Double-hydrophilic block copolymer as an effective and green scale inhibitor in industrial recycling water systems

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).

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.

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.

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.

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.

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.

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.

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.

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.

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).