A review of tungsten: From environmental obscurity to scrutiny

19 Pages·0.313 MB·English

by  Koutsospyros A., Braida W., Christodoulatos C., Dermatas D., Strigul N.

A review of tungsten: From environmental obscurity to scrutiny
A. Koutsospyrosa,b,∗, W. Braidab, C. Christodoulatosb, D. Dermatasb, N. Strigulc
aDepartmentofMechanical,Civil,andEnvironmentalEngineering,UniversityofNewHaven,WestHaven,CT06516,USA
bCenterforEnvironmentalSystems,StevensInstituteofTechnology,Hoboken,NJ07030,USA
cDepartmentofEcologyandEvolutionaryBiology,PrincetonUniversity,Princeton,NJ08540,USA
Availableonline15December2005
Abstract
Sinceitsdiscovery,tungsten,atransitionelementofGroupVIbofthePeriodicTableofElements,anditscompoundshavebeenconsidered
environmentallybenign.ItspresenceinbiologicalanddrinkingwatersamplesinFallon,Nevada,anacutelymphocyticleukemiaclusterstruck
communityhasalarmedpublichealth,environmentalandregulatoryagencies.Tungsten,ametalofextraordinarypropertiesthatmakeithardly
substitutable, is considered an essential commodity with a wide variety of uses stretching from household necessities to highly specialized
applications.Thisworkisundertakeninordertoexploreaspectsofenvironmentalbehavioroftungstenanditscompounds.Occurrencedatain
terrestrial,atmospheric,aquaticandbioticsystemsarepresented.Variousaspectsofenvironmentalchemistry,fatetransportacrossenvironmental
interfacesandtoxicologyarediscussedwiththeobjectiveofidentifyingknowledgegapsandoutliningdirectionsforfutureresearch.
©2005ElsevierB.V.Allrightsreserved.
Keywords: Tungsten;Environmentalbehavior;Fateandtransport;Toxicology
1. Introduction uously,interestonanddemandfortungsten,alreadyanessential
commodity, are projected to increase steadily in the years to
Tungstenisatransitionmetalfound,alongwithchromium, come. Unavoidably, as is the case with other natural materi-
molybdenumandseaborgium,inGroupVIofthePeriodicTable alsand/ornon-renewableresources,increaseddemandanduse
of elements. Since its discovery in the last quarter of 18th of tungsten will spawn (a) increased interactions with other
century, tungsten-based products have been in use in a wide materialsand/ornon-sustainablepractices,(b)agreaternumber
range of applications stretching from daily household neces- of possible entry points into the natural and human environ-
sitiestohighlyspecializedcomponentsofmodernscienceand ment and (c) a higher probability of deliberate or accidental
technology.Asnewapplicationsandusesarediscoveredcontin- releases.
Currently,theexistingknowledgebasedoesnotprovideclear
information about the behavior of tungsten-based products in
Abbreviations: ACGIH,AmericanConferenceofGovernmentIndustrial theenvironment.Thetoxicologicalprofileoftungsten,includ-
Hygienists;ALL,acutelymphocyticleukemia;BCF,bioconcentrationfactor; ingpossibleeffectsonlivingorganismsandexposurepathways,
BLM,bioticligandmodel;BTF,biotransferfactorforafoodproduct;BSAF,
remains rather sketchy, narrow and fragmentary. Regulation
biota/sedimentaccumulationfactor;dimensionless;CDC,CentersofDisease
of tungsten, both in terms of environmental and occupational
Control;CF,soil-to-plantconcentrationfactorofW;DOM,dissolvedorganic
matter;EEC,effectiveenvironmentalconcentration;EPA,environmentalprotec- safety and health, is, at present, limited in comparison with
tionagency;EqP,equilibriumpartitioning;ITC,interagencytestingcommittee; othermetals.Thispatternofenvironmentalobscurityhasbeen
LD50,lethaldosekilling50%ofagroupoftestorganisms;NCEH,National unequivocally disrupted by the events of Fallon, Nevada and
CenterforEnvironmentalHealth;NIEHS,NationalInstituteofEnvironmen-
the possible implication of tungsten to an acute lymphocytic
talHealthSciences(Netherlands);NIOSH,NationalInstituteofOccupational
leukemia(ALL)cluster.Tungstenisnowthefocusofscrutiny
SafetyandHealth;NTP,NationalToxicologyProgram;OSHA,Occupational
SafetyandHealthAdministration;PEL,permissibleexposurelimits;PM2.5,par- asitcurrentlyoccupiesthetopof“todo”listsofvariousregu-
ticulatematterofsizesmallerthan2.5(cid:1)m;ppb,partsperbillion;ppm,partsper latory,healthandenvironmentalagencies.
million;STEL,short-termexposurelimits;TLV,thresholdlimitvalues;TSCA, Thisreviewattemptsto:summarizetheexistingliteratureon
ToxicSubstancesControlAct;TWA,timeweightedaverage
theoccurrence,environmentalchemistryandtoxicology;review
∗ Correspondingauthor.Tel.:[phone];fax:[phone].
the existing regulatory framework in the U.S. and other coun-
E-mail addresses: [email protected], [email protected]
(A.Koutsospyros). tries;identifyknowledgegapsandoutlinedirectionsforfuture
0304-3894/$–seefrontmatter©2005ElsevierB.V.Allrightsreserved.
doi:10.1016/j.jhazmat.2005.11.007
research needs on the environmental relevance of W and W- theleukemiaclusters[12].However,anobjectiveinvestigation
basedproducts. abouttungstensourcesandmobilizationmechanismsisneeded,
beforea“finalverdict”canbereached.Herearesomequestions
2. Background thatneedanswers.Whatisthesourceoftungsten?Isitnatural
oranthropogenic?How,whenandwhatmechanismsmobilized
2.1. Tungstenandleukemiaclusters:reviewoftheevents tungsten?Areanthropogenicactivitiesresponsibleforthismobi-
lization?Isthereevidencefromotherenvironmentalmedia(e.g.
The occurrence of a childhood leukemia cluster in Fallon, biota)supportingmobilization?
Nevada[1]promptedawideinvestigationthatinvolvedseveral Itisdifficulttotakesidesinadebateontheoriginoftungsten
local, state and federal agencies led by the Centers of Disease in Fallon as ores of that element are indigenous to Nevada’s
Control(CDC)[2–7].Inessence,theobjectiveofthisinvestiga- geologicformationsandtheareahasseveral(activeorinactive)
tionwastoassesswhetherenvironmentalcauseswererespon- minesandatungstensmeltingplantthatoperatedinthetarget
sibleforthecluster.The16reportedleukemiacaseswithinthe areaforseveralyears[13].Theresultsofsubsequentbiological
timeframeof1997–2001,waswellabovetheaverageforNevada testinginthreecommunitiesnearFallon[14]alsorevealedhigh
(3.0 cases/100,000 children/5 years). Several possible causes tungstenvaluesinurinesamples,andthusmaybesupportiveof
were proposed, such as jet fuel (JP-8) from a nearby military the“naturaloption”resultingfromexposuretoelevatedvalues
baseorfromaJP-8pipelinerunningthroughthecity,highlev- oftungstenintheregion’swatersupplies.
els of arsenic and other metals in the drinking water supplies, Ontheothersideofthedebate,anthropogenicsourcesmay
industrialpollutionfromalocaltungstensmeltingfacility,and beinpartresponsibleforcontaminationofgroundandsurface
agrochemical contamination resulting from agricultural pesti- water. A tungsten smelting plant in the area operated an open
cide/fungicide use. Although the exact causes of leukemia are airkilnforover20yearsupto1994.Atmosphericdischarges,
notwell-known,geneticand/orenvironmentalfactorsmaytrig- priorto1994whenairpollutioncontrolswereinstalled,could
gerthediseaseincludingionizingandelectromagneticradiation, beapossible,butnotlikelysourceofsoilandwatercontamina-
infectiousandchemicalagents[8]. tion,asprevailingwindsplaceFallonupwindoftheplant[15].
Extensivetestingofdrinkingwater[2],surfacewater,sedi- Theplantwascitedforon/off-sitelandreleasesof240,005and
mentandbiota[3],surfacesoilsandresidentialindoordust[4], 100,000poundsofwaste[16,17].Thecompositionofthiswaste
air[5],aswellascross-sectionalexposureassessmentsofenvi- isnotdisclosedintheinventoryreports,butupto1990,liquid
ronmentalcontaminants[6],andtheJP-8fuelpipeline[7]ruled wastefromacid-leachingoperationswasdumpedintothedesert.
outenvironmentalfactorsasbeingresponsibleforthedisease. However,thesitesofthesereleaseshappentobehydrologically
However,highlevelsoftungstenwerereportedinurine(ashigh downgradientofFallon[15].
as15timesthenationalaverage)[2]anddrinkingwatersamples
(0.25–337(cid:1)gl−1)[7].Thebiologicaltestsalsorevealedelevated 2.2. Overviewofcurrentusesoftungsten
levelsofarsenic,sevenothermetals,andsomeorganophosphate
and organochlorine pesticides. None of these contaminants, Tungstenhasbeenarelativelylittleknowntransitionmetal
however,isknowntocauseleukemiaincludingarsenic,which despiteitsremarkablephysicalandchemicalpropertiesinclud-
hasbeenassociatedwithothercancers,butnotleukemia. ingthehighestmeltingpointofallelements(exceptcarbon),a
◦
Soonthereafter,twoadditionalchildhoodleukemiaclusters boilingpointof5660 C,thelowestvaporpressureandexpan-
appeared, one with 12 cases between 1997 and 2003 in Sierra sioncoefficientofallmetals,veryhighmoduliofcompression
Vista, AZ and the other one with six reported cases in Elk andelasticity,highdensity,thermalandelectricalconductivity.
Grove,CA.Alllocationsarerural,deserttownswithactiveor Tungstenisconsideredtobeoflimitedchemicalactivityand
inactivetungstenminesand/orprocessingoperationsandmili- isoftencitedasnon-reactivewithstrongmineralacidsandwater.
tarybasesnearby.Hightungstenconcentrationswerereported These unique properties, alterable upon demand by alloying
in trees from all three communities [9]. Recent tree ring sam- withcarbonand/orvariousothermetals,maketungstenhighly
plesconsistentlyshowedsignificantincreasesintungstenlevels desirable and hardly substitutable in everyday and specialized
compared to older wood. Trees have been known to translo- applications of modern technology. Consequently, tungsten is
catemetalsabsorbedthroughtherootsorleavesoutwardinthe regardedasanessentialcommodityandametalofhighstrate-
trunkthusconcentratingthemintheyoungertissue.Asaresult gicimportanceforyearstocome[18].
oftheseevents,theCDCandtheNationalCenterforEnviron- Metalworking,miningandconstructionindustriesusetung-
mentalHealth(NCEH)nominatedtungstenfortoxicologyand sten cemented carbides. Tungsten metal wires, electrodes and
carcinogenesisstudies[10]andsomepreliminaryfindingswere contactsareusedinawidevarietyofelectrical/electronicsappli-
publishedrecently[11]. cations. Tungsten heavy metal alloys find uses in heat sinks,
To this date, tungsten has not been the subject of extensive weights/counterweights, super-alloys for turbine blades, wear-
toxicologicaland/orfateandtransportstudiesandremainsone resistantmechanicalpartsandspecializedtools.Chemicaluses
of the least regulated metals. Elevated values in human bio- includecatalysts,high-temperaturelubricantsandinorganicpig-
logical tests, water and tree samples combined with a lack of ments.Widelyknowncivilapplicationsoftungstenincludeuse
toxicologicalinformationpromptedasegmentofthescientific inincandescentlampfilaments,televisionsets,magnetronsfor
communitytosuspecttungstenasthemost“probablecause”for microwave ovens and other electrical consumer products, golf
tally“benign”,anotionthatseemstopermeatethroughmuchof
theexistingenvironmentalregulationsandenvironmentalliter-
ature.
Exposure limits have been established in the U.S. for solu-
bleandinsolubletungstencompoundsinworkplaceatmospheric
environments.Currently,theNationalInstituteforOccupational
SafetyandHealth(NIOSH)permissibleexposurelimits(PEL),
expressed as time weighted average (TWA), stand at 1 and
5mgm−3 of air for soluble and insoluble compounds, respec-
tively.Therespectiveshort-termexposurelimits(STEL)estab-
lished by the same agency are 3 and 10mgm−3 of air [23].
TheAmericanConferenceofGovernmentIndustrialHygienists
Fig.1. U.S.tungstenconsumptionbyenduse. (ACGIH)hasestablishedthresholdlimitvalues(TLV)forinsol-
ubletungstencompoundsat5mgm−3(TWA)anda10mgm−3
clubs, fishing weights and hunting ammunition. Recently, in (STEL). Respective values for soluble tungsten compounds
viewoftheadverseenvironmentalnotorietyofleadandpublic standat1mgm−3(TWA)and3mgm−3(STEL)[24].
concernsaboutdepleteduranium,tungstenhasbeenthefocusof Atpresent,therearenodrinkingwaterstandardsordischarge
severalmilitaryapplicationsincludinghighkineticenergypene- limitsforair,surfacewater,groundwaterorsoilsintheU.S.The
trators,smallcaliberammunitionandarmorplating.TheGreen eventsinFallonandSierraVistaalarmedhealthandregulatory
ArmamentTechnology(GAT)program,aU.S.Armypollution agenciesandaninterimdrinkingwaterthresholdlevelandother
preventioninitiativethatcallsforsubstitutionofleadbytung- tungsten regulations are imminent. Tungsten compounds have
steninsmallcaliberammunition,hasreceivedmuchpublicity. beenincludedintheEPA’stoxicreleaseinventorysinceitwas
Concernsoveradverseenvironmentaleffects[19]havealsoled originally published in 1987. However, tungsten has recently
to the substitution of lead shotshell ammunition and lead fish- been one of thirteen metals included in the CDC National
ingweightsinvariousNorthAmericanandEuropeancountries. ReportonHumanExposuretoEnvironmentalChemicals[25].
In the U.S., the Fish and Wildlife Service, approved the use Inadditiontotheongoingtoxicologyandcarcinogenesisstudy
of tungsten–nickel–iron shots as non-toxic for migratory bird requested by the CDC, and the EPA’s Toxic Substances Con-
huntingeffectiveJanuary4,2001[20]. trolAct(TSCA)InteragencyTestingCommittee(ITC)recently
Theseinitiativeshavegeneratedarenewedinterestintung- included20tungstencompoundsinthePriorityTestingList[10].
stenandtungsten-basedproducts,whichisanticipatedtoboost TheRussianFederationregulatestungstenindrinkingwater
to even higher levels an already increasing demand. For the (0.05mgl−1) and fishing lakes and rivers (0.0008mgl−1)
period 1984–1998, U.S. reliance on foreign sources of tung- [26,27]. From the above discussion, it is evident that, to this
sten materials has increased nearly 40% compared with the date,tungstenanditscompoundsremainamongsttheleastreg-
average for the previous decade [21]. In 2001, U.S. apparent ulatedandleaststudiedmetalsubstances.
consumption of all tungsten materials, calculated on the basis
ofnetimports,primaryandsecondaryproduction,andchanges 3. Environmentalchemistryandbehavior
in Government and industry stock levels, was 14,600 metric
3.1. Physicalandchemicalpropertiesoftungsten
tonnes, virtually unchanged from that of 14,400 metric tonnes
tungstencontentin2000.U.S.tungstenconsumptionbyenduse
Table1summarizestherelevantphysicalandchemicalprop-
isgraphicallyshowninFig.1.
ertiesoftungsten.Adiversityofoxidationstates(rangingfrom
In2001,worldproductionwasat44,200metrictonnestung-
−2 to +6) and coordination numbers (5–9) make the chem-
sten content, approximately 83% of which was contributed by
istry of tungsten one of the most varied and complex among
China[21].Theamountoftungstenconsumedinproductsfor
the transition elements. Consequently, numerous possibilities
defenseandmunitionswillincreasesubstantiallyoverthenext
exist for formation of soluble complexes with many inorganic
severalyears.Itisestimatedthatincrementaltungstenconsump-
and organic ligands (aqua-, oxo-, halide-, organo- and mixed)
tionbasedonapproveddefenserelatedprojects,includingGAT
[32]withimplicationsforenhancedmobilityinsurfaceandsub-
willrangebetween2200and2700metrictonnestungstencon-
surfaceaquaticenvironments.Unliketungstenforwhichsome
tentbytheyear2006.TheGATprogramalonewouldgenerate
physicalandchemicalpropertiesareavailableintheliterature,
anadditionaldemandof1300–1500metrictonnestungstencon-
information on many other tungsten compounds is scarce and
tentperyear[22],representingabout10%ofthecurrenttungsten
thisisoftenproblematicforanestimationandpredictionofthe
demand,rankingthirdbehindcementedcarbideandmillproduct
environmentalbehaviorofthesecompounds.
enduses.
3.2. Occurrenceoftungsteninenvironmentalsystems
2.3. Regulatorystatus
3.2.1. Terrestrialsystems
Muchoftherevivedinterestintungstenandtungsten-based Theearth’scrust,byfarthemostsignificantsourceoftung-
productswasbasedonthepremisethattungstenisenvironmen- sten in the ecosphere, is estimated to contain 0.00013% W on
Table1 Table2
Physicalandchemicalpropertiesoftungsten(compiledfromreferencedsources) Tungstenoccurrenceinterrestrialsystems
Property Value Terrestrialsystem Concentration Reference
(mgkg−1)
Atomicnumber 74
Averageatomicmass,gmol−1 183.85 Earth’crust 1.55 [18]
Atomicvolume,cm3/mol 9.53 Lithosphere 0.1–2.4 [36,37]
Naturallyoccurringisotopes,(%) 180(0.135%);182(26.4%);183 Surfacerocks 1.0–1.3 [33,34]
(14.4%);184(30.6%);186(28.4%) Surfacesoils 0.68–2.7 [37,38]
Paulingelectronegativity 2.36 Topsoils(Fallon,NV,USA) 10.0–67.0 Thiswork
Vanderwaalsradius,nm 0.137 Topsoil(nearmining/smeltingsites, 56.0 [41]
Ionicradius,nm 0.068(+4);0.067nm(+6) Australia)
Electronicshell [Xe]4f145d46s2 Fertilizedagriculturalsoils(Europe) 0.5–83 [37]
Energyoffirstionization,kJmol−1 768.6 Agriculturalsoils(NewZealand) 1.9–21.4 [39]
Standardpotential,V(W+4/W) −0.05 Agriculturalsoils(Iowa,USA) 0.0–2.0 [40]
Oxidationstates −2,−1,0,+2,+3,+4,+5,+6 Deepersoil(nearmining/smelting 78.4 [41]
Heatofvaporization[28],kJmol−1 824 sites,Australia)
Heatoffusion[28],kJmol−1 35.4 Surfacesoils(afterGulfWarinSaudi 126.5 [42]
Specificheat[28],J/gK 0.13 Arabian–Kuwaitborder)
Coordinationnumbers[29] 5,6,7,8,9 Surfacesoils(300kmawayfrom 3.25 [42]
Electricalresistivity[28],(cid:1)(cid:1)cm 5.5 SaudiArabian–Kuwaitborder)
Thermalconductivity,W/cmK 1.74 Militaryfiringrangesoils(estimate 5200 [43]
Meltingpoint,◦C 3410 basedonlead)
Boilingpoint,◦C 5660 Navalfiringrangesoils(estimatefor 5500 [44]
Vaporpressure[30],Pa(3407◦C) 4.27 tungsten–tantalummunitions)
Density,gcm−3at20◦C 19.3 Huntinggrounds(estimate) 25.7–58.3 [45]
Appearance[31] Hard,brittle,steel-graytotin-white
solid
Additional properties may be found in Properties of Tungsten at (10–67mgkg−1) of W content in topsoils collected from four
http://www.tungsten.com/mtstung.html. random locations in Fallon, Nevada within a few miles from
radiusfromanabandonedtungstenmineandasmeltinglocation.
a mass basis [18], equivalent to an average (background) W Inflated W contents (126.50mgkg−1) of surface soils
concentrationof1.3mgkg−1.Basedonthisfigure,Woccupies (0–5cm depth) have been reported in areas close to the Saudi
the57th/18thpositionsintheoverallelement/metalabundance Arabian–Kuwaitborderasaresultofcombatoperationsduring
lists.SurfacerocksreportedlycontainWatlevelsintherangeof the first Gulf War (1990–1991) whereas the W content of sur-
1.0–1.3mgkg−1[33,34].Interrestrialsystems,Wexistsalmost facesoils(3.25mgkg−1)inareas300kmawayfromtheborder
exclusivelyintheformofoxo-richtungstatemineralseitheras remained at levels comparable to natural background concen-
scheelite(CaWO )orwolframite([Fe/Mn]WO ).Hydrothermal tration [42]. Other sites of military activity with potentially
4 4
mineralizationandmetamorphicprocessesareprimarilyrespon- hightungstenconcentrationareshootingrangesusingtungsten-
siblefortheformationofWdepositsinsubterraneansystemsand basedammunition.Currently,thereareseveralshootingranges
aresubsequentlytransportedviahigh-temperaturehydrothermal in the U.S. where tungsten based ammunition has been used
veins.Amoredetailedtreatmentofgeochemical/mineralogical on an experimental/developmental basis. Detailed information
aspectsofWisprovidedinRefs.[34–36]. on firing range site numbers, locations or tungsten soil con-
LiteratureassessingbackgroundWconcentrationlevelsfor tent levels are not available. However, if the level of tungsten
soilsystemsgloballyisnotavailableandonlyalimitednumber is similar to that of its lead predecessor, then typical concen-
ofstudiesexistonalocalscale.StudiesintheEuropeanUnion trationsof5200mgkg−1 andashighas200,000mgkg−1 [43]
have reported W concentrations in soils (0.5–83mgkg−1 dry can be expected. In fact, this typical concentration value is
mass) and surface soils (0.68–2.7mgkg−1 dry mass) [37,38]. closetoanestimateof5500mgkg−1reportedbytheAirForce
ThesamestudieshavereportedWcontentlevels(inmgkg−1) Research Laboratory for surface soils in firing ranges where
in the lithosphere (0.1–2.4), phosphate and phosphorite rocks tungsten–tantalum penetrator munitions are used [44]. These
(30–270), limestones and carbonate rocks (0.2–0.8), sewage values,threeordersofmagnitudehigherthanbackgroundval-
sludge (1–100), manure (8–2800) and a variety of fertiliz- uesinsoils,mayprovetobeofenvironmentalconcern.Tungsten
ers (ND-100). Elevated values in agricultural soils from New recently approved for hunting migratory birds may also pose
Zealand(1.9–21.4mgkg−1)maybeattributedtoagrochemical a concern in civilian firing ranges and hunting grounds. Data
practices[39].Conversely,valuesintherangeof0–2mgkg−1 are currently scarce, but the Department of the Interior esti-
reportedforagriculturalsoilsinIowaareincloseagreementwith mateseffectiveenvironmentalconcentrations(EEC)ofvarious
thebackgroundconcentrationof1.3mgkg−1[40].Highaverage tungsten-based shots for terrestrial ecosystems in the range of
Wconcentrationshavealsobeenreportedfortop(56mgkg−1) 25.7–58.3mgkg−1 [45]. Table 2 summarizes occurrence data
anddeepersoil(78.4mgkg−1)formationsinthevicinityofmin- forWinterrestrialsystems.
ing/smeltingsitesinNorthQueensland,Australia[41].Aspart Natural and anthropogenic transformation and transport
ofanindependentinvestigationourgroupdetectedsimilarlevels mechanisms are responsible for the occurrence of W and its
compoundsintheotherecospheredomains(hydrosphere,atmo- amountsinmunicipalsolidwasteaswellasinlandfillleachates
sphere and biosphere). Principal transport and transformation isnotsurprising,eventhoughspecificstudieshavenotbeencon-
mechanisms of relevance include deposition (wet and dry), ducted.Thetoxicologicalbehavioroftungstenhexacarbonylis
advectivetransport,colloidaltransport,chemicalprecipitation, notknown.However,accordingtotheauthorsacomparisonwith
oxidation/reduction,dissolution,complexation,adsorptionand nickeltetracarbonyl[Ni(CO) ]isinevitablegiventheproximity
4
anionexchange.Anthropogenicactivitiesthatmaycontributeto ofthetwometalsontheperiodictableofelements[55].Inview
tungsten mobilization include pollution resulting from mining ofthefactthatexposuretonickeltetracarbonylisregulated,and
and industrial operations, military combat/training operations incertaincountriesitisevenconsideredacarcinogen,concerns
usingW-containinghardware,agrochemicalpractices,suchas aboutapossiblesimilarbehavioroftungstenhexacarbonylcan-
applicationofW-containingfertilizers,non-sustainabledisposal notbereadilydiscounted.
practicesofW-containingsubstances(e.g.disposaloflightbulbs Othertungstencompoundsofrelevanceforatmosphericenvi-
in landfills, land application of wastewater residuals) or other ronmentsincludetungstencarbide(WC),tungstenhexachloride
substancesthatcouldpotentiallyinteractwithW. (WCl ),tungstenhexafluoride(WF )andtungstenoxidefibers
6 6
(WO ),Althoughvaporpressureorothersolid/liquid–gaspar-
x
3.2.2. Atmosphericsystems titioning data for most of these compounds are not generally
◦
Tungsten,withavaporpressureof1mmHgat3000 C,isthe available,aerosolsandparticulatesareassumedtobetherele-
leastvolatileofallmetals.Basedonitslowvaporpressureand vantW-compoundbearingfractionsingaseousstreams.Certain
atmosphericinterferencefactor(calculatedastheratioofanthro- W-compounds, however, can exist in the vapor phase either
pogenicovernaturalemissionstotheatmospheremultipliedby because of volatility (e.g. tungsten carbonyl has a vapor pres-
◦
100), tungsten is considered to be a lithophilic element (pre- sure of 0.1mm Hg at 20 C), or because they are gases under
ferring terrestrial over atmospheric or aquatic environments). normal conditions (e.g. tungsten hexafluoride is a gas at room
Sub-nanogram concentrations have been recorded in pristine temperature).Ampleliteratureimplicatesseveralofthesecom-
areas(Arctic/Antarctic)aroundtheglobe[46,47].Tungstencon- poundsininterstitialpulmonaryfibrosis(“hardmetaldisease”).
centrationsinambientairaregenerallylessthan1ngm−3,but Muchofthisliteratureoriginatesfromworkplaceenvironments
increase moderately in urban, mining/metal processing areas. inseveralcountriesaroundtheglobewithtungsten-richdeposits
Air sampling studies over six U.S. cities report atmospheric and/or sites of intense related mining/industrial activities and
levels of tungsten in the range of 0.1–2.0(cid:1)gm−3 [48], while is reviewed in references [9,24,56,57]. Concentrations close
other studies performed in urban areas of East Chicago, Indi- to 8.0mgm−3 in atmospheric workplace environments have
anaandMontreal,Que.,Canadahavereportedvaluesof6.0and been reported [56]. W-oxides (WO ) have attracted consider-
x
5.2ngm−3,respectively[49,50].Amoresystematicstudycon- able attention as fibers of these materials, greater than 0.5(cid:1)m
ductedineighturbanareasofCanadareportedameanWcon- size, may be generated in high numbers (>38.3fibersml−1)
centrationof0.0039(cid:1)gm−3 intheparticulatefraction(PM ) industyworkplaceenvironmentsduringvariousW-production
2.5
ofatmosphericsamples[51].Wlevelsintheplumesoffivecop- stages[56].AlthoughtheexactroleofW-oxidesinlungfibro-
persmeltingplantsinsouthernArizonavariedfromlessthan1 sis is still debated [58], W-oxides are currently under toxico-
to23ngm−3 [52].Alimitednumberofstudiesreportedvalues logical and carcinogenicity scrutiny [59]. Additional concerns
oftungsteninindustrialemissionstreams.Wlevelsintherange involving occurrence, fate and transport of W-oxides in atmo-
of5–21(cid:1)gg−1(ppm)wererecordedinflyashfromamunicipal sphericenvironmentsmayariseasaresultofnovelapplications,
wasteincineratorinBarcelona,Spain[53].TheWcontentdeter- e.g.conductancetypegassensors[60],photochemicalaerosol
mined in the smokestack gases of two coal-fired power plants generators [61,62], particle accelerators [63] and nanomateri-
varied in the range of 2.0–23.2(cid:1)gm−3 [54]. The local scale als [64] of W-oxides (particularly WO ) as a result of their
3
andlimitednumberofthesestudiescannotprovideconclusive unique electro-optical, electrochromic, ferroelectric, catalytic
evidence that supports atmospheric enrichment resulting from properties,electricalconductivityandadsorptionproperties.A
tungstenindustrialemissions. summary of occurrence data for W in atmospheric systems is
Consideringtungsten’slithophilicnature,researcherstendto givenonTable3.
reasonthatthelimitedtungstenenrichmentobservedonaglobal
andregionalscale,relativetoitsnaturalabundanceontheEarth 3.2.3. Aquaticsystems
crust, is not of anthropogenic origin. However, recent studies W can be released to aquatic systems via multiple natural
indicatethattungstencanbetransformedviamultiplereactions or anthropogenic routes from terrestrial, atmospheric and
tosolubleorevenvolatilespecies.Evidenceofthelatterispre- bioticenvironments.Anon-exhaustivelistofnaturalprocesses
sentedinastudyontheoccurrenceofvolatiletransitionmetal includes weathering of W-rich rocks and soils, dissolution,
compoundsinthreemunicipalwastelandfills.Tungstenhexacar- hydrothermal and volcanic activity, atmospheric precipitation
bonyl(W(CO) )wasmeasuredatconcentrationsintherangeof (wet and dry) and excretion of metabolites. Anthropogenic
6
0.005–0.01(cid:1)gofWperm3 ofgas[55].Itisclaimedthatreac- sourcesincludeavarietyofindustrial,commercialandmilitary
tions of metallic tungsten with carbon monoxide produced by activities along with non-sustainable disposal practices of
anaerobic reactions within a landfill environment could be the municipal, agricultural and industrial wastes. Unlike the
source.Sincemetallictungsteniswidelyusedincommonhouse- atmospherewhereparticulateformsaredominant,bothsoluble
hold products, such as light bulbs, its presence in detectable and particulate W forms can exist in aquatic environments,
Table3 Tungsten enrichment in surface and groundwater resources
Tungstenoccurrenceinatmosphericsystems resultingfromanthropogenicactivitiesisevidentinthelimited
Atmosphericsystem Concentration Reference number of studies available in the literature. Average W con-
(ngm−3unless centrationsof0.05,0.15and0.08(cid:1)gl−1havebeenreportedfor
otherwisenoted) thewatersofthreerivers(Mahanadi,BrahmaniandBaitarani)
Air(pristineareasArctic/Antarctic) <1.0 [46,47] flowing through industrial areas of the Indian peninsula [75].
Air(6majorcitiesinUSA) 0.1–2.0(cid:1)g/m3 [48] Data have been presented for two rivers in Japan, Tamagawa
Air(PM2.5from8urbanareasCanada) 3.9 [51] andSagamigawa,theformerflowingthroughaprimarilyindus-
Urbanareaair(EastChicago,IN,USA) 6.0 [49]
trial basin discharging in Tokyo Bay and the latter flowing
Urbanareaair(Montreal,Canada) 5.2 [50]
through an agricultural basin discharging in Sagami Bay. The
Smeltingplantemissions(AZ,USA) 1–23 [52]
Flyash(incineratorBarcelona,Spain) 5–21mgkg−1 [53] W load of the Tamagawa River (industrial area) is distributed
Smokestackgases(coal-firedpower 2.0–23.2(cid:1)gm−3 [54] approximately equally in the particulate (0.006–0.104(cid:1)gl−1)
plant) and dissolved (0.038–0.107(cid:1)gl−1) fractions and is generally
Landfillgas((W(CO)6reportedasW) 0.005–0.01(cid:1)gm−3 [55] enriched along the reach of the river. For the Sagamigawa
Atmosphericworkplace(Wsmelting 8.0mgm−3 [56]
River(agriculturalarea),theWcontentismostlydistributedin
facility)
thedissolvedfraction(0.027–0.028(cid:1)gl−1)withsmallamounts
present in the particulate (0.003–0.004(cid:1)gl−1) fraction and is
the former being of higher environmental concern because of virtually constant throughout its reach. The authors of Ref.
highermobilityandtoxicity.Wexistsnaturallyinoceanwater [76] suggest that W can exist in oxy-anionic soluble forms
and sediments, surface water bodies and groundwater in areas (primarily WO 2−), but also as species associated with the
4
of hydrothermal activity while anthropogenic emissions and small inorganic colloids. Similar values have been reported
transportfromothersystemsareresponsibleforWpresencein by the same group of researchers for three other rivers in
otheraquaticsystems. Japan [77]. Very high W enrichment has been documented
W is present in oceanic waters in trace amounts gener- in the sediment of a reservoir in a mining area of Germany
ally at levels below 1ngkg−1 [65], whereas a typical value of (near Dresden). W levels of 52mgkg−1 reflect an enrich-
0.2ngkg−1 is most often cited [66]. W content for Northern ment factor of 9 within a period 81 years [78]. W detected
Atlantic and Pacific Oceans reported in the literature are 100 in trace amounts in the particulate (0.14ngl−1) and dis-
and 8ngl−1,respectively [67].Duetothelackofotherverifi- solved(0.76ngl−1)fractionsofrainwaternearbyahard-metal
able data, the elevated value for the Northern Atlantic Ocean industrial plant in Russia was attributed to atmospheric emis-
cannotbeattributedwithcertaintytoenrichmentreflectingthe sions from that plant [79]. W was found in roadside soils
intensiveindustrialactivityknowntoexistinthisregionofthe (7.40–63.9mgkg−1) [80] and runoff (<25mgl−1) [81,82] at
globe.Althoughdataarenotreadilyavailable,deephydrother- high concentrations possibly indicating an impact from stud-
mal vents and marine sediments are known environments of ded tires. Sequential extraction data provided in these stud-
increasedtungstenavailability.Datawererecentlypublishedfor ies show that W is roughly distributed equally in the residual
hydrothermalventfluidsinthreeknownlocationsintheIndian (48.0%)andoxidizablefractions(47.6%)withminoramounts
Ocean(0.039(cid:1)gkg−1),NorthPacificOcean(2.8(cid:1)gkg−1)and present in the reducible (3.3%) and acid/exchangeable (1.1%)
EastChinaSea(22.6(cid:1)gkg−1).Thelatterrepresentsanincrease fractions.
offourordersofmagnitudeoverthebackgroundseawaterlevel The presence of W and its compounds is not considered
[68].Similarhighvaluesformarinesediments(10–60(cid:1)gkg−1) likelyforthemajorityofpublicdrinkingwatersuppliesinthe
arereportedinliteraturecitedbyRef.[69]. U.S. and other economically advanced countries. However, in
Surfaceandgroundwaterinareasoftungstendeposition,or theabsenceofanykindofregulationinaquaticenvironments,
hydrothermalactivityaswellasinalkalinelakesinaridregions W is not part of routine testing programs. With the exception
arealsoknowntocontainelevatedvaluesofW.Valuesashighas of some Russian studies [83–85] that probably served as the
70mgkg−1havebeenreportedforconcentratedbrinesofSear- basisofWregulationinaquaticsystemsforthatcountry,litera-
les Lake, CA [70]. Samples analyzed at multiple points along turespecificallydealingwithoccurrenceofWindrinkingwater
thereachofthreeriversystems(Truckee,WalkerandCarson)in suppliesisratherlimited.AFrenchstudyperformedinground-
NevadashowedWlevelsintherangesof0.319–72.1,0.15–4.07 water, used as production source for bottled mineral water, in
and1.51–189.7(cid:1)gl−1,respectively[71].Asystematicanalysis several sites in Vichy reported an average tungsten content of
ofgroundwaterfromanaquifer,locateddowngradientfromthe 62.73(cid:1)gl−1 (range: 0.4–112(cid:1)gl−1) with no statistically sig-
potential nuclear waste repository at Yuca Mountain, Nevada, nificantdifferencebetweenWlevelsingroundwaterandbottled
generatedresultsthatvariedintherangeof0.10–4.36(cid:1)gkg−1 water[86].TheeventsinFallon,Nevadapromptedseveralinves-
[72].StudiesonNahanniRiverwater,hotspringsandground- tigations of the town’s drinking water supply. W was detected
water in scheelite/wolframite bearing areas of the Northwest inall77tapwatersamplesinconcentrationsrangingfrom0.25
Territories of Canada showed W concentrations as high as to337(cid:1)gl−1withanaverageconcentrationof19.9(cid:1)gl−1[87].
224.5(cid:1)gl−1[73].Valuesintherangeof0.005–11.5(cid:1)gl−1were The highest W levels were detected in households using pri-
reported in glacial and thermal waters in areas known to bear vatewells.Aspartofourindependenttestingconductedatsix
formationsofnaturalWmineralsinNorthernIceland[74]. random sites in the town of Fallon in the summer of 2003,
Table4 Table5
Tungstenoccurrenceinaquaticsystems Tungstenoccurrenceinplantsystems
Aquaticsystem Concentration Reference Plantsystem Concentration Reference
(ngkg−1unless (mgkg−1unless
otherwisenoted) otherwisenoted)
Oceanwater 1 [65] Treesandshrubs(RockyMountains) 5–100 [91]
Oceanwater(typical) 0.2 [66]
Eucalyptus(NorthQueensland,Australia)
NorthernAtlanticOcean 100 [67]
Leaves 13.6 [41]
PacificOcean 8 [67]
Youngstems 2.9 [41]
Hydrothermalventfluids(Indian 39 [68]
Oldstems 4.3 [41]
Ocean)
Hydrothermalventfluids(North 2800 [68] Europeanbeechtreeleaves 7–50 [93]
PacificOcean) Oaktreebark(CzechRepublic) 0.129–4.79 [94]
Hydrothermalventfluids(East 22600 [68] Onions(Denmark) 6.3–39(cid:1)gkg−1 [95]
ChinaSea) Berries(Sweden) 0.22–7.2 [96]
Marinesediment 10–60mgkg−1 [69] Cabbage 0.2–1.7 [97]
Concentratedbrines(Searles 70mgkg−1 [70]
Lake,CA,USA)
TruckeeRiverwater(NV,USA) 0.319–72.1(cid:1)gl−1 [71]
WalkerRiverwater(NV,USA) 0.15–4.07(cid:1)gl−1 [71] 3.2.4. Bioticsystems
CarsonRiverwater(NV,USA) 1.51–189.7(cid:1)gl−1 [71]
Wcanbetransportedtobioticsystemsfromterrestrial,atmo-
NahanniRiver(river,hotsprings, 224.5(cid:1)gl−1 [73]
spheric or aquatic systems via several mechanisms including
groundwaterNorthwest
Territories,Canada) uptakebyplants,treesandvegetativematter,bioaccumulation
CarsonRiverwater 1.5–23(cid:1)gl−1 [72] inmicrobialandanimalorganisms,sorption/desorption,forma-
Groundwater(aquifer 4.36(cid:1)gl−1 [72] tionoforganotungstencompoundsandsubsequentpartitioning
downgradientofYuca
inorganicmatter,andexcessmetalsequestration.
mountain,NV,USA)
Glacialandthermalwaters 0.005-11.5(cid:1)gl−1 [74] The role of tungsten in biological systems has been estab-
(NorthernIceland) lished only recently. For years there existed a notion that this
MahanadiRiver(India) 0.05(cid:1)gl−1 [75] element,acloserelativeofmolybdenumwhoseroleinmolyb-
BrahmaniRiver(India) 0.15(cid:1)gl−1 [75] doenzymes is well established, has a functional role in living
BaitaraniRiver(India) 0.08(cid:1)gl−1 [75]
matter. Mixed indications that the presence of W may cause
TamagawaRiver(industrialarea,Japan) growthenhancementormoderatetoxicitytocertainmicrobial,
Particulatefraction 0.006–0.104(cid:1)gl−1 [76] plant,andanimalspecieshavebeenreportedinRefs.[88,89].A
Dissolvedfraction 0.038–0.107(cid:1)gl−1 [76]
thoroughreviewontungsteninbiologicalsystemswithnumer-
SagamigawaRiver(agriculturalarea,Japan) ous references, covering microorganisms, plants and animals,
Particulatefraction 0.003–0.004(cid:1)gl−1 [76]
has been published fairly recently [68]. In this section, only a
Dissolvedfraction 0.027–0.028(cid:1)gl−1 [76]
summaryandselectedworkspublishedinthelast10yearsper-
Sedimentinreservoir(mining 52mgkg−1 [78] taining to the occurrence of W in plants are presented. For a
area,Dresden,Germany)
discussion of the occurrence of W in other constituents of the
Rainwater(hard-metalplant,Russia)
Particulatefraction 0.14ngl−1 [79] bioticenvironment,thereaderisdirectedtothetoxicologysec-
Dissolvedfraction 0.76ngl−1 [79] tion of this review. A summary of occurrence data in various
Drinkingwater(Vichy,France) 0.4–112(cid:1)gl−1 [86] plantsisprovidedinTable5.
Tapwater(Fallon,NV) 0.25–337mgl−1 [87] Plantsareknowntotakeup(possiblyinanionicform,WO42−
Tapwater(Fallon,NV) 64–135mgl−1 Thiswork [90])andaccumulatetungsteninsubstantialamounts.Theextent
Drinkingwaterwells 0.27–742(cid:1)gl−1 [19] ofaccumulationappearstobedirectlyrelatedtotheWcontent
of the soil irrespective of the source nature (natural or anthro-
pogenic) and varies widely depending on the plant genotype.
W was detected in samples in concentrations varying in the Certainplants(e.g.tomatoes)actasexcludersofW,particularly
range of 64–135(cid:1)gl−1. A follow-up study has recently pub- in the presence of Mo [68]. Concentrations of 5–100mgkg−1
lished results of extensive testing of 100 drinking water wells were found in trees and shrubs in the Rocky Mountains [91],
from all aquifers in the Fallon area, where W concentration whereasvaluesbelow1mgkg−1appeartobemostcommonin
rangedfrom0.27to742(cid:1)gl−1 [15].Thisstudysuggestedthat otherplanttypes[92].ElevatedvaluesofWhavebeenreported
thesourceofWinthestudyareaisnaturalandidentifiedfour in the leaves (13.6mgkg−1), young stems (2.9mgkg−1)
majorcontributingmechanisms,namely(a)evaporativeconcen- and old stems (4.3mgkg−1) of eucalyptus trees (Eucalyptus
tration in river water, (b) intermixing with geothermal waters, melanophloia)inthevicinityofaninactivewolframiteminein
(c)dissolution/precipitationofWmineralsinsedimentsand(d) NorthQueensland,Australiacomparedtocontrolvaluesof0.1,
adsorption/reductivedissolutionofWinFeandMnoxyhydrox- 0.0and0.1mgkg−1,respectively[41].Wdetectedintheleaves
ides.OccurrencedataforWinaquaticsystemsaresummarized of European beech trees (Fagus sylvatica) varied in the range
inTable4. of 7–50mgkg−1 and was attributed primarily to atmospheric
deposition[93].ACzechstudyreportedameanWconcentration of complexing/precipitating agents, alloyed metallic elements
of0.775mgkg−1 inoaktreebark(range0.129–4.79mgkg−1) orinterfaces(e.g.soilparticles).W,abasemetalwithadomain
[94].Informationhasbeenpresentedearlieronelevatedvalues of stability below that of water, under alkaline conditions has
ofWintreeringsamplestakenfromFallon,Nevadaandother a tendency to dissolve WO 2− while forming non-protective
4
ALLstruckcommunities[9].Limitedliteratureexistsonfood oxidelayers.AccordingtoLassnerandSchubert[32],thereac-
crops.Wwasdetectedinonionsharvestedinvarioussites(Den- tion of metallic tungsten with distilled water is very slow at
mark)atameanlevelof16.7(cid:1)gkg−1freshmass(n=64;range, low temperature (38◦C), corresponding to a corrosion rate of
6.3–39(cid:1)gkg−1) [95], in two wild berry species in Sweden at 3.8(cid:1)gm−2h−1 and resulting in an equilibrium tungstate con-
levels in the range of 0.22–7.2mgkg−1 [96] and in cabbage centration in solution of 10−3M (equivalent to 184mgl−1 of
(0.2–1.7mgkg−1)growninflyash-amendedsoils[97]. W). The environmental behavior of tungsten once it dissolves
AlthoughcertainplantsarecapableofactingasWaccumu- becomesverycomplexasthetungstateanionoccursmonomer-
latorsorexcluders,planttoxicityhasalsobeenreportedinthe icallyonlyinalkalineorneutralsolutions.Underevenslightly
literature. Accumulation and toxicity of W on rye grass was acidicconditionstungstatestendtopolymerizetoformisopoly-
reported recently [98]. The accumulation increased as the W tungstateswithpossiblebiotictoxicityimplications[101,102].
soilconcentrationincreasedfrom0.1to10,000mgkg−1.Atthe TheabilitytopolymerizeiscommonforelementsinGroups
highest soil content level, toxicity became apparent through- VandVIofthePeriodicTable.Themonomericstateoftungstate
out the life of the plant and resulted in death, approximately isonlystableatpH≥6.2insolutionsfreeofcomplexingagents.
4 weeks after germination. Accumulation of W, investigated A simplified scheme of the tungstate polymerization process
using three species of Brassica (fast plants, red cabbage and alongwithrelevantpHandtimescalesisshowninFig.2[103].
Indianmustard),wasattributedtoanthocyanins[99].Thesame SpeciationcanalsobedescribedintermsofpC–pHdiagrams.
study concluded that replacement of Mo in nitrate reductase However,databasesfortungstenspeciesarenotreadilyavailable
by W is a likely toxicity mechanism and that sequestration of as most of these species are difficult to be determined analyt-
excessmetalsintheperipheralcelllayersappearstobeacom- ically. The speciation diagram shown in Fig. 3 was produced
monmechanismofplantmetalaccumulation.Sequestrationof usingVISUALMINTEQVersion2.3.Table6liststhespecies
metalsintissueswherethesesubstancesarelessharmfultothe considered along with their respective equilibrium constants.
plant,orplantdefenseagainstpathogensorherbivoresaretwo Therunswerebasedona0.005Mtotalconcentrationofsodium
plausibleexplanations. tungstate in a 0.01M solution of NaCl allowing solid species
(such as tungstic acid) to precipitate. Smith and Patrick [104]
3.3. Environmentalchemistryoftungsten studied the speciation equilibrium of sodium metatungstate
(Na [H W O ])inthepHrange4.5to10usingNMRspec-
6 2 12 40
Tungsten, with oxidation states ranging from −2 to 6 and troscopy.Sodiummetatungstatewasfoundtodecomposewith
coordinationnumbersfrom5to9,iscapableofformingalarge the addition of a base to paratungstate A (W O −6), which
7 24
number of soluble complexes with a variety of inorganic and slowlyconvertedtoparatungstateB(H W O −10).Equilibra-
2 12 43
organic ligands [32,100]. For instance, tungsten(V) gives the tionmaytakeupto8monthstobeachievedandthemajorstable
octahedral halogen complexes WX − (X=F, Cl and Br) and polytungstates species are the monomeric WO −2, W O −6,
6 4 7 24
alsoWF 3−.ForoxidationstateIII,tungstenformsface-sharing H W O −10 (solid phase) and H W O −6. The environ-
8 2 12 42 2 12 40
bioctahedralspeciesoftheformWX 3−inwhichstrongW≡W mentalimplicationsofthesetransformationsarenotaddressed.
9
triplebondsareformed.Numerousmixedligandcomplexesof However,fromtheavailableliteratureandmodelingstudies,it
the type [WXnL6−n] (L=ligand molecule other than halogen) appearsthattungstateistheprincipalsolublespeciesinalkaline
are also formed. Aqua and oxo complexes of tungsten are of environments,whereasanumberofpossibilitiesforpolymerized
special interest for assessing its subsurface mobility. From the formsprevailatacidicenvironments.Thesituationininfinitely
W O 4−aquaionmanycomplexescanbeobtainedbyreplacing complicatedinthepresenceofredox,complexingandprecipi-
3 4
someorallofthenineouterwatermoleculesbyotherligands. taingagents,interfacesandorganiccompounds.
Examplesarethe[W O F ]5−and[W O L ]4+,whereLisan Short-termdissolutionofWandWO indistilledwaterhas
3 4 9 3 4 3 3
organicligand.Tungsten(VI)formsafewoxocomplexes,such recently been reported. WO equilibrium in distilled water,
3
as[WOF −],[WO F 2−],[WO Cl 2−]and[WO F 3−][32]. attained within 48–72h, results in concentrations of about
5 2 4 2 4 3 3
The reactions of W with water play a major role in the 500(cid:1)gl−1W in the aquatic phase, while metallic W, after 9
behavior and mobility of this element and its compounds in days of equilibration, still dissolves at a rate of 500(cid:1)gday−1
the environment. Potential-pH equilibrium diagrams and per- [105,106].Inbothcasesthedissolutionprocessisaccompanied
tinent reactions of W-water systems, under conditions of ther- by a significant pH drop and dissolved oxygen depletion. The
◦
modynamic equilibrium at 25 C, in the absence of complex- effect of the presence of alloying elements (Co, Fe and Ni)
ing/precipitating substances, taking into account the WO 2− on W dissolution was also reported in long-term dissolution
4
andthesolidsubstancesW,WO ,W O andanhydrousWO , experiments [106]. Dissolved oxygen depletion accompanies
2 2 5 3
have been generated by Pourbaix [100]. Although these con- the dissolution process regardless of alloy composition and
straintslimitseriouslypragmaticinferencestonaturalsystems, initialpH,whereasapHdropisobserved,whentheinitialpH
the diagrams clearly indicate the complexity of the chemical is alkaline or neutral, but reverses direction for acidic initial
behavior of tungsten in aqueous solutions even in the absence pHs. Although a detailed mechanism for W dissolution is not
Fig.2. TungstenspeciationasafunctionofpH(basedonRefs.[32,103]).
elucidated in [106], it is suggested that anodic oxidation of Upondissolution,dependingonprevailingconditions(tem-
metallic tungsten accompanied by cathodic reduction of dis- perature,pressureandpH),presenceofotherions,and/orcom-
solvedmolecularoxygentakeplaceaccordingtothefollowing plexing agents, several possibilities exist for transformation.
simplifiedhalf-reactionscheme: Formationofinsolubleorsparinglysolubletungstates,possibly
accompanied by reprecipitation/coprecipitation, are known to
W + 8OH−→ WO42−+6e−+4H2O (1) forminaquaticenvironmentsinthepresenceofsaltsofdivalent
and trivalent metals (except Mg). Alkali tungstates are highly
O2+2H2O + 4e−→ 4OH− (2) mobileinaquaticmediaastheyareverysolubleinwater.Under
acidic pH, the formation of a number of isopolytangstates is
ThedissolutionbehaviorofWcanbeofhighenvironmental possible, but their stability and optimal H O+/WO 2− ratio is
3 4
relevance in natural ecosystems as hypoxia and acidification
influenced by the presence of other anions. For example, the
aretwoconditionswithsignificantimplicationsforspeciation,
presence of ammonium may shift polycondensation to higher
reactivity,mobilityandtoxicityoftungstenorotherstressors.
pHvalues.Competitionoftungstatespecieswithotheranionic
species is also of high environmental relevance with implica-
tionsforparticulateandcolloidaltransportasaresultofsorption
and/orionexchange.
Although the lithophilic nature of W and its compounds is
undisputablelittleinformationexistsintheliteratureonadsorp-
tion/desorption behavior [107,108]. Over the last 2 years, our
grouphasperformedseveralexperimentalstudiestodetermine
Table6
VISUALMINTEQtungstenspeciesandequilibriumconstants
Reaction Equilibriumconstant,pK
12WO42−+14H+⇔H2W12O4210−+6H2O 111.5
6WO42−+6H+⇔H2W6O226−+2H2O 48.4
7WO42−+9H+⇔HW7O245−+4H2O 71.24
WO42−+H+⇔HWO4− 3.62
WO42−+8H+⇔W7O246−+4H2O 65.19
Fig.3. pC–pHdiagramforasolutionofsodiumtungstate0.005Min0.01M WO42−+2H+⇔WO3(H2O)aq 8.7
NaCl(VISUALMINTEQVersion2.3).
adsorption and leaching behavior of dissolved W species on
model natural soils and nanocrystalline (crystal sizes 3–7nm)
titanium dioxide hydrate [105,106]. A better understanding of
theenvironmentalbehaviorofWandidentificationofpossible
remedialschemesforW-contaminatedenvironmentalmatrices
werethebasicobjectivesofthesestudies.Adsorption/desorption
experiments, carried out using solutions resulting from disso-
lution of powders containing alloying elements in proportions
found in certain munitions formulations (Ni/Fe, Ni/Co and
Ni/Co/Fe),usingfourmodelsoils(montmorilonite,illite,sigma
sandandPahokeepeat)havebeendescribedinRef.[106].The
adsorption behavior was described adequately by Freundlich
isothermswhilehystereticdesorptivebehaviorwasevidentinall
cases.Althoughasignificantuptakewasobservedinallmodel
soils,theamountofdissolvedWspeciesadsorbedbyPahokee
peat(containingabout90%organic)wasbyfarthehighest.This Fig.4. TungstenbreakthroughcurveforW/Cusolutionwithandwithoutphos-
preferenceofdissolvedWspeciesfortheorganicphasemaybe phate.
an indication of possible interactions with the organic matter
present on that particular soil. The adsoption behavior of W fected or slightly improves. Nevertheless, competitive adsorp-
ontheinorganicsoilsalsoaccompaniedbyasimultaneouspH tionbetweencertainanionicspeciesanddissolvedWformsmay
increasecanbeattributedtochemisorptionofthetungstateanion beofsignificanceinnaturalenvironmentalsystemsasthismay
onareactivemetalhydroxylsitesaccordingtothereactions: serve as a mobilization mechanism for W and is certainly an
areawheremoreresearchisneeded.
>S−OH + WO 2−→ S−WO −+OH− (3)
4 4 Thepresenceoforganicmattermayinducetwomajortypes
2 > S−OH + WO 2−→ S −WO +2OH− (4) of interactions with metals in environmental systems: (a) by
4 2 4
influencing metal partitioning between various environmental
ortoformationofternarycomplexesaccordingtothefollowing compartments and interfaces and (b) by means of biochemi-
reaction: calreactivityresultingintheformationoforganometallicsub-
stances.Thepresenceofdissolvedorganicmatter(DOM)was
> S−OH + M2++WO42−→ S−O−M−WO4−+H+ shown to affect the distribution bioavailability of other metals
(e.g.AgandCd[109])butrelevantstudiesontungstenhavenot
(5)
beenpublished.ThepresenceofDOM,specificallythedissolved
Thepresenceofphosphatesinsolutionscontainingdissolved fulvicandhumicacidfraction,mayaffectametal’stoxicitynot
Wspeciesaffectsthesorptionbehaviorofthelatterandactsas only by acting as a complexing agent in solution, but also by
acompetitorforadsorptionsites.Thisbehaviorwasevidentin interacting directly at the biological interface as a natural sur-
a series of experimental runs carried out to explore remedial factantordirectlycompetingwithWforanionexchangesites.
treatment of aquatic solutions containing dissolved W species No studies have been reported in the literature on interactions
byadsorptiononnanocrystalline(crystalsizes3–7nm)titanium betweenWandDOM.
dioxidehydrate.Solutionscontainingvariablecombinationsand W and the other transition metals (Cr and Mo) with their
amounts of dissolved W, Cu, Fe, Ni and P (as PO 3−) were diverse array of chemistry are very popular for synthetic
4
passed through a column containing the Ti-based adsorbent
mixed with sand to produce a desirable porosity bed. Results
plottedinFig.4areforaleachingsolutionresultingfromdisso-
lutionofapowderconsistingofW,Cuwithorwithoutaddition
ofphosphates.Thepresenceofphosphateanionsresultedinan
earlybreakthroughoftungsten(after800ml)comparedwiththe
solutionswithoutphosphateamendment(1200ml).Normalized
concentration values (C/C ) for phosphate amended solutions
0
reachedvalueslargerthanunitysuggestingphosphatedisplace-
mentofsorbedtungsten.Thesamebehaviorwasobservedforall
the other solutions used in our experiments. Additional exper-
imentsweresubsequentlydesignedandperformedusingthree
modelsoilsinordertoconfirmthatthiscompetitivebehavioris
notduetosomepeculiarityoftheTiO adsorbent.Theresults,
2
shown in Fig. 5, indicate a significant decrease in the adsorp-
tivecapacityforPahokeepeatsoil,butnotformontmorillonite
Fig.5. Uptakeoftungstenbymodelsoilsinthepresenceofdifferentconcentra-
and illite soils, where the adsorptive capacity remains unaf- tionsofphosphate.Errorbarsrepresentthestandarddeviationof3/4replicates.