Chemosphere 77 (2009) 597–603

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Dissolution and transport of 2,4-DNT and 2,6-DNT from M1 propellant in soil
Katerina M. Dontsova a,b,*, Judith C. Pennington b, Charolett Hayes c, Jiri Šimunek d, Clint W. Williford aaDepartment of Chemical Engineering, University of Mississippi, University, MS 38677, United States
US Army Engineer Research and Development Center, 3909 Halls Ferry Rd., Vicksburg, MS 39180, United StatescComputer Sciences Corporation, 3909 Halls Ferry Rd., Vicksburg, MS 39180, United StatesdUniversity of California, Riverside, United Statesba r t i c l e

i n f oArticle history:
Received 23 February 2009
Received in revised form 20 May 2009
Accepted 20 May 2009
Available online 2 September 2009
Keywords:
2,4-Dinitrotoluene
2,6-Dinitrotoluene
Column studies
HYDRUS-1D
Propellant

a b s t r a c t
Live-ﬁre training exercises can result in particulate propellant contamination on military training ranges
and can potentially contaminate ground water. This study was conducted to evaluate dissolution of the
2,4-dinitrotoluene (2,4-DNT) and 2,6-dinitrotoluene (2,6-DNT) from the propellant formulation, M1(87.6% nitrocellulose, 7.3% 2,4-DNT, 0.57% 2,6-DNT, 1.06% diphenylamine, 3.48% dibutyl phthalate) andtheir subsequent transport in soil. Batch dissolution studies were followed by saturated column transport
experiments. Neat, dissolved 2,4-DNT, and M1 in solid and dissolved forms were used as inﬂuent to columns ﬁlled with Plymouth loamy sand (mesic, coated Typic Quartzipsamments) from Camp Edwards,
MA. Dissolution rates and other fate and transport parameters were determined using the HYDRUS-1D
code. M1 dissolution was limited by DNT diffusion from the interior of the pellet, resulting in an exponential decrease in dissolution rate with time. The HYDRUS-1D model accurately described release andtransport of 2,4- and 2,6-DNT from M1 propellant. Dissolution rates for M1 in the stirred reactor and column studies were similar, indicating that batch dissolution rates are potentially useful to represent ﬁeld
conditions.
Published by Elsevier Ltd.

1. Introduction
Live-ﬁre training can result in soils contaminated with propellants. One of the propellant components commonly detected in soil
samples collected on military training ranges is 2,4-DNT (Jenkins
et al., 2006), an energetic binder in single-based propellants andhigh explosives formulations (Mirecki et al., 2006). When produced
for military applications, 2,4-DNT, a suspected carcinogen (Quigley,1994), is accompanied by its isomers, particularly, 2,6-DNT. Both
2,4- and 2,6-DNT are listed as US EPA priority pollutants (40 CFR
423).
Brannon and Pennington (2002) reviewed available batchdetermined fate and transport parameters for propellants usedby the military, including 2,4-DNT. Adsorption coefﬁcients are generally low (0.02–0.67 cm3 g1) for low organic matter sediments(Pennington et al., 1999; Hernández et al., 2006), but can be higher
Abbreviations: 2,4-DNT, 2,4-dinitrotoluene; 2,6-DNT, 2,6-dinitrotoluene; DPA,
diphenylamine; DBPH, dibutyl phthalate; CEC, cation exchange capacity; DNT,
dinitrotoluene; HPLC, high performance liquid chromatography; kd, adsorption
coefﬁcient; MMR, Massachusetts Military Reservation; OC, organic carbon; OM,
organic matter; RIA, Rock Island Arsenal.
* Corresponding author. Present address: Biosphere 2, University of Arizona,
Marshall Building, Room 526, 845 N. Park Avenue, Tucson, AZ 85721-0158, UnitedStates. Fax: +1 520 626 0793.
E-mail address: Kateryna.Dontsova@gmail.com (K.M. Dontsova).
0045-6535/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.chemosphere.2009.05.039

for clay soils (12.5 cm3 g1) (Pennington et al., 2001), and very highfor the phyllosilicate clays (690–7400 cm3 g1) (Haderlein et al.,1996). The 2,6-DNT has much lower adsorption coefﬁcients inclays than 2,4-DNT (10–125 cm3 g1), with the difference attributed to the steric hindrance of the NO3 group in the ortho position.
Work on clays (Haderlein et al., 1996) indicated linear adsorption
at low concentrations and a decrease in adsorption coefﬁcients asconcentrations increase and available absorption sites are taken.
Speciﬁc adsorption through electron donor–acceptor complexes
with the siloxane surfaces, analogous to other nitroaromatic compounds, was suggested as the mechanism (Haderlein et al., 1996).
Low calculated KOC values (1.79 for both 2,4- and 2,6-DNT)(Rosenblatt et al., 1991) indicate limited hydrophobic partitioning
to organic carbon in soils. However, DNT degradation products canform covalent bonds with the natural organic matter in soils
resulting in removal of DNTs from solution (Pennington et al.,
2003; Thorn et al., 2008).
In addition, some work has been performed to examine behavior
of 2,4-DNT and 2,4-DNT-contaminated soils under ﬂow conditions(Yamamoto et al., 2004; Hewitt and Bigl, 2005). Dinitrotoluenes
have relatively high aqueous solubilities: 280 mg L1 at 25 °C for
2,4-DNT (Kaye, 1980) and 208 mg L1 at 25 °C for 2,6-DNT(Rosenblatt et al., 1991). However, in propellant formulations, 2,4DNT is impregnated into an insoluble nitrocellulose matrix. This

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process will likely affect release of propellants into solution. For
explosives, binders and waxes present in formulations have beenshown to decrease dissolution rates of individual explosive compounds (Lynch et al., 2002; Phelan et al., 2002). A similar trend
was observed for dissolution of several propellant formulations inbatch studies (Mirecki et al., 2006). In columns, Hewitt and Bigl(2005) measured no 2,4-DNT release from a soil containing particulate M1 propellant residues. The effects of formulation on other fate
and transport processes are unknown.
This study focused on M1, one of the single-base, 2,4-DNT containing propellants used by the military. In addition to insoluble
nitrocellulose and DNTs, M1 contains diphenylamine (DPA) anddibutyl phthalate (DBPH). Diphenylamine is used as a stabilizer,
while DBPH is a plasticizer. These compounds were not evaluatedin this study.
The objective of this study was to determine release rates andtransport parameters for 2,4- and 2,6-DNT from solid phase M1
propellant under saturated ﬂow conditions. The parameters forsolid M1 propellant were compared with those for neat 2,4-DNT;
2,4- and 2,6-DNT in dissolved M1; and with adsorption coefﬁcients
reported in the literature.
2. Materials and methods
2.1. Soils
Plymouth loamy sand (mesic, coated Typic Quartzipsamments)
was used for this experiment. Samples were collected at Camp
Edwards, MA (Massachusetts Military Reservation [MMR]) to adepth of 20 cm. Soil was air-dried, ground, passed through a
2-mm sieve, and analyzed for cation exchange capacity (CEC) using
the NaAc method (US Environmental Protection Agency, 1986) andorganic matter using the Walkley-Black method. Particle size wasdetermined by the hydrometer method (Gee and Or, 2002). The
soil pH in 1:1 soil/water slurry, exchangeable cations, and base saturation were also measured. The soil had relatively low organic
matter (8.5 g kg1) and CEC (4.4 cmolc kg1), base saturation of
9.7%, and pH of 5.2. Collected samples contained 93.3% sand,
5.8% silt, and 0.8% clay.
2.2. Propellants
M1 propellants are produced as dark green, single- or multiperforated cylinders. Single-perforated cylinders are smaller with
one perforation in the middle, while multi-perforated are larger
and have seven perforations per cylinder. According to speciﬁcations, all M1 formulations contain 85 ± 2% nitrocellulose, 10 ± 2%
DNT, 5 ± 1% DBPH, and 1.0 ± 0.10 DPA. They can also contain PbCO3(1.0 ± 0.3%), K2SO4 (1.0 ± 0.3%) and retain some residual solvent(max 0.9%) and moisture (0.6 ± 0.2%) (Defense Ammunition Center,2003). Propellant compositions used in this study contained K2SO4
as a ﬂash reducer.
Two different M1 formulations were obtained from Rock Island
Arsenal (RIA): multi-perforated M1 propellant (8 mm in length and
3 mm in diameter) and single perforated M1 propellant (5 mm inlength and 1 mm in diameter). Propellants were manufactured
by Radford Army Ammunition Plant, Radford, VA, in 2005 and2006, respectively.
Both formulations of M1 propellant were analyzed for totalcomposition. Propellant cylinders were extracted in acetonitrile.
Extracts were analyzed for 2,4- and 2,6-DNT by High Performance
Liquid Chromatography (HPLC) (Waters HPLC, GenTech Scientiﬁc,Inc., Arcade, NY), Method 8330 (US Environmental Protection
Agency, 1994) and for DPA and DBPH by GC/MS, Method 8270C(US Environmental Protection Agency, 1996).

2.3. Dissolution in stirred reactors
In stirred reactor batch dissolution studies, 10 g samples of propellant were added to 3.5 L of de-ionized water and stirred for
6 days. The solution was sampled before adding the propellant
and at 2, 6, 12, 24, 48, 72, 96, 120, and 144 h of stirring. After
6 days, the 144 h samples were analyzed for 2,4-DNT by HPLC
Method 8330 (US Environmental Protection Agency, 1994). Stirringof the small single-perforated propellant resulted in higher
concentrations (4.69 ± 0.13 mg L1, average of two replications)than stirring of the larger multi-perforated formulation (3.01 ±0.03 mg L1). Consequently, dissolution kinetics were determined
for the single-perforated formulation by analysis for 2,4-DNT,
and 2,6-DNT in samples collected over time. Samples collected at
144 h were also analyzed for DPA and DBPH. Solutions obtained
from the single-perforated M1 propellant tests were used in column experiments.

2.4. Saturated ﬂow experiments
Methods for saturated ﬂow column experiments are describedin detail in Dontsova et al. (2006). Brieﬂy, ﬂux-controlled stainlesssteel ﬂow-through columns (10.16-cm internal diameter by 17.00cm height) were supplied with solution by a Fisherbrand UltralowFlow Peristaltic Pump (Fisher Scientiﬁc Houston, TX). Outﬂow
samples were collected continuously into 40-mL vials using an
automated Universal fraction collector with 50-vial capacity (Eldex
Laboratories, Inc. Napa, California). Soils were packed into columns
to an average bulk density of 1.67 ± 0.09 g cm3.
Columns were saturated from the bottom using 0.005 M CaBr2
background solution. Pore volume (0.462 ± 0.046 L) and volumetricwater content (h), were determined during saturation from the volume of solution necessary to ﬁll the packed column. Columns werethen connected to the pump and operated in downﬂow mode. The
average ﬂow rate was 1.12 ± 0.17 mL min1, or solution ﬂux of0.83 ± 0.13 cm h1. In 2,4-DNT or dissolved M1 experiments, studied solutes in 0.005 M CaBr2 background were applied to the column. In solid M1 experiments, M1 cylinders were placed on thesoil surface between two layers of glass wool and a 0.005 M CaBr2
background solution was applied.
Target concentration of analytical grade 2,4-DNT for the neat
2,4-DNT experiments was 10 mg L1; measured concentration
was 9.91 ± 0.68 mg L1. In experiments with dissolved M1 propellant, the input concentration of 2,4-DNT was 4.87 ± 0.15 mg L1,
while the 2,6-DNT concentration was 0.194 ± 0.005 mg L1. M1solutions also contained 358 ± 23 lg L1 of DPA and 33 ± 13 lg L1
of DBPH.
Uniformly ring-labeled 14C-2,4-DNT was added as a tracer tothe bulk solutions (speciﬁc activity 16.67 mCi mmol1) at 0.037%
of total 2,4-DNT in solution. The radiotracer allowed monitoringof 2,4-DNT concentrations in outﬂow as the experiment progressed. Comparison of breakthrough curves of labeled and unlabeled 2,4-DNT allowed us to distinguish between transformationand mineralization of 2,4-DNT. Tritiated water (3H2O) (speciﬁc
activity of 2.18 mCi mmol1 with 1.239  106% of 3H2O in solution) was applied with the solutes to determine the longitudinal
dispersivity (k) for each soil and monitor for signs of preferential
ﬂow.
After six pore volumes of propellant solution the ﬂow was
switched back to the background solution (or the solid M1 was removed), which was applied for another 14–18 pore volumes. Flowinterruption was used to observe kinetic processes (Murali and
Aylmore, 1980; Šimunek et al., 2002). For every soil/propellant
treatment two experiments were conducted. In one, ﬂow wascontinuous, while in the other, both inﬂow and outﬂow from the599

K.M. Dontsova et al. / Chemosphere 77 (2009) 597–603
Table 1
Total content of 2,6-DNT, 2,4-DNT, DBPH, and DPA in several M1 propellant
formulations.

RIAa multi-perforated
RIA single-perforateda2,6-DNT (%)

2,4-DNT (%)

DBPH (%)

DPA (%)

0.48 ± 0.002
0.57 ± 0.028

8.61 ± 0.045
7.34 ± 0.43

3.40 ± 0.16
3.48 ± 0.28

1.25 ± 0.28
1.06 ± 0.06

RIA Rock Island Arsenal.

column were stopped for 24 h to allow propellant constituents toequilibrate with the soil.
Every second outﬂow sample (approximately every 0.075 L)
was analyzed by scintillation counting (Tri-Carb 2500TX Liquid
Scintillation Analyzer, PerkinElmer Life and Analytical Sciences,
Inc., Boston, MA). Based on 14C tracer results about 16–18 samples
per breakthrough curve were selected for HPLC analysis using standard EPA Method 8330 (US Environmental Protection Agency,
1994). Detection limit was 20 lg L1 for both 2,4-DNT and 2,6DNT.
2.5. Numerical analysis
Experiments were analyzed using the HYDRUS-1D code for simulating the one-dimensional movement of water, heat and multiple solutes in variably saturated porous media (Šimunek et al.,2005). The following models were used in analysis: convection–
dispersion equation for 3H2O tracer and the two-site sorption model (with decay) (chemical non-equilibrium model) for propellants
using approach similar to one applied to dissolution of explosives(Dontsova et al., 2006). The 3H2O breakthrough curves were used
to estimate the longitudinal dispersivity, k (cm). Then, k was ﬁxed
at a value determined for tracer and the following parameters were
estimated for propellants: a fraction of sites with instantaneousadsorption, f, adsorption coefﬁcient, kd (cm3 g1), a ﬁrst-order rate
coefﬁcient for dissolved phase (degradation rate), lw (h1), and aﬁrst-order rate coefﬁcient for two-site non-equilibrium adsorption,x (h1). In 14C radiotracer experiments the rate coefﬁcient (lw)corresponded to irreversible attenuation and mineralization, whilein HPLC experiments it corresponded to the sum of attenuation,mineralization, and transformation.
For experiments with solid M1 propellant formulation, 2,4-DNT
and 2,6-DNT dissolution rate, c (lg g1 h1) was deﬁned asfollows:

c ¼ nevt

ð1Þ
1where n is the initial dissolution rate (lg gstant (h1) and t is time (T).

1h), v is decay con-Table 3Fraction of solutes recovered in outﬂow in the column transport experiments
involving 3H2O, 14C-2,4-DNT, 2,4-DNT, dissolved and solid M1 propellant in Plymouth
soil.
Treatment

3

H2O

14

2,4-DNT
2,4-DNT IFa
M1 solution
M1 solution IF
M1 solid
M1 solid IF
AverageStandard deviation

1.11
1.06
0.98
1.02
0.98
1.01
1.03
0.05

1.02
0.95
0.76
0.67
NA
NA

a
b

C-2,4-DNT

2,4-DNT

2,6-DNT

1.07
0.92
0.69
0.43
NA
NA

NAb
NA
0.88
0.60
NA
NA

IF = interrupted ﬂow.
NA = non-applicable.

Each breakthrough curve was integrated to calculate recoveryof the 14C-2,4-DNT and HPLC-measured 2,4-DNT and 2,6-DNT (Table 3). Recovery of conservative tracer was used to evaluate theaccuracy of mass balance estimates of reactive solutes. The goodness of ﬁt between measured and modeled breakthrough concentrations was evaluated using the R2 values and conﬁdence
intervals for ﬁtted parameters (Šimunek and Hopmans, 2002).
Parameter estimates were considered signiﬁcant if their conﬁdence intervals did not intersect with zero.
Interrupted and continuous ﬂow experiments were treated as
replicates for the purposes of statistical analysis of transportparameters. For comparison between treatments, differences between treatment means were considered signiﬁcant when greaterthan the sum of their 95% conﬁdence intervals.
3. Results and discussion
3.1. Dissolution in stirred reactors
Results of propellant analysis (Table 1) were in close agreementwith the speciﬁed composition. Dissolution of RIA multi-perforated
propellant resulted in lower concentrations of 2,4-DNT after 144
hours of stirring (3.01 ± 0.03 mg L1) than dissolution of RIA single-perforated M1 propellant (4.69 ± 0.13 mg L1). The same trend
was observed for 2,6-DNT. Differences were attributed to the smaller size and, therefore, larger surface area per gram of single-perforated particles.
Measured concentrations were well below the solubility of both
2,4-DNT and 2,6-DNT (Fig. 1). Solubility limit at 25 °C is 280 mg L1
for 2,4-DNT (Kaye, 1980) and 208 mg L1 for 2,6-DNT (Rosenblatt
et al., 1991). Mirecki et al. (2006) previously observed that

Table 2
Solute transport parameters obtained by HYDRUS-1D for column saturated ﬂow experiments involving 3H2O, 14C-2,4-DNT, 2,4-DNT, and dissolved and solid M1 propellant inPlymouth soil (k was estimated from 3H2O, while f, kd, lw, x, n, and v from propellants).
f

kd (cm3 g1)lw (h1)x (h1)

2,4-DNT solution, k = 0.52 ± 0.27 cm
14
C-2,4-DNT
2,4-DNT

0.48 ± 0.06
0.56 ± 0.06

0.37 ± 0.02
0.28 ± 0.03

0.0020 ± 0.0028
0.0031 ± 0.0043

0.07 ± 0.03
0.10 ± 0.06

0.989
0.992
0.988

Dissolved M1, k = 0.75 ± 0.26 cm
14
C-2,4-DNT
2,4-DNT
2,6-DNT

0.52 ± 0.05
0.53 ± 0.03
0.74 ± 0.28

1.11 ± 0.08
1.08 ± 0.19
0.97 ± 0.18

0.0129 ± 0.0021
0.0238 ± 0.0067
0.0085 ± 0.0020

0.06 ± 0.01
0.09 ± 0.01
0.12 ± 0.01

0.996
0.986
0.971
0.934

Solid M1, k = 0.70 ± 0.29 cm
2,4-DNT
2,6-DNT

0.16 ± 0.05
0.31 ± 0.04

2.01 ± 0.47
0.84 ± 0.02

0.0238c0.0085c

0.06 ± 0.03
0.11 ± 0.02

av (h1)n (lg g1 h1)b0.057 ± 0.023
0.043 ± 0.009

69.00 ± 4.13
1.27 ± 0.00

R2

0.998
0.984
0.976

a
k = longitudinal dispersivity (determined from tracer experiments); f = fraction of sites with instantaneous adsorption; kd = adsorption coefﬁcient; lw = ﬁrst-order rate
coefﬁcient for dissolved phase, degradation rate (for 14C radiotracer experiments indicates irreversible attenuation and mineralization); x = ﬁrst-order rate coefﬁcient for
two-site non-equilibrium adsorption; n = initial dissolution rate for 2,4-DNT and 2,6-DNT from solid M1; v = dissolution rate decay constant.
b(Means ± standard deviation).
c
Degradation rate determined from dissolved M1 experiments was used.

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K.M. Dontsova et al. / Chemosphere 77 (2009) 597–603

5

0.5

2,6-DNT

4

0.4

3

0.3

2

0.2

1

0.1

0
0

20

40

60

80

100

120

140

1

Measured H-3 water
Fitted H-3 water

0.8

Measured C-14 2,4DNT
Fitted C-14 2,4DNT

C/C0

2,4-DNT

2,6-DNT concentration, mg L -1

2,4-DNT concentration, mg L-1

1.2

0.6

6

Measured 2,4DNT

0.6

Fitted 2,4DNT

0.4
0.2

0
160

0

Time, h

0

5

10

15

20

25

Pore volumes
Fig. 1. Dissolution of 2,4-DNT and 2,6-DNT from solid M1 propellant in stirred
reactors. Points are average of two replications; error bars equal standard error of
the measurement.

3.2. Conservative tracer
Measured concentrations of the conservative tracer, 3H2O, areshown on the Figs. 2–4 by hollow triangles and labeled as ‘‘measured H-3 water.” Modeled breakthrough curves are shown asdashed lines. Values for longitudinal dispersivity determined fromtracer breakthrough curves were between 0.33 and 1.46 cm (Table2), similar to previously measured values for the same soil
(Dontsova et al., 2006).
Breakthrough of the conservative tracer at one pore volume(462 ± 46 mL) indicated the absence of preferential pathways for
ﬂow. Physical equilibrium in the soil columns was also supportedby the fact that 3H2O outﬂow concentrations resumed at similarvalues after the ﬂow interruption. All 3H2O water was recovered(103 ± 5%) (Table 3) as was expected for the conservative tracer.
3.3. 2,4-DNT
Fig. 2 presents measured values and HYDRUS generated ﬁt for
the transport of neat 2,4-DNT in Plymouth soil columns with andwithout interrupted ﬂow. Grey diamonds represent measured values for the 14C-2,4-DNT radiotracer (Fig. 2), while black diamonds
represent HPLC-measured 2,4-DNT concentrations.
For both radiolabeled and unlabeled 2,4-DNT breakthrough wasobserved later than for the conservative tracer, indicating adsorption to the solid phase. Calculated kds were 0.37 ± 0.02 cm3 g1
and 0.28 ± 0.03 cm3 g1, respectively (Table 2). These values are

1

Measured H-3 water
Fitted H-3 water

0.8

Measured C-14 2,4DNT
Fitted C-14 2,4DNT

C/C0

propellant constituents from solid propellants (M9, M10, and M30)
did not reach their solubility limits in stirred reactors. They concluded that aqueous solubility alone does not control the releaseof propellant compounds from the nitrocellulose matrix withleaching of soluble components becoming increasingly diffusionlimited as leaching progresses toward completion.
An exponential equation, where initial dissolution rate is equal
to 61.99 ± 35.57 lg g1 h1, and the exponential coefﬁcient is0.029 ± 0.017 h1, can be used to describe the observed decreasein dissolution rate of 2,4-DNT with time (R2 = 0.655 ± 0.004).
Apparently, as constituents were depleted in the propellant exterior, further release required diffusion from the interior of the pellets. The rapid decrease in dissolution rate indicates that for 2,4DNT the diffusion rate was low relative to dissolution rate.
Total mass transferred from the solid into the liquid phase in
6 days (17.05 ± 0.36 mg 2,4-DNT and 0.677 ± 0.012 mg 2,6-DNT)
was small relative to the total mass of these nitroaromatic compounds in the formulations. According to our propellant analysis,
10 g of M1 propellant contained 734 mg 2,4-DNT, 57 mg 2,6DNT, 106 mg DPA, and 348 mg DBPH.

1.2

Measured 2,4DNT

0.6

Fitted 2,4DNT

0.4
0.2
0
0

5

10

15

20

25

Pore volumes
Fig. 2. Breakthrough curves for 14C-2,4-DNT, 2,4-DNT, and 3H2O in Plymouth soil
for continuous (top) and interrupted (bottom) ﬂow experiments. Dashed verticallines indicate when ﬂow was interrupted or inﬂow solution switched to 0.005 M
CaBr2.

similar to the kds for the MMR soil obtained in column experiments(0.47 ± 0.11 cm3 g1), and lower than measured in batch experiments for the same soil (3.3 ± 0.2 cm3 g1) (Yamamoto et al.,2004). Batch experiments have previously been shown to over-predict kds for compounds that experience irreversible adsorption(Dontsova et al., 2006). The breakthrough curve was asymmetric,a feature that can indicate either kinetic or non-linear adsorption.
HYDRUS-1D modeling supported kinetic sorption. About 50% of
adsorption sites in Plymouth soil exhibited kinetic adsorption, withthe ﬁrst-order rate coefﬁcient of 0.07 to 0.10 h1. Chemical nonequilibrium was also supported by the decrease in concentration
following ﬂow interruption, which was observed for both 2,4DNT and 2,6-DNT.
Degradation rate calculated for the 2,4-DNT was not statisticallydifferent from zero (Table 2). This agreed with the mass balancecalculations for these experiments (Table 3). Recovery of 14C-2,4DNT was 99 ± 5%, while recovery of HPLC-measured 2,4-DNT was
100 ± 11%. Lower recoveries were observed for the interrupted
ﬂow experiments, where more time was available for degradationin agreement with previous studies of explosives (Dontsova et al.,2006).
Low values for degradation rate in the 14C-2,4-DNT experiments
indicate that no irreversible attenuation or mineralization to CO2
was occurring, while overlap between breakthrough curves forthe 14C-2,4-DNT and HPLC-measured 2,4-DNT indicated negligible
transformation of 2,4-DNT to its products.
3.4. Solution phase M1
Greater adsorption and degradation was observed when
dissolved M1 was applied to the columns, compared to neat

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1

Measured H-3 water
Fitted H-3 water

C/C0

0.8

Measured C-14 2,4DNT
Fitted C-14 2,4DNT
Measured 2,4DNT

0.6

Fitted 2,4DNT
Measured 2,6DNT

0.4

Fitted 2,6DNT

0.2
0
0

5

10

15

20

25

3H O C/C + propellant concentration, mg L-1
2
0

1.2

1.2

3H O C/C + propellant concentration, mg L-1
2
0

K.M. Dontsova et al. / Chemosphere 77 (2009) 597–603

1.2

1Measured H-3 water
Fitted H-3 water

0.8

Measured 2,4DNT
Fitted 2,4DNT
measured 2,6DNT

0.6

fitted 2,6DNT

0.4
0.2
0
0

5

10

Pore volumes
1.2
1

Measured H-3 water
Fitted H-3 water

0.8

Measured C-14 2,4DNT

C/C0

Fitted C-14 2,4DNT
Measured 2,4DNT

0.6

Fitted 2,4DNT
Measured 2,6DNT

0.4

Fitted 2,6DNT

0.2
0
0

5

10

15

20

25

Pore volumes
Fig. 3. Breakthrough curves for 14C-2,4-DNT, 2,4-DNT, 2,6-DNT, and 3H2O in
Plymouth soil for continuous (top) and interrupted (bottom) ﬂow experiments.
Dashed vertical lines indicate when ﬂow was interrupted or inﬂow solution
switched to 0.005 M CaBr2.

2,4-DNT (Table 2, Fig. 3). Non-linear sorption can explain thisphenomenon. During non-linear adsorption, lower inﬂow concentrations, as in M1 experiments, will result in greater adsorption
coefﬁcients. Haderlein et al. (1996) demonstrated non-linear sorption behavior of nitroaromatic compounds. However, non-linear
adsorption was not signiﬁcant when modeled using HYDRUS-1D(parameter B in Freundlich equation was not signiﬁcantly different
from 1).
Incomplete recovery of 14C-2,4-DNT indicated irreversible
attenuation or mineralization to CO2. Both mineralization (Nishino
et al., 1999) and reduction to aminonitrotoluenes with subsequent
covalent bonding of the products to the soil organic matter(Pennington et al., 2003; Thorn et al., 2008) have been demonstrated. Higher recovery of the tracer than of the HPLC-determined
2,4-DNT (Table 3), indicated transformation of 2,4-DNT to intermediate daughter products. Higher recoveries 2,6-DNT compared to
2,4-DNT agreed with the lower degradation rate (Table 2) thatwas observed in the solution experiments.
When ﬂow was interrupted, concentrations of DNTs in outﬂow
decreased due to continued degradation and kinetic adsorption,
but later recovered to levels similar to ones measured before ﬂow
interruption.
3.5. Solid M1
Breakthrough curves for DNTs in experiments with solid
M1 had a different pattern compared to the dissolved propellant
compound breakthroughs (Fig. 4). An initial peak in concentrationthat decreased with time was observed. This behavior indicatednon-constant dissolution rate of DNTs from M1 propellant. A con-15
Pore volumes

20

25

Measured H-3 water

1

Fitted H-3 water
Measured 2,4DNT

0.8

Fitted 2,4DNT
measured 2,6DNT

0.6

fitted 2,6DNT

0.4
0.2
0
0

5

10

15
Pore volumes

20

25

Fig. 4. Breakthrough curves for 2,4-DNT, 2,6-DNT from M1 and 3H2O in Plymouthsoil for continuous (top) and interrupted (bottom) ﬂow experiments. Dashedvertical lines indicate when ﬂow was interrupted or M1 removed from the column.

stant dissolution rate model previously used for explosives (Dontsova et al., 2006) did not adequately describe the shape of the
breakthrough curves. Therefore, a model that predicted an exponential decrease in dissolution rate of the propellant with time
was applied to the data. The difference in dissolution behavior of
explosives and M1 propellant can be explained by the presenceof the insoluble nitrocellulose matrix that prevents 2,4-DNT fromentering solution. Unlike patterns observed for explosives, breakthroughs were similar between the replicates due to the regularsize of the particles. A similar trend was observed for other propellants (Dontsova et al., 2008).
Using an exponential decrease in dissolution rate resulted in avery good ﬁt of the breakthrough curves (R2 between 0.976 and
0.984) (Table 2, Fig. 4). Estimated dissolution rates and transportparameters were also in good agreement for the two replicates.
Degradation rate for these experiments could not be estimated, because it would require a deﬁned source term. As we needed to estimate source term, degradation rate values for DNTs in solution forthe same soil and run conditions (0.0238 h1 for 2,4-DNT and
0.0085 h1 for 2,6-DNT) were used.
Adsorption coefﬁcients for 2,6-DNT determined from solid M1
were similar to the values obtained for dissolved M1. Simulated
kds for 2,4-DNT were higher than for 2,6-DNT. However, due tothe fact that estimated fractions of adsorption sites at equilibriumwith the solution were lower for 2,4-DNT than for 2,6-DNT, this did
not result in signiﬁcantly higher retardation of 2,4-DNT in thecolumns.
The total amount of propellant recovered in outﬂow during column experiments (2.957 ± 0.565 mg of 2,4-DNT and 0.156 ±0.018 mg of 2,6-DNT) was considerably lower than the total
available in the pellets (734 mg 2,4-DNT and 57 mg 2,6-DNT).

602

K.M. Dontsova et al. / Chemosphere 77 (2009) 597–603

Integration of dissolution over time using the HYDRUS-1D simulated equation showed that a smaller amount of 2,4-DNT and
2,6-DNTwasdissolved
duringcolumnexperiments(11.20 ± 2.90 mg of 2,4-DNT and 0.237 ± 0.024 mg of 2,6-DNT)
than in stirred reactors (17.05 ± 0.36 mg of 2,4-DNT and
0.677 ± 0.012 mg of 2,6-DNT). When we took into account durationof dissolution (144 h in batch studies and 44 h in column studies)and extrapolated column dissolution to 144 h, the calculated
amount dissolved was still lower (12.69 ± 4.48 mg of 2,4-DNT
and 0.276 ± 0.041 mg of 2,6-DNT), but not signiﬁcantly different
from batch dissolution. Extending time of dissolution did not impact total values to a great extent due to an exponential decreasein dissolution rates with time. Total volume of solution in contact
with solid M1 propellant was similar between batch and column
experiments.
Simulated dissolution rates were consistent between the replicates with low error and high R2 (Table 2). The values obtained for
2,4-DNT in columns (69.00 ± 4.13 lg g1 h1 and exponential coefﬁcient of 0.057 ± 0.023 h1) were not signiﬁcantly different fromthe ones obtained in batch studies (61.99 ± 35.57 lg g1 h1 andexponential coefﬁcient of 0.029 ± 0.017 h1). The values for 2,6DNT were also similar (1.27 ± 0.00 lg g1 h1 and exponential
coefﬁcient of 0.043 ± 0.009 h1 in column studies andandexponential
coefﬁcientof1.07 ± 0.82 lg g1 h10.015 ± 0.014 h1 in batch studies). This supported the accuracyof HYDRUS estimates of dissolution rates of M1 and indicated thatbatch studies may closely approximate dissolution of propellants
in the ﬁeld.
Dissolution rate was higher for 2,4-DNT than for 2,6-DNT. Previous studies of explosives formulations showed that dissolution
rate was affected by the concentration of a component in the formulation (Dontsova et al., 2006). However, in this study, releaseof 2,4-DNT was preferred to the 2,6-DNT as the ratio between
the concentration of 2,4- and 2,6-DNT was 13 in solid propellant,
25 in solution after batch dissolution and 47 in column dissolution.
This indicates that 2,6-DNT has slower dissolution kinetics evenafter we account for its lower concentration in the M1 propellant.
Flow interruption resulted in a similar pattern for neat 2,4-DNT
and dissolved M1. A decrease in outﬂow concentration was followed by an increase to the levels observed before ﬂow interruption indicating continued release of DNTs from propellant particles.
It was speculated that diffusion of 2,4-DNT from propellant matrix limits dissolution of solid propellants. Limitations on dissolution due to matrix effects may explain why 2,4-DNT is detectedin soils on military installations, but not in ground water (Clausen
et al., 2004).

4. Conclusions
A batch dissolution study evaluated dissolution of DNTs fromsolid M1 propellant. In addition, a saturated column transport
study was conducted to assess dissolution of M1 propellant andtransport of its soluble components: 2,4-DNT and 2,6-DNT in soil.
Dissolution rates, sorption and transformation coefﬁcients were
estimated and contribution of non-equilibrium sorption was established. Dinitrotoluenes showed adsorption and transformation insoil. 2,4-DNT had higher dissolution rate than 2,6-DNT even afterits greater content in the propellant was taken into consideration.
Dissolution rates of DNTs were similar in batch and column studies, and were consistent between the samples indicating applicability of batch dissolution results to ﬂow and by extension, ﬁeld,
conditions. Both studies showed that dissolution rates of DNTs
were decreasing with time. Observed non-constant dissolution
rate was consistent with diffusion limitation on the dissolution.

It was concluded that solution phase concentrations of 2,4-DNT
were diffusion-limited rather than solubility limited.
Acknowledgements
The tests described and the resulting data presented herein,
unless otherwise noted, were obtained from research conductedunder the US Army Environmental Quality Technology Basic
Research Program by the US Army Engineer Research and Development Center. Permission was granted by the Chief of Engineers topublish this information. Work of Šimunek was supported by the
Terrestrial Sciences Program of the Army Research Ofﬁce(Terrestrial Processes and Landscape Dynamics and Terrestrial
System Modeling and Model Integration). The views expressed inthis article are those of the authors’ and do not reﬂect the ofﬁcial
policy or position of the Department of the Army, Department of
Defense, or the US Government. The use of trade, product, or ﬁrm
names in this paper is for descriptive purposes only and does not
imply endorsement by the US Government.
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