z. amjad interactions of hardness ions with polymeric scale ......z. amjad interactions of hardness...

❙ Z. Amjad

Interactions of Hardness Ionswith Polymeric Scale Inhibitorsin Aqueous Systems

The interactions of hardness ions (i. e., Ca, Mg, Ba) with anionicpolymeric scale inhibitors (SI) containing different functionalgroups has been investigated using a turbidity technique. It hasbeen found that all SI tested form insoluble salts with hardnessions under the experimental conditions used (pH 7.0 to 10.0, 25to 65 °C, 100 to 1,000 mg/L Ca). The results indicate that com-patibility of hardness ions with SI decreases with increasing pH,temperature, and hardness ion concentrations and increaseswith increasing NaCl concentrations. It has also been found thatcompatibility of hardness ion with SI can be improved by incor-porating bulkier and hydrophobic monomer into SI structure. Inaddition, the data suggest that SI architecture (i. e., molecularweight, monomer functional group, and nature of polymeriza-tion solvents) influences the interactions of hardness ions withSI in aqueous systems.

Key words: Hardness ions, compatibility, polymers, scale inhibi-tor, precipitation

Wechselwirkungen zwischen härtebildenden Ionen undpolymeren Scaling-Inhibitoren in wäßrigen Systemen. DieWechselwirkungen zwischen härtebildenden Ionen (d. h., Ca,Mg, Ba) und anionischen polymeren Scaling-Inhibitoren (SI),die unterschiedliche funktionelle Gruppen besitzen, sind mittelseiner Trübungsmethode untersucht worden. Dabei wurde ge-funden, dass unter den experimentellen Bedingungen (pH 7.0bis 10.0, 25 bis 65 °C, 100 bis 1000 mg/l Ca) alle getesteten SImit den härtebildenden Ionen unlösliche Salze bilden. Die Er-gebnisse zeigen, dass die Kompatibilität zwischen den härtebil-denden Ionen und den SI abnimmt, wenn pH, Temperatur unddie Konzentration an härtebildenden Ionen erhöht wird, und zu-nimmt, bei Erhöhung der NaCl Konzentration. Weiterhin wurdegefunden, dass die Kompatibilität zwischen den härtebildendenIonen und den SI durch Einfügen von sterisch sperrigen und hy-drophoben Monomeren in die SI-Struktur verbessert werdenkann. Zusätzlich lässt sich aus den gewonnen Daten schließen,dass die SI-Konstitution (d. h., Molekulargewicht, funktionelleGruppen der Monomere und Art des Polymerisationslösungs-mittel) die Wechselwirkungen zwischen den härtebildenden Io-nen und den SI in wässrigen Systemen beeinflussen.

Stichwörter: Härtebildende Ionen, Kompatibilität, Polymere,Scaling-Inhibitoren, Ausfällung

1 Introduction

Formation of mineral scales such as calcium carbonate, cal-cium sulfate, magnesium hydroxide, and calcium phosphateon heat exchangers and reverse osmosis membranes is acommon problem in water treatment processes [1]. Mineralscale forms because of the composition and hardness of feedwater available for industrial applications. Other industrial

processes impacted by scale formation problems include oiland gas production, geothermal power generation, sugar re-fining, pulp and paper fabrication.

In the laundry application, insoluble calcium carbonatetends to accumulate on washed fabrics and washing equip-ment parts, resulting in undesirable fabric-encrustation orscaling [2]. The precipitation of scale forming salts is alsoof primary importance in biological systems. Dental calculusor tartar, consists primarily of salts of calcium, phosphate,and carbonate [3]. Calcium oxalates are the main constitu-ents of pathological deposits in the urinary tract. The medi-cal community is seeing a renewed interest in the regulationof these species [4].

The precipitation of anionic surfactants with multivalentmetal ions has been the subject of intensive research. Cal-cium salts of most of the anionic surfactants are insolublein water at room temperature and this makes soaps and de-tergents less effective in hard water. Two approaches havebeen generally applied to control the precipitation of Ca-sur-factant salt: (a) use of ethoxylated alkylsulfates, which signif-icantly decreases the Krafft points of calcium salts belowroom temperature [5] and (b) adding nonionic surfactantswith anionic surfactants where the former exert a markedKrafft point decreasing effect of the anionic surfactant. Theuse of mixed surfactant systems has been proposed in en-hanced oil recovery [6].

Researchers have proposed several options for control-ling scale formation including the use of acids, chelants, orthe addition of threshold inhibitors. The most promisingmethod is the addition of water soluble additives at verylow concentrations such as few parts per million (ppm). Ad-ditives commonly used for scale control applications arepolymers containing carboxylic acid group such as poly-(acrylic acid), poly(maleic acid), and acrylic acid and maleicacid-based copolymers. For carboxylic acid containing poly-mers, it appears that precipitation inhibition of scale form-ing salts (i. e., calcium carbonate, calcium fluoride, calciumphosphate, calcium sulfate, etc.) is dependent upon (a) poly-mer architecture (i. e., ionic charge, monomer size, mono-mer ratio, etc.) and (b) scaling salt being formed [7, 8].

In developing effective formulations industrial technolo-gist takes into consideration known factors that are commonto most water systems. These factors may include systemwater chemistry (e. g., pH, temperature, total dissolved so-lids), type and amount of pretreatment chemicals (i. e., floc-culant/coagulant) and compatibility of formulation additives(i. e., scale inhibitor, chelant, dispersant, anti-redepositionagent, surfactant, bleaching agent, color transfer agent, cor-rosion inhibitor, etc.) with each other as well as the compat-ibility of hardness ions with scale inhibitors, corrosion inhi-bitors, surfactants, and biocides.

The control of mineral scale, suspended matter, and cor-rosion in water treatment and laundry applications by var-ious additives has been well researched [1, 9]. However, the

APPLICATION

Tenside Surf. Det . 42 (2005) 2 Carl Hanser Publisher, Munich 71

interactions of various treatment additives with hardnessions have been mostly overlooked. One of the major con-cerns of the industrial technologist is compatibility or toler-ance limit of the anionic polymeric scale inhibitor (hereafterSI) with hardness ions (i. e., calcium, magnesium, barium,hereafter M). The tolerance or compatibility is defined asthe maximum amount of the SI that can be added to thewater system without significant precipitation of M-SI salt.If the compatibility is poor and the dosage of the formula-tion used is higher than the recommended level, not onlydoes M-SI precipitation occurs, but also the overall scale for-mation becomes faster and could lead to many troubles(e. g., early shutdown, wasted time, reduced production,and low profitability) [10, 11].

In previous contributions from our laboratories, we pre-sented results on the interaction of various impurities (i. e.,coagulating and flocculating agents, biocides, and multivalantmetal ions such as iron, manganese, copper, and zinc) pre-sent in aqueous system with a variety of polymeric scale inhi-bitors. It was shown that the performance of scale inhibitor ismarkedly influenced by the presence of various impuritiespresent in water [12 –14]. The present investigation is con-cerned with the interaction of hardness ions with anionicscale inhibitors. The polymers tested include: (a) homopo-lymers i. e., polyacrylic acid, P-AA; polymaleic acid, P-MA;(b) co-polymers i. e., acrylic acid : 2-acrylamido 2-methylpro-pane sulfonic acid, P-A : SA; acrylic acid:maleic acid, P-MA : A, and c) terpolymers i. e., acrylic acid : 2-acrylamido 2-methylpropane sulfonic acid: sulfonated styrene, P-A : SA : S;acrylic acid:methacrylic acid:t-butylacrylamide, P-A : MAA :sAM. In addition, poly(acrylic acids), P-AAs, made in differ-ent polymerization solvents (i. e., organic solvent and water)were also evaluated for their compatibility with calcium ions.It is hoped that the data presented in this paper will enablethe industrial technologists to recognize the impact of waterchemistry, system operating conditions, and interactionsof scale inhibitors with biocides, coagulating/flocculatingagents, and hardness ions in selecting the appropriate addi-tives for their formulations.

2 Experimental

The chemicals used to prepare the solutions were FisherScientific ACS certified grade. Stock solutions of knownconcentrations of calcium chloride, magnesium chloride,barium chloride, hydrochloric acid, sodium hydroxide, andvarious polymeric scale inhibitors were prepared and usedto make up test solutions. The inhibitors were selected fromcommercial materials. All inhibitor solutions were preparedon dry weight basis. The desired concentrations were ob-tained by dilution, using double deionized and distilledwater. A Metrohm-Brinkman pH-stat unit equipped with acombination electrode was used to maintain the experimen-tal solution pH. In order to avoid erroneous pH readingsdue to possible adsorption of polymer on electrode surface,the pH electrode was calibrated before each experiment withstandard buffers.

Metal ion tolerance towards SI was measured by moni-toring turbidity. The test set-up used a constant water bath,double walled-glass reaction cell, and a Brinkmann PC 802colorimeter with 420 nm filter (for transmittance measure-ments).

The M-SI compatibility experiments were performed in aglass bottle (125 mL capacity) placed in double walled glasscell maintained at the required temperature. The test solu-tions were prepared by adding a known volume of stock so-lution of SI to a known volume of water in the glass bottle.

After allowing the SI solution to equilibrate at required tem-perature for at least 30 minutes, the pH was adjusted to re-quired value using dilute HCl and/or dilute NaOH. AfterpH adjustment, a known volume of a metal ions stock solu-tion was added to the SI solution. The solution pH wasquickly re-adjusted to required value with dilute HCl and/or NaOH solutions. The bottles were capped and were con-tinuously stirred with stirring bars. At known time intervals(typically 40 minutes) transmittance readings were takenusing the fiber optic probe. Duplicate/triplicate experimentswere run to check the reproducibility of the compatibilitydata. In order to avoid faulty signal, extreme care was takento eliminate air bubbles in the solutions, especially in thevicinity of fiber optic probe. Table 1 shows the structures ofSI tested.

3 Results and Discussion

In domestic and industrial applications (i. e., laundry, clea-ners, water treatment, desalination, oil field, etc.) polymersare used for a variety of reasons but most importantly theyinhibit the formation of scale forming salts and disperse par-ticulate matter. Polymers prevent scale formation by adsorb-ing onto crystal growth sites of micro-crystallites therebyinterfering with crystal growth and altering the crystals mor-phology. The precipitation of M-SI salt can (a) cause foulingof various substrates (i. e., heat exchanger, reverse osmosismembrane, fabric) and (b) decrease the polymer concentra-tion in the system to the extent that severe deposition ofscale forming salts can occur. The deposition of M-SI saltsand mineral scales are known to impede heat transfer andtransport characteristics of the reverse osmosis membrane[1, 15, 16].

3.1 Tolerance of homopolymers with calcium ions

Figure 1 illustrates the typical “%transmittance” (% T) as afunction of P-Ao5 (polyacrylic acid, organic solvent polymer-ized, MW 5,000) concentration. The inflection point in thetransmittance-inhibitor profile was used to calculate thepoint of onset of turbidity. Figure 1 illustrates compatibilitydata obtained for Ca–P-Ao5 system (1,000 mg/L Ca, pH9.00, 25 °C) and shows good reproducibility (± 7 %). Thecompatibility value calculated for P-Ao5 is 13 ± 1 ppm per1,000 mg/L Ca.

The influence of polymerization solvent was investigatedby conducting a series of experiments with calcium ion(1,000 mg/L), pH 9.00, 25 °C in the presence of poly(acrylicacids), P-AAs, made in water and organic solvent. As shownin Figure 2 all polymers evaluated form insoluble salts withcalcium ion and the compatibility of these polymers de-pends on the type of polymerization solvent. In general, sol-vent polymerized P-AAs exhibit better tolerance to calciumion than do water polymerized P-AAs of similar molecularweight. For example, under the experimental conditions em-ployed (1,000 mg/L Ca, pH 9.00, 25 °C), calcium tolerancevalues obtained for P-Ao2 (solvent-polymerized P-AA, MW2,000) is greater than 800 ppm compared to 60 ppm ob-tained for P-Aw2 (water-polymerized P-AA, MW 2,000). A si-milar trend in increased calcium ion tolerance was also ob-served for P-Ao5 (solvent polymerized P-AA, MW 5,000) andP-Aw5 (water polymerized P-AA, MW 5,000). As demon-strated in Figure 2, the polymerization solvent plays animportant role in improving calcium ion tolerance to poly-(acrylic acids).

Figure 2 presents compatibility data for P-MA (polyma-leic acid, organic solvent polymerized, MW 1,000) collected

Z. Amjad: Interactions of hardness ions with polymeric scale inhibitors in aqueous systems

72 Tenside Surf. Det . 42 (2005) 2


Tenside Surf. Det . 42 (2005) 2 73

Inhibitor/Acronym Structure

poly(acrylic acid)MW 1,500

PAo1.5


P-Ao2


P-Ao5


P-AW2


P-AW5


P-AW60


P-AW240

poly(maleic acid)MW 900

P-MA

poly(acrylic acid : maleic acid)MW 10,000

P-MA : A*

poly(acrylic acid: 2-acrylamido2-methylpropane sulfonic acid)

MW < 15,000P-A : SA

poly(acrylic acid: 2-acrylamido2-methylpropane sulfonic acid:

sulfonated styrene)MW < 15,000

P-A : SA : S

poly(acrylic acid:methacrylic acid:t-butylacrylamide)P-A : MAA : sAmMW < 15,000

a polydispersity: 1.9 to 2.1; residual monomers: <0.1 %* experimental

Table 1 Structures of scale inhibitors testeda

in the presence of 1,000 mg/L Ca, pH 9.00, 25 °C. It is evi-dent from Figure 2 that under similar experimental con-ditions, P-MA exhibits a calcium ion tolerance greater thanP-Ao5 but less than the P-Ao2. The observed calcium iontolerance for P-MA versus the P-AA may be attributable toseveral factors including polymer architecture (i. e., mole-cular weight, number of functional groups i. e., -COOH,-SO3H, -CONH2 etc.), end groups, branching, polymeriza-tion solvent, and residual monomer concentration.

3.1.1 Polymer Molecular Weight

In recent years, many studies have been undertaken concern-ing the influence of molecular weight on the precipitation ofscale forming salts. Studies of polymers as scale inhibitorshave shown that polymer performance in industrial watertreatment is strongly affected by polymer molecular weight.For carboxylic acid containing polymers, it appears that preci-pitation inhibition is greatest for molecular weights of below20,000 with the optimum molecular weight being dependentupon on the particular polymer composition and the saltbeing formed [8, 12].

In order to observe the molecular weight effects forpoly(acrylic acids), P-AAs, made in different polymerizationmedia (solvent and water), two sets of experimental condi-tions were employed as follows:

❙ High stress (500 mg/L Ca and 45 °C, pH 9.00) for organ-ic solvent P-AAs, and

❙ Low stress (100 mg/L Ca, 25 °C, pH 9.00) for water poly-merized P-AAs

Figure 3 shows the calcium ion tolerance for P-AAs withmolecular weight ranging from 1,500 to 240,000. It is evi-

dent that same calcium ion tolerance trend (i. e., increasingcalcium ion tolerance with decreasing P-AA molecularweight) applies for both types (solvent and water polymer-ized) of P-AAs. For example, calcium ion tolerance valuesobtained for solvent polymerized P-AAs are 195 for P-Ao2(MW 2,000) and 12 ppm for P-Ao5 (MW 5,000) comparedto > 1,200 ppm for P-Ao1.5 (MW 1,500). As illustrated inFigure 3, similar molecular weight dependence is observedfor P-AAs made in water medium. It is interesting to note,that a similar molecular weight trend was observed in stu-dies involving the interaction of anionic polymers (i. e., poly-acrylic acid and acrylic acid based copolymers) with cationicpolymers (i. e., quaternary ammonium chloride) [17].

3.1.2 Tolerance of co- and ter-polymers with calcium ions

In order to study the effect of monomers containing differentfunctional groups (i. e., anionic, nonionic) a series of calciumion tolerance experiments were carried out under similar ex-perimental conditions (i. e., 1,000 mg/L Ca, pH 9.00, 25 °C).It is evident from the data presented in Figure 4 that calciumion tolerance strongly depends upon polymer composition.For example, replacing a portion of AA in P-Aw5 with SA (2-acrylamido 2-methylpropane sulfonic acid) results copolymer(P-A : SA) with an order of magnitude increase in calciumion tolerance values. For example, calcium ion tolerance valueobtained for P-Aw5 is 6 ppm compared to 74 ppm obtainedfor P-A : SA. As illustrated in Figure 4, replacing portion ofMA (maleic acid in P-MA) with AA (acrylic acid) decreasesthe calcium ion compatibility of the P-MA : A. Therefore, itappears that both the monomer’s charge and size play rolesin a polymer’s calcium ion tolerance.

The calcium ion tolerance of two ter-polymers (contain-ing 3 different monomers of varying size and ionic charge)was investigated. Figure 4 shows results of these experi-ments and the data indicate that the two ter-polymers (i. e.,P-A : SA : S, P-A : MAA : s-AM) tested are extremely compa-tible (> 1,000 ppm polymer/1,000 mg/L Ca) with calciumions. These data suggest that the incorporation of large andgreater amounts of non-ionic and hydrophobic groups in-crease the calcium ion tolerance of ter-polymers. It appearsfrom the compatibility data of AA-based co- and ter-poly-mers that the length of the second or third monomer sidechain causes increased calcium ion compatibility. We can ra-tionalize that carboxyl groups in P-AA and P-MA mighteasily bind calcium ion due to their close proximity. On theother hand, in the case of co- and ter-polymers i. e., P-A : SA



Figure 1 Calcium ion interactions with scale inhibitor. Plots of % transmit-tance versus P-Ao5 (polyacrylic acid, MW 5,000, organic solvent polymerized)concentrations at pH 9.00, 25 °C, and 1,000 mg/L Ca

Figure 2 Calcium ion tolerance of various homopolymers at pH 9.00, 25 °C,and 1,000 mg/L Ca

Figure 3 Calcium ion tolerance of poly (acrylic acids) of varying molecularweight made in different polymerization solvents. Experimental conditions:poly(acrylic acids), solvent polymerized: (500 mg/L Ca, pH 9.00, and 45 °C),poly(acrylic acids), water polymerized: (100 mg/L Ca, pH 9.00, and 25 °C)

and P-A : SA : S, the sulfonate groups are away from thepolymer backbone, thus suppressing or preventing the pre-cipitation of calcium-polymer salt. The observed increasedin calcium ion compatibility of P-A : MAA : s-AM may be at-tributed to the decreased charge density of the ter-polymerdue to the presence of uncharged substituted tertiary butylacrylamide.

3.1.3 Effect of solution temperature

It is well documented that solubility of scale forming saltssuch as calcium carbonate, calcium sulfate, and calciumphosphates are inversely dependent on solution temperature[1]. This solubility-temperature relationship suggests thatthe scaling tendency will be higher at the heat exchangersurfaces than in other parts of the re-circulating water sys-tem. Figure 5 presents the compatibility data for our investi-gations to determine the influence of solution temperature(25 to 65 °C) in the presence of 500 mg/L Ca, pH 9.00 fortwo polymers (i. e., P-Ao5 and P-A : SA). The data clearly in-dicate that solution temperature has a pronounced effect onthe compatibility of polymers with calcium ions. As noted inFigure 5, increasing the temperature from 25 to 45 °C re-sults in a ~2.0 to 3.5 fold decrease in the calcium ion toler-ance of P-Ao5 and P-A : SA, respectively. It should be notedthat further increase in temperature from 45 to 65°C resultsin ~1.5 to 2.0 fold decrease in calcium ion tolerance for bothpolymers. The observed decrease in calcium ion compatibil-ity of P-Ao5 and P-A : SA (Figure 5) with increasing tem-perature may be attributed to several factors includingincreased ionization of functional group (i. e., -COOH,SO3H), uncoiling of polymers, and increased mobility ofpolymer and calcium ions.

3.1.4 Effect of solution pH

It is generally accepted that increasing the re-circulatingwater pH has a two-fold effect on system performance: (a) ittends to decrease the rates of metal corrosion, and (b) it in-creases the scaling tendency by increasing the supersatura-tion of scale forming salts [1]. It is also well recognized forpolymeric inhibitors such as poly(acrylic acid) and acrylicacid-based co-polymers that the degree of de-protonation ex-plains the observed improvement in polymer performanceas the solution pH increases from 4.5 to 9.00 [18, 19].

Figure 6 shows the calcium ion compatibility data fortwo structurally different but commonly used inhibitorsnamely: P-Ao5 and P-A : SA. As illustrated solution pH inthe 7.0 to 10.0 range influences to varying degrees the com-patibility of these inhibitors with calcium ions. For boththese polymers compatibility values decrease by a factor of~1.5 as the pH is increased from 7.0 to 10.0.

3.1.5 Effect of ionic strength

In addition to the effect of solution temperature and pH, itis also worthwhile to examine the influence of ionic strengthor total dissolved solids (TDS) on the interaction of calciumions with polymeric scale inhibitors. Figure 7 compares thecompatibility values of P-Ao5 and P-A : SA polymers col-lected at pH 9.00, 25 °C, 500 mg/L Ca and in the presenceof varying (0 to 5,000 mg/L) concentration of NaCl. The re-sults presented in Figure 7 indicate that a polymer’s calciumion tolerance is strongly dependent upon the type of func-tional group and system TDS. It is worth noting for bothpolymers that increasing the NaCl concentrations from 0 to5,000 mg/L results in about 12 fold increase in calcium iontolerance. For example, compatibility values obtained for P-



Figure 4 Calcium ion tolerance of various homo-co-, and ter-polymers at pH9.00, 25 °C, and 1,000 mg/L Ca

Figure 5 Effect of solution temperature on Ca–SI precipitation for P-Ao5 andP-A : SA at pH 9.00, and 500 mg/L Ca

Figure 6 Effect of solution pH on Ca–SI precipitation for P-Ao5 and P-A : SAat 25 °C, and 500 mg/L Ca

Figure 7 Effect of sodium chloride concentration on Ca–SI precipitation forP-Ao5 and P-A : SA at pH 9.00, 500 mg/L Ca, and 25 °C

Ao5 in the presence of 1,000, 2,500, and 5,000 mg/L NaClare 35, 52, and 200 ppm compared to 19 ppm obtained inthe absence of NaCl. The observed increase in Ca ion toler-ance with increasing NaCl concentration suggests that Ca-polymer salts are more soluble in high TDS water. Thismay be attributed to the stabilization of the anions throughshielding of the charge-charge interactions by the sodiumcounter ions and the increase in random coil nature of thepolymer chains in salt water.

3.1.6 Effect of calcium ion concentration

Figure 8 demonstrates the effect of Ca ion in the rangeof 250 to 1,000 mg/L on the compatibility of P-Ao5 andP-A : SA. It is evident that compatibility of these polymerswith Ca ion decreases as the Ca ion concentration is in-creased. For example, in the presence of 500 mg/L Ca ion,pH 9.00, and 25 °C the compatibility values obtained forP-Ao5 and P-A:SA are 19 and 195 ppm respectively com-pared to 15 and 75 ppm obtained in the presence of1,000 mg/L Ca ion. Thus, calcium ion compatibility de-creases by a factor of two to three times for P-A : SA withtwo fold increase in Ca ion concentrations (from 500 to1,000 mg/L). It should be noted that under similar experi-mental conditions calcium ion tolerance value for P-Ao5decreases by a factor about 1.3 times as the calcium ion con-centration is increased from 500 to 1,000 mg/L. It is inter-esting to note under similar experimental conditions and atlow Ca ion concentration (i. e., 250 mg/L), that P-A : SA issignificantly more tolerant to calcium than P-Ao5.

3.1.7 Effect of divalent metal ions

The compatibility of divalent metal ions (e. g., Mg, Ca, andBa) with polymeric inhibitors was investigated by carrying

out a series of experiments under similar experimental con-ditions (i. e., pH 9.00, 25 °C, divalent metal ions 250 mg/L).Figure 9 shows the results for two 5,000 molecular weightpoly(acrylic acids) i. e., P-Ao5 and P-Aw5, made in differentpolymerization media (organic solvent and water, respec-tively). It is evident from Figure 9 that both polymers forminsoluble salts with divalent metal ions and based on thecompatibility data, the metal-polymer salt may be ranked(in terms of the decreasing solubility or tolerance) as fol-lows: Mg >> Ca >> Ba. The observed order for polymer toler-ance with various divalent metal ions is consistent with me-tal reactivity and metallic character.

3.2 Metal-Poly(acrylate) Salt Characterization

Metal-polymer salts appear to be of enduring interest tomacromolecular, colloid, and industrial technologists. From awater treatment and laundry perspective, understanding ofthe interactions between metal ions and common formulationadditives is especially important for aqueous systems operat-ing under stressed conditions. As discussed above, under theexperimental conditions employed metal ions (i. e., Ca, Mg,Ba) are incompatible with commonly used polymeric scale in-hibitors and this leads to the formation of insoluble salts thatcan potentially foul the equipment surfaces.

The metal-polymer salt formed during the compatibilityexperiments was characterized using a Nicolet Magna 560Fourier Transform Infra-red (FT-IR) spectrophotometerequipped with a fixed-angle horizontal – stage AttenuatedTotal Reflectance (ATR) accessory. The hot-air dried samplesof P-Aw5, sodium-P-Aw5 salt, and P-Aw5-barium salt (fil-tered through 0.22 micron and dried) were characterizedusing an FT-IR spectrophotometer according to the proce-dure described previously [20]. Figure 10 presents IR spectrafor P-Aw5 and the P-Aw5-metal ion salts. The band near1699 cm1 represents the acid carboxylate of acrylic acid inP-Aw5. The bands near 1543 and 1533 cm1 represent thecarboxylate salt carbonyl bands for the sodium and bariumsalts, respectively. It can be seen that there is significantshift in band for barium salt (1533 cm–1) versus 1543 cm–1

for the sodium salt of P-Aw5. This confirms that bariumforms an insoluble salt with P-Aw5.

4 Summary

This study has shown that:

❙ The calcium ion tolerance of SI such as poly(acrylic acid)strongly depends upon polymerization solvent and poly-mer molecular weight.



Figure 8 Effect of calcium concentration on Ca–SI precipitation at pH 9.00and 25 °C

Figure 9 Divalent metal ion-SI precipitation for P-Ao5 and P-Aw5 at pH 9.00and 25 °C

Figure 10 FT-IR spectra of P-Aw5 (curve A), P-Aw5-Na salt (curve B), andP-Aw5-Ba salt (curve C)

y System pH, temperature, and ionic strength significantlyimpact the interaction of SI with hardness ions.

y Charge, size, and the amount of co-monomers presentin SI affect the interactions that occur between SI andhardness ions.

y SI tolerance toward metal ion strongly depends uponmetal ion concentration and the character of the metalion.

y Terpolymers compared to homopolymers and copoly-mers are more tolerant to calcium ions.

y Further understanding of role of residual monomer andresidual solvent concentrations on M-SI precipitation isneeded on these polymers. Ongoing work includes metalion compatibility studies using poly(acrylamide), poly-(vinylsulfonic acid), poly(sulfonated styrene), poly(2-acryl-amido 2-methyl propane sulfonic acid), and potentio-metric titrations using calcium-sensitive electrodes tocompare the ion-binding ability in different microstruc-tural environments.

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International, Houston, TX (1999).15. Butt, F. H., Rahman, F. and Baduruthamal, U.: Desalination 101 (1995) 219.16. Amjad, Z., Workman, K. R. and Castete, D. R.: Chapter No. 7, Reverse Osmosis:

Membrane Technology, Water Chemistry, and Industrial Applications, Amjad, Z.(Ed.) Van Nostrand Reinhold, New York (1992).

17. Amjad. Z., Zuhl, R. W. and Zibrida, H. F.: Paper No. 23, Association of WaterTechnologies 2000 Annual Convention, Honolulu, Hawaii (2000).

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Received: 03.11. 2004Revised: 06. 01. 2005

y Correspondence to

Zahid AmjadPerformance Coatings GroupNoveon, Inc., 9911 Brecksville RoadBrecksville, OH 44141, USA

The author of this paper

Zahid Amjad, received his M.Sc. in Chemistry from Punjab University, Lahore, Paki-stan, and his Ph.D. in Chemistry from Glasgow University, Scotland. He is currently aResearch Fellow in the Performance Coatings Group of the Noveon, Inc. His areasof research include water soluble/swellable polymers, adsorption of polymers atsolid-liquid interface, and prevention of scaling in industrial water systems.

Z . Amjad: Interact ions of hardness ions with polymeric scale inhibitors in aqueous systems


z. amjad interactions of hardness ions with polymeric scale ......z. amjad interactions of hardness...

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