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Synthesis of a polyaminophosphonate and its evaluation as an antiscalant in desalination plant.

INTRODUCTION

Butler's cyclopolymerization protocol has led to the synthesis of a variety of cationic and anionic polyelectrolytes (1) as well as polybetaines (having charges of both algebraic signs in the same repeating units) (2-6) and polyampholytes (having repeating units containing cationic and anionic charges with or without charge symmetry) (7). The polymer architecture, having the five-membered cyclic units embedded in the backbone, has been recognized as the eighth major structural type of synthetic polymers (8), (9).

These cyclopolymers are of significant scientific and technological interest; poly(diallyldimethylammonium chloride) itself has more than 1000 patents and publications; more than 33 million pounds of the polymer are sold annually for water treatment and another 2 million pounds are used for personal care formulation (1). The technical applications based on coulombic attraction between the positively charged quaternary ammonium polyelectrolytes and negatively charged macro-ions, surfactants, etc, have resulted in different materials including membranes, modified surfaces, and coated particles (5), (10), (11).

The unquenched valency of nitrogen in diallylamine salts containing sulfonate and carboxylate, and phosphonate functionalities as pendents led to the synthesis of a variety of p1-1-responsive poly sulfo- (12), carbo- (13), and phosphono-betaines [14, 151 [i.e., polyzwitterions (PZs)] (2), (3), (5). The intragroup, intra-and interchain electrostatic dipole--dipole attractions among the dipolar groups in PZs lead to collapsed or globular conformations which undergo globule-to-coil transitions (antipolyelectro-lyte effect) in the salt-added (e.g., NaC1) solutions owing to the neutralization of the ionically cross-linked network in the collapsed conformation (2), (3), (16-18). The high dipole moment of a PZ makes it an excellent polar host matrix in which only target ions can migrate (19), (20). The pH-responsive PZs, whose structure and behavior seem to mimic biopolymers, have been utilized in various medical fields (21-24), for efficient separations of biomolecules (3) and to develop procedures for DNA assay (25). They have also drawn attention in the field of ion exchange; their abilities to chelate toxic trace metals (Hg, Cd, Cu, and Ni) have been exploited in wastewater treatment (3), (24).

The polyphosphobetaines (containing pendent with a phosphate group) seem to mimic phospholipid biomembranes and have offered many new applications (2), (26). Recently, Butler's cyclopolymerization protocol has been applied in the syntheses of some polyphosphono-betaines (having phosphonate group in the pendent) (14), (15), (27-31). Extraordinary chelating properties of aminomethyl-phosphonic acid groups have been exploited to form polymer-heavy metal ion complexes from waste water (32-34).

In pursuit of tailoring pH-responsive polymers, we describe herein the synthesis and cyclopolymerization of a new cationic phosphonium monomer 2 (Scheme 1) to cationic polyelectrolyte (CPE) 3 which was converted to pH-responsive polyzwitterionic acid (PZA) 4, dianionic polyelectrolyte (DAPE) 6, zwitterionic/anionic polyelectrolyte (ZAPE) 7. The polymerization of zwitterionic monomer 8 to PZ 9 (14) and its conversion to anionic polyelectrolyte (APE) 10, PZA 4, DAPE 6, and ZAPE 7 would allow us to correlate solution properties with the charge types and densities on the polymer backbone. Antiscalants are chemical substances that reduce or prevent the formation of scales or precipitation which is one of the major problems in operating desalination plants (35). One important objective of this work is to investigate the antiscalant properties of the synthesized polymer DAPE 6. The pH-responsive polyaminophosphonate having the phosphonate and amine groups separated by a three-carbon spacer would thus be prepared for the first time using cyclopolymerization protocol and used as a novel antiscalant in inhibiting the formation of calcium sulfate scale. The work would thus pave the way to synthesize and utilize many such polymers having different length of the spacer separating the amine and phophonate functionalities.

EXPERIMENTAL

Physical Methods

Elemental analysis was carried out on a Perkin Elmer Elemental Analyzer Series II Model 2400. IR spectra have been recorded on a Perkin Elmer 16F PC FTIR spectrometer. The [.sup.31]P, [.sup.13]C, and [.sup.1]H NMR spectra of the polymers have been measured in CDC[l.sub.3] (using TMS as internal standard) or [D.sub.2]0 (using HOD signal at [delta]4.65 ppm and dioxane [.sup.13]C peak at [delta]67.4 ppm as internal standards) on a JEOL LA 500 MHz spectrometer. [.sup.31]P was referenced with 85% [H.sub.3]P[O.sub.4] in DMSO. Viscosity measurements were made by Ubbelohde viscometer (having Viscometer Constant of 0.005718 cSt/s at all temperatures) using C[O.sub.2]-free water under [N.sub.2] to avoid C[O.sub.2], absorption that may affect the viscosity data. Molecular weights of some synthesized polymers were determined by GPC analysis using Viscotek GPCmax VE 2001. The system was calibrated with nine polyethylene oxide monodispersed standards at 30[degrees]C using two Viscotek columns G5000 and G6000 in series. The Atomic Absorption Spectrometer AAnalyist-100 PerkinElmer was used to determine the concentration of metal ions in evaluating the effectiveness of the antiscalant.

For the determination of molecular weights, the polymer DAPE 6 derived from CPE 3 and PZ 9 were analyzed using an aqueous solution of 0.1 N NaNO3 as the eluant. Refractive Index and viscometer detectors were used to determine molar mass of the polymers.

Materials

Ammonium persulfate (APS), t-butylhydroperoxide (TBHP) (70% aqueous solution), anhydrous CaC[l.sub.2], N[a.sub.2]S[O.sub.4], 1,3-dibromopropane, triethyl phosphite, and diallyl amine from Fluka Chemie AG (Buchs, Switzerland) were used as received. For dialysis, a Spectra/Por membrane with a molecular weight cut-off (MWCO) value of 6000-8000 was purchased from Spectrum Laboratories. All glassware was cleaned with deionized water. N,N-Dially1-3-(diethylphosphonato)propylamine (1) was prepared as described (14) by reacting diallylamine and 1-bromo-3-(diethyl phosphonato)propane.

Monomer and Polymer Synthesis

N,N-Diallyl-3-(Diethylphosphonato)propylammonium Chloride (2). To an ice-cooled stirred solution of 1 (41.3 g, 0.15 mol) in diethyl ether (300 c[m.sup.3]), moisture-free HCI was bubbled until the supernatant ether layer became clear and further production of milky solution seized. The separated chloride salt was washed several times with ether to obtain 2 as a viscous liquid which was dried under vacuum at 40[degrees]C to a constant weight (45.2 g, 97%). The viscous liquid was solidified when kept inside a freezer. An analytical sample was prepared by crystallizing 2 from acetone/ether/methanol mixture. Mp 57-59[degrees]C (closed capillary); the highly hygroscopic salt gave an approximate elemental analysis: (Found: C, 49.6; H, 8.97; N, 4.3. [C.sub.13][H.sub.27]CIN[O.sub.3]P requires C, 50.08; H, 8.73; N, 4.49%); [v.sub.max] (KBr) 3435, 2985, 2930, 2630, 2639, 1646, 1457, 1218, 1163, 1025, 963, and 768 c[m.sup.-1]; [[delta].sub.H] ([D.sub.2]0) 1.21 (6 H, t, J 7.0 Hz), 1.89 (4H, m), 3.14 (2 H, m), 3.69 (4 H, m), 4.03 (4 H, m), 5.51 (4 H, m), 5.82 (2 H, m); [[delta].sub.C] (125 MHz, [D.sub.2]O): 16.49 (s, 2C, Me), 17.74 (s, 1C, PC[H.sub.2]C[H.sub.2]), 21.84 (d, 1C, d, PC[H.sub.2], [.sup.1]J(PC) 140 Hz), 52.30 (1C, d, PC[H.sub.2]C[H.sub.2]C[H.sub.2], [.sup.3]J(PC) 17.5 Hz), 55.80 (s, 2C, =CH--C[H.sub.2]), 64.37 (d, 2C, OC[H.sub.2]C[H.sub.3], [.sup.2]J (PC) 7.2 Hz), 126.22 (s, 2C, =CH), 127.61 (s, 2C, C[H.sub.2]=); bp (202 MHz, [D.sub.2]0): 31.40 (m, IP). [.sup.13]C spectral assignments were supported by DEPT 135 NMR analysis.

TABLE 1. CyclopolymerizaLion of CaLionic Monomer 2 (a) to CPE 3.

Entry  Initiator    Temperature  Time  Yield      Intrinsic
       (mg)        ([degrees]C)   (h)    (%)  Viscosity (b)
                                                        (dL
                                                [g.sup.-1])

1      APS (400)             85    48     49         0.0934

2      TBHP (270)            85    48     61          0.101

(a.) Polymerization reactions were carried out in aqueous solution
of monomer (20 mmol) (70 w/w% monomer) in the presence of ammonium
persulfate (APS) or tertiary butyihydroperoxide (TBHP).

(b.) Viscosity of 1-0.25% polymer solution in 0.1 N NaC1 at 30
[+ or -] 0.1[degrees]C was measured with an Ubbelohde Viscometer
(K = 0.005718). The average of three readings (for the time of how)
having standard deviation within 0.3 s are taken for the viscosity
plots.


General Procedure for the Polymerization of Cationic Monomer 2

To a solution of monomer 2 in deionized water in a 10 c[m.sup.3] round bottomed flask purged with [N.sub.2] was added the required amount of the initiator (as listed in Table 1). The mixture was stirred in the closed flask at the specified temperature for a specified time. The transparent reaction mixture was dialyzed against deionized water for 24 h. The polymer solution of CPE polydiallyl(diethylphospho-nato)propylammonium chloride (3) was then freeze-dried. The onset of thermal decomposition (closed capillary): 300-310[degrees]C (decomposed, turned black); (Found: C, 49.8; H, 8.8; N, 4.4. [C.sub.13][H.sub.27]ClN[O.sub.3]P requires C, 50.08; H, 8.73; N, 4.49%); [v.sub.max] (KBr) 3400 (very broad almost engulfing the CH stretching vibrations), 2989, 2945, 1652, 1461, 1398, 1219, 1127, 1025, 968, 794, 705, 619, and 542 c[m.sup.-1]; [[delta].sub.P] (202 MHz, [D.sub.2]0): 33.84 (s, major) and 26.09 (s, minor) in a 75:25 ratio.

Acidic Hydrolysis of CPE 3 to PZA 4. A solution of polymer 3 (122 g, 10.3 mmol) (entry 2, Table 1) in HCI (25 c[m.sup.3]) and water (20 c[m.sup.3]) was hydrolyzed in a closed vessel at 90[degrees]C for 48 h. The homogeneous mixture was cooled to room temperature and dialyzed against deionized water for 48 h. Some polymer settled down on the bottom of the dialysis bag. The whole mixture was then freeze-dried to obtain poly [3-(N,N-diallylammonio)propanephosphonic acid] 4 as a creamy white powder (2.12 g, 94%). The onset of thermal decomposition (closed capillary): 310-320[degrees]C (dec, turned light brown). (Found: C, 49.0; H, 8,4; N, 6.2%. [C.sub.9][H.sub.18]N[O.sub.3]P requires C, 49.31; H, 8.28; N, 6.39%); [v.sub.max] (KBr) 3450, 2952, 2730, 1653, 1463, 1144, 1053, 907, 768, 709, and 543 c[m.sup.-1].

Basification of PZA 4 to Dianionic Polyelectrolyte (DAPE) 6. A mixture of PZA 4 (derived from hydrolysis of CPE 3 from entry 2, Table 1) (0.876 g, 4.0 mmol) in water (5 c[m.sup.3]) was neutralized with 1.0 N NaOH (8.0 c[m.sup.3], 8.0 mmol). After the mixture became homogeneous, the solution was freeze-dried to obtain poly[disodium 3-(N,N-diallylamino)propanephosphonate] 6 as a creamy white powder (1.02 g, 97%). The onset of thermal decomposition (closed capillary): above 300[degrees]C (turned brown), did not melt even at 370[degrees]C; (Found: C, 40.8; H, 6.4; N, 5.1. [C.sub.9][H.sub.16]NN[a.sub.2][0.sub.3]P requires C, 41.07; H, 6.13; N, 5.32%); [v.sub.max] (KBr) 3475 (very broad engulfing the CH stretching vibrations), 2920, 2820, 1660, 1451, 1390, 1306, 1222, 1060, 973, 783, and 613 c[m.sup.-1]. [[delta].sub.P] (202 MHz, [D.sub.2]0) 22.60 (s, 1P).

Basification of PZA 4 to Zwitterionic Anionic Polyelectrolyte (ZAPE) 7. A mixture of PZA 4 (derived from hydrolysis of CPE 3 from entry 2, Table 1) (0.876 g, 4.0 mmol) in water (8 c[m.sup.3]) was neutralized with 1.0 N NaOH (4.0 c[m.sup.3], 4.0 mmol). After the mixture became homogeneous, the solution was freeze-dried to obtain poly[-sodium 3-(N,N-diallylammonio)propanephosphonate] (7) as a creamy white powder (0.894 g, 93%). The onset of thermal decomposition (closed capillary): Neither change in color nor melted up to 330[degrees]C; (Found: C, 44.5; H, 7.35; N, 5.6. [C.sub.9][H.sub.17]NNa[O.sub.3]P requires C, 44.82; H, 7.10; N, 5.81%); [v.sub.max] (KBr) 3790, 2962, 2929, 2852, 1458, 1413, 1381, 1060, 974, 715, and 559 c[m.sup.-1]; [[delta].sub.P] (202 MHz, [D.sub.2]0: 20.81 (s, 1P).

Synthesis and Polymerization of Zwitterionic Monomer 8 and Conversion of the Resultant PZ 9 to PZA 4, DAPE 6, ZAPE 7 and Anionic Polyelectrolyte (APE) 10. Zwttreionic monomer 8 was synthesised and polymerized to obtain PZ 9 using procedure as described (14). A solution of monomer 8 (6.06 g, 24.3 rnmol) in 0.5 N NaC1 (2.02 g) was polymerized using initiator TBHP (125 mg) at 85[degrees]C for 48 h. The reaction mixture was dialyzed against deionized water for 48 h to remove the unreacted monomer and NaCl. The polymer solution was then freeze-dried to obtain PZ 9 in 59% yield. The intrinsic viscosity [eta] at 30[degrees]C was determined to be 0.0988 dLig in salt-free water (Table 2). PZ 9 was hydrolyzed to obtain PZA 4 (85%) which was then converted to DAPE 6, ZAPE 7 using procedures as described above. Simlarly, PZ 9 was converted to APE 10 using 1 equivalent NaOH (14).

TABLE 2. EtTect of NaCI on the intrinsic viscosity ([Z] (a) and
Huggins constant (K) of PZE 8 (Entry 2, Table 1).

Polymer  NaCI   [eta]  Correlation    Slope      k  [M.sub.W]
          (N)  (dL/g)  coefficient
                       ([R.sup.2])

CPE 3     0.1   0.101       0.9968  0.00290  0.284

DAPE 6    0.1   0.125       0.9839  0.00961  0.614     27,300
(c)

ZAPE 7    0.1   0.114       0.9970   0.0215   1.65
(c)

          0.5   0.114       0.9887  0.00821  0.631

          1.0   0.113       0.9893  0.00901  0.705

PZ 9        0  0.0988       0.9945   0.0192   1.97

          0.1   0.114       0.9987   0.0253   1.95

DAPE 6    0.1   0.266       0.9998   0.0287  0.406     62,500
(d)

ZAPE 7    0.1   0.244       0.9918   0.0436  0.732
(d)

APE 10    0.1   0.147       0.9933    0.502   2.32
(d)
Polymer  (PDI)
           (b)

CPE 3

DAPE 6     2.2
(c)

ZAPE 7
(c)

PZ 9

DAPE 6     2.1
(d)

ZAPE 7
(d)

APE 10

(d)

(a.) Viscosity of 1-0.25% polymer solution in 0. 1 N NaCI at 30
[+ or -] 0.1[degrees]C was measured with an Ubbelohde Viscometer
(K = 0.005718). The average of three readings (for the time of Aow)
having standard deviation within 0.3 s are taken for the viscosity
plots.

(b.) Polydispersity index.

(c.) Derived via CPE 3.

(d.) Derived via PZ 9.


Evaluation of Antiscalant Behavior

Analysis of feed water and reject brine in reverse osmosis (RO) Plant at King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia, revealed the concentration of C[a.sup.2+] as 281.2 ppm in the brackish feed water and 867.7 ppm in the reject brine; the corresponding concentration of [S[0.sub.4].sup.2-] in the feed water and reject brine was found to be 611 and 2100 ppm, respectively (36). The composition of the above reject brine at 70% recovery is referred to as 1 concentrated brine (1 CB). The evaluation of the newly developed scale inhibitor DAPE 6 was performed in a solution containing 2.3 times the concentration of C[a.sup.2+] and [S[0.sub.4].sup.2-] than 1 CB. The 2.3 CB solutions would be supersaturated with respect to CaS[O.sub.4].

Solutions containing C[a.sup.2+] and [S[0.sub.4].sup.2-] ions equals to 4.6 times of the 1 CB were prepared by dissolving the calculated amount of CaC[l.sub.2] and N[a.sub.2]S[O.sub.4], respectively, in deionized water. 4.6 CB calcium chloride solution (60 mL) at a dose level of 20 ppm DAPE 6 was taken in a two necked round bottom flask and heated to 50[degrees]C [+ or -] 1[degrees]C. A preheated (50[degrees]C) solution of 4.6 CB sodium sulfate (60 mL) was added quickly to the flask, the content of which was stirred at 300 rpm using a magnetic stir-bar. The resultant solution containing 10 ppm of DAPE 6 thus becomes 2.3 CB which is 2.3 x 866.7 mg/L, i.e., 1993 mg/L in C[a.sup.2+] and 2.3 x 2100 mg/L, i.e., 4830 mg/L in [S[0.sub.4].sup.2-]. 200 [micro]L samples were taken at different interval of time through 0.45 micron filter (millipore) to measure the C[a.sup.2+] ions remaining in the solution using Atomic Absorption spectrometer (Table 3).

TABLE 3. Concentration of C[a.sup.2+] at various times at
50[degrees]C in the absence (a) and presene (a) of antiscalant
additive DAPE 6 (10 mg/L).

Time      Solution (a) with  Blank solution (a)  Scale inhibition
(mm)  inhibitor C[a.sup.2+]  C[a.sup.2+] (mg/L)               (%)
                     (mg/L)

0                      1990                1990                --

6                      1988                 660              99.8

180                    1984                 640              99.6

420                    1960                 630              97.8

1800                   1930                 620              95.6

(a.) Both solution contained C[a.sup.2+] and [S[0.sub.4].sup.2-] at
a concentration of 2.3 times the concentration of concentrated brine
(CB), i.e., [C[a.sup.2+]] = 1993 mg/L and [[S[0.sub.4].sup.2-]]
= 4830 mg/L.


RESULTS AND DISCUSSION

Synthesis of Monomer and Polymers

New cationic monomer 2 was prepared in excellent yield by bubbling anhydrous HCI through a solution of 1 [14] in ether (Scheme 1). The monomer underwent cyclopolymerization in aqueous solution containing 70 wt % monomer to give CPE 3. The results of the polymerization reaction are given in Table 1. Initiator TBHP (Entry 2) gave the polymer in better yield than initiator APS (Entry 1). The lower yields and intrinsic viscosity values (Table 1) could be a result of chain transfer between polymer and monomer through the alkoxy group attached to the P atom (37). Acidic hydrolysis of CPE 3 afforded PZA 4 (>90% yield). PZA 4 on treatment with 2 and 1 equivalent of NaOH afforded DAPE 6 and ZAPE 7, respectively. ZAPE 7 contains an interesting blend of zwitterionic as well as anionic groups in the same repeating unit.

For the purpose of comparison, PZ 9 was prepared (14) via cyclopolymerization of zwitterionic monoester 8 (Scheme 1). PZ 9 was treated with one equivalent NaOH to obtain a solution of APE 10. Using acidic hydrolysis, PZ 9 was also converted into PZA 4, DAPE 6, and ZAPE 7.

Polyphosphonates 3-7 were found to be very stable and did not show any appreciable decomposition or color change up to 300[degrees]C. While CPE 3, DAPE 6, and ZAPE 7 were found to be readily soluble in salt-free water, PZA 4 remained insoluble. A cloudy mixture of 3 wt % PZA 4 in salt-free water became transparent in 0.03 M NaCI. The PZA was also found to be soluble in 0.1 N HCI. Even though overwhelming majority of the reported PZs were found to be insoluble in water (12), (16), (38), (39), PZ 9 remained soluble both in salt-free and salt-added water as a result of the extensive hydration of the [O.sup.-] and steric crowding exerted by OC[H.sub.2]C[H.sub.3] which prevent effective zwitterionic interaction with the cationic nitrogens (14).

Infrared and NMR Spectra

The strong absorptions at 1127 and 1025 c[m.sup.-1] in the IR spectrum of CPE 3 were attributed to the stretching frequency of P=O and P--O--C, respectively. For PZA 4, DAPE 6, and ZAPE 7, the peaks at 1053, 1060, and 1060 c[m.sup.-1] were assigned to the stretching frequency of P.O. [.sup.1]H and [.sup.13]C NMR spectra of the monomer and polymers are displayed in Figs. 1 and 2, respectively. The absence of any residual alkene proton or carbon signal in the spectra suggested the chain transfer process (40) for the termination reaction.

The assignments of the [.sup.13]C peaks are based on earlier works (41-45) on quaternary ammonium salt monomers which undergo cyclopolymerization to afford kinetically favorable five-membered ring structure whose substituents at the C-b can either be in the symmetrical cis (major) or unsymmetrical trans (minor) dispositions (Scheme 2). Integration of the signals yielded the cisltrans ratio of the ring substituents to be 75:25; similar ratios are observed for many a Butler's cyclopolymerization process (41), (46). [.sup.31]P NMR signal *for monomer 2, CPE 3, DAPE 6 and ZAPE 7 appeared at [delta]31.4, 33.8, 22.6, and 20.8 ppm, respectively. The upfield shift of P signal in 6 and 7 is attributed to the higher electron density around P as a result of negatively charged oxygens.

Viscosity Measurements

Viscosity data as evaluated by the Huggins equation: [[eta].sub.sp]/C = [eta] + k [[eta].sup.2] C are given in Table 2. The Huggins viscosity plots for CPE 3, DAPE 6 and ZAPE 7 (having identical degree of polymerization) in salt-free water remain concave upwards like any polyelectrolytes (Fig. 3). In the lower concentration range, the sequence of decreasing reduced viscosity was found to be:

CPE 3 > ZAPE 7 > and DAPE 6.

One would have expected dianionic 6 to have higher viscosity values than zwitterionic/anionic 7 since the former has higher net charges than the later. A possible rationale behind this observation could be attributed to the increased repulsion among positively charged nitrogens in 7, hence expansion of the polymer chain. Note that the increased distance between the negatively charged oxygens in the neighboring repeating units in DAPE 6 leads to a less effective repulsion. Viscosity plots became linear in the presence of 0.1 N NaC1, and the sequence of decreasing intrinsic viscosity was found to be (Fig. 3, Table 2):

DAPE 6 > ZAPE 7> and CPE 3

The lowest value for the Huggins constant k in the case of 3 in comparison to 6 and 7 may be associated with an increased polymer--solvent interaction. Hydration shell of N[a.sup.+] is generally fairly large; hence, the distance of closest approach is not sufficient to effectively neutralize the charge on the pendent phosphonate anions in 6 and 7. However, CL ions are very effective in shielding, hence minimizing the repulsion among positive nitrogens in 3 and 7 which leads to lower hydrodynamic volumes and intrinsic viscosities [eta].

The decrease and increase of intrinsic viscosity in the presence of an added salt is a demonstration of "polyelectrolyte" and "anti-polyelectrolyte" behavior of a polyelectrolyte (cationic or anionic) and a PZ, respectively (12), (16), (38), (39). Note that the intrinsic viscosity of ZAPE 7 in 0.1 N. 0.5 N, and 1.0 N NaC1 remained almost unchanged (~0.114dL/g) (Table 2). ZAPE 7 has dual groups of zwitterion and anion, and it is the anionic portion that dictates the viscosity plot (Fig. 3). The presence of NaCl leads to a contraction and expansion of the polymer chain of a polyelectrolyte and PZ, respectively (12), (16), (38), (39). These opposite forces cancel the overall effect of NaCl in the solution, thereby leading to the unchanged intrinsic viscosity values for ZAPE 7 in the presence of various concentrations of the salt.

The viscosity plot for PZ 9 in the absence or presence of added salt (NaCl) remains linear (Fig. 4). The intrinsic viscosities in salt-free and 0.1 N NaCl were found to be 0.0988 and 0.144 dL/g, respectively; an increase in the intrinsic viscosity in the presence of NaCl is a demonstration of the "anti-polyelectrolyte" behavior of the PZ 9. As discussed earlier, the [Cl.sup.-] ions shield the positive nitrogens more effectively than the shielding of phophonate anions by the hydrated [Na.sup.+] ions. As a result, the zwitterionic moiety is left with a net negative charge that leads to repulsion hence expansion of the polymer chains. While CPE 3, ZAPE 7, and DAPE 6 (derived from CPE 3) have intrinsic viscosity values of 0.101, 0.114, and 0.125 dL/g, respectively, the corresponding values for PZ 9, ZAPE 7, and DAPE 6 (derived from PZ 9) are found to be 0.114, 0.244, and 0.266 dL/g in 0.1 N NaCl (Table 2). Note that CPE 3 and PZ 9 have similar [eta] (0.101 vs. 0.114 dL/g) in 0.1 N NaCl, while the DAPE 6 and ZAPE 7 derived from PZ 9 have twice the [Os of the corresponding polymers derived from CPE 3 (Fig. 5. Table 2) as confirmed by the higher Mw of DAPE 6 derived from PZ 9 (Table 2). This indicates that even though PZ 9 has higher molar mass than that of CPE 3, yet they have similar [eta]s as a result of the former having zwitterionic groups that leads to contraction of the polymer chains.

Antiscalant Evaluation

The deposits commonly encountered in desalination process include mineral scales [i.e., CaC[O.sup.3], CaS[O.sup.4], Mg[(OH).sub.2]], corrosion products, polymeric silica, and suspended matter. Davey [35] presented an excellent review on the role of additives in precipitation processes. The specific mechanism of inhibition of scaling is sequestration or the capability of forming stable complexes with polyvalent cations. The antiscalant treated solutions are evidently stabilized in some manner involving alteration in crystal morphology at the time of nucleation and subsequent inhibition in growth rate [47]. Commonly used antisealants are derived from three chemical families: condensed poly(phosphate)s, organophosphates, and polyelec-trolytes [48, 49]. The anionic form of the antiscalants helps prevent scale formation by sequestering the cations. Extraordinary chelating properties of compounds containing aminomethylphosphonic acid groups in the molecule have attracted considerable attention to synthesize low molecular-weight chelating agents containing these functional groups that should be able to form polymer-heavy metal ion complexes from waste water (32-34).

In the RO process, the feed water splits into product water and reject brine streams. The dissolved salts in the feed water are concentrated in the reject brine stream. If supersaturation occurs and their solubility limits are exceeded, precipitation or scaling will occur. The percent scale inhibition is calculated using the following Equation: where [[CA.sup.2+].sub.inhibited([t.sup.0]) is the initial concentration at time zero, [[CA.sup.2+].sub.inhibited] (t) and [[Ca.sup.2+]]blank (t) are the concentration in the inhibited and blank solutions at time t. The blank at the zero time was a supersaturated solution of Ca[SO.sup.4] containing 1993 mg/L [Ca.sup.2+] (using Ca[Cl.sup.2]) and 4830 mg/L [S[0.sup.4].sub.2] (using [Na.sup.2][SO.sup.4]) which is equivalent to 2.3 times the concentration of a CB (Experimental). Spontaneous precipitation of Ca[SO.sup.4] was observed in the case of the blank solution with the decrease in the concentration of [Ca.sup.2+] (Table 3, Fig. 6). However, with the addition of DAPE 6 to the CB solution, [Ca.sup.2+] remained almost constant for about 180 min with 99.6% scale inhibition. After the elapse of 30 h (i.e., 1800 min), [Ca.sup.2+] dropped from 1990 mg/L to 1930 mg/L resulting in a scale inhibition of 95.6%. This indicates that the additive is very effective against precipitation of Ca[SO.sup.4] at 50[degrees]C in a 2.3 CB and thus suitable for its application in the prevention of scaling in RO desalination plants.

CONCLUSIONS

Using Butler's cyclopolymerization process, we have, for the first time, synthesized CPE 3 containing a pendant having three-carbon spacer separating the diethylphosphonate and NH. On acidic hydrolysis of the phophonate esters, CPE 3 was readily converted into pH-responsive PZA 4, DAPE 6, and ZAPE 7. The solution properties of the polymers were correlated to the structurally similar PZ 9 having monoethylphosphonate and [NH.sup.+] groups. Evaluation of the antiscalant properties using CBs revealed that DAPE 6 at a meager concentration of 10 ppm is very effective in inhibiting the formation of calcium sulfate scale, and as such it can be used effectively as an antiscalant in RO plant.

ACKNOWLEDGMENTS

The facilities provided by the King Fahd University of Petroleum and Minerals, Dhahran, are gratefully acknowledged.

Correspondence to: Shaikh A. Ali; e-mail: shaikh@kfupm.edu.sa Contract grant sponsor: King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM); contract grant number: No. 08-WT76-04; contract grant sponsor: National Science, Technology and Innovation Plan.

DOI 10.1002/pen.23548

Published online in Wiley Online Library (wileyonlinelibrary.com). [C] 2013 Society of Plastics Engineers

I.W. Kazi, (1) F. Rahman, (2) Shaikh A. Ali (1)

(1.) Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia

(2.) Center for Refining & Petrochemicals, Research Institute, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia

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ABBREVIATIONS
APE    Anionic polyelectrolyte
APS    Ammonium persulfate
CB     Concentrated brine
CPE    Cationic polyelectrolyte
DAPE   Dianionic polyelectrolyte
MWCO   Molecular weight cut-off
PZ     Polyzwitterion
PZA    Polyzwitterionic acid
RO     Reverse osmosis
TBHP   t-Butylhydroperoxide
ZAPE   Zwitterionic/anionic polyelectrolyte
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Author:Kazi, I.W.; Rahman, F.; A. Ali, Shaikh
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:7SAUD
Date:Jan 1, 2014
Words:5754
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