, surface iron oxides appeared to play a role in petroleum binding. An assessment of any soil treatment approach, including soil washing, should thus use bench-scale treatability studies to develop or refine the available technology and to see if the desired treatment goals are achievable.

Try as we might, soil washing did not appear to be capable of reducing TPH concentrations to acceptable levels for the New River soil. Studies comparing the sand fraction of the two different soils suggest that the crystalline iron oxide content of the New River soil was responsible for the retention of TPH, which made attainment of acceptable levels difficult. Additional treatment will be required to achieve TPH levels acceptable for unrestricted disposal of the New River sand.

Admittedly, any post-wash treatment will add to the cost and timeliness of cleanup. However, it would be far too simplistic to assume that state- of-the-art soil washing systems are capable of removing contaminants in ,the majority of soils. Judicious use of bench-scale and pilot-scale treatability studies are imperative in determining the rate of material handling as well as the effectiveness of the contaminant removal.

In the case of the soils in this study, soil washing would not be recommended for contaminated New River soil. For the Eagle Point soil, the soil washing option remains viable and should be considered as parz of the final decision process regarding the remediation of contaminated soils at this site. At this time, final decisions regarding cleanup at this site have not been made.

ACKNOWLEDGMENTS

The authors would like to acknowledge the financial support of Coastal Remediation Co. of Roanoke, Virginia, and the Virginia Center for Innovative Technology in Reston, Virginia.

REFERENCES

Daley, P. S. 1989. "Cleaning up Sites with On-Site Process Plants," Enuiron. Scf. Technol.

Jardine, P. M., N.L. Weber, and J.F. McCarthy. 1330. "Mechanism of Dissolved Organic Carbon Adsorption on Soil," Soil Scf. SOC. Am.J 53, 1378-85.

Murphy, E. M., J.M. Zachara, and S.C. Smith. 1991. "Influence of Mineral Bound Humic Substances on the Sorption of Hydrophobic Organic Compound." Enufron. Scf. Tecbnol.

Nash, J. and R.P. Traver. 1988. FfeUApplicatfon of Blot Soil Wizshing System. EPM68-03- 3450.

Phieffer, T. H. 1990. "EPA's Assessment of European Contaminated Soil Treatment Techniques," Enutron. Progress. 9 (21, 79-86.

Zielke, R. C., T.J. Pinnavaia, and M.M. Modand. 1983. "Adsorption and Reactions of Selected Organic Molecules on Clay Mineral Surfaces," Reactions undMovement of O?gunfc Chemfculs fn Soils, Sofl Scf. Soc. Am. Spl. Pub. No. 22.

23 (81, 912-16.

24 (101, 1507-16.

RE.MEDIATION/WINT%R 1992/93 67