3.1. Structure of Prepared Polymers
The condensation polymerization may occur between carboxyl groups of EDTA and the hydroxyl groups or amine groups of DEA (as shown in Figure 1
). There are four carboxyl groups in each EDTA molecule (see Figure 1
a), and two hydroxyl groups and one amine group in each DEA molecule (see Figure 1
b). According to chemical principles, an esterification reaction occurs between a carboxyl group and a hydroxyl group or an amidation reaction occurs between a carboxyl group and an amine group, catalyzed by acid or base as a catalyst. Thus, EDTA and DEA can be thermally polycondensed to produce polymers. As mentioned above, the configuration of the polymer is not necessarily a mono-linear polymerization, but may be branched. The polymer presented in Figure 1
consists of a plurality of carboxyl groups, and can continue to react to form an ester bond or an amide bond. In short, there are many carboxyl, ester and amide groups in PEDTA-DEA.
Infrared spectra of EDTA and DEA are obviously different from the polymer products. In the infrared spectrum of EDTA (curve a in Figure 2
A), the stretching vibration peaks of -CH2
- are 3017 cm−1
and 1439 cm−1
and the C=O bond in the carboxylic acid group is 1683 cm−1
. The peak at 1388 cm−1
is the C-N bond, 1310 cm−1
is the bending vibration of the C-C bond and 765 cm−1
is the stretching vibration peak of the C-O bond in C-OH. In the infrared spectrum of DEA (curve b in Figure 2
A), 3296 cm−1
is the stretching vibration peak of -OH while 2837 cm−1
is the stretching vibration peak of -CH2
-. -NH- bond vibration peaks are at 1453 cm−1
(in-plane bending) and 861 cm−1
(out-of-plane vibration), respectively. The in-plane bending vibration peak of the O-H bond is 1361 cm−1
, 1122 cm−1
is the stretching vibration peak of the C-N bond and 1050 cm−1
is the stretching vibration peak of the C-O bond. In the infrared spectrum of the polymer (curve c in Figure 2
A), the stretching vibration peak of -CH2
- is at 2975 cm−1
and 1501 cm−1
. The peaks at 1654 cm−1
and 1648 cm−1
are the carbonyl groups in the suspension carboxylic -COOH, amide groups R-CO-NR2
and ester groups -CO-O-. Moreover, the vibration peak of the C-O bond in the carbonyl group is at 1466 cm−1
Comparing the three infrared spectra in Figure 2
A, the newly generated peak at 1397 cm−1
is the stretching vibration peak of the C-N-C bond and the peak at 1167 cm−1
is the stretching vibration peak of the C-N in the amide bond (curve c in Figure 2
A). Simultaneously, the intensity of the peaks of -C-O-(H) and -NH- in curve c of Figure 2
A are both weaker than that of the peaks in curve a and curve b of Figure 2
A. It can be known these groups will reduce when the polymer is formed, all of which indicate that the two monomers have successfully transferred into the polymer.
H NMR was recorded on Bruker AVANCE spectrometer (as shown in Figure 2
B–D). Chemical shifts (δ) were expressed in ppm relative to the residual of solvent (DMSO 2.5 ppm for 1
H NMR). Coupling constants (J) were recorded in hertz (Hz). Multiplicities explained by the following abbreviations: s, singlet; d, doublet; t, triplet; q, quartet.
1H NMR (500 MHz, DMSO-d6, ): EDTA: 2.76 (s, 4H); 3.33 (s, 8H); 3.45 (s, 4H). DEA: 2.56 (t, J1 = 10 MHz, 4H); 3.43 (t, J2 = 0.5 MHz, 4H); 3.84 (s, 3H). PEDTA-DEA: 2.86 (d, J3 = 80 MHz, the peak of H in the unreacted carboxyl group); 2.96 (d, J4 = 55 MHz, the peak of H in the methylene group linked to the carboxyl group.); 3.56 (s, the peak of H in amino group).
The stability of the polymer was determined by thermogravimetry and the results are shown in Figure 2
E. When the test temperature of thermogravimetric analysis (TGA) is at the range of 100 °C–200 °C, the stability of the polymer is similar to that of the raw material. When the temperature is over 250 °C, the polymer is much more stable than EDTA and DEA. The highest temperature used in this experiment was 180 °C, so PEDTA-DEA has good stability.
The molecular weight of the polymer is measured by size exclusion chromatography. The number average molecular weight (Mn) of the PEDTA-DEA was 1.97 × 104 Da, and the polydispersity coefficient of the polymer was 1.4. All of these illustrate that the condensation polymerization was carried out efficaciously.
3.2. Precipitation Inhibition Performance of Prepared Polymers
The precipitation inhibition of the prepared polymers was evaluated by a static scale inhibition test. It can be observed that all of the polymers have distinct inhibition performance against calcium scale crystal precipitation. When the dosage of the polymer inhibitor is 20 mg/L, the inhibition rate can reach 93.2%, and the inhibition effect will not increase much as the dosage of polymer increases. Some of other inhibitors show similar performance to calcium sulfate [8
] and all manifested threshold effects.
The precipitation inhibition properties of the polymers are related to the types and amounts of functional groups on the polymer. The main functional group that is capable of inhibiting calcium sulfate may be a carboxylic acid group (-COOH) [26
]. The molecular structure of the polymer is shown in Figure 1
, and the molecular structures of PBTCA, HEDP and PASP are presented individually in Figure 3
. Although prepared polymer and PBTCA molecules all have a large number of carboxylic acid groups, there are differences between the polymer, PBTCA and HEDP. Polymer molecules include ester groups (-CO-O-) and amide groups (-CO-NH-), compared with each PBTCA molecule with a phosphonate group [-PO(OH)2
] and three carboxylic groups. In a further step, HEDP has two phosphonate groups [-PO(OH)2
] and a hydroxyl group (-OH), EDTA has four carboxylic groups (-COOH) and DEA has two hydroxyl groups (-OH), respectively.
The differences of molecular structure lead to different precipitation inhibition performances. According to the static scale inhibition tests, the inhibition rates of the synthesized polymer on calcium sulfate and calcium carbonate are better than those of EDTA, DEA, hydroxyethylenediphosphonic acid (HEDP), 2-phosphonbutane-1,2,4-tricarboxylic acid (PBTCA), polyaspartic acid (PASP) and the compound (the combination of two monomers) (as shown in Figure 4
A). The little molecule compounds, EDTA, DEA or the combination of monomers, are not suitable inhibitors for calcium sulfate scale, because the scale inhibition rate is still less than 10% even when the dosage is 25 mg/L. Although the inhibition rate of 25 mg/L PBTCA is up to 50% on the calcium sulfate scale, it is not as functional an inhibitor as the polymer. This is due to the fact the carboxyl group, ester bond and amide bond have scale inhibition effects on calcium sulfate and can chelate with calcium ion, making it difficult to form scale. After adding a scale inhibitor, the scale inhibition performance has not been obviously improved. Although test conditions are different, similar test results [26
] were obtained.
Compared with other inhibitors, the scale inhibition effect of the synthesized polymer on calcium carbonate is inclined to be better (refer to Figure 4
B). When the polymer concentration is 15 mg/L, the precipitation inhibition rate can reach about 70%. Therefore, diethylenediamine tetraacetate diethanolamine is a kind of polymer with inhibition on calcium sulfate and calcium carbonate precipitation.
3.3. Different Conditions of Inhibition Test
The initial Ca2+
concentration in the solution also has influences on the precipitation inhibition performance of prepared polymers against calcium sulfate. As the calcium ion concentration increases, the inhibition rate decreases. Figure 5
A reveals the inhibition of the polymer against calcium sulfate at different initial Ca2+
concentrations. It can be found that when the Ca2+
concentration is 2040 mg/L, the scale inhibition effect of the polymer achieves the best performance. When the polymer concentration is 3 mg/L, the scale inhibition rate reaches 89.58%. As the calcium ion concentration increases, the inhibitor effect of the polymer becomes worse. However, when the calcium ion concentration is 4000 mg/L and the amount of polymer is 25 mg/L, the scale inhibition rate can also reach 55.6%.
With the rise of the concentration of calcium ions, the probability of collision and recombination between calcium ions and sulfate ions increases, the precipitation is easier to appear, and the inhibition effect weakens.
The inhibition performance of polymers on calcium sulfate precipitation is affected by the heating time of the water bath, as shown in Figure 5
B. The inhibition rates of the polymer were determined at 6 h, 10 h, 14 h, 24 h and 36 h, and did not change much at different water bath times. When the polymer concentration is 10 mg/L, its scale inhibition rate reaches 90%. It can be considered that the synthetic polymer scale inhibitor possesses a strong ability to chelate calcium ions. The calcium ions’ precipitation from the solution was still delayed despite the increase of heating time. The results show that the effect of heating time in the water bath on the polymer is small, so that it can inhibit the precipitation of calcium sulfate for a long time.
Temperature is a significant factor affecting precipitation inhibition efficiency. Figure 5
C demonstrates the inhibition performance of polymer precipitation inhibitor on calcium sulfate when the water bath temperature is differently set at 50 °C, 80 °C, 120 °C, 150 °C and 180 °C.
With the temperature of the water bath increasing, the precipitation inhibition performance of precipitation inhibitor against calcium sulfate is getting worse (according to Figure 5
C). When the temperature of the water bath rises, the movement of the molecules will become more intense. The probability that calcium ions collide with the sulfate ions will also increase, so that the precipitation will be formed more easily. As a result, the effect of precipitation on calcium sulfate becomes worse and worse.
3.4. Characterization and Analysis of Calcium Sulfate Precipitate
The precipitation of CaSO4
prepared by adding different doses of precipitation inhibitor is compared with that without precipitation inhibitor. The changes of crystal morphology are manifested in Figure 6
Most of the CaSO4
precipitation particles without polymer inhibitor are slim and tenuous. The surface of the needle-like precipitation is flat and smooth. With the addition of the polymer inhibitor, the originally smooth surface becomes irregular and needle-like precipitation becomes wide and thick. When the dosage of precipitation inhibitor increases, the surface of the CaSO4
precipitation particles becomes uneven and many defects appear. The crystal also changes from a regular, long strip to a short, irregular crystal. These changes illustrate that adding precipitation inhibitor has a great influence on the growth process and morphology of CaSO4
]. Calcium precipitation added with precipitation inhibitor is more likely to flow with water rather than adhere to the pipeline. There are inevitably solid impurities in the water [37
] which will become the nucleation center of gypsum. After adding scale inhibitor, the scale inhibitor will be adsorbed on the surface of solid impurities in the form of calcium salt, thus blocking the nucleation of calcium sulfate and preventing the formation of scale [37
In order to further investigate the calcium sulfate crystal, the precipitation obtained from the static scale inhibition experiment was subjected to XRD testing. The diffraction patterns of calcium sulfate crystals with and without the inhibitor are shown in Figure 7
In the absence of precipitation inhibitor, the diffraction peaks are mainly distributed at 14.7°, 25.6° and 29.7°, which are the diffraction peaks of calcium sulfate crystals lattice planes of (100), (110) and (200), respectively. After adding polymer inhibitor, the main diffraction peaks appear at 11.6°, 20.7°, 23.4°, 29.1° and 31.1°, corresponding to lattice faces of (020), (021), (040) (041) and (−221) of gypsum (syn) [39
]. What is mentioned above indicates that the polymer has altered the crystal type of calcium sulfate precipitation.
The calcium sulfate precipitation is characterized by infrared spectroscopy (shown in Figure 8
The infrared absorption peak of calcium sulfate precipitation mainly appearing at 3487 cm−1
, 3396 cm−1
and the peak at 1681 cm−1
are the stretching vibration peaks of the hydroxyl group, the peaks at 666 cm−1
and 597 cm−1
represent the stretching and bending vibration of sulfate, the peak at 1096 cm−1
is the stretching vibration of S-O and the peak at 1619 cm−1
is the stretching vibration of S=O [41
]. If we choose the intensity of absorption peaks at 1096 cm−1
(I1096) as standard, the intensities of absorption peaks at 1681 cm−1
, 666 cm−1
and 597 cm−1
will obviously become weaker, while the intensities of absorption peaks at 3487 cm−1
, 3396 cm−1
and 1619 cm−1
will be stronger. Compared with the hydroxyl vibration peak of calcium sulfate, the tendency of hydroxylation is strong, which indicates that the crystal form of calcium sulfate has changed [43
]. This is also consistent with the results of the XRD.
As mentioned above, the crystal structure of the calcium sulfate precipitation can be significantly changed by adding the polymer inhibitor. That is the lattice distortion caused by the polymer inhibitor in the precipitation formation process. The polymer molecules adsorbing to the surface of the CaSO4 crystals not only hinder the growth of CaSO4 crystals but also make the crystal structure unstable and distorted, causing the formed CaSO4 precipitation to be easily washed away.