Synthesis and evaluation of amino acid modified polyepoxysuccinic acid as inhibitor of calcium carbonate scale Open Access

Industrial recycled water treatment technology is one of the most effective ways to conserve industrial water and reduce water pollution (X. H. Liu et al. 2020; Z. Liu et al. 2020). Nevertheless, the recycling of cooling water can cause problems (such as scaling inside condenser tubes and PVC fillers in cooling towers) (Chhim et al. 2020), which lead to a decrease in productivity and waste of energy during the operation of companies (such as power plants) (Haklidir & Haklidir 2017). Many techniques can solve the problem of scaling (Moya & Botella 2021), and the most convenient and effective solution among chemical treatment methods is the addition of water treatment agents (Gitis & Hankins 2018). With the development of industrial circulating cooling water treatment technology, negative effects, such as the eutrophication of water bodies caused by phosphorus-based formulations, have gradually attracted attention (Y. M. Yin et al. 2019; Z. L. Yin et al. 2019), and green inhibitors that are easy to synthesize and environmentally friendly have become an important development direction in the field of water treatment (Chaussemier et al. 2015; Gao et al. 2021).

Polyepoxysuccinic acid (PESA) is phosphorus-free, non-nitrogenous, and easily biodegradable. PESA has a strong chelating effect on Ca²⁺ and especially on CaCO₃, and it is better than other carboxylate-containing scale inhibitors (Chen et al. 2021; Nie et al. 2022). However, PESA has problems, such as by-products and contaminants, in the production and low atomic utilization of raw materials (Leng et al. 2020). Researchers mostly adopted physical or chemical modification techniques to improve their comprehensive performance (Liu et al. 2019). Physical modification is the compounding technology (Xue et al. 2021), whereas chemical modification is the use of ring-opening or graft copolymerization to introduce functional groups on the main chain or branch the chains of PESA molecules to change the polarity and spatial structure of the molecule, making its scale inhibition targeted, multifunctional, and adaptable to complex water quality (Bai & Xu 2020). In terms of the reported research results on PESA modification worldwide, although the synthesized products have improved the scale inhibition rate of PESA to a certain extent, some defects remain, such as high synthesis cost a, certain toxicity of the modifier, secondary pollution of the environment caused by the degraded products, and other technical problems (X. H. Liu et al. 2020; Z. Liu et al. 2020; Zou et al. 2021; Htet & Zeng 2022).

Amino acids (AAs) are amphiphilic organic compounds, and in addition to environmental factors, the presence of heteroatoms (such as S, N, and O) and their conjugated p-electron systems on the molecular structure have attracted the attention of scientists. El Ibrahimi et al. (2020) showed that AAs and their derivatives can be employed as corrosion inhibitors for metals and their alloys. However, studies on their use as scale inhibitors have not been reported. Furthermore, although AAs have been used as raw materials to obtain AA-modified PESA (AA-PESA) with relatively improved properties, a series of PESA functional materials has not been formed (Liu 2015; K. F. Zhang et al. 2021; Y. J. Zhang et al. 2021). In this paper, we have obtained ‘serialized’ AA-PESA by using various AAs as green grafting agents. The synthesis process of AA-PESA is discussed, their scale inhibition rate is evaluated, and their scale inhibition mechanism is analyzed. Given that AAs are widely available, nontoxic, and biodegradable, their use as green grafting agents improves the scale inhibition rate of PESA, and reduces the synthesis cost and secondary pollution to the environment, with a view to providing an effective way to improve the comprehensive performance of PESA and realize its industrialization.

Maleic acid anhydride, sodium tungstate, calcium hydroxide, sodium hydroxide and asparagine were purchased from Sinopharm Group Chemical Reagent Beijing Corp. Ltd. The glutamine, cysteine, hydrochloric acid, calcium carboxylic acid indicator, anhydrous calcium chloride, sodium bicarbonate, potassium hydroxide, disodium EDTA, sodium tetraborate, and potassium chloride were purchased from Tianjin Damao Chemical Reagent Factory. All reagents used were of analytical purity.

An electronic balance (e = 10d) was used to weigh the test materials. Scanning electron microscopy (SEM, SIGMA 300) was used to observe and analyze the microscopic morphology and scale inhibition mechanism of AA-PESAs. Fourier transform infrared (FTIR, IRPrestige-21) spectroscopy was used to analyze changes in the functional groups before and after AA-modified PESA, and contrast morphology change of calcium carbonate scale samples. Nuclear magnetic resonance hydrogen (¹HNMR, Pulsar TM) spectroscopy was used to analyze changes in the functional groups before and after AA-modified PESA. X-ray diffraction (XRD, D/max 2200 PC) was used to contrast morphology change of calcium carbonate scale samples, and analyze the scale inhibition mechanism of AA-PESA. Gel permeation chromatography (GPC, PL-GPC220) was used to analyze the molecular weights and obtain the dispersion coefficients of PESA and three AA-PESAs.

The 9.8 g of maleic anhydride (MA) and 7.5 g of NaOH were weighed and completely dissolved in 15 ml of deionized water to obtain sodium maleate. The oil bath was raised to 50–60 °C, 0.8 g Na₂WO₄, was added, and after waiting for complete dissolution, slowly dropwise 12 mL of 30% H₂O₂ was added. The pH of the mixture was adjusted to pH 7 with an appropriate amount of 50% NaOH solution, the temperature was raised to 70 °C, and the reaction was cyclized for 2 h to get epoxy succinic acid (ESA). The solution was cooled to room temperature and purified by adding 15 mL of acidified methanol to obtain the mucilaginous substance PESA. The reaction process was as follows.

(1) Synthetic Mechanism

(2) Synthetic Process

The optimal synthesis process of AA-PESA scale inhibitors was determined by single-factor experiment and orthogonal test. The process is as follows. Certain amounts of PESA and AA were weighed in a three-necked flask in accordance with a certain molar ratio. An appropriate amount of deionized water was added to the mixture and stirred to dissolve. The oil bath was raised to a certain temperature. The pH of the mixture was adjusted to pH 5 with an appropriate amount of acetic acid, and the mixture was cooled to room temperature after a certain reaction time, 15 mL acidified methanol was added to purify and obtain AA-PESA, and it was dried at 50 °C for use.

Quantities of 10 mg of PESA and three AA-PESA were weighed respectively, and the molecular weight was determined by applying the gel permeation chromatography (GPC) technique using water as the solvent. The instrument parameters were as follows: injection rate of 0.5 mL/min, mobile phase of sodium nitrate solution, temperature of 45 °C, and protection column of Ultrahydrogel.

The samples PESA, ASN-PESA, GIN-PESA and CYS-PESA were mixed with spectrally pure KBr in the ratio of 1:100 and scanned with an IRPrestige-21 infrared spectrometer in the range of 500 cm⁻¹–4,000 cm⁻¹.

Using 0.50 mL of D₂O as the solvent, 10 mg of PESA and three AA-PESA were dissolved in the NMR test tube, and the NMR hydrogen spectra of the four were measured by a Pulsar TM NMR hydrogen spectrometer at room temperature.

IR tests were performed on the blank scale samples and the scale samples after adding scale inhibitor, and the testing conditions were as described in Section 2.5.

The D/max 2200 PC powder diffractometer was used to perform XRD tests on calcium carbonate samples under different conditions. The test conditions were as follows: tube current 40 mA, tube voltage 40 kV, scan range 3° ≤ 2θ ≤ 85°.

A SIGMA 300 energy field emission scanning electron microscope was used for SEM testing of CaCO₃ scale. The test conditions were as follows: accelerating voltage 20 kV, magnification 5,000 times.

The effect of molar ratio on the scale inhibition rate of AA-PESA is investigated under the conditions of reaction temperature of 95 °C, reaction time of 2 h, and scale inhibitor concentrations of 8 mg/L. Results are shown in Figure 5(a). As the amount of AA added increases, the scale inhibition rate tends to increase gradually, and reaches the maximum when n(PESA):n(AA) = 1:0.625. If AA is further increased, the scale inhibition rate decreases. This finding is due to the large molecular weight and the long water-soluble polymer chain, which increase the adsorption bridging in the solution and accelerate the flocculation and precipitation of fine particles in the solution. This phenomenon results in the weakening of scale inhibition (Yang et al. 2019). Therefore, from the economic point of view, n(PESA): n(AA) = 1:0.625 is chosen.

Under the conditions of n(PESA):n(AA) = 1:0.625, reaction time of 2 h, and scale inhibitor concentration of 8 mg/L, the effect of reaction temperature on the scale inhibition rate of AA-PESA is explored, and results are shown in Figure 5(b). With the increase in reaction temperature, the scale inhibitions of the three AA-PESAs are the first to increase and then decrease and reach the maximum value of 100% at 95 °C. The reason is that when the temperature is low, the reaction activity is low, the reaction rate is slow, and the grafting rate of the products is low, leading to a decrease in scale inhibition. When the reaction temperature is too high, problems such as difficulty in controlling the reaction rate and the decomposition of AA-PESA arise, resulting in a drop in scale inhibition (Jia et al. 2020). Hence, the optimal temperature for AA-PESA synthesis is 95 °C.

Under the conditions of n(PESA):n(AA) = 1:0.625, reaction temperature of 95 °C, and concentration of scale inhibitor of 8 mg/L, the effect of reaction time on the scale inhibition rate of AA-PESA is studied. Results are illustrated in Figure 5(c). With prolonged reaction time, the scale inhibition of ASN-PESA and GIN-PESA increase and then decrease, and the scale inhibition rate reaches the maximum (98.64% and 100%, respectively) at 2 h. The primary reason is that the short time is not conducive to the full reaction between AA and PESA, and the grafting rate is low, which leads to decreased scale inhibition. Subsequently, as the reaction reaches equilibrium with the increasing time, a side-reaction or decomposition reaction occurs, which diminishes the chelation effect and leads to the decline of scale inhibition (Liu et al. 2021). For CYS-PESA, the two highest points are observed at 1.5 and 2.5 h. This finding is because the short chain length and low spatial site resistance of CYS can fully react with PESA within 1.5 h, and the grafting rate is high, with a scale inhibition rate of 98.9%. At 2 h of reaction, the repulsion and entanglement between branched chains lead to a decrease in scale inhibition rate to 86.8%. At 2.5 h of reaction, the grafting rate can be further increased and still maintain its effective adsorption of Ca²⁺ with a scale inhibition rate of 95.8%. Therefore, the optimal reaction times of ASN-PESA, GIN-PESA, and CYS-PESA are 2, 2 and 1.5 h, respectively.

In summary, the results of the single-factor experimental study for the optimization of the AA-PESA synthesis process conditions are: n(PESA):n(AA) = 1:0.625, reaction temperature of 95 °C, and optimal reaction times of 2, 2 and 1.5 h for ASN-PESA, GIN-PESA and CYS-PESA, respectively.

The synthesis conditions of AA-PESA are further optimized by orthogonal experiments based on the evaluation of the inhibition performance of AA-PESA on the CaCO₃ scale formation process. The effects of four factors, namely, type of AA (A), n(PESA):n(AA) (B), reaction temperature (C), and reaction time (D), on the scale inhibition rate of the product are investigated. Three levels are used for each factor. Orthogonal experiment factor levels are shown in Table 2, and orthogonal experiment results are shown in Table 3.

Table 3 shows that R_C > R_A > R_B > R_D, indicating that the C factor has highest influence on the scale inhibition rate among the four factors. The best scheme for AA-PESA is A₁B₂C₂D₂ from the visual analysis, i.e. the grafting agent is ASN, n(PESA):n(AA) = 1:0.625, the reaction temperature is 95 °C, and the reaction time is 2 h. At this time, the scale inhibition rate is 100%. Additional experiments are conducted for the orthogonal experiments to further investigate the inhibition of CaCO₃ by the two other grafting agents under optimal scheme conditions, and results are shown in Table 4.

As shown in Tables 3 and 4, the three AA-modified PESAs have the same synthesis process parameters, i.e. n(PESA): n(AA) = 1:0.625, reaction temperature of 95 °C, and reaction time of 2 h. The scale inhibition rate of 100% can be achieved under the conditions of a scale inhibitor concentration of 8 mg/L. Compared with the scale inhibition rate of 92.5% at 10 mg/L for unmodified PESA described in Section 2.2.1 of this paper, those for AA-PESAs are improved. This finding indicates that the introduction of reactive groups, such as amide groups on the side-chains of PESA molecules by AA, can significantly improve the scale inhibition effect.

In summary, the three AA-PESA scale inhibitors can be used in the following environments: pH 7.0–8.5, agent concentration of 8–10 mg/L, action period of 10–14 h, ambient temperature < 80 °C, and ρ(Ca²⁺) < 250 mg/L water system. These conditions result in the inhibition rate of CaCO₃ reaching 100%, and GIN is advisable to adopt for the graft modification of PESA.

• (1)

  Three AA-PESA scale inhibitors were obtained by functionalizing PESA with AAs as green grafting agents. The comparison of FTIR, ¹HNMR, and GPC spectra before and after PESA modification showed that the target molecular structures were synthesized correctly and that the heavy average molecular weight of all three AA-PESAs was more than 3,500 g/mol. These values were larger than those reported in the literature (Chhim et al. 2020). The D values of PESA and the three AA-PESAs were all around 1.20, indicating that the damage to the long chain of PESA was minimal after modification. This finding demonstrated the feasibility and reproducibility of the modification experiments and the rationality of the process and improved the idea for industrial production.

• (2)

  The synthesis process of AA-PESA was carried out by single-factor experiment and orthogonal test. Results showed that the three AA-PESAs had the same synthesis process parameters, i.e. n(PESA): n(AA) = 1:0.625, reaction temperature of 95 °C and reaction time of 2 h, and the scale inhibition rate could reach 100% at a scale inhibitor concentration of 8 mg/L. Compared with the existing CYS-PESA, AA-PESA achieved 100% scale inhibition of CaCO₃ at the same concentration of 8 mg/L, which could reduce the cost of water treatment (Liu et al. 2015; Yang et al. 2020). The introduction of reactive groups, such as amide groups, on the side chains of PESA molecules by AAs could lead to a significant increase in scale inhibition.

• (3)

  The scale inhibition rate of AA-PESA was evaluated by the static scale-inhibition method. Results showed that the inhibition rate of CaCO₃ by the three AA-PESAs could reach 100% in the water system with pH 7.0–8.5, agent concentration of 8–10 mg/L, action period of 10–14 h, ambient temperature < 80 °C, and ρ(Ca²⁺) < 250 mg/L. Concentration and hardness threshold effect were observed, and GIN for graft modification of PESA was advisable to recommend.

• (4)

  The mechanism of scale inhibition of CaCO₃ by AA-PESA was analyzed using IR, XRD, and SEM techniques. Results showed that the presence of AA-PESA could cause the lattice distortion of CaCO₃ scale from calcite-type to spherulite-type and inhibit the formation and growth of the calcite (104) crystalline surface. In the process of the alternating arrangement of Ca²⁺ and CO₃²⁻ in layers along the c-axis, the oxygen coordination number of Ca²⁺ increased from 6 to 12, and spherulite-type CaCO₃ crystals were formed. The texture of calcium scale was changed into loose and porous ‘soft scale’, thus achieving the purpose of scale inhibition.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.