In an attempt to control CaCO3 deposits in industrial recycling water systems, the performance of acrylic acid (AA)–allylpolyethoxy carboxylate (APEL) copolymer as an economical and environmentally friendly inhibitor have been investigated by static experiments, scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The experimental results revealed that AA–APEL (acrylic acid–allylpolyethoxy carboxylate copolymer) achieved the maximum scaling inhibition efficiency of 99.1%. The results of SEM and XRD studies revealed that both the morphology and aggregation of calcium carbonate crystals had been changed, when the inhibitor was added. Moreover, the results of TGA further confirmed the scaling mechanism of the copolymer.

double-hydrophilic block copolymer, industrial recycling water, non-phosphorus, scale inhibitor, surface morphology

INTRODUCTION

For environmental and economic reasons, a greater number of cycles for industrial water should be used. However, this cannot be realized without development of scale control methods (Ling et al. 2012; Wang et al. 2014; Al-Hamzah & Fellows 2015; Liu et al. 2016). The potential of mineral precipitation continues to be by far the most costly design and operating problem in recycling water systems (Al Nasser et al. 2011; Zhang et al. 2016). Commonly, mineral precipitation consists of calcium scales, zinc scales, magnesium hydroxide, ferric hydroxide, barium sulfate, etc., among which calcium carbonate scales are considered most frequent in cooling water systems (Alimi et al. 2006; Hasson et al. 2011; Wang et al. 2016).

The most common and effective method of scale control is the use of chemical additives as scale inhibitors that retard or prevent scale formation even in very small concentrations (Zhang et al. 2016). At present, amino trimethylene phosphonic acid (ATMP) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) are well-known scale inhibitors in cooling water systems. Although the nitrogen- and phosphorus-containing scale inhibitors are highly efficient, their use is limited because these compounds are nutrients for algae, which has the potential to ruin the environment (Koelmans et al. 2001). Under the pressure of worsening global ecological and environmental problems, the concept of ‘Green Chemistry’ was proposed and green scale inhibitors became a focus of water treatment technology.

In recent years, no-phosphorus copolymers have attracted great interest, both in industry and in academia. Polycarboxylates such as polyacrylic acid (PAA), polymaleic acid (PMA) and polyepoxysuccinic acid (PESA) are environmentally benign inhibitors. But they will react with calcium ions to form insoluble calcium–polymer salts, so they have a low calcium tolerance (Wang et al. 2010). Thus, novel scale inhibitors should be further developed to offer a high calcium tolerance and should be environmentally acceptable water additives.

In the present work, a polyether-type scale inhibitor, double hydrophilic block copolymer acrylic acid–allylpolyethoxy carboxylate (AA–APEL) was synthesized. In comparison with traditional scale inhibitors, AA–APEL derived from capped polyether is easily prepared with non-toxic, biodegradable, lower cost, reliable reproducibility and lower dosages, and has superior scale inhibitive performances. In addition, AA–APEL is an environmentally friendly scale inhibitor, only containing the three elements of carbon (C), hydrogen (H), and oxygen (O), and is free of phosphorus (P) and nitrogen (N).

EXPERIMENTAL SECTION

Materials and characterization

APEL was synthesized (Figure 1) from allyloxy polyethoxy ether (APEG) in our laboratory according to Fu et al. (2011). AA was analytically pure grade and was supplied by Zhongdong Chemical Reagent Co. (Nanjing, Jiangsu, China). Distilled water was used for all the studies.
Figure 1
Synthesis of APEL.
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Synthesis of APEL.

Figure 1
Synthesis of APEL.
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Synthesis of APEL.

Structures of APEG, APEL and AA–APEL were also explored with a Bruker NMR analyzer (AVANCE AV-500, Bruker, Switzerland) operating at 500 MHz. The X-ray diffraction (XRD) patterns of the CaCO3 crystals were recorded on a Rigaku D/max 2400 X-ray powder diffractometer with Cu Kα(λ = 1.5406) radiation (40 kV, 120 mA). Powder samples were mounted on a sample holder and scanned at a scanning speed of 2° min−1 between 2θ = 20–60°. The shape of the calcium carbonate scale was observed with a scanning electron microscope (S-3400N, Hitech, Japan). Thermogravimetric analysis (TGA) was performed on samples at temperatures ranging from 25 °C to 600 °C. Such signals were obtained at a heating rate of 20 °C/min in air using a Perkin-Elmer Derivatograph instrument.

Synthesis of AA–APEL

A five-neck round bottom flask, equipped with a thermometer and a magnetic stirrer, was charged with 100 mL distilled water and 0.1 mol APEL and heated to 65 °C with stirring under a nitrogen atmosphere. After that, 1.0 mol AA in 16 mL distilled water (the mole ratio of APEL and AA was 1:10) and the initiator solution (0.5 g ammonium persulfate in 15 mL distilled water) were added separately at constant flow rates over a period of 2.0 h. The reaction was then heated to 75 °C and maintained at this temperature for an additional 1.5 h, ultimately affording an aqueous copolymer solution containing approximately 30% solid. The synthesis of AA–APEL is given in Figure 2.
Figure 2
Synthesis of AA–APEL.
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Synthesis of AA–APEL.

Figure 2
Synthesis of AA–APEL.
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Synthesis of AA–APEL.

Precipitation conditions

All precipitation experiments were carried out in flask tests and the inhibitor dosages given below are on a dry-inhibitor basis. Tests of the inhibitor were carried out using supersaturated solutions of CaCO3 at 60 °C. The solutions were prepared by dissolving in distilled water reagent grade CaCl2 and NaHCO3 (Zhongdong Chemical Reagent Co.). Each inhibition test was carried out in a flask of 500 mL immersed in a temperature-controlled bath for 10 h. Precipitation of CaCO3 was monitored by analyzing aliquots of the filtered (0.22 μm) solution for Ca2+ ions using EDTA complexometry as specified in code GB/T 15452–2009. Inhibitor efficiency was calculated from the following equation: 
formula
where [Ca2+]final is the concentration of Ca2+ ions in the filtrate in the presence of the inhibitor after calcium carbonate supersaturated solutions were heated for 10.0 h at 60 °C, [Ca2+]blank is the concentration of Ca2+ ions in the filtrate in the absence of the inhibitor after calcium carbonate supersaturated solutions were heated for 10.0 h at 60 °C, and [Ca2+]initial is the concentration of Ca2+ ions at the beginning of the experiment.

RESULTS AND DISCUSSION

Characterization of inhibitor

APEG ((CD3)2SO, δppm): 2.50 (solvent residual peak of (CD3)2SO), 3.00–3.80 (−OCH2CH2−, ether groups), 3.80–6.00 (CH2·CH-CH2−, propenyl protons), 4.40–4.60 (−OH, active hydrogen in APEG) (Figure 3(a)).
Figure 3
1H-NMR spectra of (a) APEG, (b) APEL and (c) AA–APEL.
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1H-NMR spectra of (a) APEG, (b) APEL and (c) AA–APEL.

Figure 3
1H-NMR spectra of (a) APEG, (b) APEL and (c) AA–APEL.
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1H-NMR spectra of (a) APEG, (b) APEL and (c) AA–APEL.

APEL ((CD3)2SO, δppm): 2.25–2.55 (−CH2CH2−, protons in −COCH2CH2COOH), 2.50 (solvent residual peak of (CD3)2SO), 3.00–3.80 (−OCH2CH2−, ether groups), 3.80–4.10 and 5.00–6.00 (CH2·CH-CH2−, propenyl protons) (Figure 3(b)).

The δ4.40–4.60 ppm (OH) active hydrogen in (a) disappeared completely and (−CH2CH2−) protons in −COCH2CH2COOH appear obviously in δ2.25–2.55 ppm in (b). This proves that −OH in APEG has been entirely replaced by −COCH2CH2COOH.

AA–APEL ((CD3)2SO, δppm): 2.50 (solvent residual peak of (CD3)2SO), 3.00–3.80 (−OCH2CH2−, ether groups) (Figure 3(c)). The δ3.80–6.00 ppm in (b) double bond absorption peaks completely disappeared in (c). This reveals that free radical polymerization among APEL and AA has happened. From 1H-NMR analysis, it can be concluded that synthesized AA–APEL has the anticipated structure.

Influence of AA–APEL dosage on CaCO3 inhibition

The scale inhibition performance of AA–APEL in simulated scale inhibition solution at different concentrations of the inhibitor is shown in Table 1. For CaCO3 inhibition, 98.6% inhibition were obtained at the concentration of 8 mg/L.

Table 1

Comparison of CaCO3 inhibition (%)

 Dosage (mg/L)
Scale inhibitors246810121416
AA–APEL 17.8 52.6 88.5 98.6 98.3 99.1 98.9 97.9 
T-225 17.6 38.2 51.2 62.3 73.1 78.8 80.7 80.1 
HPMA 11.1 13.9 33.6 49.5 60.2 65.7 68.3 67.8 
PAA 15.0 33.5 45.2 57.6 61.9 67.1 70.6 69.5 
PESA 16.7 26.8 65.9 75.1 78.6 81.2 85.1 84.8 
 Dosage (mg/L)
Scale inhibitors246810121416
AA–APEL 17.8 52.6 88.5 98.6 98.3 99.1 98.9 97.9 
T-225 17.6 38.2 51.2 62.3 73.1 78.8 80.7 80.1 
HPMA 11.1 13.9 33.6 49.5 60.2 65.7 68.3 67.8 
PAA 15.0 33.5 45.2 57.6 61.9 67.1 70.6 69.5 
PESA 16.7 26.8 65.9 75.1 78.6 81.2 85.1 84.8 

Furthermore, to understand the performance, some inhibition experiments were conducted with commercial inhibitors under identical conditions. Compared to commercial inhibitors, AA–APEL had a superior ability to inhibit the CaCO3 scale, with 88.5% inhibition at a level of 6 mg/L, whereas it was 65.9% for PESA at the same dosage (the best inhibitor among them). So when compared to these nonphosphorus inhibitors, CaCO3 inhibition of AA–APEL is much better than that of PESA, T-225, HPMA, PAA at the same dosage. It can be shown that the order of preventing precipitation in the flask tests was AA–APEL>PESA>T-225>PAA ≈ HPMA.

We found that PAA and HPMA contain carboxyl groups and possess molecular structures similar to AA–APEL inhibitor but can hardly control CaCO3 scale even at a high dosage. It may be that the side-chain polyethylene (PEG) segments of APEL and carboxyl groups of AA might play an important role during the control of calcium carbonate scales. Taking Table 1 into account, it can be concluded that the studied copolymer AA–APEL not only solves the water eutrophication problems caused by phosphorus but also has significant inhibition efficiency in cooling water systems.

Influence of solution property on CaCO3 inhibition

Solution properties have a great influence on the precipitation of calcium carbonate. In order to optimize the parameters of the recycling water process on an industrial scale, we investigated the effect of solution parameters on calcium carbonate inhibition by AA–APEL. The results are shown in Figure 4.
Figure 4
Inhibition at a level of 8 mg/L AA–APEL as a function of (a) solution Ca2+ concentration and (b) pH.
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Inhibition at a level of 8 mg/L AA–APEL as a function of (a) solution Ca2+ concentration and (b) pH.

Figure 4
Inhibition at a level of 8 mg/L AA–APEL as a function of (a) solution Ca2+ concentration and (b) pH.
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Inhibition at a level of 8 mg/L AA–APEL as a function of (a) solution Ca2+ concentration and (b) pH.

Figure 4(a) indicates AA–APEL provides unexceptionable calcium carbonate inhibition under conditions of water with a much higher hardness (HCO3− concentration kept constant and at 732 mg/L level). As illustrated in Figure 4(b), calcium carbonate inhibitory power drops 24.1% with increasing the solution pH from 7 to 12. The reason is probably that the solubility of calcium carbonate decreases when increasing the pH. At pH 8.0–9.5, the usual pH values of industry recycling water, AA–APEL still shows superior calcium carbonate inhibition. Thus, the incorporation of the high performance scale inhibitor AA–APEL into recycling water ensures a better overall system performance.

Characterization of calcium carbonate scale

The scanning electron microscope (SEM) images for collected CaCO3 particles with and without inhibitors are shown in Figure 5. Compared with the two images, both the size and shape of the calcium carbonate precipitation were different due to the addition of the AA–APEL copolymer. Without the inhibitor, the CaCO3 crystals had a regular rhombohedron shape with average particle size of about 10–40 μm. They also had a glossy surface and compact structure. This indicated that the CaCO3 crystals without scale inhibitor were mainly composed of calcite, which is the most thermodynamically stable form of CaCO3 crystal. In contrast, when the scale inhibitor was added into the sample, the CaCO3 crystals lost their sharp edges, and the morphology was modified from rhombohedron forms to an irregular spherical with relatively loose accumulation. The irregular spherical CaCO3 particle diameters were 0.5–2 μm.
Figure 5
SEM images of the CaCO3 crystals formed (a) in the absence of AA–APEL and (b) in the presence of AA–APEL.
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SEM images of the CaCO3 crystals formed (a) in the absence of AA–APEL and (b) in the presence of AA–APEL.

Figure 5
SEM images of the CaCO3 crystals formed (a) in the absence of AA–APEL and (b) in the presence of AA–APEL.
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SEM images of the CaCO3 crystals formed (a) in the absence of AA–APEL and (b) in the presence of AA–APEL.

The major components of the scale inhibitor were PAA and PEG. During CaCO3 crystal growth, the PAA and PEG groups could affect the scale inhibition efficiency by occupying the active sites on the surface of the CaCO3 crystals and changing the extent of chemical bonding with the surface.

In addition, the PEG group and −COO− group had a high chelating ability toward calcium ions to form stable chelation compounds. These would interfere with the nucleation and growth of CaCO3 crystals so that the crystals became irregular. The distortion in the CaCO3 crystals increased their internal stress, which would lead to crystal fractures and inhibition of the deposition of microcrystals. Previous studies suggested that vaterite could be more thermodynamically stable than calcite at certain temperatures or in the presence of some inhibitors (Kralj et al. 1997). Thus it was illustrated that the vaterite possessed higher thermodynamic stability than calcite in the presence of the scale inhibitor. Because vaterite has a higher solubility product and free energy than calcite, the scale was easy to dissolve and can be washed away by water.

In order to further investigate calcium carbonate crystals, the XRD was measured (Figure 6). In the absence of a copolymer, only sharp calcite reflections appeared, which coincides with the structure of the calcite crystal standard substance (Figure 6(a)). This suggested that the morphology of calcium carbonate was mainly a component of calcite. However, when the novel copolymer inhibitor was added (Figure 6(b)), both the intense peaks of calcite and vaterite were discovered by comparison of literature data with their XRD. In a typical aqueous system, vaterite is the first phase of calcium carbonate which then changes to a more stable phase (aragonite or calcite) over time (Greenlee et al. 2010). Therefore, the polymer not only can chelate with Ca2+, but can also modify the formation of CaCO3, which illustrates that the studies of XRD gave results consistent with SEM.
Figure 6
XRD image of the CaCO3 crystal formed (a) in the absence of AA–APEL and (b) in the presence of 5 mg/L AA–APEL.
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XRD image of the CaCO3 crystal formed (a) in the absence of AA–APEL and (b) in the presence of 5 mg/L AA–APEL.

Figure 6
XRD image of the CaCO3 crystal formed (a) in the absence of AA–APEL and (b) in the presence of 5 mg/L AA–APEL.
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XRD image of the CaCO3 crystal formed (a) in the absence of AA–APEL and (b) in the presence of 5 mg/L AA–APEL.

The TGA curves for CaCO3 formation in the solution in the absence and presence of the polymer are shown in Figure 7. A comparison between the weight losses of the two kinds of calcium carbonate provided several new and important findings. Apparently, in Figure 7(a), the onset of scale degradation was governed by the decomposition of CaCO3 at a temperature of 647 °C. However, the scale formed in the solution adding inhibitors presented two distinct stages of mass loss. The first peak at 228 °C can be assigned to thermal decomposition of AA–APEL on the sample surface (Song et al. 2010), while the second peaks at 632 °C can be assigned to the degradation of CaCO3 particles. Beyond expectation, CaCO3 decomposed completely at 766 °C (Figure 7(b)), which was lower than the classic temperature of 795 °C (Aubert et al. 2006). This phenomenon is probably related to the polymer embedded in the crystal, making the crystal lattice distorted.
Figure 7
TGA image of the CaCO3 crystal formed in the absence of AA–APEL and in the presence of 15 mg/L AA–APEL.
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TGA image of the CaCO3 crystal formed in the absence of AA–APEL and in the presence of 15 mg/L AA–APEL.

Figure 7
TGA image of the CaCO3 crystal formed in the absence of AA–APEL and in the presence of 15 mg/L AA–APEL.
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TGA image of the CaCO3 crystal formed in the absence of AA–APEL and in the presence of 15 mg/L AA–APEL.

These three kinds of characterization of calcium carbonate scale described in Figures 5–7 indicate that the inhibitor did perform well in reducing the calcium carbonate particles. On the one hand, reduced carbonate scale formation was associated with the chelating of Ca2+ ions, which was attributed to the incorporation of PAA and PEG in the copolymer. On the other hand, the inhibitor was absorbed even embedded in the crystal to modify crystal structure and prevent the formation of dense, well-shaped, strongly adherent particles.

AA–APEL is a structurally well-defined diblock copolymer, depicted in Figure 8(a); the main chains are composed of allyl-terminated AA, denoted as PAA, and the side chains are made of carboxylate-capped polyethylene glycol (PEG) segments. Both PAA and PEG segments are hydrophilic blocks and exist randomly in water (Figure 8(b)). When calcium ions are added into AA–APEL solutions, carboxyl groups in AA–APEL matrixes can recognize and encapsulate or react with positively charged calcium ions either in solutions or on the surface of inorganic minerals, such as CaCO3 (Harada & Kataoka 1999; Yu et al. 2003). Encapsulation or interaction, between calcium ions and carboxyl groups, leads to the spontaneous formation of AA–APEL–Ca complexes (Figure 8(c)). On the other hand, when negatively charged CO32− ions are added into solutions, the positively charged calcium ions also interact with these negatively charged ions thereby forming CaCO3 crystal embryos. As a result, calcium ions acting as ties simultaneously link AA–APEL through carboxyl groups and CO32− ions through an electrostatic attractive force. At the same time, water-compatible PEG segments, that is to say, long side chains of AA–APEL, surrounding the surfaces of crystal embryos are stable toward the aqueous phase because of their high hydrophilic properties (Figure 8(d)). Thus, CaCO3 embryos incorporate into the polymer matrix of AA–APEL, and they are coated with double layers of PAA (inner layer) and PEG (outer layer). As a consequence, the aggregation of CaCO3 solid particles is blocked. In addition, the long PEG side chains in the AA–APEL matrix result in steric and electrostatic repulsion. Therefore, the existing minerals do not precipitate in the presence of AA–APEL through its excellent ability to disperse solid particles.
Figure 8
Encapsulation route of calcium ions via carboxyl groups.
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Encapsulation route of calcium ions via carboxyl groups.

Figure 8
Encapsulation route of calcium ions via carboxyl groups.
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Encapsulation route of calcium ions via carboxyl groups.

CONCLUSIONS

A green calcium carbonate scale inhibitor AA–APEL was successfully synthesized by free polymerization of AA and APEL. 1H NMR identified that AA–APEL has the expected structures. The dosage of AA–APEL copolymer has a strong effect on the formation of calcium carbonate precipitation. It can be concluded that the order of preventing the precipitation was AA–APEL>PESA>T-225>PAA ≈ HPMA.

AA–APEL provides unexceptionable calcium carbonate inhibition under conditions of water with a much higher hardness. At the usual pH values of industry recycling water, AA–APEL shows superior calcium carbonate inhibition.

SEM images indicated that AA–APEL changes highly the morphology and size of calcium carbonate crystals during the inhibition process. XRD images indicated that AA–APEL could not only greatly inhibit the crystal growth of calcite but also transform a large amount of calcite phase to the vaterite phase.

Evaluation of the thermal stability of the CaCO3 scale particles produced in the solution in the absence and presence of AA–APEL copolymer suggest that the copolymer existed in the calcium carbonate crystals. The copolymer embedded in the crystal to modify the crystal structure and prevent the formation of dense, well-shaped, strongly adherent particles.

ACKNOWLEDGEMENTS

The National Natural Science Foundation of China (No. 51077013), China Postdoctoral Science Foundation (No. 2014M560381), Jiangsu Planned Projects for Postdoctoral Research Funds (No. 1401033B), the Project of the Young Scientist Foundation of Nanjing Xiaozhuang University (No. 2013NXY89). The Municipal Key Subjects of Environmental Science and Engineering, Nanjing Xiaozhuang University, Nanjing. University Student Technology Innovation Project of Jiangsu Province (No. 201611460008Z). University Student Technology Innovation Project of Jiangsu Province (School-enterprise cooperation) (No. 201611460085H).

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