Inhibition of homogenous formation of calcium carbonate by poly

8/10/2019 Inhibition of homogenous formation of calcium carbonate by poly

1/13

Inhibition of homogenous formation of calcium carbonate by poly(acrylic acid). The effect of molar mass and end-group functionality

Ali A. Al-Hamzah a,b, Christopher P. East c, William O.S. Doherty c, Christopher M. Fellows a,a School of Science and Technology, The University of New England, Armidale, NSW 2351, Australiab Saline Water Desalination Research Institute, SWCC, P. O. Box 8328, Al Jubail 31951, Saudi Arabiac Sugar Research and Innovation, Centre for Tropical Crops and Biocommodities, Queensland University of Technology, Brisbane, Queensland 4001, Australia

H I G H L I G H T S

End-groups strongly affect the effectiveness of polymeric scale inhibitors of CaCO3. Poly(acrylic acids) with end-groups of moderate hydrophobicity are most effective. This is true for temperatures between 25 C and 100 C. Effective scale inhibitors also stabilize less stable polymorphs of CaCO3.

a b s t r a c ta r t i c l e i n f o

Article history:

Received 20 December 2013Received in revised form 23 January 2014Accepted 24 January 2014Available online 22 February 2014

Keywords:

CrystallizationCalcium carbonatePolyelectrolytesScale inhibitorSEM (Scanning Electron Microscopy)XRD (X-ray Diffraction)Poly(acrylic acid)ATRP (Atom Transfer Radical Polymerization)

The ability of poly(acrylic acid) (PAA) with different end groups and molar masses prepared by Atom TransferRadical Polymerization (ATRP) to inhibit the formation of calcium carbonate scale at low and elevated tempera-tures was investigated. Inhibition of CaCO3deposition was affected by the hydrophobicity of the end groups ofPAA, with the greatest inhibition seen for PAA with hydrophobic end groups of moderate size (610 carbons).The morphologies of CaCO3crystals were signicantly distorted in the presence of these PAAs. The smallestmorphological change was in the presence of PAA with long hydrophobic end groups (16 carbons) and the rel-

ative inhibitionobserved forall specieswere in thesame order at 30 C and100 C.As well as distortingmorphol-ogies, the scale inhibitors appeared to stabilize the less thermodynamically favorable polymorph, vaterite, to adegree proportional to their ability to inhibit precipitation.

2014 Elsevier B.V. All rights reserved.

1. Introduction

Calcium carbonate is one of the most common scales found in boththermal (e.g., multi-stage ash, MSF) and membrane (e.g., reverse os-mosis) desalination processes. At lower temperatures, it is the maincomponent of alkaline scale, while at higher temperatures it is foundmixed with magnesium hydroxide in desalination plant scales[1]. Theexact temperature at which Mg(OH)2deposition becomes competitivewith CaCO3 deposition will depend on the extent to which carbon diox-ide is degassed from the brine[2].

In MSF desalination, CaCO3generally appears above 45 C as a resultof thethermaldecomposition of thebicarbonateion (Re.(1)); increasing

temperature pushes bicarbonate to carbonate by the entropically-favored reaction.

2HCO3 aq CO3

2 aq CO2 aq H2O l Re:1

The precipitation of CaCO3 occurs when theion product exceeds theKsp (Re. (2)). The concentration of Ca

2+ and HCO3 ions in standard sea-

water (Salinity = 35 g/kg) are 10.3 and 1.8 mM respectively, but theconcentration of CO3

2 will be much lower and sensitively dependenton conditions[3].

Ca2 aq CO32 aq CaCO3 s Re:2

Calcium carbonate can be found as an amorphous solid and in threedifferent crystallineforms, calcite (Ksp at25C=3.310

9), aragonite(Ksp at25C=4.610

9) and vaterite (Ksp at25C=1.2108) [4,5]

listed in order of increasing solubility and decreasing thermodynamic

Desalination 338 (2014) 93105

Corresponding author. Tel.: +61 2 6773 2470.E-mail address:[email protected](C.M. Fellows).

Contents lists available atScienceDirect

Desalination

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / d e s a l

http://dx.doi.org/10.1016/j.desal.2014.01.020

0011-9164 / 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.desal.2014.01.020http://dx.doi.org/10.1016/j.desal.2014.01.020http://dx.doi.org/10.1016/j.desal.2014.01.020mailto:[email protected]://www.sciencedirect.com/science/journal/00119164http://dx.doi.org/10.1016/j.desal.2014.01.020http://dx.doi.org/10.1016/j.desal.2014.01.020http://www.sciencedirect.com/science/journal/00119164mailto:[email protected]://dx.doi.org/10.1016/j.desal.2014.01.020http://crossmark.crossref.org/dialog/?doi=10.1016/j.desal.2014.01.020&domain=pdf


2/13

stability. The crystal shape and morphology of calcium carbonateprecipitation is affected by factors such as temperature, pH, supersatu-ration, the ratio of [Ca2+]/[CO3

2]and the presence or absence of addi-tive[6]. At high temperatures (T N70 C), aragonite is favored, whilecalcite is favored at low temperature (T b 30 C). At any temperatureall polymorphs eventually recrystallize to the thermodynamically-favored calcite[7]. Rhombohedral calcite is favored at a 1:1 [Ca2+]/[CO3

2] ratio, while scalenohedral calcite is favored when the [Ca2+]/

[CO32

] ratio is

1.2[7].To control scaling in desalination plants, several methods of controlhave been adopted. The most important methods are acid treatment,mechanical cleaning and the use of polymeric scale inhibitors. The pri-mary method used historically to control scale formation in MSF desali-nation plants has been acid treatment. In this treatment, the pH ofseawater is maintained around 4.5 using acid, most often sulfuric aciddue to its low cost. Controlling the pH of treated seawater is crucial forboth inhibition of scale and prevention of corrosion. Acid reacts withthe CO3

2 and HCO32 ions present in seawater yielding H2O and CO2

and preventing formation of CaCO3and Mg(OH)2[8].Scale inhibitors are chemical additives used to control the forma-

tionand/ordepositionof scale. There are three common groups of poly-meric scale inhibitors: polymers containing carboxylic acids such aspoly(acrylic acid) and poly(maleic acid); polymers containing phos-phate groups, such as polyphosphates and polyphosphate esters; andpolymers containing sulfonate groups[9]. The attractive features ofthese chemical additives include ease of handling, relatively low cost,low dose rate, and ability to inhibit hard calcium sulfate scale formation[10,11].

It has been suggested that scale inhibitors may operate by threedistinct mechanisms; by sequestration, dispersion, or adsorption[9].As solubility is dened as the maximum concentration of dissolvedions in equilibrium with solid phase at a xed temperature and back-ground ionic composition, that maximum concentration of dissolvedions can be reduced prior to nucleation by sequestration with scaleinhibitors.

Scale forms primarily heterogeneously, on interfaces (of bubbles,plant surfaces, and particles of suspended matter), but this heteroge-

neous nucleation is still primarily in the bulk phase. If scale particlesformed in solution can be prevented from aggregating onto surfacesand remainsuspended in thenal brine,they will notcontributeto scal-ing. By adsorbingto thesurface of these particles and providingadditionelectrostatic and/or steric stabilization, scaleinhibitors can retard aggre-gation of these particles, effectively dispersing them in suspension untilthey are ejected in the waste brine.

Differences in crystal form can arise from selective adsorption ofscale inhibitors on the points of crystal growth, causing differential re-duction in the growth rate of different crystal planes. For example, inthe case of calcium carbonate, scale inhibitors can stabilize the vateriteand prevent its transformation into calcite or aragonite[12]. The newcrystal morphologiesmay grow more slowly overall, may not aggregateas effectively as the native crystal morphologies, or may form a deposit

that is more porous and more easily removed[13].We have previously found in investigations of inhibition of the

related calcium oxalate (CaC2O4) scaling system (prevalent in sugarmanufacture) thatthe effectiveness of poly(acrylic acid) (PAA) scale in-hibitors is sensitively dependent not only on dose and molar mass, butalso on the end-groups attached to the polymer[1416]. Reversible-termination radical polymerization methods, such as ReversibleAdditionFragmentation chain Transfer polymerization (RAFT)[17]and Atom Transfer Radical Polymerization (ATRP)[18]provide a wayin which the molar mass and end-group functionality of PAA canbe controlled to a high degree of precision, enabling such structureproperty relations to be conclusively established. For CaC2O4scaling,we found that most effective scale inhibition by PAA was obtained notfor hydrophilic end groups, or hydrophobic end-groups large enough

to generate signicant surface activity, but moderately hydrophobic

end-groups[14]. These polymers had the greatest impact on crystalspeciation and morphology[15].

The main aim of this paper is to determine if similar trends occurin CaCO3scaling as have been observed previously for CaC2O4. Theefciency of PAA of controlled molar mass with different end-groupsas inhibitors of CaCO3crystallization in the bulk solution was deter-mined at ambient and elevated temperatures, using conductivity andturbidity measurements. We have also previously observed differing

impacts of this same set of PAAs on the decomposition of the HCO3

ion which suggests that they should have different effects on CaCO3scaling[19].

2. Experimental

2.1. Synthesis and characterization of poly(acrylic acid)

Poly(acrylic acid) with different end-groups (Table 2) and molarmasses were synthesized by Atom Transfer Radical Polymerization(ATRP)oft-butyl acrylate and subsequent hydrolysis with triuoroaceticacid, as described previously[15]. Molar masses of PAA were estimatedby 1H and NMR spectroscopy (Bruker-300) and Gel Permeation Chroma-tography (GPC Waters 1525 HPLC, Waters auto-sampler 712 WISP and

Waters 2414 RI detector).The following PAAs were tested: carboxymethyl-1,1-dimethyl-

PAA (CMM-PAA, Mn = 1500, 7600, 11,800); ethyl-isobutyrate-PAA,(EIB-PAAMn= 1700, 5100, 7200); cyclohexyl-isobutyrate-PAA (CIB-PAA, Mn= 1700, 1900, 3500, 5100, 8400, 11,000, 13,200); n-hexyl-isobutyrate-PAA (HIB-PAA,Mn= 1400, 2000, 3600, 4200, 6700, 8900,13,100);n-decyl-isobutyrate-PAA (DIB-PAA,Mn= 2400, 4500, 6200);andn-hexadecyl-isobutyrate-PAA (HDIB-PAA,Mn= 1700, 4100, 9400,7200) (see Supplementary material, Fig. S1).

2.2. Crystallization test conditions

Two stock solutions, 0.167 M CO32 as Na2CO3(10,000 ppm) and

0.413 M Ca2+

as CaCl2(16,500 ppm), were prepared. These solutionswere ltered and degassed using a 0.45 m Millipore solvent lter.PAA solutions were prepared by dissolving 0.015 g of PAA in 20 mLwater (750 ppm) and were used after three days to ensure completedissolution of the polymer.

Tests were conducted at a pH of 9.2 under four sets of conditions ofvarying temperatures and supersaturation levels (SL=Qsp/Ksp, whereQsp is the solubility quotient), as outlined in Table 1. It should benoted that the Ca2+ concentrations are much less than those expectedunder typical thermal desalination conditions, and at a correspondinglylower [Ca2+]/[CO3

2] ratio: thus while the supersaturation values arecomparable to thermal desalination conditions, they correspond to dis-tinctly different environments for crystal growth[20].

Time-resolved measurements of crystal formation, and microscopic

examination of the crystals formed in these experiments was carried

Table 1

Crystallization test conditions with PAA.

Condition set 1 2 3 4

pH 9.2T C 25 60 100 100[Ca2+] ppm 66 66 36 66[CO3

2] ppm 100 100 30 100[Ca2+]/[CO3

2] 1:1 1:1 1.8:1 1:1[PAA] ppm 1.50 1.50 0.50 6.70Ksp 10

9 4.95 2.80 1.69 1.69SL 556 983 277 1629

94 A.A. Al-Hamzah et al. / Desalination 338 (2014) 93105


3/13

out to see if any changes in speciation and morphology analogous tothose seen in the calcium oxalate system could be observed for CaCO3.

2.2.1. Determination of turbidity and conductivity at 25 C

Filtered (0.45 m cellulose acetate membrane) deionized water(246 mL)was placed in a 500mL cleaned beakercontaining theconduc-tivity probe and meter (Beta 81, CHK Engineering). Under magnetic

stirring, one drop of 0.1 M NaOH, Ca

2+

as CaCl2solution to give a

nalconcentration of 66 ppmand scale inhibitor solution to give a nal con-centration of 1.5 ppm were added. The solution was pumped continu-ously through a 1 cm Quartz ow cell using a Gilson peristaltic pump(Minipuls 2) with 4 mm silicon tubing and then returned to the mainmixture. CO3

2 as Na2CO3solution was added to the beaker to give anal concentration of 100 ppm. A UVvis spectrophotometer (Unicamspectrophotometer SP6-550) was used to measure the increase in tur-bidity with time by recording apparent absorption at 900 nm (whereabsorbance is negligible). Recording of absorbance at 900 nm started20s after addition of CO3

2 solution. Due to the formationof air bubblesat high temperatures whichinterfere with turbidity measurements, tur-bidity measurements were only recorded at 25 C (condition set 1).

Analog outputs from conductivity probe and spectrophotometerwere digitally converted using a Picolog A/D Converter 16 (16 Bit) andPicolog recording software and data was acquired every 5 and 10 s.After each experiment all equipment was ushed multiple times withweak acid followed by R/O water.

2.2.2. Determination of conductivity at 60 C

Filtered (0.45 m cellulose acetate membrane) deionized water(98.25 mL) was placed in a clean cell containing the conductivityprobe and thermometer. One drop of 0.1 M NaOH, Ca 2+ as CaCl2solu-tion to give a nal concentration of 66 ppm and scale inhibitor solutionto give 1.5 ppm were added to thecell under stirring. Recording of con-ductivity started when the solution reached the target temperature and20 s later CO3

2 as Na2CO3solution to give a nal concentration of100 ppm was added to the cell. Conductivity was measured by a con-

ductivity probe and meter (Beta 81, CHK Engineering).

2.3. Steady state and induction time

The steady state (SS) is dened as the conductivity of the systemwhen it is under equilibrium, that is, the measured conductivity whenthere is no more precipitation and conductivity value remain stable.Inhibition efciency supersaturation level (% IE) was determined byapplying the following equation (Eq.(1))

% IE SSCBlank

CC

Blank

100 1

where SS is the steady state valueof conductivity,CBlankis the equilibri-um conductivity value of the system with no PAA under the givenconditions, and C0is the conductivity value before any CaCO3scale for-mation under the given conditions. For example % IE of PAA at withoutany CaCO3scale formation = 100% and IE of blank = 0%.

Induction time (IT) is a measure of the time before any effects ofcrystal formation on macroscopic properties of the reaction mixturecan be observed. It was measured by tting a straight line to the de-creasing part of the conductivity curve (Fig. 1a) or increasing part ofthe turbidity curve (Fig. 1b) and a straight line to the initial invariantpart of the curve. The induction time is then measured as the intersec-

tion of these two lines, as illustrated inFig. 1.

2.4. Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and

Fourier Transform Infrared Spectroscopy (FTIR)

Crystals of CaCO3in the presence and absence of PAA (Mn 2000)under condition set 4 were collected after 1000 s and ltered through0.45 m pore size cellulose acetate lter paper and then characterizedby SEM, ATR-FTIR and XRD. Scale samples were gold-coated and SEMimagesobtained using a FEI Quanta 200 EnvironmentalSEM at an accel-

erating voltage of 15 kV. XRD was carried out using a Rigaku diffractioncamera with an X-ray generator with Cu Kradiation of wavelength1.5418 at the X-ray Analysis Facility, Queensland University of Tech-nology, Brisbane, Australia. FTIR was carried out using a Varian 660-IRSpectrometer.

3. Results

The conductivity and turbidity measurement curves of solutionscontaining Ca2 + and CO3

2 ions and PAA with different end groupsand molar masses under the conditions described inSection 2are de-tailed below. First, results obtained at 25 C (condition set 1), then 60C (condition set 2) and then 100 C at relatively low (condition set 3)

and relatively high (condition set 4) supersaturation values. Following

Conduc

tivity(S/cm)

Absorbance(900nm

)

570

0

Induction Time

450

200

180

160

140

120

100

80

60

40

20

0

470

490

510

530

550

590

610

500 1000 1500 2000

Time (s)

0 500 1000 1500

Time (s)

(a)

(b)

Steady State

Induction Time

Fig. 1.Determination of induction time and steady state for three different experiments(ne gray lines) and their average (thick gray line) for a solution containing Ca 2 +

and CO32 ions and 1.5 ppm HDIB-PAA (Mn= 17,200) under condition set 1 (SL= 556,T = 25 C) by conductivity (a) and turbidity (b).

95A.A. Al-Hamzah et al. / Desalination 338 (2014) 93105


4/13

Table 2

Induction times (IT)and % inhibitionefciency (%IE)of PAA forCaCO3 formationunder condition set1. Unclear indicates that no clear transitioncorrespondingto an induction timecouldbe observed. (1)Groupone, no precipitation andno signicant change in conductivity. (2)Grouptwo, minorprecipitation withlong IT.(3) Group three,more signicant precipitationandshorter IT.

End groupterminatedPAA

Mn SS

conductivity(S/cm)

% IE IT

turbidity IT conductivity Group

Blank 419 0 225 400

CMM

1457 1.5 592 100 >5000 >5000 1

7633 1.5 582 94.2 Unclear Unclear 2

11773 1.6 576 90.5 Unclear Unclear 2

EIB

1669 1.3 592 100 >5000 >5000 1

5065 1.3 481 35.8 796 +110

225 866

+208332

3

7180 1.3 514 54.9 460 +81

185 610

+21223

3

CIB

1689 1.4 592 100 >5000 >5000 1

3518 1.2 592 100 >5000 >5000 1



9954 1.3 497 45.1 510 +50

75 749

+20425

2

10988 1.2 505 49.7 620 +86

160 790

+20425

3

13209 1.1 456 21.4 318 +33

23 433

+14650

3

HIB

1403 592 100 >5000 >5000 1

1981 1.2 592 100 >5000 >5000 1

3563 1.2 592 100 >5000 >5000 1

4224 1.2 592 100 >5000 >5000 1

6723 1.2 592 100 >5000 >5000 1

8928 1.1 511 53.2 829 +70

99 1190

+15209

3

13094 1.1 461 24.3 314 +30

13 421

+10313

3

DIB




HDIB

1687 491 41.6 706 +6

16 959

+266239

3

4135 497 45.1 1095 +92

397 1150

+238365

3

9391 489 40.5 609 +40

27 827

+16615

3

17167 470 29.5 507 +16

35 676

+104139

3



5/13

the presentation of these results, the results of characterizations carried

outon crystals formedat 100C (condition set4)

SEM, FTIR, and XRD

will be given.

3.1. Inhibition of CaCO3 crystallizationat roomtemperature (condition set 1)

Induction times were determined by conductivity and turbiditymeasurements in the absence and presence of PAAs under conditionset 1. No statistically signicant difference between the IT valuesdetermined by conductivity and turbidity measurements for CaCO3 for-mationat room temperature.However, at short times it can be seen thatsystematically longer IT values are reported using conductivity rather

than turbidity, suggesting that the initially formed colloidal particlesscatter considerable light while their effect on conductance is stillmarginal compared to the starting conditions of the solution (see Sup-plementary material, Fig. S2). The IT values measured by the twomethods thus correspond to different physical states of the systemand are not directly comparable.

For condition set 1, CBlank = 419S/cm and C0 = 592S/cm. The in-hibition efciency, conductivity and turbidity measurements of PAAunder condition set 1 allowed the PAA to be divided into three groups(Table 2,Fig. 2).

Group One For the lowest molar mass of PAA ( Mn ~ 2000) withhydrophilic end group and short and medium hydropho-bic end groups, such as carboxymethyl-1,1-dimethyl(CMM), ethyl isobutyrate (EIB), hexyl isobutyrate (HIB)

and cyclohexyl isobutyrate (CIB) terminated-PAAs, ex-cellent % IE were obtained. No precipitation wasobserved

Table 3

Induction times (IT) and % inhibition efciency (% IE) of PAA for CaCO3formation under condition sets 2, 3 and 4.

End groups

terminated

PAA

Mn SS

(S/cm)

IT

conductivity % IE Group

Blank 453 175 Condition

set 2

Condition

set 3

Condition

set 4

CMM

2106 1.3 588 217 +166

80 68.3 67.8 65.9 1

7633 1.5 521 110 +12

5 34.4 2

EIB

1669 1.3 573 202 +58

61 61.1 84.3 75.3 1

7180 1.3 494 128 +42

16 21.1 2

CIB

1689 1.4 600 304 +126

121 74.4 84.3 93.2 1

5088 1.3 521 158 +41

28 34.9 2

HIB

1403 589 250 +1290

68.8 100 100 1

3563 1.3 522 176 +34

49 50.3 1

DIB

2422 1.3 503 163 25.5 25.5 21.6 18.2 2

4472 1.3 504 130 +26

9 25.6 2

HDIB

1687 506 182 +68

17 26.9 12.7 45.5 2

4135 484 188 +66

29 15.8 2

0

10

20

30

40

50

60

70

80

90

100

0 5000 10000 15000 20000

%I

E

Mn

Fig. 2. Inhibition efciency (% IE) ofCaCO3formation by PAA with different molar massesand end groups ( CMM, EIB, CIB, HIB, DIB and HDIB) under condition set1 (SL= 556, T = 25 C).



6/13

and the turbidity and conductivity changed very slowly,therefore no induction time could be reported (Table 2).

Group Two For decyl isobutyrate (DIB) terminated-PAA, HIB-PAA andCIB-PAA of moderate molar mass (Mn~ 6000), % IE weregenerally good, and little precipitation was observed,with high induction times.

GroupThree Forthe highest molar mass of PAA (Mn N9000) forall endgroups except CMM, and for all molar masses for the lon-

gest end group hexadecyl isobutyrate (HDIB), % IE is lessthan 50%, with distinct induction times.

3.2. Inhibition of CaCO3crystallization at elevated temperatures (condition

sets 2, 3 and 4)

Since CaCO3is the predominant alkaline scale in MSF plants at tem-peratures less than 100 C, the % IE was determined at 60 and 100 C.Under condition set 2, conductivity measurements were used to deter-minethe % IE of PAAwith differentend groups and molar massto inhibitCaCO3 crystallization in bulk solution for the polymers which performedvery well (IE = 100%) under condition set 1.

3.2.1. Inhibition of CaCO3crystallization at 60 C

Under condition set 2, conductivity measurements was used to de-termine the % IE and induction time of PAA with different end groupsand molar mass in the range of (14009000) to inhibit CaCO3 formationin bulk solution (Table 3,Fig. 3).

For condition set 2, CBlank =453S/cmandC0 =650S/cm.The%IEand induction time of PAA with different end groups and molar massallow the PAA to be divided into two groups.

Group One Low molar mass PAA with hydrophilic end group (CMM)and short and medium hydrophobic end groups, such asEIB-PAA (Mn= 1669), HIB-PAA (Mn= 1403) and CIB-PAA (Mn = 1689) showed good inhibition efciency

(% IE N50) and high induction times.Group Two PAA with molar mass more than 4000 for hydrophilic end

group (CMM) and short and medium hydrophobic endgroups (EIB, CIB and HIB), and low molar mass PAA withlong hydrophobic end groups (DIB and HDIB), whichshowed low inhibition efciency (% IE b 30%) and low in-duction times.

3.2.2. Inhibition of CaCO3crystallization at 100 C (condition sets 3 and 4)

As the conductivities of Ca2+ and CO32 at 100 C were greater than

at lower temperatures,causing thetotal conductivity to exceed thescaleof the conductivity meter used, the system of CaCO3crystallization re-cording was changed by increasing the cell constant. Considering thedecrease in % IE and induction time of PAA with molar mass morethan 2000 under condition set 2 (60 C), only the lowest molar massPAA withdifferentend groupswere chosen for investigation under con-dition sets 3 and 4 (Table 3). The concentration of PAAwas increased to6.7 ppm under condition set 4 to make the trends in % IE and inductiontime of PAA with different end groups more evident. While conditionset 4 maintained the same equimolar Ca2+:CO3

2 ratio as conditionsets 1 and 2, condition set 3 employed an excess of Ca 2+.

Conductivity results for condition sets 3 and 4 are summarized in

Figs. 4 and 5respectively.It can clearly be seen that at higher temperatures the difference be-

tween PAA bearing different end-groups is reduced, but not to any dra-matic extent, and that the relative effectiveness of the different PAA isretained under the two sets of conditions; the only difference in the re-versal in order of HDIB-PAA and DIB-PAA in orderof effectiveness at thehigherSL.

0

10

20

30

40

50

60

70

80

90

100

0 2000 4000 6000 8000 10000

%I

E

Mn

Fig. 3.Inhibition efciency (% IE) of CaCO3formation by PAA with different end groupsand molar masses ( CMM, EIB, CIB, HIB, DIB and HDIB) under condition set

2 (SL= 983, T = 60 C).

600

650

700

750

800

850

0 200 400 600 800 1000

Con

d.uctivity(S/cm)

Time (s)

1403 (HIB)

1669 (EIB)

1689 (CIB)

2106 (CMM)

2422 (DIB)

1687 (HDIB)

Blank

Fig. 4.Conductivity measurements of solutions containing Ca2+ and CO32 ions and 0.50

ppm PAA (Mn 2000) with different end groups under condition set 3 (SL= 277, T =100 C) with end groups: HIB, CIB, EIB, CMM, DIB, HDIB and Blank.

820

860

900

940

980

0 100 200 300 400 500 600

Conductivity(S/cm

)

Time (s)

1403 (HIB)

1689 (CIB)

1669 (EIB)

2106 (CMM)

1687 (HDIB)

2422 (DIB)

Blank

Fig. 5.Conductivity measurements of solutions containing Ca2+ and CO32 ions and 6.7

ppm PAA (Mn 2000) with different end groups under condition set 4 ( SL= 1629, T =

100 C) with end groups: HIB, CIB, EIB, CMM, DIB, HDIBand Blank.



7/13

The similarity between the results under condition set 3 and condi-

tion set 4 requires explanation. At the same supersaturation level, lessscale control would be expected at 0.5 ppmthan 6.7 ppm; butvery sim-ilar results are seen in both cases. However, the concentration of thescale-forming ions is considerably lower in condition set 3 and theratio ofSL values in the two systems (5.9) is roughly comparable tothe difference in PAA concentrations (13.4).

3.3. SEM, XRD and FTIR results

In the absence of PAA, CaCO3 crystals occurred primarilyas a mixtureof calcite and aragonite in a rod-like morphology as shown by SEM(Fig. 6) with traces of rhombohedral calcite (Fig. 11(a)) and hexagonal

orettes of vaterite (Fig. 12(a)). FTIR results showed two peaks at711 cm1 indicating calcite polymorph and at 1082 cm1 indicatingaragonite formation (Fig. 7(a)) [21], while theXRD of these crystalssug-gest that they are largely aragonite (Fig. 8(a)).

After background correction, XRD results showed that the CaCO3

present was a mixture of calcite and aragonite (Fig. 8). Those results

Fig. 6. SEM micrograph magnication(500) of rod-like CaCO3 crystals, identiedbyXRDaslargelyaragonite,in the absenceof PAAunderconditionset 4 (SL = 1629, T = 100 C).

Fig. 7.FTIR spectra of CaCO3crystals prepared under condition set 4 ( SL= 1629, T =100 C). A: aragonite, C: calcite, V: vaterite. (a) in the absenceof PAA;(b) in the presence

of DIB-PAA (Mn= 2400).

Fig. 8.XRD ofCaCO3crystals after subtraction of background under condition set 4 (SL=1629, T = 100 C). A: aragonite, C: calcite, V: vaterite. (a) in the absence of PAA (Blank);(b) in the presence of CMM-PAA (Mn= 2100); (c) in the presence of DIB-PAA (Mn=2400); (d) in the presence of HDIB-PAA (Mn= 1700).

http://localhost/var/www/apps/conversion/tmp/scratch_6/image%20of%20Fig.%E0%B8%80http://localhost/var/www/apps/conversion/tmp/scratch_6/image%20of%20Fig.%E0%B7%80


8/13

mayindicate that therod-like morphology consistedoriginally of arago-nite polymorph as was observed by Yang et al. [22].

In the presence of low molar mass PAA with different end groups,

two points can be observed from the SEM micrographs (500) of

CaCO3crystals. First is the signicant reduction in the population ofCaCO3 crystals in order of HIB N CIB N CMM N HDIB N DIBwhich is com-patible with the conductivity measurements under condition set 4

(Fig. 9). Second is the distortion in crystals of the different polymorphs

Fig. 9. SEM micrographs (500) of CaCO3crystals in the presence of PAA (Mn 2000) with different end groups under condition set 4 (SL= 1629, T = 100 C). (a) CMM-PAA;(b) CIB-PAA; (c) HIB-PAA; (d) DIB-PAA; (e) HDIB-PAA.

http://localhost/var/www/apps/conversion/tmp/scratch_6/image%20of%20Fig.%E0%B9%80


9/13

of aragonite (Fig. 10), calcite (Fig. 11), and vaterite (Fig. 12) in the sameorder. This is presumably due to adsorption of PAA with different endgroups to the active faces of growing nuclei of CaCO3[23].

The results of SEM (Fig. 9) and XRD (Fig. 8(b)) forPAA with a hydro-philic end group (CMM) showed theCaCO3 crystals to be mostly calciteof rhombohedral morphology, with distorted edges. The hydrophilicityof the CMM group may make the effect of PAA on the growth stage ofcalcium carbonate precipitation more pronounced than at the nucle-ation stage, and it appears to be much less selective in its effect, in

that all crystals formed in the presence of CMM-PAA are signi

cantly

distorted, while the rod-like crystals formed in the presence of DIB-PAA and HDIB-PAA show little distortion (Fig. 10).

In contrast to PAA-CMM, the SEM of crystals grown in the presenceof PAA with long hydrophobic end groups (DIB and HDIB) shows crys-tals of CaCO3were a mixture of rod-like aragonite, vaterite orettesand a few single crystals of rhombohedral calcite (Figs. 912). The for-mation of vaterite ower polymorph may be due to the adsorption ofPAA on active vaterite surfaces, preventing the transformation ofvaterite into aragonite or calcite. The distortion of different polymorphs

by DIB-PAA and HDIB-PAA was the least in comparison to other

Fig. 10.SEMmicrographs (2000) of aragonite rod-like CaCO3 morphology under condition set4 (SL = 1629,T = 100 C) inthe absence (a) and inthe presenceof PAA(Mn ~ 2000) withdifferent end groups; (b) CMM-PAA; (c) HIB-PAA; (d) DIB-PAA; (e) HDIB-PAA.

http://localhost/var/www/apps/conversion/tmp/scratch_6/image%20of%20Fig.%E0%B1%B0


10/13

hydrophobic end groups of PAA. Moreover, the rod-like crystals wereshorter (~ 28 and 29 m in the presence of PAA with HDIB and DIB re-spectively) than the same morphologies in the absence of PAA (~63m) (Fig. 10).

Due to the control of CaCO3formation by HIB-PAA and CIB-PAA, nopeaks could be observed by FTIR or XRD. However, there were tracesof single crystals in different habits with rod-like crystals (14 m),rhombohedral calcite and vaterite all detected by SEM. Those imagesillustrate the highest distortion in different forms, which may be dueto the mid-length hydrophobic end groups discouraging PAA chains

fromdesorbing fromthe nuclei of CaCO3 as fast as PAA with hydrophilic

end groups, delaying the growth of the CaCO3nuclei. Alternatively, thehydrophobic end-groups may lead to preferential adsorption at theedges between more and less charged surfaces, giving a higher degreeof distortion per amount of polymer adsorbed.

3.4. Discussion

Thetarget molar mass of scale inhibitors for good control of scale for-mation is 10003000 [9]. In theresults presented here for CaCO3 scaling,PAAs in that molar mass range were generally most effective in inhibi-

tion of CaCO3crystallization. The results for all conditions showed that

Fig. 11. SEM micrographs(30,000) of calcite rhombohedral morphology under condition set 4 (SL = 1629, T = 100 C) in theabsence (a) andin thepresence of PAA(Mn ~ 2000)withdifferent end groups; (b) CMM-PAA; (c) HIB-PAA; (d) DIB-PAA; (e) HDIB-PAA.



11/13

the nature of the end groups terminating PAA played a very importantrole, as previously reported for calcium oxalate scaling. In general, thetwo measurements of effectiveness (induction time, IT, which relatesto the nucleation stage of scale formation and inhibition efciency,% IE, which relates to the growth stage of scale crystals) tracked eachother closely, with an additive giving a good result by one measurealso giving a good result with the other.

Induction time was affected by the hydrophobicity of end groups,with the hydrophobic end groups having a usually longer inductiontime than the hydrophilic end groups, as previously found in the calci-um oxalate system[14]. This may possibly be because the hydrophobicendgroupsdiscourage the PAAchainsfrom desorbingfrom the nuclei of

CaCO3as fast as PAA with hydrophilic end groups, delaying the growth

of the CaCO3nuclei. However, IT measurements showed a higher vari-ability and a reduced difference between PAA than measurements of %IE, consistent with the greater variability in the nucleation stage ofscaling.

The % IE of CaCO3precipitation was affected strongly by the size ofthe end groups. PAAs with short end groups with both hydrophilic(CMM) and hydrophobic (EIB) and middle hydrophobic end groups(CIB and HIB) of PAA were found to have excellent inhibition efciency(100%) at room temperature and are more efcient than PAA with longhydrophobic end groups (DIB and HDIB) at all temperatures investigat-ed (25100 C). At high temperature, the inhibition efciency of PAAwith hydrophobic end groups of moderate size (CIB and HIB) remained

superior. The inhibitionefciency and induction time of PAAs decreased

Fig. 12. SEM micrographs(10,000) of vaterite morphology under condition set 4 (SL = 1629,T = 100 C) in theabsence (a)and in the presenceof PAA(Mn ~ 2000)with different end

groups; (b) CMM-PAA; (c) CIB-PAA; (d) HIB-PAA; (e) DIB-PAA; (f) HDIB-PAA.



12/13

with the increasing temperature, which is reasonable since at highertemperatures, the solubility product of CaCO3will decrease to give agreater thermodynamic driving force for crystallization. Therefore,more rapid and less reversible adsorption of PAA to delay the crystalgrowth of CaCO3will be more required than at low temperature.

It has been found for calcium oxalate that the best performance ofPAA inhibitors occurs at an equimolar ratio of cations to anions [24],while the similarity between results obtained in this work at very differ-ent inhibitor concentrations (0.5 and 6.7 ppm) at different Ca2+:CO3

2

ratios (1.8 and 1.0) may suggest that in the calcium carbonate scalingsystem an excess of cations is preferable. However, the concentrationsof scale-forming ions were also different in the two systems, making itimpossible to draw any conclusion.

Differentcharge densitiesreadily develop on differentfaces of grow-ing crystals, with molecular dynamics being an ideal tool to estimatethese values[25]. Based on our previous results on calcium oxalatewhere marked changes in crystal morphology were observed[15], weadvanced the hypothesis ofedge activity[26], where end-group mod-ied polyelectrolytes might partition selectively to edges betweengrowing interfaces with different charge densities specically, to theedge between a highly negatively-charged and a near neutral face as a mechanism to achieve greatereffectiveness than indiscriminate ad-sorption of scale inhibitor to crystallites. In order to investigate this fur-ther, SEM and XRD measurements were carried out on the crystalproducts formed under condition set 4.

Theformationof different polymorphs of CaCO3 crystals precipitatedunder condition set 4 ([Ca2+]/[CO3

2] = 1,SL= 1629 and T = 100 C)

can be explained by a dissolutionrecrystallization mechanism, wherethe least thermodynamically stable phase is initially formed, anddissolves and reprecipitates in the absence of scale inhibitor to a morestable phase.

We suggest that the less thermodynamically stable vaterite phase isformedrst, consistent with Ostwald's rule of stages which hold thatthe least thermodynamically stable phase is usually kinetically pre-ferred[27]. All scale inhibitors can bind to this phase to some extent,retarding its dissolution and leading to distorted crystals. Aragoniteand calcite then both form from these vaterite crystals by a process ofdissolution and recrystallization, with the crystals formed being consid-erably more distorted and showing a higher degree of twinning then inthe absence of scale inhibitor.

The nature and length of end groups of PAA play a signicant role in

the distortionof differentpolymorphs of calcium carbonate. The highestdistortion in those polymorphs was in the presence of PAA with endgroups of moderate hydrophobicity (HIB and CIB) while the lowest dis-tortion was in the presenceof PAA with long end groups (DIB and HDIB)(Figs. 1012). This distortion is likely to be dependent both on the selec-tivity of PAA with different end groups to adsorb on different activefaces of CaCO3crystals and its likely rate of adsorption/desorption to/from those active faces[28]. It is to be expected that PAA will adheremost readily to the most positively charged faces of the growing crys-tals. Hydrophobic end-groups would be expected to prefer relativelyuncharged surfaces, possibly directing PAA toward edges; while longerhydrophobic groups may self-assemble, leading to unselective PAA ad-sorption and retarding desorption.

InTable 4it can be seen that effective scale inhibitors also gave

shorter rod-like crystals and smaller vaterite orettes, suggesting

again that they have a strongimpacton growthas well as nucleation. Al-though the inhibition efciency of PAA with DIB and HDIB end groupswas the lowest (18.2 17 and 45.5 5.7 respectively), the action ofthese polymers as scale inhibitors can clearly be seen in the reductionof rod-like population(comparison with theblank)and thestabilizationof vaterite metastable polymorph(Fig.9(d, e)). The correlation between% IE and amount of rod-like polymorph formed canbe observed in theseimages.

Finally, in contrast to the action of these same scale inhibitors previ-ously reported on the calcium oxalate system[15], all changes in mor-phology observed in the calcium carbonate system tended to a loss ofdenition in the structures. This suggests a relatively indiscriminate ad-sorption of polymer to all surface and/or edges, which is not surprisingconsidering the high (6.7 ppm) concentration of inhibitor employed incondition set 4. Work is currently underway on the behavior of PAA atlower concentrations and over a range of ion ratios.

4. Conclusion

The inhibition efciency of PAA with different end groups and molarmasses as scale inhibitors to prevent CaCO3formation in bulk solutionwasstudied at temperatures ranging between 25 and 100 C using con-ductivity and turbidity. The results showed that both molar mass andthe nature of the end group of PAA affect the inhibition of CaCO3scale.Low molar mass PAA with short and moderately-sized end groups hadrelatively high inhibition efciency and long induction times under allconditions investigated, while all high molar mass PAAs had poor

inhibition efciency of CaCO3. Low molar mass PAA with long hydro-phobic end groups had poor inhibition efciency of CaCO3scale. Atroom temperature, the lowest molar mass of PAA with hydrophilicend group showed good efciency in the inhibition of CaCO3 scalemaking it suitable for use as a scale inhibitor in RO desalination.However, with increasing temperature, the lowest molar mass of PAAwith different middle hydrophobic end groups gave a better inhibitionefciency and induction time than PAA with hydrophilic end groups.These results suggest that the lowest molar mass PAA with end groupsof moderate hydrophobicity are more suitable as scale inhibitors in MSFdesalination.

Effectivenessof the inhibitorsdeclined with increasing temperature,but the relative effectiveness of the inhibitors according to end-groupremained essentially the same. At 100 C, 0.5 ppm PAA applied to a

system with SL = 277 gave approximately the same inhibition efcien-cy (%IE) as6.7ppmappliedat SL = 1629, with theorder of efciency ofthe different end-groups being almost identical.

The nature and length of end groups of PAA had a major effect onthemorphologies of CaCO3 obtainedat 100 Cwith6.7 ppm.The highestdistortion in the CaCO3 polymorphs was for PAA with mid-hydrophobicend groups. However, the lowest distortion in different CaCO3poly-morphs was for PAA with long-hydrophobic end groups. These resultsmay due to its rate of adsorption/desorption on the active faces ofCaCO3polymorphs. The conductivity and morphology results are com-patible, where the % IE of CaCO3 formation and the distortion of its mor-phologies have the same order of HIB NCIB NCMM NHDIB NDIB. Aspreviously observed with the calcium oxalate system, the PAA appearsto be stabilizing the initially-formed kinetically-favored product so it

is not transformed to the thermodynamic product, though less

Table 4

The relationship between the nature and length end groups of PAA and the distortion and speciation of CaCO3polymorphs under condition set 4.

End-group % IE Distortion Average rodlength (m)

Average vateritediameter (m)

Predominant polymorph

None 0 None 63 20 Rod-likeCMM 66 Moderate 19 9.5 Rhombohedral calciteHIB/CIB 100/94 High 14 9 Mixture of rod-like, vaterite and

rhombohedral calciteDIB 20 Low 29 13HDIB 45 Low 28 21



13/13

comprehensively. The crystals formed are more ill-dened than ob-served in the calcium oxalate system, which may be related to thehigher concentration of polymer employed, a smaller difference incharge density between crystal growth faces, or some other unidenti-ed feature of the system.

List of acronyms

ATRP Atom Transfer Radical Polymerization

CMM-PAA carboxymethyl-1,1-dimethyl-terminated poly(acrylic acid)EIB-PAA ethyl-isobutyrate-terminated poly(acrylic acid)CIB-PAA cyclohexyl-isobutyrate-terminated poly(acrylic acid)DIB-PAA n-decyl isobutyrate-terminated poly(acrylic acid)FTIR Fourier Transform Infrared spectroscopyHDIB-PAA n-hexadecyl isobutyrate-terminated poly(acrylic acid)HIB-PAA n-hexyl isobutyrate-terminated poly(acrylic acid)IT induction timeMSF multi-stageash thermal desalinationPAA poly(acrylic acid)RAFT Reversible AdditionFragmentation chain Transfer

polymerizationSEM Scanning Electron MicroscopySL supersaturation levelSS steady state value of a parameterXRD X-ray diffraction% IE Percent inhibition efciency

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.desal.2014.01.020.

References

[1] A.M. Shams El Din, M.E. El-Dahshan, R.A. Mohammed, Inhibition of the thermal de-composition of HCO3

: a novel approach to the problem of alkaline scale formationin seawater desalination plants, Desalination 142 (2002) 151159.

[2] F. Rahman, Z. Amjad, Scale formation and control in thermal desalination systems,

The Science and Technology of Industrial Water Treatment, CRC Press, Boca Raton,2010, pp. 271296.[3] K. Al-Anezi, N. Hilal, Effect of carbon dioxide in seawateron desalination:a compre-

hensive review, Sep. Purif. Rev. 35 (2006) 223247.[4] G. Wolf, E. Knigsberger, H.G. Schmidt,L.-C. Knigsberger, H. Gamsjger, Thermody-

namic aspects of the vateritecalcite phase transition, J. Therm. Anal. Calorim. 60(2000) 463472.

[5] D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, S.M. Bailey, I. Halow, K.L.Churney, R.L. Nuttall, Selected values of thermodynamic constants, in: R.C. Weast,D.R. Lide, M.J. Astle, W.H. Beyer (Eds.), Handbook of Chemistry and Physics, 70thed., CRC Press, Boca Raton, Florida, 1989.

[6] O. Cizer, K. Balen, J. Elsen, D. Gemert,Crystal morphology of precipitatedcalcite crys-tals from accelerated carbonation of lime binders, ACEME08, 2nd International

Conference on Accelerated Carbonation for Environmental and Materials Engineer-ing, Rome, 2008.

[7] W. Jung, S. Kang, C. Kim, C. Choi, Particle morphology of calcium carbonate precipi-tated by gasliquid reaction in a CouetteTaylor reactor, Chem. Eng. Sci. 55 (2000)733747.

[8] D. Hasson, R. Semiat, Scale control in saline and wastewater desalination, Isr. J.Chem. 46 (2006) 97104.

[9] E. Senogles, W.O.S. Doherty, O.L. Crees, Scale inhibitors, polymeric, in: J. Kroschwitz(Ed.), Encyclopediaof PolymerScienceand Technology, 2nd ed.,Wiley-Interscience,Boca Raton, 1996.

[10] C. Gabrielli, M. Keddam, H. Perrot, A. Khalil, R. Rosset, M. Zidoune, Characterization

of the efciency of the antiscale treatment of water. Part I: chemical processes,J. Appl. Electrochem. 26 (1996) 11251132.[11] O.A. Hamed, H.A.Al-Otaibi, Prospects of operation of MSF desalination plants at high

TBT and low antiscalant dosing rate, Desalination 256 (2010) 181189.[12] Z. Amjad, R.W. Zuhl, Effect of heat treatment on the performance of deposit control

polymers as calcium carbonate inhibitors, Corrosion 2007 NACE International,Nashville2007.

[13] O.A. Hamed, M.A.K. Al-So, G.M. Mustafa, A.G. Dalvi, The performance of differ-ent anti-scalants in multi-stage ash distillers, Desalination 123 (1999)185194.

[14] A.D. Wallace, A. Al-Hamzah, C.P. East, W.O.S. Doherty, C.M. Fellows, Effect of poly(acrylic acid) end-group functionality on inhibition of calcium oxalate crystalgrowth, J. Appl. Polym. Sci. 116 (2010) 11651171.

[15] C.P. East, A.D. Wallace, A. Al-Hamzah, W.O.S. Doherty, C.M. Fellows, Effect of poly(acrylic acid) molecular mass and end-group functionality on calcium oxalate crys-tal morphology and growth, J. Appl. Polym. Sci. 115 (2010) 21272135.

[16] W.O.S. Doherty, C.M. Fellows, S. Gorjian, E. Senogles, W.H. Cheung, Inhibition of cal-cium oxalate monohydrate by poly(acrylic acids) with different end-groups, J. Appl.Polym. Sci. 91 (2004) 20352041.

[17] M. Siauw, B.S. Hawkett, S. Perrier, Short chain amphiphilic diblock co-oligomers viaRAFT polymerization, J. Polym. Sci. A Polym. Chem. 50 (2012) 187198.

[18] H. Minami, A. Tanaka, Y. Kagawa, M. Okubo, Preparation of poly(acrylicacid)-b-polystyrene by two-step atom transfer radical polymerization in supercrit-ical carbon dioxide, J. Polym. Sci. A Polym. Chem. 50 (2012) 25782584.

[19] A. Al-Hamzah, C.M. Fellows, Apparent inhibition of thermal decomposition ofhydrogencarbonate ion by poly(acrylic acid). The effect of molar mass and end-group functionality, Desalination 332 (2014) 3343.

[20] Y. Wang, University of New South Wales (Sydney), 2005.[21] K. Naka, D. Keum, Y. Tanaka, Y. Chujo, Control of crystal polymorphs by a latent

inductor : crystallization of calcium carbonate in conjunction within situradicalpolymerization of sodium acrylate in aqueous solution, Chem. Commun. (2000)15371538.

[22] D. Yang, L. Qi, J. Ma, Well-dened star-shaped calcite crystals found in agarose gels,Chem. Commun. (2003) 11801181.

[23] E. Hadicke, J. Rieger, I. Ursula Rau, D. Boeckh, Molecular dynamics simulations of theincrustation inhibition by polymeric additives, Phys. Chem. Chem. Phys. 1 (1999)38913898.

[24] W. Al-Thubaiti and C. M. Fellows, unpublished work, 2013.[25] J. Rieger, E. Hadicke, I.U. Rau, D. Boeckh, A rational approach to the mechanisms of

incrustation inhibition by polymeric additives, Tenside Surfactants Deterg. 34(1997) 430435.

[26] C.M. Fellows, A. Al-Hamzah, C.P. East, W.O.S. Doherty, E.J. Smith, Edge-active poly-mers for tailoring crystal morphology, 12th Pacic Polymer Conference, Jeju,Korea, 2011.

[27] T. Threlfall, Structural and thermodynamic explanations of Ostwald's rule, Org. Pro-cess Res. Dev. 7 (2003) 10171027.

[28] A.S.Schenk, I. Zlotnikov, B. Pokroy, N. Gierlinger, A.Masic, P. Zaslansky, A.N. Fitch, O.Paris, T.H. Metzger, H. Coelfen, P. Fratzl, B. Aichmayer, Hierarchical calcite crystalswith occlusions of a simple polyelectrolyte mimic complex biomineral structures,Adv. Funct. Mater. 22 (2012) 46684676.

http://dx.doi.org/10.1016/j.desal.2014.01.020http://dx.doi.org/10.1016/j.desal.2014.01.020http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0005http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0005http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0005http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0005http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0005http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0005http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0005http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0005http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0155http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0155http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0155http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0155http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0155http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0015http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0015http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0015http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0015http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0160http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0160http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0160http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0160http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0160http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0160http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0160http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0110http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0110http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0110http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0110http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0165http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0165http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0165http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0165http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0030http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0030http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0030http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0030http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0030http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0030http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0030http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0030http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0030http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0035http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0035http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0035http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0035http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0120http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0120http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0120http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0040http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0040http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0040http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0040http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0040http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0040http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0040http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0045http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0045http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0045http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0045http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0125http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0125http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0125http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0055http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0055http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0055http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0055http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0055http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0055http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0055http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0055http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0055http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0060http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0060http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0060http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0060http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0060http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0065http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0065http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0065http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0065http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0065http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0070http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0070http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0070http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0070http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0070http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0130http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0130http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0130http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0130http://refhub.elsevier.com/S0011-9164(14)00036-8/rf9000http://refhub.elsevier.com/S0011-9164(14)00036-8/rf9000http://refhub.elsevier.com/S0011-9164(14)00036-8/rf9000http://refhub.elsevier.com/S0011-9164(14)00036-8/rf9000http://refhub.elsevier.com/S0011-9164(14)00036-8/rf9000http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0075http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0075http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0075http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0075http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0075http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0140http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0140http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0140http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0140http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0140http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0140http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0140http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0140http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0140http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0140http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0145http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0145http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0145http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0145http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0145http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0145http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0085http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0085http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0085http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0085http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0085http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0090http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0090http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0090http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0090http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0090http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0150http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0150http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0150http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0150http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0150http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0100http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0100http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0100http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0100http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0105http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0105http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0105http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0105http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0105http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0105http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0105http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0105http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0105http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0105http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0100http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0100http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0150http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0150http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0150http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0090http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0090http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0090http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0085http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0085http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0085http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0145http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0145http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0140http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0140http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0140http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0140http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0075http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0075http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0075http://refhub.elsevier.com/S0011-9164(14)00036-8/rf9000http://refhub.elsevier.com/S0011-9164(14)00036-8/rf9000http://refhub.elsevier.com/S0011-9164(14)00036-8/rf9000http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0130http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0130http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0070http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0070http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0070http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0065http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0065http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0065http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0060http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0060http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0060http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0055http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0055http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0055http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0125http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0125http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0125http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0045http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0045http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0040http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0040http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0040http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0120http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0120http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0120http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0035http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0035http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0030http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0030http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0030http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0165http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0165http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0165http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0165http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0110http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0110http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0110http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0110http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0160http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0160http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0160http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0015http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0015http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0155http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0155http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0155http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0005http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0005http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0005http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0005http://refhub.elsevier.com/S0011-9164(14)00036-8/rf0005http://dx.doi.org/10.1016/j.desal.2014.01.020http://dx.doi.org/10.1016/j.desal.2014.01.020

Documents

Inhibition of homogenous formation of calcium carbonate by poly