Impact of biodegradable trenching slurry on iron treatment wall performance

e:>

Pergamon

War. Sci. Tech; Vol. 38, No.7, pp. 49-53.1998. IAWQ

e 1998 Published by ElsevierScience Ltd. Printedin GreatBritain.All rightsreserved PH: S0273-1223(98)OO606-4

0273-1223198 $19'00 + 0'00

IMPACT OF BIODEGRADABLE TRENCHING SLURRY ON IRON TREATMENT WALL PERFORMANCE D. Navon, R. C. Loehr, H. M. Liljestrand and D. E. Daniel, Jr Civil Engineering, 8.6 ECJ, University ofTexas, Austin, TX 78712. USA

ABSTRACT Biodegradable trenching slurries have been proposed as a cost effective method for the emplacement of reactive treatment barriers. In this study. the impact of a bio-polymer slurry on the ability of zero-valent iron to reductively dehalogenate trichloroethene was measured. First-order trichloroethene degradation rates were determined for three types of iron. with and without slurry in batch reactions . For the commercial grade iron. a significant difference was determined between the surface area normalized rate without slurry (1.78E-4 L m-2 hr l ) and with slurry (I.23E.4 L m-2 hr- I ). However. no significant difference was determ ined with or without bio-slurry for the reagent grade irons. ICl 1998 Published by Elsevier Science Ltd. All rights reserved

KEYWORDS Bio-slurry; in situ treatment; reactive barriers; trichloroethene degradation; zero-valent iron. INTRODUCTION In situ treatment wall technologies have been proposed to transform or remove pollutants from contaminated groundwater (e.g. Starr and Cherry, 1994). Zero-valent metals are known to react with chlorinated organics (e.g., Reynolds et al., 1990), the process has been patented (Blowes and Ptacek, 1994), and full-scale pilot field tests have begun (e.g., Appleton, 1996). One of the main costs has been the installation of containment barriers and reactive media for the in situ removal or destruction of pollutants. Standard trenching can aChieve depths of 18 m with the use of steel sheet piles to stabilize the sidewalls until backfilling is completed (Rumer and Ryan, 1995). An alternative is to pump into the trench a solution of about 1% biopolymers to create use a viscous, pseudo-plastic slurry during excavation. The hydrostatic force supports the sidewalls and allows excavation to depths of more than 45 m (Gavaskar, 1997). The slurry is broken and displaced by the containment or reactive media. This study addresses whether the use of bio-polymer impacts the effectiveness of zero-valent iron as a ~eactive barrier for the reductive dehalogenation of trichloroethene. In construction of a treatment wall, the Iron is suspended in the slurry, and its surface would be coated during emplacement. The slurry is degraded or "broken" to a less viscous fonn and displaced from the trench, leaving the reactive iron in a permeable sand matrix. Potentially, residual organics could remain on and deactivate parts of the iron surface. The

49

50

D. NAVON et aL

reaction rate of trichloroethene has been determined to be first-order in the mass sorbed on the iron surface, where the amount sorbed is limited and described by a competitive Langmuir isotherm (Burris et al., 1995). The mechanisms for trichloroethene reaction with zero valent metals are fairly well known (Roberts et al., 1996), and sorption to the metal surface is a key step in its degradation as well as its degradation products (Ortha and Gillham, 1996). Alternatively, the bio-polymers could sorb pollutants, depress their free aqueous concentration and make them less available for reaction at the solid surface (Schwarzenbach et al., 1993). To investigate whether bio-polyrners could impair reaction rates, an experimental study of the degradation rates of trichloroethene by iron was performed in the presence and in the absence of a ground guar bean biopolymer. MATERIALS AND METHODS Batch tests were performed in 40 mL glass vials. Three types of samples were prepared: blanks (3-5 mglL trichloroethene in filtered MiIIi-Q water with 0.05 M Na2S04 as an ionic strength adjustment), reactive samples (5 giron, 0.5 g pyrite for pH/pe buffering, and the solution of 3-5 mgIL trichloroethene with 0.05 M Na2S04)' and reactive samples containing an additional I g of guar bean based biodegradable trenching slurry (GeoCon. Inc.). For the samples with slurry, the iron, pyrite and slurry were placed in the vials and mixed on an orbital shaker for eight hours in order to ensure coating of the surfaces. The final 1 g guar/40 mL concentration contributes approximately a TOC of 50 mgIL, BOD of 73 mgIL, and a COD of 160 mgIL to the slurry samples. The vials were filIed with the trichloroethene solution leaving no head-space and sealed immediately with open-top screw caps containing Teflon-faced septa. The vials were placed in a tumbler (10 rpm) at room temperature (22°C) and extracted by pentane with trichloromethane at 1 mgIL as an internal standard over time (up to 200 hours) to measure the concentration remaining. The extraction procedure required two syringe needles. One open needle was first inserted through the Teflon septum to alIow sample fluid to be displaced as extraction fluid was introduced. The vial was inverted and 2 mL of the pentane with trichloromethane was injected directly using a Hamilton gas-tight 5 mL syringe, fulIy inserted to about 3.8 em. After injection of the pentane, the syringe and second needles were removed and the vials were always kept inverted to prevent the extraction fluid from contacting the pierced septum, iron or pyrite. The inverted vials were placed in a tilted angle on an orbital platform shaker for one hour at 180 rpm. After this extraction, the vials were turned cap up and opened, and the pentane was rapidly skimmed using disposable Pasteur pipettes to minimize volatilization losses. The extraction fluid was placed in 2 mL gas chromatograph vials containing 0.25 mL inserts. These vials were sealed by crimping on rubber-lined metal covers and stored at -20°C no longer than 10 days prior to analysis. Extracts were analyzed by GC-ECD using a Hewlett Packard 5890A gas chromatograph equipped with a Hewlett Packard 7673A automatic injector and Vetra VL2 data acquisition system. Triplicate injections of 1 Ill. were analyzed for each sample, using a 30 m column, 0.25 mm I.D. with 0.25 mm thickness of DB-I701 stationary phase. The initial oven temperature was 50°C for 5 minutes and then ramped 10°C/minute for 2 minutes to the maximum temperature of 70°C. Three types of iron were used. The commercial grade was industrial scrap filings, -8 to +16 mesh of irregular shape supplied as Peerless Iron (Detroit MI). This material was not pretreated to remove any oil or grease, and BET analysis using nitrogen gas indicated it had a surface area of 0.900 m 2/g. A degreased , laboratory grade iron was obtained from Alfa Chemicals. This iron consisted of spherical particles of -40 to +70 mesh with a BET surface area of 0.199 m2/g. Its surface was further cleaned by rinsing with 3% hydrochloric acid, rinsing with Milli-Q water, and drying at 102°C. The third iron, degreased, laboratory grade, 40 mesh irregular filings with a BET surface area of 2.98 m 2/g from Anachemia (Canada), was also acid washed. RESULTS The pseudo first-order decay of trichloroethene is given by eq. I,

Biodegradable trenching slurries

51

(I)

where C is the concentration (mglL) of trichloroethene at time t (hr) and k l is the first-order rate constant (hr l ) . The concentration is monitoredfrom its initial concentration (CO> of 3-5 mgIL to the detection limit. The integral form of eq. I is given by eq. 2, and the slope of a plot of (n(ClCO> versus time were used to determine the pseudo first-orderrate constant (k I)' In(ClCo) =- k l t

(2)

The results for both cases of with and without bio-polymerslurry using the commercial grade Peerless iron, laboratory grade Alfa Chemicals iron, and laboratory grade Anachemia iron are presented in Fig. 1-3, respectively. These pseudo first-order rate constants in units of hr l are the slopes of the regression lines in the figures below. These implicitly assume a constant surface area (SA in m2) of iron to volume (V = 0.04 L) of trichloroethene solution. The surface-area normalized rate constants (kSA) are summarizedin Table I, where kSA =k l VISA, which allows a more direct comparison of the reactivity of irons of different size and shape (Johnson et al., 1996).

o"t"-----------------, y = -0.0374 - 0.0139x

-1

-.=~

R"2 = 0.99n

0

·2

-3

•

Fe

A. Fe+Blopolymer

-4+-...-,-...-...........-,-~ ......-r-...._r.........,.._.....-_r__.,.......,...-~ o 20 40 60 80 100 120 140 160 180 200 Time (hours) Figure I. Trichloroethene degradation by Peerless Iron (5 g/40 ml.), where the initial concentration Co = 3.4 mgIL.

A StUdent's T-test was used to compare the variances of the rate constants with the difference between the rate constants for the cases of with and without bio-polymerslurry present. The calculatedT for the Peerless Iron was 48.4, as compared with t = 2.16 for the 5% significance level for 4 degrees of freedom. The calculated T values for the Alfa iron with and without biopolymer was 0, and that for the Anachemia iron With and without biopolymer was 0.282. Thus, there was not a statistically significantdifference between the ~ates in the presence or absence of biopolymer slurry for the two degreased, acid washed, laboratory grade Irons. There was a significant slowing of the rate in the presence of biopolymer slurry for the commercial grade industrial scrap filings which had been neither acid washednor degreased. The surface-area normalized rate constants presented in Table I are comparable to those reported by JOhnson et al, (1996). Literature reported batch and column first-order trichloroethene dehalogenation rates using iron metal were converted to their surface-area normalized values, with (3.9±3.6)xlO·4 L m,2 hr'( beinga representative value. Values in this study range from 0.3x I0-4 to 2.0x I0-4 L m·2 hr' I.

52

D. NAVON et al.

0.0 -0.2

-

••

-0.4

0

~ 0

-0.6

.5 -0.8

• •

-1.0

v » -0.0034 - 0.0052x Fe R"2 .. 0.9452 v » -0.0052x Fe+Blopolymer R"2 .. 0.919

-1.2 +----,r----r--r--or---r--r---r--r-----r---! o 20 40 60 80 100 120 140 160 180 200 Time (hours) Figure 2. Trichloroethenedegradationby AlphaChemicalsiron (5 g/40 mL), where the initial concentrationCo", 3.8mgIL.

0.0 -0.2

0

~

0

.5

y .. -0.0374 • 0.0139x

-0.4

R"2 .. 0.99n

-0.6 y

-0.8

=0.0287 •

0.0201 X R"2 • 0.9954

-1.0 • •

-1.2

Fe. Biopolymer Fe

-1.4 0

10

20

30 40 50 Time (hours)

60

70

80

Figure 3. Trichloroethenedegradation by Anachemiairon (5 g/40 rnl.), where the initialconcentration Co =4.5 mgIL.

The economic implications result from the size of the reaction zone and the amounts of iron required. For slowerrates, the reaction zone must be largeror more iron reactive ironsurfacearea must be suppliedin the same reactor volume. For example. providing 45% more Peerless iron in the same volume would compensate for the reduced reactivity in the presence of biopolymer. Increasing the volume reaction zone would be morecostly than increasing the concentration of iron. The concentrations (e.g.•Co =3-5 mgIL) in this study are trace levels wellbelowthe maximum solubility. In the limitof low concentrations. the Langmuir isotherm approaches a linear isotherm, and the iron surface is not saturated with trichloroethene. Under these conditions, competitive sorption of pollutants and guar on the metal surface would not be a limiting factor. If the reactive barrier were to be emplaced using the bioslurry method near a highly contaminated source, further studies would be needed to examinethe effect of high concentrations of pollutant with residual biopolymer.

Biodegradable trenching slurries

53

Table I . Surface-area normalized rate constants for the destruction of trichloroethene

ksA' (L m,zh(l) Without Biopolymer

With Biopolymer Present

Peerless Iron, Commercial Grade

0.000178

0.000123

Alpha Chemicals iron, Degreased, Acid Washed Laboratory Grade

0.000209

0.000209

Anachemia iron. Degreased, Acid Washed Laboratory Grade

0.0000408

0.000031 ]

Partitioning onto soluble guar is also be expected to be linear with respect to the concentration of free neutral organic pollutant (Schwarzenbach et al., 1993). Thus. a constant fraction of the total aqueous trichloroethene concentration is bound to the biopolymer. In cases where the bioslurry concentrations remain high, the fraction of bound pollutant would be more important. CONCLUSIONS In this study, the degradation of trichloroethene by zero valent iron was demonstrated under conditions in which an organic polymer is present. Large molecular weight organics bind with free aqueous organic species. making the nonpolar pollutants such as trichloroethene less available for reaction. Moreover. natural organic compounds competitively sorb onto solid surfaces, thereby decreasing available reaction sites. The guar bean biopolymer used in this study did result in a statistically significant decrease in reactivity of the commercial grade iron. However, the biopolymer did not result in a statistically significant decrease in reactivity of the two degreased, acid washed laboratory grade irons. Thus. biopolymers used to create viscous. pseudo-plastic slurries to stabilize deep trench walls appears to be a technology compatible with zero-valent metal reactive barriers . The 40% decrease in reactivity of the commercial grade iron should be accommodated by a safety factor in the design of the reactive wall thickness . REFERENCES Appleton. E. L. (1996). A nickel-ironwall againstcontaminated groundwater. Environ: Sci. Technol.• 30(12). 536A-539A. Blowes, D. W. and Ptacek. C. J. (1994). United States Patent: System for Treating Contaminated Groundwater. Patent No.: 5.362,394. Burris. D. R.• Campbell. T. J. and Manoranjan, V. S. (1995). Sorption of trichloroethylene and tetrachloroethylene in a batch reactivemetallic iron-watersystem. Environ. Sci. Technol.• 29(II), 2850-2855. Gavaskar, A. R. (1997). PermeableBarriers for GroundwaterRemediation: Design.Construction and Monitoring, Battelle Press, Columbus.Ohio. Johnson.T. L.• Scherer, M. M. and Tratnyek, P. G. (1996). Kineticsof halogenated organic compounddegradation by iron metal. Environ. Sci. Technol•• 30(8), 2634-2640. Orth, W. S. and Gillham, R. W. 1996. Dechlorination of trichloroethene in aqueous solution using Fe", Environ. Sci. Technol.• 30(1l, 66-71. Reynolds, G. W.• Hoff. J. T. and Gillham. R. W. (1990). Sampling bias caused by materials used to monitor halocarbons in groundwater. Environ. Sci. Technol., 24(1), 135-141. ROberts. A. L.• Totten. L. A., Arnold. W. A., Burris. D. R. and Campbell. T. J. (1996). Reductive elimination of chlorinated ethylenes by zero-valentmetals. Environ. Sci. Technol.• 30(8}, 2654-2659. Rumer. R. R. and Ryan, M. E. (1995). Barrier Containment Technologies for Environmental Remediation Applications. John Wiley & Sons, Inc.• New York. Schwarzenbach, R. P.. Gschwend, P. M. and Imboden,D. M. (1993). Environmental OrganicChemistry, Wiley-Intersciencc. New York. Starr. R. C. and Cherry, J. A. (I994). In situ remediation of contaminated ground water: the funnel-and-gate system. Ground Water. 32(3). 465-476.