application of photo degradation for remediation of cyclic...

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REVIEW

Application of ph

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mania. His research interest inclhazardous organic compounds, ominescent detection for hazardouslight sources for analytical andchemometrics. Parvez is a professitralia since 2006.

Australian Centre for Research on Separat

University of Tasmania, Hobart 7001, Aust

au; [email protected]; Tel: +61 3 6226

Cite this: RSC Adv., 2016, 6, 77603

Received 14th May 2016Accepted 10th August 2016

DOI: 10.1039/c6ra12565d

www.rsc.org/advances

This journal is © The Royal Society of C

oto degradation for remediationof cyclic nitramine and nitroaromatic explosives

P. Mahbub* and P. N. Nesterenko

The advantages of photo degradation for remediation of potentially hazardous and widely used explosives

deposited in ground and surface water reserves has received continuous attention from researchers over

the past 20 years, particularly due to energy efficiency, rapidness and the environmental safety

associated with photo degradation processes. Hence, this review has introduced an up-to-date collation

of knowledge regarding the radiation sources for effective photo degradation of cyclic nitramine and

nitroaromatic explosives, irradiation time, photocatalysts as well as important physical–chemical

parameters such as pH and substrate concentrations. Most importantly, this review has highlighted the

fact that recent advances in continuous-flow photo chemistry, micro photo-reactor technology, energy

efficient light sources at low wavelengths as well as advances in synthesizing photosensitive

nanostructures have opened up new possibilities in regards to the application of advanced oxidation

technology using photon energy for rapid, safe and energy efficient remediation of hazardous organic

contaminants in the environment.

Introduction

Contamination of surface and ground water reserves resultingfrom the release of energetic materials (EM) contained in pinkwater (generated during the demilitarisation of munitions in

r Parvez Mahbub Parvezompleted his PhD in environ-ental engineering fromueensland University of Tech-ology (Brisbane, Australia) in011 and worked as a postoctoral fellow at the school ofhemistry in University of Tas-ania (Hobart, Australia) from012 to 2014. He currently holdsUniversity Associate position

t the School of Physicalciences in University of Tas-udes remediation of extremelyw injection analyses, chemilu-compounds, characterisation ofindustrial purposes as well asonal member of Engineers Aus-

ion Science, School of Physical Sciences,

ralia. E-mail: [email protected].

1070

hemistry 2016

the factories by sinking the munitions in hot water basins) aswell as in red water (generated during the production ofmunitions in ammunition plants) is a serious problem world-wide.1–3 In 2010, more than 12 million pounds of EMs werereleased into waterways by the US Army's Radford AmmunitionPlant.4 Additionally, EMs contained in unexploded ordnancesin warfare zones and live ring ranges were reported to inltrateinto the near-surface and ground water reserves via leachingthrough soil.5,6 Certini et al.7 attributed the modern era

Prof. Pavel N. Nesterenkoreceived PhD (1984) and DScdegrees (2000) in chemistry fromLomonosov Moscow StateUniversity (Moscow, Russia). In2006 moved to Australia, wherehe holds a strategic position ofNew Stars Professor in School ofPhysical Sciences of the Univer-sity of Tasmania, Hobart.Author of more than 300 scien-tic publications including 3monographs, 9 Chapters in

books, 290 regular papers and 12 patents. Research interest isassociated with development of new advanced adsorbents and owthrough technologies for various separation and detection tech-niques. Member of advisory and editorial boards for 6 interna-tional journals in the eld of analytical chemistry and separationsciences. Editor-in-Chief of journal Current Chromatography.

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RSC Advances Review

chemical contamination of soils by warfare activities and theresultant negative impact on ground water quality largely toEMs such as, cyclic nitramine and nitroaromatic explosives.Both cyclic nitramine and nitroaromatic explosives were clas-sied as high explosives (HE) in the United Nations listing dueto their mass explosion hazards through secondary detonation.8

Hence, the environmental remediation of cyclic nitramine andnitroaromatic explosives through degradation is an importanttask as it aims to abating the detrimental impacts of suchsubstances on human health and ecosystem. Depending on thechemical and physical characteristics of the EM in the targetexplosives to be degraded, fours types of degradation tech-niques namely, acid degradation, photo degradation, thermaldegradation and bio degradation were employed by the reme-diation practitioners and chemists.9,10 Fig. 1 illustrates thechemical structures of the cyclic nitramine and nitroaromaticexplosives currently being used worldwide.

The fate and transport of cyclic nitramine and nitroaromaticexplosives were extensively studied by researchers with a view tounderstand the far reaching impact of such EMs on humanhealth and environment through potential transport pathways.

Fig. 1 Structures of cyclic nitramine and nitroaromatic explosives: octahy3,5,7-trinitro-1,3,5,7-tetrazocine (1NO-HMX), hexahydro-1-nitroso-3,5triazine (DNX), hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX), he2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), 5-nitro-2,4-dihydro-(1,2,4)talso known as picric acid (TNP), 2,4,6-trinitrophenylmethylnitramine alstoluene (2,4,6-TNT), 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (picronitrate (AP), 2,4-dinitrobenzoic acid (2,4-DNBA), 1,3,5-trinitrobenzeDNB), nitrobenzene (NB), 4-chloronitrobenzene (CNB), ortho-nitroaniso2,6-diamino-3,5-dinitropyrazine-1-oxide also known as LLM-105 (LLMtrostilbene (HNS), hexanitrobenzene (HNB).

77604 | RSC Adv., 2016, 6, 77603–77621

Sorption and transport studies of various nitramine and nitro-aromatic explosives established the fact that nitramine explo-sives such as RDX and CL-20 as well as photo degraded productsof nitroaromatic explosives such as 2-amino-4,6-dinitrobenzoicacid can reach groundwater through sorption of these HEmolecules/degradation products to the colloidal fractions ofsoil.11–13 To the contrary, undegraded nitroaromatic explosivessuch as 2,4,6-TNT was reported to be increasingly partitioned tosoil-immobile phase which inhibited its transport to ground-water.14 The relatively high aqueous solubility of TNT (130 mgL�1 at 20 �C;15) compared to that of RDX (38.9 mg L�1 at 20 �C;16)and CL-20 (3.11 mg L�1 at 20 �C;17) may contribute towardsfaster dissolution and transport of TNT into the surface waterreserves via surface runoff rather than groundwater reserves viasorption and leaching. Brannon and Myers18 described thedissolution of cyclic nitramine and nitroaromatic explosivesinto the aqueous phase as one of the primary processes deter-mining their fate, once released into the environment. BothKalderis et al.9 and Pichtel10 reported photolysis of cyclic nitr-amine and nitroaromatic explosives in aqueous phase as themajor transformation process in waste streams and surface

dro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), octahydro-1-nitroso--dinitro-1,3,5-triazine (MNX), hexahydro-1,3-dinitroso-5-nitro-1,3,5-xahydro-1,3,5-trinitro-1,3,5-triazine (RDX), 2,4,6,8,10,12-hexanitro-riazol-3-one also known as nitrotriazolone (NTO), 2,4,6-trinitrophenolo known as N-methyl-N,2,4,6-tetranitroaniline (tetryl), 2,4,6-trinitro-2,6-DNT), mononitrotoluenes or MNT (2-MNT, 4-MNT), ammoniumne (1,3,5-TNB), 1,3-dinitrobenzene (1,3-DNB), 1,4-dinitrobenzene (1,4-le (2-NAN), 2,4-dinitroanisole (DNAN), triaminotrinitrobenzene (TATB),-105), trinitroresorcinol also known as styphnic acid (TNR), hexani-

This journal is © The Royal Society of Chemistry 2016

Fig. 2 The number of journal papers published in every 5 year on thephotolysis of cyclic nitramine and nitroaromatic explosives since 1993to present are shown in black circles while the dotted line illustratingthe upward trend; a total of 59 papers on the topic was published up toJuly 2016.

Review RSC Advances

water bodies in terms of degree of mineralisation and rapidnesscompared to other transformation processes such as hydrolysis,thermolysis and bio degradation. Additionally, Bordeleauet al.19 ascribed photolysis as a major attenuation mechanismwhile investigating photo degradation of RDX in inltratedwater and surface water bodies aer the release of this HE froma military training range nearby.

A number of cyclic nitramine and nitroaromatic HEs such asRDX and 1,3,5-TNB (a degradation product of TNT) were identi-ed as potential water contaminants by the USEPA.20 The detri-mental effects of cyclic nitramine and nitroaromatic explosivesand their residues on human health and ecosystem were welldocumented in literatures.16,21–24 Detailed accounts of bitter taste,burning eyes and discolouration of skin and hair of workers inTNT and DNT disposal facilities were presented by Letzel et al.25

In 2002, a total of 6.2 metric tons of nitroaromatic explosivessuch as, NB and DNT were released in soil in the United Stateswith 70 sites contaminated with nitroarene and their chemicalprecursors.21 In 2006, 3130 metric tons of cyclic nitramineexplosives such as RDX was produced in the USA.26 The 2015Australian explosive manufacturing market research reportstated 5.2% annual growth of explosive production from 2011 to2016.27 In the 2013–2014 nancial year, Australia had a 3.5billion dollar explosive industry which mainly produced cyclicnitramines, nitroaromatics and ammonium nitrate basedcommercial explosives.28 A site specic policy by the Departmentof Defence (DOD) in Australia stipulated the use of the chemicaloxidation/reduction technologies for the remediation of cyclicnitramine and nitroaromatic explosives in groundwater mainlydue to the shorter clean-up time compared to bio degradationtechnologies.29 Additionally, Sheremata et al.30 reported that onlytraces of RDX were mineralised to CO2 and N2O by the indige-nous microorganisms in nonsterile topsoil with production oftoxic nitroso metabolites such as MNX, DNX and TNX inaqueous phase and 55–99% recovery of RDX from sterile topsoilaer 5 weeks. Although the chemotaxis-mediated biodegrada-tion of 20 mM cyclic nitramine explosives such as RDX, HMX andCL-20 resulted relatively faster mineralisation within 50 hours31

and photo assisted biological transformation of 30 mM RDXrequired 40 hours,32 the inuence of initial concentrations ofHEs and pH in aqueous solutions on such degradation wereunknown. The nitroaromatic explosives contain at least onenitro (–NO2) group attached to the aromatic ring whichcontributes to formation of stable complexes as well as recalci-trance to oxidative degradation in the environment.21 Addition-ally, cyclic nitramine explosives contain at least one nitro (–NO2)group along with amine (–NH2) or nitroso (–NO) group, with atleast three nitrogen atoms forming the cyclic structure.33 Thedenitration of RDX in aqueous solutions by photolysis was re-ported to bemore rapid than the enzymatic denitration of RDX.34

Hence, during the past 20 years, researchers utilised the highabsorption of photon energy by the nitro (–NO2) groups presentin the cyclic nitramine and nitroaromatic explosives at variouswavelengths with a view to initiate photo degradation andcomplete mineralisation of these HEs rapidly. The two earliestworks on photo degradation of nitroaromatic explosives inaqueous solutions were undertaken by Mabey et al.35 and


Simmons and Zepp.36 Since then a steady growth of interest inphotolysis of both nitroaromatic and cyclic nitramine explosiveswas observed as illustrated in Fig. 2.

Despite the fact that photo degradation is a very cost effec-tive, safe and rapid remediation technique for cyclic nitramineand nitroaromatic explosives, only one review paper regardingremediation of trinitrotoluene in water partially addressedphoto degradation along with other processes.15 Hence, there isa need to collate the detailed knowledge regarding the absorp-tion wavelength, characteristics of the light sources, photodegradation products, pH of the aqueous solutions, inuence ofphoto catalysts and photo sensitizers as well as initial concen-trations of these explosives for better implementation of photodegradation based remediation technology of these HEs. Theaim of this review is, therefore, to collate the knowledge fromthe photo degradation studies of cyclic nitramine and nitro-aromatic explosives from the past 20 years with a view to informremediation practitioners with up-to-date scientic knowledgeregarding the applicability of photo degradation of these HEs asa rapid, safe and energy efficient technology.

Methods of photo degradation

The photo degradation process starts with the initial selectionof the phases of the reactants and the catalysts in the solution.Depending on the reactants and catalysts in the same ordifferent phases, the photo degradation methods can be eitherhomogeneous or heterogeneous, respectively. Interestingly,both the homogeneous and heterogeneous photo degradationcan only incorporate two different modes of operating condi-tions inside the physical reactors where the degradation takesplace. Depending on costs of reagents, degradation timerequired to mineralising the target hazardous substances aswell as fate and toxicity of the degradation products, the photodegradation process may employ reactors which incorporateeither batch mode or continuous-ow mode. The photo

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Fig. 3 Photo degradation methods are classified in terms of solutionphase, operating mode and reaction type.

RSC Advances Review

degradation of the target analytes in homogeneous phase maytake place without any additives such as catalysts, promoters,photosensitizers or reagents. Hence, we termed the photodegradation reaction in the absence of any additives in thehomogeneous phase as direct photo degradation. However, inmany instances, the photo degradation reactions employdifferent additives to enhance the reaction rates in bothhomogeneous and heterogeneous phases. Fig. 3 illustrated thedifferent methods of photo degradation.

Photo degradation of explosives

Photo degradation of explosives in soil and water is an effectivetechnique that employed the energy of photons from various lightsources to cause dissociation of bonds within the explosivemolecule to create simpler molecules. The high absorption ofphoton energy by the nitro (–NO2) groups present in cyclic nitr-amine and nitroaromatic explosivemolecules fromultraviolet andvisible light sources is the main driving force behind the photodegradation of these HEs. The cyclic nitramine and nitroaromatic

Table 1 Photo degradable explosives with corresponding groups areabsorptivity coefficients, 3max. Places were left blank where data were un

Explosives Solvent

Cyclic nitramines HMX EthanolRDX EthanolCL-20 Methanol

Nitroaromatics TNP Dimethylsulfoxide (DMSTetryl Ethanol2,4,6-TNT Water2,4-DNT 5% ethanol in water2,6-DNT Water1,3,5-TNB Water1,3-DNB Water1,4-DNB Water

NB Water2-NAN —DNAN WaterTNR DMSO

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explosives include 2,4,6-TNT, 1,3,5-TNB, 2,4-DNT, 2,6-DNT, 1,3-DNB, 1,4-DNB, tetryl, TNP, RDX, HMX, HNS, HNB, NAN, DNAN,NTO, TATB and TNR.21,37 Since 2011, the TNT based explosives inartillery shells of US Army were gradually being replaced by IMX-101 which is a mixture of DNAN, NTO and nitroguanidine (NQ).38

Recently, Mahbub et al.39 reported another type of explosiveknown as organic peroxide explosives (OPEs) that manifestedefficient photo degradation phenomenon despite having very lowmolar absorptivity coefficients. As opposed to cyclic nitraminesand nitroaromatic explosives, the OPEs are extremely unstableand very sensitive to temperature, shock and friction. Hence,OPEs are not suitable for mass production asmilitary/commercialexplosives. Additionally, the hazardous effects of OPEs on humanhealth and ecosystem are not yet established. Hence, this reviewwill not discuss the photo degradation of OPEs.

The photo degradability of these HEs will depend on thephoton absorption characteristics of the explosive molecules ata particular wavelength. Dubroca et al.40 investigated theabsorption spectra of TNT, RDX, HMX and tetryl in UV andvisible region and reported that the intermolecular hydrogenbonding of these molecules was the key parameter thatcontrolled the photon absorption. In an earlier kinetic study offulvic acid initiated photolysis of 19 nitroaromatic compoundsin aqueous solution with explosive potential, Simmons andZepp36 demonstrated that the largest enhancement in the rateof photolysis, ranging from 2 to 26 fold, occurred for nitro-aromatics that were methylated ortho to the nitro group.

Table 1 compiled the molar absorptivity coefficients atmaximum absorption wavelengths of various cyclic nitramineand nitroaromatic explosives.

Photo degradation methods applied to the cyclic nitramineand nitroaromatic explosives

There were many examples when researchers simply utiliseddifferent light sources without employing any additives in termsof catalysts, promoters, photosensitizers or reagents in the

listed with their maximum absorption wavelengths, lmax and molaravailable

lmax, nm 3max, M�1 cm�1 � 103 Ref.

228–229 21 41213 11 41228 23 42

O) 378 19 41225 25 41233 16 43240 29 41245 — 44220 25 45231 17 45262 13 45200 18 45260 7.5 45267 5.4 45300 11 46352 10 41


Review RSC Advances

photochemical reaction. Therefore, the subsequent discussionsof photo degradation of cyclic nitramine and nitroaromaticexplosives will cover photo degradation methods applied fordegrading these HEs with and without additives. The discus-sions also focussed on key information needed for remediationpractice such as energy requirement by the light source, yieldand products of degradation as well as irradiation time.

Direct photo degradation

Just and Schnoor47 employed direct photo degradation of RDXusing simulated sunlight (170–2200 nm) generated from a 1000Wmetal halide lamp. Direct photo degradation of nitrobenzenewas reported by Li et al.48 at a wavelength of 172 nm. However,they found that UV photocatalytic degradation of nitrobenzenein presence of H2O2 was 3 times more efficient than UV only interms of irradiation time. A comparison between direct photodegradations of CL-20 and RDX was carried out by Hawariet al.34,49 The initial photo degradation of the monocyclic nitr-amine (RDX) lead to ring cleavage and decomposition yieldingHCHO, HCOOH, NH2CHO, N2O, NO2

� and NO3+ aer 16 hour

at 350 nm and at pH 5.5.34 The direct photolysis of the rigidmolecule CL-20 at 254–350 nm produced NO2

�, NO3�, NH3,

HCOOH, N2 and N2O.49 The natural photo degradation of RDXand nitroglycerin in aqueous and solid samples collected frommilitary training camps were undertaken by Bordeleau et al.19

They reported UV degradation of both compounds when dis-solved in water, with half-lives between 1 and 120 days. Muchslower rates of photo degradation (half lives 2–4 months) werereported for RDX and nitroglycerin bearing solid particles byBordeleau et al.19 Recently, Capka et al.50 employed 125 W high-pressure mercury lamp to convert –NO2 and –ONO groups,typical for cyclic nitramine and nitroaromatic explosives such asHMX, RDX, tetryl, TNT, 2,4-DNT, 2,6-DNT, TNB, 1,3-DNB andNB, to peroxynitrite by absorption of UV light and peroxynitritewas detected by the chemiluminescence reaction with thealkaline solution of luminol. The chemical and physical fates oftrace amount of TNT and RDX in environment were reported byKunz et al.51 through direct UV photo degradation using 150 W

Table 2 Pseudo-first-order constants for Fenton/photo-Fenton oxidatio

ExplosivesReagent [Fe(II)](mM)

Reaction rate constantin Fenton, kF (min�1)

TNP 0.144 0.09350.288 0.1691

AP 0.144 0.09470.288 0.2000

DNT 0.090 0.00490.180 0.0108

TNT 0.360 0.01351.440 0.0588

Tetryl 0.360 0.00830.720 0.0264

RDX 0.720 0.02421.440 0.0407

HMX 0.720 0.00151.440 0.0017


halogen source and formation of 2-aminodinirotoluene and 4-aminodinirotoluene were reported along with net loss of massof TNT through sublimation within 30 min irradiation. Inter-estingly, no degradation product of RDX was reported by Kunzet al.,51 which suggested that the initial concentration of RDXmay play a crucial role on the yield of direct photo degradationof RDX. The direct photo degradation as well as humic acid andsalt (NaCl) sensitised homogeneous photo degradation of 2,4-DNT in aqueous solution resulted in 50% degradation in 4hour, 2 hour and 1 hour, respectively.52

Homogeneous photo degradation

The homogeneous photo degradation of nitroaromaticsemployed UV oxidation with ozone,53–55 or with hydrogenperoxide,53,56 or with the Fenton's reagent (H2O2 + Fe2+).57,58

Rodgers and Bunce22 termed these UV oxidation processes ashomogeneous photolysis where UVC (290–200 nm) radiationproduced OHc radicals by breaking down H2O2 or O3. There arecurrently no research studies on homogeneous UV oxidationwith peroxone (H2O2 + O3) systems in regards to the degradationof nitroaromatics or cyclic nitramines.

Beltran et al.54 and Mcphee et al.59 suggested that UV oxida-tion with O3 was superior to the UV oxidation with H2O2,possibly due to higher molar extinction coefficient of O3 thanH2O2. However, Mcphee et al.59 criticised that despite removing2,4,6-TNT from solution, 1,3,5-TNB was one of degradationproducts of TNT resulting from UV oxidation with O3. Hence,the toxicity of the solution was retained even aer the UVoxidation with O3.

Liou et al.60 reported oxidative destruction of TNP, AP, 2,4-DNT, tetryl and 2,4,6-TNT, RDX and HMX using Fenton andphoto-Fenton processes. For all explosives, Liou et al.60 foundthat the oxidation rates signicantly increased with increasingthe concentration of Fe2+ in the Fenton system, as well asthrough illumination with UV light in the photo-Fenton system.The added UV efficiency of photo-Fenton compared to Fentonsystem was reported within a range of 1–3 by Liou et al.60 and isreproduced in Table 2.

n of explosives (adapted from Liou et al.60)

Reaction rate constantin photo-Fenton (40 W UV), kP (min�1)

Added UV efficiency,kP/kF

0.1087 1.160.1741 1.030.1260 1.330.2190 1.100.0089 1.800.0158 1.460.0359 2.570.0918 1.260.0140 1.690.0437 1.660.0274 1.130.0411 1.010.0031 2.100.0032 1.90

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Interestingly, Li et al.57 reported the increased oxidation rateof mononitrotoluenes (MNT) at 254 nm UV light using Fenton'sreagent (H2O2 + Fe

2+) at pH 6 whilst adjusted aqueous solutionsof pH 3.0 in UV-Fenton systems were required for the degra-dations of higher nitro-substituted toluenes. The low molarabsorptivity coefficient of H2O2 at 254 nm was unsuitable foremploying UV with H2O2 for the degradation of nitrotoluenes,according to the Li et al.57 study. Under simulated solar light,Carlos et al.61 demonstrated an autocatalytic behaviour duringthe reaction between nitrobenzene and Fe3+ and concludedthat simulated solar light with cut-off wavelength from 300 nmhad negligible impact on Fe3+/H2O2 initiated degradation ofNB. Therefore, conventional UV oxidation with H2O2 at 254 nmas well as solar light (l > 300 nm) induced photo degradationmay not be very effective for the photo degradation of eithercyclic nitramine or nitroaromatic explosives. In this regard, Liet al.48 illustrated nearly 90% degradation of NB in combina-tion of UV and H2O2 (7 : 1 molar ratio of H2O2 : NB) with172 nm excimer (Xe*2 7.21 ev) UV light where, 4.07 mM NBsolution drastically decreased to 0.41 mMNB aer treatment ofonly 20 min. Although the excimer and exciplex UV sourceshave illustrated characteristic features such as, incoherence,near monochromaticity, more than 22 wavelengths, high UVand vacuum UV intensities, large absorption cross section, lowtemperature and mercury-free operation as well as long life,62

these innovative UV sources have been under-utilised in thedegradation of nitroaromatics and cyclic nitramine explosives.Other methods such as ushing with cyclodextrin solution inphoto-Fenton treatment of TNT for its removal from contami-nated soil was employed by Yardin and Chiron63 and they re-ported 1.3 times increase in the TNT degradation rate whichwas ascribed to the formation of a ternary complex (TNT–cyclodextrin–iron) that directed hydroxyl radical reactiontoward the oxidation of TNT.

Heterogeneous photo degradation

Rodgers and Bunce22 termed heterogeneous photolysis as theprocesses where OHc radicals were generated at the surface ofa semiconductor (such as TiO2) in presence of UVA light (400–320 nm). Due to competition between electron–hole recom-bination and water oxidation in aqueous solution, the semi-conductor assisted photolysis was deemed to be an inefficienttechnique for the degradation of organic pollutants.22 AnataseTiO2 suspension with UV radiation was employed by Lee et al.64

to photodegrade aqueous form of TNT, RDX and HMX withoutsufficient information regarding the support mechanism forimmobilisation of TiO2 in suspension. Recently, Alam et al.65

demonstrated graphene oxide (GO) supported Ag–TiO2 nano-lms for effective photo degradation of 4-nitrobenzenethiol(4-NBT) upon UV irradiation. Different support mechanismsfor TiO2 catalyst such as, TiO2 lm on borosilicate glass,66

nano-TiO2 on activated carbon bre or ACF,67 surface modi-cation of nanosize TiO2 by chemical adsorption68 wereemployed by researchers to develop efficient UV-TiO2 photo-catalytic processes for degradation of nitroaromatic explo-sives. Schmelling and Gray69 reported that coloured sensitizers

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did not play any detectable role in photo degradation of 2,4,6-TNT in a TiO2 slurry reactor in near UV radiation (l > 340 nm)with 90% degradation of TNT achieved in 120 min. Theanticipation of considerable economic savings in large scaleoperations prompted researchers to utilise sunlight or nearvisible UV light (400–320 nm) in semiconductor assistedheterogeneous photo degradation of nitroaromatic contami-nants in different waste matrices.15 Rajamanickam andShanthi70 utilised ZnO supported heterogeneous photodegradation of 4-nitrophenol (4-NP) under solar light irradia-tion. However, they reported that the degradation was stronglyenhanced in the presence of electron acceptors such as H2O2,K2S2O8 and KBrO3. This was attributed to the fact that theconduction band electron hole recombination in ZnO wasreduced effectively with addition of electron acceptors such asH2O2, K2S2O8 and KBrO3. Fe-doped TiO2 dispersions alongwith H2O2 was employed by Zhao et al.,71 who reported 67.53%removal of total organic carbon of a solution containing 20 mgL�1 4-NP at pH 6.17. In this regard, new narrow-band-gapsemiconductor photocatalyst with high negative value of theconduction band was investigated by Qusti et al.72 Silverreduced graphene oxide supported nanocomposites (Ag-RGOwith conduction band gap 2.4 eV) were utilised by Qustiet al.72 to reduce nitrobenzene to aniline under visible light.Approximately 100% decomposition of aqueous 0.8 mMnitrobenzene to aniline under 4 hour irradiation under visiblelight source from xenon (Xe) lamp with maximum emission at470 nm was reported in this study.

The weak reactivity of RDX and HMX and their poor degra-dation yield in TiO2-UV photo degradation scheme was reportedby Perchet et al.73 While they attributed the weak reactivity ofRDX and HMX to their incapacity to approach the TiO2 catalystsurface, the effect of experimental pH condition of such reac-tivity was not discussed by Perchet et al.73 To the contrary,wavelengths >320 nm from a xenon source at pH ranging 3 to 11were employed by Dillert et al.74 for homogeneous and hetero-geneous photo degradation of TNT and TNB. At pH 7, Dillertet al.74 found that TiO2-UV photo degradation reaction rate ofTNT was similar to the photo degradation reaction rate of TNTin H2O2-UV. However, photo degradation reaction rate ofdegradation product of TNT (i.e., TNB) was reduced by 16 fold inH2O2-UV homogeneous system at pH 7. There was a generalincrease in the photo degradation reaction rate of TNT in TiO2-UV suspension than in H2O2-UV solution for an increase of pHfrom 3 to 11. However, this effect was not observed for the TNBsolutions. Although it was ascribed by Dillert et al.74 that thebathochromic and hypsochromic shis of UV-vis absorption ofTNT with increased pH were more pronounced in homoge-neous solutions than TiO2-UV, the slower rate of TNB photodegradation in homogeneous solutions prompted them torecommend in favour of TiO2-UV system for TNT and TNBphoto degradation. More recently, Khue et al.75 reported 100%degradation of 2,4,6-trinitroresorcine (TNR) in 30 minutes inhomogeneous H2O2-UV system. Nevertheless, they found thatthe combined application of nano-TiO2 (anatase)/UV/H2O2 wasthe most efficient system for the 98% removal of the degrada-tion product of TNR in 60 minutes.


Review RSC Advances

Parameters affecting photo degradation of cyclic nitramineand nitroaromatic explosives

The radiation source and wavelength, radiation time and cata-lysts and photosensitizers used in photo degradation, initialconcentrations of explosives as well as pH of the solution canaffect the yields of photo degradation. Tables 3–5 and thesubsequent discussions summarised the parameters affectingall three types of photo degradation discussed in this review.

Radiation source and wavelength

To apply the photo degradation techniques effectively,researchers utilised the ultraviolet absorption wavelength of theenergetic molecules, where applicable. For example, the molarabsorptivity coefficient of HMX was reported in the order of21 000 M�1 cm�1 at a maximum absorption wavelength of 225nm in Table 1. Hence, Wang et al.114 employed 229 nm laser UVirradiation for direct photo degradation of HMX in solidsamples without reporting details of photo degradation yieldsand photo degradation products. Additionally, Tanjaroonet al.86 reported photo dissociation rate of NB as 1.7 � 107 s�1 at266 nm using picosecond tunable laser where NB was excited tohigher absorption band at 250 and 266 nm than at 280 nm fromits ground state. This was in conformity with the maximumabsorption wavelength of NB at 260 nm as illustrated in Table 1.

For both direct and homogeneous photo degradation ofcyclic nitramine and nitroaromatic explosives, the radiationsources primarily employed were mercury vapour lampsranging from low to medium pressure. For heterogeneousphoto degradation of these explosives, high pressure xenonlamps and simulated sunlights were employed. This generallyimplied to the higher energy requirements in the heterogeneousprocess for the photo degradation of these explosives. To thecontrary, the usual 254 nm wavelength of the mercury vapourlamps was not sufficient for complete mineralisation of theseHEs in the direct photo degradation schemes. Near 100% photodegradation was reported in the homogeneous photo degrada-tion schemes that employed the 254 nm wavelength. Theheterogeneous photo degradation systems reported 67–100%photo degradation of HEs using wavelengths ranged from 300to 800 nm.

Radiation time

The irradiation times varied 0.37 to 15 minutes for direct photodegradation of cyclic nitramine and nitroaromatic explosiveswhere low pressure mercury sources were used. Hence, thedirect photo degradation mechanisms are suitable for imple-menting ultra-fast continuous-ow micro-photoreactor tech-nology. To the contrary, the homogeneous photo degradationmechanisms required 10 to 240 minutes irradiation while theheterogeneous photo degradation mechanisms required 90 to1200 minutes irradiation times. Although the irradiation timewill depend on initial concentrations of explosives, the usuallylonger irradiation times in homogeneous and heterogeneousphoto degradation systems will require special design consid-erations for implementing fast and continuous-ow micro-


photoreactor technology for the degradation of nitroaromaticsand cyclic nitramine explosives.

Catalysts and photosensitizers used

All of the homogeneous photo degradation mechanisms ofnitroaromatics and cyclic nitramine explosives employed eitherFenton's reagent (H2O2 + Fe2+) or ozone as catalysts while theheterogeneous photo degradation mechanisms of nitro-aromatics and cyclic nitramine explosives employed metaloxides such as graphene oxide, ZnO, TiO2 as well as sulphidessuch as CdS as catalysts. Yang et al.84 and Cui et al.85 employedriboavin as a photosensitizer for photo degradation of 2,4,6-TNT under natural sunlight. The combined effect of photo-sensitizer such as humic acid and photocatalyst such as TiO2 onthe photo degradation of 2,4,6-TNT in near UV radiation (l > 340nm) was reported by Schmelling et al.110 The strong absorbanceof humic acid in the visible region was attributed to theincreased rate of sensitised degradation (both photolytic andphotocatalytic) of TNT. Nevertheless, the scavenging of reactivespecies such as OHc by the photosensitizers may lower the rateof direct photocatalytic transformation of nitroaromatic andcyclic nitramine explosives. Recently, Vione et al.115 reportedthat 10–55% degradation of NB in deep water bodies (10 mdepth with dissolved organic carbon concentration 10 mg L�1)occurred due to homogeneous photo degradation in presence ofchromophoric dissolved organic matters (CDOMs) where OHc

radicals were scavenged by the dissolved organic matters (eitherchromophoric or not). Hence, the choice of catalysts or photo-sensitizers in photo degradation studies of cyclic nitramine andnitroaromatic explosives depends on the reactivity of the radi-cals towards these explosives during photo reaction.

Initial concentrations of explosives

Lee et al.64 reported increased rates of heterogeneous photodegradation of HMX in anatase TiO2 suspension at decreasinginitial concentrations of HMX of 2, 1 and 0.5 mg L�1. It shouldbe noted that while the rate of photo degradation can increasewith decreasing initial concentrations of explosives, the initialreaction rate actually increases with increasing initial concen-trations of explosives. For example, Priya and Madras97 reportedthe increased initial reaction rates with increasing initialconcentrations (from 0–0.7 M) of various nitroaromatic explo-sives such as NB and 1,3-DNB in both combustion synthesizedTiO2 (CST) and Degussa P-25 TiO2 suspensions. Priya andMadras97 found faster initial reaction rates in the CST mainlydue to the higher surface area, lower band gap and higherhydroxyl species content of CST compared to that of Degussa.The effects of initial concentrations of explosives on thedegradation efficiency in both homogeneous and heteroge-neous photo degradation systems were also studied by Herrera-Melian et al.116 who reported that solar-TiO2 photocatalysiscombined with adsorption in bench-scale constructed wetland(CW) efficiently degraded up to 200mg L�1 4-nitrophenol aer 4hour irradiation and 16 hour adsorption whilst higherconcentration up to 500 mg L�1 4-nitrophenol required Fentonplus photo Fenton treatment.

RSC Adv., 2016, 6, 77603–77621 | 77609

Tab

le3

Summaryofdirect

photo

degradationstudiesofcy

clic

nitraminean

dnitroaromatic

exp

losive

s

Com

poun

dsRad

iation

Source

Irradiation

Mod

ePh

otode

grad

ation

prod

ucts

Yield,%

Ref.

t,min

l,n

m

RDX

Natural

sunligh

t—

—Batch

Nitrate

(NO3�),nitrite

(NO2�)

Halflives1–12

0da

ysin

aque

ous

solution

san

d2–4mon

thsin

solid

matrices

19

Simulated

sunligh

tfrom

1000

Wmetal

halide

lamp

8400

170–22

00Batch

Nitrous

oxide(N

2O),NO2�

and4-nitro-2,4-

diazab

utan

al

99%

degrad

ationof

8.45

mM

RDXin

aque

oussolution

was

achieved

47

150W

Xesh

ortarc

9025

4Batch

1,3-Dinitro-1,3,5-

triazacycloh

ex-5-ene,

1-nitro-1,3,5-triazacyclohex-

3,5-en

e

95%

degrad

ationof

170mM

pure

RDXin

aque

oussolution

was

achieved

76an

d77

HMX,R

DX,

tetryl,T

NT,

2,4-DNT,2

,6-

DNT,

TNB,D

NB,

NB

125W

high-pressure

mercu

ry—

—Con

tinuo

us-

ow

Peroxynitrite

—50

RDX,C

L-20

1000

WHg–Xearc

5—

—cN

O2radical

Inpe

rdeu

tero-dim

ethylsu

lfoxide(D

MSO

-d6)

solution

,freecN

O2prod

uction

ratiowas

1:1

forRDXan

d4:1

from

CL-20

78

2,4-DNT

125W

med

ium

pressu

remercu

ry

60—

Batch

—In

aque

oussolution

,45%

removal

ofDNTa

er1hou

r,39

%removal

oftotal

orga

nic

carbon

(TOC)a

er2hou

ran

d18

%removalof

totaln

itrogen(TN)a

er

2hou

rwas

achieved

79

2-MNT,2

,4,6-

TNT,

2,4-DNT,2

,4-

DNBA

Simulated

sunligh

tfrom

1500

Wxenon

lamp

240–48

029

0–80

0Batch

Interm

ediate

prod

uctof

degrad

ationof

TNTwas

5-am

ino-2,4-dicarboxy-3,3,5-

trinitroazoxyb

enzene

100%

degrad

ationof

MNT,T

NTan

dDNT

inaq

ueou

ssolution

sa

er8hirradiation

at70

0W

m�2was

achieved;

10%

degrad

ationof

DNBAat

that

time

80

2,6-DNT,T

NP

Simulated

solar

radiation

(SSR

)43

2020

0–80

0Batch

—In

seawater

solution

,100

%de

grad

ation

for2,6-DNTwas

achievedin

72h;n

oph

otode

grad

ationob

served

forTNPin

seawater

81

2,4-DNT,2

,6-

DNT

Solarsimulator

from

1.5kW

Xelamp

300

<305

—Dinitrobe

nzaldeh

ydes

and

dinitrobe

nzoic

acids

Inseawater,8

5%an

d27

%de

grad

ation

of55

mM

2,6DNTan

d2,4-DNT,

resp

ectively

was

achieved

82

Solarsimulator

from

1.5kW

Xelamp

360

>295

Batch

—In

seawater,8

3%an

d42

%de

grad

ation

of0.06

mM

2,6-DNTan

d2,4-DNT,

resp

ectively

was

achieved

83

77610 | RSC Adv., 2016, 6, 77603–77621 This journal is © The Royal Society of Chemistry 2016

RSC Advances Review

Tab

le3

(Contd.)

Com

poun

dsRad

iation

Source

Irradiation

Mod

ePh

otode

grad

ation

prod

ucts

Yield,%

Ref.

t,min

l,n

m

RDX,T

NT

150W

halog

en30

——

2-Aminod

inirotolue

nean

d4-am

inod

inirotolue

nefrom

10mgTNT;R

DX

degrad

ationprod

ucts

were

repo

rted

tobe

completely

volatilized

—51

2,4,6-TNT

Natural

sunligh

t—

>400

Batch

3,5-Dinitroan

iline(3,5-

DNA)an

d1,3,5-TNB

Inaq

ueou

ssolution

,halflife

of50

mM

TNTin

thepresen

ceof

1mM

ribo

avin

photosen

sitizerwas

21.87min

84

Natural

sunligh

t—

—Batch

1,3,5-TNBin

absence

ofribo

avin,1

,3,5-TNBan

dtw

oun

know

ninterm

ediatesin

presen

ceof

ribo

avin

Insu

rfacefreshwater

solution

,halflife

of20

mgL�

1TNTin

thepresen

ceof

1mM

ribo

avin

photosen

sitizerwas

23.96min

85

NB

Xe* 2

excimer

6017

2Batch

p-Ben

zoqu

inon

e,ph

enol,o-

nitroph

enol,1

,2-

benzened

iol,p-

nitrocatech

ol,m

-nitroph

enol,p

-nitroph

enol

57%

of2.44

mM

aque

oussolution

ofNB

was

degrad

edin

60min

48

20–25ps

tunab

leLa

ser

—25

0,26

6an

d28

0—

106molecules

percm

3NO

perpu

lse

NBdissociation

rate

inga

sph

asewas

1.7

�10

7s�

1at

266nm.N

Bwas

excitedto

higher

absorption

bandat

250an

d26

6nm

than

280nm

from

itsgrou

ndstate

86

DNAN

3differen

tset-up

:UVA,

UVBan

dasolar

simulator

1700

WXe

—UVA(316

–400

nm),UVB

(280

–315

nm),solar

simulator

(300

–400

nm

withcu

t-off

lter

<300

nm)

Batch

NO3�,N

O2�an

d2,4-

dinitroph

enol

Inaq

ueou

ssolution

,half-livesof

0.7,0.37

and0.11

days

wereachievedfor1mgL�

1

DNAN

insu

nligh

t,UVAan

dUVB,

resp

ectively

46

Solarsimulator

1440

—Batch

2-Hyd

roxy-4-nitroan

isole

and2,4-dinitroph

enol

were

interm

ediate

prod

ucts;a

tcompletede

grad

ation,

each

moleof

DNANyielde

d0.7M

NO3�,1

MNH

4+,

totalof

0.9M

HCHO

and

HCOOH

Inaq

ueou

ssolution

,100

%de

grad

ation

was

achieveda

er21

days

ofexpo

sure

87

HNS

10nsNd-YAGlaser

—53

2—

NO2–O

NO

isom

erof

HNS

—88

This journal is © The Royal Society of Chemistry 2016 RSC Adv., 2016, 6, 77603–77621 | 77611

Review RSC Advances

Tab

le4

Summaryofhomogeneousphoto

degradationstudiesofcy

clic

nitraminean

dnitroaromatic

exp

losive

s

Com

poun

dsRad

iation

Source

Irradiation

Mod

eOxidiseran

d/or

sensitizer

used

Photode

grad

ation

prod

ucts

Yield,%

Ref.

t,min

l,n

m

TNP,

AP,

2,4-

DNT,tetryl,

2,4,6-TNT,R

DX

andHMX

40W

UV

25–90

254

Batch

H2O2(0.29mM)+Fe

2+

(0.09–0.72

mM)

—In

aque

oussolution

,theresidu

alTNP,

AP,

DNT,T

NT,tetryl,RDXan

dHMXwere6.76

%,4

.65%

,67.1%

,19

.5%,5

5.6%

,7.1%

and76

.1%,

resp

ectively

60

CNB

Lowpressu

reHg(15W

and

90W)

4–35

—Batch

and

continuo

us-

ow

H2O2/O

3Cl�,N

O3�

andCO2

Inaq

ueou

ssolution

,95%

degrad

ationof

90mM

CNBwas

achievedin

35min

inba

tchmod

e(U

V+1mM

H2O2)an

d92

%de

grad

ation

of25

mM

CNBwas

achievedin

4min.

Incontinuo

us-ow

mod

ean

din

20mM

aque

ousO3solution

of,7

5%de

grad

ationof

2.2mM

CNBwas

achievedin

4min

53

RDX

150W

Xe

shortarc

4025

4Batch

H2O2

PFBHA(O-

pentauo

robe

nzyl-

hyd

roxylamine)

derivatised

form

amidean

durea

98%

degrad

ationof

20mM

RDXin

aque

oussolution

76an

d77

Lowpressu

reHgUV

2.5

—Con

tinuo

us-

ow

O3

N-H

ydroxyform

adide,

nitroform

alde

hyd

e99

%de

grad

ationof

6.7mM

RDXin

aque

oussolution

76an

d77

TNT,R

DX,H

MX

15W

HgUV

2525

4Batch

H2O2(0.38–65

mM)

—In

aque

oussolution

,near10

0%de

grad

ationof

23–55mgL�

1RDX,4

mgL�

1HMXan

d29

–80mgL�

1TNT

89

NB,2

,6-DNT

Lowpressu

reHg

10–20

—Batch

O3

Inaq

ueou

ssolution

,6�

10�5M

DNT

completelyde

grad

edin

10min

atpH

7an

d1.2�

10�4M

NB93

.3%

degrad

edin

20min

atpH

8.5

54

NB

125W

med

ium

pressu

reHg

5>1

90Batch

H2O2

—In

aque

oussolution

,100

%de

grad

ationof

0.25

mM

NBin

presen

ceof

10mM

H2O2was

achieved

90

125W

med

ium

pressu

reHg

90—

Batch

H2O2

NO3�,N

O2�,H

COO�,

C2O42

�In

aque

oussolution

,73.6%

degrad

ationof

1.9mM

NBin

presen

ceof

11mM

H2O2was

achieved

91

1,3-DNB

1kW

med

ium

pressu

reUV

4522

0–58

0Batch

H2O2

Interm

ediatessu

chas

resorcinol

(RS),c

atechol

(CC),hyd

roqu

inon

e(H

Q),

benzoqu

inon

e,nitrohyd

roqu

inon

e(N

HQ),

4-nitrocatech

ol(4-NC),2,4-

dinitroph

enol,p

hen

ol,3

-nitroph

enol

aswella

snal

mineralisationprod

ucts

such

asNO3�,N

O2�an

dCO2

Inaq

ueou

ssolution

,aroun

d97

%de

grad

ationof

0.11

mM

1,3-DNBin

presen

ceof

9.3mM

H2O2was

achievedin

30min.C

omplete

mineralisationwas

achievedin

45min

92


RSC Advances Review

Tab

le4

(Contd.)

Com

poun

dsRad

iation

Source

Irradiation

Mod

eOxidiseran

d/or

sensitizer

used

Photode

grad

ation

prod

ucts

Yield,%

Ref.

t,min

l,n

m

2-MNT,2

,4-DNT,

2,4,6-TNT

6W

low

pressu

reHg

—25

4—

H2O2/O

3/H

2O2+Fe

2+

—40

%de

crease

ofinitialDOClevelfor

2,4,6-TNTin

aque

oussolution

inpresen

ceof

12–14mM

H2O2.

Evide

ncesforother

UVmethod

swere

inconclus

ive

56

2,4-DNT

125W

med

ium

pressu

reHg

60—

Batch

H2O2/H

2O2+Fe

2+

—In

aque

ousph

ase,

90%

and10

0%removal

of10

0pp

mDNTin

photo-

peroxide

system

andph

oto-Fe

nton

system

,respe

ctivelya

er1h96

%an

d57

%removalof

TOCan

dTNin

photo-

Fentonsystem

aer

2hirradiation

79

Lowpressu

reHgUV

425

4Batch

Cationic

surfactant

cetyltrimethylam

mon

ium,

brom

ide(CTAB)withthe

anionic

nuc

leop

hile,

borohyd

ride

(BH

4�)

2-Methyl-5-nitroan

iline,

4-nitrotoluen

e,2-

nitrotoluen

e,4-methyl-3-

nitroan

iline,

2,4-

diam

inotolue

ne,

o-toluidine,

1,3-

dinitrobe

nzene,

3-nitroan

iline,

p-cresol,a

nd

2,4-diam

inop

hen

ol

Inaq

ueou

ssolution

s,70

%de

grad

ationof

0.1mM

2,4-DNTwith

overallhalf-life

2.1min

in3mM

CTABwas

achieved

93

MNT,D

NT,T

NT

—15

–60

254

Batch

H2O2+Fe

2+

Oxalicacid,C

O2,N

O3�an

dH

2O95

%de

grad

ationof

TNTin

aque

ous

soilextractwas

achievedin

60minutes

atpH

3.Fo

rMNTan

dDNT,

15an

d30

minutes

wererequ

ired

,resp

ectively

toachieve

similar

degrad

ationat

pH3

57

2,4,6-TNT

Med

ium

pressu

remercu

ry

240

254

Batch

Methylated

-b-cyclode

xtrin

(MCD),hyd

roxyprop

yl-b-

cyclod

extrin

(HPC

D),H

2O2

+Fe

2+

Ternarycomplex

(TNT–

cyclod

extrin–iron),oxalic

acid

andform

icacid

Insoilextractsolution

,95%

ofthe

initialTOCvalueab

atem

entwas

achieved

63

8�

10W

low

pressu

remercu

ry

120

—Batch

H2O2/H

2O2+Fe

2+

1,3,5-Trinitrobe

nzene,

1-methyl-2,4-dinitrobe

nzene,

2,5-dinitrobe

nzoicacid

and

1,3-dinitrobe

nzene

Inaq

ueou

ssolution

s,TNTresidu

alratios

were43

.4%

and5%

inph

oto-

peroxide

andph

oto-Fe

ntonsystem

s,resp

ectively

for2.4mW

cm�2UV

intensity

94

TNP

Natural

sunligh

tas

wella

s15

0W

med

ium

pressu

reHg

UV

120

254

Batch

H2O2+Fe

2+

NO3�

Inaq

ueou

ssolution

,>92

%of

0.87

mM

TNPwas

mineralised

insolar

Fentonas

wellas

UVFe

ntonprocess

atpH

3

95


Review RSC Advances

Tab

le5

Summaryofheterogeneousphotoca

talyticdegradationstudiesofcy

clic

nitraminean

dnitroaromatic

exp

losive

s

Com

poun

dsRad

iation

Source

Irradiation

Mod

eSemi-c

ondu

ctor

photocatalyst

Photode

grad

ation

prod

ucts

Yield,%

Ref.

t,min

l,n

m

NB

Xelamp

240

470

Batch

Ag-Red

uced

grap

hen

eoxide(RGO)

nan

ocom

posite

(Ag-RGO)

Aniline

100%

decompo

sition

of0.8mM

NBin

ethan

olsolution

toan

ilinein

presen

ceof

50mgAg-RGO

powde

r

72

3W

blue

LED

Upto

1200

>420

Batch

CdS

nan

ostruc

ture

Aniline

19–97%

decompo

sition

of10

mM

NB

inalcoholic

solution

toan

ilines

inpresen

ceof

20mgCdS

nan

opow

der

96

125W

high

pressu

reHg

240

365

Batch

Com

bustionsynthesized

nan

o-TiO

2

—87

%de

grad

ationof

75mgL�

1NBin

aque

oussolution

withan

ataseTiO

2

load

ingof

75mgL�

1.H

ydroxy

and

chloro

subs

titution

onNBaccelerated

thede

grad

ationrate

whilenitro

subs

titution

onNBde

celeratedthe

photode

grad

ationrate

97

125W

high

pressu

reHg

150

<340

Batch

TiO

2Nitrohyd

roqu

inon

e,p-

benzoqu

inon

e,nitroph

enols(N

PhOH),

nitrosobe

nzene,

dinitrobe

nzenes,

CH

3COO�,H

COO�,N

O3�

Aroun

d10

0%de

grad

ationof

3mM

NB

inaq

ueou

ssolution

withan

ataseTiO

2

load

ingof

2.5gL�

1.C

hem

ical

yields

of2-,3

-and4-NPh

OH

were6.3%

,7.5%

and6%

,respe

ctivelyat

irradiationtimes

correspo

ndingto

25min,3

5min

and45

min

98

1,3-DNB

125W

low

pressu

reUV

240

254

Batch

and

continuo

us-

ow

TiO

2o-,m

-,p-Nitroph

enol,2

,4-

dinitroph

enol,C

O2,H

2O

65%

degrad

ationof

1,3-DNBin

aque

oussolution

with0.2%

(w/v)

anataseTiO

2load

ingwas

achieved

99

TNP

500W

supe

r-highpressu

reHg

60–180

330–39

0Batch

TiO

2NO3�,formic

acid

and

acetic

acid

98%

degrad

ationof

0.2mM

aque

ous

solution

ofTNPin

60min

whilst

60%

degrad

ationof

1mM

aque

ous

solution

ofTNPin

180min

with

anataseTiO

2load

ingof

75mgin

25mLsample(w

/v);33

%TOCof

1mM

TNPredu

cedin

240min

whilst

completeredu

ctionof

TOCof

0.2mM

TNPin

120min

100

Simulated

sunligh

tan

dmed

ium

pressu

reHg

120

—Batch

ZnO,T

iO2

—12

0min

UVirradiationresu

lted

100%

degrad

ationof

1mM

TNPan

d15

min

UVirradiationresu

lted

100%

degrad

ationof

0.1mM

TNP,

both

in2

gL�

1Zn

Osu

spen

sion

.Insu

nligh

t,80

hou

rirradiationwas

requ

ired

toachieve

90%,8

0%an

d55

%de

grad

ationof

0.1mM

TNPthroug

hZn

O,T

iO2an

ddirect

photolysis,

resp

ectively

101

RDX

Simulated

sunligh

t12

040

0–80

0—

Form

icacid,C

O2

When

ACFclothload

edwithnan

o-TiO

2im

mersedin

simulated

67


RSC Advances Review

Tab

le5

(Contd.)

Com

poun

dsRad

iation

Source

Irradiation

Mod

eSemi-c

ondu

ctor

photocatalyst

Photode

grad

ation

prod

ucts

Yield,%

Ref.

t,min

l,n

m

Nan

o-TiO

2(anatase:rutile

¼1:1

)onactivatedcarbon

bre(ACF)

cloth

wastewater

solution

containing40

mg

L�1RDX,9

5%de

grad

ationof

aque

ousRDXwas

achieved

RDX,H

MX

415W/05UV

480

300–45

0Batch

TiO

2—

AtpH

7.4,

13.7%

RDXan

d0%

HMX

degrad

ationin

1:1

00dilutedslud

gesolution

was

achievedwith1gL�

1

TiO

2load

ing

73

100W

Hg

and50

0W

halog

en

120

—Batch

ZnSdo

pedwithCu

(Zn0.95Cu 0

.05S)

—Atp

H7,

91%

and93

%de

grad

ationof

40mgL�

1RDXan

d4mgL�

1HMXin

aque

oussolution

,respe

ctivelywas

achievedwithof

150mgL�

1

photocatalystload

ing

102

15W

UV

150

254

—TiO

2HCOO�,N

O3�,N

O2�

and

NH

4+AtpH

7,20

%redu

ctionin

TOCfor20

mgL�

1RDXan

d60

%redu

ctionin

TOCfor5mgL�

1HMXin

aque

ous

solution

was

achievedwith0.7gL�

1

TiO

2load

ing

103

NTO

—18

0>2

90Batch

TiO

2P2

5(D

egus

sa)

NO3�an

dNO2�

100%

degrad

ationof

150mgL�

1NTO

inaq

ueou

ssolution

was

achieved

with0.4gL�

1TiO

2load

ing

104

2,4,6-TNT

8W

UV

210

254

Batch

TiO

2–SiO

2aerogel+H

2O2

(0.35mM)

—99

%mineralisationof

100mgL�

1

TNTin

aque

oussolution

with0.3g

L�1TiO

2–SiO

2(1

:1)aerogelpo

wde

rload

ingwas

repo

rted

105

15W

UV

150

254

Con

tinuo

us-

ow

TiO

2CH

3COO�,H

COO�,N

O3�,

NO2�an

dNH

4+

AtpH

7,10

0%de

grad

ationof

30mg

L�1TNTin

aque

oussolution

with1g

L�1TiO

2load

ing

106

500W

halog

en+9

WUV

3525

4,36

5,>4

00Batch

TiO

2,W

O3/TiO

2,A

u/TiO

2

andAu/WO3/TiO

2in

presen

ceof

O3or

O2–O

3

mixture

—AtpH

7an

dat

254nm

wavelen

gth,

100%

degrad

ationof

0.53

mM

TNTin

aque

oussolution

inpresen

ceof

O2–

O3with1gL�

1load

ingof

Au/WO3/

TiO

2ph

otocatalyst

107

200W

HgUV

300

>290

Batch

TiO

2NO3�an

dNH

4+

50mgL�

1TNTin

aque

oussolution

was

degrad

ed98

%in

60min

but

100%

mineralisationof

degrad

edTNTprod

ucts

requ

ired

300min

irradiationin

O2bu

bbledTiO

2slurry

with0.1gL�

1TiO

2load

ing

108

400W

low

pressu

reHg

UV

25–90

—Batch

TiO

2Trinitrobe

nzene,

3,5-

dinitroan

iline,

2-am

ino-

4,6-dinitrotoluen

e,4-

amino-2,6-dinitrotoluen

e

With1gL�

1TiO

2load

ing,

100%

oxidativede

grad

ationof

aque

ous

solution

of50

mgL�

1TNTin

absence

of1%

methan

olwas

achievedin

30minutes.H

owever,1

00%

redu

ctive

109


Review RSC Advances

Tab

le5

(Contd.)

Com

poun

dsRad

iation

Source

Irradiation

Mod

eSemi-c

ondu

ctor

photocatalyst

Photode

grad

ation

prod

ucts

Yield,%

Ref.

t,min

l,n

m

degrad

ationof

TNTin

presen

ceof

1%methan

olwas

achievedin

90minutes

450W

med

ium

pressu

reHg

UV

60>3

40Batch

TiO

21,3,5-TNB

With0.25

gL�

1TiO

2load

ing,

100%

degrad

ationof

aque

oussolution

of50

mgL�

1TNTat

both

pH5an

d8.5.

To

thecontrary,

60%

mineralisationof

50mgL�

1TNTat

pH8.5compa

redto

90%

mineralisationof

50mgL�

1TNT

atpH

5

110

Red

water

containing

2,4,6-TNT,2

-MNT,4

-MNT,

2,6-DNT,2

,4-

DNT,2

,5-

DNT,1

,3,5-

TNB

500W

Hg–W

300

—Batch

ZnO:C

u 2O

(4:2

.5)on

diatom

ite

1,3,5-TNB

With5gL�

1ph

otocatalystload

ing,

72.8%

ofredwater

degrad

edin

420

nm

wavelen

gth

1

DNT

300W

Xe

240

250–38

0Batch

H3PW

12O

40/TiO

2an

dH

4SiW

12O40/TiO

2

—AtpH

2with0.8gL�

1H

3PW

12O40/

TiO

2catalyst

load

ing,

95%

degrad

ationof

aque

oussolution

of40

mgL�

1DNTwas

repo

rted

111

TNR

15W

UV

3025

4Con

tinuo

us-

ow

TiO

2+H

2O2

TNT,C

O2,N

O3�

With1gL�

1TiO

2load

ingan

din

presen

ceof

10mM

H2O2,9

0%de

grad

ationof

aque

oussolution

of0.1mM

TNRwas

achievedbu

t40

%TOCfrom

thede

grad

ationprod

ucts

ofTNRwas

removed

75

NB,2

,4,6-

TNT,1

,3,5-

TNB,D

NT,

DNB

150W

Xe

10–20

>320

Batch

TiO

2In

neu

tral

andalka

linepH

,1,3,5-TNBwas

the

interm

ediate

prod

uctof

2,4,6-TNT

photod

egrada

tion

and1,3-

DNBwas

themainprod

uct

of2,6-DNT

photod

egrada

tion

.In

acidic

pH,n

osu

chprod

ucts

wereob

served

With1gL�

1TiO

2load

ing,

16%,1

6%an

d4%

degrad

ationof

aque

ous

solution

of10

0mM

2,4,6-TNT,1

,3-

DNBan

d1,3,5-TNBin

20minutes,

resp

ectively

atpH

3.20

%an

d28

%de

grad

ationof

100mM

2,6-DNTan

dNBin

10minutes,respe

ctivelyat

pH3

112

TNTan

dNB

mixture

15W

high

pressu

reHg

60–300

>290

Con

tinuo

us-

ow

TiO

2+O2

—With0.6gL�

1TiO

2load

ingan

dcontinuo

usbu

bblingof

O2,90

%de

grad

ationof

aque

oussolution

of50

mgL�

1TNTan

d30

mgL�

1NB

mixture

was

repo

rted

in60

minutes

atpH

6.5

113


RSC Advances Review

Table 6 Effects of pH and incident light intensity on the initial reaction rates (mM L�1 min�1) of nitroaromatic explosives (100 mM) with 1 g L�1 TiO2

suspension (partially adapted from Dillert et al.112)

CompoundsLight intensity,mM photons per min

Initial reaction rates, mM L�1 min�1

pH 3 pH 4 pH 5 pH 7 pH 9 pH 10 pH 11

2,4,6-TNT 3.5 1.13 1.63.1 0.94 1.01 1.19 1.31 1.2 1.26 1.192.6 1.13 1.19 1.19 1.13 1.1 1.22 1.3

2,4-DNT 3.5 3.24 2.062,6-DNT 3.1 2.37 1.661,3,5-TNB 3.9 0.38 0.39 0.38 0.32 0.3 0.3 0.22

2.9 0.32 0.35 0.32 0.45 0.33 0.28 0.291,3-DNB 3.0 1.95 0.91 0.81 0.82 0.83 0.93 0.741,4-DNB 3.1 1.2 0.83NB 3.0 3.96 3.5 4.19 4.01 3.79 4.2 3.31

Review RSC Advances

pH

Dillert et al.112 demonstrated the fact that whilst pH did notaffect the initial reaction rate of a wide range of nitroaromaticexplosives in TiO2 suspensions, pH of the suspension hada pronounced effect on the degradation pathways of thesenitroaromatic explosives. For example, at pH 9 and pH 11, about18% of the reacted 2,4,6-TNT were identied as 1,3,5-TNB aer10 minutes of irradiation, in acidic media, to the contrary, no1,3,5-TNB was found. Similar observations were made in thedegradation of 2,6-DNT where 1,3-DNB was identied as anintermediate in alkaline but not in acidic suspensions.Recently, Sisco et al.117 also re-established the fact that salinityalone had a negligible impact on the degradation of TNT andRDX in aqueous solutions and RDX was more stable than TNTin aqueous solutions aer 3 days of UV exposure. Table 6 isreproduced from Dillert et al.112 to demonstrate the relativelyunchanged initial reaction rate for a number of nitroaromaticcompounds at acidic and alkaline pH.

Future direction of application of photodegradation in explosive remediation

The future direction of application of photo degradation inremediation of extremely hazardous organic substances such asexplosives will rely on the integration of engineering advance-ments facilitating the pilot or industry scale application offollowing photo-oxidative processes with principles ofphotochemistry:118

� UV only.� UV + Fe2+ + H2O2.� UV + H2O2.� UV + O3.� UV/visible light + new generation semiconductors.In this context, the recent advances of the continuous-ow

photochemistry have been described as the next generationmean for ‘rapid, safe and efficient’ application of photonsciences in a number of industries such as, water treatment,polymer synthesis, nanoparticle and small moleculesynthesis.119 It is important to note that all of the above listedphoto-oxidative processes can be applied in both batch photo


reactors and continuous-ow photo reactors. At present thebatch photo reactors overwhelmingly outnumber continuous-ow photo reactors in the context of large scale application ofphoto degradation. This could be attributed to lack of researchstudies regarding the effects of the irradiation time, radiationsources, radiation and mass transport constraints as well assolvent constraints on the quantum yields of various photo-chemical reactions to facilitate “scale-up” in continuous-owphoto reactors.120 However, the recent evolution of micro-reactor technology and its impact on continuous-ow photo-chemistry have the potential to overcome the “scale-up” issuesencountered in continuous-ow photo reactors.121 In thiscontext, Su et al.122 demonstrated 2n (n¼ 0, 1, 2, 3) parallel microphoto reactors to scale-up the photocatalytic aerobic oxidationof thiols to disuldes with excellent yield result. Varioushomogeneous and heterogeneous photocatalytic reactions incontinuous-owmicro photo reactors with comparable yields ata much faster rate compared to batch photo reactors were re-ported by Oelgemoller.121 Additionally, recent advances in thedevelopment of light emitting diodes (LEDs) in vacuum UVrange with high radiometric energy efficiency123 as well as LEDbased accurate and facile techniques to measure the radio-metric outputs of different light sources124 will surely playa signicant role in regards to the development of highly energyefficient light sources at low wavelength particularly in indus-trial applications of continuous-ow photochemistry in comingdecades. Nonetheless, the remediation of cyclic nitramine andnitroaromatic explosives in liquid form using the micro photoreactor technology is not reported yet and it is envisaged thatthe requirement of fast, safe, green and cost effective remedi-ation of these hazardous explosives in liquid form will surelydraw researchers' attention into the integration of continuous-ow micro photo reactor technology with the aforementionedphoto-oxidative processes.

Neither homogeneous nor heterogeneous photocatalyticsystems currently have complete upper-hand for the degrada-tion of nitroaromatic explosives. While the electron–holerecombination is a major and well-known disadvantage in largescale heterogeneous photocatalytic systems, sustainablemethods to suppress electron–hole recombination process in

RSC Adv., 2016, 6, 77603–77621 | 77617

Table 7 A qualitative comparison of various degradation techniques of EMs in water in terms of existing or future potential (3) as well as absenceof existing or future potential (7)

Remediation method Portability Rapidity Low cost Energy efficiencyEnvironmentfriendly Green practice Scalability

Photo degradation (continuous-owmode)

3 3 3 3/7 3 3/7 3

Photo degradation (batch mode) 7 7 7 3/7 3/7 7 7

Thermal degradation 7 3 7 7 7 7 3

Bio degradation 7 7 3 3 3/7 3 7

Acid degradation 7 3 7 3 7 7 7

RSC Advances Review

graphene modied TiO2 photo catalytic systems using cheapindustrial organic waste products such as, glycerols wereinvestigated by Ibadurrohman and Hellgardt.125 Recently,signicant efforts have also been made to shi the light sensi-tivity of TiO2 from UVA region to visible region through chem-ical additives (electron donors and suppression of backwardreaction), noble metal loading, ion doping, sensitization andmetal ion-implantation with a view to produce visible lightresponsive TiO2 or VLR-TiO2.126,127 However, efforts to increasethe quantum efficiency of VLR-TiO2 under solar light is still inits infancy126 and hence, practical applications of these tech-nologies for remediation of cyclic nitramine and nitroaromaticexplosives in large scale are currently not viable. Balkus Jr128

introduced nanoparticles in a variety of shapes includingnanotubes, nanorods as well as quantum dots for heteroge-neous photocatalysis in various applications including envi-ronmental remediation. In this context, Eskandari et al.96

illustrated that CdS nanostructures under visible LED lightsources degraded various aromatic nitro compounds to corre-sponding amines with maximum irradiation time up to 20 hand percentage yield ranged from 40% to 97%. The compro-mise between photoabsorption efficiency and quantum effi-ciencies of CdS nanostructures under visible LED light sourcesmight have played a role in the prolonged irradiation time asreported by Eskandari et al.96 Recently, a series of mesoporousmaterials such as, H3PW12O40/TiO2, Y–H3PW12O40/TiO2 and La–H3PW12O40/TiO2 were introduced by Liu et al.129 for rapid andenvironmentally safe degradation of 2,4-DNT and 2,6-DNT.Hence, this review envisage that the future of application ofphoto degradation in remediation of hazardous photo degrad-able explosives relies on several research domains, namely,continuous-ow photochemistry, micro photo reactor tech-nology incorporating energy efficient light sources at lowwavelength as well as advances in photosensitive nanostructuredevelopment. Additionally, continuous-ow photo degradationhas the potential to incorporate rapid, energy efficient, costeffective, portable and green remediation practices not only foreffluents from military ranges and ammunition plants but alsofor the remediation of water in swimming pools, rain watertanks, surface water reserves, sh tanks and ponds used insheries industries as well as wastewater from agricultural,livestock and poultry industries. With the advent of energyefficient light sources, green reagents and environmentally safephotocatalysts, the principles of continuous-ow photochem-istry will strengthen the futuristic views of cheap and portable

77618 | RSC Adv., 2016, 6, 77603–77621

solution for ‘scaling-up’ the yield of photo degradation inminimal time through direct, homogeneous or heterogeneousphoto degradation, where the resultant reactive species fromphoto degradation will be utilised for remediation of water. Inlight of the discussions based on future directions of remedia-tion of EMs in water using photo degradation, Table 7 illus-trates a qualitative guidance for chemists and remediationpractitioners to compare photo degradation with the traditionaltechniques.

Conclusions

Three types of photo degradation techniques namely direct,homogeneous and heterogeneous photo degradation can beapplied for remediation of cyclic nitramine and nitroaromaticexplosives. The percentage yield of degradation and degree ofmineralisation through photo degradation are inuenced byradiation sources, irradiation time, photocatalysts as well asimportant physical–chemical parameters such as pH and initialsubstrate concentrations. While the photo degradation of theseexplosives in batch mode are widely practiced at present, theneed for ‘fast, safe, cost effective, and energy efficient’ remedi-ation of these hazardous explosives has resulted signicantadvances towards the next generation photo degradation tech-nology. In this context, the recent advances in the synthesis ofphotosensitive nanostructures as well as highly energy efficientlight sources at low wavelength (e.g., deep UV LED) possess thepotential to facilitate “scale-up” in continuous-owmicro photoreactors incorporating direct, homogeneous or heterogeneousphoto degradation processes.

Acknowledgements

Both authors gratefully acknowledge the logistic supportprovided by Australian Centre for Research on SeparationScience (Across) at University of Tasmania for publishing thisreview.

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