Low-phosphorus maleic acid and sodium ρ-styrenesulfonate copolymer as calcium carbonate scale...

Low-Phosphorus Maleic Acid and Sodiumq-styrenesulfonate Copolymer as CalciumCarbonate Scale Inhibitor

Chen Wang, Deyi Zhu, Xikui Wang

Department of Light Chemical and Environmental Engineering, Shandong Institute of Light Industry,Jinan 250353, People’s Republic of China

Received 24 January 2009; accepted 15 August 2009DOI 10.1002/app.31300Published online 7 October 2009 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: In this article, a multifunctional and envi-ronment-friendly agent for industry circulation coolingwater, maleic acid and sodium q-styrenesulfonate copoly-mer containing low-phosphorus was synthesized in aque-ous solution with hypophosphorous acid/hydrogenperoxide as redox initiators. The effects of monomer feedratio, temperature, dose of hypophosphorous acid, anddrop speed of hydrogen peroxide on calcium carbonate in-hibition were investigated, and the experimental condi-tions were optimized using orthogonal method. Thecomposition and structure of the copolymer were analyzedby enery-dispersive X-ray analysis, fourier transform infra-red spectroscopy, and nuclear magnetic resonance spec-

troscopy. The results of the static scale inhibition testsshowed that the copolymer was effective in inhibiting theformation of calcium carbonate scale. The calcium carbon-ate precipitates formed in the static tests were examinedby scanning electronic microscopy and X-ray diffractionspectra. The results indicate that in the presence of the co-polymer the calcium carbonate precipitate is the mixtureof calcite and vaterite that could be stabilized kineticallyin the presence of the copolymer. VC 2009 Wiley Periodicals,Inc. J Appl Polym Sci 115: 2149–2155, 2010

Key words: copolymerization; water-soluble polymers;phosphonate; scale inhibition; cooling water

INTRODUCTION

Open recirculating cooling water systems are fre-quently used because they not only provide econom-ical heat removal but also recirculation of water con-servation is accomplished with substantial costreductions. The cooling water circulates to operatingunits where it picks up heat and the resulting warmwater is cooled through vaporization by returning tothe cooling tower. Dissolved and suspended mattercontained in the water is concentrated in coolingwater recirculation. The precipitation of calcium car-bonate scale on heat transfer surfaces widely occurs,which involves the deposition of an insulating layeronto the internal walls owing to its inverse tempera-ture-solubility characteristics.1–3 Deposits formationmay cause severe corrosion and deterioration of heatexchange. The study of the mechanism of calciumcarbonate crystal growth inhibition is an importanttheme for developing an effective technique to pre-vent scale formation. The most common and effec-

tive method of scale controlling is the use of chemi-cal additives as scale inhibitors that retard orprevent scale formation even in very small concen-trations.4,5 Several studies about calcium carbonatescale formation in the absence and presence of inhib-itors have been carried out.6–10

Copolymer has been used satisfactorily as a spe-cific scale inhibitor in the circulating cooling watersystems is developed in the late 1970s11 because ofits strong complexation of multifunctional groupsand superior dispersion characteristic of macromole-cule. This kind of chemical inhibitors is appliedwidely in circulating cooling water treatmentcontribute to their excellent performances such as in-hibiting formation of calcium carbonate scale, dis-persing ferric oxide (Fe2O3), and stabilitating Zn2þ inwater. In addition, some of them can perform as ametal corrosion inhibitor. With the improvement ofenvironmental consciousness, the content of phos-phorus in water treatment agent has been of rigor-ous control. Concerning over accelerated aquaticeutrophication, the popularity of inhibitors contain-ing high phosphorus is diminishing. As a result, thecurrent trend for inhibitor usage is toward moreenvironmentally friendly ‘‘green’’ chemicals.In this article, a maleic acid–sodium q-styrenesul-

fonate (MAc-SS) copolymer containing low-phospho-rus was prepared in aqueous solution with redox

Correspondence to: C. Wang ([email protected]).Contract grant sponsor: Research Award Program for

Yong Scientists in Shandong Province; contract grantnumber: 2008BS09021.

Journal ofAppliedPolymerScience,Vol. 115, 2149–2155 (2010)VC 2009 Wiley Periodicals, Inc.

system of hypophosphorous acid and hydrogen per-oxide as initiator. The experimental factors of mono-mer feed ratio, reaction temperature, dosage ofhypophosphorous acid, and drop speed of hydrogenperoxide were investigated. Elemental compositionof the optimization copolymer was analyzed withenergy-dispersive X-ray analysis (EDX), and infraredspectroscopy was used to study the structure of thecopolymer, and the chain structure of the copolymerwas identified by31P-NMR and 13C-NMR spectra. Inaddition, the viscosity–average molecular weight ofthe copolymer were calculated. The CaCO3 scale in-hibition performance of the optimization copolymerin the artificial cooling water was studied throughstatic scale inhibition tests,12 and the effect on forma-tion of CaCO3 was investigated with scanningelectronic microscopy (SEM) and X-ray powderdiffraction (XRD), respectively.

There are few reports of low-phosphoruscopolymer of MAc-SS used as scale inhibitor incooling water.11,13,14 It is believed to represent apotentially new environmentally safe water treat-ment agent suitable for cooling water systemsbecause of its low-phosphorus and decreasingeutrophication.

EXPERIMENTAL

Materials

All reagents were commercially available and wereused without further purification. Maleic anhydride(MA), hydrogen peroxide, sodium hydroxide, cal-cium chloride, and sodium bicarbonate were analyti-cal reagent grade purchased from SinopharmChemical Reagent Beijing. Sodium q-styrenesulfo-nate (SS) was industrial grade afforded by ZiboXingzhilian Chemical, and hypophosphorous (Guar-anteed reagent) was supplied by Nanjing ChemicalReagent. All solutions were prepared usingdeionized water.

Synthesis of MAc-SS copolymer

A standard orthogonal array L9 (34) was used toexamine reaction conditions such as the ratio of MAto SS, reaction temperature, the ratio of H3PO2 tomonomer, and the drop time of H2O2 in three levels.The four factors and the levels of the factors studiedare shown in Table I. Accordingly, the optimal reac-tion conditions were determined by combining thelevels of factors that had the highest main effectvalue.

The copolymerization reaction was carried out in250-mL flasks with mechanical stirrer, reflux con-denser, dropping funnel, and thermometer. A totalof 10 g MA and 50 mL of deionized water were

mixed stirring continuously with nitrogen as protec-tive gas. SS and hypophosphorous acid were addedin definite proportion and heated to the reactiontemperature. In fixed temperature and stirring con-ditions, hydrogen peroxide was added drop bydrop. After hydrogen peroxide had been added, itwas kept reacting for 60 min. Finally, the pH valueof solution was adjusted to 7–8 by adding 20%NaOH solution.To test the properties of the low-phosphorus

MAc-SS copolymer as a recirculating cooling watertreatment agent, inhibition of calcium carbonatescale efficiency was developed for controlling experi-mental parameters in orthogonal experiments. Allthe parameters were determined according to thestandard method of static evaluation for circulatingcooling water treatment agent.12 All calculationswere performed using Design Expert software(version 6.0.10, Stat-Ease, USA).

Inhibition test

The ability of the low-phosphorus MAc-SS copoly-mer to inhibit calcium carbonate scale was comparedwith that of the free-inhibitor in flask tests. The inhi-bition test was carried with artificial water, whichwas prepared by dissolving a certain quantity ofCaCl2 and NaHCO3 in deionized water, and theinitial total concentrations of Ca2þ(CCa,i) andHCO�

3 (CHCO3,i) were 2.45 � 10�3 mol L�1 and 5 �10�3 mol L�1, respectively. Hence, the ratio of initialconcentrations, Ri(¼CCa,i/CHCO3,i), was 0.49. Theartificial waters containing different dosages of thelow-phosphorus MAc-SS copolymer were thermo-stated at 80�C for 10 h.The concentrations of soluble calcium ions in dif-

ferent solutions were analyzed using a standard so-lution of EDTA. The inhibition efficiency u wasdefined as:

u ¼ q1ðCa2þÞ � q2ðCa2þÞq0ðCa2þÞ � q2ðCa2þÞ

� 100%

where q0(Ca2þ) was the total concentrations of

Ca2þ(mg L�1), q1(Ca2þ) was the concentrations of

Ca2þ(mg L�1) in the absence of the copolymer inhib-itor, q2(Ca

2þ) was the concentrations of Ca2þ(mgL�1) in the presence of the copolymer inhibitor. Themorphologies of CaCO3 precipitates were examinedby SEM on a scanning electronic microscope(QUANTA 200, Netherlands). Precipitated phaseswere identified by XRD on an X-ray powder diffrac-tometer (SHIMADZU XRD-600, Japan, using Ni-fil-tered CuKa radiation).

2150 WANG, ZHU, AND WANG

Journal of Applied Polymer Science DOI 10.1002/app

Analysis

The low-phosphorus MAc-SS copolymer sampleobtained under the optimal reaction condition wasprecipitated (not adjusted pH by adding NaOHsolution) and purified with ethanol.

The intrinsic viscosity [g] of the copolymer (0.005g mL�1) was measured in sodium thiocyanate aque-ous solution (0.15 mol L�1) with an Ubbelohde vis-cometer thermostated at 25�C � 0.1�C, and the vis-cosity–average molecular weigh was calculated byMark-Houwink empirical equation15:

½g� ¼ KMa

where [g] is intrinsic viscosity, K is a Mark-Hou-wink constant, here 7.65 � 10�6 mL g�1 wasadopted, a is the other Mark-Houwink constant,here 0.924 was adopted, M is molecular weight

The elemental compositions of the low-phospho-rus MAc-SS copolymer were measured using X-rayPhotoelectron Spectroscopy (EDAX, USA). FourierTransform Infrared (FTIR) spectra were recorded bya Tensor 27 spectrophotometer (BRUKER OPTICS,Switzerland) using KBr pellet at room temperature.NMR spectra were acquired on BRUKERAVANCE600 spectrometer (BRUKER, Switzerland)at 300 K, and all the experiments were recordedusing D2O as solvent, The 13C-NMR and 31P-NMRspectra were operated at 150.90 MHz with DSS asan internal reference and at 242.93 MHz with 85%H3PO4 as an external reference, respectively.

RESULTS AND DISCUSSION

Results of orthogonal experiment

Table I presents the contribution of the four inde-pendent factors at different levels to calcium car-

bonate scale inhibition performance of the low-phosphorus sodium MAc-SS copolymers. MA/SSratio has shown the highest positive impact on thecalcium carbonate scale inhibition rate. The nextmost important factor is reaction temperature. Theratio of H3PO2 to monomer and the drop time ofH2O2 rank the last. Therefore, the optimum param-eter level for the calcium carbonate scale inhibitionperformance is A2B2C3D1. The optimal reaction con-ditions to synthesize the copolymer are 1.25 ofMA/SS ratio, 0.18 of the ratio of H3PO2 to mono-mer, 80�C reaction temperature, and 60 min ofH2O2 drop time.

Structure analysis of the copolymer

The viscosity–average molecular weight of the opti-mization low-phosphorus MAc-SS copolymer wascalculated as 1.86 � 105.EDX was performed to get information about the

elemental composition of the optimization low-phos-phorus MAc-SS copolymer, including C, O, P, S, andNa. Importantly, the phosphorus content is 3.63% inweight. In fact, the actual phosphorus contentshould be lower than 3.63% because the hydrogenhad been reckoned without in the EDX result. Con-sequently, the phosphorus content in the copolymeris lower than 4% for certain, which carried thepoints of low-phosphorus content and environmentfriendly.The FTIR spectra of sodium q-styrenesulfonate (a)

and the optimization low-phosphorus MAc-SScopolymer (b) were shown in Figure 1. In contrast tothe spectrum of sodium q-styrenesulfonate, the char-acteristic absorption bands of alkene, i.e., out ofplane bending vibration of CH2 at 909 cm�1, out ofplane bending vibration of CH at 991 cm�1, andCAHstr at 3051 cm�1 all disappeared indicating that

TABLE IThe Arrangement of Orthogonal Array L9 (3

4) and Corresponding Results for Calcium Carbonate Scale Inhibition

Trial no.MA /SSratio

Reactiontemperature (�C)

H3PO2/monomerratio

Drop timeof H2O2 (min)

Scale inhibition rate oncalcium carbonate (%)

1 A1(1 : 1.0) B1(65) C1(0.14 : 1) D1(60) 72.352 A1(1 : 1.0) B2(80) C2(0.16 : 1) D2(90) 88.743 A1(1 : 1.0) B3(95) C3(0.18 : 1) D3(120) 85.724 A2(1 : 0.8) B1(65) C2(0.16 : 1) D3(120) 92.835 A2(1 : 0.8) B2(80) C3(0.18 : 1) D1(60) 98.566 A2(1 : 0.8) B3(95) C1(0.14 : 1) D2(90) 96.377 A3(1 : 0.6) B1(65) C3(0.18 : 1) D2(90) 68.588 A3(1 : 0.6) B2(80) C1(0.14 : 1) D3(120) 75.659 A3(1 : 0.6) B3(95) C2(0.16 : 1) D1(60) 72.46k1 82.27 77.92 81.46 81.12k2 95.95 87.65 84.68 84.56k3 72.23 84.85 84.29 84.73R 23.72 9.73 3.22 3.61

k, average of every factor scale inhibition rate on calcium carbonate, %; R, range; the copolymer concentration is 16 ppm.

CALCIUM CARBONATE SCALE INHIBITOR 2151


a polymerization reaction must be carried out. Theslender peak at 776 cm�1 is attributed to PACdef,1126 cm�1 is attributed to P¼¼Ostr, 2309 cm�1 isattributed to PAOstr. These appearance can beexplained by H3PO2 that had been concerned withthe copolymerization. At the same time, anotherthree visible peaks (C¼¼Ostr at 1719 cm�1, symmetri-

cal and asymmetrical stretching vibration of S¼¼O at1038 cm�1, and 1181 cm�1) are observed in the FTIRspectrum, which reflects the copolymerization reac-tion between MAc and SS coming true. Therefore,the FTIR spectra revealed that the optimizationMAc-SS copolymer contain multifunctional groupsincluding carboxyl, phosphoric acid and sulfonic.

Figure 1 The FTIR spectrums of sodium q-styrenesulfonate (a) and the MAc-SS copolymer (b).

Figure 2 13C NMR spectra of the copolymer (ppm).



These groups provide a base for its varied applica-tion properties such as inhibiting the formation ofcalcium carbonate, dispersing ferric oxide, and stabi-litating Zn2þ.

The 13C NMR spectrum and 31P NMR spectrum ofthe optimization low- phosphorus MAc-SScopolymer were shown in Figures 2 and 3, respec-tively. 13C NMR spectral analysis reveals that thecopolymer produced in the copolymerization is aalternating copolymer with maleic acid and sodiumq-styrenesulfonate. The peak at 179.7 ppm isassigned to the carbonyl carbon adjacent the phenyl.The chemical shifts in the region from 172.3 to 178.3ppm are assigned to the carbonyl carbon away fromthe phenyl. The chemical shifts in the region from136 to 139 ppm are assigned the phenyl carbon adja-cent to the copolymer backbone. The presence ofthese peaks provides evidence that the copolymerbackbone chain is the alternating units of maleicacid and sodium q-styrene sulfonate. The 31P-NMR

spectra in Figure 3 implied that the chemical shiftsin the region from 8.6 to 10.0 ppm are a terminalphosphoro-to-carbon bond in the copolymer molecu-lar structure. Therefore, the molecular structure ofthe copolymer was presented in Figure 4.

Inhibition of the copolymer

The effect of the low-phosphorus MAc-SS copolymeras an antiscale agent for calcium carbonate was

Figure 3 31P NMR spectra of the copolymer (ppm).

Figure 4 Diagram of the molecular structure of thecopolymer.

Figure 5 Influence of the copolymer concentration on theinhibition efficiency.



shown in Figure 5. A series of artificial cooling waterwith different dosages of the low-phosphorus MAc-SS copolymer ranging from 8 to 20 ppm were heatedat 80�C for 10 h, respectively. Figure 5 illustrates theability of the low-phosphorus MAc-SS copolymerunder identical conditions. When the copolymerconcentration increased, the scale inhibition effectenlarged correspondingly. Noticeably, there is a sud-den increase of scale inhibition when the concentra-tion increases from 8 to 14 ppm, where the scale in-hibition result is 90.3% at 8 ppm, but 97.4% for 14ppm, and the curve has a tendency to moderatewhen the concentrations increase from 14 to 20 ppm,where reaches 98.2% at 16 ppm, and 98.4% for 20ppm.

Figure 6 shows the SEM micrographs of CaCO3

precipitate formed in artificial cooling water. Asshown, in the absence of the low-phosphorussodium MAc-SS copolymer, the form of CaCO3 pre-cipitates is essentially well-regulated orthorhombiccalcite particles with average particle size of about

10 lm. In the presence of the low- phosphorus MAc-SS copolymer [Fig. 6(b)], obvious changes can benoted in distribution, size, and morphology of theCaCO3 precipitate comparing with Figure 6(a). TheCaCO3 precipitate lost its sharp edges, and the formwas distorted. In addition, the size was increased to25–30 lm, larger than those deposited in uninhibitedsolution.It is generally accepted that the inhibition of scale

formation is influenced by both the location of theadsorbed inhibitor at the crystal surface andthe extent of chemical bonding with the surface.16–18

The more effective the inhibitor is, the higher thesurface binding capability. Organic phosphonic canstrongly adsorb onto a CaCO3 crystalline substrateowing to the high chelating ability of theAP(O)(OH)2 group.19–21 The low-phosphorus MAc-SS copolymer contains both carboxyl and phospho-rus groups in one molecule. Therefore, its surface-binding capability is stronger, and as a result, itpossesses higher inhibition effect under identical

Figure 6 SEM micrographs of CaCO3 precipitates. (a) In the absence of the low-phosphorus copolymer and (b) In thepresence of 8 ppm the low-phosphorus copolymer.

Figure 7 The XRD pattern of CaCO3 precipitates. (a) In the absence of the low-phosphorus copolymer and (b) in thepresence of the low-phosphorus copolymer.



conditions. The low-phosphorus MAc-SS copolymeradsorbs on CaCO3 surfaces by binding the carbox-ylic or phosphonate anions to surface calcium ions.

The CaCO3 precipitated phases were identified byXRD, and the spectra were shown in Figure 7. In theabsence of the low-phosphorus MAc-SS copolymer,calcite is the main crystal form [Fig. 7(a)]. As indi-cated in Figure 7(b), in the presence of the low-phos-phorus MAc-SS copolymer, there are a number ofvaterite crystals interlarding.

It is well known that calcite is the most thermody-namically stable, and vaterite is the least stable formin the three polymorphic forms of CaCO3.

1,22,23

Vaterite is the initial phase formed when CaCO3

supersaturated; calcite can be formed from the trans-formation of aragonite or vaterite in the absence ofinhibitors.24,25 Although calcite has the greatest ther-modynamic stabitily under ambient conditions, thethermodynamically less stable aragonite and/orvaterite phase may be stabilized under certain condi-tions of temperature or in the presence of other ionsor inhibitors.26–29 The calcium carbonate precipitateobtained from the artificial cooling water withoutthe low-phosphorus MAc-SS copolymer has XRDpatterns [Fig. 7(a)] corresponding to calcite. Figure7(b) shows the X-ray diffraction spectrum for CaCO3

precipitate in the presence of the low-phosphorusMAc-SS copolymer, in which there are the (004),(110), (112), (114), (300), and (118) weak peaks corre-sponding to vaterite, and a strong peaks for calcite.These results indicate that in the presence of thelow-phosphorus MAc-SS copolymer the calcium car-bonate precipitate is the mixture of calcite and vater-ite. It was found that thermodynamically unstablevaterite could be stabilized kinetically in the pres-ence of the low-phosphorus MAc-SS copolymer.

CONCLUSIONS

In this article, MAc-SS copolymer containing low-phosphorus was synthesized in aqueous solutionwith hypophosphorous acid/hydrogen peroxide asredox initiators to inhibit cooling water scale. Basedon the earlier discussions, we reach the followingconclusions:

1. The optimal reaction conditions to synthesizethe copolymer are 1.25 of MA/SS ratio, 0.18 ofthe ratio of H3PO2 to monomer, 80�C reactiontemperature, and 60 min of H2O2 drop time.

2. Phosphorus content in the copolymer is lowerthan 4% (mass percentage); the viscosity–aver-age molecular weight of the copolymerobtained under the optimal reaction conditionsis 1.86 � 10�5 g mol�1; the copolymer containscarboxyl group, sulfonic group, and terminalphosphonate group, and the main chain struc-

ture of the copolymer is the alternating units ofstyrene sulfonic acid and maleic acid.

3. The results of static scale inhibition test showedthat the MAc-SS copolymer is effective in in-hibiting calcium carbonate scaling. The MAc-SScopolymer sharply inhibited the formation ofcalcium carbonate scale with a lower dose inthe solution. The adsorption of the MAc-SS co-polymer on the crystal surface caused a defor-mation of the crystal morphology.

We would also like to thank Dr. Chu Libing for valuablediscussions.

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Low-phosphorus maleic acid and sodium ρ-styrenesulfonate copolymer as calcium carbonate scale...

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