SYNTHESIS AND EVALUATION OF PHOSPHORUS-FREE POLYMER-BASED INHIBITORS FOR SCALE CONTROL IN INDUSTRIAL SYSTEMS

ABSTRACT

Scale inhibitors are an efficient and cost-effective method to prevent scale formation in cooling water systems. This study reports the synthesis of a novel phosphorus-free scale inhibitor obtained by chain extension of N,N′-ethylenebismaleamic acid with ethylenediamine. The structure of the synthesized compound was confirmed by infrared and nuclear magnetic resonance spectroscopy. The effects of reaction conditions on product yield and viscosity were investigated. The results showed that an increase in viscosity leads to improved scale inhibition efficiency. The maximum inhibition efficiency reached 83.61% at a concentration of 20 mg·L⁻¹, which is higher than that of polyaspartic acid. Mechanistic analysis demonstrated that chelation, lattice distortion, adsorption, and dispersion play important roles in the inhibition process. This study provides a practical approach for the development of environmentally friendly, phosphorus-free scale inhibitors.

АННОТАЦИЯ

Ингибиторы отложений являются эффективным и экономичным методом предотвращения образования накипи в системах охлаждающей воды. В данной работе представлен синтез нового бесфосфорного ингибитора, полученного путем удлинения цепи N,N′-этиленбисмалеаминовой кислоты с использованием этилендиамина. Структура синтезированного соединения подтверждена методами инфракрасной и ядерно-магнитно-резонансной спектроскопии. Исследовано влияние условий реакции на выход продукта и вязкость. Установлено, что увеличение вязкости приводит к повышению эффективности ингибирования накипи. Максимальная эффективность ингибирования составила 83,61 % при концентрации 20 мг·л⁻¹, что выше по сравнению с полиаспарагиновой кислотой. Механистический анализ показал, что процессы хелатообразования, искажения кристаллической решётки, адсорбции и диспергирования играют важную роль в ингибировании. Данная работа предлагает практический подход к разработке экологически безопасных бесфосфорных ингибиторов накипи.

Keywords: Scale inhibitor, phosphorus-free inhibitor, cooling water systems, calcium carbonate, scale prevention.

Ключевые слова: Ингибитор накипи, бесфосфорный ингибитор, охлаждающая вода, карбонат кальция, предотвращение отложений.

Introduction

Water scarcity is a pressing global issue, highlighting the need for efficient water management in industrial processes, often achieved through circulating cooling water systems. However, scale formation on heat exchange surfaces, caused by ions such as Ca²⁺, Mg²⁺, HCO₃⁻, and SO₄²⁻, significantly reduces heat transfer efficiency and threatens equipment safety [1, 2]. Scale inhibitors are widely used to mitigate this problem due to their high efficiency at low dosages, mainly by affecting nucleation and crystal growth of calcium salts [3, 4].

Polyphosphates and phosphonates exhibit excellent inhibition performance, but their phosphorus content contributes to eutrophication and environmental pollution [9, 10]. Therefore, the development of low-phosphorus and phosphorus-free inhibitors has become increasingly important. Polycarboxylic acid-based inhibitors, such as polymaleic anhydride, polyacrylic acid, and related copolymers, are widely applied due to their effectiveness, although their poor biodegradability remains a concern [5, 7].

Recently, environmentally friendly inhibitors based on biodegradable materials have attracted considerable attention. Compounds such as polyepoxysuccinic acid and polyaspartic acid show good performance and improved environmental compatibility [3, 6]. Natural macromolecules, including starch, chitosan, and tannins, also exhibit strong complexation ability with metal ions; however, their high cost and relatively low efficiency at elevated temperatures limit their industrial application [1, 6].

In recent years, ethylenediamine-based compounds have shown promising scale inhibition performance. However, studies on high-performance phosphorus-free inhibitors derived from such compounds remain limited.

In this work, a new phosphorus-free scale inhibitor was synthesized through a reaction between maleic anhydride and ethylenediamine followed by chain extension. The structure of the obtained product was confirmed using infrared and nuclear magnetic resonance spectroscopy. The scale inhibition performance against calcium carbonate was evaluated, and the mechanism was investigated using modern analytical and computational methods.

The aim of this study is to synthesize a novel phosphorus-free scale inhibitor and evaluate its efficiency and mechanism of action in preventing calcium carbonate scale formation.

Materials and methods

2.1. Materials

Ethylenediamine (EDA), maleic anhydride (MAH), anhydrous ethanol (EtOH), sodium hydroxide, hydrochloric acid, acetone, ethyl acetate, sodium tetraborate, potassium hydroxide, disodium ethylenediamine tetraacetic acid (EDTA), sodium thiocyanate, anhydrous sodium sulfate, anhydrous calcium chloride, sodium bicarbonate, and polyaspartic acid (PASP) were obtained from commercial suppliers. All chemicals were analytical grade with a purity ≥99.5%.

2.2. Synthesis of EBMAA

EBMAA was synthesized by reacting MAH with EDA under ice-bath stirring. After the slow addition of EDA, the mixture was stirred for 30 min. The crude product was collected, recrystallized with distilled water, washed, and dried to obtain EBMAA (melting point 195.2–195.5 °C).

2.3. Synthesis of PEBMAA-EDA

EBMAA, water, and NaOH were mixed, followed by the addition of EDA under stirring. The mixture was refluxed at 90 °C for 3 h to yield a yellowish viscous solution. The product was precipitated three times with anhydrous ethanol and dried at 140 °C to constant weight, yielding PEBMAA-EDA.

2.4. FT-IR and NMR Characterization

Functional groups of EBMAA and PEBMAA-EDA were analyzed by FT-IR (Nicolet-310, Thermo Electron, USA) with ATR attachment. Structures were further confirmed using ^1H NMR for EBMAA and ^1H/^13C NMR for PEBMAA-EDA (Bruker AV500M, Germany) in DMSO-d6 and D2O solvents, respectively.

2.5. Determination of Intrinsic Viscosity

The intrinsic viscosity ([η]) of PEBMAA-EDA was determined according to HB/T 3822–2020. The sample (0.8–1 g) was dissolved in 1.25 mol·L⁻¹ sodium thiocyanate solution, filtered, and measured using an Ubbelohde viscometer at 30 °C. [η] was calculated as:

where t₀​ is the flow time of the blank, t is the flow time of the solution, and c is the concentration (g·dL⁻¹).

2.6. Static Scale Inhibition Tests

Scale inhibition performance against CaCO₃ was evaluated according to GB/T 16,632–2019. Solutions containing the inhibitor were heated at 80 °C for 10 h. Ca²⁺ concentration was determined via titration with EDTA, and the scale inhibition rate (ε, %) was calculated as:

where ρ is the Ca²⁺ concentration before heating, ρ₁ after heating the blank, and ρ₂ after heating the test solution.

Results and discussions

3.1. FT-IR Analysis

The FT-IR spectra of MAH, EDA, EBMAA, and PEBMAA-EDA are shown in Fig. 1. The characteristic absorption peaks of MAH and EDA, such as the cyclic anhydride peaks at 1854 cm⁻¹ and 1777 cm⁻¹ and the amino group peaks at 3361 cm⁻¹ and 3289 cm⁻¹, disappear in EBMAA and PEBMAA-EDA. New peaks corresponding to amide and carboxyl groups confirm the successful synthesis of EBMAA and its polymerization with EDA.

Figure 1. FT-IR spectra of MAH, EDA, EBMAA, and PEBMAA-EDA

3.2. NMR Analysis

¹H and ¹³C NMR spectra further validate the structures of EBMAA and PEBMAA-EDA (Fig. 2). In EBMAA, peaks at δ = 14.62 ppm and δ = 9.01 ppm correspond to -COOH and -CONH- protons, while the vinyl proton signals disappear in PEBMAA-EDA, indicating complete polymerization. ¹³C NMR spectra show characteristic carbon signals confirming the formation of the target polymer.

Figure 2. H and ¹³C NMR spectra of EBMAA and PEBMAA-EDA

3.3. Effect of Reaction Conditions on EBMAA Yield

The effects of solvent type, reaction temperature, and reactant molar ratio on EBMAA yield were investigated (Fig. 3). Ethanol provided the highest yield (95.08%), while water gave the lowest (52.20%). The optimal reaction temperature was between 0–5 °C, and the best molar ratio of EDA to MAH was 1:2.1, balancing yield and cost-effectiveness.

Figure 3. Influence of reaction temperature and reactant molar ratio on EBMAA yield

3.4. Scale Inhibition Performance of PEBMAA-EDA

The intrinsic viscosity and scale inhibition rate of PEBMAA-EDA increased with reaction temperature, time, and optimal monomer ratio, reaching 0.13 dL⋅g⁻¹ and 83.61%, respectively (Fig. 4). Compared to commercial PASP, PEBMAA-EDA demonstrated superior scale inhibition due to the higher content of carboxyl and amine groups, which enhance chelation with Ca²⁺ ions.

Figure 4. Scale inhibition rate of PEBMAA-EDA and PASP under different conditions

3.5. Mechanistic Insights

Molecular simulations and analyses revealed that the carboxyl and amide groups of PEBMAA-EDA interact strongly with Ca²⁺ ions, providing multiple active sites for scale inhibition. XRD and SEM analyses showed that PEBMAA-EDA altered the crystal morphology of CaCO₃ from calcite to vaterite and aragonite, effectively preventing further deposition (Fig. 5). The binding energies between the inhibitor and calcite surfaces confirmed the stronger adsorption and enhanced inhibition compared to PASP.

Figure. 5. Mechanistic illustration and representative XRD/SEM images of CaCO₃ crystals

4. Conclusion

In this study, a novel phosphorus-free scale inhibitor, PEBMAA-EDA, was successfully synthesized. The optimal synthesis conditions were determined to be a reaction temperature of 90 °C, a reaction time of 3 h, and a molar ratio of n(EBMAA):n(EDA) = 1:1.

The obtained results demonstrated that PEBMAA-EDA exhibits significantly higher scale inhibition performance against CaCO₃ compared to PASP, achieving a maximum inhibition efficiency of 83.61% at a concentration of 20 mg·L⁻¹.

Molecular dynamics and DFT analyses revealed that both carboxyl (–COO⁻) and amide (–NH–) groups in PEBMAA-EDA actively participate in binding with Ca²⁺ ions, resulting in stronger chelation ability than PASP.

Furthermore, XRD and SEM analyses confirmed that PEBMAA-EDA alters the crystal structure of CaCO₃ from dense calcite to more dispersed vaterite and aragonite forms, leading to lattice distortion and reduced scale deposition.

Overall, the enhanced interaction between functional groups and Ca²⁺ ions plays a key role in improving scale inhibition performance, providing valuable insights for the design of efficient and environmentally friendly scale inhibitors.

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