Science of the Total Environment 624 (2018) 758–768

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Science of the Total Environment
journal homepage: www.elsevier.com/locate/scitotenv

Dissolution and transport of insensitive munitions formulations IMX-101
and IMX-104 in saturated soil columns
Jennifer D. Arthur a, Noah W. Mark a, Susan Taylor b, Jiří Šimůnek c,Mark L. Brusseau a,d, Katerina M. Dontsova a,e,⁎a

Soil, Water and Environmental Science Department, University of Arizona, United States
U.S. Army Engineer Research and Development Center, United Statesc
Department of Environmental Sciences, University of California, Riverside, United StatesdDepartment of Hydrology & Atmospheric Sciences, University of Arizona, United StateseBiosphere 2, University of Arizona, United Statesb

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T• Dissolution and transport experiments
were performed for IMX-101 and IMX104• IM constituents dissolved in order of
their solubility• NTO and NQ concentrations were highinitially but decreased with time
• Modiﬁed HYDRUS-1D successfully described both dissolution and transportof IMs• Determined transport parameters agreed
with values obtained in batch studies

a r t i c l e

i n f oArticle history:
Received 5 October 2017
Received in revised form 11 November 2017
Accepted 27 November 2017
Available online xxxx
Editor: Dr. Ouyang Wei
Keywords:
Insensitive munitions
Dissolution
HYDRUS-1D modeling
IMX formulations
2, 4-Dinitroanisole (DNAN)
3-Nitro-1,2,4-triazol-5-one (NTO)a b s t r a c t
Military training exercises can result in deposition of energetic residues on range soils, which ultimately can contaminate groundwater with munitions constituents. Column experiments followed by HYDRUS-1D modelingwere conducted to evaluate dissolution and transport of energetic constituents from the new insensitive munitions(IM) formulations IMX-101, a mixture of 3-nitro-1,2,4-triazol-5-one (NTO), nitroguanidine (NQ), and 2, 4dinitroanisole (DNAN), and IMX-104, a mixture of NTO, 1,3,5-hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and
DNAN. NTO and DNAN are emerging contaminants associated with the development of insensitive munitions as
replacements for traditional munitions. Flow interruption experiments were performed to investigate dissolution
kinetics and sorption non-equilibrium between soil and solution phases. The results indicated that insensitive munitions compounds dissolved in order of their aqueous solubility, consistent with prior dissolution studies conducted in the absence of soil. Initial elution of the high concentration pulse of highly soluble NTO and NQ was followed
by lower concentrations, while DNAN had generally lower and more constant concentrations in leachate. The sorption of NTO and NQ was low, while RDX, 1,3,5,7-octahydro-1,3,5,7-tetranitrotetrazocine (HMX, an impurity intechnical grade RDX), and DNAN all exhibited appreciable sorption. DNAN transformation was observed, with formation of amino-reduction products 2-ANAN (2-amino-4-nitroanisole) and 4-ANAN (4-amino-2-nitroanisole).
HYDRUS-1D model, incorporating one-dimensional advective-dispersive transport with particle dissolution andﬁrst-order solute transformation was used to simulate the measured breakthrough curves. Optimized dissolution
parameters varied widely but were correlated between compounds in the same formulation. Determined⁎ Corresponding author at: Biosphere 2, University of Arizona, 845 N. Park Avenue, Tucson, AZ 85721-0158, United States.
E-mail address: dontsova@email.arizona.edu (K.M. Dontsova).

https://doi.org/10.1016/j.scitotenv.2017.11.307
0048-9697/© 2017 Published by Elsevier B.V.

J.D. Arthur et al. / Science of the Total Environment 624 (2018) 758–768

759

adsorption coefﬁcients generally agreed with values determined from batch and column studies conducted withpure NTO and DNAN, while mass-loss rate coefﬁcients were in better agreement with ones from batch than column studies possibly due to suppression of microbial transformation during elution of high concentrations of explosives. Even in the low organic matter soils selected in this study DNAN experienced signiﬁcant retardation andtransformation, indicating potential for its natural attenuation.
© 2017 Published by Elsevier B.V.

1. Introduction
The United States Army and BAE systems (Arlington, VA) have developed insensitive munitions (IM) formulations to replace 2,4,6-trinitrotoluene (TNT) and Composition B, which contains 1,3,5-trinitro-1,3,5triazine (RDX) and TNT, because TNT and RDX can be vulnerable to temperature and shock causing unintended detonations. Two explosive
compounds, 3-nitro-1,2,4-triazol-5-one (NTO) and 2, 4-dinitroanisole(DNAN) have been selected as replacements. The IM formulations thathave been developed using NTO and DNAN are IMX-101 (19.7% NTO,
36.8% nitroguanidine (NQ), and 43.5% DNAN) and IMX-104 (53.0%
NTO, 15.3% RDX, and 31.7% DNAN). DNAN serves as a binder in thesemelt-cast explosive formulations. IMX-104 also has 1,3,5,7-octahydro1,3,5,7-tetranitrotetrazocine (HMX) present because technical grade
RDX contains approximately 10% HMX.
When munitions do not completely detonate (a low-order detonation), a signiﬁcant mass of the explosive compounds originally present
in a round can be deposited on the ground. Training ranges are a particular concern due to the great number of rounds ﬁred in the same location, resulting in some cases in groundwater contamination withmunitions constituents (Jenkins et al., 2001). The likelihood of IM rounds
experiencing low-order detonations is higher than for rounds ﬁlled withtraditional explosives (Walsh et al., 2013). Hence, more IM explosives
will likely be deposited on the ground compared to traditional explosives. When explosives are exposed to rainfall, they dissolve, introducing
the munitions constituents into the environment. Transport of explosive
compounds off military ranges can result in closure of the ranges or limit
training.
Health and environmental effects associated with exposure to explosive compounds vary. Toxicological reports for NTO indicate that is it nottoxic to rats and mice when given orally (LD50 N 5 g·kg−1) (London andSmith, 1985). Nitroguanidine has low toxicity in mammals, as well: thereported oral LD50 for NQ is 3.9 g·kg−1 in mice and 10.2 g·kg−1 in rats
(Deeter, 2000). NQ also has low acute and chronic toxicity to a range ofaquatic organisms; however, products of NQ photolysis have beenshown to be 100-fold more toxic (Burton et al., 1993; Sunahara, 2009).
RDX and DNAN have higher mammalian and ecotoxicity: RDX's reported
LD50 ranges from 44 to 300 mg·kg−1 in rats and the major toxic effects
include anemia with secondary splenic lesions, hepatotoxicity, possible
effects on the central nervous system, and urogenital lesions (Etnier,1986; Deeter, 2000). Toxicological reports for DNAN indicate that it ismore toxic than TNT for mammals (LD50 (rat) of 199 mg·kg−1)(Davies and Provatas, 2006) and is toxic to bacteria Vibrio ﬁscheri andfreshwater green algae Pseudokirchneriella subcapitata in the mg·L−1
range and to earthworm Eisenia andrei and perennial ryegrass Lolium
perenne in the mg·kg−1 range (Dodard et al., 2013).
The high aqueous solubilities of NTO and NQ indicate that both compounds will more readily dissolve in water than DNAN and RDX, which
are substantially less soluble (Table 1). Laboratory drip tests show thatthe aqueous solubility of the IMX formulations controls their dissolution,with NTO dissolving 1st followed by NQ, and DNAN in IMX-101 and by
RDX and DNAN in IMX-104 (Taylor et al., 2013; Taylor et al., 2015b).
Concentrations of NTO and NQ in aqueous solutions were high initiallywhen the compounds were dissolving from the outer shell of IM particles, but decreased with time as the distance between particle surface
and dissolving compounds increased. Dissolution of NTO and NQ from

IMX-101 and NTO from IMX-104 left a porous particle comprised of
DNAN (and RDX for 104) whose dissolution was more constant overtime (Taylor et al., 2013). This dissolution pattern was also observed in
outdoor dissolution studies, where in addition photo-transformation of
IM compounds was observed (Taylor et al., 2015a).
Parameters that describe interaction of munitions constituents withsoils, linear (Kd) and Freundlich (Kf) soil adsorption coefﬁcients, soil organic carbon (OC) adsorption coefﬁcients (KOC), as well as octanol–
water partition coefﬁcients, Kow, have been reported for the traditional
components of these formulations, NQ and RDX, and to a lesser extent
for NTO and DNAN (Table 1). NQ and NTO are both very mobile andhave low Kd, KOC, and Kow values (Mirecki et al., 2006; Mark et al.,2016; Mark et al., 2017) (Table 1). Mark et al. (2016) examined NTO interactions with a number of soils and concluded that adsorption was
low and not related to the amount of OC in the soil, but was strongly inﬂuenced by soil pH due to the ionic character of NTO. Mark et al. (2016,2017) also indicated that NTO transformation rates are sensitive to soil
OC content. In agreement with its low Kow values (Burrows et al., 1989;
Dave et al., 2000), NQ experiences little adsorption in soils (Dontsova etal., 2007; Dontsova et al., 2008; Taylor et al., 2012). It also does nottransform, resulting in conservative transport through soils (Dontsova
et al., 2007; Dontsova et al., 2008; Taylor et al., 2012).
DNAN has a higher Kow value than NQ, NTO and RDX, and thus is anticipated to be less mobile in soil (Table 1). Batch and column soil adsorption experiments for DNAN (Arthur et al., 2017) indicated that DNAN
strongly adsorbed to soils, resulting in signiﬁcant retardation duringtransport. Linear adsorption coefﬁcients measured in batch studies for
11 different soils ranged between 0.6 and 6.3 L·kg−1, and Freundlich coefﬁcients between 1.3 and 34 mg1 − n·Ln·kg−1 and positively correlatedwith cation exchange capacity and the amount of OC in the soil. It wasalso shown that DNAN is readily reduced to amino transformation products 2-amino-4-nitroanisole (2-ANAN) and 4-amino-2-nitroanisole (4ANAN) (Hawari et al., 2015; Arthur et al., 2017). RDX has intermediate
behavior with higher Kd, KOC, and Kow values than NTO and NQ, but
lower than for DNAN (Haderlein et al., 1996; Singh et al., 1998). The results of a study examining NTO and DNAN sorption from IMX-101 solutions (Richard and Weidhaas, 2014) showed that the Freundlich
isotherm provided the best ﬁt of both data sets, and that desorption
was limited.
The objective of this study was to examine the dissolution and fate(adsorption and transformation) of IMX-101 and IMX-104 in two soils
collected from military training ranges in Camp Swift, TX and Camp
Guernsey, WY. The results are assessed and compared to the dissolution
studies without soils and transport and fate behavior observed for individual constituents of the insensitive munitions formulations.
2. Materials and methods
2.1. Soils
Camp Swift (Bergstrom sandy clay loam, mixed, superactive, thermic
Cumulic Haplustolls) collected at Camp Swift, TX and Camp Guernsey(Keeline-Turnercrest loam, mixed, superactive, calcareous, mesic Ustic
Torriorthents) collected in Camp Guernsey, WY were the soils used forthis study. These two soils were previously used in batch and column
transport experiments conducted with NTO and DNAN (Dontsova et760

J.D. Arthur et al. / Science of the Total Environment 624 (2018) 758–768

Table 1
Environmentally relevant chemical and physical properties of DNAN, NTO, NQ, RDX, and HMX including solubility at 25 °C, octanol-water partition coefﬁcient (Kow), soil organic carbon
adsorption coefﬁcient (KOC), and soil adsorption coefﬁcient (Kd).
Property

DNAN

NTO

NQ

RDX

HMX

Solubility, mg·L−1
Log Kow
Log KOCKd, L·kg−1

276.2a1
1.7–1.92b, 1.62a6, 1.58a7
2.2b10, 2.33–2.56a7, 1.93–2.63a11
0.6–6.3a11

16642c2
0.37–1.03b6
2.1b10, 0.60–1.79a12
0.02–0.51a12

2600–5000b3−0.83b8−0.356b13, 0.82–2.22a14
0.15–0.43a16, 0.24–0.60a17

59.9a4
0.81–0.87b4
0.88–2.4b4
0.12-36a14

55
0.19a90.54–2.8a15
0.12–12.1a14

aMeasured; bEstimated; cInterpolated from measured values. 1(Boddu et al., 2008); 2(Spear et al., 1989); 3(Mirecki et al., 2006); 4(Brannon and Pennington, 2002); 5(Glover andHoffsommer, 1973); 6(Sokkalingam et al., 2008);7(Hawari et al., 2015); 8(Dave et al., 2000); 9(Yoon et al., 2002); 10(Chakka et al., 2008); 11(Arthur et al., 2017); 12(Mark et al., 2016);
13(Burrows et al., 1989); 14(Dontsova and Taylor, 2018); 15(Boyer et al., 2007); 16(Pennington et al., 2004); 17(Taylor et al., 2012).

al., 2014; Mark et al., 2016; Arthur et al., 2017; Mark et al., 2017). Thesesoils were selected for their relatively low organic matter content to provide conservative estimates of soil attenuation of IM compounds. Soilsamples were collected to a depth of 30 cm; they were air-dried andsieved (b 2 mm) prior to being analyzed for pH in 1:1 soil:water, electrical conductivity (EC), cation exchange capacity (CEC), OC content, andclay mineralogy (Mark et al., 2016). Both soils have high pH (7.83 and8.21) and low OC content (0.34 and 0.77% for Camp Swift and Camp
Guernsey, respectively), but differ in their particle size distribution.
Camp Swift is ﬁner textured (23.7% clay, 20.8% silt, and 55.5% sand)while Camp Guernsey is coarser (4.1% clay, 12.5% silt, and 83.4% sand).
Cation exchange capacity is higher in Camp Swift: 6.5 cmol·kg−1 compared to 2.9 cmol·kg−1 in Camp Guernsey due to higher clay content;
and both soils have mixed mineralogy with Camp Guernsey having
more smectite (Mark et al., 2016).
2.2. Column experiments
To measure dissolution and transport behavior of IMX-101 and IMX104, saturated ﬂux-controlled column experiments were conductedunder steady-state and transient conditions. Supelco (Belfonte, PA)
glass columns with internal diameter of 1.18 cm and 7-cm length wereused in this study. Columns were packed with Camp Guernsey andCamp Swift soils. The columns were packed in small increments to ensure uniform bulk density and to minimize preferential ﬂow. Glasswool was placed on the top and bottom of the soil proﬁle to prevent
soil movement. The average packed bulk density (ρb) was 1.61 ±0.05 g cm−3 for Camp Guernsey soil and 1.51 ± 0.04 g cm−3 for Camp
Swift soil. The bulk density was determined from the dry mass of thesoil used to pack the glass columns of known volume.
Columns were saturated from bottom to top with 0.005 M CaCl2
background solution for one hour to avoid air entrapment. Pore volume(PV) of the columns was determined by measuring the volume of solution necessary to saturate the packed column, corrected for column
dead volume. After this, approximately 50 mg of undetonated or detonated IMX-101 and IMX-104 was placed on top of the column on alayer of glass wool (Figure S1). Technical grade IMX-101 and IMX-104
used in the experiments (undetonated samples) were provided by the
US Army Armament Research, Development and Engineering Center(ARDEC), Picatinny Arsenal, while detonated samples were residues of
low order detonations (Walsh et al., 2014).
PTFE caps were used to seal the tops of the columns. Tygon microboretubing connected PTFE caps with the Cole-Parmer (Vernon Hills, IL) Master ﬂex peristaltic pump that supplied solution. Columns were connectedto the pump on the top and ﬂow of 0.005 M CaBr2 tracer solution wasstarted. Measured average ﬂow rate was 0.008 ± 0.001 mL·min−1,
equivalent to Darcy ﬂux of 0.42 ± 0.05 cm·h−1 and mean pore-watervelocity of 1.06 ± 0.17 cm·h−1. After three to ﬁve PV) for IMX-101(ﬁve to eight PVs for IMX-104) depending on the soil, IM particles
were removed and ﬂow was switched to background solution(0.005 M CaCl2) that was applied for another three to ﬁve PVs to monitor
the elution of the munitions constituents from the soil. In addition, ﬂow
interruption studies were conducted, where ﬂow was stopped for 24 h tobetter characterize potential mass-transfer processes. This technique isused to differentiate between different mechanisms responsible for sorption non-equilibrium (Brusseau et al., 1989; Brusseau et al., 1997) andstudy dissolution kinetics (Dontsova et al., 2008; Dontsova et al., 2009;
Taylor et al., 2012).
Efﬂuent samples were collected continuously into 4-mL amber vials
using a Teledyne ISCO (Lincoln, NE) Foxy 200 Fraction collector with a
200-vial capacity. A conservative tracer, Br−, was used to determine
the longitudinal dispersivity (λ) and observe for preferential ﬂow foreach soil. Br− was analyzed using Ion Chromatography (Dionex ICS
5000 with diode array). High performance liquid chromatography wasused to quantify eluting IM components as described below. Upon completion of each experiment, columns were subdivided into thirds (top,middle, bottom) and soil extractions were performed using acetonitrile(1:2 soil to acetonitrile). The suspensions were agitated for 24 h, centrifuged and ﬁltered using 0.45 μm Millex-HV PVDF ﬁlter (EMD Millipore
Darmstadt, Germany) (U. S. Environmental Protection Agency, 1994),
and subjected to HPLC analysis.
Mass balance calculations were performed for all IM components byintegration of each breakthrough curve (BTC), performing soil extractions, and extractions of remaining IM particles. Initial amount of IM
compound in each particle was estimated assuming ideal mixing of thecomponents.
Eight continuous ﬂow dissolution and transport experiments (2 soils∗ detonated / undetonated ∗ 2 replications) and four interrupted ﬂow experiments (2 soils ∗ detonated only ∗ 2 replications) were conducted foreach IM formulation. A total of twenty-four experiments were conducted, with a fresh pack used for each one.
2.3. Analytical method
Column efﬂuent was analyzed for energetic compounds and theirtransformation products using a high performance liquid chromatograph (HPLC, Dionex Ultimate 3000) equipped with a diode array detector (ThermoFisher, MA). The method for NTO was adapted from Le
Campion et al. (1999). The parameters used for analyzing NTO samples
were the following: mobile phase ratio of acetonitrile (ACN): deionizedwater of 75:25 with 0.1% triﬂuoroacetic acid (TFA) was run isocracticallyat 1 mL·min−1. Oven temperature was set at 32 °C. NTO and its transformation products were separated using a Thermo Scientiﬁc Hypercarb
Column. NTO was detected at approximately 2.8 min using 315 nm
wavelength. The UV detector was set at 220 nm to monitor for presenceof potential NTO transformation product, 5-amino-1,2,4-triazol-3-one(ATO) (Le Campion et al., 1999). No ATO was detected. The operational
method for DNAN, RDX, NQ, HMX and DNAN transformation products
was adapted from Olivares et al. (2013). The following parameters
were used to analyze these samples: a mobile phase ratio of 43:57 methanol and water; the Thermo Scientiﬁc Acclaim reversed phase column C18 with 5-μm particle size; and a ﬂow rate of 1 mL·min−1. The wavelengths used for detection and quantiﬁcation were 300 nm for DNAN,
NQ, 254 nm for 2-ANAN, 4-ANAN, RDX, HMX, and 210 nm for DAAN.
The retention times for each compound on HPLC were for DNAN approximately 21 min, 11 min for 2-ANAN, 6 min for 4-ANAN, 8 min for RDX,
5 min for HMX, and 3 min for NQ. Detection limits were 0.015 mg·L−1
for DNAN, 2-ANAN, 4-ANAN, NQ, RDX, HMX, and 0.020 mg·L−1 forJ.D. Arthur et al. / Science of the Total Environment 624 (2018) 758–768

NTO as determined by visual evaluation of the standard curve for eachcompound.
2.4. Numerical analysis
All 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 (Šimůnek et al., 2008;Šimůnek and van Genuchten, 2008). HYDRUS-1D is a numerical modelthat solves the governing ﬂow and transport equations using the methodof ﬁnite elements. The software was used in the inverse mode to analyze
BTCs obtained in the column experiments. HYDRUS-1D was modiﬁed toaccount for dissolution of constituents from solids. Similar modiﬁcations
have been used in previous studies (Dontsova et al., 2006; Dontsova etal., 2008; Dontsova et al., 2009; Taylor et al., 2012) to estimate the dissolution rates of energetic materials, explosives and propellants.
2.4.1. Dissolution
For explosive materials in this study, the DNAN, NTO, NQ, HMX, andRDX dissolution rate, Υ, was deﬁned as:ϒ ¼ max ϒ max e−χt ; ϒ min

ð1Þ

where Υmax is the initial (maximum) dissolution rate [ML−3·T−1], χ isthe decay constant [T−1], Υmin is the minimum or steady-state dissolution rate, deﬁned as a product of Υmax and a, the dimensionless constant,and t is time [T]. If Υmaxe−χt N Υmin then Υmaxe−χt is used in the equation.
If Υmaxe−χt b Υmin then Υmin is used. Dissolution rates are reported normalized to the mass of the IMX-101 or 104 added to the column (Taylor
et al., 2012).
2.4.2. Reactive transport
The transport equation for constant ﬂux density, water content, dispersion coefﬁcient, bulk density, and distribution coefﬁcient is!
2∂ϴC∂qC ρb ∂S
 ∂ C
−
¼D
−
−φ þ ϒϴ
∂t
∂z2
∂z
ϴ∂t

ð2Þ

where ϴ represents the volumetric water content [L3·L−3], C is the concentration of the solution [ML−3], z represents the spatial coordinate [L],q is the volumetric ﬂuid ﬂux density [LT−1] evaluated by Darcy–
Buckingham law, ρb is the bulk density [ML−3], ϒ is the dissolution rate
as described in Eq.1 [ML−3·T−1], and D is the dispersion coefﬁcientwhich accounts for both hydrodynamic dispersion and diffusion[L2·T−1] which is represented in Eq. 3.
D ¼ λv þ D

ð3Þ

where v is the average linear velocity [LT−1], λ is the longitudinal
dispersivity [L], and D∗ represents the effective diffusion coefﬁcient[L2·T−1]. Sorption, S, [MM−1] is assumed to be instantaneous and is deﬁned as:
S ¼ KdC

ð4Þ

where Kd is the linear adsorption coefﬁcient [L3·M−1]. φ is the sink/
source term which accounts for zero-, ﬁrst- and other-order reactions[ML−3·T−1]. In this model, φ is the ﬁrst-order rate for the solution only
[ML−3·T−1] and is deﬁned for IM constituents as:
φ¼kϴC

ð5Þ

where k represents the mass-loss rate coefﬁcient due to transformation
and/or irreversible adsorption [T−1]. Lastly, for the daughter products,
the ﬁrst-order reaction rate [ML−3·T−1] is deﬁned as:
φ ¼ k θC w −kp θ C wp

ð6Þ

761

where the ﬁrst term on the right side represents transformation of adaughter product (i.e., 2-ANAN) and the second term represents production of a daughter product by transformation of a parent compound (i.e.,
DNAN). In eq. 6, kp and Cwp represent the transformation rate constant[T−1] and solute concentration [ML−3] of a parent compound. Bromide
tracer values for k and Kd were equal to zero.
2.4.3. Parameter estimation
Numerical analysis of the experimental data was performed by ﬁrst
using the nonreactive tracer data to determine λ (cm). The dispersivity
for explosives was ﬁxed at the values determined for the tracer. The following parameters were optimized for the munitions constituents: Kd(L·kg−1) and dissolution parameters, Υmax, χ, and constant, a, which
multiplied by Υmax gives the steady-state dissolution rate (Υmin)(mg·L−1·h−1). The mass-loss rate coefﬁcient for dissolved phase, k(h−1) (transformation rate constant) was set to values determinedfrom batch studies for DNAN and NTO (Mark et al., 2016; Arthur et al.,2017) and derived from literature for NQ, RDX, and HMX. Conversely,
they were optimized via HYDRUS-1D for 2-ANAN. The 95% conﬁdence
intervals and R2 values for optimized parameters were obtained by statistical analysis of the predicted versus measured breakthrough curves.
Parameter estimates were considered signiﬁcant if they were statisticallydifferent than zero. For the replicate experiments parameter estimates
were averaged and differences determined based on conﬁdence intervals of the mean.
3. Results and discussion
3.1. Tracer
Measured bromide BTC are symmetrical, occur at one PV, and are
well described using the advection-dispersion transport model (Figs. 1,
2, 4, and 5). This indicates ideal advective-dispersive transport, with nomeasurable impacts of preferential ﬂow or air entrapment within thecolumn (e.g., Brusseau, 1994). Longitudinal dispersivity was slightly larger for the coarser Camp Guernsey soil (0.57 ± 0.32 cm) (mean ± CI)
than for Camp Swift (0.20 ± 0.13 cm) but differences were not signiﬁcant. The dispersivity values are consistent with values measured inprior column experiments (e.g., Brusseau, 1994).
3.2. IMX-101
All IMX-101 components, NTO, NQ, and DNAN, as well as 2-ANAN
and 4-ANAN, transformation products of DNAN, were observed in the efﬂuent from both detonated and undetonated IMX-101 (Fig. 1). The IM
constituents BTCs followed a general pattern of high initial peak in efﬂuent concentration, followed by a rapid decrease to a relatively steadystate stage. This pattern was very pronounced for NTO and NQ, while
for DNAN efﬂuent concentrations were relatively constant once breakthrough occurred. NTO had the highest maximum outﬂow concentrations in all experiments despite representing only 19.7% of the total
mass of IMX-101. The high concentrations of NTO seen in the efﬂuent
are consistent with its high solubility and rapid dissolution observed inlaboratory drip studies (Taylor et al., 2013; Taylor et al., 2015b). Initial
peak concentrations of NQ, on the other hand, were lower than thoseof NTO, despite higher content in the formulation (36.8%) but consistent
with NQ solubility being lower by a factor of four (Table 1).
The suggested mechanism explaining the shape of the BTCs is thatthe initial higher concentrations are produced by ready dissolution of
the IM constituents adjacent to the external surfaces of the IM particles,
faster for more soluble components, and slower for less soluble ones.
Whereas the reduction in concentrations that follows is related to slower
release of constituents deeper within the matrix, where it is limited bytime needed for water to reach particle interior and for solutes to exit
the particle through convection and diffusion. Observed dissolution patterns are similar to ones described for propellant constituents that have

762

J.D. Arthur et al. / Science of the Total Environment 624 (2018) 758–768

1.2

a. NTO 1200

1.2

1

1000

1

250

0.8

800

0.8

200

0.6

600

0.6

150

0.4

400

0.4

100

0.2

200

0.2

50

0

0

0
0

2

4

6

8

0

10 12 14 16

0

2

4

Pore Volume
1.2
1

70

1.2

60

1

50

0.8

40

0.6

30

0.4

20
10
0

0

8

10 12 14 16

0.7

d. 2-ANAN and 4-ANAN

0.6
0.5
0.4
0.3

0.4

0
6

10 12 14 16

0.6

0.2

4

8

0.8

0.2
2

6

Pore Volume

c. DNAN

0

300

b. NQ

0.2
0.1
0
0

2

Pore Volume

4

6

8

10 12 14 16

Pore Volume−

Fig. 1. Breakthrough curves for IM constituents dissolving from detonated IMX-101 in Camp Swift soil. Measured Br concentrations are presented in light grey diamonds, IM constituents,
NTO, NQ, DNAN in dark grey triangles; and products of DNAN transformation, 2-ANAN and 4-ANAN, in solid and hollow circles, respectively. Lines present HYDRUS-1D simulation results.
Vertical grey solid line indicates switch to background solution.

an insoluble nitrocellulose matrix (Dontsova et al., 2008; Dontsova et al.,2009; Taylor et al., 2011; Taylor et al., 2012), but are unlike what was observed for traditional explosive formulations that have overall less soluble constituents than IMs (Dontsova et al., 2006; Taylor et al., 2009).
While constituent concentrations in solution and estimated dissolution parameters differed between individual columns (Tables 2 ands1), HYDRUS-1D simulated Υmax parameters had a highly signiﬁcant correlation between NTO and DNAN (Fig. 2, P = 0.000423), indicating a link
between dissolution of constituents from the IM particles. There was no
signiﬁcant difference between Υmax, Υmin, and χ parameter estimates for
all IMX-101 constituents between experiments with undetonated anddetonated IMs (Table 2 and s1). Parameter estimates for IMX-101 fromlab drip studies (Taylor et al., 2015b) (Table s2) underestimated theYmax NTO (mg L-1 h-1 mg -1)

1600

dissolution parameters in this study, possibly due to larger size, andtherefore lower speciﬁc surface area, of the particles they used (Tables1).
NTO breakthrough occurred at the same time as nonreactive tracer
Br− for all IMX-101 experiments, detonated and undetonated, withand without ﬂow interruption (Figs. 1 and 2). This indicates NTO experienced no measurable sorption by either soil. In solution NTO is present in
its deprotonated form and is negatively charged at environmentally relevant pHs (Smith and Cliff, 1999), limiting its adsorption to largely negatively charged soil surfaces. Previous research conducted on soil
Table 2
Mean dissolution parameters obtained by HYDRUS-1D for saturated column experiments
involving IMX-101 in Camp Swift and Camp Guernsey soils. Decay constant, χ, andinitial and steady-state dissolution rates, Υmax and Υmin, were estimated from explosives
BTCs. FI = ﬂow interruption and n = number of experiments included in the mean.
Treatmentχ

95% CI Υmax
mg·L

1200
800
y = 14.90x + 94.86
R² = 0.93

400
0
0

50

Ymax

100

DNAN (mg L-1 h-1 mg -1)

Fig. 2. Correlation between simulated initial dissolution rates, Υmax, for NTO and DNAN,
components in IMX-101 formulation from continuous ﬂow experiments. The Thompson
Tau method was used to reject 1 outlier in the data set.

95% CI
−1

·h

−1

·mg

−1

Υmin
mg·L

95% CI
−1

·h

−1

·mg−1

NTO
Detonated (n = 4)
Undetonated (n = 4)
Detonated FI (n = 4)
Mean across IMX-101(n = 12)

0.66
1.04
0.34
0.68

0.36
1.13
1.12
0.40

349.99585.25926.70620.65223.13564.20306.40248.15

10.30
11.40
3.13
8.28

11.12
8.11
0.87
4.69

NQ
Detonated (n = 4)
Undetonated (n = 4)
Detonated FI (n = 4)
Mean across IMX-101(n = 12)

0.76
1.91
0.20
0.96

0.82
3.07
0.18
1.05

177.11143.86541.44287.47136.16120.40476.74187.06

19.15
8.65
21.85
16.55

20.63
5.96
16.35
8.81

DNAN
Detonated (n = 4)
Undetonated (n = 4)
Detonated FI (n = 4)
Mean across IMX-101(n = 12)

2.84
35.96
4.00
14.27

5.38
62.63
7.35
21.13

17.19
57.04
35.18
36.47

8.92
48.92
16.21
18.48

4.71
3.26
13.97
7.31

1.97
3.83
10.07
4.33

J.D. Arthur et al. / Science of the Total Environment 624 (2018) 758–768

adsorption of NTO showed high mobility and low sorption (Braida et al.,2012; Hawari et al., 2012; Richard and Weidhaas, 2014; Mark et al.,2016; Mark et al., 2017). NTO Kd's were not signiﬁcantly different fromzero in HYDRUS-1D simulations for IMX-101 and therefore were set tozero for ﬁnal optimization simulations (Table s1).
Similarly to NTO, NQ arrived with the nonreactive tracer indicating
low soil adsorption (Figs. 1 and 2). Nitroguanidine is a relatively inert
compound in soil environments: previous research has indicated limited adsorption of NQ in soils and little degradation/transformation (Haaget al., 1990b; Dontsova et al., 2007; Dontsova et al., 2008; Taylor et al.,2012). Estimated Kd's (Table s1) (0.06 ± 0.03 L·kg−1 for Camp Swiftand 0.06 ± 0.04 L·kg−1 for Camp Guernsey) were on the lower rangeof literature values (Table 1) (Haag et al., 1990a; Pennington et al.,
2004; Taylor et al., 2012) consistent with low OC content of the studied
soils.
DNAN was the last compound to break through (Fig. 1). DNAN also
had the lowest efﬂuent concentrations. Previous research has shownthat DNAN adsorbs to soil and that its adsorption is inﬂuenced by thesoil OC and clay content (Hawari et al., 2015; Arthur et al., 2017). Linear
adsorption coefﬁcient values estimated by HYDRUS-1D for DNAN (Tables1) were similar for the two soils (0.63 ± 0.21 L·kg−1 for Camp Swift and
0.53 ± 0.07 L·kg−1 for Camp Guernsey). For Camp Swift soil, they werecloser to batch experiment Kd's (0.60 ± 0.20 L·kg−1) then those obtained from column experiment Kd values (1.2 ± 0.08 L·kg−1) (Arthur et al.,2017).
2-ANAN and 4-ANAN, DNAN transformation products detected incolumn efﬂuent also exhibited retardation (Figs. 1 and 2). They both appeared at the same time as DNAN for detonated and undetonated IMX101. 2-ANAN concentrations in the efﬂuent were approximately anorder of magnitude lower than DNAN concentrations and 4-ANAN concentrations were smaller than 2-ANAN concentrations, indicating
regio-selectivity of DNAN amino-transformation (Hawari et al., 2015;
Arthur et al., 2017). Tailing occurred at the end of the experiments for
2-ANAN and 4-ANAN. Soil adsorption coefﬁcients estimated for 2ANAN (0.71 ± 0.52 L·kg−1 for Camp Swift and 1.56 ± 1.16 L·kg−1 forCamp Guernsey) (Table s1) were similar to the values determined in acolumn transport study of pure DNAN (0.43–1.93 L·kg−1) and estimatedk values for 2-ANAN (0.01 ± 0.01 h−1 for Camp Swift and 0.06 ±0.03 h−1 for Camp Guernsey) were close to the estimated ks determinedfor 2-ANAN in Camp Swift soil in column transport studies (0.03 ±0.04 h−1) (Arthur et al., 2017). There were no signiﬁcant differences between undetonated and detonated IMX-101 Kd values for all constituents (Table s1).
Flow interruption experiments were conducted for detonated IMX101 only. After stopping ﬂow and then resuming it in 24 h and elutionof 1 PV, efﬂuent NTO and NQ concentrations increased for both soils
and then returned to steady-state values (Fig. 3). Increase in concentration after ﬂow interruption is consistent with continuing dissolution of
IM particles when ﬂow is stopped causing solute accumulation. Once
this solute passes through the column concentrations return to lower
values. No or little decrease in concentration was observed for NTO andNQ immediately after ﬂow interruption due to continued transformationin agreement with used k values (zero for both soils for NQ (Taylor et al.,2012) and 0.0009 and 0.0004 h−1 for NTO in Camp Swift and Camp
Guernsey soils, respectively (Mark et al., 2016)).
For DNAN, no clear decrease in concentration was observed after ﬂow
interruption. However, concentrations of its transformation products, 2ANAN and 4-ANAN, increased immediately after ﬂow was resumed, indicating continued transformation and accumulation of transformation
products throughout the columns. Absence of corresponding decreases
in DNAN concentration is explained by a low transformation rate relative
to error of the measurement. DNAN also did not experience increase inconcentration following ﬂow resumption like NTO and NQ due to its
slower dissolution rate.
Transformation rate constants for DNAN were ﬁxed in HYDRUS-1D
simulations at the values determined in batch studies (Arthur et al.,763

2017), 0.0006 and 0.0033 h−1 for Camp Swift and Camp Guernsey, respectively. However, good agreement between simulated and measured
BTCs for IF experiments that allow separating effects of dissolution andtransformation on leachate concentrations, indicates that these values
described observed patterns well.
Transformation rate constants previously obtained for NTO and DNA
in column transport experiments are signiﬁcantly higher than in batch
experiments (Mark et al., 2016; Arthur et al., 2017; Mark et al., 2017)supposedly due to decrease in redox potential of the soil during saturated ﬂow. However, current work indicates that batch transformation rate
constants describe transport of dissolving explosives better. This phenomenon may be explained by DNAN toxicity to soil microorganisms(Dodard et al., 2013). High concentrations of DNAN during dissolution
experiments may suppress microbial growth and cause decrease intransformation.
3.3. IMX-104
IMX-104 contains 53.0% NTO, 15.3% RDX, and 31.7% DNAN, 33.3%
more NTO and 11.8% less DNAN than IMX-101. The behavior of NTO
was similar to that of NTO in IMX-101, however it was retarded slightly
more in both Camp Swift and Camp Guernsey soils. Similarly to patterns
observed for IMX-101, NTO arrived before all other IM constituents in
the IMX-104 and had the highest peak and steady-state concentrations(Fig. 4). NTO Kd's ranged from not signiﬁcantly different from zero to
0.23 L·kg−1 (Table s3) which in some cases was higher than Kd values
obtained for NTO in the IMX-101 dissolution studies and Kd values obtained in batch study experiments performed by Mark et al. (2016).
RDX was the second IM compound to break through (Fig. 4). RDX exhibited retardation due to sorption, as demonstrated by the delay in RDX
breakthrough relative to the conservative tracer. Kd values obtained for
RDX ranged between 0.20 and 0.85 L·kg−1 with mean of 0.48 ±0.14 L·kg−1 for Camp Swift and 0.37 ± 0.10 L·kg−1 for Camp Guernsey(Table s3). These values were on the lower end of the range of 0.12 to
8.2 L·kg−1 RDX Kd values reported by Brannon and Pennington (2002),
as expected for soils with low OC content.
Concentrations of HMX in solution were smaller than those of RDX.
HMX Kd's equaled 0.94 ± 0.17 L·kg− 1 for Camp Swift and 0.72 ±0.20 L·kg− 1 for Camp Guernsey (Table s3), higher than RDX values
and consistent with HMX Kd values reported by Brannon andPennington (2002) (b1 L·kg−1) and Dontsova et al. (2006) (0.22–
0.36 L·kg−1).
Breakthrough for DNAN, 2-ANAN, and 4-ANAN occurred at about 3
PVs (Fig. 4). All curves were asymmetrical. DNAN Kd values (0.66 ±0.06 L·kg−1 for Camp Swift and 0.90 ± 0.26 L·kg−1 for Camp Guernsey)
agreed very well with Kds determined in in batch studies for the same
soils (0.60 ± 0.20 L·kg− 1 and 0.90 ± 0.10 L·kg−1, respectively) andwere similar to 1.2 ± 0.08 L·kg− 1 determined in column studies forCamp Swift soil (Arthur et al., 2017). 2-ANAN Kd values were 1.20 ±0.73 and 1.78 ± 1.13 L·kg−1 in Camp Swift and Camp Guernsey soils(Table s3).
Flow interruption during dissolution and transport of detonated IMX104 had no effect on NTO in Camp Swift soil (Fig. 5). RDX concentrations,about 10 times HMX concentrations, ﬁrst decreased and then increased
after ﬂow resumption, but predicted BTC showed smaller range of bothdecrease and increase, indicating that used RDX transformation rate constant (Brannon and Pennington, 2002), as well as dissolution rate calculated using it were lower than observed. Both RDX and HMX exhibited
greater adsorption to the soil than NTO with RDX breakthrough occurring around 2 PVs and HMX at 4 PVs.
DNAN, 2-ANAN and 4-ANAN also adsorbed to the soil as indicated by
the delay in the BTC (Fig. 5). Before ﬂow interruption, DNAN and 2ANAN concentrations decreased with time on the arrival wave. After resuming ﬂow DNAN concentrations slightly decreased and 2-ANAN and
4-ANAN concentrations increased indicating continued DNAN transformation in Camp Swift soil, while concentrations of NTO, RDX, HMX,

764

J.D. Arthur et al. / Science of the Total Environment 624 (2018) 758–768

1.2

a. NTO

3000

1.2

1

2500

1

0.8

2000

0.8

0.6

1500

0.6

0.4

1000

0.4

0.2

500

0.2

0

0

0
0 2 4 6 8 10 12 14 16 18

b. NQ

2000
1500
1000
500
0
0

2

4

Pore Volume
1.2

c. DNAN2500

6

8 10 12 14 16 18

Pore Volume
300

1.2

d. 2-ANAN and 4-ANAN

12

1

250

1

10

0.8

200

0.8

8

0.6

150

0.6

6

0.4

100

0.4

4

0.2

50

0.2

2

0

0

0

0

2

4

6

8 10 12 14 16 18

Pore Volume

0
0

2

4

6

8 10 12 14

Pore Volume

Fig. 3. Breakthrough curves for IM constituents dissolving from detonated IMX-101 in Camp Swift soil with ﬂow interruption. Measured Br− concentrations are presented in light grey
diamonds, IM constituents, NTO, NQ, DNAN in dark grey triangles; and products of DNAN transformation, 2-ANAN and 4-ANAN, in solid and hollow circles, respectively. Lines present
HYDRUS-1D simulation results. Vertical dashed line indicates start of 24-hr ﬂow interruption and vertical grey solid grey line indicates switch to background solution.

DNAN and 2-ANAN all increased after ﬂow interruption in Camp Guernsey soil (Figure S2). There were no signiﬁcant differences in Kd values between undetonated and detonated IMX-104 nor were there differencesin Kd values between the two soils used in the experiments.
A signiﬁcant or highly signiﬁcant correlation exists between
HYDRUS-1D estimated values for Υmin NTO and Υmin RDX (P =0.0061); Υmax NTO and Υmax DNAN (P = 0.010); and Υmin HMX andΥmin DNAN (P = 0.018) (Fig. 6). The Υmax NTO and Υmax DNAN correlation was also observed for the IMX-101 system. Some deviation from thepredicted lines is expected for these correlations because of variability incomposition of individual particles.
When comparing estimated dissolution parameters between treatments, we found that none of the parameters were signiﬁcantly different
between detonated and undetonated samples (Table 3, Table s3).

Fig. 4. Breakthrough curves for IM constituents from detonated IMX-104 particle in Camp Swift soil. Measured Br− concentrations are presented in light grey diamonds, IM constituents,
NTO, RDX, DNAN in dark grey triangles, HMX in dark grey squares; and products of DNAN transformation, 2-ANAN and 4-ANAN, in solid and hollow circles, respectively. Lines present
HYDRUS-1D simulation results. Vertical grey solid line indicates switch to background solution.

J.D. Arthur et al. / Science of the Total Environment 624 (2018) 758–768

765

Fig. 5. Flow interruption breakthrough curves for IM constituents from detonated IMX-104 particle in Camp Swift soil. Measured Br− concentrations are presented in light grey diamonds,
IM constituents, NTO, RDX, DNAN in dark grey triangles, HMX in dark grey squares; and products of DNAN transformation, 2-ANAN and 4-ANAN, in solid and hollow circles, respectively.
Lines present HYDRUS-1D simulation results. Vertical dashed grey line indicates start of 24-h ﬂow interruption. Vertical solid grey line indicates switch to background solution.

Comparison of the dissolution parameters estimated here with the estimates obtained by Taylor et al. (2015b), same as for IMX-101 showedthat their parameter estimates were lower than the estimates obtained
from this study (Table s2) likely due to the size of the particle dissolved
during analysis.
3.4. Comparison of IMX-101 and IMX-104
Dissolution rates are affected by the solubility of the compound, temperature, particle size and surface area, and agitation. During these experiments, constant temperature was maintained and the solid IMX
particles were stationary on top of the soil proﬁle, leaving the particle
surface area (or exposed surface area of individual component of the formulations) and the solubility of the compounds (Table 1). Relative solubility of compounds manifest itself in higher estimated maximum andminimal dissolution rates for NTO compared to other IM constituents,
while changing exposed surface area of individual IM components was
responsible for observed patterns of dissolution.
IMX-101 and IMX-104 are made by melt-casting during which
NTO, NQ, and RDX crystals are added to the molten DNAN. Since
DNAN is less soluble than NTO and NQ, it can shield these compounds from dissolution after they are lost from the particle surface
and thereby cause concentration decreases over time. Similarly, in
IMX-104, RDX and DNAN remain after NTO is dissolved from the surface, constraining further dissolution of NTO. The shrinking coremodel is one way of conceptualizing dissolution behavior of the
IMX particles: reactions occur ﬁrst on the outer skin of the particle,
then the zone of the reaction moves into the solid, leaving behindinert (in our case, less soluble) materials as well as exposing anotherlayer of the reactant. As NTO is depleted from the outer surface of theparticle, the dissolution front progresses deeper within the particle.
This process continues and thus the unreacted core of the particle,
where NTO remains, shrinks. Taylor et al. (2013) examined thephysical changes undergone by insensitive munitions particles during dissolution. They conﬁrmed that for IMX-101, NTO and NQ dissolved from the outside of the particles leaving behind thin bridgesof DNAN, which dissolves more slowly. The resulting porous matrix
allowed greater access of water to NTO and NQ in the particle interior, promoting their dissolution and subsequent advection with themoving water and diffusion to the particle exterior. While an analytical solution associated with the shrinking core model exists (e.g.
Levenspiel, 1999, pages 569–570), an empirical equation (Eq. 1)
was used to describe the observed patterns to simplify calculations.
IMX-101 and IMX-104 both contain NTO and DNAN in their formulations. IMX-104 contains 2.6 times more NTO than IMX-101 and 1.4 times
less DNAN. The maximum and minimum dissolution rates are higher for
NTO in IMX-101 than IMX-104 consistent with its larger percentage inthe formulation. Though ratios between formulations for maximum
and minimum dissolution rates were not the same (1.2 for Υmax and
4.0 for Υmin), they averaged to 2.6. When comparing the maximum andminimum dissolution for DNAN, we found that the dissolution rates of
DNAN in IMX-104, were in fact higher than in 101, despite smaller
DNAN percentage in the formulation (Tables 2, 3, s1, and s3). This
could not be explained by differences in exposed surface area as wewould expect that in IMX-101 fast dissolution of NTO and NQ will expose
more of DNAN surfaces, while in IMX-104 less soluble RDX would persist
and prevent further DNAN dissolution.
Dissolution model employed for HYDRUS-1D in these simulations, as
previously used in Taylor et al. (2012), resulted in good ﬁt to the initial
parts of the BTC when concentrations rapidly increased and then decreased due to decrease in dissolution. In the later part of the BTC
when model predicted no change in dissolution it worked better for
RDX, HMX, DNAN, and NQ which had more constant dissolution, andless well for NTO which experienced continued slow decrease in concentrations (Figs. 1, 2, 4, 5). Mean R2 value across all HYDRUS-1D simulations was 0.92 (Tables s1 and s3).

J.D. Arthur et al. / Science of the Total Environment 624 (2018) 758–768

Y

766

Y

Y

Y

Y

Y

Fig. 6. Correlations between simulated initial and steady-state dissolution rates, Υmax and Υmin, of IM constituents, NTO, RDX, HMX, and DNAN, in IMX-104 formulation from continuous
ﬂow experiments.

3.5. pH of the IM solutions
Table 3
Mean dissolution parameters obtained by HYDRUS-1D for column saturated experiments
involving IMX-104 in Camp Swift and Camp Guernsey soils. Decay constant, χ, and initialand steady-state dissolution rates, Υmax and Υmin, were estimated from explosives BTCs.
FI = ﬂow interruption, n = number of experiments included in the mean.
Treatmentχ

95% CI Υmax

95% CIΥmin

95% CI

mg·L−1·h−1·mg−1

mg·L−1·h−1·mg−1

NTO
Detonated (n = 4)
Undetonated (n = 4)
Detonated FI (n = 4)
Mean across IMX-104(n = 12)

0.07
0.20
0.16
0.15

0.08
0.11
0.04
0.05

996.42564.88725.34762.211138.57
365.88252.10383.24

36.36
43.66
20.41
33.48

53.73
26.86
14.62
19.50

RDX
Detonated (n = 4)
Undetonated (n = 4)
Detonated FI (n = 4)
Mean across IMX-104(n = 12)

0.18
0.21
0.16
0.18

0.22
0.18
0.16
0.10

6.10
138.57
4.35
49.68

6.15
259.66
2.85
86.68

0.98
37.21
2.33
13.51

1.39
69.87
1.48
23.29

HMX
Detonated (n = 4)
Undetonated (n = 4)
Detonated FI (n = 4)
Mean across IMX-104(n = 12)

84.04
1.35
0.05
28.48

61.46
0.96
0.07
29.71

8.83
23.02
0.31
10.72

8.70
6.25
0.20
6.41

0.19
0.14
0.18
0.17

008
0.09
0.06
0.04

DNAN
Detonated (n = 4)
Undetonated (n = 4)
Detonated FI (n = 4)
Mean across IMX-104(n = 12)

0.28
0.24
0.06
0.19

0.23
0.43
0.09
0.16

93.70
28.53
17.07
48.43

96.95
28.63
5.47
36.46

8.77
5.97
9.72
8.15

9.17
4.64
3.98
3.45

The pH of efﬂuent solutions of IMX-101 and IMX-104 was measured
after the water interacted with the explosives and soil column. In solution, NTO is acidic as a result of the N\\H bond dissociation and has a
pKa of 3.67–3.76 (Lee et al., 1987; Smith and Cliff, 1999). Taylor et al.
(2015a) showed that pH of NTO solutions is inversely related to theirconcentrations and can reach pH values as low as 2–3. During transport
NTO solutions are buffered by reactions with the soil and the average pH
of IMX-101 the outﬂow efﬂuent solution was 8.44 ± 0.19 and the average pH of IMX-104 outﬂow efﬂuent solution was 8.27 ± 0.26. However,for several PVs collected (Figure s3) when NTO concentrations were thehighest, solutions also had lower pH values. pH of solutions entering soil
matter both for fate of NTO itself, which can become deprotonated at
higher pHs decreasing its adsorption to soils that possess net negative
charge (Mark et al., 2016) and because of potential increase in mobilityof metals in soils at low pHs.

3.6. Mass balanceIMX-101 (NTO, NQ, DNAN) and IMX-104 (NTO, RDX, DNAN) constituents were recovered in column efﬂuent, soil extracts, and residual particle extracts. In addition to parent compounds, DNAN transformation
products, 2-ANAN and 4-ANAN, were also recovered in both IMX-101
and 104 and HMX was recovered in IMX-104 studies. Mass balance calculations (Tables 4, s4) indicate that the majority of the individual compounds were recovered in efﬂuent solution and in the residual particle.
In IMX-101, NTO had the highest recoveries in solution followed by NQ,
DNAN, 2-ANAN and 4-ANAN (Tables 4, s4). There were no signiﬁcant

J.D. Arthur et al. / Science of the Total Environment 624 (2018) 758–768

767

Table 4Mass recoveries of IMX-101 and IMX-104 constituents in efﬂuent, soil, and extractions of remaining IM particles relative to initial amount estimated based on deﬁned composition during
dissolution column transport studies. FI = ﬂow interruption for 24 h, n = number of experiments included in the mean.
IMX-101

Solution recoveries

Treatment

NTO

Detonated Mean (n = 4)
95% CI
Undetonated Mean (n = 4)
95% CI
Detonated FI Mean (n = 4)
95% CI
Mean across IMX-101 (n = 12)
95% CI

NQ

Soil recoveries
DNAN

2-ANAN

4-ANANIMX particle recoveries

NTO

NQ

DNAN

2-ANAN

4-ANAN

NTO

NQ

DNAN

%

%

%

%

%

%

%

%

%

%

%

%

%

46.03
32.96
34.38
21.54
73.43
22.13
51.28
16.70

19.07
17.33
12.91
8.53
80.51
35.67
37.50
21.79

11.41
14.02
18.16
28.99
32.13
5.70
20.57
11.10

0.10
0.05
0.21
0.20
2.52
2.26
0.95
0.95

0.07
0.07
0.02
0.01
0.25
0.20
0.11
0.08

1.92
2.09
0.89
0.08
0.07
0.06
0.96
0.77

0.22
0.02
0.22
0.03
0.08
0.14
0.17
0.06

1.35
0.46
1.18
0.24
0.37
0.21
0.97
0.30

0.29
0.11
0.25
0.11
0.02
0.01
0.18
0.08

0.01
0.00
0.00
0.00
0.01
0.02
0.01
0.01

43.92
30.94
75.55
10.25
5.25
2.02
41.36
18.81

46.90
29.67
67.06
8.15
10.82
5.50
41.11
15.16

29.90
17.60
84.27
43.44
52.89
12.22
58.03
19.31

IMX-104

Solution recoveries

Soils, treatment

NTO

DNAN

2-ANAN

4-ANAN

RDX

HMX

NTO

DNAN

2-ANAN

4-ANAN

RDX

HMX

NTO

DNAN

RDX

HMX

%

%

%

%

%

mg

%

%

%

%

%

mg

%

%

%

mg

50.28
8.34
56.51
9.66
79.07
16.07
67.74
13.57

15.66
8.67
9.37
6.10
26.50
15.07
18.88
7.84

0.62
0.91
0.16
0.05
0.53
0.70
0.48
0.37

0.03
0.01
0.04
0.02
0.12
0.10
0.07
0.04

5.40
2.75
4.28
2.74
11.74
9.38
7.89
3.99

0.04
0.02
0.04
0.02
0.07
0.04
0.06
0.02

0.03
0.00
0.04
0.01
0.12
0.17
0.07
0.06

0.40
0.12
0.28
0.08
0.45
0.41
0.41
0.15

0.02
0.01
0.02
0.00
0.01
0.01
0.02
0.01

0.03
0.03
0.01
0.01
0.04
0.04
0.03
0.02

0.04
0.01
0.03
0.00
0.28
0.52
0.14
0.18

0.01
0.01
0.00
0.00
0.01
0.01
0.00
0.00

16.18
13.34
36.43
4.11
7.07
7.12
20.15
8.53

39.83
4.70
56.71
23.35
45.05
10.02
49.01
8.81

53.58
25.37
35.66
48.03
45.36
13.90
67.81
17.45

0.30
0.12
0.37
0.34
0.27
0.06
39.43
0.11

Detonated Mean (n = 4)
95% CI
Undetonated Mean (n = 4)
95% CI
Detonated FI Mean (n = 4)
95% CI
Mean across IMX-104 (n = 12)
95% CI

Soil recoveries

differences in solution recoveries of individual compounds in detonatedversus undetonated IMX-101 but more NQ, DNAN, and 2-ANAN was removed in solution during ﬂow interruption experiments for detonatedsamples due to their longer duration. For IMX-104, NTO had the highest
recoveries in efﬂuent followed by DNAN, RDX, 2-ANAN, 4-ANAN, andHMX (Tables 4, s4) and more NTO was recovered in solution for theinterrupted ﬂow experiments.
Recoveries in the soil were b 1% for all IM constituents. Low recoveriesof the explosive compounds from the soil (Tables 4, s4) are related to experimental design: majority of munitions constituents should bedesorbed from the soils and eluted by the time experiment was ﬁnishedand soils extracted with acetonitrile. DNAN had the highest recoveries insoil for both IMX-101 and 104 consistent with the fact that its elution
was not complete by the time experiment was ended due to its adsorption in the soil (Figs. 1, 2, 4, 5). It should be noted that while on the scaleof individual munitions these compositions are well mixed, the particles
are small and because of that can have somewhat varying composition
amongst different particles. Hence, recoveries of each component in individual particles may exceed 100% of the calculated amount.
4. Conclusion
This study examined the dissolution, fate and transport behavior of
IMX-101 and IMX-104, new IM formulations, in two soils. Insensitive
munitions dissolution resulted in distinct elution patterns with high initial concentrations for highly soluble NTO and NQ quickly followed bylower and relatively constant concentrations in column efﬂuent. Less soluble DNAN, RDX, and HMX had smaller or no initial peaks and efﬂuent
concentrations were lower and fairly steady over time. Flow interruption, as would be observed under natural rainfall conditions, resulted inincreases in concentrations of energetics due to their continued dissolution from the particles. HYDRUS-1D software for modeling water ﬂow
and reactive transport of chemicals, modiﬁed to also simulate energetic
compound dissolution from IM particles, successfully simulated dissolution and subsequent transport and transformation of these compounds,
as well as the production and transport of their transformation products.
Dissolution was represented by an initial (maximum) rate followed byexponential decrease to a constant (steady-state or minimum) rate. Estimated dissolution rates varied widely between the particles but werecorrelated between different components in the formulation.

IMX particle recoveries

NTO and NQ exhibited minimal retardation, while elution of DNAN,
RDX, and HMX was delayed by adsorption to the soil. Estimated adsorption coefﬁcients from HYDRUS-1D for each explosive compound weresimilar to those of pure NTO, NQ, DNAN, RDX, and HMX obtained frombatch experiments, indicating that adsorption coefﬁcients obtained
from batch studies of pure explosives in solution can be used to reasonably represent transport of mixtures. DNAN transformed to 2-ANAN and
4-ANAN and under ﬂow interruption conditions. NTO concentrations
also decreased due to transformation. This indicates the potential for natural attenuation of IM constituents in soils through adsorption andtransformation.
Acknowledgements
This work was funded by the Strategic Environmental Research andDevelopment Program, SERDP, project ER-2220. We thank Dr. Andrea
Leeson, SERDP Environmental Restoration Program Manager for her continued support. We are grateful to Anthony Di Stasio and Erika Rivera, US
Army Armament Research, Development and Engineering Center(ARDEC), Picatinny Arsenal for providing IMX-101 and IMX-104; Bonnie
M. Packer, Rosa Gwinn, Lisa DeGrazia, Bethany Keller, Jessica Milose,
Amibeth Sheridan, Laurie Stenberg, Sarah Gettier, Army National
Guard–Environmental Directorate and URS Corporation, Germantown,
MD for collecting soils used in the experiments on Army National Guard
installations.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.scitotenv.2017.11.307.
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