Environ. Sci. Techno/. 1995,29,2528-2534
Intemted In Situ Soil Re&diation Technology: The Lasagna Process SA V. H O , * P. WAYNE S H E R I D A N , C H R I S T O P H E R J . ATHMER, MICHAEL A. HEITKAMP, JOAN M. BRACKIN, DEBORAH WEBER, AND PHILIP H. BRODSKY Monsanto Company, U4E,800 North Lindbergh Boulevard, St. Louis,Missouri 63167
Contamination in low-permeability soils poses a significant technical challenge to in situ remediation efforts, primarily due t o poor accessibility to the contaminants and difficulty in uniform delivery of treatment reagents. This paper discusses an integrated in situ remedial approach in which electrokinetics is coupled with sorption/degradation in treatment zones that are installed directly in the contaminated soils. Called Lasagna due to the layered appearance of electrodes and treatment zones, the technology could conceptually treat organic and inorganic contamination as well as mixed wastes. Laboratory results obtained with p-nitrophenol as a model organic contaminant in kaolinite support the feasibility of the approach. Key operating characteristics of the process are discussed.
Introduction During the last decade, a great deal of research has been conducted to develop in situ technologies for treating contaminated soils and groundwater. In situ methods are attractivebecause of the potential lower cost, less disruption to the environment, and reduced worker exposure to the hazardous materials. However, promising in situ treatments such as bioremediation, vapor extraction,and pumpand-treat have been found rather ineffective when applied to low permeability soils present at many contaminated sites. Recently, the use of electrokinetics as an in situ method for soil remediation has received increasing attention due to its unique applicability to low-permeability soils (1-10). Electrokinetics includes the transport of water (electroosmosis) as well as ions (electromigration) as a result of an applied electric field. Electroosmosis in particular has been used since the 1930s for dewatering clays, silts, and fine sands ( 1 1 ) . For remedial applications, water is typically introduced into the soil at the anode to replenish the water flowing toward the cathode due to electroosmosis. The * Corresponding author telephone: 314-694-5179; Fax: 314-6941531; e-mail address:
[email protected].
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ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 10, 1995
water flow is utilized to flush contaminants from the subsurface soil to the ground surface at the cathode region for further treatment or disposal. Advantages with electroosmosis include uniform water flow through heterogeneous soil, high degree of control of the flow direction, andvery low power consumption. There are, however, several major drawbacks associated with electroosmosis for remedial applications. These include low speed (liquid velocity induced by electroosmosis is typically about 1 in./day for clay soils), additional aboveground treatment, and unstable long-term operation resulting from soil consolidation and cracking, steep pH gradient in the soil bed, and precipitation of metals and minerals near the cathode (4-7). The slowness of electroosmosis is less of an issue for cleanup of metal contamination where transport by electromigration is normally dominating and can be several times faster than the optimum electroosmotic flow (9,10). The pH changes primarilyresult from water electrolysisas the predominant electrode reactions, generating acid at the anode and base at the cathode according to the following reactions: anode: 2H,O - 4ecathode: 2H,O
+ 2e-
- 0, +
-
H,
4H’
+ 20H-
E, = -1.23 V
E, = -0.83 V
where Eo is the standard electrochemical reduction potential. Recent efforts to mitigate the pH problem primarily involve conditioning the anode and cathode solutions through external recirculating loops (8, 9). In this paper, we describe an integrated approach coupling electrokinetics with complementary in situ technologies to eliminate or minimize the drawbacks associated with the use of electrokinetics alone and present experimental results supporting the feasibility of the approach. Integrated In Situ Remediation Concept (Lasagna). The general concept is to use electrokinetics to move contaminants from the soils into “treatment zones” where the contaminants are removed from the water by adsorption, immobilization, or degradation. One possible configuration of the process has the following components: (a) Create highly permeable zones in close proximity sectioned through the contaminated soil region and turn them into treatment zones by introducing appropriate materials (sorbents, catalytic agents, microbes, oxidants, buffers, etc.). Hydraulic fracturingand related technologies may provide an effective and low-cost means for creating such zones horizontally in the subsurface soil (12). The treatment zones can also be vertical, which can be constructed using sheet piling, trench, slurry wall, etc. Instead of being discrete, the treatment zones can be continuous using in situ soil mixing devices. (b)Utilize electrokineticsfor transporting contaminants from the soil into the treatment zones. Since these zones are deliberatelylocated close to one another, the time taken for the Contaminants to move from one zone to the adjacent one can be short. In the horizontal configuration,the zones above and below the contaminated soil area can be injected with graphite particles during the hydrofracturing process to form in-place electrodes. For highly nonpolar contaminants, surfactants can be introduced into the water or
0013-936X/95/0929-2528$09.00/0
@ 1995 American Chemical Society
borehole
a
m u nnmde Incm
-
,IhrnY*E k l r a d e
i27rrn
ascn,--aiic.-
-ii"n--r
FIGURE 2. Electroosmotic cell for studying the integrated technologylolid graphite electrodes. W
NOW rlmmorrnoll~flow i s m a n e d upon i r l l t h l n g rlrddrrl polidty.
FIGURE 1. Schematic diagram of the integrated in situ process-horizontal la1 and vertical (b) configurations.
incorporated into the treatment zones to solubilize the organics. For a mixture of organics and metals, the treatment zones cancontainsorbents for bindingthe metals and contain microbes or catalysts for degrading the organics. (cl Liquid flow can be periodically reversed, if needed, simplybyswitchingtheelectricalpolarity.This modewould enable multiple passes of the contaminants through the treatment zones for complete sorption/destNction. The polarity reversal also serves to minimize complications associated with long-term operation of uni-directional electrokinetic processes as discussed above. Optionally. the cathode effluent (highpH) can he recycled directlyback to the anode side (low pH), which provides a convenient means for pH neutralization as well as simplifies water management. Electrodesand treatment zones can be of any orientation depending upon the emplacement technology used and the sitelcontaminant characteristics. Schematic diagrams of two typical configurations, horizontal and vertical, are shown in Figure 1. The process has been called Lasagna due to thelayered configurationofelectrodesand treatment zones. The technology could be effective for treating contaminated low-permeabilitysoils (clayey, silty soils) or heterogeneous soils (clay lens in permeable soils).
Experimental Methods Setup for Electroosmotic Removal of PNP Coupled Wlth In Sihr Adsorption. Electroosmotic studies were carried out using either solid graphite electrodes or granular activated carbonlgraphite electrodes. The cell containing solid graphite plate electrodes (with holes to allow liquid passage1 is depicted in Figure 2. It is a modification of the cell purchased from Electrokinetics, Inc. (61, to allow investigationof the electroosmotic process for soil column
containingmanylayers. Each electrode (10 cm in diameter and 0.64 cm thick) was housed in a compartment of about 310 cm3 in volume (230 cm3 in front and 80 cm3 in back of the electrode). Electrical connections were made via stainless steel rods screwed into the side of the electrodes through the tube wall. The soil was packed in a cylindrical tube made of clear plastic having the dimensions of 10 cm inside diameter and21.6cmlong. Packedinthemidsectionofthecellwas Georgia kaolinite clay (air-dried, air-floated kaolinite obtained from Thiele Kaolinite Co., Wrens, MI. The characteristics of this clay arewell documented in the literature (3).with hydraulic conductivity in the range of cmls. The clay was uniformly contaminated with an aqueous solution containing p-nitrophenol (PNP) as the model organiccontaminant. Typically. about 500 gofdrykaolinite clay was mixed with 300 g of aqueous solution containing 1050 mg of PNPIL, which resulted in a clay paste of 37.5 wt % moisture and a loading of 0.39 mg of PNPIg of wet clay. This PNP-contaminated clay section was bracketed ateachendwitha1.27-cmlayerofsandandcarbonparticles (2.49bcarbonbyweight]. Thecarbonusedwasanactivated carbon obtained from Calgon and exhibited high-adsorption capacityfor PNP (0.5gofPNP/gof carbon). The sandcarbon layers thus represented permeable adsorption zones. Uncontaminated kaolinite clay (about 38 wt % moisture1 was packed next to each sand-carbon layer to facilitate detection of any PNP that could escape the carbon adsorption zones. Glass fiber filters were placed between different soil layers to facilitate soil sampling. Ports were installedalongtherubewall toserveas posts formeasuring the voltage drops across the various soil sections between the electrodes. The well water used in the experiments to simulate groundwater had a pH ofabout 8 and conductivity of about 600 pS1cm. To simulate the configuration encountered in using hydrofracturing to install electrodes and treatment zones in subsurface soils, the cell design shown in Figure 3 was used. The cylindrical tube was 10cm i.d. and 23.5 cmlong. The packed granular carbon electrodes were in direct contact with the soil instead of being separated by a water layer as in the Electrokineticscell. Each electrode consisted of about 50 g of granular activated carbon packed into a 1.27-cm layer. Connection to each electrode was made with a carbon rod through the side ofthe tube wall. Water inlet and outlet were conducted through the packed electrodes. A layer of clean clay was packed in the back of each electrode to both keep the carbon particles in place and prevent water leakage. At the end of each experiment, several small clay samples at different locations in the contaminated clay section as well the whole clay section itself were analyzed for PNP. The analysis involved extracting PNP from clay samples VOL. 29. NO. 10.1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY. 2529
Liquid Inlout
It)
Llquld InlOvl
+
1-1
FIGURE 3. Electroosmotic cell for studying the integrated technology-granular carbon electrodes.
with 0.1 N NaOH solution and measuring the level of PNP insolution byspectropbotometricabsorptionat 400 nmor by high-performance liquid chromatography (HPLC). One extraction was sufficient to remove all PNP from clay. For carbon, which binds PNP much more tightly, the extraction solution contained 0.1 N NaOH and 2 wt % methylene chloride. and repeated extractions were carried out to maximize PNP recovery. Setup for ElectmasmosisExperlmentswith Concurrent Biodegradation. Experiments coupling electroosmosis with in situ biodegradation of PNP utilized Pseudomonas sp. strain PNPl, a bacterium reponed to mineralize high concentrations ofPNPas asole source ofcarbonand energy (13). Bacterial solutions used for inoculation into the electroosmotic unit were typically prepared by growing Pseudomonassp. PNPl in L-salts media (14) that contained about 100 mg of PNP/L. A known volume of this bacterial solution was injected into the treatment zones prior to the electroosmosis experiments. Microbial supports packed into the treatment zones included sand, powdered and granular activated carbon, and sawdust. Oxygen was supplied for biodegradation by sparging air through tubes connected to the bottom of the treatment zones. Due to its extremelylowvolatility(Henry's constant =4.15 x atm m3mol-l (la), PNP's loss through gas stripping was insignificant: less than 0.02 mglweek or
