Characterization of Soluble Algal Products (SAPs) after electrocoagulation of a mixed algal culture

Delft University of Technology

Characterization of Soluble Algal Products (SAPs) after electrocoagulation of a mixed

algal culture

Rafiee, Poorya; Ebrahimi, Sirous; Hosseini, Maryam; Tong, Yen Wah

DOI

10.1016/j.btre.2020.e00433

Publication date

2020

Document Version

Final published version

Published in

Biotechnology Reports

Citation (APA)

Rafiee, P., Ebrahimi, S., Hosseini, M., & Tong, Y. W. (2020). Characterization of Soluble Algal Products

(SAPs) after electrocoagulation of a mixed algal culture. Biotechnology Reports, 25, [e00433].

https://doi.org/10.1016/j.btre.2020.e00433

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Characterization

of

Soluble

Algal

Products

(SAPs)

after

electrocoagulation

of

a

mixed

algal

culture

Poorya

Ra

_fiee

,

Sirous

Ebrahimi

**

,

Maryam

Hosseini

,

Yen

Wah

Tong

*

a

BiotechnologyResearchCentre,FacultyofChemicalEngineering,SahandUniversityofTechnology,Tabriz,Iran b

FacultyofChemicalEngineering,AzarbaijanShahidMadaniUniversity,Tabriz,Iran c

ChemicalEngineeringDepartment,NationalUniversityofSingapore,Singapore d_{DepartmentofBiotechnology,DelftUniversityofTechnology,Delft,Netherlands}

ARTICLE INFO Articlehistory: Received30July2019

Receivedinrevisedform9February2020 Accepted9February2020

Keywords: Electrocoagulation Microalgae Mixedculture SolubleAlgalProducts Harvest

ABSTRACT

Thedewateringofalgalculturerequirescoagulationofthealgalcells.However,thecoagulationina continuous operation is slowed down through the excretion of Soluble Algal Products (SAPs). Electrocoagulation(EC),alreadyutilizedasacoagulationtechnique,hasbeeninvestigatedforitseffects on SAPscharacterizations.A mixedcultureofChlorellavulgaris,ScenedesmusObliquus,Botryococcus braunii,Botryococcussudeticus,andAfrocarpusfalcatuswaspreparedandSAPscharacteristics,including SpecificUltraVioletAbsorbance(SUVA),Zetapotential,MolecularWeight(MW)fractionation,Dissolved OrganicCarbon(DOC),proteinandcarbohydratecontent,Excitation-EmissionMatrix,and hydropho-bicityusingXADresins,weremeasuredandevaluatedbeforeandafterelectrocoagulationusingmild steelandaluminumelectrodesat5and10min.TheresultsshowedseveralimprovementsafterEC. Accordingtoresults,ECcanrenderSAPshydrophobicityupto95%,andthefluorescencepeakresults showedthecompleteremovalofhumic-like.Moreover,theSAPswereremovedupto21,60,and47%for protein,carbohydrateandDOC,respectively.Resultscollectivelyshowedthatelectrocoagulationmight beabletomitigatethenegativeeffectsofgrowthonflocculation.

©2020TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

The combination of wastewater treatment and microalgal cultivationhasbecomeapromisingwastewatertreatmentstrategy [1]. For a successful result, however, the biomass must be efficiently harvested from the cultivation broth. In addition, microalgaerequire3726kgfreshwatertoproduce1kgbiodiesel withoutrecycling[2].Althoughmembranebioreactorshavebeen proved to be successful [3], the membrane fouling by algal, bacterial, and SAPs sources highly reduces the efficiency and membranelifespan[4].Moreover,theeffectofSAPsisprovedtobe moredominantthanalgalandbacterialsources[5].Furthermore, Dischargingthemediumintothepipenetwork,rivers,orlakes, willleadtobacterialgrowth,sinceSAPsserveasagreatcarbon source. Additionally, SAPs impact the taste and odor of fresh waters.Ontheotherhand,keepingSAPsinthemediumhasshown toaffectthemicroalgalgrowth[6].Finally,studieshaveshownthe

linksbetweentheSAPsandproductionofprecursorsof disinfec-tionby-products(DBPs)[7–9].Thus,removalofSAPseffectseems tobecriticalformanyoperations.

SAPs include carbohydrates, protein, lipid, polysaccharides, aminoacids,peptides,glycolicacid,organicphosphorus,enzymes, vitamins, hormonal substances, pigment, inhibitors and toxins [7,10].ProteinandcarbohydratesdominatemostofSAPs[11].They originatefromthreesources:intracellularorganicmatter(IOMs) lysedfromdeadcells,desorbedsurface-retainedorganicmatters (SOMs) from cell surfaces, or secreted from living cells as extracellular organic matter (EOMs) [11]. Some SAPs like polyunsaturatedaldehydesandprecursorsactassignalmolecules andcontrolthepopulationdensity[12].ThepresenceofSAPsalso largelyaffectsthedosageofcoagulantsnecessaryforflocculation of microalgae in harvesting stage [13–16]. SAPs with high molecular weight (MW) can aid coagulationwhereas those of lowMWcanincreasethenegativesurfacecharge[17].

Althoughtreatmentstrategies,suchasbiologicalmethods[18], coagulation[14], and ozonation[19] havebeenintroduced,the applicationofelectrocoagulationanditseffectsonthedecreaseof SAPs amounthavenot beenfullyinvestigated.In addition,this paperaims tostudya mixedcultureof microalgaewhich with furtherdevelopmentinlargerscalecansimplifytheoperationsin

* Correspondingauthor.

** Correspondingauthorat:BiotechnologyResearchCentre,FacultyofChemical Engineering,SahandUniversityofTechnology,Tabriz,Iran.

E-mailaddress:chetyw@nus.edu.sg(Y.W.Tong).

https://doi.org/10.1016/j.btre.2020.e00433

2215-017X/©2020TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

ContentslistsavailableatScienceDirect

Biotechnology

Reports

industrybyremovingthecostsnecessaryformonoculturegrowth. Understanding the effects of the electrocoagulation on the characterization of SAPs in a mixed microalgal cultivation can facilitatethecontinuousalgaldewateringandharvesting. 2.Materialsandmethods

TwosetsofECexperimentswereconductedindependentlyto ensure the reproducibility of the results. Samples of each set underwentSAPscharacterizationtests.SAPswerecharacterizedby compositionthroughproteinandcarbohydratetests, Excitation-Emission Matrix (EEM) by Fluorescence Spectrophotometer, hydrophobicity/hydrophilicityusingXADresins,MolecularWeight (MW) fractionation using fractionation membranes, dissolved organiccarbon(DOC),specificultravioletabsorbance(SUVA)using spectrophotometricmeasurements,andzetapotential(ZP) utiliz-ingaZetasizerbeforeandafterelectrocoagulationandtheresults werecomparedaccordingly.Thedetailsofeachmeasurementhave beenprovidedbelow.

2.1.Microalgaemediumandcultivation

A mixed culture containing Chlorella vulgaris, Scenedesmus Obliquus,Botryococcusbraunii,Botryococcussudeticus,andAfrocarpus falcatus was prepared and inoculated into a 4-liter cylindrical photobioreactor(PBR)fedwithBold’sBasalMedium(BBM)up to3.5 liters.ThePBRwasilluminatedusingfour13W6700Kflorescent lampsandaeratedwith0.5vvmofamixedflowofairandCO2(1.75

LPMairandits5%CO2flow).Inadditiontotheairflow,amagnet

stirrerwasutilizedforfurthermixing.ThefinalDryWeight(DW)of thePBRwas2mg.l 1.Allanalyseswereconductedwithin4daysof extractionandkeptat4Cinthemeantime.

2.2.Electrocoagulationcell

TheECcellconsistedofa250-mililiterbeakerequippedwith aluminum (Al)or mild steel (Fe)electrodes connectedto a DC powersupply. The sample volume was 200 milliliters, and EC lengthswere5and10min.Thesamplenamingwasaccordingto

Table1.Al5andAl10fromnowforthwillrefertosamplesfrom aluminumelectrodeswithECtimeof5and10minrespectively,as wellasFe5andFe10willdetermineelectrocoagulationusingmild steelelectrodesfor5and10min.Eachsamplewaslefttosettlefor 5minbeforesampling.Thecurrentdensityforallexperimentswas 250A.m 2_{,andtheinter-electrodedistancewas1cm.Thevoltage}

wasalsomonitoredalongallexperiments.Thecellsweremixed usingamagnetstirreratafixedspeedforallsamples.Toobtain two samples of each Fe5, Fe10, Al5, and Al10, two sets of electrocoagulationexperimentswererun.

2.3.SAPsextractionmethod

SAPswereextractedbycentrifugingthesamplesat12857g RCF for 15min at the roomtemperature, and the subsequent

filteringofthesupernatantthroughaglassmicro-fiber(0.7mm WhatmanGF/Fglassmicro-fiber)filter.

2.4.SAPscharacterizations 2.4.1.Biochemicalcompositions

ProteinsweremeasuredthroughmodifiedLowrymethod[20] and carbohydrates were measured using the phenol-sulfuric method[21].Allanalyseswerecarriedoutintriplicatesanderror barswereprovidedusingstandarddeviationforthegraphs. 2.4.2.Fluorescencespectroscopy

EEMvalueswere obtainedusing a CaryEclipseFluorescence Spectrophotometer(Agilent,Netherlands)anda4-mililiterlength cuvettewith1cmpath.Emissionspectrawerescannedfrom300to 500nmat0.5nmincrementsandexcitationspectrawerescanned from 250 to 400 nmwith 5 nm increments [22]. The slits for excitationandemissionwere5nm.Inaddition,thedeionizedwater blankswereusedbetweenevery4analyses.Ithastobenotedthat EEMvalueshavebeenusedtoprovideamorequalitativeratherthan aquantitativeperspectivetotheinvestigationresults[7].

2.4.3.XADresinfractionation

A pairof XAD-7HP/XAD-4 columns was usedto fractionate SAPs into hydrophobic and hydrophilic components [23]. A 250-ml SAPs samplewas acidified topH 2, and it waspassed consecutively through the XAD-7HP and XAD-4 resins (5 ml resinsin10mmglasstubes).Thenon-retainedsamplecomprised thehydrophilicfraction(HPI).Eachcolumnwasback-elutedwith NaOH(0.1M,120ml)suchthattheXAD-7HPandXAD-4resin back-effluent comprised the hydrophobic fraction (HPO) and transphilicfraction(TPI),respectively.Thecarbohydratecontent ofallfractionswasmeasuredaspreviouslydescribed.

2.4.4.MWfractionation

Vivaspin membrane (Sartorius, UK) was used to separate differentsizesofbiomolecules.Membranesizesof2,5,10,30,100, and300kDawereselected.Themembraneswerecentrifugedat thespeedandtimegivenbythecompanymanual.Thesupernatant of each size centrifugation was collected and tested for total carbohydrateandproteincontent.

2.4.5.DOCandSUVA

Dissolved Organic Carbon (DOC) was measured using an analyzer(ShimadzuTOC-5000A,Japan)bymeasuringthe differ-ence between total organic carbon and inorganic carbon. A UV/visible spectrophotometerwas used at 254nm to measure theabsorbanceatthiswavelength.Next,theSUVAwascalculated as UV254/DOC to reflect the concentration of the unsaturated

compounds(includingaromaticcompounds)inthetotaldissolved organiccarbon.

2.4.6.Zetapotentialandchargedensity

ZetapotentialwasmeasuredusingaZetasizer(Malvern,UK). ThesampleswerekeptatpH=7usingphosphatebufferandthe constanttemperatureof25C.

3.Resultsanddiscussion 3.1.Molecularweightfractionation

ElectrocoagulationwasconductedusingAlandFeelectrodes andtheSAPscharacteristicswereanalyzedtoinvestigatetheeffect of EC on SAPs. The results of total carbohydrates vs. MW

Table1 Samples’Naming.

Name Description

Al5 Samplewith5minutesofelectrocoagulationusingaluminum electrodes

Al10 Samplewith10minutesofelectrocoagulationusingaluminum electrodes

Fe5 Samplewith5minutesofelectrocoagulationusingmildsteel electrodes

Fe10 Samplewith10minutesofelectrocoagulationusingmildsteel electrodes

fractionationforcontrolandstudysamplesafterECaredepictedin theFig.1.AlthoughMWoftheSAPsvariessignificantlyindifferent species,themoleculesaremainlyintherangeoflessthan3kDaor morethan30kDa[11].Thecontrolsampleincluded40%oftotal carbohydrateinSAPsMWabove30kDaand42%inSAPsMW below2kDa.Similar resultswerealso foundinthe literature [17].Aftercoagulationbiggercarbohydratesareexpectedtobe separatedthroughsettlingorfloatation.Thus,thepercentageof carbohydratesbiggerthan30kDais expectedtofallwhilean increaseinpercentageof carbohydratessmallerthan3kDais predicted to be observed. Except forAl10, the total carbohy-dratesvaluesforalltheothersamples_fittothepredictions.The resultsforAl10mightbeassociatedwithover-dosageofelectric charge leading to smaller EC efficiency. In addition, the increasing results of carbohydrate between 30 and 100 kDa forAl10 may also suggest the inefficiencyof EC to coagulate thesemedium-sized particlesresultingin their suspension in finalmedium.Accordingtotheresults,thepercentageoftotal carbohydratecontainingmoleculesbiggerthan30kDafellfrom 40%incontrolsampleto26%,14%and8%inAl5,Fe5,andFe10, respectively. On the other hand, the percentage for carbohy-dratessmallerthan2kDarosefrom42%inthecontrolsample to,respectively,52%,76%,and72%intheAl5,Fe5,andFe10.It mustbe notedthat the carbohydrateanalysis shows thatthe totalamountincontrolsamplewas18.75mg.l 1_{.Thisvaluefell}

into15.41,14.78,18.03, and 13.15 mg.l 1_{. These values might}

suggestthata part ofcarbohydrates wasseparated,since the samples were taken from the solution and notthe top layer separatedbyelectrocoagulation.

Thecarbohydrateseparationissmallerthanreportedvalueby thechemical coagulation[14].However,thetotalcarbohydrates valuewasapproximately1mg,which,consideringthevolume,is almostaquarteroftheamountinthisstudy.

TheproteincontentofthesampleafterECwasalsomeasured andtheresultsfromMWfractionationareillustratedinFig.2.The controlsamplecontained16%ofitstotalproteinatMWlargerthan 30kDa.Also,67%oftheSAPscontainedoligopeptides,proteins smallerthan 2kDa.Theseresultscontradictwiththeresultsin previous works [4,17] where the major part of the proteins containedlargerMWfractions.Theseresultsmightbeassociated withtheextractiontechnique[4].ExceptforAl10,theproteinsize followedthesamepatternascarbohydrates.Theproteincontent tendedtowardsthesmallerMWfractionsafterEC.Theproteins largerthan30kDaconsisted%13,5,5ofSAPsinAl5,Fe5,andFe10, respectively.

Theprotein:carbohydrateratiointhecontrolsamplewas1.66 mg mg 1. This ratio fell into 0.84 and 1.29 for Al5 and Fe5, respectively,butroseto3.2and2.05mgmg 1_{forAl10andFe10,}

respectively.Ononehand,theprimaryfallinprotein:carbohydrate ratioin both Fe5and Al5maybe associatedwiththefact that proteinsareconsiderdaschargenuetralizers[24].Inotherwords, thesurfacechargeontheproteinswillinteractwithionsgenerated byelectrocoagulation.Thesubsequentchargeneutralizationwill leadtoenhancmentofcoagulation,sinceparticlescanapproachto formflocs.Thisprocedureconclusivelycausestheseperationfrom theculture(theprimaryfallofprotein).Ontheotherhand,SAPs aredominatedbyhydrophobicproteinsandhydrophylic carbohy-drates[17],andthedataobtainedinthisresearch(refertoXAD fractionation resluts) show that EC operation largely improves hydrophobicity leading to sepration of more carbohydrates by floatation. This conclusion is supported by the fact that the percentage of total carbohydrate value with MW larger than 30kDahasfallen.Collectively,thebigriseofprotein:carbohydrate ratio inFe10 and Al10ismore likelytobe associatedwiththe seperationoflargehydrophiliccarbohydratescomparedtosmall proteins.

Itmustbenotedthataccordingtothereactionsattheanode [25],withthesameamountofmass,aluminiumgeneratesmore ions, neutralizing more charges and leading to better results compared to steel. Furthermore, studies have shown that the affinity of protein to form complexes with organic matter is differentwiththeaffinityofproteinwithiron.[26].Nevertheless, onlytheaffinityat60kDaontwopuremicroalgaespecieshasbeen investigated,andfurtherinvestigattionsarenecessaryto under-standtheimpact.

3.2.Fluorescencespectroscopy

Theresultsoffluorescencespectroscopyareillustratedinthe

Fig.3.Qualitatively,humic-likesubstances,whichcompriseofan importantpartofSAPs,willshowapeakattheExcitationvalueof 350nmandanEmissionrangeof420 480nm[27].Asitcanbeseen inFig.3.a,thispeak,markedbytheletterH,canbeobservedbya relativelymoderateintensity.However,onalltheotherEEMs,the intensityhasdramaticallyfallen.AccordingtoFig.3,inAl5,Al10,Fe5, and Fe10, the H peak intensity representing the humic-like substances has decreased. This weakened intensity might be interpretedastheseparationofhumic-likesubstancesthroughEC application.Nevertheless,theremainingstripsonalltheflorescence mapshavealsobeenpresentinotherworksonSAPsanalysis[17].

3.3.Zetapotential

To reacha senseofcomparison andconsistency,alltheZeta potentialmeasurementswerecarriedoutforECrunsof5and10 min.However,theZetapotentialsofthesamplesincreasedalmost twicethe-14.1mVincontrol sample.Thiscouldbedue tothe chargeoverdose[28].Asaresult,onlyinZetapotential measure-ments,theexperimentswereexecutedfor60s.Consequently,a

sharpchangeofZetapotentialwasobtainedforboth AlandFe electrodes.TheZetavaluesforAlandFewere-0.5and-1.2mV, respectively.TheZetapotentialmaybelinkedtotheionizationof functionalgroupsinthecolloidalorganics.Althoughthedecrease inZetapotentialcanvaryindifferentspecies[14],thefactthatthe SAPschargecanbepartiallyorcompletelyneutralizedmaysuggest thefunctionalityofECforthereductionofSAPsinthemedium.The results can be comparable with chemical coagulation where a range of 90 5000 mg.meq 1 _{coagulant dose:charge equivalent}

Fig.2. MWdistributionsofSAPsintermsofproteincontentpercentageforcontrolsample,andtheelectrocoagulatedsamplesofAl5,Al10,Fe5,andFe10.

Fig.3.Fluorescenceexcitation–emissionmatrix(EEM)spectrafor(a)controlsamplewithamoderate-intensitypeakforhumic-likesubstances(with“H”mark);(b) Electrocoagulatedsamplewithaluminumelectrodesfor5minshowingnohumic-likemoderate-intensitypeak(Al5);(c)Electrocoagulatedsamplewithaluminum electrodesfor10minshowingnohumic-likemoderate-intensitypeak(Al10);(d)Electrocoagulatedsamplewithaluminumelectrodesfor10minshowingno moderate-intensity humic-likepeak (Fe5); (e)Electrocoagulated sample with aluminum electrodesfor 10 min showing nomoderate-intensity humic-likepeak (Fe10). Z-axis=excitation(nm);X-axis=emission(nm);andY-axis=intensity.

was usedtoachieve similarneutralization[14]. Hereagain the aluminum worked better in neutralizing the charge, since for constantmassitcanproducemoreions.

3.4.DOCandSUVA

Thecarbohydrate:DOCratiofor thecontrolsamplewas 0.49 mg. mg 1_{. Although it is suggested by other studies that}

carbohydrate:DOC ratio is consistent around 1 mg. mg 1 _for

differentspecies [17], it hasbeenshown that this ratio largely dependsontheextractionmethodincludingtemperature,pH,and centrifugationspeed.Theresultsinthatstudyshowthatathigher centrifugation rates, such as this study, DOC values can be significantlylargerthan carbohydratevalues leading tosmaller carbohydrate:DOCratio.Inaddition,centrifugationtimecanaffect the DOC values much more than carbohydrate values [4]. Accordingtothemeasurementscarbohydrate:DOC ratiosforall samplesslightlychanged.(Table2).

SUVA is an indicator for the presence of aromatic carbon compounds.TheSUVAofthecontrolsamplewasmeasuredtobe 0.67l.m 1_.mg 1_{whichmatcheswithotherworks[17].Thisvalue}

forAl5andAl10fellinto0.44and0.46l.m 1_.mg 1_{,respectively.}

However,theSUVAvaluesforFe5andFe10increasedto0.69and 0.71 l.m 1. mg 1, respectively. These results indicate that the aromaticcarboncontentincaseofAlelectrodeshasfallenby30%. TheincreaseinSUVAvaluesforFeelectrodes,ontheotherhand, mighthaveoccurredasaresultofinterferenceofferric(Fe+3)and ferrous(Fe+2_{)ironaspreviouslydiscussedbyresearchers[29].}

3.5.XADresinfractionation

AsitcanbeseenfromFig.4,thealgalcultureinthisstudyis highly hydrophilic(54 %). Similarresults wereobtained inthe literature[4,17].Thishighamountofhydrophiliccontentusually includespolysaccharidesandhydroxylacids[30].Inmembranes, fouling is largely caused through hydrophilic compounds [8]. AfterEC,thehydrophilicitydecreasedinto4and5percentforAl5 and Al10 samples, respectively, and almost zero in both Fe samples. The hydrophobicity is the primary force for the hydrophobic articles and air bubbles [31]. In addition, cross-linkingofproteinscoagulationwhichissequenttodenaturation oftheproteinsusuallyinvolvespolyfunctionalagentsincluding metal ions in solution [32]. Given the fact that _floatation by bubblesisconsideredoneofthemajormechanismsof electro-coagulationinseparatingcolloids,theincreaseinhydrophobicity will, therefore, enhance the efficiency of coagulation. As the resultsshow,ECitselfcanincreasethehydrophobicity.Inother works in literature, hydrophobicity was improved by adding

collectors and raising the dosage [33]. The main measure of hydrophobiccompoundscomprisesoflargemolecules(>10kDa). On the other hand, hydrophilic compounds largely contain smaller molecules (<1 kDa) [34].Consequently, by raising the amount of hydrophobic molecules in the algal culture, the subsequent coagulationwill begreatlyimproved.Furthermore, hydrophilic compoundsmainlyincludeneutralpolysaccharides andacidicsugars[30].Giventhecarbohydrate:DOCratioafterEC and thefactthatbothDOCandtotalcarbohydratevalueshave decreasedindependently,thedatasuggesttheseparationofmore hydrophiliccompounds.

AsummaryofcharacterizationofSAPsinallsampleshasbeen collectedinTable2.

3.6.SAPremoval

Accordingtoonestudy,algalorganicmatterwasremovedat anefficiencybetween46and71%[35]whileinanotherstudy this efficiencyrangedbetween18and50%[36].Here,inthis studySAPs,intermsofDOCwasremovedby35,40,42,and47% for Al5,Al10, Fe5, andFe10, respectively. In terms of protein contentonly,resultsshowthatSAPswerereducedby5,20,8, and21%forthesamesamples.Andfinallyasforcarbohydrate perspective,ECseparatedtheSAPsby41,60,39,and56%.These resultsarequitecomparablewiththeabovementionedstudies. ItmustbenotedthattheefficiencyoftheSAPsremovalislargely dependent on factors like pH, and MW fractionation of thecontent. pH,forinstance,determines whichcomplexation

Table2

AsummaryofsamplecharacteristicsafterECoperationscomparedwithcontrolsample.

Parameters Control Al5 Al10 Fe5 Fe10

SUVA(lm 1_mg 1_{) 0.67 0.44 0.46 0.69 0.71} Hydrophobicity(%) 39.6 95.5 85.8 78.6 76.9 Hydrophilicity(%) 54.7 4.3 5.6 0.13 0.27 Zetapotential(mV) 14.1 0.5† 1.2a Carbohydrate:DOC(mgmg 1 ) 0.49 0.44 0.32 0.5 0.41 Protein:carbohydrate(mgmg1 ) 1.66 0.84 3.2 1.29 2.05

Fluorescencepeak Humic-likesubstances none none none none SAPscarbohydrates>30kDa(%) 40.64 25.82 56.19 13.73 8.27 SAPscarbohydrates<2kDa(%) 42.40 52.85 12.44 75.69 71.98 SAPsproteins>30kDa(%) 16.44 13.23 17.15 5.11 5.13 SAPsproteins<2kDa(%) 66.92 63.21 50.86 81.06 74.69 †_{Experimentswerecarriedoutin60sasexplainedinthecontent.}

Fig.4. PercentageoftotalcarbohydratesintheSAPpresentintheHPO,TPIandHPI fractions for control sample (without EC) and other samples undergoneEC experiments.

wouldoccurandthereforechangescanaffecttheseparation[37]. In other words, the SAPs separation can be improved by parametricoptimizationofEC using pH,currentdensity, inter-electrodegap,time-length,etc.Whilethe“optimization”ofSAPs removalmaybethesubjectofaseparatestudy,inthispaperthe “possibility”ofthisremovalusingECwasmostlyfocused.Onthe otherhand,theresults hereshowthata largeportionof SAPs contains low MW molecules. These small molecules cause inefficient flocculation, since the SAPs must cross-link metal compoundstoformflocs[14].Asaresult,evenincaseofsmaller efficienciesofremoval,anincreaseinthedemandofcoagulants willbemostlikelytoresultinhigherremovalef_ficiencies.Itmust beremarkedthatwithproperpreparationslikesufficientdosage ofcoagulants,andoptimumpH,metal-organiccomplexescanbe removedrapidly using theprocess of sweep flocculation [38]. Thus,aseparatestudyisrequiredtoinvestigatetheoptimization ofECforSAPsremoval.

4.Conclusion

Inthepresentresearch,theSolubleAlgalProducts(SAPs)were characterizedafteranelectrocoagulationoperation.Basedonthe resultsobtained,electrocoagulationpositivelyaffectstheSAPs.In otherwords,thoseSAPsfeatureswhichdecreasetheefficiencyof coagulationovertimewereeffectivelyremovedormitigated.Asa result,incaseofcontinuouselectrocoagulationforalgalharvesting anddewatering,theeffectofSAPs whichremainin theculture aftereach cycle of operation can be moderated. Moreover, for similarreasons if other technologiesare aimed tobe used for separatingalgalbiomass,electrocoagulationseemsquite promis-ing as a pretreatment unit. However, a further study on the parametersofelectrocoagulationseemsnecessarytooptimizethe workofsuchtarget.

Authorstatement

Thismanuscriptisajointworkbyalloftheauthors.PRafieeis responsiblefor writing themanuscript, conducting the experi-mentsandanalyzingthedata.SEbrahimiand YWTongjointly helpedtoconceptualizetheideaand developthemethodology. MHosseiniprovidedguidanceindataanalysis.

DeclarationofCompetingInterest

Wewishtoconfirmthattherearenoknownconflictsofinterest associatedwiththispublicationandtherehasbeennosignificant financial support for this work that could have influenced its outcome.

Acknowledgements

Thisresearchdidnotreceiveanyspeci_ficgrantfromfunding agenciesinthepublic,commercial,ornot-for-profitsectors. AppendixA.Supplementarydata

Supplementarymaterialrelatedtothisarticlecanbefound, intheonlineversion,atdoi:https://doi.org/10.1016/j.btre.2020. e00433.

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