Synthesis and evaluation of phosphate-free antiscalants to control CaSO4·2H2O scale formation in reverse osmosis desalination plants

Desalination 357 (2015) 36–44

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Synthesis and evaluation of phosphate-free antiscalants to control CaSO4·2H2O scale formation in reverse osmosis desalination plants Shaikh A. Ali a, I.W. Kazi a, F. Rahman b,⁎ a b

Chemistry Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Center for Refining & Petrochemicals, Research Institute, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia

H I G H L I G H T S • Pre-treatment of reverse osmosis (RO) desalination process for seawater and brackish water • Synthesis of phosphate-free antiscalants to control calcium sulfate scale formation • Comparative evaluation of different antiscalants for seawater and brackish water RO plants

a r t i c l e

i n f o

Article history: Received 13 July 2014 Received in revised form 3 November 2014 Accepted 5 November 2014 Available online xxxx Keywords: Reverse osmosis Gypsum scale Phosphorus-free antiscalant Polyaspartate Brackish water

a b s t r a c t Polysuccinimide (PSI) of various molecular weights (MW) was synthesized from aspartic acid. The PSIs were hydrolyzed with NaOH to obtain sodium polyaspartates. Partial hydrolysis of a PSI gave the copolymer poly(SI-cosodium aspartate). PSIs were treated with one equivalent of cyclic amines: pyrrolidine, piperidine, and azepane to afford the corresponding polyaspartamides in excellent yields. Treatment of PSI with 0.5 equivalents of the cyclic amines led to the formation of 1:1 copolymer poly(SI-co-aspartamide) which on basic hydrolysis gave the corresponding 1:1 copolymer poly(sodium aspartate-co-aspartamide). The antiscalant behavior of the newly synthesized compounds was evaluated at various concentrations (2.5 ppm– 10.0 ppm) for the inhibition of gypsum scale formation. The antiscalant evaluation experiments were performed by measuring conductivity and using concentrated brines. The study revealed the performance of low MW antiscalants to be superior to their high MW counterparts. Thus, a polyaspartate is found to be a better antiscalant than its corresponding copolymer of SI/aspartate, i.e. a partially hydrolyzed PSI. Derivatives of 7-membered cyclic amine azepane performed better than their 5- and 6-membered counterparts. At the same degree of polymerization, performance with decreasing order of effectiveness was found to be: polyaspartate N poly(aspartamide-co-aspartate) N (SI-co-aspartate) N poly(SI-co-aspartamide) N polyaspartamide. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The world desalting capacity including contracted or under construction plants as of July 2013 was 80.5 × 106 m3/d. The share of GCC countries in the world desalting capacity is about 45–48% whereas the share of Saudi Arabia is largest, about 18–20% of the world desalting capacity [1]. The share of reverse osmosis (RO) process has increased to 63% of the world desalting capacity. Rapid urbanization, phenomenal industrial growth along with high population growth and agricultural development make water one of the most precious resources in Saudi Arabia. Scaling is one of the major problems in both RO and MSF desalination plants. In membrane-based desalination plants, deposit of scale on the membranes reduces product recovery and salt rejection, and increases pressure drop. All these factors result in decreasing the product ⁎ Corresponding author. E-mail address: [email protected] (F. Rahman).

http://dx.doi.org/10.1016/j.desal.2014.11.006 0011-9164/© 2014 Elsevier B.V. All rights reserved.

water quantity as well as lowering the product water quality besides increasing the pumping cost. The feed waters to the desalination plants in the Middle East region are seawater and aquifer brackish waters, which consist of mostly NaCl; but they also contain ionic species such as Ca2+, − Mg2+, Sr2+, SO2− 4 , and HCO3 which, under suitable conditions of temperature, pressure, and concentration, may combine to form CaCO3, Mg(OH)2, CaSO4·2H2O, SrSO4, etc. These salts have very low solubility; therefore, they come out of the brine and deposit on the heat transfer tubes in the case of thermal plants, and on membrane surfaces in case of RO plants. In this paper, we present synthesis and performance evaluation of phosphate-free antiscalants to inhibit scale formation in RO plants. 1.1. Literature review Davey [2] presented an excellent review on the mechanism of additives in precipitation processes. Logan and Walker [3] reported the role

S.A. Ali et al. / Desalination 357 (2015) 36–44

of additives in the prevention of scale formation. Harris [4] discussed the effect of various additives in inhibiting the precipitation of alkaline scale (e.g., calcium carbonate, magnesium hydroxide) and sulfate scale (e.g., calcium sulfate, strontium sulfate) in desalination plants. Amjad [5] presented the influence of various antiscalants such as polyphosphates, phosphonates, polystyrene sulfonate, polyacrylamide, polyacrylate and formulated polyelectrolytes on crystallization and crystal modification process of gypsum scale at 25 °C. It is reported [6] that some correlation exists between the structure of the inhibitors and the inorganic scale. Effectiveness of inhibitors may even vary for different hydration forms of the same scale. Dihydrate and hemihydrate forms of calcium sulfate have markedly distinct interactions with inhibitors like polyvinyl sulfonate (PVS) with respect to change of the crystal habit. The effective interaction among the negatively charged groups on the inhibitor and Ca2+ imparts the changes in precipitate morphologies in the crystal lattice of calcium sulfate hemihydrates and dihydrate and scale inhibition. Carboxylate antiscalants are usually less effective in the presence of multi-valent metal ions like Ca2+. “AQUAKREEN KC-550” is reported [7] to be effective against such multi-valent ions. Effect of polyelectrolyte architecture on scale inhibition has been discussed by Oner et al. [8]; it was found that polyacrylates imparting maximum surface charge density were more effective than polymethacrylates. Le Gouellec and Elimelech [9] reported the effect of hydrodynamics and dosage of additives on inhibition of gypsum scaling in a nanofiltration system. Effectiveness of several commercial antiscalants against gypsum scale formation was evaluated by Shih et al. [10]. Based on precipitation studies, Rahardianto [11] found that increasing bicarbonate concentrations led to minimized membrane scaling with gypsum. Amjad [12] examined the inhibition of gypsum precipitation by a variety of polymeric materials and concluded that compared to polymers, surfactants perform poorly against gypsum precipitation. Polyphosphate additives added for controlling scale formation in the reject brine of desalination plants when discharged in the sea act as nutrient to algae, leading to their increased growth, and local flourishing and influence over the marine biota picture at the intakes and outfalls as compared to that of the open sea [13]. Poly(acrylic acid) and poly(acrylamide), often used as antiscalants, do not exhibit biodegradability [14] and as such affect the natural world and impact the environment negatively. Therefore, there is a demand for biodegradable water-soluble polymers that would be able to solve the scaling problem in the desalination industry without also damaging the environment. Poly(amino acids), with amide linkage of a peptide, are completely biodegradable [15–17] and as such have attracted considerable attention as candidates for biodegradable antiscalants. Sodium polyaspartate (SPASP), a poly(amino acid) with carboxylic acid side chains, exhibits both biodegradability and functionality such as chelating ability and dispersibility. Scale inhibition and delay in deposition of precipitates onto the membrane surface have been investigated using polyaspartic acids and their mixtures with surfactants and emulsifiers in the presence of polyacrylates or phosphonates [18–21] or 1-hydroxyethylidene-1,1-diphosphonic acid [22]. Formulated compounds prepared by mixing scale inhibitor containing polymer and organic phosphonic acid have been applied for water treatment [23]. A study on the function and mechanism of phosphonic acid modified polyaspartic acid scale inhibitor has been reported by Liang et al. [24]. A scale inhibitor composed of components such as hydroxyethylidene diphosphonic acid, sodium polyacrylate, hydrolyzed poly(maleic anhydride), and poly(epoxysuccinic acid) and auxiliary agents like sodium bisulfite, chlorine dioxide, and benzotriazole have been used [25]. Sodium hexametaphosphate is a popular additive that has been used as a gypsum-scale inhibitor to prevent or minimize scale formation in concentrated brines [26]. In accordance with the principles of responsible care, Hasson et al. [27] proposed a general product profile for ecologically benign inhibitor systems as follows: excellent scale inhibition, low aquatic and human toxicity, high biodegradability, good price/performance ratio, and free of phosphorus, nitrogen, and heavy metals [28]. Keeping these entire

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challenging requirements in view, there is a need to develop antiscalants that readily biodegrade and have low mobility for minimum environmental impact and also perform at cost-effective dosage of antiscalants. 2. Experimental Poly(aspartic) based inhibitors are currently the most promising alternative to conventional non-degradable inhibitors. In the present research, we have synthesized polysuccinimide (PSI) of various molar masses, which were converted into various homo- and copolymers of aspartates, aspartamides, poly(SI/aspartamides), poly(SI/aspartates), and poly(aspartates/aspartamides). The structures of newly synthesized polymers along with their codes are illustrated in Table 3. The scale inhibiting properties of these new inhibitors have been investigated using aqueous solution containing 3 times the concentration of calcium and sulfate ions found in RO plant concentrated brine (CB) corresponding to the concentrations of reject brine at 70% recovery for brackish water as feed. 2.1. Materials L -Aspartic acid (ASP) (Fluka Chemie AG), sulfolane N,Ndimethylformamide (Fluka Chemie AG) o-phosphoric acid (85% certified ACS Fischer Scientific), sulfuric acid (98% certified ACS Fischer Scientific), sodium hydroxide (Fluka Chemie AG), pyrrolidine, piperidine and azepane (Fluka Chemie AG) were used as received. A commercially available leading antiscalant for reverse osmosis desalination plant applications was obtained in the local market.

2.2. General procedure for polymer synthesis 2.2.1. Polysuccinimide with degree of polymerization (n) of 100: PSI 2-100 A slightly modified procedure [28] was adapted for the preparation of the PSI 2-100. Thus a mixture of H2SO4 (2.5 g) and L-aspartic acid (1) (50 g, 376 mmol) was thoroughly ground in a porcelain mortar at room temperature. The mixture was then transferred to a two-necked round bottomed flask fitted with a large air condenser and heated in an oil bath at 203 °C under nitrogen atmosphere. Water vapor, deposited in the condenser, was swept off by N2 purge. The mixture was stirred continuously using a magnetic stir-bar. Clumps, formed after 30 min, were finely ground again and heated for a further 7 h. A pale white powdered product PSI 2-100 was washed with several portions (300 mL) of hot water to remove unreacted aspartic acid as well as H2SO4. Final wash was done by methanol (150 mL) and the product was dried under vacuum at 85 °C for 2 h (until achieving constant weight, 33 g, 90%). A 0.5% solution of the PSI 2-100 in dimethyl formamide (DMF) showed reduced viscosity (ηred) of 8.57 mL/g as determined at 25 °C using an Ubbelohde viscometer. The molecular weight and the degree of polymerization (n) were estimated to be approximately

Table 1 Percent inhibition against precipitation at various times in the presence of 10 ppm of the synthesized polymers in 3CB supersaturated CaSO4·2H2O solution at 40 °C. Entry

1 2 3 4 5

Sample

3-100 4-100 10-100 11-100 12-100

Percent inhibition at different times (min) 100

500

1000

1500

2000

2500

3200

3500

100 100 100 100 99

100 98 98 100 99

100 77 93 100 99

100 7 85 99 98

98 7 9 96 95

97 7 9 90 91

92 7 9 10 73

20 7 9 7 12

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Table 2 Percent inhibition against precipitation at various times in the presence 10 ppm of the synthesized polymers in 3CB supersaturated CaSO4·2H2O solution at 40 °C. Entry

1 2 3 4 5 6 7 8 9

Sample

Percent inhibition at different times (min)

3-230 5-100 6-100 7-100 8-100 9-100 9-230 9-600 Commercial

10

100

500

800

100 29 100 100 100 100 99 100 99

100 23 99 50 96 99 4 15 6

98 22 82 29 47 94 1 4 4

9 22 9 20 12 89 2 5 4

9744 g mol−1 and 100, respectively, as calculated from the following empirical equation [29]: n ¼ 3:52  ηred

1:56

ð1Þ

where n is the degree of polymerization and ηred is the reduced viscosity of a 0.5% solution in DMF expressed in the unit of mL/g. 2.2.2. Polysuccinimide with degree of polymerization (n) of 230: PSI 2-230 PSI 2-230 was prepared using the procedure as described in [30]. A suspension of aspartic acid (1) (37.7 g, 283 mmol) and 85% phosphoric acid (1.62 g, 14.1 mmol) in sulfolane (120 g) was refluxed under N2 at 160 °C in an oil bath for 5 h. The reaction mixture containing the insoluble white product PSI 2-230 was filtered off and washed several times

Table 3 Chemical structure of synthesized polymers of different molar masses.a Polymer

2.2.3. Polysuccinimide with degree of polymerization (n) of 600: PSI 2-600 PSI 2-600 was prepared using the procedure as described in [31]. L-Aspartic acid (1) was mixed with 30 wt.% phosphoric acid, and the mixture was heated at 180 °C for 3 h under reduced pressure (25 mm Hg) and then for 1 h under a higher vacuum of 1 mm Hg. The mixture was cooled to room temperature and dissolved in DMF. The solution was then poured into a large excess of water. The precipitated polymer PSI 2-600 was filtered, washed with water and methanol and then dried to a constant weight under vacuum at 85 °C to obtain the PSI with 85% yield. 2.2.4. Aspartamide derivatives of PSI 2 Procedure reported for the amidation of PSI with primary amines is used to carry out amidation reaction using cyclic amines [32]. To a stirring solution of PSI-2 (1.95 g, 20 mmol) in DMF (10 mL) at 0 °C, pyrrolidine (m = 1) or piperidine (m = 2) or azepane (m = 3) (10 mmol or 20 mmol) was added drop-wise for 5 min. The mixture was stirred at room temperature for 20 h and then poured drop-wise into a vigorously stirred acetone (100 mL). The product was filtered and washed with acetone and dried under vacuum at 70 °C to a constant weight to obtain pale white products with yields in the range of 85–93%. Reaction of PSI 2-100 with 1 equivalent of cyclic amines pyrrolidine (m = 1) and azepane (m = 3) gave poly[(pyrrolidine-1-yl)aspartamide] (5-100) and poly[(azepane-1-yl)aspartamide] (6-100), respectively. Reaction of PSI 2 with 0.5 equivalent of cyclic amines afforded poly[(pyrrolidine-1-yl)aspartamide-ran-succinimide] (7-100); poly[(piperidine-1yl)aspartamide-ran-succinimide] (8-100); and poly[(azepane-1-yl) aspatamide-ran-succinimide] (9-100, 9-230 and 9-600). The synthesized compounds gave satisfactory elemental analyses.

Chemical structure

Polyaspartate

α β Poly(aspartamideran-aspartate)

Poly(succinimide-ranaspartate)

with water until the washing becomes neutral and finally with methanol (100 mL). The residue was dried under vacuum at 90 °C (till constant weight) (24 g, 88%).

α β

Poly(succinimide-ranaspartamide)

2.2.5. Alkaline hydrolysis of PSI/PSI derivatives Solid NaOH [103 mmol (1 equivalent) and 79.3 mmol (0.77 equivalent)] for the hydrolysis of PSI 2 and 51.5 mmol of NaOH for PSI/ aspartamides 7, 8 and 9 were added portion-wise to a stirring mixture of PSI 2 or PSI/aspartamide 7, 8 or 9 (103 mmol) in water (30 mL) for 5–8 min at 0 °C under N2. The solution was stirred at room temperature for 3 h. After the completion of the reaction, the mixture was poured drop-wise onto a vigorously stirred acetone (200 mL). Hygroscopic white sticky product of polyaspartate 3-100, 3-230, poly(aspartateran-succinimide) 4-100, poly[(pyrrolidine-1-yl)aspartamide-ran-aspartate] (10-100), poly[(piperidine-1-yl)aspartamide-ran-aspartate] (11-100), and poly[(azepane-1-yl)aspartamide-ran-aspartate] (12100) was precipitated out. The products were filtered and washed with acetone and dried under vacuum at 70 °C to constant weights. The yields of the hydrolysis reactions remained over 90%. The synthesized compounds gave satisfactory elemental analyses. 2.3. Evaluation of antiscalant behavior

Polyaspartamide

α β

a Polymer structures are given as bold numbers and the hyphenated numbers refer to the degree of polymerization (n) values of 100, 230 and 600 corresponding to the MW of 10,000, 22,000 and 58,000 g mol−1, respectively, of the precursor polymer PSI 2.

The precipitation and inhibition of calcium sulfate (gypsum) scale formation were evaluated using a series of the newly synthesized environmentally benign antiscalants. The concentration of reject brine at 70% recovery and 98% salt rejection was denoted as 1CB. A mathematical model was applied on the basis of feed water analysis to determine the concentrations of ions in the reject brine (1CB). Analysis of brine in an RO Plant at King Fahd University of Petroleum & Minerals (KFUPM) revealed the presence of 281.2 and 943.3 ppm Ca2+ ions and 611 and ions in the brackish feed water and in the concen2100 ppm of SO2− 4 trated brine (1CB) respectively [26]. The evaluation of the new scale inhibitors was performed in 3CB solutions containing three times the concentrations of Ca2+ and SO2− 4 in 1CB solutions. The solutions were supersaturated with respect to

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Scheme 1. Synthesis of polysuccinimide of various molar masses.

CaSO4·2H2O as confirmed from its solubility data. Solutions containing ions equal to 6 times the concentrated brine of 1CB were prepared by dissolving the calculated amount of CaCl2 and Na2SO4 respectively, in

deionized water. The solutions were filtered (0.45 μm membrane filters, Millipore) and standardized by complexometric titration (EDTA) (Metrohm autotitrator) and ion chromatography (Dionex ICS-3000)

Scheme 2. The monomer in different forms and protonation constants.

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for calcium (Ca2+) ions and sulfate (SO2− 4 ) ions respectively. A three step procedure was adopted to evaluate the performance of newly synthesized antiscalants. 2.3.1. Visual inspection method-fast screening Equal volume (20 mL) of 3CB calcium chloride solutions and sodium sulfate solutions was introduced in the glass tubes or vials and 10 ppm antiscalant formulations were added to each tube before mixing the two solutions. All these tubes were kept in the constant temperature water bath at 50 °C equipped with continuous shaking mechanism. About 30 antiscalants were evaluated in the glass tubes simultaneously. Visual inspection of each antiscalant was carried out and time was recorded when the solution started to indicate turbidity or precipitation. Antiscalants that showed better inhibition, meaning clear solutions or no turbidity for about 10 h were selected for further testing in stirred beaker experiments. The initial fast screening method at accelerated conditions provided performance of successful antiscalants in a qualitative manner. 2.3.2. Conductivity measurement method A solution of 6CB calcium chloride (60 mL) containing (1.2 mL of 1000 ppm) antiscalant in a round bottom flask was heated at 40 °C ± 1 °C. A preheated (40 °C) 6CB sodium sulfate solution (60 mL) was added to the stirring solution (300 rpm) in the flask. The resultant solution of 3CB concentration with respect to calcium ions and sulfate ions now contained 10 ppm of antiscalant. The performance of all the selected antiscalants from fast screening method was evaluated by recording the conductivity at every 10–20 s through data-logger. These conductivity measurements provided accurate data on performance of antiscalants.

2.3.3. Calcium ion concentration measurement method The antiscalants which demonstrated very good inhibition properties in conductivity experiment measurement method were further evaluated by measuring the Ca++ ion concentration of the solutions with time. The percent inhibition (PI) of antiscalant against scaling is calculated using the following equation: h i h i Ca2þ − Ca2þ inhibited ðtÞ blankðtÞ    100 % Scale Inhibition ¼  2þ  Ca inhibitedðt Þ − Ca2þ blankðtÞ 0

where

h i 2þ Ca

inhibitedðt0 Þ

is the initial concentration at time zero,

[Ca2 +] inhibited (t) and [Ca2 + ] blank (t) are the concentrations in the inhibited and blank solution (without antiscalant) at time t respectively [35]. 3. Results and discussion 3.1. Synthesis of aspartic acid-based antiscalants Polysuccinimide (PSI) 2 with degree of polymerizations of 100, 230 and 600 has been prepared from L-aspartic acid (1) using the procedure as outlined in Scheme 1. A sample, for instance, designated as 2-100 identifies the structure by the bold number followed by the regular number 100 as the degree of polymerization. The degree of polymerization and molar mass of the polymers were determined using viscosity measurements as described in the experiment [29]. Samples of 2-100 and 2-230 upon treatment with 1 equivalent of NaOH were converted into sodium polyaspartates 3-100 and 3-230, respectively. A partially

Fig. 1. 1H NMR spectra of (a) 3-100 in DMSO-d6, (b) 9-100 in D2O, (c) 6-100 in D2O, and (d) 12-100 in D2O.

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Fig. 2. Precipitation behavior of a supersaturated solution (3CB) of CaSO4·2H2O at 40 °C in the presence (10 ppm) and absence (blank) of (a) 9-100, 8-100, 7-100; (b) 3-100, 3-230; (c) 9-100, 9-600, 9-230; and (d) 9-100, 6-100, 7-100, 5-100.

hydrolyzed sample of 4-100 was prepared by treating PSI 2-100 with 0.77 equivalent of NaOH. Aspartamide 5-100 and 6-100 were obtained by treating PSI 2-100 with one equivalent of pyrrolidine and azepane, respectively (Scheme 2). However, reaction of PSI 2 with 0.5 equivalent of 5-, 6-, and 7- membered cyclic amines (pyrrolidine, piperidine and azepane) afforded the copolymers poly(SI/aspartamide) 7, 8 and 9, respectively. Finally, the PSI part of the 1:1 poly(SI/aspartamides) 7-100, 8-100, and 9-100 was hydrolyzed with an equivalent amount of NaOH to the corresponding poly(aspartate/aspartamide) 10-100, 11-100, and 12-100 (see Table 3). Fig. 1 represents the 1H NMR spectra of 2, 6, 9, and 12. The 1:1 composition of the copolymers 9-100 (Fig. 1b) and 12-100 (Fig. 1d) was confirmed using area integration of the 8H protons (marked ‘d’ and ‘e’) appearing in the δ range of 1.25–1.75 ppm versus the rest of the protons.

3.2.1. Effect of MW on CaSO4·2H2O scale inhibition The effect of degree of polymerization (n) on percent inhibition (PI) is revealed by the performances of 3-100 (Table 1, entry 1) and 3-230 (Table 2, entry 1). Low molecular weight 3-100 demonstrates much higher PI than its higher MW counterpart 3-230 (Fig. 2b). The dramatic effect of MW on antiscalant performances is shown in entries 6–8 (Table 2); the copolymers 9-100, 9-230 and 9-600 at 100 min gave PI of 99, 4 and 15%, respectively (Fig. 2c).

3.2. Scale inhibition properties of the synthesized polymers In the reverse osmosis (RO) process, the inlet stream is feed water and outlet streams are product water and reject brine. If the dissolved salts in the reject brine stream become supersaturated due to increased recovery ratio, then scaling occurs. For the current work, the precipitation behavior of a supersaturated solution of CaSO4·2H2O containing 2830 ppm of Ca2+ and 6300 ppm of SO24 − in the absence and presence of antiscalants was investigated based on the method used by Amjad and Zuhl [35]. Percent inhibition (PI) of newly synthesized antiscalants at a concentration of 10 ppm is shown in Tables 1 and 2.

Fig. 3. Precipitation behavior of a supersaturated solution (3CB) of CaSO4·2H2O at 40 °C in the presence (10 ppm) and absence (blank) of 12-100, 11-100, 10-100 and 4-100.

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3.2.2. Effect of charges on the polymer backbone on CaSO4·2H2O scale inhibition The data reveals that a fully hydrolyzed sample 3-100 (Table 1, entry 1) imparts higher PI than its partially hydrolyzed counterpart 4-100 (Table 1, entry 2) having the same degree of polymerization (Figs. 2b and 3). Higher amount of negative charges on the polymer backbone 3-100 has thus greater beneficial effect; while at 1500 min, 3-100 imparted a PI of 100%, the 4-100 displayed a PI of 7% only. A comparison between the poly(SI/aspartamide) copolymers 7, 8 and 9 (Table 2, Fig. 2a) with the corresponding aspartamide/aspartate copolymers 10, 11 and 12 (Table 1, Fig. 3) revealed the superiority of the later antiscalants, thus yet again displaying the importance of the negative charges on the polymer backbone in suppressing the precipitation of CaSO4·2H2O. 3.2.3. Effect of amine ring size in the aspartamides on CaSO4·2H2O scale inhibition Aspartamide 6-100 (Table 2, entry 3) of 7-membered azepane performed better than the aspartamide 5-100 (Table 2, entry 2) of 5-membered pyrrolidine (Fig. 2d); both of which, however, are outperformed by either aspartate 3-100 (Table 1, entry 1) or 3-230 (Table 2, entry 1) (Fig. 2b). Copolymers 7-100, 8-100, and 9-100 with a 1:1 aspartamide/SI ratio (Table 2, entries 4–6, Fig. 2a) imparted PI of 29, 47 and 94%, respectively, at 500 min. Thus, azepane derivative 9-100 demonstrated the best antiscalant behavior. Note that the aspartamide/SI copolymers 7-100, 8-100, and 9-100 performed better than their corresponding aspartamide homopolymers 5-100 and 6-100 (Table 2, entries 2, 3).

3.2.4. Effect of copolymer type on CaSO4·2H2O scale inhibition As discussed under Sections 3.2.2 and 3.2.3, poly(aspartamide/SI) copolymers 7-100, 8-100, and 9-100 performed better than their corresponding aspartamide homopolymers 5-100 and 6-100 (Table 2), while the corresponding aspartamide/aspartate copolymers 10, 11 and 12 (Table 1, Fig. 3) revealed the best performance. 3.2.5. Effect of antiscalant concentration on CaSO4·2H2O scale inhibition The effect of dosage of antiscalant 3-100, 10-100, 12-100, and (d) a commercial antiscalant on the gypsum scale inhibition is shown in Fig. 4. Induction time was recorded to measure the performance of each inhibitor. Induction time is defined as the time required for the commencement of precipitation or it is the time elapsed until the moment at which the onset of precipitation can be detected. The commencement of precipitation in our experiments was detected by drop in conductivity. When the conductivity decreases rapidly, precipitation is visible, as the solutions become turbid. It is apparent from Fig. 4 that the induction time increases with the increase in dosage of antiscalants. As shown in many graphs, in the presence of copolymer, onset of precipitation is preceded by an induction time of several minutes to several thousand minutes. Later on the crystal growth recommences with a measurable rate observed by a sharp drop in conductivity. The equilibrium inhibition percentages in almost all experiments are low. As evident from the figures and tables, all the synthesized compounds performed much better than the commercial antiscalant (Fig. 4d). In addition, the duration of the induction time depends on the concentration of the antiscalants. Involvement of polyaspartates into the

Fig. 4. Precipitation behavior of a supersaturated solution (3CB) of CaSO4·2H2O at 40 °C in the absence (blank) and presence of various concentrations of (a) 3-100; (b) 10-100; (c) 12-100; and (d) a commercial antiscalant.

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nucleation process during the induction period leads to the prevention of the growth of CaSO4·2H2O by poisoning of active growing sites in the crystal. However, a few of the growth sites of lower energy may still be free to grow at a slow rate which leads to a very small change in ion concentrations which are difficult to be detected by conductivity measurement. Later on, after the adsorbed molecules are incorporated into growing crystal, the crystal growth can resume at a rate comparable to that of the un-poisoned systems. The results presented in this study clearly show that the polyaspartates are able to prolong the induction period and may thus minimize the fouling of membranes by CaSO4·2H2O. 3.2.6. Summary of antiscalant behaviors Low MW antiscalants are found to be superior to their high MW counterparts. Higher amount of negative charges on the polymer backbone is found to have a greater beneficial effect on the gypsum scale inhibition. Derivatives of 7-membered cyclic amine azepane, performed better than their 5- and 6-membered counterparts. At

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the same degree of polymerization, performance in decreasing order of effectiveness was found to be: polyaspartate N copolymer poly (aspartamide-co-aspartate) N copolymer poly(SI-co-aspartate) N copolymer poly(SI-co-aspartamide) N polyaspartamide. 3.3. EDX and SEM images As shown in Fig. 5a, scanning electron microscopy (SEM) analysis revealed that a monoclinic CaSO4·2H2O crystal of a regular shape and compact structure is formed in the absence of inhibitors. In the presence of antiscalants, crystals having irregular shapes and loose structures are formed. Fig. 5b shows that in the presence of sodium polyaspartate 3-100, a CaSO4·2H2O crystal loses its sharp edges, and its morphology is modified from elongated stick forms to thin flakes. In the presence of 12-100 and 10-100 the crystal morphology also changes to smaller fragments (Fig. 5c, d). Since various types of crystal faces have different surfaces, inhibitor molecules are adsorbed at different rates onto each type of face lattice structure. Consequently, alterations in crystal shape

Fig. 5. EDX and SEM images of gypsum formed from 3CB at 40 °C with (a) blank; (b) 3-100; (c) 12-100; and (d) 10-100.

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occur during outgrowth that is, the crystal cannot grow normally in accordance with an array of crystal lattices strictly, causing the crystals to become distorted or increasing the internal stress of crystals. Increase in stress could result in crystal fractures and prevention of deposition of microcrystalline [33]. 4. Conclusions In comparing the efficacy of the inhibitors, the performance criterion proposed by Reitz [34] can be adopted. In RO desalination, the residence time of the saline water inside the permeators is less than 15 min. If, according to Reitz [34], a scale inhibitor at a certain dose level can keep a scale-forming salt in solution for at least 15 min (i.e., if it can help achieve an induction period of 15 min), it can be considered as an effective inhibitor at that dose level against that particular scaling. Based on the Reitz [34] criterion, the results indicate that almost all of the synthesized green additives in the present study are very effective with only 10 ppm dose concentration against precipitation of gypsum at 40 °C and 3CB brine concentrations except 5-100. Hydrolyzed derivatives are more efficient than unhydrolyzed PSI-derivatives. Antiscalant 3-100 was found to be the most effective with very high induction time at a 10 ppm dose concentration. Antiscalants 3-100, 4-100, 12-100 and 10-100 can be used as effective antiscalants for CaSO4·2H2O even at a concentration of only 2.5 ppm in brackish water RO plant. It is planned to evaluate the most promising antiscalants using a minimum size commercial RO unit with two-stage membrane configuration according to the procedure adopted by Butt et al. [36]. Actual brackish feed water will be used with appropriate filtration system and results will be reported in next publication. Acknowledgments The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum and Minerals (KFUPM) for funding this work through project No. 08WT76-04 as part of the National Science, Technology and Innovation Plan. The laboratory facilities provided by the Chemistry Department and Center for Refining & Petrochemicals, Research Institute, KFUPM are also acknowledged. References [1] http://www.filtsep.com/view/33597/50th-anniversary-desalination-50-years-ofprogress-part-2/2013. [2] R.J. Davey, The role of additives in precipitation processes, in: S.J. Jancic, E.J. de Jong (Eds.), Industrial Crystallization 81, North-Holland Publishing Co., 1982, pp. 123–135. [3] D. Logan, J.L. Walker, Scale Control in High Temperature MSF Evaporators with Concurrent Acid and Inhibitor Treatment, Calgon Corporation, Pittsburgh, Pennsylvania, 1982. [4] A. Harris, in: A. Porteous (Ed.), Desalination Technology, 1983, pp. 44–48. [5] Z. Amjad, Applications of antiscalants to control calcium sulfate scaling in reverse osmosis systems, Desalination 54 (1985) 263–276. [6] J.S. Gill, R.G. Varsanik, Computer modelling of the specific matching between scale inhibitors and crystal structure of scale forming minerals, J. Cryst. Growth 76 (1986) 57–62. [7] Y. Fukumoto, K. Isobe, N. Moriyama, F. Pujadas, Performance test of a new antiscalant ‘AQUAKREEN KC-550’ under high temperature conditions at the MSF desalination plant in Dubai, desalination and water re-use, Proceedings of the Twelfth International Symposium, Malta, 15–18 April 1991, 1991, pp. 65–75. [8] M. Oner, O. Dogan, G. Oner, The influence of polyelectrolytes architecture on calcium sulfate dihydrate growth retardation, J. Cryst. Growth 186 (1998) 427–437.

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