Phytoremediation: Modeling Removal of TNT and Its Breakdown Products

Victor F. Medina Steven C. McCutcheon

Victor F. Medina i s a National Research Council Research Associate at tbe EPA National Exposure Research Laboratory, Ecosystems Research Dlvision (USEPA-NEW ERD) in Athens, Georgia. Steven C McCutcheon, P.E., is a Research Environmental Engineer at USEPA-NEWERD.

Contamination of soil and groundwater by trinitrotoluene is a widespread problem confronting milita y bases and ammunition manu- factu ring facilities throughout the United States. Phyto remediation pro- vides a promising treatment of ;rlVT-contaminated groundwater and wastewater because many plants contain the necessa y enzymes to de- grade explosives such as M. Twophytoremediation methods are proposed in this article: controlled reactors and constructed wetlands. Controlled reactors provide greater control of operating parameters, a reduced possi- bility of contaminant migration, control of animals feeding on theplants, and minimization of competition from other plant species. Constructed wetlands have relatively low capital costs, and the wetland becomes a desirable ecological resource. Because cost, as opposed to reactor size, appears to be the most significant factor for military base cleanup, this project focused on the constructed wetland approach.

To estimate the disappearance of W a n d its breakdown products from a constructed wetland, a first-order, nonreversible reaction, plug-flow, finite-difference model was developed. Batch scale experiments were con- ducted to define disappearance kinetics for individual chemical species. The results of the model suggest that reasonably sized wetlands may be used to treat a wastestream with an influent TNTconcentration of 2.25ppm atflow rates ranging from 10 to 5,000 gpm. Economic comparisons to other published costs for competing technologies are promising.

Contamination of soil and groundwater by trinitrotoluene (TNT) is a persistent and widespread problem confronting existing and former military bases and ammunition manufacturing facilities throughout the United States and the world. TNT-contaminated water has been generated by a number of means. The manufacture of TNT generates large quantities of wastewater, which is contaminated with TNT and other products. TNT- contaminated groundwater is found at many sites. Leachate systems have also been used to remediate TNT-contaminated soil, and these systems generate wastewaters that must be treated.

Cost-effective treatment of TNT-contaminated water is an important issue. Existing technologies include:

CCC 1051-5658/96/070131-15 0 1996 John Wiley & Sons, Inc.

31

VICTOR P. MEDINA STEVEN C. MCCUTCHEON

Phytoremediation is the use of plants to remediate, or assist in remediating, air, soil, groundwater, or wastewater.

Microbiological Treatment (USEPA, 1993); Adsorption on Activated Carbon (Griswold and Estrada, 1995);

Ultraviolet (W)-Oxidation/W-Ozone Oxidation (USEPA, 1993; Wujcik et al., 1995; Ritchie and Lechner, 1995; Griswold and Estrada, 1995); Wet Air Oxidation (USEPA, 1993; Hao et al., 1993; Hao et al., 1994); Supercritical Water Oxidation (SCWO) (Griswold and Estrada, 1995); Photochemical Remediation (Showalter et al. , 1995); and Electron Beam Destruction (Griswold and Estrada, 1995).

All of the above-mentioned technologies have certain advantages that make them promising. However, they also have disadvantages, ranging from high cost, to ineffective treatment under certain conditions, to the formation of undesirable breakdown products. There is a need for cost- effective alternatives. One alternative is phytoremediation.

PHYTOREMEDIATION Phytoremediation is the use of plants to remediate, or assist in

remediating, air, soil, groundwater, or wastewater. Plants have many features that make them useful for environmental purposes. Plants are adaptive to a wide range of environmental conditions (McCutcheon et al., 1995). Plants have unique protein and enzyme systems that may be of environmental use. These systems derive from the fact that plants have a different life style than heterotrophic organisms. Plants photosynthesize, as well as respire, and they lack mobility. Therefore, plants have enzyme systems for photosynthesis as well as unique biochemical systems for detoxification, nutrient acquisition, and the control of local geochemical conditions.

There are four techniques in which plants can be used to assist in environmental remediation. Phytoaccumulation involves the removal of metals, radionuclides, and organic contaminants from soil (Moffat, 1995; Kelly and Guerin, 1996; Cornish et al., 1995; Azadpour and Matthews, 1995; Wildemannet al., 1993; Chaney et al., 1995; Reeves et al., 1995; Cunningham, 1995) and water (Dushenkov et al., 1995; Wang et a1.,1995) and the accumulation of them in plant tissue. A second phytoremediation tech- nique is the use of plants as pumps, removing water and contaminants (Gatliff, 1994; Schnoor et al., 1995). A third phytoremediation technique takes advantage of the fact that rhizosphere, the area of the soil in which root penetration occurs, is rich in microbial activity (April1 and Sims, 1990; Anderson et all 1993; Anderson et al., 1994; Sorenson et al., 1994; Schwab et al., 1995).

A fourth phytoremediation technique is phytotransformation, which relies on plant proteins and enzymes to degrade or transform contami- nants. Plants can degrade aromatic rings in the absence of microorganisms (Ellis and Towers, 1970). Harms and Kotutz (1995) showed that plant tissues can transform pentachlorophenol, 4-chloroaniline, anthracene, and phenanthrene. Polychlorinated biphenyls (PCBs) have been metabolized by sterile plant tissues (Lee and Fletcher, 1995). Phenols have been

32 I~MEDIATION~~INTER 1996

PHYTORE~DIATION: MODELING REMOVAL OF TNT AND ITS BREAKDOWN PRODUCTS

destroyed by plants containing the peroxidase enzyme, including horse- radishes, potato, and white radish (Dec and Bollag, 1994). Poplar trees are capable of removing and degrading trichloroethylene from soil and groundwater (Strand et al., 1995).

Phytotransformation of Explosives The use of phytoremediation to treat explosives is gaining interest as

a cost-effective alternative to existing technologies. Numerous studies have documented that plants can uptake and transform TNT from both soil and water (Palazzo and Leggett, 1986; Folsom et al., 1988; Cataldo et al., 1989; Young, 1995; Schnoor et al., 1995; Carreira and Wolfe, 1996; Vanderford, 1996). The rates of transformation are rapid and there is no lag phase (McCutcheon et al., 1995).

Plant-mediated reduction of TNT involves a two-enzyme system (Exhibit 1). First, TNT is sequentially reduced by a nitroreductase enzyme to aminodinitrotoluene (ADNT) to diaminonitrotoluene (DANT) to triaminotoluene (TAT). ADNT and DANT each have two isomers. TAT is an unstable molecule that can auto-oxidize in aerobic conditions. How- ever, its oxidation is catalyzed by another plant enzyme, the laccase enzyme. This reaction scheme has been verified in a number of aquatic plants, including parrot feather and stonewort (Carreira and Wolfe, 1996).

Exhibit 1. A Two-Enzyme System for TNT Reduction

TNT

I

j4\ \ NIboredudase

N02 \ 'r J

other * Reaction PrOdUCtS

LaouIse

TAT "? * Ring Cleevago

N I

~ ~~

REMEDIATION/~INTER 1996 33

VICTOR F. MEDINA STEVEN C. MCCUTCHEON

Aquatic Phytoremediation Reactors The ability of plants to break down TNT is the basis for treatment of

TNT-contaminated water using aquatic phytoremediation reactors. Aquatic phytoremediation may be applied using one of two techniques:

Constructed wetlands Controlled phytoremediation tank reactors

Exhibit 2 is a schematic of a constructed wetland system. A con- structed wetland consists of a lined, man-made lagoon with active plants. The wetland system is outdoors, relying on solar power. Exhibit 3 shows a controlled tank reactor. The reactor can use greenhouse glass to take advantage of the sun, but also may have artificial lighting. Coils and other heaters can be used to maintain an ideal temperature.

Constructed wetlands are less expensive for both capital costs and operating costs. Tank reactors are less prone to temperature and weather fluctuations. Further, specialized monocultures or limited populations could be more easily maintained in a tank reactor. The biggest potential advantage of a tank reactor would be operation at optimal conditions and, therefore, smaller size. However, space is not a major concern at most military bases and ammunition plants where TNT contamination has occurred. Cost is the overriding factor. Therefore, this project focused on a constructed wetland approach.

I l l

The Of

to treat various wastewaters is an established technology.

Constructed Wetlands The use of wetlands to treat various wastewaters is an established

technology. Constructed wetlands have been used for treatment of municipal wastewater, paper mill effluent (Kadlec and Knight, 19951, food processing wastes, petroleum refinery effluent, surfactants, pesticides, metals and sulfides from mining waters (Wildemann et al., 1993; Thomp- son and Turney, 1996), and effluent from aquaculturing operations.

The chief advantage of constructed wetlands is low capital and operating costs. Cueto (1993) estimated annualized costs for a constructed wetland versus a conventional treatment plant for municipal wastewater. For flows ranging from 0.1 to 100 gpm, the estimated cost for a wetland was 43 to 51 percent of the cost of the conventional plant.

Wetlands are a valuable ecological resource and are areas of great biological diversity. They may provide for recreational opportunities such as bird and wildlife watching, canoeing, and hiking. However, use of the wetlands may be precluded while treating contaminants. Nonetheless, many sites have definite life spans after which the contaminated water will be completely treated. Then, the benefits of the wetlands can again be realized.

MATHEMATICAL MODEL A model was developed to determine the potential effectiveness of

phytoremediation for the treatment of TNT-contaminated wastewater. The goals were to estimate which compound (TNT or its breakdown product)

34 R.EMEDIATION/WINTER 1996

PHYTORZMEDIATION: MODELING REMOVAL OF TNT AND ITS BREAKDOWN PRODUCTS

Exhibit 2. Conceptual Design of Constructed Wetland Phytoremediation Reactor

Yhytoremediation Reactor

I /

Plant

Exhibit 3. Conceptual Design of Controlled Tank Phyto-Reactor

Grwn Hotix: Glass

? IIeatinglCooling Coil

fight

REMEDIATION/WINTER 1996 35

VICTOR P. MEDINA STEVEN C. MCCUTCHEON

Wetlands have traditionally been modeled as idealized plug flow.

would be treated, predict detention time, and use the detention time to estimate volume and costs of a constructed wetland system. The model is based on the following assumptions:

The reactions are irreversible.

The reactions follow the sequential reaction scheme shown in Exhibit 1. The isomers of ADNT and DANT have approximately the same rate constant and can be grouped for simplicity.

The reactions follow first-order kinetics in regard to contaminant concentration. Idealized plug flow is approximately maintained.

Wetlands have traditionally been modeled as idealized plug flow (USEPA, 1988). Nonetheless, objections have been raised to the validity of plug flow (Kadlec et al., 1993, Tchobanoglous, 1993; Wetzel, 1993; Kadlec and Knight, 1996). These objections are based on the impact of dispersion, channelized flow, and other disruptions to idealized plug flow. Use of this model must take into consideration these objections. The use of safety factors can account for deviations due to mixing. Using idealized plug flow simplifies the issue of modeling consecutive reactions.

First-order decay rate constants were determined for each compound from batch tests of the aquatic plants-parrot feather (Myraphyllum aquaticum) and stonewort (Algae nitella). The model assumes a first-order reaction with respect to contaminant concentration. However, a second factor is plant density. The more plants, the faster the reaction. Therefore, for each first-order rate constant:

k = KP,

where K is an intrinsic rate constant. Adjusting K for the plant density, P, gives the first-order rate constant, k, allowing for the comparison of different tests with different plant densities. Parrot feather plant densities up to 40 g/liter have been observed at a natural wetlands in Shaking Rock Park in Crawford, Georgia. For the purposes of the model, a conservative plant density of 15 g/ liter was chosen. Exhibit 4 summarizes the rates for each compound.

TAT disappearance is due to oxidative reactions, as opposed to reduction. This reaction is catalyzed by the laccase enzyme, as compared to the nitroreductase enzyme. TAT is inherently unstable. Consequently, TAT is removed much faster than the other compounds. Because the decay rate of TAT is two orders of magnitude faster than the other compounds, its disappearance is virtually instantaneous. TAT is never detected in aerobic phytoremediation experiments. Therefore, it is assumed that TAT does not build up appreciably in the system; therefore, its concentration was not modeled.

Based on the above assumptions, the following equations are adapted to describe the concentrations of TNT, ADNT, and DANT (Capellos and Bielski, 1972). The reduction of TNT concentration over time, t, is

36 ~~ ~

REMEDIATION/WINTER 1996

PHYTOREMJDIATION: MODBLING REMOVAL. OF TNT AND ITS BREAKDOWN PRODUCTS

Exhibit 4. First-Order Decay Rate Constants for TNT and Its Phytoremediation Breakdown Products

where kmt is the first-order decay rate constant for TNT. The formation and degradation of ADNT is

where kaht is the first-order decay rate constant for ADNT. The formation and degradation of DANT is

where kdvu is the first-order decay rate constant for DANT. The equations for ADNT and DANT cannot be solved analytically as

a function of time. A numerical approach is necessary. In this case, an iterative, finite difference approach was selected to calculate concentra- tions of TNT, ADNT, and DANT at different time steps, until the target concentration was reached for each compound.

Assumed Operating Conditions for the Model The influent TNT concentration was assumed to be 2.25 mgAiter, as

observed at an actual groundwater contaminated site. The treatment goal

RB~~~IJD~TIoN/WINTBR 1996 37

VICTOR P. MEDINA STEVEN C. MCCUTCHEON

Exhibit 5. Simulated Evolution of TNT and Breakdown Products in a Plug-Flow, Constructed Wetlands Reactor

2,500

I 2,000

1,500

DANT = 2 ppb Q 35 days

TNT = 2 ppb a12.56 days

\ ADNT = 2 ppb Q 32:6 days

0 10 20 30 40 Elapsed Days

TNT -m- ADNT --e DANT +

for TNT was assumed to be its maximum contaminant level (MCL) in drinking water, 2 pg/liter. For the breakdown products, there are no MCLs established. Therefore, the same level as TNT was used.

RESULTS OF THE MODEL

Detention Time Detention time is the time necessary for a reactor to hold a unit of water

to achieve the desired treatment. Exhibit 5 shows the results of the model for a constructed wetland with a plant density of 15 grams per liter. The removal of DANT is the limiting factor for complete treatment, requiring a detention time of 35 days. This is 2.8 times longer than the removal of TNT. Detention time could be shortened by using higher plant densities, a lower influent concentration, or a lower treatment goal.

Reactor Volume Calculations

reactor volume, V,, can be calculated for a given flow rate, Q: Using the detention time, t, calculated from the model, an empty bed

V , = Q t

However, the plants take up a volume, so that the actual volume, Va,, is

38

~

REMEDIATION/WINTER 1996

PHYTOREMEDIATION: MODELING REMOVAL OP TNT AND ITS BREAKDOWN PRODUCTS

vact = ve 1+ - ( l.0'00)

assuming P is in g/l. To calculate the design volume, Vde,, a 25-percent safety factor was added to account for deviations in the actual system:

Vde, = 1.25 Va,

Exhibit 6. Constructed Wetland Reactor Sizes for Various Flow Rates for the Conditions Used in the Model

500

1,000

5.000

Volume SurfaceArea

76,690 25,563 ft2 0.59 acres

127,816 ft2 2.93 acres

383,449

3,834,496 1,278,165 ft2 29.34 acres

7,668,991 2,556,330 ft2 58.69 acres

12,781,653 ft2 293.43 acres 30,675,966

Size Visualization 1 square (side)

160 ft circle (radius)

90 ft

square (side) 358 ft

circle (radius) 201 ft

square (side) 506 ft

circle (radius) 285 ft

square (side) 1,130 ft

circle (radius) 638 fl

square (side) 1,599 ft

circle (radius) 902 ft

square (side) 3,575 ft

circle (radius) 2,017 ft

REMEDIATION/WINTER 1996 39

VICTOR P. MEDINA STEVEN C. MCCUTCHEON

Flow Rate (8Pm)

10

50

Exhibit 7. Constructed Wetland Costs for Various Flow Rates for the Example Used in the Model

Capital Cosb AMua o&M costs

$29,500 $2,950

$167,500 $16,750

1,000

5,000

I 100 I $293,500 I $29,350 I

$2,934,500 $293,450

$14,67 1,500 $1,461,750

I $146,700 I 500 I $1,467,000 I

The amount of land needed for the wetland is determined by the surface area. Because of the need for light penetration, the wetland is limited to a depth of three feet. Therefore, the surface area was calculated by the following:

where d is the depth, in this case 3 ft. Exhibit 6 summarizes volumes and surface areas for constructed

wetland reactors treating TNT with an influent concentration of 2.25 ppm. In addition, two means of visualizing the size of the wetland are provided: as a square or a circle. Optimum configurations may be a rectangle to provide plug-flow conditions. Radial flow may also be promising.

costs Costs for constructed wetlands can vary significantly. Kadlec and

Knight (1996) report a range from $2,000 to $80,000 per acre for capital costs. Factors that may increase costs include the need for liners, automatic sampling, planting costs, complex structures, control equipment, pumps (as opposed to gravity flow), and land costs. For a constructed wetland treating nitroaromatics, it is anticipated that a liner will be necessary and that automatic sampling and flow control devices may be needed. Therefore, the cost per acre is anticipated to be on the higher end of the scale. However, since most of the sites are at military bases, land costs are not anticipated to be significant because most military bases and ammu-

40 REMEDIATION~~INTER 1996

PHYTOREMEDIATION: MODELING REMOVAL OF TNT AND ITS BREAKDOWN PRODUCTS

Treatment

nition plants have an abundance of land. Therefore, a capital cost of $50,000 per acre was used for this analysis.

Operation and maintenance ( O M ) costs for constructed wetlands are generally low, Median O&M costs for four constructed wetlands was reported to be $400 per acre annually (Kadlec and Knight, 1996). However, like capital costs, O&M costs can increase as certain requirements are added, such as plant harvesting, maintaining an individual species, and, especially, ancillary research and intensive monitoring costs for the initial installations. For this analysis, a higher- end cost of $5,000 per acre per year is used. Exhibit 7 summarizes estimated capital and O M costs for a range of flow values for the example used for the model.

Wujcik et al. (1995) compared costs of four ultraviolet-ozone systems treating TNT and trinitrobenzene (TNB) for a 50 gpm wastestream with an inlet concentration of 3 ppm and a treatment goal of 7 ppb. These numbers are comparable to the 50 gpm system analyzed for the constructed wetland. The capital costs for the UV-Ozone systems ranged from $393,000 to $687,000. The annualized O M costs ranged from $96,813 to $447,496. The cost of the constructed wetland compares well.

Griswold and Estrada (1995) compared operating costs for activated carbon, UV oxidation, supercritical water oxidation (SCWO), and electron beam destruction (Exhibit 8). Based on the analysis given above, phytoremediation compares favorably to the latter three technologies and is competitive with activated carbon adsorption. A two-chambered carbon vessel was estimated to cost $250,000 (Griswold and Estrada, 1995); however, the appropriate flow rate was unclear.

Operating Cost per 1,000 gallons

CONCLUSION This model, based on actual experimental data and conservative

Exhibit 8. Operating Cost Comparisons

Activated Carbon Adsorption

LJV Oxidation

scwo Electron Beam Destruction

Phytoremedia tion

$0.40 to $1.00

$1 to $3 $100 to $250

$1 to $5 $0.64

REMEDIATION/WINTER 1996 41

VICTOR F. MEDINA STEVEN C . MCCUTCHEON

The consideration of breakdown products of TNT is important for phytoremediation and for other technologies as well.

estimates, indicates that the use of constructed wetlands is a promising technology to treat TNT-contaminated groundwater and wastewater. Wetlands are large in area, but most army ammunition plants and military bases have abundant land, so cost is the primary consideration. Economic evaluation is promising. However, the technology is still relatively new compared to other technologies, and field investigations are necessary before widespread application is possible.

This analysis is conservatively based on the assumption that plant- mediated degradation of TNT would be the only significant mechanism for its breakdown. However, there are many additional transformations that may occur in the wetland, including microbial reactions (both aerobic and anaerobic), photochemical reactions and inorganic reductive reactions with sediment, and metals in the sediment. These other transformations may potentially increase the reaction rates in the wetland. However, other reactions may also produce undesirable breakdown products such as azoxynitrotoluene compounds. The effect of these additional reactions needs to be further studied.

The consideration of breakdown products of TNT is important for phytoremediation and for other technologies as well. The model predicts that a phyto-reactor constructed for TNT only could be approximately three times smaller than one constructed to satisfactorily remove its breakdown products as well.

A key design consideration involves avoiding of groundwater con- tamination by leaching. Liner systems and possibly subsurface drains should minimize this problem.

A second concern involves the control of animals drinking the wetlands water or feeding on the plants. This technology destroys TNT and its breakdown products, thereby removing toxicity. However, at the influent of the wetland, removal of TNT and its breakdown products is limited. Therefore, control of wildlife may be necessary. Ecological risk assessments should define the control necessary for each site. Fortunately, the use of constructed wetlands to treat hazardous compounds is estab- lished, so many of these issues have been dealt with. For example, Kadlec and Knight (1996) report that control of mosquitos and rodents is not particularly expensive. Nonetheless, monitoring for toxic effects and controlling, if necessary, would be an important part of an annual monitoring program. Therefore, the O&M costs per acre used for this analysis were adjusted appropriately.

The application of phytoremediation to treat TNT is an important but narrow step in dealing with explosives contamination. Commonly, TNT is mixed with other explosives including RDX and HMX. Unpublished data indicate that treatment of these compounds is slower (Medina, 1996), requiring larger wetlands. Also, there is not sufficient information to account for seasonal variation in the performance of the system. Studies of this kind would also be most useful.

ACKNOWLEDGMENTS The Strategic Environmental Research and Development Program (SERDP)

42 REMEDIATION~~INTER 1996

PHYTOREMEDIATION: MODELING REMOVAL OP TNT AND ITS BREAKDOWN PRODUCTS

provided funding that supported this project. Much of the kinetic data used in this article were collected by Dr. T.Y. Ou.

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