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Environmental factors influencing remediation of TNT-contaminated water and soil with nanoscale zero-valent iron particlesWaraporn Jiamjitrpanich a , Chongrak Polprasert b , Preeda Parkpian a , R. D. Delaune c &Aroon Jugsujinda ca Program on Environmental Engineering and Management, School of EnvironmentalResources and Development , Asian Institute of Technology , Klong Luang , Pathumthani ,Thailandb Biochemical Engineering and Technology, Sirindhorn International Institute of Technology ,Thammasat University , Pathumthani , Thailandc Wetland Biogeochemistry, Department of Oceanography and Ocean Science, School of theCoast and Environment , Louisiana State University , Baton Rouge , Louisiana , USAPublished online: 28 Jan 2010.

To cite this article: Waraporn Jiamjitrpanich , Chongrak Polprasert , Preeda Parkpian , R. D. Delaune & Aroon Jugsujinda(2010) Environmental factors influencing remediation of TNT-contaminated water and soil with nanoscale zero-valent ironparticles, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering,45:3, 263-274, DOI: 10.1080/10934520903468012

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Journal of Environmental Science and Health Part A (2010) 45, 263–274Copyright C© Taylor & Francis Group, LLCISSN: 1093-4529 (Print); 1532-4117 (Online)DOI: 10.1080/10934520903468012

Environmental factors influencing remediation ofTNT-contaminated water and soil with nanoscalezero-valent iron particles

WARAPORN JIAMJITRPANICH1, CHONGRAK POLPRASERT2, PREEDA PARKPIAN1, R.D. DELAUNE3

and AROON JUGSUJINDA3

1Program on Environmental Engineering and Management, School of Environmental Resources and Development, Asian Institute ofTechnology, Klong Luang, Pathumthani, Thailand2Biochemical Engineering and Technology, Sirindhorn International Institute of Technology, Thammasat University, Pathumthani,Thailand3Wetland Biogeochemistry, Department of Oceanography and Ocean Science, School of the Coast and Environment, Louisiana StateUniversity, Baton Rouge, Louisiana, USA

This study evaluated the application of nanoscale metallic particles (nanoscale zero-valent iron (nZVI) particles) in the remediationof TNT in contaminated water and soil samples. The effects of treatment dosages of synthesized nZVI particles and reaction timeon degradation rate of TNT were determined. The synthesized nZVI particles (99.99% pure) size distribution was between 20–100nm (average particle size 80 nm), with a surface area of 21.63 ± 0.24 m2/g. The optimum dosage of nZVI for degradation of 10mg/L TNT in the contaminated water was 2000 mg/L (w/v) at a reaction time 20 min. However, trace level of TNT remainedsince the BOD5 and COD levels at the optimum nZVI treatment dosage were 834 ± 8 mg/L and 1280 ± 900 mg/L, respectively. TheBOD5/COD ratio was 0.65, which was higher than the BOD5/COD ratios for the other nZVI dosages which supports the beneficialeffect of using nZVI particles for enhancing degradation of TNT. The observed first-order degradation rate of TNT at 25◦C was0.137 min−1 corresponding to a degradation rate of 0.156 L/m2·h. In experiments using sandy clay loam soil containing 20 mg/kgTNT in slurry form (1:2 soil to solution ratio, the optimum nZVI treatment dosage that resulted in 99.88% TNT removal was 5000mg/kg soil. Less toxic intermediate products and their concentrations following degradation were 2-ADNT and 4-ADNT at 0.90and 0.10 mg/kg, respectively. Results of this study indicate it is feasible to use nZVI for the remediation of TNT-contaminated waterand soil samples as a pre-treatment step however secondary treatments such as phyto-remediation or other biological processes maybe needed to remove any residue or intermediate products of TNT degradation.

Keywords: Trinitrotoluene (TNT), nanoscale zero-valent iron particles (nZVI), remediation, intermediate products, degradation rate.

Introduction

Trinitrotoluene (TNT) or C6H2(NO2)3CH3 is explosive,flammable, and toxic. The U.S.EPA has designated it asa hazardous waste, possible human carcinogen[1–4] andmutagenic.[5] TNT-contaminated water and soil can be acontaminant problem at military munition depots and ar-tillery test ranges.[6–7] Many military testing grounds arecontaminated with TNT including both surface water andgroundwater. Since its colour may become pink, the water

Address correspondence to Dr. Aroon Jugsujinda, Wetland Bio-geochemistry, Department of Oceanography and Ocean Science,School of the Coast and Environment, Louisiana State Univer-sity, Baton Rouge, LA 70803, USA. E-mail: ajugsv@lsu.eduReceived August 26, 2009.

containing TNT is sometimes called “pink water”. TNTtransformations include reduction of one, two, or all nitro(-NO2) groups to amino (-NH2) groups (Fig. 1).

TNT is reduced to 2-amino-4,6-dinitrotoluene (2-ADNT) or 4-amino-2,6-dinitrotoluene (4-ADNT), de-pending on the location of the amino group. ADNTs (2-ADNT and/or 4-ADNT) are less toxic than TNT becauseof the loss of one nitro group. The main TNT transforma-tions resulting from the reduction of the nitro groups toamino groups are given in Equations 1–4. Amino groupsare more stable than nitro and hydroxylamino groups. InEquation 4, six electrons reduce each nitro group to anamino group such as ADNTs, DANTs (2,4-DANT and/or2,6-DANT) and triaminotoluene (TAT), respectively.

The DANTs are less toxic than both parent compoundssince two nitro groups have been eliminated. DANTs

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Fig. 1. TNT transformation in environment (adapted fromWelch[8]).

production is the slowest of all transformations becauseDANTs are thermodynamically difficult to reduce whileTNT is easily reduced. ADNTs are slightly more resistantto reduction. DANT reduction to TAT will occur only ina strongly reducing environment. TAT is the least toxiccompounds since all nitro groups have been removed.[8]

−NO2 + 2e− + 2H

+ → −NO + H2O nitro → nitroso (1)−NO + 2e

− + 2H+ → −NHOH nitroso → hydroxylamino (2)

−NHOH + 2e−+2H

+ → −NH2 + H2O hydroxylamino → amino (3)−NO2 + 6e

− + 6H+ → −NH2 + H2O net reaction (4)

The transformation of TNT is influenced by a numberof environmental factors such as TNT concentration, oxy-gen concentration, moisture content, temperature, micro-bial activity, photolysis, organic matter (OM) content andmetallic oxides in soil. Brannon and Pennington[9] reportedthat TNT transformations in soils can occur both biolog-ically and abiotically. The rate of TNT transformation byFe+2 in the presence of montmorillonite or kaolinite in-creased as pH increased. The study showed that aluminosil-

icate surfaces also catalyze the nitro-to-amino reduction ofTNT in the presence of ferrous iron at a molar ratio of ap-proximately 10 (0.00134 mol/L Fe2+ and 0.00011 mol/LTNT). A study by Nefso et al.[5] showed that the directreduction of TNT by aqueous Fe2+ is a very slow reac-tion; however, in the presence of an iron hydroxide mineralsurface or surface coating, it was observed that 10 mono-substituted nitrobenzenes were readily reduced to their cor-responding anilines in a pH-dependent reaction.

In the case of TNT, soil organic matter and clay min-erals can act as sorbents which reduce TNT toxicity. Clayminerals usually have a permanent negative charge, act-ing as a cation exchanger with a given cation exchangecapacity (CEC). TNT is not adsorbed to sand, as indi-cated by the high hydraulic conductivity for TNT in sandas compared to clay. As a result of the electron-attractingnitro-groups the aromatic ring of TNT is electron defi-cient and hence can be sorbed to negative charged sites.Thus, TNT can be adsorbed easily to clay minerals byforming electron donor-acceptor complexes.[10] The sur-faces of the clay minerals serve as e−-donors and the ni-troaromatic compound as an e− acceptor. Thus, these sur-faces are easily accessible with sorption occurring withinminutes.[11]

For soil contaminated with explosives, incineration isthe most widely used remediation method. Incineration ofTNT is very expensive, with some air emission and noiseproblems. Bioremediation technologies are being soughtas less expensive and more energy efficient alternatives.However, bioremediation is often slow and may be in-complete because of the electron-withdrawing nitrogroupsthat impede electrophilic attack by oxidative enzymes ofaerobic bacteria. In order to cope with these bioremedia-tion limitations, chemical treatments are also used to pro-vide efficient and environmentally acceptable remediationalternatives.

Nanotechnology has been suggested for use in enhanc-ing contaminant transformation and stimulating micro-bial growth. Such nanoparticles include zero-valent iron(ZVI) particles. Iron typically exists in the environment asFe2+ and Fe3+ oxides, and ZVI is a manufactured material.The nZVI particles are effective in the treatment of manypollutants commonly found in groundwater. These com-pounds includes perchloroethene (PCE), trichloroethene(TCE), carbontetrachloride (CT), nitrate, energetic mu-nitions such as TNT, legacy organohalogen pesticidessuch as lindane and dichloro-diphenyl-trichloroethane(DDT), and heavy metals including chromium andlead.[12]

Current research on the remediation of TNT includes theassessment of zero-valent iron (Fe0) or ferrous iron (Fe2+)to transform TNT and other nitroaromatic and nitraminecompounds to less hazardous or more biodegradableproducts.[5] Metallic iron is inexpensive, non-toxic, and al-ready common in the environment and serves as a strongreductant, reducing the targeted chemicals to less toxic or

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Remediation of TNT-contaminated water and soil 265

non-toxic products. The reaction between TNT and nZVIparticles occurs under reducing conditions. In the presenceof water, oxygen of the nitro group is removed and re-placed with hydrogen, rendering the compound less toxicand more amenable to biological breakdown. The net re-action between TNT and nZVI is shown in Equation 5.

−NO2 + 2Fe0 + H2O → −NH2 + Fe2O3 (5)

where -NO2 represents the nitro group of TNT.Microscale and larger ZVI particles are known to re-

move TNT from water. Bandstra et al.[13] reported completebreakdown of 176 µM TNT by 17 g/L ZVI within 200 min-utes. The rapid removal suports efficiency of ZVI in TNTremoval. However, movement of ZVI particles through soilis restricted, making in situ applications limited; since theZVI particles generally cannot move with groundwater flowthrough soil pore space.[14] Pink water, effluent from muni-tions manufacturing plants, can be pretreated with ZVI todegrade TNT and RDX. According to Oh et al.[15] pretreat-ment has proven to enhance fenton oxidation and increaseremoval of TNT and RDX. Removal of these compoundswas 20–60% greater with ZVI pretreatment.

The overall objective of this study was to evaluate thefeasibility of applying nZVI particles to remediate TNT-contaminated water and soil. The specific objectives were:(i) to determine the optimum nZVI dosage and conditionsfor promoting TNT degradation in water samples, (ii) to de-termine biodegradability of the TNT-contaminated watersamples treated with nZVI particles including the degra-dation rates, and (iii) to determine the optimum dosageand conditions for TNT degradation in soil using nZVIparticles and quantify in production of any intermediateproducts.

Materials and methods

Materials

The major chemical stocks used in this study were 2,4,6-trinitrotoluene (TNT), 2-amino-4,6-dinitrotoluene (2-ADNT), 4-amino-2,6 dinitrotoluene (4-ADNT) (99.90%purity at 1000 mg/L concentration) purchased from SU-PELCO Co., USA. Water used in the degradation exper-iments was deionized using a Milli-Q deionized system.Uncontaminated soil samples were collected from a mili-tary school site in central Thailand. The physicochemicalcharacteristics of this soil determined at the Land Develop-ment Department, Ministry of Agriculture and Coopera-tives, Bangkok, Thailand, were found to be sandy clay loamwith pH of 5.3, OM of 1.44% and CEC of 32.88 meq/100 g.

Nanoscale zero-valent iron synthesis

The nZVI particles were synthesized by the reductive pre-cipitation process using two chemicals, namely, sodium

borohydride (NaBH4) and iron (III) chloride (FeCl3). Theprocedure for nZVI synthesis followed the method of Wangand Zhang[16], Choe et al.[17] and Sun et al.[18] in whichNaBH4 (0.25 M) aqueous solution was added dropwise toFeCl.36H2O (0.045 M) aqueous solution at 1:1 volume ratio.To ensure complete mixing the solution was homogenizedfor 20 min in a nitrogen gas purged reactor allowing thereaction to come to completion. The nZVI particles wereimmediately formed according to Equation 6 as follows:[19]

2FeCl3 + 6NaBH4 + 18H2O → 2Fe0(s) + 6B(OH)3

+ 21H2 + 6NaCl (6)

The synthesized nZVI particles were separated from su-pernatant using a magnetic bar. The supernatant was laterdecanted and discarded, while the synthesized nZVI par-ticles were rinsed with de-oxygenated milli-Q water threetimes, and methanol three times, before drying with nitro-gen gas and stored in methanol at 4◦C.

Nanoscale zero-valent iron characterization

The particle size, shape and distribution of the nZVI mate-rial were characterized using a transmission electron micro-scope (TEM; JEOL 2010 TEM). The nZVI material wereprepared by placing nZVI material solution on a TEM gridwhich was placed on a glass slide. The TEM grid had cop-per on one side and carbon on the other side. The nZVImaterial sample was added on the copper side. After 5 min-utes, the grid was placed on a filter paper and dried at roomtemperature before being characterized by TEM. Also, ascanning electron microscope (SEM; JEOL JSM-6301F)was used for taking magnified images of the nZVI parti-cles. The nZVI material samples required coating with avery thin layer of a conducting material before SEM mea-surement.

Specific surface area of nZVI particles was determinedby Brunauer-Emmett-Teller (BET) specific surface area an-alyzer using QuantaChome Autosorb-1 analyzer (Gas ad-sorption technique). The nZVI material samples were pre-pared by placing 0.1 g of samples into a sample cell forgas drying before XRD analysis. X-ray diffraction (XRD;RIGAKU TTRAX III theta-theta rotating anode X-Raydiffractometer) was used to determine the chemical com-position, purification and crystalline structure of the syn-thesized nZVI particles. The nZVI compositions were an-alyzed before and after TNT treatment.

Preparation of TNT-contaminated water and soil samples

TNT-contaminated water samples were prepared by spik-ing aliquots of the TNT stock solution into milli-Q water toobtain a TNT concentration of 10 mg/L. The collected soilsamples were air-dried at a room temperature of 25◦C for3–4 days, ground and sieved to obtain an average particlesize of 2 mm (Hundal et al.[20] and Arienzo[21] ). TNT stock

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solution was added to the prepared soil sub samples. Thesamples were allowed to stand for at least one hour in thedark at 20◦C, as stated in previous reports,[22−23] to obtainan initial TNT concentration of 20 mg/kg soil (Hundalet al.[20] and Arienzo[21]). Milli-Q water was then added tosoil to form slurry before being used in the experiment.

Experimental conditions for remediation ofTNT-contaminated water and soil with synthesized nZVIparticles

For determining the remediation of TNT-contaminatedwater, 50 mL of the TNT-contaminated water was addedto serum bottles (100 mL). The added nZVI dosages were0, 10, 50, 100, 150, 200, 1000, 2000, 3000, 4000, 5000, and10,000 mg/L. To determine the optimum nZVI dosages, pHand temperature were fixed at approximately 7 and 25◦Cto simulate natural conditions in the tropics. The reactiontime was maintained at 20 min. A temperature controlledshaker was used in these experiments. The optimum nZVIdosage was based on conditions that resulted in maximumTNT degradation or removal. The optimum nZVI dosagewas determined from the TNT degradation rate under var-ious reaction times ranging from 0–32 min (0, 2, 4, 8, 16,20, and 32 min). Under each experimental condition, theserum bottles were shaken at 150 rpm using an oscilla-tor and supernatant samples were collected for analyses ofTNT and intermediate product concentrations.

For the experiments determining remediation of TNT-contaminated soil, 10 g of the prepared soil sample wasadded to each serum bottle, 20 mL of milli-Q water andTNT stock solution was added to obtain the initial TNTconcentration of 20 mg/kg soil (Hundal et al.[20] andArienzo[21]). The added nZVI dosages were 0, 50, 250,500, 2500, 5000 and 10000 mg/kg. The optimum reac-tion time were determined from the experiments of TNT-contaminated water was used, while pH and temperaturewere maintained under ambient conditions and not ad-justed. The water and soil experiments were conducted intriplicates.

Adsorption capacity of TNT in soil particles

Adsorption capacity of TNT by soil particles was con-ducted using batch adsorption test modified from Metcalfand Eddy.[24] Soil particles were dried at room temperatureat approximately 25◦C for 3–4 days to obtain a moisturecontent of less than 1%. The soil was then ground andsieved to obtain an average particle size of 2 mm for us-ing in the adsorption test. Seven 50-mL serum bottles wereused in this test. Each 50-mL serum bottle was filled with 10g of the prepared soil and 20 mL of the TNT-contaminatedwater. The concentrations of the TNT-contaminated wa-ter used were 10, 20, 40, 60, 80, 100 and 200 mg/L. Allserum bottles were capped and shaken on a shaker at 150rpm for 24 h at room temperature (25◦C). The supernatants

were then filtered through 0.45 µm polytetrafluoroethylene(PTFE) filters and TNT concentrations determined.

Analytical method of TNT and its metabolites

For analyzing TNT and intermediate products, the super-natant and nZVI particles were separated by transferring1 mL of the mixed liquid to a 2 mL polypropylene micro-centrifuge tube containing 1 mL of 35% methanol. Aftercentrifugation at 13,000 rpm (≈12470 g-force) for 5 min,the supernatant was filtered through a 0.45 µm PTFE filterprior to analysis of TNT and intermediate product concen-trations using gas chromatography with an electron capturedetector (GC-ECD: Hewlett Packard 5890 series II, USA)based on U.S.EPA method 8095.[25]

TNT, 4-ADNT and 2-ADNT standards (purity of99.90%) were used to prepare stock solutions in pure ace-tonitrile. The calibration curves consisted of 5 standard so-lutions (ranging from 0–20 mg/L) provided linearity withcorrelation coefficients (R2) greater than 0.99.

Results and discussion

Characteristics of synthesized nZVI particles

Most of the synthesized nZVI particles were in the rangeof 20–100 nm (average particles size 80 nm) (Figs. 2a and3a) with round shape and forming chain-like aggregatessimilar to those reported by Sun et al.[18,26], Li et al.[27],Uzum et al.[28], and Kim et al.[29] The average surface areaof the synthesized nZVI particles (21.63 ± 0.24 m2/g) washigher than that reported for microscale ZVI (0.29 m2/g[30]

and 0.06 m2/g[31]). Zhang and Elliott[32] determined thespecific surface areas of iron nanoparticles to be in therange 10–50 m2/g, similar to the values reported by Sunet al.(14.5 m2/g)[26], Li et al. (30–35 m2/g)[27], Uzum et al.(14.2 m2/g)[28], Zhang (35 m2/g)[12] and Choe et al. (31.4m2/g).[31] The XRD results (Fig. 4) revealed the apparentpeaks of Fe0 at 44.6◦ of degree two-θ whose purity was99.99%. Morphology and particle size of synthesized nZVIparticles measured by SEM are shown in photograph beforeand after TNT reaction (Fig. 2).

Morphology and particle size of synthesized nZVI par-ticles detected by TEM are shown in photographs beforeand after the TNT reaction (Fig. 3). XRD analysis of syn-thesized nZVI particles are shown in photographs beforeand after the TNT reaction (Fig. 4).

Effects of dosages of synthesized nZVI particles onremediation of TNT-contaminated water

The effect of nZVI dosages on the degradation of TNT inthe TNT-contaminated water at a reaction time of 20 min,pH of 7.0 and temperature of 25◦C are shown in Figure 5.The TNT concentrations in the supernatant samples were

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Remediation of TNT-contaminated water and soil 267

Fig. 2. Morphology and particle size of synthesized nZVI particles were detected by a SEM: (a) SEM photograph of nZVI particlesbefore TNT reaction; (b) SEM photograph of nZVI particles after TNT reaction.

found to decrease with increasing nZVI dosages becomingnon-detectable at nZVI dosages of 2000 mg/L and be-yond. The TNT degradation efficiency at the nZVI dosageof 2000 mg/L was near 100%, suggesting an optimum ratioof 200/1 of nZVI/TNT (w/w). This was higher than the re-sults of Riefler and Bryson,[33] who reported a nZVI/TNTratio of 50/1 (5 g of nZVI were needed to transform 1 Lof 100 mg/L TNT) for TNT degradation under anaerobicconditions which resulted in the complete removal in lessthan 30 min of TNT from water. The rather low nZVI/TNTratio reported by Riefler and Bryson[33] was attributed toanaerobic conditions employed in the experiments whichminimized oxidation of nZVI particles prior to the reactionwith TNT. To simulate actual conditions in remediation ofTNT-contaminated water, anaerobic conditions was notmaintained in the serum bottles, hence some nZVI parti-cles could have been oxidized, causing the nZVI/TNT ratioto be higher than that reported by Riefler and Bryson.[33]

The supernatant from the samples (after reacting withnZVI particles) were measured for concentrations of inter-mediate products such as 2-ADNT and 4-ADNT known

to occur during TNT degradation[11,34] (Fig. 1). Figure6a showed the GC-ECD peaks of TNT, 2-ADNT, and4-ADNT standards. The measured GC-ECD peaks ofthese intermediate products were small, or undetectable,although there were peaks of other possible intermediateproducts (Fig. 6b) when compared with the standard peaks(Fig. 6a). According to Hundal et al.[20] an aqueous solu-tion of TNT (70 mg/L) could be treated with as little as1% Fe0 (w/v) within 8 h of contact time. These researchersdetected only small amounts of 2-ADNT and 4-ADNTduring the initial 30 min of Fe0 treatment which later dis-appeared, indicating rapid transformation of these inter-mediate products on the iron surface.

Supernatant samples from this experiment with pH andtemperature initiallly fixed at approximately 7 and 25◦Cwere analyzed for the BOD5 and COD concentrations todetermine the extent of organic matter removal. The datashown in Figure 7 confirmed the trend reported in Fig-ure 5 that the optimum nZVI dosage of 2000 mg/L re-sulted in the lowest BOD5 and COD concentrations of834 ± 8 and 1280 ± 900 mg/L, respectively (Note: 10 mg/L

Fig. 3. Morphology and particle size of synthesized nZVI particles were detected by a TEM: (a) TEM photograph of nZVI particlesbefore TNT reaction; (b) TEM photograph of nZVI particles after TNT reaction.

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Fig. 4. XRD analysis of synthesized nZVI particles: (a) XRD analysis of nZVI before TNT reaction; (b) XRD analysis of nZVI afterTNT reaction.

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Remediation of TNT-contaminated water and soil 269

0123456789

10

0 10 50 100 150 200 500 1000 2000 3000 4000 5000 10000

nZVI dosages (mg/L)

TN

T c

on

ce

ntr

ati

on

s (

mg

/L)

0102030405060708090100

TN

T r

em

ova

l e

ffic

ien

cy

(%

)

TNT concentration (mg/L) TNT removal efficientcy (%)

Fig. 5. Effects of nZVI dosages on TNT degradation in water.

of TNT in the TNT-contaminated water was equivalentto 2370 and 6400 mg/L of BOD5 and COD concentra-tions, respectively). This optimum nZVI dosage resulted ina BOD5/COD ratio of 0.65, which was higher than theBOD5/COD ratios of the TNT-contaminated water beforenZVI reaction (including ratios of supernatants for othernZVI dosages).

Since the higher the BOD5/COD ratio reflectsbiodegradability of TNT in the water sample, these resultsdemonstrated the beneficial effects of the nZVI particles inpromoting bacterial decomposition of TNT compounds.The residue BOD5 and COD level at the nZVI dosage of2000 mg/L and other dosages indicated that the spikedTNT was not completely degraded by the nZVI reaction,but was partly degraded to form intermediate productswhich could be further degraded by bacterial. At nZVIdosages higher than 2000 mg/L, there were increase inBOD5 and COD levels in the treated supernatant samplesprobably due to interfering effects of Fe0 on the BOD5 andCOD measurement.[35] Another reason could be becauseof the magnetic properties of the nZVI particles resultingin aggregate formation at high nZVI dosages (Uzum etal.[28]), resulting in less surface areas of nZVI for reactingwith TNT.

Analysis of the nZVI particles after reacting with TNTshowed larger floc aggregates with shapes similar to mon-oclinical needles[11] (Figs. 2b and 3b) resulting from oxideformation. The XRD analysis (Fig. 4b) also showed thatmost of the synthesized nZVI particles were converted toFe2O3 (hematite or lepidocrocite) and Fe3O4 (magnetite)as a result of TNT degradation similar to that reported bySun et al.[26] and Uzum et al.[28] The XRD peaks in Figure4b showed that positions of two-theta (deg) of the nZVIparticles after reacting with TNT to be Fe2O3 (hematiteor lepidocrocite) and Fe3O4 (magnetite) at 36.0◦ and 35.8◦

which indicated that the nZVI particles were converted toFe2O3 (hematite or lepidocrocite) and Fe3O4 (magnetite).

Effect of reaction times on TNT degradation by nZVI anddegradation rates

Using the optimum nZVI dosage of 2000 mg/L, the ef-fect of reaction time on TNT degradation and TNT re-moval efficiencies are shown in Figure 8. TNT concentra-tion decrease with increasing reaction times and becomingnon-detectable after 20 min, resulting in near 100% TNTremoved.

The data shown in Figure 8 were plotted as semi-log(Fig. 9) in order to determine the first-order reaction rateof TNT degradation according to Equation 7:

− ln(Ce / Co) = k · t (7)

where Ce and Co are TNT concentrations in supernatantsamples after and before reacting with nZVI (mg/L), k is afirst-order reaction (min−1), and t is reaction time (min).

Based on the linear plot shown in Figure 9, the k valueat 25◦C for TNT degradation by nZVI particles was 0.137min−1 (R2 = 0.9737) which was somewhat higher than thatreported by Riefler and Bryson,[33] who found the range ofk values at 22◦C to be 0.135–0.034 min−1. Nefso et al.[5]

reported that the reaction rates for TNT degradation bynZVI particles at 10◦C were less than the reaction ratesmeasured at 20◦C because higher temperatures supports afaster chemical reactions.

The k value determined from this study was normal-ized using the specific surface area of the nZVI parti-cles. Based on surface area of the nZVI particle (21.63m2/g) and the nZVI dosage (2000 mg/L) added to the50 mL TNT-contaminated water, the specific surface areaof nZVI in each serum bottle was calculated to be 52.6

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Fig. 6. GC-ECD graphs of TNT and its intermediate products: (a) standard peaks; (b) peaks in a treated water sample.

m2/L. Therefore, the specific k value was equal to (0.137min−1)/(52.6 m2/L) or 0.156 L/ m2·h. This specific k valuewas about 40 times higher than the specific k value reportedby Chompuchan et al.[36] who employed nZVI particles todecolorize reactive dyes. The high specific k value deter-mined from this study suggested that TNT was degradedby nZVI more rapidly than that reported for reactive dyes.

Therefore, countries situated in tropical areas withambient temperatures in the range of 20–30◦C couldemploy nZVI particles to pre-treat TNT-contaminatedwater. The pre-treated water may, however, may re-quire remediation by other biological processes to re-move any remaining organic compounds or intermediateproducts.

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Remediation of TNT-contaminated water and soil 271

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 50 100 150 200 500 1000 2000 3000 4000

nZVI dosages (mg/L)

CO

D a

nd

BO

D5

(mg

/L)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

BO

D5/

CO

D

COD BOD5 BOD5/COD

Fig. 7. Effects of nZVI dosages on TNT biodegradability.

Similar to temperature effects, Nefso et al.[5] have re-ported that high pH (between 9 or 10) increased TNTdegradation using nZVI. This is due to the formation ofiron hydroxide solid suspension, which acts as a surfacecatalyzing agent[26] for the reduction of TNT. Hence, itcould be stated that the k value or specific k value of TNTdegradation would be higher than those found in this study,if the water pH were higher than pH 7, which was used asthe initial condition in this experiment. However, accordingto Equation 5, pH of the TNT-contaminated water sam-ples treated with nZVI particles would increase due to theformation of the amino group (-NH2) and contributing toincreased TNT degradation.

0

1

2

3

4

5

6

7

8

9

10

0 2 4 8 16 20 32

Reaction times (min)

TN

T c

on

cen

trat

ion

(m

g/L

)

0

10

20

30

40

50

60

70

80

90

100

TN

T r

emo

val

effi

cien

cy (

%)

TNT concentration (mg/L) TNT removal efficiency (%)

Fig. 8. Effects of reaction time on TNT degradation by nZVI.

y = 0.137x

R2 = 0.974

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

Reaction time (min)

-ln

(Ce/C

o

Fig. 9. First-order reaction rate of TNT degradation by nZVI.

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Fig. 10. Effects of nZVI dosages on TNT degradation in soil.* Initial TNT concentration in the soil samples was 20 mg/kg.

Effect of dosages of synthesized nZVI particles onremediation of TNT-contaminated soil

The effect of nZVI dosages on TNT-degradation in soil (20-min reaction time) are shown in Figure 10. TNT removalof 99.90% was achieved at nZVI dosages of 2500 mg/kgand above, resulting in the residual TNT concentration inthe soil sample of about 0.02 mg/kg. The highest concen-tration of the intermediate products such as 2-ADNT and4-ADNT (1.07 and 0.27 mg/kg, respectively), were foundat an nZVI dosage of 5000 mg/kg. The occurrence of 2-ADNT and 4-ADNT was likely due to adsorption of theTNT compound on the soil particles which reflected the re-actions of Equations 1–4, showing the formation of theintermediate products (Fig. 1).

Because these intermediate products are reported to beless toxic[11] and to be more biodegradable than TNT aspreviously shown in Figure 7, there should be an overallbeneficial effect in applying nZVI particles for remedia-tion of TNT-contaminated soil. Nefso et al.[5] and Parket al.[37] also reported 2-ADNT and 4-ADNT as inter-mediate products formed during the remediation of TNT-contaminated soils. Since the occurrence of high concen-trations of intermediate products suggests significant TNTtransformation, the nZVI dosage of 5000 mg/kg was con-sidered to be most effective for use in TNT degradation(Fig. 10).

This nZVI dosage corresponded to the nZVI/TNT ratioof 250/1, which was higher than that found in the remedia-tion of TNT-contaminated water. It could be hypothesizedthat the OM contained in the soil sample was partly re-sponsible for TNT sorption reducing the amount of TNTdegradated by the nZVI particles.[11,38] In treating TNTsoil with Fe0, Hundal et al.[20] reported the occurrenceof intermediate products such as 2-ADNT and 4-ADNT

attributed nitro groups of TNT which are susceptible toreduction by Fe0 producing amino degradation products(Fig. 1). Also, the treatment of TNT with Fe0 would in-crease biodegradability and promote detoxification in soilthrough sorption and binding process.[20] Moreover, nZVIused in the treatment of dinitrotoluene (DNT) resulted inthe formation of diaminotoluene (DAT),[17] an intermedi-ate product in accordance with that shown in Figure 1. Re-sults of this study and those reported in the literature sup-ports the use of nZVI particles in the remediation of TNT-contaminated soils. There however would likely be produc-tion of some intermediate products such as 2-ADNT and4-ADNT. As shown in Figure 7. These intermediate prod-ucts are more biodegradable and less toxic than TNT.[34]

The treated soils would require further remediate usingbiological processes such as phyto-remediation or biore-mediation for removal of any toxic intermediate productin order to insure suitable for use in agricultural or otherpractices.

Table 1. Calculated results for the adsorption capacity of TNT insoil.

Soilweight,

g (Mm)

SpikedTNT,mg/L

TNT insupernatant,mg/L (C)

TNTadsorbed,mg (x) x/Mm 1/x/Mm 1/C

10.00 10.00 1.03 (±0.00) 0.18 0.02 55.74 0.970910.00 20.00 5.03 (±0.05) 0.30 0.03 33.40 0.198810.00 40.00 14.47 (±0.02) 0.51 0.05 19.58 0.069110.00 60.00 25.70 (±0.40) 0.69 0.07 14.58 0.038910.00 80.00 37.68 (±0.50) 0.85 0.08 11.81 0.026510.00 100.00 54.10 (±0.00) 0.92 0.09 10.89 0.018510.00 200.00 127.77 (±0.62) 1.44 0.14 06.92 0.0078

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Remediation of TNT-contaminated water and soil 273

y = 130.27x + 8.39

R2 = 0.97

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

50.00

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

1/C (1/ mg/L TNT)

1/x/

Mm

(1/

mg

TN

T/g

so

il)

Fig. 11. TNT adsorption capacity of soil.

TNT adsorption capacity of soil

The experiments on TNT adsorption capacity were con-ducted in triplicates and the results are tabulated in Table1. These results were then plotted in linear form in accor-dance with the Langmuir Equation 8.[38]

1 / x / Mm = (1 / C) · (ka + b) (8)

where Mm is soil weight (g), x is TNT adsorbed (mg), Cis TNT concentration in supernatant (mg/L), ka and b areconstants.

The linear Equation (Y = aX+b) from Figure 11 wasused to calculate the maximum TNT adsorption capacity(1/b) of the soil, which was shown to be 120 mg/kg soil.

With respect to the experiments on TNT degradation insoil by nZVI particles, 0.2 mg of TNT was spiked into 10 gsoil to obtain a TNT concentration of 20 mg/kg. Becausethe TNT adsorption capacities of the soil was 120 mg/kg,the 10 g of soil in the serum bottle was able to adsorb 1.2mg of TNT which was more than the spiked amount of 0.2mg. The adsorbed TNT would reduce the enhancement ofdegradation from application of nZVI particles resulting inthe formation of intermediate products such as 2-ADNTand 4-ADNT. The TNT adsorption data suggested thatthe sandy clay loam soil has a high capacity to adsorbTNT, which could reduce the benefit derived from the nZVIaddition. Therefore, TNT-contaminated soils pre-treatedwith nZVI may require further remediation using biologicalprocesses in order to remove any residue TNT and anyintermediate products formed.

Conclusions

Base on the experimental results obtained from this study,the following conclusions are made:

1. The optimum nZVI dosage to remediate 99.99% ofTNT-contaminated water was found to be 2000 mg/Lat a reaction time of 20 min, corresponding to annZVI/TNT ratio (w/w) of 200/1.

2. At the optimum nZVI dosage, the BOD5/COD ratioof the treated supernatant was 0.65, which was higherthan the BOD5/COD ratios of the other nZVI dosagesevaluated which supported the beneficial effect of nZVIparticles in increasing the TNT degradation.

3. The first-order degradation rate of TNT with nZVI at25◦C, and pH 7 was found to be 0.137 min−1. The specificdegradation rate was 0.156 L/m2·h.

4. The optimum nZVI dosage to remediate 99.88% ofTNT-contaminated soil was 5000 mg/kg at the reactiontime of 20 min, corresponding to the nZVI/TNT ratio(w/w) of 250/1. The higher requirement of nZVI in soilremediation as compared to water was likely due to thepresence of organic matter and TNT adsorption on thesoil particles which interfered with the TNT degrada-tion.

5. The remediation of TNT-contaminated water and soilby nZVI particles should be considered as a pre-treatment since the formation of intermediate productssuch as 2-ADNT and 4-ADNT may require further re-mediation by phyto-remediation or bioremediation.

6. These results support the use of nZVI particles to pre-treat TNT-contaminated soil and water.

Acknowledgments

The authors acknowledge the Office of the Higher Edu-cation Commission, Thailand for supporting a grant un-der the program Strategic Scholarships for Frontier Re-search Network for the Joint Ph.D. Program. Moreover, the

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authors would like to acknowledge the Royal Thai Govern-ment (RTG) fellowships and French scholarship 2007 forthe partial support given to the first author during her studyat AIT.

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