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RemovalofIronfromDyffrynAdda,ParysMountain,N.Wales,UKusingSono-electrochemistry(ElectrolysiswithassistedPowerUltrasound)

SarahAMorgan1,ZoeNMatthews1,PhilipGMorgan1andPeterStanley2

1KP2MLtd.,C10AshmountBusinessPark,Swansea,SA68QR,UK(email:pmorgan@powerandwater.com)

2NaturalResourcesWales,TŶCambria,NewportRd.,Cardiff,CF240TP,UK(email:Peter.Stanely@cyfoethnaturiolcymru.gov.uk)

AbstractTheDyffrynAddafromParysMountain,N.WalesisoneofthemostpollutingminewatersintheUKreleasingc.10tonnesofcopperperannumand24tonnesofzincperannumintotheIrishSea.TheMetalMinesStrategyforWaleshasrankeditfirst.Anacid,ironrichminewatervisiblebyitsochreousstainingalong3kmoftheAfonGochAmlwchtoitscoastaldischargeatPorthOffeiriad(PriestPort)hasanegativeimpactonbothriverandcoastalwaterqualityandlocalbusinessesandcommunities.SeveralinvestigationsusingActive,PassiveandHybridtreatmentprocessesemployingconventionaltreatmenttechnologiesaswellasPumptoseahavebeenconsidered,howeversuccessfultreatmenthasnotprovento be cost beneficial to date. This study shows that sono-electrochemical treatment(combinedelectrolysisandpowerultrasound)toproducemagnesiumhydroxidecanraisethepHof thewater, precipitate ironas insoluble ironhydroxide [Fe(OH)2] andhas thepotential to preferentially precipitate other metals in their stable hydroxide forms.Extrapolating the laboratoryresultsandmethodsto fullscale treatment(12 lsec-1 flowrate) indicates that it isaviableActive treatmentprocesscomparedtoother treatmentoptionsbeingconsideredandcanaidfailingwaterbodiesachievecompliancewiththeEUWaterFrameworkDirective.IntroductionParysMountain,N.WalesisoneofthemostpollutingminewatersintheUK,dischargingmoremetalsintotheIrishSeathantheRiverMerseydespitehavinglessthan0.3%oftheflowoftheRiverMersey.ParyshasanumberofdiscretedischargesofwhichDyffrynAddaisconsideredthemostchallengingwaterchemistrybeingmoreacidicandmetalrichthanmostsurfacewaterdischargesintheUK.TheDyffrynAddapollutes3kmoftheAfonGochAmlwchbeforedischargingintotheseaatPortOffeiriadcausingochreousstaininganddepositingcopper(c.10tonnesannum-1),zinc(c.24tonnesannum-1)andcadmium(~45kgannum-1)intotheIrishSea.Thedischargechemistryalsoincludesothercontaminants(Table1).Table1.DyffrynAdda–FlowConcentrations(12lsec-1[1040m3day-1])Total(ugl-1) Minimum Mean MaximumAcidity(asCaCO3) 1,194 1,743 2,210pH 2 3 3Iron 453,000 599,000 708,000Copper 30,600 38,130 52,600Zinc 55,900 65,311 92,900Manganese 14,300 20,470 30,400Aluminium 4 63,571 87,800Sulphate(asSO4) 1,940,000 2,534,000 3,020,000Cadmium 153 175 196Arsenic 129 360 662Nickel 154 189 240Lead 19 28 46A ‘ParysMountain Treatment Options Report’was commissioned in 2012 byNatural ResourcesWales(formerly Environment Agency Wales) (URS 2012) to review treatment options to ameliorate theenvironmental impactofParysMountainon theeconomicandwellbeingbenefits to local andnationalcommunities.Infurtherdiscussionsitwasconsideredthatthefirststageofameliorationwouldbetoreducethe total iron discharged from Dyffryn Adda to <1.0 mgl-1 and to reduce the ochreous visual impact

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dischargeintheAfonGochAmlwchandintothesea,nowtheNorthAngleseyMarinepossibleSpecialAreaofConservation.Thereleasesofiron,sulphateandhighlevelsofacidityareprimarilyaresultofpyriteoxidationcatalyzedbysulphuroxidizingbacteria(Barbesetal.,1968):

4FeS2+15O2+14H2O=4Fe(OH)3+8H2SO4(Sulphuricacid)TraditionaltreatmentmethodsforMinewatercangenerallybedividedinto2techniques:-

1. Active treatment which require ongoing mechanical / electrical operations and manualmaintenance e.g. aeration, liquid chemical pH neutralization, chemical cationic coagulants andprecipitation; membrane processes; ion exchange and chemical and / or biological sulphateremoval;

2. Passivetreatmenttypicallyreferstoprocessesthatdonotrequirehumanintervention,operation,

or maintenance, use gravity flow for water movement and promote the growth of naturalvegetatione.g.reedbedsystems.

Given the high concentrations of metals and low pH passive treatment at Parys Mountain is broadlyaccepted as being impractical and technically unfeasible. Active treatment comprising of High DensitySludge(HDS)withpHadjustmentusinglimeishoweverwidelyusedfortreatingacidminedrainage(AMD)water.Inadditionsomesuccesshasbeenachievedusingsulphidereduction(additionofsodiumsulphide)afterpHadjustment(causticsodadosing)asapotentialtreatmentoption.Insomecircumstanceshybridactive/passivesystemsmaybeemployede.g.semi-passive ironremovalwith targetedremovalofothermetalsbysulphideprecipitationorbiologicalremovalofironasschwertmanniteandremovaloftargetedmetalsbysulphidereductionalongwithpart treatmentandpartpumpedseadispersaloptions.Budgetcostsforthesevariousoptionsarepresented(Table2).Table2.CostEstimatesforIronremoval/reductionfromDyffrynAddaaditElement/Description CostEstimatePumpedseadispersalofpartiallytreatedminewater £0.6to£0.7MHighDensitySludge(HDS)Plant £1.8to£2.0MCapex/£0.5to£0.7MOpexHybridActive/PassiveTreatment(incl.Sulphidetreatment) £1.8to£2.5MCapex/£0.25MOpexThepresenceofmetalspecieseitherintheirionicformorinequilibriumastheirstableoxide,hydroxideforms,inawaterphasesuchasminewater,aredeterminedbytheirrespectivePourbaixsolubilitydiagrams(Pourbaix1964).Forexample,thePourbaixdiagramforIroninWater(Fig.1)indicatesthepredominanceofironasaqueousions(Fe2+,Fe3+)orasstableinsolubleoxideorhydroxideforms[Fe(OH)2,Fe(OH)3]basedonthepHofthewater(horizontalaxis)anditsEH(voltagepotential)i.e.oxidizingorreducingpotential(verticalaxis).ForexampleinDyffrynAdda(pH=3.13andEH=0.287)ironwillbepresentassolubleFe2+aqueousions.If theminewater is aerated (EH > 0.0) and pH corrected to < pH 6.5 ironwill precipitate as insolubleorange/brownferrichydroxide[Fe(OH)3].Underanoxygenreducedstate(EH<0.0)theironwillprecipitateas green ferrous hydroxide [Fe(OH)2]. Similar reactions can be achieved within an electrochemicaltreatmentprocessbyselectingappropriateelectrodematerialsandapplyingavoltageacrosstheanodeandcathode electrodes to control current density (Amps/Electrode area [Am-2]) (Fig. 2). Increasing –decreasingcurrentdensitycanberegardedaswayofspeedinguporslowingdownreactionsanddependingupontheelectrodematerialmakingreactionsmoreoxidizingormorereducing.Byapplyingavoltagetoanoxidizingover-potentialanodeelectrode(e.g.MMO–mixedmetaloxide)thewaterintheelectrochemicalreactorcanbeoxidizedandmadeacidic. 2H2O=H2O2(oxidized)+2H+(acid)+2e-Similarlybyemployingavoltagetoanalkalineearthmetalanodeinwater,thewaterbecomesreducedandalkalineduetotheformationofthehydroxideMg(OH)2. Mg2+(anode)+2OH-(cathode)=Mg(OH)2

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Fig1.Iron–WaterPourbaixdiagram Fig2.pHandEHvariationswithinanelectrochemicalreactor

Oncemagnesiumhydroxideisgenerateditoperatesbytheprincipalofionexchange,whereamagnesiumion(Mg2+)exchangeswithametalion(M2+)asshownbythefollowingequation: Mg(OH)2(adsorbent)+M2+(aq.)=M(OH)2(adsorbent)+Mg2+Although themetal is shown tobedivalent (2+) in thisequation forsimplicity, itmaybeofanyvalencyprovidedthationexistsasacationinsolution.Themagnesiumions(Mg2+)mayfurtherreactwithsulphateions(SO42-)toproduceinsolublemagnesiumsulphate.

Mg2++SO42-=MgSO4Ingeneral,thetendencyisforthesolubilityofthehydroxidesofothertreatedmetalstobelowerthanthatofmagnesiumhydroxideasshowninTable3andtoprecipitateinpreferencetomagnesiumhydroxide.Table3.SolubilityDataforMetalsMetalHydroxide KsP Solubility(molel-1)Copper-Cu(OH)2 2.2x10-20 2.8x10-7Cadmium-Cd(OH)2 1.7x10-15 1.2x10-5Chromium-Cr(OH)3 1.7x10-24 1.2x10-8Nickel-Ni(OH)2 6.5x10-18 1.9x10-6Zinc-Zn(OH)2 1.7x10-16 5.5x10-6Lead-Pb(OH)2 1.1x10-20 2.2x10-7Iron-Fe(OH)2 8.8x10-16 6.0x10-6Magnesium-Mg(OH)2 1.1x10-11 2.2x10-4TraditionalchemicalmethodsoftreatingAMDusingmagnesiumhydroxidearealreadydescribed(Bologoetal.,2009,2012)andreportgoodremovalratesformetalsfromsimilarAMDwatersinWitwatersrandBasininS.Africaandothersurfacefinishingandwastewaterstreams(Walteretal.,2015).Thesacrificialdissolutionofamagnesiumelectrodebyelectrolysistoelectro-generatemagnesiumhydroxidein-situinatreatmentprocesshoweverhasnotbeen reported.Theuseof electrolysis,which encompasses electro-coagulation,-flocculationand–flotation,havepreviouslybeentrialledandreportedfortreatmentofAMDwaters(Florence2013)usingiron,aluminiumsacrificialelectrodesandoxygenover-potentialelectrodessuchasMMOandEbonex(Hayfield2001).One major draw back with electrolysis (electro-coagulation) is passivation of the anode and cathodeelectrodesurfacesduringoperation.Suchfoulingcanleadtodeteriorationoftreatmentperformanceandexcessiveelectricalvoltagesbeingusedtoachieveacceptabletreatmentcurrentdensitiesinthetreatmentprocess.Passivationispartiallyovercomeinsomeelectrochemicalequipmentbyusinghighshearvelocitiesacrosstheelectrodesurfaces,polarityreversalandoff-lineelectrodeacidwashing.Thispaperreportsthenoveluseofsono-electrochemistry,electro-coagulationwithcombinedpowerultrasound(Morgan2014)for treatmentofAMDusingamagnesiumanode toproducemagnesiumhydroxide in-situ.Usingpowerultrasoundsimultaneouslywithelectrolysisremovestheionicboundaryandpassivationlayers(Sternand

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Helmholtzlayers)thatcandevelopalongtheelectrodesurfaceduringoperation,making‘fresh’electrodematerialavailablefortreatment.Thisreducestheelectricalresistanceofreactorcircuit,reducesthepowerrequirementandincreasestreatmentefficiencyandeffectiveness.MaterialsandMethodsWatersampleswerecollectedfromthesamplingflumeatDyffrynAdda(GISSH4380791223).TwolitrealiquotsamplesweretreatedusingaSoneco®(Power&Water)sono-electrochemicalreactorconsistingofstainlesssteelcathode,foursetsof28kHzultrasonictransducers,magnesiumanode,twolitreB-Kersquareflocculator jar and Watson Marlow peristaltic recirculation pump. Water samples were re-circulatedbetween the B-Ker and Soneco® reactor using the peristaltic pump. Following sonication and electro-coagulation,sampleswereflocculatedonaPhipps&Birdbenchflocculatorforoneminuteat250rpmwithadditionof5gofdriedmicro-sandballast(<150umdiameter,stirredat150rpmwithadditionof0.2mlpolymer for threeminutesandsettled for3minutesat0 rpm.50mlaliquot sampleswere then filteredthroughaWhatmanNo.10filterbeforebeingtestedforirononaHach-LangeDR3900spectrophotometerusingtheHachLangeIronTestkit(LCK320).DuetolimitedtimeconstraintsandtestmethodstheonlyothermetaltestedwascopperusingQuantofixCopper(0–100mgl-1)dip-test.Reactiontime,pH(HannaInstruments),EH(mV)(HannaInstruments)andamperageswerenotedduringthetreatmentprocedureandphotographsofthetreatmentreactionsweretaken.ThesewereusedalongwiththemechanicalandelectricalspecificationsoftheSoneco®reactortocalculateCapexandOpexforafull-scaletreatmentplant.ResultsTable4.pHchangeoverreactionperiod(13mins.)

Table5.EH(mV)changeovertreatmentperiod(13mins.)

Table6.Changesinsamplethroughtreatmentprocess

T0min

T3min T10min T12min T12min T13min

Untreatedsample

PrecipitationatpHc.3.5-3.7

PrecipitationatpHc.7.5

Additionofmicro-sandand

polymer

Rapidsettlement

Clarifiedsample

Table7.IronreductionTest Amperage Time [Fe]Start(mgl-1) [Fe]End(mgl-1)Run1 2 13 800 0.069Run2 1.5 17 800 0.007

3.00

4.00

5.00

6.00

7.00

8.00

9.00

1 3 5 7 9 11 13 15 17

pHUnits

No.Observations -400 -300 -200 -100

0

100

200

300

400

1 3 5 7 9 11 13 15 17

EH(mV)

No.Observations

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Table8.CopperreductionTest [Cu]Beforemgl-1 [Cu]aftermgl-1Run1 40 <1.0Run2 40 <1.0Based on the above tests its is estimated that the magnesium (Mg2+) dissolution required to both pHneutralizeandprecipitate ironto<1mgl-1 is325mgl-1.Basedonthisratiothepredictivescale-upcosts(capitalandoperating)basedonaflowof12lsec-1will:-

1. Capex-£840,000GBP-Soneco®Reactor&PowerSupplyexcl.solid-liquidseparationtank2. Opex-£220,000GBPperannum-Sono-electrochemicalplantexcl.replacementanodes

DiscussionTheresultsconfirmthatelectricaldissolutionofamagnesiumelectrodeproducesmagnesiumhydroxidewhich exhibits the same characteristics asMg(OH)2 powders and granules currently employed inAMDtreatmentbutovercomesthepotentialHealthSafetyandEnvironmentalimpactsofhavingliquidchemicalsandpHcorrectingchemicalsonsite.ThereactionraisesthepHofacidwaters,precipitatesironasferroushydroxide and other metal hydroxides. In common with Bologo et. al., 2012 ferric hydroxide is firstprecipitatedasaresultofraisingthepHfromc.3.0pHunitsto3.5-3.7pHunitstotheinsolubilityproductofFe(OH)3.AsthepHfurtherrisesandthewaterbecomesreducedironisfinallyprecipitatedasFe(OH)2.Theremovalofcopperduringthereactionsupportsthefindingsthatmagnesiumhydroxidewillprecipitateothermetalsbeforeitself.TheBologoet.al.,2009paperpurportsthatthesemetalhydroxidesprecipitateat1pHunitbelowtheirnormal insolubilityproduct.Accordingly it is likely that this treatmentmethodshould provide a ‘sweep’ removal of most alkaline mine water metals dissolved in Dyffryn Adda aditincludingcadmium.Extrapolatingthebenchscaleoperatingparametersandresults toa full-scale treatmentplant(12 lsec-1flowrate)indicatesthattheCapexandOpexwillbe£840,000GBPand£220,000GBPrespectively.Thesecostsareslightlylowerthanthebudgetcostsalreadypresented(Table2,above).

Fig.3P&IDSono-electrochemicaltreatmentplantforAMD

Fig.4Soneco®Sono-electrochemicalwatertreatmentforremovalofPhosphorusfromwastewater

Forfutureconsideration,theCapexandOpexcostscouldbeoff-setbytherecoveryofmetalfromthesludgeaftertreatment,andasreportedbyBologoetal.,2009,eventhesolublemagnesiuminthetreatedwatercouldberecoveredbyprecipitatingwithcarbondioxidegasandrecycledasmagnesiumhydroxidebacktotheheadofworkswithinthetreatmentprocess.Further,thepotentialuseofrenewableenergysourcesatsitecouldbeusedasanelectricalpowersourceforasono-electrochemicaltreatmentprocess.Forillustrationpurposes,aP&IDSono-electrochemicaltreatmenttotreatDyffrynAddaaditwaterisshownin (Fig. 3), together with a typical Soneco® (sono-electrochemical treatment plant) for removal ofPhosphorusfrommunicipalwastewater(Fig.4).Conclusion

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1. Electro-generation of magnesium hydroxide by electrolysis of a magnesium electrode has beensuccessfully demonstrated and that its properties for water treatment are similar to proprietarypowdersandgranularmedia.Thesepropertiesinclude:-

a. RaisingthepHofanacidwateranduseonAMDschemes;b. Electro-generatedmagnesiumhydroxidesuccessfullyprecipitatesFe2+aqueousas insoluble

ferroushydroxide;theresidualironconcentrationsofthetreatedwaterbeing<1.0mgl-1;c. Magnesium hydroxide will simultaneously precipitate other metals as their insoluble

hydroxideforms.2. ThepredictedCapexandOpexofasono-electrochemicaltreatmentplantisbroadlyattractivetoother

treatment/disposaloptionsbeingconsideredforDyffrynAddaaditwater.Thesecostscouldbeoff-setbytherevenueearned formmetalsrecovered fromthetreatedsludge.Electricalenergy forasono-electrochemicalplantcouldbesourcedfromarenewableenergysupplies.

3. Thispresentstudywasundertakenunderatimeconstraintandas‘proofofconcept’toquicklyassess

the viability of sono-electrochemistry (electro-generation of magnesium hydroxide) for watertreatmentforAMD.Havingachievedfavourableresultsandconclusions,furtherbenchtrials,extensivesamplingtestingandsitepilotstudywouldberecommended.

4. Thistechniqueistransferableandcanbeaccordinglyscalesformetalminewatertreatmentprojectsat

othermineschallengedbysteepterrainandexhibitingacidorochreousminedischarges,likethoseatCwmRheidolandCwmystwythminesalsobeingconsideredbyNaturalResourcesWales.

ReferencesURS (2012). ParysMountain – Treatment Options Report. Draft 30March 2012., Environment AgencyWales.ProjectReference46399817/MARP002.Barbes,H.J.andRomberger,S.B.(1968).ChemicalAspectsofAcidMineDrainage.J.WaterPoll.Contr.Fed.40(3)371–384.Pourbaix,M. (1964). Atlas of Electrochemical Equilibrium in Aqueous Solutions. Published by NationalAssociationofCorrosion(1974).ISBN10:0915567989.Bologo,V.,Maree, J.P.andZvinowanda,C.M.(2009).TreatmentofAcidMineDrainageUsingMagnesiumHydroxide.AbstractsoftheInternationalMineWaterConference,ProceedingsISBNNo.878-0-9802623-5-3,19th-23rdOctober2009,Pretoria,SouthAfrica.Bologo,V.,Maree,J.P.,andCarlsson,F.(2012).ApplicationofMagnesiumHydroxideandBariumHydroxidefortheremovalofMetalsandSulphatefromMineWater.WaterSAvol.38n.1Pretoria.Walter,M.D.,Witkowski,J.T.,andGibson,A.(2015).RemovalofMetalsfromMetalFinishingWasteWaterUsingaGranular,Magnesium-BasedAdsorbent.MartinMariettaMagnesiaSpecialties,LLC.8140CorporateDrive,Suite220,Baltimore,Maryland21236USA.Florence,K.M,andMorgan,P.G.(2013).Anovelelectrochemicalprocessforaqueousoxidationofacidminedrainageusingadvancedpowerelectronicsandrealtimecontrol.IMWAConference“ReliableMineWaterTechnology”IMWA31stMay2013,GoldenCO;USA.Hayfield, P.C.S. (2001). Development of a New Monolithic Ti4O7 Ebonex Ceramic. Published by RoyalSocietyofChemistry.ISBN10:0854049844.Morgan,P.G.(2014).BritishPatentNo.GB1503638.7–Methodandapparatusfordecontaminationoffluids(Soneco). International Patent PCT/GB2016/050692 – Method and apparatus for decontamination offluids.