Integrating phytoremediation with biomass valorisation and critical element recovery

12 Pages·2015·2.13 MB·English

by  Ying Jiang

BiomassandBioenergy83(2015)328e339
ContentslistsavailableatScienceDirect
Biomass and Bioenergy
journal homepage: http://www.elsevier.com/locate/biombioe
Review
Integrating phytoremediation with biomass valorisation and critical
element recovery: A UK contaminated land perspective
Ying Jiang a, Mei Lei b, Lunbo Duan c, Philip Longhurst a,*
aCentreforBioenergy&ResourceManagement,SchoolofEnergy,Environment&Agrifood,CranfieldUniversity,Cranfield,MK430AL,UK
bCentreforEnvironmentalRemediation,InstituteofGeographicSciencesandNaturalResourcesResearch,ChineseAcademyofSciences,Beijing,100101,
China
cKeyLaboratoryofEnergyThermalConversionandControl,MinistryofEducation,SchoolofEnergyandEnvironment,SoutheastUniversity,Nanjing,
210096,China
a r t i c l e i n f o a b s t r a c t
Articlehistory: IntheUK,thewidespreadpresenceofelementalcontaminantssuchasarsenicandnickelincontami-
Received27May2015 natedsitesandmorewidelyreleaseofplatinumgroupmetalsintothebiospherearegrowingconcerns.
Receivedinrevisedform Phytoremediation has the potential to treat land contaminated with these elements at low cost. An
30September2015
integratedapproachcombininglandremediationwithpost-processbiomasstoenergyconversionand
Accepted14October2015 highvalueelementrecoveryisproposedtoenhancethefinancialviabilityofphytoremediation.
Availableonline24October2015
Ananalyticalreviewofplantspeciessuitableforthephytoremediationofnickel,Arsenicandplatinum
groupmetalsisreported.Additionally,apreliminarymodelisdevelopedtoassesstheviabilityofthe
Keywords:
proposedapproach.AfeasibilityappraisalusingMonteCarlosimulationtoanalyseprojectrisksuggests
Phytoremediation
Elementalcontaminant highbiomassyieldplantspeciescansignificantlyincreasetheconfidenceofachievingfinancialreturn
Biomasstoenergy fromtheproject.Theorderoffinancialreturnfromrecoveringelementswasfoundtobe:Ni>Pt>As.
Montecarlosimulation CrownCopyright©2015PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense
Riskanalysis (http://creativecommons.org/licenses/by/4.0/).
Contents
1. Introduction................................................................ ...................................................... 328
2. Phytoremediationandplantselection.................................................... ............................................ 329
2.1. Arsenicphytoremediation.....................................................................................................330
2.2. Nickelhyperaccumulators.....................................................................................................331
2.3. PGMphytoremediation .......................................................................................................332
3. Improvementoffinancialfeasibilityofphytoremediationprojectbyplantbiomassutilisationandelementrecovery.............. ............ 333
3.1. Economicmodelofaphytoremediationproject ........................................... ......................................335
3.2. Riskmanagementoftheintegratedphytoremediationproject .................................... ................................337
4. Conclusion ................................................................ ....................................................... 338
Acknowledgement ............................................................ ....................................................338
References.........................................................................................................................338
1. Introduction identifiedelementalcontaminants,Arsenic(As)andnickel(Ni)are
twoofthemostcommonones.Duetotheirubiquitousoccurrence
Soilscontaminated with metaland metalloidelements pose a on contaminated sites, concentration levels and high risk factors,
major environmental and human health risk. Amongst the bothelementsarelistedaspriorityinorganiccontaminantsunder
theUKPart2Aregime[1].Platinumgroupmetals(PGMs)onthe
otherhand,haveonlylimiteddistributionintheenvironmentand
* Correspondingauthor. inertchemicalandbiochemicalproperties;thereforehavenotbeen
E-mailaddress:P.J.Longhurst@cranfield.ac.uk(P.Longhurst).
http://dx.doi.org/10.1016/j.biombioe.2015.10.013
0961-9534/CrownCopyright©2015PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).
Y.Jiangetal./BiomassandBioenergy83(2015)328e339 329
recognisedasprioritysoilcontaminants.However,theincreasing translocatecontaminantstoabove-groundtissuesforlaterharvest,
useofPGMsinthepastfewdecadesinvehicleexhaustcatalysts,as i.e.phytoextraction;convertingtheelementtoalesstoxicchemical
wellasinseveralotherindustrialandmedicalapplicationshasled species, i.e. transformation; or at the very least sequestering the
to a heightened soil concentration of PGMs, especially in urban elementinrootstopreventleachingfromthesitei.e.phytostabi-
high-trafficareas[2]aswellashighvaluelossesinminingareas. lisation.Asacompetingtechnology,phytoremediationoffersalow
Consequently, these increases have given rise to public health cost,albeitsloweralternativetophysicalandchemicaltreatment
concerns[2]. methods[9]andisviableinmitigatingcontaminationlevelsfora
IntheUK,metalsandmetalloidsarethemostwidespreadsoil widerangeoforganicandinorganiccontaminants.However,asa
contaminantspresentinover80%ofallidentifiedsitesinEngland biological method, phytoremediation is limited by a number of
and Wales [3]. Management and remediation of these sites is factors such as the long treatment time and site/contaminant
clearlyofpublicinterest.Fromanenvironmentalperspectiveitis specificityetc. In addition, a key inhibiting factor for commercial
desirabletorehabilitatecontaminatedsitestothehighestpossible implementation of phytoremediation is the disposal of large
standard, regardless of the potential costs. In practice, such ap- quantitiesofcontaminatedplantbiomassmaterialthataccumulate
proachesimposeaheavyfinancialburdenongovernmentexpen- throughouttheprocess[10,11].Whencontaminantconcentrations
diture, as demonstrated by the Dutch government since their in the biomass exceed specific levels, the biomass material is
adoptionofthisapproachintheearly1980s.AccordingtoHonders regarded as potentially hazardous, therefore must be stored or
etal.[4],itwasestimatedthatifalltheidentifiedsitesinHolland disposedofappropriately[12].Here,aradicalapproachtoaddress
were treated to the standard required by legislation, the total thisdisposalproblembyincorporatingathermochemicalconver-
remediationcostswouldbeintheorderof50billioneuros.By1997, sionofbiomasstorenewableenergyfollowedbyametal(loid)re-
itwasevidentthatthe‘Dutchsystem’wasnotfinanciallysustain- covery stage to the process is proposed. The feasibility of using
able and the government changed their system to a more cost- phytoremediationtechnologytoremediateselectedelementsfrom
effective ‘function-orientated’ approach adopting a risk-based contaminatedsiteswhicharenotonthelocalauthorities'priority
managementsystem,similartotheUK[5]. listisreviewed,thenfollowsdiscussionofthefeasibilityofsuchan
UKcontaminatedlandisregulatedbyaframeworkoflegislation integrated approach to maximise economic benefit from phytor-
and policies underpinned by the contaminated land regime (as emediation alongside biomass energy production and high value
stipulatedinPart2AoftheEnvironmentalProtectionAct1990,or metalrecovery.
simplyPart2A)andland-useplanningregimeWithinthisregime
theTownandCountryplanningAct1990isthemostimportant). 2. Phytoremediationandplantselection
The underlying concept of the UK system emphasises on a risk-
basedapproach[6]andrelianceontheland-useplanningsystem Phytoremediation asadisciplinein environment scienceswas
(87%inEnglandand79%inWales)tofundremediationworkwhen established in late 1970s following the discovery of a series of
the site is developed and redeveloped [3]. This approach, in hyperaccumulators [13]. Since then the field has developed
contrasttothe‘Dutchsystem’hasprovedtobemorecost-effective attractingnotonlyscientificinterestbutattentionfromprivateand
forgovernmentintervention.Howeverthisapproachislimitedto industrial site owners, regulators and the environmental engi-
urbanareaswherethereisarapidlyexpandinglandrequirement neeringcommunity[9].Todate,intensiveresearchinthisareahas
for residential and commercial development, and no lack of resulted in a significant improvement in knowledge of hyper-
financial drive for developers to undertake remediation work. In accumulators and their elements of affinity. It is now generally
ruralandlowervalueareaswherecommerciallanddevelopmentis agreedthatinordertodistinguish‘hyperaccumulator’fromnormal
less competitive, there remain a large number of contaminated oraccumulator,asetof thresholdvaluesofelementalconcentra-
siteswithremediationworkpendingduetofinancialbarriers.Ac- tions in plant biomass (dry weight) are used to define hyper-
cording to the latest survey carried out by UK Environmental accumulation:MnandZnhyperaccumulatorscontain>10,000mg/g
Agency,bytheend of2007,of the746contaminatedsiteswhich [14],hyperaccumulatorsofAs,Co,Cu,Ni,Se,andPbhave>1000mg/
had been identified under Part 2A, only 144 were reported as g[14,15],andhyperaccumulatorsofCdhave>100mg/g[14].
completelyremediated[3]. The mechanism and rationale of phytoremediation has been
Remediation of elemental soil pollutants presents distinct sci- discussed in a number of reviews [16e20]. Depending on con-
entificandtechnicalchallenges,asunlikeorganicpollutantsthese taminants, the siteconditions, level of clean-up requiredand the
cannotbedegradedfurtherintonon-harmfulproducts.Therefore plantspecies,itinvolvestheuseofplantstoextract,sequester,and/
theonlywaytoremediatetoxicelementalpollutantsistoremove ordetoxifypollutants[21].Theconceptofusingplantstouptake
orsequesterthemfromthesoil.Currenttechnologiesavailablefor environmental contaminants from soil is not new, however it is
remediation of elemental pollutant including in-situ or ex-situ onlyinthetwentiethcentury,afteraseriesofdiscoveryofhyper-
chemicaltreatment,biologicaltreatment,soilwashing,soilflush- accumulator and vast advance of analytical techniques, has the
ing,vitrification,incinerationandlandfilling[7]. conceptofphytoremediationbeenrapidlydeveloped[14].
Remedial treatments for contaminated sites in the UK are In recent years, research on phytoremediation has shown the
currentlydominatedbyexcavationandoff-sitedisposalofmaterial. overallenvironmentalandeconomicbenefitsfromlandremedia-
Thispractice isusedalmostexclusivelyforremedialwork of this tion.Currentresearchtrendsarefocusingonmaximisingtheuseof
typeandregardedasthelikelysolutionforallfutureworkinthe by-products fromphytoremediationprocess.Researchersarealso
view of Environmental Agency [3]. Preference for this ‘dig and exploringtheusephytoremediationbiomassasarenewableenergy
dump’approachisduetoitsstraightforwardoperationandshort source [22,23]. In addition, the concept of moving from ‘phytor-
project time frame. However, volatile emissions, odour nuisance emediation’ to ‘phytomining’ to reclaim potentially valuable ele-
andnoiseduringtheexcavationstageaswellaspossiblesecondary mentsforfurthereconomicbenefitsisunderway.
contamination during transport and landfill are evident risks. In The greatest advantage of phytoremediation is low cost. Ac-
additiontotheenvironmentalconcern,increasinglandfilltaxation cordingtoaEuropeanscalestudy[7],theaveragecostforon-site
resultinthismethodnotbeingvariable/feasibleinthelongterm phytoremediation and off-site landfilling are 122 and 231 Euro
[8]. perm3,respectively.IntheAmericanmarket,similarcostadvan-
Phytoremediation technology uses plants to extract and tagesfromphytoremediationexist.Itisgenerallyagreedthatthe
330 Y.Jiangetal./BiomassandBioenergy83(2015)328e339
estimated cost for phytoremediation of soil is in the range of largelyattributedtotheproductionofintracellular thiolssuchas
25e100 US dollars per ton [9], in contrast to approximately glutathione (GSH) and phytochelatins (PCs) which are chelators
150e350 US dollars per ton for conventional excavation-landfill withastrongaffinitytoarsenic[29e31].
approaches [24]. In addition to the cost, phytoremediation offers Among the numerous arsenic hyperaccumulating plants re-
better performance compared to the conventional approach, e.g. ported to date, the majority belong to the fern Pteris family
permanentlyremovalofthecontaminants,lessdisturbancetothe (Table1).ThefirstarsenichyperaccumulatorPterisvittatawasre-
site. It has to be noted that phytoremediation will more readily portedin2001byMaetal.[15].Theplantisamesophyticfernthat
remove the bioavailable fraction of the contamination, and is capableofaccumulatingarsenicintheabovegroundfrondwithin
therefore more compatible with risk-based contaminated land the range of 2500e22,630 mg kg(cid:1)1 on a dry weight (DW) basis,
management systems [9]. The common perception towards dis- ~100-fold higher than soil concentrations [15,28]. This discovery
advantages of phytoremediation is the substantially longer time has led to intensive screening of the fern Pteris family for other
scalesrequiredforremediationtobecompleted.Thisdisadvantage potential arsenic hyperaccumulators [28,32]and as a result, a
hasexcludedphytoremediationasamainstreamtechnologysolu- number of species in the family such as Pteris cretica and Pteris
tionforurbancontaminatedsites.Itshouldalsobenotedthatthe umbrosa,havebeenrecognisedasarsenichyperaccumulators.
specificity of hyperaccumulators result in selective remediation The high BF, TFand reasonable biomass yield of P. vittata has
whichislesseffectiveforsiteswithmultiplecontaminants[25]. prompted notable attention from commercial phytoremediation
Selectingtherightplantsforphytoremediationfromthewide projects.Fieldstudieshavebeenconductedonanumberofocca-
rangeofcandidatesisthemostimportantstageofsuchproject.In sions.Grayetal.[26]demonstratedinfieldstudiescarriedoutin
generalruleachievingahighbioaccumulationfactor(BF,definedas southwestEnglandthatP.vittataandP.creticaarebothsuitablefor
theratioofelementconcentrationinplantbiomasstothatinsoil) climateconditionsinthatregion.Afterexposureofsoiltotalarsenic
andhightranslocationfactor(TF:definedastheratio ofelement concentrationsof471mgkg(cid:1)1,bothspeciesexhibitedahigheffi-
concentration in above ground shoots to that in roots) is key. ciency of bioaccumulation and root to shoot translocation, with
However,whenlandremediationisnotthesolegoaloftheproject most of the arsenic accumulated in the frond (4371 and
and downstream processes for element and energy recovery are 2344mgkg(cid:1)1forP.vittataandP.cretica,respectively.)Howeverthe
desired,otherfactorssuchashighbiomassyieldandtoleranceto relatively low above ground biomass yield from P. vittata which
the contaminants also become relevant. The rationale to support averagedat0.76tha(cid:1)1of(ondryweighbasis)hasbeenconcluded
thisdecisionmakingisdiscussedinthefollowingsection. tobe the maindrawbackforachieving higharsenicextraction in
thefieldstudy.
2.1. Arsenicphytoremediation Kertulis-Tartaretal.[33]carriedouta2-yearfieldstudyusing
P. vittata for phytoremediation of the soil contaminated with
Arsenic is a metalloid which is considered non-essential and chromated copper arsenate on a 30.3 m2 plot. Soil arsenic con-
toxic at high concentration to plants and animals. In the UK, centrationsatthebeginningofthestudyweremeasuredbetween
particularly in the Southwest, large areas of soil are considered 190 and 278 mg kg(cid:1)1 from samples taken at depths within the
contaminatedwithAs,eithergeogenicallyorfromanthropogenic rangeof1e60cm.Duringthe2-yrperiod,atotalof26.3gofarsenic
activities such as mining and smelting [26]. In other part of the wasremovedfromtheplot.Reportedbiomassyieldwasapproxi-
world, Arsenic-contaminated soil is one of the major sources of mately 1.3 t ha(cid:1)1. This improved yield was possibly due to the
arsenicindrinkingwater[27],andalsoresultsinhigharseniclevel subtropical climate in Florida where the test was carried out.
incereals,vegetablesandfruitsgrownonthecontaminatedsoil.All Similarly,elevatedbiomassyieldhavealsobeenreportedbyChen
chemicalspeciesofarsenicarebioactive[28]andthereforecanbe etal.[34]inafieldstudycarriedoutinasubtropicalclimateregion,
readilyabsorbedbyanimalsandplants.Thisbiochemicalproperty in which an average above ground biomass yield of just below
ofarsenicgivesrisetointensiveresearchintohyperaccumulators 2tha(cid:1)1wasachieved.
which can be applied for the phytoremediation of arsenic. A Biomassyieldofthepollutantaccumulatorsisthedetermining
numberofstudiesreportthattheabilitytotolerantandaccumulate factorforthesuccessanddurationofthephytoremediationprocess
arsenic in many plant and phytoplankton species. This can be [35]. For P. vittata, although it has a significantly higher biomass
Table1
Plantspecieswithpotentialforarsenicphytoremediation.
Species Planttype Reportedaccumulationrates(mgkg(cid:1)1DWa) Reference
AgrostiscaninaL. Perennialherbb 460 [40]
AgrostisstoloniferaL. Perennialherb 1350 [40]
AgrostistenuisSibth. Perennialherb 3470 [40]
Callunavulgaris Perennialshrub 4131 [40]
Helianthusannuus Annualherb 1550 [39]
Holcuslanatus Perennialherb 560 [32,40]
JasionemontanaL. Annual/biennialherb 6640 [40]
Pityrogrammacalomelanos Fern 5000e8350 [67]
PterisbiauritaL. Fern 2000 [68]
Pteriscretica Fern 3500e4000infrond;2200e2600inroot [28,32]
Pterislongifolia Fern 4308 [28,32]
Pterisquadriaurita Fern 2900 [68]
Pterisryukyuensis Fern 3700 [68]
Pterisumbrosa Fern 3735e5000 [28,32]
Pterisvittata Fern 2500e22,630 [15,28]
Reynoutriasachalinensis Perennialshrub 1900 [69]
Note.
a DW¼DryWeight.
b Herb¼Herbaceousplant.
Y.Jiangetal./BiomassandBioenergy83(2015)328e339 331
yieldcomparedtomostofthehyperaccumulators,itisstillfarless
than those of high yield economic crops such as sunflower and
cultivarsinthewillowfamily.
Shelmerdineetal.[36]examinedthesuitabilityofP.vittatafor
phytoremediationof21siteshistoricallycontaminatedwitharsenic
atvariouslevelsaroundEngland.Thestudyfoundthatthefraction
ofAsremovedgenerallydeclinedassoilAsconcentrationincreased.
Anuptakemodelwasdevelopedusingexperimentaldatatopredict
the time frame required for site cleanup to the target level. It is
concludedbytheauthorsthatP.vittataisonlysuitableforsoilswith
minorlevelsofarseniccontaminationandthatmajorlimitationto
successfulphytoremediationislowbiomassyieldofP.vittata.
Increasingly, research evidence suggests that although species
in the Pteris familyexhibit a high capacity of bioaccumulation of
arsenic,thelowbiomassP.vittataproductionhindersitsapplica-
tion on heavily contaminated sites [37,38]. High yield common
plantsandeconomiccrops,ontheotherhand,havebeendemon-
strated in a number of studies to have more promising field
application[38e40].
Amongtheplantspeciesproposedinthesestudies,shrubwil-
low (Salix spp.) and sunflower (Helianthus annuus) are the most
promising for phytoremediation field applications due to their
relativelyhighaccumulationabilityandsubstantialbiomassyield.
According toJanuaryetal. [39] H.annuus is capableof uptake of
arsenic upto1550 mg kg(cid:1)1 in the plant shootunderhydroponic
conditions.ThestudyalsosuggestsH.annuusiscapableofhyper-
accumulatingsimultaneouslyarangeofothermetalcontaminants
suchasnickel,cadmiumandchromium.Anumberofrecentstudies
have addressed the potential of Salix spp. for a range of phytor-
emediation applications [38,41]. Purdy and Smart [38] examined Fig.1. NiconcentrationintopsoilinEnglandasapercentileclassifiedinterpolated
arsenicaccumulationaffinityinfourwillowclonesgrownhydro- image.
ponically.Inthehighestaccumulatingclone,arsenicwasaccumu- Source:DefraTechnicalGuidanceSheetNo.TGS05[45].
lated at 329, 201 and 5800 mg kg(cid:1)1 in the leaf, stem and root,
respectively.Inasimilarhydroponicstudy,Puckettetal.[41]found other hyperaccumulators [14]. The plant family most strongly
accumulation of an As-tolerant willow (Salix viminalis (cid:3) Salix represented are Euphorbiaceae, Brassicacceae, Asteraceae, Fla-
miyabeana) reached 66.8, 34.2, and 3170 mg kg(cid:1)1 and an As- courtiaceae,BuxaceaeandRubiaceae[14].Table2selectivelylistsa
sensitive willow (Salix eriocephala) reached 20.3, 16.8, and number of plant species with potential for application inphytor-
2380mgkg(cid:1)1(DW)forleaf,stem,androot,respectively. emediationprojects.Detailedsummariesofspeciescanbefoundin
Although it seems the arsenic accumulation abilities of these earlier works by Baker and Brooks [46], Reeves et al. [47,48] and
plantsare2e3timeslessthaninP.vittata,thesignificantlyhigher ReevesandBaker[14].Thereasonforthelargenumberofnickel
biomass yield (at least 10e20 fold) drastically reduces the reme- hyperaccumulatorsispartiallyduetotheextensiveanalyticalwork
diationtime.Itisalsorecognisedthatbiomassproducedduringthe carriedoutonultramaficfloras,butmorefundamentalexplanation
phytoremediation process can be reconsidered as a locally pro- canbeattributedtomillionyearsofevolutionofplantscolonisedin
duced,renewablefeedstockforbioenergyandbioproducts[42]. theNi-enrichedultramafic,whichisbyfarthemostwidespreadon
aglobalscale[14].
2.2. Nickelhyperaccumulators Theextensivedistributionofnickelinsoilandthelargeselec-
tion of nickel hyperaccumulators has encouraged intensive
IntheUK,nickelwaschosenasoneoftheeightcontaminants research for phytoremediation of land contaminated by nickel.
examined by a study conducted by the British Geological Survey Additionally, the high biomass yield and high bioaccumulation
(BGS)inordertogivefurtherguidanceontherecentlypublished factorsexhibitedinsomeofthenickelhyperaccumulatorsmakesit
revised Part 2AContaminated Land Statutory Guidance [43]. It is possibletousetheseplantstoextractnickelfromlowgradenickel
alsorecognisedbytheEnvironmentalAgencyasoneofthefiftysix oreswhichcoverlargeareasoftheEarthcrust[49].
prioritycontaminantsintheUK[44]. Amongst hundreds of nickel hyperaccumulators, there are a
DistributionandconcentrationofnickelintheUKsoilisinflu- numberofspeciesthathavesofarbeenappliedinfieldstudiesand
encedmostlybytheunderlyinggeology,i.e.parentmaterialofsoil; have demonstrated their potential for commercial phytor-
whereasnickelpollutioninsoilcausedbyhumanactivityisnotas emediation and phytomining. Alyssum bertolonii was reported by
significantasseenwithsomeothercontaminants[45].Inarecent Robinsonetal.[50]inafieldtrialascapableofaccumulatingNiat
studyconductedbyBGS[43,45],significantlyhighconcentrations 0.8%(8gkg(cid:1)1)drymatterofitsbiomass.Reasonablygoodbiomass
of nickel wereidentified in areas at the southern tip of Cornwall yield was achieved with moderate fertilization (N, P and K) at
(Lizardserpentinites),ironstonerockrichareasinOxfordshireand 9.0 t ha(cid:1)1, which gave a metal yield of 72 kg ha(cid:1)1 assuming all
areasinthePeakDistrictwheremineralisationandNi-richparent nickelinthebiomassisrecovered.Theauthorsconcludedthatthe
materialareresponsibleforhighNiconcentrationinsoil(Fig.1). net return from this Ni hyperaccumulator per hectare could be
Sincethediscoveryoftheworld'sfirst‘nickelaccumulator,sofar comparable tothatof wheat based ona conservative calculation.
no less than 320 plant species have been reported, which makes However if energy yield from biomass via thermochemical con-
nickel hyperaccumulators possibly the largest family amongst version,e.g.gasificationisconsidered,evenhigherreturnscanbe
332 Y.Jiangetal./BiomassandBioenergy83(2015)328e339
Table2
Plantspecieswithpotentialfornickelphytoremediation.
Family Species Planttype Reportedaccumulationrates(mgkg(cid:1)1 Reference
DW)
Asteraceae Berkheyacoddii Perennialherb 11,600 [51]
Berkheyazeyheri Perennialherb 17,000 [47,48]
Pentacalia(10species) Herb 16,600 [47,48]
Helianthusannuus Annualherb 510e1070 [39,70]
Seneciocoronatus Herb 24,000 [71]
Brassicaceae Alyssum(48taxa,allinsectionOdontarrhena) Annualorperennialherbs 1280e29,400 [46,72,73]
Bornmuellera(6taxa) 11,400e31,200 [48,74,75]
Thlaspi(23taxa) Annualorperennialherbs 2000e31,000 [48]
Buxaceae Buxus(17taxa) Shrub 1320e25,420 [47]
EuphorbiaceaeLeucocroton(27species) Herbs 2260e24,600 [76]
Phyllanthus(16taxa) Herbs 1090e38,100 [77]
Phyllanthuschamaecristoides(2subsp:chamaecristoidesand Herbs 3400e31,740 [47]
baracoensis)
Cleidionviellardii Herbs 9900 [48]
Baloghiasp. Herbs 5380 [48]
Flacourtiaceae Homalium(7species) Shrub(withinwillow 1160e14,500 [78]
family)
Xylosma(11species) Shrub(withinwillow 1000e3750 [78]
family)
Rubiaceae Psychotriadouarrei Shrub 14,900e27,700 [79]
achieved.Inalaterstudycarriedoutbythesameresearchgroup exposedto5.7mgl(cid:1)1Pt.Duringasix-weekexposuretothePt,an
[51],ahighbiomassyieldNihyperaccumulatorBerkheyacoddiiwas accumulationfactorof6952intherootswasachieved.
reported. In the field test, the plant was capable of accumulating Ballach and Wittig [57] carried out a hydroponic experiment
1.8e7.8 g kg(cid:1)1 Ni in the above ground biomass (on dry weight usingpoplar(Populusmaximowiczii)toexaminetheaccumulation
basis)whilstachieving22tha(cid:1)1ofdrybiomass.Additionally,the ofPtanditstoxiceffectonbiomassgrowth.Thegrowthnutrient
ease of propagation and culture, as well as its tolerance to cool solutionwasspikedwith34.8mgl(cid:1)1PtCl4.Thestudyagreedwith
climaticconditionrenderthisspeciesasuitableagentforphytor- previousliteraturethatthePtwaspredominatelyaccumulatedin
emediationparticularlyintheUK.Theeconomicaspectsofusing therootandthetranslocationfactortootherpartsofthetissuewas
B.coddiiarediscussedbytheauthorsinthisstudy.Itisconcluded verylimited.Howevertheauthorsnotedthattheaccumulationof
thatatthehighestNiconcentrationinthebiomassarchivedinthis Ptsimultaneouslycausedagradualdepletionoftheplants’water
study(7.8gkg(cid:1)1),1haofB.coddiicropcanremove168kgofNi supply.
assuming the biomass yield of 22 t ha(cid:1)1. When combined with DuetothestablechemicalandbiochemicalpropertiesofPGMs,
energy from biomass combustion, assuming at 25% of the total bioaccumulation of these elements from soil by plants is heavily
biomasscalorificvalue,anestimatedreturnofUS$1548perhais dependent on their chemical forms. Despite this, hydroponic ex-
predictedbytheauthorsatthetimethestudy. periments where plants were exposed to high concentrations of
dissolved Pt-salts can provide insight of metal distribution after
2.3. PGMphytoremediation uptake.Inordertoassessthefeasibilityofphytoremediation/phy-
tomining of PGMs as a commercially viable option, experimental
In the UK, PGMs such as Pt and Pd are not listed as soil con- datacollectedfromrealisticfieldconditionsisofhighimportance.
taminantsinthePart2Aregime.Howeverduetothewideusageof Todate,onlyalimitednumberofstudieshavebeenconductedto
catalyticconvertor,highlevelofthesemetalsinroadsidesoiland sufficientlysimulate naturalconditions.HelmersandMergel[58]
roaddusthavebecomeagrowingconcern.Studieshaveidentified analysed PGM concentration in grass samples collected within
in soils,dusts andplantsexposedtohigh-trafficdensity,concen- close radius of highways and monitored the concentration trend
trationsofPGMsfarexceedingnaturalbackgroundlevels[52].Long overa3-yearperiod.ItwasfoundthataverageconcentrationsofPt
termmonitoringoftheseenvironmentalsamplesshowsanupward increasedfrom3.6to10.6mgkg(cid:1)1andRhfrom0.65to1.54mgkg(cid:1)1.
trend of PGM concentration and a strong correlation with traffic The study also reported one particular sample which exposed to
conditions[53]. streetdustforamuchlongerperiod,thatcontained96mgkg(cid:1)1(Pt)
AutomobilederivedPGMsreleasesaremainlyintheoxidation and15mgkg(cid:1)1(Rh)nearly10timeshigherthanaverageconcen-
statusofzeroorasoxide[54];thereforearecommonlyassumedto trationlevels.
be inert and immobile in the environment. However solubility In a greenhouse experiment, Scha€fer et al. [59] investigated
studiesofexhaustfumeandroaddustssuggestPGMsofsuchorigin concentrationsofPt,RhandPdinplantsgrownoncontaminated
areatleastpartlysolubleandthereforemobileintheenvironment highways soils. The PGM concentrations analysed in plants (dry
[55].Todate,littleisknownaboutthebiologicalmechanismofhow material) reached upto 8.6,1, and 1.9 mg kg(cid:1)1 for Pt, Rh and Pd,
thesenoblemetalsinteractwithplants. respectively.Theorderofuptakeratesforthethreeelementsinall
A few early works which studied bioaccumulation rates and plantstestwerefoundtobe:Pd>Pt(cid:4)Rh.
effects of platinum by plants were carried out under hydroponic MostoftheexistingliteratureconcerningplantuptakeofPGM
conditions. discussesthequestiononlyinthecontextofeffectsoftheirrelease
Pallas and Jones [56] have exposed 9 horticultural important tothebiosphere.Totheauthors'bestknowledge,onePGMuptake
crops to 0.057,0.57and 5.7mg l(cid:1)1 Ptin a Hoaglandnutrient so- study was carried out in context of phytoremediation/phytoex-
lution.AllspeciesaccumulatedasignificantlyhighamountofPtin tractionunderfieldconditions.Nemutandanietal.[60]examined
their roots. For cauliflower and tomato in particular, the Pt con- the bioaccumulation capability of an indigenous species B. coddii
centration exceeds 1000 mg kg(cid:1)1 on dry weight basis when grown on contaminated land where platinum and palladium
Y.Jiangetal./BiomassandBioenergy83(2015)328e339 333
concentrationswere0.04±0.03and0.18±0.07mgkg(cid:1)1(ondry 3. Improvementoffinancialfeasibilityofphytoremediation
weightbasis),respectively.Platinumwasfoundaccumulatedinthe projectbyplantbiomassutilisationandelementrecovery
leavesandrootsat0.22±0.15and0.14±0.04mgkg(cid:1)1dryweight,
respectively. The concentrations of palladium in the leaves and CurrentlyintheUK,thehighlandvaluesinurbanareasarethe
roots were 0.71 ± 0.52 and 0.18 ± 0.07 mg kg(cid:1)1 dry weight, drivingforcefordeveloperstoremediatecontaminatedsitesunder
respectively.DuetothelackofPGMcontaminatedsitesanddiffi- theplanningsystem.Whereasinmanyareasawayfromprofitable
cultyofanalysis,phytoremediationofthesevaluableelementsare land development, funding for remediation is limited. Conse-
certainlynotintensivelystudiedcomparedtootherelementssuch quently, a large number of contaminated sites are left untreated.
asarsenicandnickel.ThestudyusingB.coddiiasaccumulatorfor According to the latest report published by Environment Agency
uptake of PGM demonstrats the potential of phytoremediation/ [3], by 2007 only 18.4% of the determined contaminated sites in
phytoextractiontechnologyforremediationandmoreimportantly England and Wales have been remediated. Additionally, the ma-
recoveryofthesescarcemetals. jority of these contaminated sites were dealt with through the
Fig.2. Logicflowchartoftheproposedintegratedphytoremediationproject.
334 Y.Jiangetal./BiomassandBioenergy83(2015)328e339
Fig.3. Determinantsfortheeconomicmodelofintegratedphytoremediation.
planning system and it was estimated only 10% was dealt with levels of metal(loid)s in soil within biomass tissues. Additionally,
under Part 2A. Following the gradually reduced government largequantitiesofbiomassproducedduringphytoremediationare
funding for contaminated land and the announcement of the amixtureofhemicellulose,cellulose,ligninandminoramountsof
closingoftheContaminatedLandCapitalProgramme(CLCP)atthe other organics, which contains substantial amount of calorific
endof2013,itislikelythesesiteswillberemainuntreatedinthe value.Whentreatedthermochemically,e.g.throughgasificationor
futureandcontinuetoposerisktoecosystemandpublic. pyrolysis,rapidvalorisationcanbeachievedtoprovidefuelgasthat
Thereis,therefore,anurgentrequirementforfinanciallyviable canbeusedforheatandelectricitygeneration[64].Themetal(loid)
technologieswhichofferfinancialincentivetoremediatecontam- contentinprocessashretrievedatthethermochemicalprocessis
inatedsitesoflowlandvalue.Asmetal(loid)sarethepredominant furtherconcentratedcomparedtoitsconcentrationintheoriginal
soil pollutants, phytoremediation offers an opportunity for site biomass, due to the efficient bulk reduction during the thermo-
withsuchpollutantswhichlackfundingtocarryoutremediation chemical process. Therefore recovery of these elements can be
work. muchmorecost-effective,whistavoidingthedisposalcostforlarge
From a resource security perspective, most of the metal(loid) quantitiesofpotentiallytoxicbiomass.
soil contaminants are alsovaluable natureresources, which have Here we propose an integrated phytoremediation concept
been dispersed throughout the environment via industrial and coupling remediation with renewable energy production from
commercial activities in much lower concentrations than their biomass and subsequent metal(loid)s recovery. Each of the inte-
naturaldeposits.Ithasalreadybeenrecognisedthattherecoveryof gratedstagesandtheirinteractionsareillustratedintheflowchart
theseelementsiscriticalforthesustainabilityofindustrialdevel- below(Fig.2).
opment,asnaturalreservesaredepletinga.Arsenicispredictedto From an environmental perspective, integrated phytor-
berunoutbetween5and50yeariftheconsumptioncontinuesat emediation addresses both land contamination and renewable
present rates [61]. Nickel and PGMs are also identified as critical energy demand simultaneously. However, as the remediation in-
materialstothe UKeconomyatriskofdepletioninthe Resource dustryislargelymarket-driven,itisessentialtoassesswhetherthis
SecurityActionPlan(RSAP)[62].However,despiteincreasingde- approach is financially viable. Furthermore, optimisation of the
mand,noneofthissupplyissupportedbyrecycling[63];duetothe profitabilityof this approach is key. Following this, a preliminary
highcostofrecoveryfromlowconcentrationswhencomparedto model is defined to analyse the profitability of a single biomass
conventionalmining.Thus,lowcosttechnologiesthatofferhigher harveston1haofcontaminatedland.Guidancefrompriorresearch
economicincentiveraisecommercialinterest. isusedwithinthemodeltoassesstheprofitabilityofanintegrated
Phytoremediation technology satisfies both requirements for landremediationprojectforvariousscenarios,i.e.targetelement
lowcostlandclean-upandelementrecoverybyconcentratinglow andplanttype.Itshouldbenotedthattheresultsfromthemodel
Y.Jiangetal./BiomassandBioenergy83(2015)328e339 335
Table3 financialreturnfromcropproductionfromthelandafterremedi-
Deterministicparametersofthemodel. ation has been achieved. The work demonstrated the economic
Symbol Parameter Value benefitof thephytoremediationbysubsidisingthecostofselling
biomass alongside the potential long-term income from the
Vth Heatfeedintariff(£/kWh) 0.01a
Ep Costofgrowingplantsperhectare(£/ha) 245b cleaned area. However, this model offers only limited immediate
Es Costofmetalsmelting£/kg 0.4b incometothestakeholder,asremediationcantakedecadesforthe
Vm MarketvalueofAs(£/kg) 0.88 soiltoreachasuitableconditionforgrowingcommercialcrops.
MarketvalueofNi(£/kg) 8.69 Robinson et al. [50] calculated the required biomass for a
MarketvalueofPt(£/kg) 27,086.6
hyperaccumulator having a metal content of 1% (on dry weight
a Sourceofdata:DepartmentofEnergy&ClimateChange,UK.http://chp.decc. basis)toachieveafinancialreturnof500USdollarssolelyfromthe
gov.uk/cms/renewable-heat-incentive/.
recoveredmetal.Thestudyconcludedthatundertheassumption
b Sourceofdata:Grayetal.[26].
thataverageannualbiomassyieldof30t/ha,onlycobalt,nickel,tin,
are not intended to provide an accurate estimation of financial cadmium,manganeseandnoblemetals(Au,AgandPtetc.)would
return; rather to provide an insight into how decision on plant befinanciallyviable.
selection and the prioritised targetelement can affect the profit- Clearlythereareeconomiclimitsintermsofbiomassproduc-
abilityandoverallfinancialriskofsuchaproject. tionandmetalcontentwhenusingphytoremediationformetal(-
loid)elementrecovery.Whenonetakesintoconsiderationenergy
3.1. Economicmodelofaphytoremediationproject productionandlandreclamation,theoverallenvironmentalben-
efitsincreasethusaffectingtheoverall economicbalanceofsuch
A number of previous studies have attempted to address the projects.
financialaspectsofaphytoremediationproject.Lewandowskietal. Tosimplifythemodel,intangibleandindirecteconomicbene-
[65] evaluated the economic value of combination of biomass fits,suchascostreductionsfromusingphytoremediationinplace
production from cadmium contaminated land and the potential of more costly ex-situ clean-up technology and avoidance of
Table4
Stochasticparametersofthemodelandtheirprobabilitydistributions.
Symbol Parameters Grapha Min Mean Max 5% 95%
Y P.vitattayield(kg/ha) 798 1353 1945 932 1789
H.annuusyield(kg/ha) 7814 9777 12994 8047 12200
B.coddiiyield(kg/ha) 9170 13473 20491 9775 18701
P.vitattaAsuptake(g/kg) 3.01 9.89 20.89 3.81 17.95
C H.annuusAsuptake(g/kg) 1.25 1.47 1.61 1.30 1.59
B.coddiiNiuptake(g/kg) 2.00 5.03 7.44 2.78 6.94
B.coddiiPtuptake(g/kg) 0.21(cid:3)10(cid:1)4 1.82(cid:3)10(cid:1)4 3.57(cid:3)10(cid:1)4 0.62(cid:3)10(cid:1)4 3.09(cid:3)10(cid:1)4
Cv RangeofHHVofwoodyBiomass(MJ/kg) 18.00 19.17 21.30 18.08 20.70
Ve Elec.FeedinTariff(£/kWh)b 0.0803 0.0810 0.0986 0.0830 0.0965
he Electricalefficiency(%) 27.1 33.7 39.4 29.0 38.0
hth Heatefficiency(%) 40.6 51.0 61.7 43.2 59.0
a Thegraphsshowthedistributionofprobabilityoftheinputdata,verticallengthofthebarindicatestheprobabilityofoccurrence.
b Assumingadvancedthermochemicalbiomasstoenergytechnologyisused,andthereforereceiving2RenewableObligationCertificates(ROCs)/MWh.ValueofROCsvaries
between£40e50.Sourceofdata:DepartmentofEnergy&ClimateChange,UK.http://chp.decc.gov.uk/cms/renewables-obligation-2/.
336 Y.Jiangetal./BiomassandBioenergy83(2015)328e339
disposalofbiomassarenotconsidered. Y¼Biomassyield(kgdryweightha(cid:1)1):Canrangefrom2000to
A schematic overview of the main factors that influence the 20,000dependingonplants
profitofanintegratedphytoremediationprojectisshowninFig.3. C¼Metalconcentrationinthebiomass(gkg(cid:1)1ondryweight
Twodirectincomestreamsaretakenintoaccountinthemodel: basis)
1.)harvestedbiomassofwhichbiomasscalorificvalue(CV),elec- Cv¼Biomasscalorificvalue(CV)(MJkg(cid:1)1)(16.7e18.6MJkg(cid:1)1)
tricity and thermal efficiency of the combined heat and power Vm¼Marketvalueofmetal(£Kg(cid:1)1)
(CHP) unit, heat and electricity tariff are the determinants Ve¼ElectricityfeedingTariff(£/kWh)
(Assumingadvancedgasificationtechnologyisusedtoproducefuel Vth¼HeatfeedingTariff(£/kWh)
gasforasmallscale(50e1000KW)gasengineCHPunit).Currently, Rm¼Metalrecovery(%):Assuming100%here
asaincentivetotherapidandsustaineddeploymentofrenewable he¼ElectricalefficiencyofaCHPunit
energy, a feed-in tariffs (FITs) or similar schemes have been hth¼HeatefficiencyofaCHPunit(Generallyhth/he¼1.2e1.8)
implementedin63jurisdictionsworldwidebytheregulators.This Es ¼ Cost of metal recovery per kg of dry biomass (£ kg(cid:1)1):
schemes offer guaranteed prices for fixed periods of time for Estimatedat£0.4kg(cid:1)1frombiomassash(10%ofDMbiomass)by
electricity/heatproducedfromrenewableenergysources[66].Asa smeltingaccordingtoGrayetal.[26].
result, this significantly reduces the uncertainties to the project Ep¼Costofgrowingplantsperhectare(£ha(cid:1)1)
income by eliminating price fluctuation in energy market. 2.)
Elemental recovery from biomass, of which market value of the Equation (1) attempts to capture the major determinants that
elementisthedeterminant.Concomitantprocesscosts,i.e.costof affect gross margin of an integrated phytoremediation project.
plantingandmaintainingthecropandcostofmetalrecoveringare Within the variables in Equation (1), Vth, Vm, Es and Ep are more
deductedfromtheincome.Thereforethenetprofitmodelonthe deterministic and tend to be project specific. Once the project
1st harvest (per each hectare land) of an integrated phytor- location,scaleofoperation,localgovernmentincentivepoliciesare
emediationprojectiscalculatedasfollowing: determined,thesevariableswillnotcontributesignificantlytothe
financial risk of the project. To present a UK scenario, values of
P¼Y(cid:3)C(cid:3)Vm(cid:3)Rmþ0:2778kWhMJ(cid:1)1(cid:3)Y(cid:3)Cv(cid:3)ðVe(cid:3)he thesevariablesusedinthisstudyareUKspecific(typicalvaluesthat
þVth(cid:3)hthÞ(cid:1)0:1(cid:3)Y(cid:3)Es(cid:1)Ep ubesefodrefoarpcpalylcinuglatthioenmineththoidsdsetuscdryibaerdehliesrteediniannTyarbelaell3if)e. pHroowjeecvte,irt,
(1)
iscriticallyimportanttocollectthesedeterministicdataperproject
inordertoobtainrealisticresults.
Where:
Fig.4. Outputoffinancialriskanalysisof2hypotheticalscenarioseusingPterisvittataandHelianthusannuusinintegratedphytoremediationprojectsonanarseniccontaminated
site.
Y.Jiangetal./BiomassandBioenergy83(2015)328e339 337
Othervariablessuchasbiomassyield(Y),metalconcentrationin theeconomicmotivationsneededtoestablishwiderapplicationof
biomass (C), biomass calorific value (Cv), and electrical and heat thistechnology.
efficiency(heandhth)oftheCHPunithaveawiderangeofreported Inthescenariousingthelowbiomassyieldhyperaccumulatorp.
values as can be seen in the previous review. This causes uncer- vittatainanintegratedlandremediationprojecttorecoverarsenic
tainty,andthusa‘risk’tothefinancialreturnoftheproject.Indeed, and produce energy, the probability distribution of the profit (P)
therangeanddistributionofinputdataforthesestochasticvari- clearlysuggestsahighriskofloweconomicreturn.Inallpossible
ablesdeterminethelevelofprofitthatcanbemadealongsidethe datacombinationssimulated,96%oftheoutcomesfailedtoachieve
probabilitiesofachieving,orfailingtoachieveaprofit.Tounder- a positive margin and only 4% of outcomes achieve a limited
standhowthesestochasticvariablesinfluencetheoutcomeofthe financial gain of £0e44.63 per ha (Fig. 4a). Among all stochastic
economic model, a quantitative analysis based on Monte Carlo variables, biomass yield is the most significant variable affecting
simulation method was carried out using risk analysis software themargin(Fig.4b).Thisisunderstandableasalargeproportionof
@RISK(PalisadeCorp.Ithaca,NY,USA).Fourscenariosofdifferent incomeobtainedinthisscenarioisachievedfromenergygenerated
target element and their corresponding accumulating plant (As/ frombiomass.
P.Vitatta,As/H.annuus,Ni/B.coddiiandPt/B.coddii)werestudiedto Inadifferentscenario,P.vittataisreplacedwithahighbiomass
compare the profitability of an integrated phytoremediation yield plant H. annuus. Although it has a lower accumulation ca-
approachundereachscenario.Fromthecomprehensiveliterature pacity for arsenic, its high biomass energy value gives a signifi-
reviewof(hyper)accumulatorsfortheelementsofinterestinthe cantlyimprovedfinancialreturn.Thereisahighcertainty(90%of
previoussection,setsofdataforthestochasticvariableshavebeen simulated combinations) of achieving a margin of between
compliedandtheirrangesandtriangularprobabilitydistributions £610e1434perhawiththehighestcalculatedmarginof£1730per
usedforthesimulationareshowninTable4.Basedontheseinput ha(Fig.4c).Inbothscenarios,thebioaccumulationcapacityisless
distributions,allvalidcombinationsofthevalueswerecalculated importantduetothelowmarketvalueofAs(Fig.4d).Thereforeit
tosimulateallpossibleoutcomesofthemodel. canbeconcludedthatrecoveryoflowvaluearsenicisnotfinan-
cially viable unless high value products can be subsequently
3.2. Riskmanagementoftheintegratedphytoremediationproject developed. This finding is consistent with previous work carried
outbyGrayetal.[26].
Whilstphytoremediationisamaturetechnology,anintegrated Based on simulation results from these 2 scenarios, it can be
approachincreasestheuncertaintyoftheoverallfinancialviability expected that the financial gain can be further improved if high
oftheproject.Thereforeeconomicchallengesandrisksthatreside biomass producing plants with high accumulating capacity are
withintheintegratedremediationhavetobeappraisedalongside usedinprojectstorecovermetalssuchasnickel.
Fig.5. Outputsoffinancialriskanalysisof2hypotheticscenariosinanintegratedphytoremediationprojectusingBerkheycoddiiintorecoverNiandPt,respectively.