Performance of green antiscalants and their mixtures in controlled calcium carbonate precipitation conditions reproducing industrial cooling circuits

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To cite this version:

Chhim, Norinda

and Haddad, Elsi and Neveux, Thibaut

and Bouteleux,

Céline

and Teychené, Sébastien

and Biscans, Béatrice

Performance

of green antiscalants and their mixtures in controlled calcium carbonate

precipitation conditions reproducing industrial cooling circuits. (2020) Water

Research, 186. 116334. ISSN 0043-1354

Official URL :

https://doi.org/10.1016/j.watres.2020.116334

Performance

of

green

antiscalants

and

their

mixtures

in

controlled

calcium

carbonate

precipitation

conditions

reproducing

industrial

cooling

circuits

Norinda

Chhim

a,b

_,

_Elsi

_Haddad

b

_,

_Thibaut

_Neveux

a,b

_,

_Céline

_Bouteleux

a,b

_,

Sébastien

Teychené

b

_,

_Béatrice

_Biscans

b,∗

a EDF Lab Chatou, 6 Quai Watier, 78401 Chatou Cedex, France

b Laboratoire de Génie Chimique, Université de Toulouse, CNRS, INPT, UPS, 4 Allée Emile Monso CS84234, 31432 Toulouse, France

a

b

s

t

r

a

c

t

Coolingcircuits inmanyindustrialsectorsarefacedwithdailyissuesofscaling.Onepreventive treat-mentconsistsininjectingapolymeradditiveinthecircuittoinhibitprecipitationofcalciumcarbonate. Amongtheusedadditives,veryfeware“green” andtheefficiencyofnewcandidatesaredifficulttotest directlyinindustrialconditions.Thepresentstudycomparedperformancebetweentwo“green” polymer additives,polyasparticacid(PASP)andpolyepoxysuccinicacid(PESA),versusatraditionalgold-standard, homopolymerofacrylicacid(HA)inalaboratoryscaleset-updesignedtoberepresentativeofan indus-trialcircuit.Resultsshowed thatHAandPASParebothinhibitorsofcalciumcarbonatecrystalgrowth. Thisinhibitionresultedfrom adsorptionofpolymeradditivemolecules onthe crystalsurface,as con-firmedby adsorption measurement.Under thesame conditions,PESAadditive, showedahigh rateof calciumioncomplexationandaverylowinhibitionrate.But,PESAwasshowntobeanucleation de-layer.MixingPESAandPASPcangavenucleationretardationofabout19h,whichapproximatesthe24 hwater residencetime inindustrialcooling circuits,as wellas almost90% calciumcarbonatecrystal growthinhibition.Thissynergyofferspromisingprospectsforpreventivescalingtreatment.

1. Introduction

Industrial cooling circuits are subject to clogging anddeposit of mineral particles such as calcium carbonate (CaCO3)

associ-ated to the suspended matter naturally present in cooling wa-ters. Deposits are localized inside the condenser tubes and on the PVC packing in cooling towers. They can lead to layers of hard scale, increasing heat transfer resistance and thus impair-ing heat exchange. This is a serious problem in industry, espe-cially in high energy consumption equipment such as thermal and nuclear power plant cooling systems (Neveux et al., 2016;

Rahmanietal.,2015),reverseosmosismembranedesalination( Al-HamzahandFellows,2015;Phuntshoetal., 2014),distillation( Al-Rawajfeh, 2008;GhaniandAl-Deffeeri, 2010) andpetroleum pro-duction(Hemingetal.,2015).Inelectricpowerplantcooling sys-tems, CaCO3 precipitation and/or deposition depends on various

factors such as circuit operating conditions (temperature range,

∗ _{Corresponding author.}

E-mail address: beatrice.biscans@toulouse-inp.fr (B. Biscans).

concentration factor, water residencetime) andcirculating water quality (notably, concentrations of calcium and bicarbonate and ofsuspendedmatter)(Chhimetal., 2017). Varyinghydrodynamic regimes (turbulent in condenser tubes and laminar on the PVC packing of cooling towers) and construction materials (concrete housing,condensermaterial,PVCpacking)alsocontributetoscale deposition.Technically,depositsimpairheatexchange,partiallyor totallyclog tubingand reduce equipment life-expectancy, and fi-nanciallyleadtoloss ofproductivityandincreasedoperatingand preventiveandcurativemaintenancecosts.

Polymeradditiveshavebeenwidelystudied,fortheireffecton dispersionofsuspendedmatter toreduceclogging andon inhibi-tionofscaling(Hassonetal.,2011;Buetal.,2016).

Additives commonly used for scale inhibition are poly-mers containing carboxylic acid groups such as polyacrylic acid (Moulaya et al., 2005; Al-Hamzah et al., 2014; Amjad et al., 2014), polymaleic acid (Shen et al., 2012; Amjad et al., 2014) and polyaspartic acid (Quan et al., 2008; Pramanik et al., 2017), whichisa well-known greeninhibitor intermsof biodegradabil-ity,non-toxicity andnon-bioaccumulation (Martinod etal., 2009;

including low concentration effectiveness and high temperature endurance.

Those last years, a lot of efforts have been done to gener-ategreen inhibitors.Intheir review,Chaussemieretal.(2015) fo-cusedonsomegreenantiscalants,obtainedeitherbyusing “natu-ral” organic molecules orextracted fromplants. The authors de-scribed some positive performance in laboratory tests but un-derlined that trials of these molecules in pilot plant are limited (Schweinsbergetal.,2003).However,astudyontheinhibition per-formancesofpolyasparticacidonacooling-waterpilotinstallation by Laborelec company (Belgium) can be found in Girasa and De Wispelaere(2004).

Several works in the literature were focused on polyepoxy-succinic acid (PESA) and polyaspartic acid (PASP) because they are green scale inhibitors given their non-nitrogenous, non-phosphorus, and biodegradable features. Experimental results of

Zhou et al. (2011) obtained by static beaker test on inhibition to CaCO3, demonstrated that PESA functioned excellent nucle-ation inhibition to CaCO3. The scale inhibition ratio was calcu-lated from the change in cation concentration. Tests were per-formed at temperature 80°C, solution pH 9.0, and reaction time 10 hr. Above 99.0% of the scale inhibition ratio was obtained whenPESA dosageis30.2mg/Lwith500 and720 mg/LofCa2₊ concentrations.

In another work, Sun et al. (2009) used the reverse osmosis scaleinhibitor forstaticanddynamicstudyofPESA inhibition at 30°C andpH7.4. PESA (5, 10,15 mg/L) wasaddedto thetesting watersamples.TheirresultssuggestthatPESA shouldbean effec-tivenucleationscaleinhibitorthatisapplicabletoreverseosmosis treatment ofwaters with a wide range ofion compositions.The scaleinhibitionratioofPESA isabove90%andatthesamewater qualityisdirectlyproportionaltothedosageofPESA,andscale in-hibitionratioatthesamePESAdosagedecreasedwiththeincrease ofcalciumconcentration.

Liuetal.(2012)comparedtheanti-scalingperformanceofPESA andPASPusingthestaticandrapidcontrolledprecipitation meth-ods(Ca2+:500mgL-1, pH:9,T:80°C,t:8h). Thescaleinhibition efficiency,wascalculatedfromthechangeincationconcentration. During the experiments, the variation in pH and resistivity with time of the water samples was measured. The maximum in the pH–timecurves corresponds tothe precipitationthresholdinthe water.In theseconditions PESA showedbetter performance than PASPonnucleationinhibition.TheirtestsshowedthatPESAcould beveryefficientatverylowconcentration.

More recentlyPeronnoetal.(2015) investigatedtheinhibition effectofpoly(acrylicacid-co-maleicacid)(PAMA)andpolyaspartic acid(PASP)onthe nucleation/growth process ofCaCO3ina bulk solutionandonametallic surfaceusingfastcontrolled precipita-tion(FCP)methodandelectrochemical quartzcrystalmicrobalance (EQCM).They showedthat theseinhibitorswere veryefficient at lowconcentration(4mg·L-1_{)andthattheymodifiedthe}

morphol-ogyofcalciumcarbonatecrystals.

Moreover,littleisknownofpolymeradditive actiononscaling underconditions representingriver-waterquality (pH, supersatu-ration,suspended matter,etc.)andindustrialcoolingcircuit oper-ation(temperature,residencetime,etc.).

Different testsconditions were applied in the previous works citedaboveforevaluatingtheinhibitioneffectsofPASPandPESA. ItisimportanttonoticethatPESA wasdemonstratedto bea nu-cleation inhibitor at low concentration, in water initially free of calciumcarbonate particles. But considering the previous experi-mentalset-ups, it is difficultto withdrawnucleation and growth kineticslawsseparately andto includein theselawsprocess pa-rameterssuch assupersaturation. Indeed,one ofthe mainissues isoftheseprevious studies istoextrapolate theresultsto indus-trialpilotplants.

In our work, seeds of CaCO3are introduced inthe reactor in ordertomimicsuspended matterinriverwaterandfavorcrystal growthincontrolled conditions.Inhibition rateisthendefinedin thisworkbycomparingR0andRawhicharerespectivelycalcium carbonategrowthrate,inmol·m−2_·min-1_{,withoutandwith}

addi-tive.Thisenabletouseverylowquantityofpolymers.

Ina previous study (Chhim etal., 2017), the constant compo-sitionmethod,wasadaptedforcalciumcarbonateprecipitation,in ordertoreplicateexperimentallytheoperatingconditionsof recir-culating coolingwatersystems,especially withrespecttothe su-persaturation range,presenceof suspendedmatter (suchasillite, silica, calcite) and the temperature range encountered. Determi-nationof the thermodynamic driving force (supersaturation)was basedontherelevantchemical equilibria,totalalkalinityand cal-culationoftheactivitycoefficients,usingPHREEQCsoftware.

In this study, this constant composition method precipitation (constantsupersaturation, pHand ionicforce) wasappliedto in-vestigatetheinfluenceofpolyepoxysuccinicacid(PESA), polyaspar-ticacid(PASP)andtheirmixtureandtocomparetheinhibition re-sultstohomopolymerofacrylicacid(HA),aclassicalpolymeric ad-ditive.So,scalinginhibitionperformancewasinvestigatedthrough dynamicconstantcompositionprecipitationexperimentstoobtain CaCO3growthkineticsinconditionsrepresentativeofanindustrial

circuit.Thecalciumcarbonatescalesampleswerecharacterizedon scanning electron microscopy (SEM) and X-ray diffraction (XRD), andpossiblemechanismsofinhibitionareproposed.Theseresults provideabasisforusingnewgreenadditivesinindustrial circulat-ingwatersystemsbysynergictreatmentofchemicals.

2. Materialandmethods 2.1. Materials

Commercial calcite powder (99% pure, Merck, reference

1.02066) was used as seeds in all experiments. Measurement of calciteparticlesizedistributioninsolutionwasperformedbylaser lightscatteringgranulometryshowingameansizeof34.7μmand a specific area of 0.266m2_·g−1_{. Calcium chloride dihydrate}

(pu-rity 99-105 % ref.141232.1210) waspurchased fromPanreac. An-hydroussodiumcarbonate(99.95to100.05% purity,ref.223484) andsodiumchloride(ACSreagent,purity≥99.0%)werepurchased fromSigma-Aldrich.

Allthereagentsbenzethoniumchloride,trisodiumcitrate dehy-drate, ethylenediaminetetraacetic acid (with purity ≥ 99%)were purchasedfromSigmaAldrich.

Green polymer scale inhibitors have been chosen according to the their biodegrability (Hasson et al., 2011; Harris, 2011;

Wilson and Harris, 2011), their lethal concentration and their bioaccumulation properties defined by Hasson et al (2011). They were used asreceived without any further modification. Table 1

showsthepolymersusedinthisstudyandtheircharacteristicsand suppliers.

2.2. Constantcompositionmethod

Fig. 1 showsthe main elements of the constant composition set-up. Precipitation in presence of seeds particles took place in a1,000mLagitateddouble-jacketedreactor.APTFEcoverwas de-signedto minimizethegaseous headspaceabove thesolutionto preventambientCO2exchangeliabletoreducesolutionpH.

Ineach experiment, solutiontemperature, supersaturationand pHwereheldconstant.Atemperatureregulatorkeptworking tem-peratureswithintheindustrialcoolingcircuitoperationalrangeof 25-45°C.Aconductivityelectrodemonitoredvariationoftheionic forceinsolution(Chhimetal.,2017).

Table 1

Characteristics of polymer additives used for calcium carbonate growth inhibition performance tests.

The constant composition method relies on the use of

two automated burettes, controlled by TiamoTM _{2.5 software}

(Metrohm), filled with solutions of calcium chloride (reagent 1) andsodiumcarbonate(reagent2)correspondingtoconcentrations of 7.08 × 10−2 _mol·L−1 _{of calcium and carbonate ions}

(calcu-latedby PHREEC software),respectively andasensor ableto fol-low reagent consumption due to crystal growth. In the case of calciumcarbonate, accordingtoEq.(1),calciumcarbonate precip-itation leads to consumption of Ca2+ _{and HCO3}− _ions

accompa-niedbyformationofH+protons,reducingpHbelowthethreshold valuedefinedinitially.

Ca2+_{+ HCO}

3− CaCO3 +H+ (1)

Consequently, a high-precision pH electrode (Unitrode Easy-clean, Metrohm) was used to detect pH variations duringCaCO3

precipitation andto trig reagentinjection inorder to maintaina constantpHduringtheprecipitationprocess.

Tocompensatethereactantconsumptionduetocrystalgrowth andthustorestorethepHatitsinitialvalue,theburettesinjected equal volumes of reagents1 and 2 witha precision of 0.001pH. Injected reagent volume variation over time showed a dV

dt slope on the basis of which the growth rate (R) of calcium carbonate crystalsintroducedinthereactor,expressedasmol·m−2_·min−1_,is

givenbyEq.(2): R= dV dt CR ST (2) where dV

dt isthereagentcompensationslopeinL·min−1,CRandST

arerespectivelythereagentconcentrationinmol·L−1 _andthetotal

surface area ofthe seeds initially introduced inthe precipitation reactor(inm2_).

The typicalcurvesobtainedduringconstant composition crys-tallizationexperimentsareschematicallypresentedinFig.2.

In orderto studytheeffect ofadditiveson calcium carbonate precipitation, in conditions that mimic the operating conditions of recirculatingcooling water systems, forall the experiments, a set of operating conditions were kept constant and are given in

Table2.

2.3.Calcitecrystalscharacterization

Each synthesized sample was characterized by powder X-ray diffraction (XRD), and scanning electron microscopy (SEM). The powder X-ray diffraction data were collected with a Bruker D8-2 diffractometer equipped with a Bragg−Brentano

θ

−

θ

geome-try and a copper anode (

λ

(Cu K

α

1) ₌ 1.54060 ˚A and

λ

(Cu K

α

2) = 1.54439 ˚A). The X-raydiffraction patterns were obtained forarangeof 2

θ

anglesfrom5to 50° withastep of0.2°. Scan-ningelectron microscopy(SEM)micrographswere obtainedusing a SEM-FEG JEOL JSM 7100Fmicroscope. The samples were gold-platedbeforeobservation.

2.4.FreeCa2+_{ionvolumetricassayprotocol}

FreeCa2+ _{ionconcentration wasassessed by volumetricassay}

in titrated EDTA (ethylene-diamine tetra-acetic acid) solution at 4.10−3 mol·L−1 _{in presenceof30 μLofcolored Patton-Reeder}

in-dicatorat1g·L−1 _{and1mLNaOH solutionat2mol·L}−1_.Tolimit

solutionvolume variationinthe precipitationreactordueto suc-cessivesampling,samplevolumewaslimitedto1mLandsample numberto3or4.

2.5.Measurementofpolymeradsorptionisotherms

Adsorptionexperimentswereperformedinaseriesof8 identi-calhermeticallycappedbottles.Eachbottlewasfilledwith200mL of supersaturated solution of calcium carbonate, 20mg of calcite seed, and polymer with a concentration ranging from 0.1 to 1.2 mg_·L−1.The bottleswere immersedina waterthermostatic bath andthetemperaturewaskept constantat35°C.After 6days,the solutions were filtered at 0.2 μm, collecting filtrate for assay of residualadditiveandofcalciteseedsforSEMcharacterization.

Residual additive concentration (Ce) in the solutionat 7.6

su-persaturation at end of each test was assayed on UV-Visible spectrophotometry using 2 reagents: benzethonium chloride and trisodiumcitratedihydrate.Theassayprincipleisbasedon propor-tionalitybetweenturbidityandadditiveconcentration.Turbidityis

Fig. 1. Laboratory constant composition set-up for kinetic study of calcium carbonate precipitation under conditions representative of industrial cooling circuits.

Table 2

Operating conditions for constant composition precipitation of calcium carbonate experiments.

Parameter Value

pH 8.5

Ionic Strength (mol ·L −1 ) _{4.24 × 10} −2

Supersaturation = aCa2+·aCO2− 3 Ks aCa2+ and a _CO2− 3 activity coefficients 7.6

Supersaturation ratio is equivalent to a river-water quality with a mean hardness from 20 to 25 °f (200 to 250 mg CaCO3 ·L −1 )

Calcite seed concentration (mg ·L −1 ) ₁₀₀

The seeds concentration corresponds to values measured in the river during moderate flooding ( Idlafkih et al., 1995 )

Fig. 2. Typical curve of constant composition crystallization experiment. The step in the volume curve ( V) corresponds to the volume resolution of the burette,

pH = 0.001 corresponds to the resolution of the pH probe. The time between two pH oscillations corresponds to the time resolution of the experiment. The typical blue curve consists of an initial plateau, characteristic of the delay time and of an increase of the volume added corresponding to crystal growth. In some cases, a change of the injected volume is observed and is characteristic of a change in the crystallization mechanism (growth, nucleation or different phase crystallization).

Table 3

Experimental growth rates at constant supersaturation = 7.6 and constant calcite seeds 100 mg ·L −1 _.

Temperature ( °C) 25 °C 35 °C 45 °C Growth rate (mol ·m −2 _·min −1 _{) 3.07 × 10} −5 _{6.07 × 10} −5 _{11.90 × 10} −5

caused by the reaction between benzethonium chloride and the functionalgroupsofthepolymeradditive. Trisodiumcitrate dihy-drateisneededtocomplexthefreecalciumionsinthesamplein ordertoreduceinterferenceresultingfromcomplexationby poly-mer additive molecules.All assays were performedon a double-beamShimadzu1800spectrophotometerwith10cmquartzoptical trajectorytanks,withworkingwavelengthsetat306nm.Thistank length reducesthe limit ofquantification to0.1mg_·L−1, enabling assay oflowpolymeradditive concentrationsaftertheadsorption tests(moredetailsontheprocedurearegiveninAppendix).

3. Results

3.1. Crystalgrowthexperimentswithoutadditives

To assess the inhibitory effect of the selected additives, ref-erence experiments were performed using the constant compo-sition method for three different temperatures (25°C, 35°C and 45°C).Thechosentemperaturescorrespondtotheoperational tem-perature range of an industrial cooling circuit. Each experiment was repeated at least three times. The evolution of the volume added during the experimentfor each experiment are presented inSupplementaryMaterial(Fig.SM1)andcalciumcarbonate crys-tal growthratewithout additive (R0), calculatedfrom Eq.(2)are

giveninTable3.

Calcium carbonate crystal growth rate (R0) without additive,

basedonEq.(2),increasedwithtemperature(Table3)inthe

oper-ationalrangeofanindustrialcircuit,suggestingthatkineticsunder ourexperimental conditionsfollowedArrhenius’slawrelating cal-ciumcarbonate growth(R0) to activation energy(Ea) by Eq. (3):

R0=k0e−

Ea

Rg T ₍₃₎

where R0 is the growth rate without additive (mol·m−2·min−1),

k0 is a pre-exponential constant, Ea is the activation energy

(kJ·mol−1_{),Rgtheuniversalconstantforperfectgas(undernormal}

conditions,R₌8.314J_·mol−1_·K−1)andTtheabsolutetemperature (Kelvin).

The activation energy of calcium carbonate growth thus ob-tainedis53.4kJ_·mol−1 suggestingthatthecalciumcarbonate pre-cipitationcouldbe surface-controlledordiffusioncontrolled. This value is in good agreement with the literature involving CaCO3

precipitation on crystal seeds, where it ranges between 22 and 155kJ·mol−1_{.Forexample,ParsieglaandKatz(1999)found45± 4}

kJ·mol−1_{andRodriguezBlancoetal.(2011)found66± 2kJ·mol}−1_.

IntherecentworkofCheap-Charpentieretal.(2018)theactivation energy(22kJ_·mol–1_{)isrelatedtoascalingprocessonan}

immobi-lizedCaCO3layer.

At 25°Cand 35°C,Powder X-Ray Diffraction (PXRD) measure-ments(notshownhere)haveshownthatonlycalcitecrystalswere obtainedduringtheseexperiments.Inaddition,SEMpicturesshow welldefinedcalcitecrystalswiththeirtypicalrhombohedralshape, andsmooth surface(seeFig. SM2 ofSupplementary Material). At 45°C, Powder-XRD characterization have shown that calcite and aragonitewere obtainedat the end of the experiment. The SEM image presented in Fig. SM2 (supplementary material) shows a typicalimageofthepowderobtainedat45°C.Theobtained pow-derisamixtureofrhombohedralshapecrystalsaswell asneedle shape crystalscharacteristicof thearagonite phase.These results agreewiththoseofRodriguezBlancoetal.(2011).

3.2.Effectofadditivenatureoncalciumcarbonatecrystallization

Asitwasalreadymentionedinliterature(Sunetal.,2009)the scaleinhibitionratiooftestedadditivesatthesamewaterquality isdirectlyproportionaltotheirdosage.Intheconditionsofour ex-periments,wefoundthat0.5mgL-1wasthethresholdvalueabove which,theinhibitionefficiencydoesnotevolve.Thisadditive con-centrationwasthenchosenfortestingtheireffect.Afirstseriesof experimentswasperformedat35°Ctounderstandtheeffectofthe nature of the additives on the crystallization of calcium carbon-ate.Theresultsoftheconstantcompositioncrystalgrowth experi-mentsobtainedforthethreetestedpolymeric additives(HA,PASP andPESA)ataconcentrationof0.5mgL−1,areshowninFig.3.This figure clearly showsthat for PASP and HA, crystalgrowth starts fewminutesaftertheintroductionofseedsandthevolume addi-tionrateismuch lowerthan theone obtainedwithoutadditives. Thecorrespondingslopesandthecalculatedgrowthratearegiven inTable 4.Onthe contrary,when PESA isadded to thesolution, the addition of the reagent starts few hours after the introduc-tionoftheseeds inthemedium.Inaddition,whencrystallization starts,thereagentadditionrateisofthesameorderofmagnitude orhigherthantheoneobtainedwithoutadditives.Another signif-icant phenomenon, whichhas beenobserved only inthe experi-mentsperformedwithPESA,istheincreaseofpHatthebeginning oftheexperiments,asshowninFig.3.Toexplainthisunexpected evolutionofpHduringtheexperiment,theamountoffreecalcium ions in solution was titrated using EDTA. The results show that quicklyafterthebeginningoftheexperiments,theamountoffree Ca2+ _{ionsdropsfrom2.7× 10}−3_mol·L−1 _{to0.65× 10}−3_mol·L−1_.

The decrease offree Ca2+ _{induces excess negativecharge due to}

excessHCO3− ionsinthesolution.Toensuresolution

Fig. 3. Temporal evolution of pH and added reagent volume during constant composition experiments performed at T = 35 °C and = 7.6 with (a) HA (b) PASP (c) PESA.

Table 4

Effect of type of polymer additive on calcium carbonate crystal growth inhibition rate and apparent delay time. Polymer additive Apparent latency time (min) R a (mol ·m −2 ·min −1 ) Inhibition rate (%)

Homopolymer of acrylic acid (HA) 5.7 4.79 × 10 −6 _92.1

Polyaspartic (PASP) 13.3 7.45 × 10 −6 _87.7

Polyepoxysuccinic (PESA) 190.3 6.02 × 10 −5 _1.0

bya reactionHCO3− ⇔ CO2gasseous + OH− (Pesonenetal.,2005).

OH− ionformationinthesolutionduringCO2 degassingaccounts

fortheriseinpHseeninFig.3.

During the pH decrease, formation of CaCO3 can occur:

Ca2+ _{+ HCO3}− _{⇔ CaCO3 + H}+_{, forming H}+ _{ions which}

neutral-ize OH− ions. It is why the delaytime before crystal growth is long.AfterpH reachedthesetpoint(pH₌8.5)andasthequantity ofPESAintroducedisverylow(0.5mgL−1) thecrystalgrowthrate obtainediscomparable tothe one obtainedwithout additive:all thePESAwasconsumedbycomplexation.So,PESAturnsouttobe anucleationinhibitorwithanefficiencyof80%,andhardlyinhibits growth(1%).

Table 4 presentsresults for these three additives at35°C, 7.6 supersaturationandcalcite seedsconcentrationof100 mg·L−1_,for

additiveconcentrationat0.5mg·L−1_.

Powdercharacterizationshowsthatwhencrystallization exper-imentswere performedinpresence ofHAandPESA, onlycalcite crystalwere obtained asfor the crystallization experiments per-formedwithoutadditives.

SEM imagesperformedoncrystalsobtainedinpresenceofHA (Fig. 4(A)) show that the crystals havethe typical rhombohedral shapeof calcite.However, compared tothe initial seeds,the sur-faceofthecrystalsseemstoberougher.

In the case of PESA (Fig. 4(C)), the roughness of the crystal surfacesismorepronouncedandthesesurfacespresentsome va-cancies,andthecrystalsseem tobe formedwithan assembly of sheets.

When crystallization experiments were performed in pres-ence of PASP, XRPD patterns show that vaterite is preferentially crystallized (see Fig. SM3 in Supplementary Material). In addi-tion, SEM images (Fig. 4 (B)) show that a wide range of crys-tal shapes are obtained: smooth rhombohedral calcite (probably initialseed crystals), porousspherical particles (witha core-shell structure)andspherulitic crystals(wheatsheaf crystalsortype 2 spherulites).

3.3. Effectoftemperatureonscaleinhibition

With0.5mgL−1polymeradditivesintheoriginalsolution, un-der the same conditions as without additive, calcium carbonate growth inhibition decreases with increasing temperature (Fig. 5) within the operating range for an industrial cooling circuit (25-45°C).TheinhibitionrateisgivenbyEq.(4):

Inhibitionrate

(

%

)

=100 R0− Ra

R0

(4)

where R0 and Ra are respectively the growth rate, in

mol_·m−2_·min−1,withoutandwithadditive.

HA showed the best inhibition performance (94.8% at 25°C, compared to87.9% forPASP or3% forPESA at thesame temper-ature),andlikewiseatothertemperatures.

Activationenergy wascalculated forthe three additives. Acti-vation energywith PASP (84.3 kJ·mol−1_{) andHA (76.8 kJ·mol}−1₎

weresimilar,andmuchhigherthantheoneobtainedwithout ad-ditives,confirmingtheinhibitoryeffectofbothpolymerson crys-talgrowth.ActivationenergywithPESA (54.3kJ·mol−1_)wasmore

orless thesame asthat found previously forcalcite seeds with-out additive (53.4 kJ·mol−1_{), suggestingabsence of inhibitory}

ac-tion, in contrast to HA andPASP. However, with PESA, apparent delaytime(beforereagentburettetriggeringasstartofgrowth)is longer(around3hours).Theseresultssuggestthat,PESA polymer playsadifferentroleincrystalgrowthinhibitioncomparedtothe twootherpolymers.

3.4. Effectofpolymeradditiveconcentrationoncrystalgrowth

Whentheconcentration ofPESAinthesolutionincreasesfrom 0.1 mg·L−1 _{to 1 mg·L}−1_{, the addition rate of reagent is almost}

not affected and is of the same order of magnitude or higher than the one obtained for the experiments performed without additives.However,asshowninFig.6,theapparentdelaytimeof

Fig. 4. SEM pictures of grown calcium carbonate seeds with additives: (A) HA; (B) PASP; (C) PESA, at 35 °C and 7.6 supersaturation ratio.

crystallization is increased from 63min to 438 min for

CPESA=0.1 mg·L−1 to CPESA=1 mg·L−1. It is worth to note that

anincreaseofpHisobservedatthebeginningoftheexperiments foralltheconcentrationstested.

Inthe caseofHAandPASP,theopposite isobserved.The de-laytimeofcrystallizationisalmostnotaffectedbytheincreaseof additiveconcentration(from0.1to0.5mg·L−1_{)anditrangesfrom}

1to 10min.However, whencrystallizationstartsthereagent ad-ditionrate, andthus thecrystalgrowthrate, decreases whenthe additiveconcentrationincreases.

By takingthe assumption that the crystalgrowth rate inhibi-tionisduetothepolymer adsorptionatthesurfaceofthe

grow-ingcrystal, thepolymer additivemolecule adsorption characteris-ticscanbederivedfromtheseexperiments.

The various studies in the literature (Klepetsanis etal., 2002;

KırbogaandOner,2012;Lioliouetal.,2006)agreeonagrowth in-hibitionmechanisminvolving polymeradditive adsorptionon the growing crystalsurface. We therefore representedthe concentra-tioneffectbyLangmuir’sisotherm(Eq.(5))intheinhibitionratio:

R0 R0− Ra = 1

(

1− b

)

+ 1

(

1− b

)

KaCadd (5) where R0 and Ra are the crystal growth rate respectively

Fig. 5. Evolution of calcium carbonate growth inhibition rate according to temper- ature for HA, PASP and PESA additives: = 7.6, C additive = 0.5 mg ·L −1, calcite seeds Cseeds = 100 mg ·L −1.

additive efficacy factor;Ka is the adsorption affinity constant for

theadditive,inL·mol−1_;andC

add istheadditiveconcentration,in

mol_·L−1.

Theadditive efficacyfactor(1– b)andadsorptionaffinity con-stant(Ka)canbederivedexperimentallyby _RR0

0− Ra =f

(

1

C_add

)

.Fig.7 showsthe experimental representationof theinhibition ratio for HAandPASP.

TheadsorptionconstantsobtainedarereportedinTable5. In ordertotest thishypothesis, adsorptionexperiments ofHA and PASP additives on the surface of calcite crystals were per-formed (the details of the experimental procedure and the data treatmentsare givenin Appendix).The linearizedplot ofthe ad-sorptionisotherm(i.e.theevolutionof Ce

Qe vsCe) arepresentedin

Fig. 7. HA and PASP concentration effects on crystal growth inhibition. = 7.6, at 35 °C and 100 mg ·L −1 calcite seeds.

Table 5

Adsorption characteristics at 35 °C, with 100 mg ·L −1 cal-

cite seeds and 7.6 supersaturation ratio. Adsorption characteristics HA PASP

(1 – b) 0.971 0.900

K a (L ·mol −1 ) 5.0 × 10 7 1.0 × 10 8

SupplementaryMaterial Fig.SM4.Inthecaseofexperiments per-formed with HA polymer, the correct fit confirmsthat the main mechanismofcrystalgrowthinhibitionisduetoadsorptionofthe polymeronthesurfaceofcalcitecrystals.

WhenPASPisaddedtothesolution,theobtainedresultisless clear.The hypothesis ofadsorption mechanismseems tobe valid toaconcentration upto5mg_·L−1.But,forhigherconcentrations, adeviationfromlinearityisobservedthatcouldbeduetoan ad-sorptionmechanismthatisnotdescribedbytheLangmuirmodel,

Fig. 6. Temporal evolution of pH and added reagent volume during constant composition experiments performed at T = 35 °C and = 7.6 with concentrations of PESA ranging from 0.1 to 1 mg ·L −1 .

Fig. 8. Temporal evolution of pH and added reagent volume during constant composition experiments performed at T = 35 °C and = 7.6 with different mixtures of (a) PESA and PASP (b) PESA and HA.

Table 6

Adsorption characteristics on adsorption bench test in a solution with 7.6 supersaturation at 35 °C in presence of calcite seeds at 100 mg ·L −1 .

Adsorption characteristics HA PASP Q m (mol ·g −1 seeds) 2.5 × 10 −6 10−6

K a (L ·mol −1 ) 2.3 × 10 7 1.1 × 10 8

orbyanotherphenomenonthatconsumethepolymerinsolution. Intheseexperiments,the adsorptionconstantsare obtainedfrom thelinearizedLangmuirmodelandaregiveninTable6.

So, the value of the adsorption affinity constant (Ka)

ob-tained by two different types ofexperiments (constant composi-tionmethod andadsorption measurement)areofthesameorder ofmagnitude.

3.5. EffectofmixingPASPandPESAorHAandPESA

It is therefore deemed interesting to studythe effect of mix-tures of growth and nucleation inhibitor additives, under the sameexperimentalconditions:i.e.,Ω=7.6,temperature35°C, cal-cite seeds at 100 mg_·L−1. Two series of tests were carried out: PASP+PESAandHA+PESA.Growthinhibitor(HAorPASP) concen-trationwasheldconstantat0.5mg.L−1whilenucleationinhibitor concentrationwasvariedbetween0.3and1.5mg·L−1_.The

perfor-manceindicators ofscaleinhibition were the delaytime of crys-tallizationandtheinhibitionrate.

Fig. 8A shows the evolution of reagent volume addition dur-ing theexperimentsatconstant compositionforPESA₊PASP mix-turesandTable7showsthevaluesofthedelaytimeof crystalliza-tion andinhibitionrateforvarious concentrationsofPESA inthe PESA₊PASPmixture.

In the case of PESA and PASP mixtures (Fig. 8A), the results show that a positive synergetic effect is obtained on delay time when additives are mixed. For instance, for the experiment per-formed with 0.5mg·L−1 _{of PESA in the mixture,a delaytime of}

666minismeasuredwhereasitisof190minwiththesame con-centration of PESA used alone. In addition, this experiment was performedwithatotaladditiveconcentrationof1mg·L−1_,andcan

be compared withexperimentperformedwithPESA aloneatthe

sameconcentrationwithalowerdelaytimeforcrystallization(i.e. 438min).

ApparentdelaytimeincreasedwithincreasingPESA concentra-tioninthemixture.Inaddition,asPESAconcentrationinthe mix-turewas increasedfrom 1mg·L−1 _{to 1.5mg·L}−1_{, apparent delay}

increasedfrom1,072.3minutesto1,128.6minutes(Fig.8A),which is more than twice the delay time obtained when PESA is not mixedwithPASP.Afterthedelaytimeofcrystallization, crystalliza-tionstartswithapproximativelythesamerateastheoneobtained withPASP used alone. The inhibition rate is 87% for PASP alone andrangesbetween85% to89% when PASPisused ina mixture withPESA.

Fig.8Bshowstheevolutionofreagentvolumeadditionduring the experiments at constant composition for the PESA₊HA mix-turesandTable8showsthevaluesofthedelaytimeof crystalliza-tion,the growth rateand the inhibition ratefor various concen-trations of PESA in the PESA₊HA mixtures. The results obtained show that no synergetic effect is obtained when both additives are mixed. The resultobtained is a combinationof both effects: PESA additionincreases thedelaytime of crystallizationwhereas HAadditiondecreasesthecrystalgrowthrate.Theexperiment per-formedwith0.5m.L−1 ofPESA(correspondingtoatotal concentra-tionofadditives of1mg·L−1_{) inthe mixturehavea muchlower}

delaytimeforcrystallizationthantheoneobtainedforPESAalone. In addition, under the same conditions as for the PESA + PASP mixture,apparent delaytime wasmuch shorterwithPESA + HA (Table8).Theinhibitionrateofcrystalgrowthisslightlyincreased (from92%to97%)bythepresenceofPESAwhencomparedtothe experimentperformedwithHAalone.

Fig.9showsSEM imagesofcrystals grown withvariousPESA andPASPmixtures. The mixtureshowed predominanceofcalcite andvateritecrystals,whatevertheconcentrationofPESA.

4. Discussion

Theresultsobtainedinthisstudy,raise threeimportant ques-tions. Whyvaterite ismainly formed whenPASP is addedto the supersaturated solution even in presence of stable calcite? Why such crystallization delaytime is obtained in presence of PESA? Why onlyPESA andPASP mixture shows a significant synergetic effect?

Table 7

Effect of PESA concentration in a PESA + PASP mix on various performance indicators, at 7.6 supersat- uration at 35° C in presence of calcite seeds at 100 mg· L −1_.

PESA + PASP mix Apparent delay time (min) R a (mol ·m −2 ·min −1 ) Inhibition rate (%)

PESA 0.3 + PASP 0.5 191.0 6.39 × 10 −6 _89.5

PESA 0.5 + PASP 0.5 665.6 7.98 × 10 −6 _86.9

PESA 1.0 + PASP 0.5 1072.3 9.05 × 10 −6 _85.1

PESA 1.5 + PASP 0.5 1128.6 6.92 × 10 −6 _88.6

Table 8

Effect of PESA concentration in a PESA + HA mix on various performance indicators, at 7.6 supersat- uration at 35 °C in presence of calcite seeds at 100 mg ·L −1 .

PESA + HA mix Apparent delay time (min) R a (mol ·m −2 ·min −1 ) Inhibition rate (%)

PESA 0.5 + HA 0.5 11.0 1.60 × 10 −6 _97.4

PESA 1.0 + HA 0.5 216.3 2.66 × 10 −6 _95.6

PESA 1.5 + HA 0.5 411.0 2.13 × 10 −6 _96.5

Fig. 9. SEM images for mixtures of various PESA concentrations with PASP at 0.5 mg ·L −1 .

When HA isadded tothe growing medium, no crystallization delay is observed, but crystal growth is strongly slowed down. Based on adsorption experiments and simulations results found in literature (Tribello et al., 2009; Bulo et al., 2007) it is more likely that crystal growth inhibition is mainly due to adsorption ofthe polymer on the crystalsurfacerather than binding of the acrylatepolymer to Ca2+ _{ion/ carbonate ionsin solution.}

More-over, because of the very low concentration of additive and due totheuseoftheconstantcomposition set-up(presenceofcalcite seeds),noeffectonnucleationwasobservedattheoppositeofthe studyof Gebauer et al.(2009). In these conditions, HAinhibitor canbe referenced asa crystalgrowth inhibitorby adsorption on crystal faces (type IV of the Gebauer et al., 2009 classification: “the additive adsorbs onto the nucleated particles and stabilizes them”).

The influence of PASP on calcium carbonate crystallization is subtler to analyze. The results of constant composition crystal growthexperimentsshow thatwiththeadditionofPASP,aslight increase of delaytime is observed and crystalgrowth on calcite seedsisalsostronglysloweddown.Adsorptionexperimentsshow that the adsorption of PASP on crystal surfaces is less effective, and a change on the adsorption isotherm is observed beyond a certainconcentration.Inaddition,the introductionofPASPinthe

growing medium, movesthe crystallization of calcium carbonate towards the crystallization of vaterite, even in presence of cal-cite. As proposed by Grower and Odom (2000), it is clear that the adsorption of polymer on crystal surface cannot provide a full explanation of the PASP additive action on crystal morphol-ogy. As presented in Fig. 4(B), the crystallization pumpkin parti-clesandthe“double-leaf” type ofparticles suggestthat,basedon the workof Xuet al.(2014),the main crystallizationmechanism proceeds via the formation of an amorphous calcium carbonate (ACC)phase.Comparedtothereferenceexperiments(without ad-ditives)orexperimentsperformedinpresenceofHA,the orienta-tion of crystallizationof calcium carbonatetowards a metastable phase with PASP(i.e. vaterite) suggeststhat one of the main ef-fects of PASP is to control the local structure of the nucleated secondary phase (typeVIof additivesinthe Gebauer’s classifica-tion). Consequently,PASPinhibitsthe formationofcalcite. A pos-siblemechanismofcrystalinhibitionduetothepresenceofPASP, could be that this additive promotes the formation ofa Polymer InducedLiquidPhase(PILP).Indeed,astheconcentrationof poly-mer is low, this liquid phase could not be stabilized and was quickly transformed in ACC, as suggested in some conditions by

Gebaueretal.(2009)andGrowerandOdom(2000).Then,thisACC turnsintovaterite.

Unliketotheothertwo polymers,PESAactsdifferently:no in-hibition ofcrystalgrowthbutstrong inhibitionofnucleation (see

Fig.8) isobtained. Inaddition,asforHA,XRD andSEM analyzes show that only calcite crystals are obtained. As pointed out by

Pesonenetal.(2005),PESA isaveryefficientchelatorleadingtoa decreaseofavailableCa2+_{ionsinsolutionandthustoadecrease}

ofsupersaturation.Thiscouldexplaintheinhibitionofnucleation. Moreover,thesimulationofPesonenetal.(2005) showedthatthe formedcomplexesareverystable.

However, this chelation effect does not explain the fact that when crystallizationstarts afterthe delaytime, the consumption ofreagent isofthe same orderofmagnitude orhigherthan the one obtained without additives (higher growth rate). To explain thisbehavior,complementaryfreeCa2+_{titrationexperimentswere}

performed, (see Fig. SM5 in Supplementary Material). These ex-periments were performed at 35°C at constant polymer concen-tration (0.5 mgL−1) and different supersaturation ratio (from 7.6 to 12). The results show that whatever the supersaturation ra-tio, the free Ca2+ _{concentration drops always to the same value}

around 8.310−3 mol/L−1 and remains stable until the beginning ofthe crystallization.Itis alsoimportantto noticethat the crys-tallization delay time depends either on supersaturation either onpolymer concentration.Thecorresponding ionactivityproduct (logIAP=a_Ca2+aCO2−

3 =−7.

56)isclosedtothesolubilityproductof ACCphases(IandII)calculatedbyGebaueretal.(2009)logkACC_I=

−7.51 and logkACC_II=−7.42. This result suggests that the addi-tionofPESAinduces theprecipitationofanamorphousphase(for whichwecannotstatewhetheritisLIPSorsolid)andthispolymer stabilizes thismetastable intermediate phase, precursorof calcite crystals. Theformation ofcalcite isthen eitherdue toan abrupt transformation of ACC to calcite (secondary nucleation)either to deposition/aggregationofthisamorphousACContheexisting sur-face calcite seeds which explain the roughness of the observed crystals.

The three additivestested in this study(HA, PASP andPESA) show three differentaction ways, that explain thesynergetic ef-fects found when two additives are mixed. For instance, mixing an inhibitor ofcrystalnucleation (PESA) andacrystalgrowth in-hibitor (HA)allows todelaycrystalnucleation(withtheactionof PESA),butalsotodecrease crystalgrowthrate, thanksto the ad-sorption of HA on the surface the newly formed crystals on the seeds. However, the synergetic effect of mixing these two addi-tivesis not asefficientasitcan be expected. Indeed,evenifthe crystalgrowthrate(or theflow rateofreagentaddition) is close to the one obtained with HA alone, the delay time observed in the caseofthemixture (PESA+HA)is muchlower than theone obtained withPESA alone. This could be due to molecular inter-actionoroligomerization betweenthetwopolyelectrolytes in so-lutionthatdecreasestheamountoffreegroupsavailable forCa2+

complexation (smaller increaseof pHatthebeginning ofthe ex-perimentscomparedtoPESAalone).Surprisingly,thereverseis ob-servedwhenPASPandPESAaremixed:eveniftheinhibitionrate isalmost thesame (thanin theprevious case),thedelaytime of crystallizationisatleasttwicetheoneobtainedwithPESAalone. By taking the assumption that the presence of PASP cancels the formationrouteofcalcite(andthuspromotestheformationof va-terite), then the ACC stabilized by the presence of PESA, cannot turnintocalcite,whichdelayscrystallization.

From a practical point of view, using a mixture of PESA and PASPprovidesapromisingapproachforscaleinhibition.Underthe experimentalconditionsappliedinthisstudy,maximumapparent delaytime wasnearly 19hours, withinhibitionratecloseto89% for a mixture of PESA at1.5 mg·L−1 _{andPASP at 0.5 mg·L}−1_{. In}

comparison, meanresidence time in industrial cooling circuits is around24hours;thus,aPESA-PASPmixture(bothgreenadditives) should significantly reduce scaling by acting on nucleation and crystal growth. In addition, the formation of aggregated vaterite particleswithpumpkinsshapemakethedepositmoreporousand thuslessadherenttothesurface.

5. Conclusion

The scaling inhibition efficacy ofthree polymer additives,HA, PASP and PESA, was assessed under conditions representative of industrialcoolingcircuitsusingaconstantcompositionmethod de-velopedinourpreviousresearch.Technically,thisapproachallows to study,closeto industrialconditions (waterhardness, tempera-ture,suspendedmatter…)thescalingbehaviorofrealwater.From apracticalpoint ofview,byunderstandingandmeasuring the in-fluenceofeachadditivealoneonthecrystallizationofcalcium car-bonate, this approach has enabled to design efficient mixture of additivestoavoidordelayscaleformation.Wehaveshownthatby mixingtwoadditivesshowingtwodifferentactingways,itis pos-sibleeithertocombinetheir effectsortoimprovetheirinhibitory effect.Forinstance, nearly 19hours, withinhibition ratecloseto 89%wasfoundforamixtureofPESAandPASPevenforasolution withrelatively highsupersaturation ratioandcontainingseedsof calcite.Thesefindingsopenhighlypromisingperspectivesfor pre-ventive scaling treatment, improvingthe thermalefficacy ofheat exchangersinindustrialcoolingcircuits.

DeclarationofCompetingInterest

Theauthorsdeclarethattheyhavenoknowncompeting finan-cialinterestsorpersonalrelationshipsthatcouldhaveappearedto influencetheworkreportedinthispaper.

Acknowledgments

TheauthorswishtothankMarie-Linede Solan-Bethmalefrom theLaboratoiredeGénieChimique(LGC)fortheSEManalysesand Cédric Charvillat from the Centre Interuniversitaire de Recherche etd’IngénieriedesMatériaux(CIRIMAT)forthepowderXRD mea-surements.

Supplementarymaterials

Supplementary material associated with this article can be found,intheonlineversion,atdoi:10.1016/j.watres.2020.116334.

Appendix:Adsorptionbenchtestandexperimentalconditions

Three models are widely used for isotherms: Langmuir, Fre-undlichandRedlich-Peterson(Liuetal.,2010).Forcalcium carbon-atecrystal growthinhibition by polymer additives,the Langmuir modelisthe mostwidelyused todescribe additive molecule ad-sorptionongrowthsitesurfaces(Dimovaetal.,2003;Dalasetal., 2006; Zhang et al., 2016; Lisitsin et al., 2009; Klepetsanis etal., 2002).

Themodelisbasedon3assumptions:

• The adsorbent exposesa limited number of adsorption sites: i.e.,hasamaximumadsorptioncapacity,Qm.

• Alladsorptionsitesareidenticalandreceiveonly1moleculeat atime.

• At binding,theenergybetweensiteandadsorbedmolecule is constant,unaffectedbyadsorptionatneighboringsites. Undercontrolledtemperature,adsorptionisothermsdependon thetypeofadsorbent (calciteseedsorsuspendedmatter)andon thetypeofadsorbedmolecules(polymeradditivemolecules).This dependenceisrepresentedbyanequation relatingquantityof ad-sorbedpolymer(Qe)toadditive moleculesequilibrium

concentra-tion(Liuetal.,2010;Blachieretal.,2009).

Qe=

V

(

Cadd− Ce

)

m (A1)

where Qe is the quantity of polymer adsorbed by the adsorbent

(mol_·g−1),Visthesolutionvolume(L),C_addandCearerespectively

theinitialandtheresidualpost-equilibriumpolymerconcentration (mol·L−1_{),andmistheadsorbent(seeds)mass(g).}

TheLangmuiradsorptionisothermmodelisarepresentationof Qe=f(Ce)withthefollowingnon-linearexpression:

Qe=Qm Ka

Ce

1+KaCe

(A2) whereQm isthemaximumadsorptioncapacity(in molepolymer

pergcalciteseeds),andKaistheLangmuiradsorptionaffinity

con-stant(L·mol−1_).

Constants Qm and Ka can be determined experimentally by

linearizingrelation (2)as Ce Qe= 1 Qm Ce+ 1 QmKa andtracing Ce Qe= f

(

Ce

)

.

To consolidate the adsorption characteristics obtained on the constantcompositionset-up,anadsorptiontest benchwas imple-mented(Fig.A1).

Thetestbenchcomprised:

- athermostatic bathat 35°C in which a magnetic stir table is immersed;

Fig. A1. Adsorption test bench for polymer additive molecules in solution at constant supersaturation in presence of calcite seeds at constant concentration.

- aseriesof8identicalhermeticallycappedbottleswithidentical magneticbars;

- eachbottleisfiledwith200mLsolutionat7.6supersaturation; - solutiontemperatureisheldat35°C;

- ineachbottle,aconstantmassofcalciteseed(20mg)is intro-duced,foraconcentrationof100mg_·L−1;

- att = 0,thetest additiveis addedatconcentrationsfrom0.1 to 1.2mg·L−1_{,a rangeset by the assay limitof quantification}

forresidualadditiveatendoftest;

- after6 days’contact, thesolutions are filteredat0.2μm, col-lectingfiltrateforassayofresidualadditiveandofcalciteseed forSEMcharacterization.

Residual additive concentration (Ce) inthe solution at 7.6

su-persaturation at end of each test was assayed on UV-Visible spectrophotometry using 2 reagents: benzethonium chloride and trisodiumcitratedihydrate.Theassayprincipleisbasedon propor-tionalitybetweenturbidityandadditiveconcentration.Turbidityis caused by the reaction between benzethonium chloride and the functionalgroupsofthepolymer additive.Trisodiumcitrate dihy-drateisneededtocomplexthefreecalciumionsinthesamplein ordertoreduce interferenceresultingfromcomplexationby poly-mer additive molecules. Allassays were performed on a double-beamShimadzu1800spectrophotometerwith10cmquartzoptical trajectorytanks,withworkingwavelengthsetat306nm.Thistank lengthreduces thelimit ofquantification to0.1 mg·L−1_,enabling

assayoflowpolymer additiveconcentrationsaftertheadsorption tests.

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