Removal of Nitrate from Drinking Water by Ion-Exchange Followed by nZVI-Based Reduction and

19 Pages·2017·2.02 MB·English

chemengineering
Article
Removal of Nitrate from Drinking Water by
Ion-Exchange Followed by nZVI-Based Reduction
and Electrooxidation of the Ammonia Product to N
2(g)
InbalFux,LiatBirnhack,SamuelC.N.TangandOriLahav*
FacultyofCivilandEnvironmentalEngineering,Technion-IsraelInstituteofTechnology,Haifa32000,Israel;
[email protected](I.F.);[email protected](L.B.);[email protected](S.C.N.T.)
* Correspondence:[email protected];Tel.:[phone]
AcademicEditor:AlírioEgídioRodrigues
Received:5April2017;Accepted:4May2017;Published:10May2017
Abstract: Ion-exchange(IX)iscommonforseparatingNO − fromdrinkingwater. Frombothcost
3
andenvironmentalperspectives,theIXregenerationbrinemustberecycled,vianitratereductionto
N . Nanozero-valentiron(nZVI)reducesnitrateefficientlytoammonia,underbrineconditions.
2(g)
However,tobesustainable,theformedammoniashouldbeoxidized. Accordingly,anewprocess
was developed, comprising IX separation, nZVI-based nitrate removal from the IX regeneration
brine, followed by indirect ammonia electro-oxidation. The aim was to convert nitrate to N
2(g)
while allowing repeated usage of the NaCl brine for multiple IX cycles. All process steps were
experimentally examined and shown to be feasible: nitrate was efficiently separated using IX,
whichwassubsequentlyregeneratedwiththetreated/recoveredNaClbrine. Thenitratereleasedto
thebrinereactedwithnZVI,generatingammoniaandFe(II).FreshnZVIparticleswerereproduced
from the resulting brine, which contained Fe(II), Na+, Cl− and ammonia. The ammonia in the
nZVIproductionprocedurefiltratewasindirectlyelectro-oxidizedtoN attheinherenthighCl−
2(g)
concentration,whichpreparedthebrineforthenextIXregenerationcycle. Thedominantreaction
between nZVI and NO − was described best (Wilcoxon test) by 4Fe + 10H+ + NO − → 4Fe2+
3 (s) 3
+NH ++3H O,andproceededat>5mmol·L−1·min−1atroomtemperatureand3<pH<5.
4 2
Keywords: nitratereduction;nZVI;electrooxidation;drinkingwater;ionexchange
1. Introduction
Regulationsrestrictingnitrateconcentrationindrinkingwaterareinplaceinboththedeveloped
anddevelopingworld.TheUSenvironmentalprotectionagency(USEPA)setamaximumcontaminant
concentration of 10 mg N/L (i.e., 45 mg/L as NO −) for drinking water. Similarly, the maximum
3
acceptableconcentrationofnitrateinpotablewater,recommendedbytheWorldHealthOrganization
(WHO) is 50 mg/L as NO − [1]. The Israeli Public Health Regulations threshold for nitrate
3
concentrationis70mgNO −/L,andincasethewateristreatedtoreducethenitrateconcentration,
3
theallowedconcentrationdropsto50mgNO −/L.
3
Noconsensusexistsonthemostappropriatetechnologytoremovenitratefromdrinkingwater.
Thetechnologiesappliedatfullscalemaybedividedintoseparationtechniques,biologicaltreatments,
such as denitrification and assimilatory nitrate reduction, chemical reduction techniques and
combinationsthereof.Directnitratereductionindrinkingwateroftennecessitatesaposttreatmentstep
inordertoremoveresidualmattercomprisedoforganicmatter,microorganismsandturbidity-causing
agents. Thisisespeciallyproblematicinthecaseofbiologicaltreatment. Althoughdirectbiological
treatmentappliedondrinkingwatertendstoinducepsychologicalpublicrejection[2],suchmethods
have been successfully implemented at full scale. Common NO − separation techniques include
3
ChemEngineering2017,1,2;doi:10.3390/chemengineering1010002 www.mdpi.com/journal/chemengineering
ChemEngineering2017,1,2 2of19
electro-dialysis,reverseosmosisandion-exchange(IX).AnionicIXisarelativelyselectiveseparation
techniqueandprobablythemostpopularmethodforNO − removalfromdrinkingwater. IXinvolves
3
passageofnitrate-contaminatedwaterthroughstrongbaseanion(SBA)exchangeresins,onwhich
nitrateionsareadsorbed,typicallyinexchangeforchlorides. Regenerationisusuallyconductedwith
aconcentratedNaClsolution. ThespentIXbrineistypicallycomprisedofseveralhundredmg/Lof
nitrate(asN)alongwithaveryhighNaClbackground. Suchasolutionisproblematicfordisposalto
thesewerortoreceivewaterbodies,andnormallyrequiresfurthertreatmentpriortodischarge. ForIX
tobeaneconomicsolution,thebrinehastobetreatedfornitrateremovalandrecycled. Apossible
optionfornitrateremovalathighNaClbackgroundconditionsismetal-inducedchemicalreduction.
Several authors already suggested combining ion-exchange separation and nitrate reduction
from the regeneration solution (preferably to N ) to enable the reuse of the latter, and thereby
2(g)
reduce the cost of the IX process and its environmental impact. Pintar et al. [3] and Kim et al. [4]
combinedanIXstepwithaPd-Cubasedcatalyticnitratereductionprocess. Shietal.[5]mounted
bimetallicFe-Pdnanoparticlesonachelatingresin(DOW3N)andshowednitrateremovalof>95%
andincreasedselectivitytoN asthenitratereductionendproductatincreasedPdcontentand
2(g)
solutionpH.Mengetal.[6]proposedadesorption–bioregenerationtechniqueforreducingnitratein
dilutedIXbrines(9to31mS/cm)tonitrogengas. Dortsiouetal.[7]applieddirectelectrochemical
reductionofnitrateonthebrineofanion-exchangeoperationandreported(ascouldbeexpected)low
currentefficiencythattranslatedintohighoperationalcosts. Kabayetal.[8]usedanelectro-dialysis
ion-exchangehybridprocessforremovalofnitratefromgroundwater.Bergquistetal.[9]alsocombined
ion-exchangewithcatalystnitratereduction.
Chemicalnitratereductionhasbeenwidelystudiedusingmostlyzerovalentiron(ZVI),Mg and
(s)
Al as the electron donors [10]. Upon the use of these metals as reducing agents, nitrate has
(s)
been reported to be reduced to TAN (total ammonia nitrogen, i.e., the combined concentration of
NH + andNH ),tonitrite(NO −),andalsotonitrogengas(N ),whichisclearlythepreferable
4 3 2 2(g)
productfromthewaterqualitypointofview. Zero-valentmagnesiumisahighlyreactivereducing
agent; however, it is relatively costly and also becomes rapidly covered with a thin magnesium
oxide layer upon contact with water, which eventually hinders its reactivity towards nitrate [11].
Drawbacksreportedforaluminumasareducingagentwerelowefficiency(40–50%),loweffectiveness
atinitialnitrateconcentrationshigherthan30mgN/Landpotentialwatercontaminationwithresidual
Al3+concentrationremaininginthewater[12]. Ingeneral,theuseofmetalsasreducingagentsresults
inreleaseofreducedmetalions(Fe2+,Mg2+andAl3+,respectively),thatsometimesrequirefurther
treatment. Moreimportantly,theproductofnitratereductionbymetalsisapparently(mostly)TAN,
whichalsohastoberemovedfromthewater.
Considerable efforts have been dedicated in recent years to studying the chemical reduction
ofnitratebynanozerovalentiron(nZVI)duetoitslargespecificsurfaceareaandhighreactivity,
as compared to other reducing agents [13]. In this respect, different stoichiometric reactions have
been suggested for describing the nZVI enhanced NO − reduction [13–15]. Apparently, only one
3
study[16]concludedthatnitratewasreducedpredominantlytoN astheendproduct. Allother
2(g)
studiesreportedthatbetween85%and90%ofthenitratewasreducedtoTANandonlytherestofthe
nitrate(i.e.,lessthan15%)wasreducedtoN (e.g.,[17–19]).Otherstudies(e.g.,[13,14])reportedthat
2(g)
almost100%oftheNO − wasreducedtoTAN.ThereportsontheportionofN vs. TANformation
3 2(g)
havebeenthusfarbasedmainlyonnitrogenspeciesmassbalancecalculations[16,17]. Accordingly,
the following reaction is considered by many (e.g., [17,20]) to represent the predominant reaction
betweennZVIandNO − attheacidictoneutralpHrange:
3
4Fe +10H++NO − →4Fe2++NH ++3H O. (1)
(s) 3 4 2
However,severalstudiesreportedthat,ontopofammoniumformation(themainendproduct),
nitriteisalsoproduced,althoughatmuchlowerconcentrations[14,21].
ChemEngineering2017,1,2 3of19
ThefactthatnitratereductionbynZVIleadspredominantlytotheformationofTAN(ortonitrite)
significantlydetractsfromtheattractivenessoftheapproachfordrinkingwatertreatment[22],anditis
clearthat,inorderfortheprocesstobefeasible,itneedstobecoupledwithanefficient(andrelatively
cheap)physical-chemicalTANremovalstrategy.
Kineticswise,severalparametershavebeenreportedtoaffectthereactionratebetweennitrate
andnZVI.TheseincludethepHvalue,presenceofnZVImassbeyondthestoichiometricrequirement,
the initial nitrate concentration and the chloride ion concentration. Contradicting reports exist
regardingtheeffectofpHonthereactionrate. Comparingresultsfromdifferentworksisproblematic
since,insomestudies,thepHwasnotcontrolledandthusfluctuatedinawiderangecoveringbasic
andacidicvalues[15,19,23],while,inothers,itwasmaintainedatarelativelywide,butacidicrangeof
pH2–5[17,24]. Severalworks(e.g.,[19,20,24,25])reportedthatnitratereductionbynZVIproceeded
morerapidlyatacidicconditions. Thisobservationseemsreasonable,sinceprotonsareconsumedin
Equation(1). Siciliano[24]alsosuggestedthatthehigherreactionrateatlowpHmightberelatedin
parttoH+-inducedironoxidationoccurringattheacidicsolutions. Accordingly,theseauthorsdefined
themechanismas“acid-drivensurfacemediatedprocess”.
Anotherparameterthatvariesinreportedstudiesistheinitialmolarconcentrationofthereactants,
i.e.,nitrateandnZVI,andtheinitialmolarratiobetweenthem. ThestoichiometricratiobetweenFe
andNO − accordingtoEquation(1)is4. However,inmostexperimentalworks,Fewasappliedin
3
large excess. For example, Yang and Lee [17] tested two Fe to NO − molar ratios: 14.72 and 7.36,
3
whilemaintainingthesameinitialnitrateconcentration. Theyreportedonincreasedreactionrateat
thehigherFetoNO − ratio. AsimilarobservationwasreportedbySiciliano[24]. Ontheotherhand,
3
YangandLee[17]reportedthatthenitratereductionratedecreasedatahigherFetoNO − ratiowhen
3
lowernitrateconcentrationswereapplied,whiletheFedosagewasmaintainedconstant(atpH4).
Thisapparentcontradictioncanbeaconsequenceoftheeffectofthereactantconcentrations. Thatis,
theconcentrationofNO − andFethemselveshaveaneffectontherateandnotonlytheratiobetween
3
them. Thisisalsoshownbyanothercomparison,inwhichahigherreactionratewasreportedatthe
sameFetoNO − molarratiowhentheinitialFeandNO − concentrationswerehigher[17]. However,
3 3
examiningtheresultsreportedbySiciliano[24]leadstooppositeconclusions:lowerreactionrateswere
observedwhenthereactantconcentrationswereelevated,whilethemolarratiobetweenFeandNO −
3
waskeptalmostconstant. Inaddition,adecreaseinthereactionrateathighernitrateconcentrations
(accompaniedbyadecreaseinthemolarFetoNO − ratio)wasobservedinexperimentsinwhich
3
thepHwasbothcontrolledandnotcontrolled([24]and[15],respectively). WhenthepHwasnot
controlled,highnitrateconcentrationspromotedfasterreactionandthusarapidpHincrease(dueto
protonconsumption),atwhichconditionsofthereactionkineticssloweddown,asmentionedabove.
Tosumup,boththeinitialreactantconcentrationsandtheFetonitrateratioseemtoaffectthereaction
rate,and,atdifferentconditions,theratelimitingfactorsareapparentlydifferent.
Contradictingresultshavealsobeenreportedontheeffectofthechlorideionconcentrationon
thekineticsofthenZVI-NO − reaction: Choeetal.[26]concludedthatahighchlorideconcentration
3
enhancedFe0 oxidationbyH+,therebyenhancingthereactivityofFe0 forNO − reduction. Onthe
3
otherhand,Hwangetal.[23]claimedthatNaCladsorptiononthesurfaceofthenZVIinhibitedtherate
ofironoxidation. Similarly,Xiongetal.[18]claimedthatthepresenceof6%NaCl(w/w)decreasedthe
observedreactionrateconstant(K )by30%comparedtotherateinfreshwater(0.30min−1compared
obs
to0.21min−1),dueto“double-layercompressioneffects”,whichledtoincreasedagglomerationofthe
nZVIparticles.
To sum up the kinetics part: the results of most of the works reviewed by us show that
the time required for complete nitrate removal from an initial nitrate concentrations lower than
120mgN/L was typically 30 min or more [13,16,19,24]. On the other hand, working with a very
high initial nitrate concentration of 10,000 mg N/L, Ryu et al. [14] reported a very rapid removal
rate(reductionof5000mgN/Linoneminute). Itseems, however,thatthereactionrateofnitrate
reductionbynZVIhasnotbeentestedthusfarattheconditionsrelevanttothecurrentwork,i.e.,NO −
3
ChemEngineering2017,1,2 4of19
concentrationexpectedinIXregenerationbrines(~250mg/L)andinthepresenceofaveryhighCl−
background(1–2M).ExcludingRyuetal.[14],thereactionbetweennZVIandnitratehasbeenthus
farinvestigatedwithrelativelylownitrateconcentrations,i.e.,<120mgN/L,andalsoatrelatively
low Cl− concentration background (e.g., [13,15,16,23,26]), since these are the conditions typical to
nitrate-polluteddrinkingwater.
The current work examines the application of nZVI-enhanced-nitrate-reduction for reusing
the regeneration solution that forms in nitrate separation ion-exchange applications. In order
to fully recycle the regeneration solution, the end product should be N , rather than TAN.
2(g)
To this end, an additional step should be applied to oxidize TAN to N . The indirect ammonia
2(g)
electrochemicaloxidationapproachhasbeenprovenhighlyeffectiveinsolutionscharacterizedby
highCl− concentration,forexample,fortheremovalofTANfromIXregenerationsolutionstreating
recirculatedaquaculturesystemwaterandswinewastewater[27,28]. Fromthemechanismstandpoint,
GendelandLahav[27]showedthatundertheconditionsprevailinginelectrochemicalreactorstreating
aTAN-containingsolutioncharacterizedbylowbuffering-capacityandveryhighCl− concentration,
theCl generatedontheanodereactswithNH +toformtrichloramine(Equation(2)),whichfurther
2 4
reactsextensivelyinthebulkelectrolytesolutiontoproducenitrogengas(Equations(3)–(5)):
3Cl +NH + →NCl +3Cl−+4H+, (2)
2 4 3
NCl +OH− →NHCl +OCl−, (3)
3 2
NCl +NH →HNCl +NH Cl, (4)
3 3 2 2
NCl +NHCl +5OH− →N +2OCl−+3H O+3Cl− (5)
3 2 2 2
ThecurrentefficiencyrecordedforTANremovalinbatchelectrolysisexperimentswasashigh
as~92%[27].
The current work focuses on two topics: (a) the development of a complete process
which combines IX separation and nZVI-based nitrate reduction, followed by indirect ammonia
electro-oxidation; (b)determiningthenZVI-basednitratereductionreactionanditskineticsinthe
unique conditions prevailing in the suggested process. The investigation of the reaction included
quantitative determination of end products as a function of pH and time; the effect of high Cl−
concentrationonthereductionkinetics;theabilitytore-synthesizenZVIfromFe(II)presentinthe
brineasaresultoftheNO − reduction,thatis,athighbackgroundCl− concentrationandrelatively
3
lowFe(II)andTANconcentrations.
DescriptionoftheProposedProcess
A schematic layout of the combined process appears in Figure 1. The process comprises an
integrationofanion-exchangestagewithchemicalreductionbynZVIandelectrochemicaltreatment
stage. TheprocessgoalistotreatdrinkingwatercontainingnitrateandrecycletheIXregeneration
solution. Thefirstandsecondsteps(Figure1)aretheion-exchangeadsorptionandregenerationsteps,
respectively. Regenerationwasconductedwith8%NaClsolution. Thethirdstep(nitratechemical
reductionbynZVI)wasappliedonthebrineleavingtheIXregenerationprocess. Thisbrinecontains
ahighNaClconcentrationalongwithaconsiderableconcentrationofNO −(severalhundredmgN/L).
3
Attheendofthethirdstep,thebrinecontains(mainly)dissolvedFe(II),ammoniumandpracticallyno
nitrate,alongwithhighchlorideandsodiumionconcentrations. Thisbrineflowstothefourthstepin
whichnZVIparticlesarereproducedfromdissolvedFe(II)inordertobeusedinthesubsequentthird
step. Inthefifthandfinalstep,electrochemicaloxidationofNH +toN ,isappliedonthefiltrateof
4 2(g)
thefourthstep,whichcontainsTANandahighCl− background. Thisallowsforrecirculationofthe
regenerationsolution.
ChemEngineering2017,1,2 5of19
ChemEngineering 2017, 1, 2 5 of 18

FFigiguurree1 1.. SScchheemmaatticic oof fththe esusguggegsteesdte pdropcreoscs.e ss.
2. M2a. tMeraitaelrsiaalns danMd eMthetohdosds
2.1. M2.1a. tMeraiatelsrials
Synthetic drinking water was prepared with characteristics typical to contaminated wells in
SyntheticdrinkingwaterwaspreparedwithcharacteristicstypicaltocontaminatedwellsinIsrael,
Israel, i.e., nitrate, sulfate, chloride and alkalinity concentrations of 27 mg N/L, 19 mg S/L, 265 mg/L
i.e.,nitrate,sulfate,chlorideandalkalinityconcentrationsof27mgN/L,19mgS/L,265mg/Land
and 120 mg/L as CaCO3, respectively, by dissolving analytical sodium-based salts in distilled water.
120mg/LasCaCO ,respectively,bydissolvinganalyticalsodium-basedsaltsindistilledwater.
Ion-exchang3e resins: Purolite A520E and Purolite A300 were examined. Purolite A520E is a
Ion-exchange resins: Purolite A520E and Purolite A300 were examined. Purolite A520E
macroporous strong base resin, often applied for removal of nitrate from drinking water. Purolite
is aAm30a0c riso ap sotrroonugs bsatsreo rnegsinb wasiteh rheigsihn o,poerfatetinnga cpapplaiceidty afonrd greomodo rvegaelnoefrantiiotnra etfeficfireonmcy. Bdoritnh kreinsigns water.
Purowlietree Aob3t0a0iniesda frseter oonf cghbaragsee frroesmin Puwriotlhiteh Iingth. Lotpd.e (rBaatrinceglocnaap, aScpiatiyn)a. ndgoodregenerationefficiency.
BothresinswereobtainedfreeofchargefromPuroliteInt. Ltd. (Barcelona,Spain).
2.2. Ion-Exchange Experiments
2.2. Ion-ExchangeExperiments
NO3− adsorption and desorption experiments were carried out using two 34 mm internal
dNiaOme−tera PdVsoCr pcotiluomnnasn, deacdhe sfiollrepdt iwonithe 6x3p5e.5r immLe notfs rewsienr.e Ac aprerriiestdaltoicu tpuumsipn gwatws uose3d4 tmo amttaiinnt ernal
3
constant flow rates of 23.6 bed-volumes per hour (BV/h) into each column during the adsorption step,
diameter PVC columns, each filled with 635.5 mL of resin. A peristaltic pump was used to attain
and 4.25 BV/h during the regeneration step. In addition, 8% (i.e., 1.37 M) NaCl solution was used for
constantflowratesof23.6bed-volumesperhour(BV/h)intoeachcolumnduringtheadsorptionstep,
regeneration. Adsorption and regeneration steps were carried out alternately for three consecutive
and4.25BV/hduringtheregenerationstep. Inaddition,8%(i.e.,1.37M)NaClsolutionwasusedfor
cycles on each column.
regeneration. Adsorptionandregenerationstepswerecarriedoutalternatelyforthreeconsecutive
cycle2s.3o. Snyenathcehsics oolfu Nmanno. Zero Valent Iron (nZVI)
The entire procedure (preparation of nZVI and subsequent reaction with nitrate) was carried out
2.3. SynthesisofNanoZeroValentIron(nZVI)
inside an anaerobic compartment to prevent reaction with atmospheric oxygen.
Theentireprocedure(preparationofnZVIandsubsequentreactionwithnitrate)wascarriedout
insid2e.3a.1n. Saynnatehreosibsi cofc noZmVpI avritam Fee3n+ tRteodpurcetivoenn treactionwithatmosphericoxygen.
The nZVI synthesis method was as described in Wang and Zhang [29]. In brief, 3.0 M NaBH4
2.3.1a.qSuyenotuhse ssoilsuotifonn ZwVasI avdiadeFde 3d+roRpewdiusec ttioo an 1.0 M FeCl3 aqueous solution at ambient temperature and
fully stirred anaerobic conditions. Fe3+ was reduced to form nZVI by the following reaction:
ThenZVIsynthesismethodwasasdescribedinWangandZhang[29]. Inbrief,3.0MNaBH
4
aqueoussolutionwasaFded(Hed(cid:2870)Od)(cid:2874)r(cid:2871)o(cid:2878)p+w3isBeHt(cid:2872)o(cid:2879)a+13.0H(cid:2870)MO→FeCFel3(cid:2868)(asq)u+e3oBu(sOsHo)l(cid:2871)u+tio1n0.5aHt(cid:2870)a.m bienttempera(6tu) reand
fullystirredanaerobicconditions. Fe3+wasreducedtoformnZVIbythefollowingreaction:
The solid cake was filtered and washed with acetone and distilled water. The wet solid cake was
weighed and immFee(dHiatOely) u3s+ed+ in3 BthHe n−itr+at3e HredOuc→tionF ee0x(pse)ri+me3nBt(sO. H) +10.5H . (6)
2 6 4 2 3 2
Thesolidcakewasfilteredandwashedwithacetoneanddistilledwater. Thewetsolidcakewas
weighedandimmediatelyusedinthenitratereductionexperiments.
ChemEngineering2017,1,2 6of19
2.3.2. SynthesisofnZVIviaFe2+Reduction
Examination of Step IV (Figure 1) was conducted by modifying the nZVI synthesis method
offeredby[29],asfollows: nZVIweresynthesizedbyadding550mL0.285MNaBH aqueoussolution
4
dropwiseto550mL0.09MFe2+aqueoussolution(preparedbydissolvingFeCl ·4H O)atambient
2 2
temperatureandstirredanaerobicconditions. TheFe(II)solutioncontainedalso1.27MNaCland
0.012MNH Clasbackground. Thatis,theFe(II)solutionsimulatedthesolutionleavingthenitrate
4
reductionstep(StepIIIinFigure1). Fe2+wasreducedaccordingtothefollowingreaction:
Fe(H O) 2++2BH − →Fe +2B(OH) +7H . (7)
2 6 4 (s) 3 2(g)
The washing and filtration procedures of the nZVI were identical to those mentioned in
Section2.3.1.
2.4. NitrateReductionbynZVI:BatchExperimentalProcedure
nZVI-induced nitrate reduction was examined in the spent solution collected from the
ion-exchange regeneration step. A weighed mass of fresh nZVI wet cake (corresponding to 2.5 g
of nZVI on a dry basis) was dosed into each beaker containing 200 mL of spent IX regeneration
solutiontoattainaratioof~12betweeninitialFeandNO − (M/M).Thebeakerswereconstantly
3
mixedusingamagneticstirrer. ThepHwasmaintainedatthechosenacidicrangebymanualdosage
ofconcentratedHClsolution. HClconsumptionwasrecorded. Whenthereactionhadcompleted,
theFeparticlesthatremainedinthebeakerwereallowedtosettleandthesupernatantwasfiltered
through0.45µmfilterpaper,andsubjectedtoanalysis. Thewholeprocedurewasconductedinside
ananaerobicchamberundernitrogengasatmosphere.
2.5. ElectrolysisExperiments
AsyntheticsolutioncontainingNH + andCl− (andnaturallyalsoNa+)atconcentrationssimilar
4
tothoseattainedattheendofthenZVIre-synthesisstep(stepIVinFigure1),i.e.,220mgN/L(lower
than the initial concentration due to acid dilution) and ~45,000 mg/L, respectively, was prepared
by dissolving NH Cl and NaCl in distilled water. Note that this step was applied following the
4
re-synthesisofnZVIfromthedissolvedFe(II),thusthedissolvedFe(II)concentrationinthesolution
of the electrolysis step was negligible. pH and ORP were recorded throughout the experiment.
Samples for TAN analysis in the effluent were collected every 7 minutes. The electrolysis cell
(Klorogen M10, monopolar, parallel plate, TiTan, Chennai, India) was operated at 5 A. The pH
oftheregenerantsolutionwascontrolledatpH7duringtheelectrolysisstepbyautomaticNaOH
(0.7N)dosage.
2.6. Analyses
Chlorideionconcentrationwasmeasuredbytheargenometricmethod,accordingtoStandard
Methods[30]. SO 2− anddissolvedFeconcentrationswereanalyzedbyICP-OES(ThermoScientific,
4
Schaumburg, IL, USA). TAN was determined by the modified salicylate method [31]. NO − and
3
NO − concentrations were measured colorimetrically using an auto-analyzer (QuickChem 8500,
2
Lachat Instruments, Milwaukee, WI, USA) according to Standard Methods [30]. Alkalinity was
measuredbytheGrantitrationtechnique. Fe(II)wasanalyzedusingthemodifiedphenanthroline
methodproposedbyHerreraetal.[32].
2.7. StatisticalAnalysis
DifferencesbetweenactualandcalculatedacidconsumptionwereexaminedusingtheWilcoxon
test(usingMatlabcode). FortyindependentcouplesofactualHCldosageandcalculatedconsumption
ChemEngineering2017,1,2 7of19
werecompared. Thepurposeofthisstatisticaltestwastoevaluatethevalidityoftheassumptionthat
Equations(1)and(8)areindeedthedominantreactionsinthenZVInitratereductionstep:
Fe +2H+ →Fe2++H . (8)
(s) 2(g)
ChemEngineering 2017, 1, 2 7 of 18
3. ResultsandDiscussion
Fe((cid:2929))+2H(cid:2878)→Fe(cid:2870)(cid:2878)+H(cid:2870)((cid:2917)). (8)
3.1. NitrateAdsorptionandRegeneration
3. Results and Discussion
Figure2a,bshowsaverage(n=3)breakthroughcurveresultsobtainedbypassingthesimulative
contaminated well water through the ion-exchange resins (A520E and A300) at 2.5 min hydraulic
3.1. Nitrate Adsorption and Regeneration
retentiontime. Thetargetnitrateconcentrationinthetreatedwater(50mgNO −/L)appearsonthe
Figure 2a,b shows average (n = 3) breakthrough curve results obtained by passing3 the simulative
plotsasasolidline. Comparingtheresultsofthetworesinsrevealedthat,asexpected,bothadsorbed
contaminated well water through the ion-exchange resins (A520E and A300) at 2.5 min hydraulic
NO3− wreteelnl.tiAonf tteimre4.0 T0hBe Vta,rtgheet nciutrmateu lcaotnicveentNraOtio3n− icno tnhec etrnetartaetdi ownastewr (e5r0e m0g.7 N27Om3−/LM) aapnpdea0r.s8 o2n0 tmheM inthe
producptlosotsl aust iao snoliodf liAne5.2 C0oEmapnardinAg t3h0e0 r,esruelstps eofc tthive etwlyo (rtehsiensa rlleovweaeledd nthiattr,a atse ecxpoenccteedn,t broatthio andsiosrb0e.8d1 2mM).
NeverthNeOle3−s ws,eAll.3 A0f0teard 4s0o0r BbVed, thme ocruemsuulalftaivtee NanOd3− acolknacelinntritaytiothnse nweAre5 200.7E27. TmhMer aenfodr 0e.,8f2r0o mmMb oint hthteh ewater
product solution of A520E and A300, respectively (the allowed nitrate concentration is 0.812 mM).
qualityandeconomicstandpoints,A520Ewasfoundtobeslightlyadvantageous. Figure3a,bshows
Nevertheless, A300 adsorbed more sulfate and alkalinity then A520E. Therefore, from both the water
the average (n = 3) cumulative concentrations of relevant anions at the outlet of the regeneration
quality and economic standpoints, A520E was found to be slightly advantageous. Figure 3a,b shows
stepsofA520EandA300,respectively. Figure3showsthatbothresinswereregeneratedefficiently.
the average (n = 3) cumulative concentrations of relevant anions at the outlet of the regeneration steps
As expoefc tAe5d2,0Ed uanedt oA3t0h0e, rheisgphecetrivaeldys. oFrigputrieo n3 sohfoswusl fthaatet baonthd raelskinasl iwneitrye rbeygetnhereatAed3 0e0ffircieesnitnly,. iAtss effluent
consisteexdpeocftehdi,g dhueer tcoo nthcee nhtigrahteiro andssoorfpbtiiocna robf osnualftaetea nandd saulklfaalitnei.tyB bayse tdheo nA3m00a sressbina, laitns ceefflcuaelnctu lations,
itwasccoonnscislutedde odf htihgahter9 c5o%ncoenfttrhateioNnsO of −bicaadrbsoonrabtee danidn stuhlfeataed. Bsaosrepdt oionn msatsesp bawlaanscer ecalelcausleatdioanfst, eitr 24BVs
3
fromthweaAs 5co2n0cElurdeesdin t,haast 9c5o%m opf athreed NtOo3−9 a0d%sorrebleeda sine dthfer aodmsotrhpetioAn3 s0te0p. Swiams irlealreacsoendc alfutesri o2n4 sBVarse frroempo rtedby
the A520E resin, as compared to 90% released from the A300. Similar conclusions are reported by
Primoetal.[33]andShams[34]. Inconclusion,bothresinsareadequateforthesuggestedprocess;
Primo et al. [33] and Shams [34]. In conclusion, both resins are adequate for the suggested process;
howevehro,wAe5v2e0r,E A’s5p20eEr’fso rpmerafonrcmeawncaes wslaisg hsltiglyhtblye tbteetrt.eAr. cAccocrodridningglyly,,a allllf ufurrtthheerr wwoorrkk rreeppoortretde dini nthtihs ispaper
wasconpdapuecrt wedaso cnontdhuecbterdin oen rtehseu blrtiinneg refsruolmtintgh ferormeg tehne erergaetinoenraotiofnA o5f2 A05E2.0E.
2--Oand HCOCumulative concentrations 43during adsorption (meq/l)1234 NO3-MCL NSCHOOlC-43O2--3- 36912mulative concentrations during adsorption (meq/l)
-O, S3 0 0−ClCu
N 0 50 100 150 200 250 300 350 400
Bed volumes
(a)
-2--, SOand HCOCumulative concentrations NO343during adsorption (meq/l)01234 NO3-MCL NSCHOOlC-43O2--3- 036912−ClCumulative concentrations during adsorption (meq/l)
0 100 200 300 400
Bed volumes
(b)
Figure 2. Results of ion exchange (IX) adsorption experiments: average (n = 3) cumulative
Figure 2. Results of ion exchange (IX) adsorption experiments: average (n = 3) cumulative
concentrations during adsorption step using A520E (a) and A300 (b) resins (HRT = 2.5 min)
concentrationsduringadsorptionstepusingA520E(a)andA300(b)resins(HRT=2.5min).
ChemEngineering2017,1,2 8of19
ChemEngineering 2017, 1, 2 8 of 18

(a)
0.1 1.5
2--Oand HCOCumulative concentrations 43during regeneration (eq/l)0000....00002468 NSCHOOlC-43O2--3- 0001....3692 ulative concentrations during regeneration (eq/l)
S m
, u
-NO3 0 0 −ClC
0 5 10 15 20 25
Bed volumes

(b)
Figure 3. Results of IX regeneration experiments: average (n = 3) cumulative concentrations during
Figure3.ResultsofIXregenerationexperiments:average(n=3)cumulativeconcentrationsduring
regeneration step using A520E (a) and A300 (b) resins (HRT = 14 min)
regenerationstepusingA520E(a)andA300(b)resins(HRT=14min).
3.2. nZVI-Induced Nitrate Reduction
3.2. nZVI-InducedNitrateReduction
3.2.1. Quantitative Determination of the Dominant Reactions
3.2.1. QuantitativeDeterminationoftheDominantReactions
Reduction of nitrate using nZVI as the reducing agent was conducted by adding nZVI slurry to
ReductionofnitrateusingnZVIasthereducingagentwasconductedbyaddingnZVIslurry
the solution attained from regenerating the A520E resin (see Table 1 for solution composition) at
tovtahreiosuosl ucotinodnitaiottnasi.n Tehdef rroesmultrse goebntaeirnaetdin ign tahlle tAhe5 2e0xEperreimsinen(tsse weeTraeb rleou1gfholry ssoimluitliaor,n acso smhopwosni tiino n)at
vaTraiobules 2c. onditions. The results obtained in all the experiments were roughly similar, as shown in
Table2.
Table 1. Characterization of the two solutions used in the NO3− reduction experiment: The cumulative
TaAb5le201E. Crehgaernaecrtaetriiozna tiborninoef t(hafetetwr o24s oBluVt)i;o nansdu sae dsyinntthheetiNc Oso3−lutrieodnu cutsieodn einx ptehreim neZnVt:I Trheeducuctmiounl ative
A5e2x0pEerrimegeenntes rfaotri oenxabmriinnein(ga fttheer e2f4feBcVt o);f aCnld− aabssyenncthe e(stieces Soelcuttiioonn 3u.3s)e. dinthenZVIreductionexperiments
forexaminingtheeffectofCl−absence(seeSection3.3).
Solution SO42− (mg S/L NO3− (mg N/L) Cl− (mg/L) Alkalinity (mg CaCO3/L)
RegSenoeluratitoionn solution SO42−(4m4 gS/L) NO32−53(m gN/L) 45C2l4−6 (mg/L) Alka1l6in2 ity(mgCaCO3/L)
Synthetic solution 64 256 0 281
Regenerationsolution 44 253 45,246 162

Syntheticsolution 64 256 0 281
ChemEngineering2017,1,2 9of19
Table 2. Average operational conditions and results (n ≥ 4) of nZVI-enhanced nitrate reduction
experiments:ratiobetweenmassofrelevantcomponentsattheendoftheexperimentandtheinitial
NO −mass.
3
RatiobetweenAcid MassBalanceError
EndProductConcentration
Exp.No. pHRange Duration Relativeto[NO3−]initial DosagReedanudceNditrate (AAcctuidalCvosn.sAunmtipctiipoante)d
min TAN(%) Fe2+(mol/mol) meq/mgNO3−asN %
#1 2–4 41 90.13 8.074±0.715 1.28±0.09 0.054
#2 3–5 52 91.76 7.277±0.169 1.17±0.02 –253
#3 4–6 38 87.89 6.968±1.007 1.13±0.11 0.939
#4 5–7 30 98.70 4.300±0.291 0.68±0.12 –12.153
#5 3–5 25 100.0 4.673±0.114 0.82±0.01 0.676
#6* 3–5 28 100.0 4.264±0.158 0.73±0.01 –4.172
#7** 3–5 28 100.0 5.540±0.403 0.89±0.06 –4.361
*nZVIproducedviaFe(II)reductionfromasolutionatacompositionsimulatingthefiltrateofStepIII.**ThenZVI
reactionwasconductedinsyntheticsolution,asdescribedinTable1.
Underthetestedexperimentalconditions,almostallthenitrate(typicallyabove90%)wasreduced
to TAN (Table 2). The results show unequivocally that TAN is the main nitrogen species product.
Moreover,basedontheresults,itcannotbeexcludedthatTANwastheexclusiveendproductand
thatnonitrogengaswasformedatall,sincethe~10%differenceobservedbetweentheinitialNO −-N
3
massandthefinalNH +-Nmassfellwithinthemeasurementerror.
4
Acid-basemassbalancecalculationswereconductedinordertodeterminethepercentagesof
nZVIthatwasoxidizedtoproduceTAN,H andN (i.e.,accordingtoEquations(1),(8)and(9),
2(g) 2(g)
respectively)atthevariousexperimentalconditions. Tothisaim,theconcentrationsofthedissolved
productsofthesereactionsweremeasured,andtheaciditymass(i.e.,H+)requiredformaintaining
aconstantpHwasrecorded. Inalloftheexperiments, themassbalancecalculationsshowedthat
thereactionsdescribedbyEquation(1)(reductionofnitratetoTAN)andEquation(8)(Feoxidation
by H+) were the dominant, and likely the sole occurring reactions. The only exception were the
pH5–7experiments,inwhichthesereactionswerethemainones,butpossiblyotherminorreactions
(seeelaborationbelow)alsotookplace. Thisfindingcorroboratesthehypothesisthatthemainend
productofnitratereductionisTANandthattheproductofoxidationofnZVIisdissolvedFe(II)rather
thanFe(III).Massbalancecalculationswereconductedandtheirclosure(intermsofpercentageerror)
isshowninTable2(righthandcolumn). Anexampleofthiscalculationispresentedherein,basedon
theresultsobtainedinExperiment#2(secondrowinTable2): 3.28mmolofTANweremeasuredat
theendoftheexperiment. TheinitialNO − masswas3.57mmol. Itwasassumedthat3.28mmol
3
ofNO − werereducedtoTANaccordingtoEquation(1). SincethemolarratiobetweenTANand
3
Fe(II)inEquation(1)is1:4,3.28×4,i.e.,13.11mmolofFe(II)shouldhavebeenproducedthrough
NO − reduction. However,theFe(II)massmeasuredattheendoftheexperimentwas25.99mmol.
3
Thus, it was assumed that 25.99–13.11, i.e., 12.88 mmol of Fe(II) resulted from iron oxidation by
H+, asdescribedinEquation(8). Accordingtothiscalculation13.11and12.88mmolofFe were
(s)
oxidizedaccordingtoEquations(1)and(8),respectively. Thus,inordertomaintainaconstantpH,
32.77+25.76i.e.,58.53mmolofH+ shouldhavebeenaddedtothesolution,ifindeedEquations(1)
and(8)dominated. Inpractice,57.81mmolofH+weredosedinExperiment#2,i.e.,adifference(or
measurementerror)of−1.25%:
5Fe +12H++2NO − →5Fe2++N +6H O. (9)
(s) 3 2(g) 2
Another example can be given from the results of Experiment #6, in which 0.998 mmol of
TAN were measured at the end of the experiment. The initial measured NO − was 0.942 mmol.
3
Assuming that TAN analysis is more accurate than nitrate analysis, and that TAN was produced
viaEquation(1),3.99(i.e.,0.998×4)mmolofFe(II)shouldhavebeenreleased. Inthisexperiment,
themeasuredFe(II)masswas4.02mmol. Thatis,practicallyalltheFe(II)foundinsolutionwasaresult
ChemEngineering2017,1,2 10of19
ofthereactiondescribedinEquation(1). Therefore,itwasassumedthatfurtherFeoxidationbyH+
(Equation(8))didnottakeplace. AccordingtoEquation(1)thestoichiometricratiobetweenNO −
3
reducedandH+ consumedis1:10;therefore,9.98meqofaciditywereexpectedtobeconsumedto
maintainconstantpH,while9.63meqwereactuallydosed. Alternatively,sincethestoichiometric
ratiobetweenFe(II)formedandH+consumedis0.4:1,4.02/0.4=10.05meqofaciditywereexpected
tobeconsumed(adifferenceof~4.2%). Table2showsthatthedeviationsinallotherexperiments
werealsolow(righthandsidecolumn). TheWilcoxontestresults,appliedtodeterminethelikelihood
thatEquations(1)and(8)accountedsolelyfortheobservedresults,suggestedthattheprobability
of calculated vs. observed differences being drawn from a distribution with zero median is 0.84.
Thisvaluereflectsahighprobabilitythattheexperimentsresultswereaccountedsolelybyusingthe
twoconsideredreactions.
Examinationofthemassbalanceerrors(Table2)suggeststhatatthe(high)pHrangeofpH5–7
otherreactionsalsooccurred,ontopofEquations(1)and(8). Thisisdemonstratedbythehigherrorin
thecalculationoftheanticipatedsupplyofacidityinExperiment#4,(i.e.,−12.15%). Atthisrelatively
highpHrange,thelikelyexplanationisthatsomeFe(II)precipitatedaccordingtoEquation(10)toform
ferroushydroxide,areactionwhichconsumesOH−ions,therebyreducingH+demand. Theinevitable
conclusionisthatTANis,toallpracticalintentsandpurposes,theonlynitrogenendproductofthe
reactionbetweennitrateandnZVIandthatN ,whichisoftenreportedinthisregard,doesnotform,
2(g)
atleastnotundertheconditionsinvestigatedinthiswork:
Fe2++2OH− →Fe(OH) (10)
2(s)
Tosummarize,theresultsshowninTable2leadtothefollowingconclusions: (a)TANandFe(II)
arethemainendproducts,resultingfromEquations(1)and(8);(b)nZVI-enhancednitratereduction
shouldbeconductedatanacidicpHrange,inwhichthenitratereductionrateishigher. Accordingly,
furtherworkwasconductedatpH4±1(i.e.,3<pH<5). ThereasonwasthatatthispHrangethe
oxidationrateofFe(II)byresidualO isconsiderablyslow[35],acidconsumptionforpHreductionis
2
notexcessiveandalsotherateofFe(II)oxidationbyH+isnotveryhigh.
3.2.2. ObservedReactionRates
Ourresultsshowed,asexpected,thatthereactivityofthesynthesizednZVIwasslightlydifferent
fromexperimenttoexperiment. Theminordifferencesinthereactionratecanreasonablybeattributed
to the differences in the nZVI reactivity rather than to the different conditions applied in each
experiment. Nevertheless,theeffectofpHandCl− concentrationonthekineticsofnitratereduction
bynZVIwerefurtherstudied. Figure4showstheevolutionofNspecieswithtimeunderdifferent
experimentalconditions. Evidently,nonitratewasdetectedinsolutionfollowingfivereactionminutes
atthepHrange3–5,irrespectiveofthebackgroundCl− concentration(Figure4a,b). Incontrast,ittook
around 20 min for all the NO − to be reduced at pH 5–7 (Figure 4c). Comparable findings were
3
reportedbyYangandLee[17]whoobservedfasternitratereductionatpH2,3and4,ascomparedto
pH5. Itisnoted,however,thattheconditionsinYangandLee[17]’sexperimentweredifferent,i.e.,
nobackgroundchlorideconcentration,lowerinitialnitrateconcentrationandhigherFetoNO − ratio.
3
DissolvedFe(II)profilesduringthenZVIexperimentsareshowninFigure5. “Totaldissolved
Fe(II)”reflectsthemeasuredamountofferrousironresultingfrombothnitrateandH+reductionwhile
"DissolvedFe(II)fromnitratereduction"representsthecomputedamountofFe(II)resultingsolely
fromnitratereductionaccordingtomeasuredTANandEquation(1). Thedifferencebetweenthetotal
measuredFe(II)andcalculatedFe(II)values(fromNO − reduction)emanatesfromnZVIreduction
3
byH+(Equation(8)). AtpH3–5,theincreaseintheamountofFe(II)resultingfromnitratereduction
levelledoffafterfiveminutes,whilethetotaldissolvedFe(II)massconstantlyincreasedthroughout
theexperiment(Figure5a,b). ThissuggeststhatthereactionbetweennZVIandnitrate(Equation(1))is
fasterthanthereactionbetweennZVIandH+(Equation(8))atpH3–5. Ontheotherhand,atpH5–7,