Introduction

The pipeline inner walls of circulating cooling water system often face the challenge of inorganic scale deposition upon exposure to a large amount of poorly soluble inorganic salts, which could cause huge economic losses and operational problems in facilities such as reverse osmosis desalination system, multi-stage flash desalination system, and multi-effect distillation system1,2,3.

Therefore, it is imperative to introduce a scale inhibitor to get rid of the scale formation on the inner walls of the pipelines of industrial circulating water. Some plant extracts and phosphorous compounds are of particular significance for inhibiting scale formation4,5. For example, Gambier extract as a green inhibitor can efficiently inhibit the scale formation of calcium carbonate; and the natural extract of tobacco rob can function as a scale- and corrosion-inhibitor in artificial seawater6,7. The reason lies in that the plant extracts contain a large amount of amino and carboxyl functionalities which have the strong complexing ability with Ca2+. However, the huge amount of plant extracts could cause potentially deteriorating the ecological environment8; and phosphorous compounds might lead to serious environmental risks of water eutrophication9,10. Therefore, the exploitation of environmentally friendly scale inhibitors for water treatment is of vital importance.

Polyaspartic acid (denoted as PASP) as an environmentally friendly additive is nontoxic, biodegradable, and free of phosphorus10,11,12. However, its scale inhibition performance is poor, which limits its application in the industry. Since the scale inhibition performance of polymer scale inhibitors is highly dependent on their carboxyl, hydroxyl, and acylamido groups with strong ionic chelating ability13,14,15, we introduce these functional groups into the PASP molecule through the copolymerization of l-aspartic acid or the ring-opening modification of polysuccinimide (denoted as PSI) to obtain functionalized PASP with improved scale inhibition performance. The copolymerization modification mainly involves the reaction of aspartic acid with other monomers, which, in combination with the introduction of other functional groups into the molecular chain of aspartic acid, could contribute to improving the scale inhibition performance to some extent. For example, Dong et al.16 reported that curcumin, a fluorescent monomer, can be introduced into the PASP molecular chain in the presence of l-aspartic acid and citric acid; and the as-synthesized scale inhibitor exhibits superior fluorescent-tagged properties and excellent scale inhibition performance. However, l-aspartic acid is expensive and faces challenges with industrial production. Fortunately, highly active linear PSI can be easily cyclized by amino to form PASP with side chains17, which provides a feasible pathway to yielding functionalized PASP. Chen et al.18 reported an interesting exploration in this respect: they synthesized a scale inhibitor with PSI and histidine as the starting materials. Thanks to the introduction of an amino group and carboxyl group to inhibit the reaction crystallization of Ca-scale crystals, the as-synthesized PASP-based scale inhibitor has a promising anti-scaling ability. Similarly, Xu et al.19 introduced hydroxyl into the side chain of PASP with PSI and 2-amino-2-methyl-1-propanol (AMP) as raw materials. The as-obtained PASP/AMP exhibits improved scale- and corrosion-inhibition performances, and its biodegradation rate is acceptable in terms of the Convention for the Protection of the Marine Environment of the Northeast Atlantic20. These researches demonstrate that introducing strong chelating groups into PASP molecular chain through ring-opening reaction is favorable for improving the scale inhibition performance; and the ring-opening reaction, with desired environmental acceptance, could be of special significance for the development of polymer–matrix scale inhibitor.

Viewing the abovementioned perspectives, here we attempt to introduce amino-functionalized silicon dioxide into the ring-opening reaction of PSI, hoping to obtain a modified PASP scale inhibitor with improved scale inhibition performance. We pay special attention to amino-functionalized nano-silica (nano-SiO2) with small size, large specific surface area, high surface activity, and multiple reaction sites, because it has been found to be intriguing in the petroleum industry, electronics industry and coating industry and so on21,22,23,24. The surface-capped nano-SiO2 has little steric hindrance and contains many reaction sites, and it can react with PSI to generate SiO2–NH2/PASP containing organic functional groups that can chelate with calcium ions, thereby effectively preventing scaling. This article deals with the preparation of SiO2–NH2/PASP, a potential green scale inhibitor applicable to the industrial circulating water system; and it also reports its scale inhibition performance and mechanism.

Results and discussion

Synthesis and structural characterization of SiO2–NH2/PASP

The TEM image of SiO2–NH2 nanoparticles is shown in Fig. S1. It can be seen that SiO2–NH2 nanoparticles appear as aggregates and have a size of about 10–20 nm. The synthetic route of SiO2–NH2/PASP scale inhibitor is shown in Fig. 1. There is a large amount of amino group on the surface of SiO2–NH2; and PASP is connected to the surface of nano-silica by CO–NH covalent bond. As a result, the as-synthesized SiO2–NH2/PASP product contains a large amount of amide group and carboxyl group that can inhibit the formation of scale.

Fig. 1: Synthesis route of SiO2-NH2/PASP scale inhibitor.
figure 1

SiO2-NH2 nanosilica containing reactive amino group, PASP polyaspartic acid.

The FTIR spectra of SiO2–NH2/PASP and PASP are shown in Fig. 2a. The stretching vibration absorbance peaks of N–H, C=O, and C–N in the amide bond appear at 3417 cm−1, 1624 cm−1, and 1402 cm−1, respectively; and these FTIR data prove that PSI successfully participates in ring-opening reaction to generate PASP25. The asymmetric stretching vibration and stretching vibration peaks of Si–O–Si can be detected at 1053 and 872 cm−1, respectively26. This indicates that SiO2–NH2 is involved in the polymerization of PSI yielding SiO2–NH2/PASP. Figure 2b shows the 1H NMR spectra of SiO2–NH2/PASP in D2O. The signals at 4.5 and 2.8 ppm are assigned to -CH- and -CH2- of SiO2–NH2/PASP, respectively. The FTIR and 1H NMR data give good evidence of the successful synthesis of SiO2–NH2/PASP.

Fig. 2: Structural characterization of polymer.
figure 2

a FTIR spectra of PASP and SiO2–NH2/PASP, b 1H NMR spectra of SiO2–NH2/PASP.

The EDS characterization and Si element mapping of SiO2–NH2/PASP are shown in Fig. S2. The Si peak is assigned to SiO2–NH2 in the as-synthesized SiO2–NH2/PASP scale inhibitor (Fig. S2a). The uniform Si element distribution in SiO2-NH2/PASP indicates that SiO2–NH2 nanoparticles do not agglomerate therein while they participate in the polymerization reaction (Fig. S2b).

To further verify the successful synthesis of SiO2–NH2/PASP scale inhibitor, we synthesized PASP under the same conditions for synthesizing SiO2–NH2/PASP. The as-obtained PASP was then mixed with SiO2–NH2 under 10 min of mechanical stirring. The mixed dispersion of PASP and SiO2–NH2 tends to sediment at the bottom of the beaker; and that of the as-synthesized SiO2–NH2/PASP is homogeneous, possibly due to the bonding of a large amount of PASP molecular chains on the surface of SiO2–NH2. This further proves that the ring-opening reaction of PSI in the presence of SiO2–NH2 indeed gives rise to SiO2–NH2/PASP.

One milliliter of PASP and SiO2–NH2/PASP solutions of 0.0230 and 0.0166 g/mL were taken into two beakers, respectively, acidified to pH ≈2 with 0.1 mol/L HCl solution, then diluted and shaken with 10 mL distilled water, titrated with 0.104 mol/L NaOH standard solution. When 0.108 mL was added to each drop, the primary conductivity value was recorded. The titration curve is shown in Fig. S3. The carboxyl group contents of PASP and SiO2–NH2/PASP were calculated to be 15.06% and 15.59%, respectively.

Effect of SiO2–NH2/PASP against CaSO4 and CaCO3 scales

Figure 3 shows the variations of scale inhibition efficiency with inhibitor concentration as well as static inhibition test temperature and time. Generally, the as-synthesized SiO2–NH2/PASP exhibits better scale inhibition performance than PASP; and its scale inhibition efficiency varies diversely with varying concentration as well as static inhibition test temperature and time. The inhibitory performance of SiO2–NH2/PASP with 5 mg/L was 21% higher than that of the unmodified PASP (Fig. 3a). Particularly, the scale inhibition efficiency of SiO2–NH2/PASP reaches the maximum of nearly 100% at a concentration of 6 mg/L. We further explore the influence of the dosage of PASP and SiO2–NH2/PASP on the inhibition rate of CaSO4, as is shown in Fig. S4. In the absence of a scale inhibitors, a large amount of calcium sulfate precipitate is formed. After the addition of PASP inhibitor, there is a little scale at the bottom of the conical flask; and there is nearly no scale after the introduction of 5 mg/L SiO2–NH2/PASP. This indicates that SiO2–NH2/PASP can effectively prevent the formation of the CaSO4 scale. The reason might lie in that the carboxylic group of SiO2–NH2/PASP can coordinate with calcium ions to adsorb free calcium ions in the solution, and thus reduce the combination of Ca2+ and sulfate ions, to prevent the generation of CaSO4 scale27. As can be seen from Fig. S3, compared with PASP, the carboxyl group content in the molecular structure of SiO2–NH2/PASP is higher, indicating that SiO2–NH2/PASP exhibits more excellent scale inhibition performance.

Fig. 3: Inhibition efficiency of PASP and SiO2–NH2/PASP against CaSO4 scale.
figure 3

a different inhibitor concentration, b different test temperature, c different test time. Error bars indicate the standard deviation.

Static inhibition test temperature also plays an important role in the scale inhibition performance. As shown in Fig. 3b, the scale inhibition efficiency of SiO2–NH2/PASP decreases rapidly with rising test temperature; and that of SiO2–NH2/PASP remains at nearly 100% in the temperature range of 70–75 °C. Besides, SiO2–NH2/PASP with the optional concentration of 6 mg/L always exhibits much higher scale inhibition efficiency than PASP in the whole temperature range of static inhibition test; and the scale inhibition efficiency tends to decline at the higher static inhibition test temperature. Yang reported that “the nucleation rate of CaSO4 scale can be influenced by temperature and supersaturation ratio (S)28,29. The temperature has little effect on S. When S remains constant, the higher the temperature is, the higher the nucleation rate is, and the easier scaling is29,30.

$$J = A\cdot {exp}\left[ - \frac{{\beta \gamma ^3V_m^2f\left( \theta \right)}}{{k^3T^3\left( {{ln}S} \right)^2}}\right]$$
(1)

where A is a frequency factor, β is a geometric (shape) factor of 16 π/3 for the spherical nucleus, and ƒ(θ) is a correction factor, when heterogeneous nucleation occurs ƒ(θ) = 0.0131. Vm is the molar volume of the phase forming, γ is the surface energy (J/m2). S is the supersaturation ratio, k is the Boltzmann constant, T is the absolute temperature29.”

Moreover, SiO2–NH2/PASP (at the optimal dosage of 6 mg/L) has excellent inhibition performance against the CaSO4 scale over a long test time; and its inhibition efficiency stays nearly at 100% after 25 h of static scale inhibition test (Fig. 3c). It is possible that the chelation stability between PASP and calcium ions becomes worse with the increase of test time, leading to the decrease of scale inhibition efficiency. This demonstrates that SiO2–NH2/PASP exhibits excellent scale inhibition performance and could have promising potential for industrial water treatment.

Figure 4 shows the static scale-inhibition performance of SiO2–NH2/PASP and PASP against calcium carbonate precipitate. The inhibition efficiency of SiO2–NH2/PASP on the CaCO3 scale is significantly higher than that of pure PASP. Particularly, at a concentration of 5 mg/L, the complexation and solubilization effect of SiO2–NH2/PASP for CaCO3 is increased by 53% as compared with those of the PASP scale inhibitor (Fig. 4a). This is caused by the chelation of the carboxyl group in the scale inhibitor molecule and calcium ions. The results in combination with Fig. S3 show that SiO2–NH2/PASP has more carboxyl group content and is easier to combine with the free calcium ions in the solution compared with PASP, thus inhibiting the formation of calcium carbonate scale. The scale inhibition efficiency initially increases with increasing concentration of SiO2–NH2/PASP; and the inhibition efficiency of SiO2–NH2/PASP is close to 68% under an inhibitor concentration of 30 mg/L. After that, the inhibition efficiency remains stable with increasing concentration, showing a so-called “threshold effect”. This is because, on the one hand, the solubilization performance of SiO2-NH2/PASP on calcium carbonate precipitate is enhanced with the increasing concentration of scale inhibitors. On the other hand, the adsorption capacity of the scale inhibitor and scale crystal is enhanced under increased concentration to reduce the formation of precipitate32.

Fig. 4: Inhibition efficiency of PASP and SiO2–NH2/PASP against CaCO3 scale.
figure 4

a Different inhibitor concentration, b different test temperature, c different test time. Error bars indicate the standard deviation.

The static scale inhibition test temperature and time also have some effects on the inhibition efficiency against the CaCO3 scale. The inhibition performance of SiO2–NH2/PASP with 30 mg/L tends to gradually decrease with the increase of the test temperature (Fig. 4b); and its inhibition efficiency maintains at 100% in the test temperature range of 50–70 °C. As the test temperature rises from 70 to 80 °C, the inhibition efficiency of SiO2–NH2/PASP decreases from 100 to 68%, but it is still much higher than that of PASP. This indicates that the introduction of SiO2–NH2 contributes to improving the scale inhibition performance of PASP at elevated static scale inhibition temperature. With the increase of temperature, the solubility of calcium carbonate decreases, and the formation of calcium carbonate crystals is easier. Meanwhile, temperature also affects the nucleation rate of calcium carbonate, the higher the temperature, the higher the nucleation rate29.

The effect of static inhibition test time on the inhibition efficiency against the CaCO3 scale is shown in Fig. 4c. The inhibition efficiencies of SiO2–NH2/PASP decrease with the increase of time; and it is as much as 76% after 8 h of static scale inhibition test, higher than 50% of PASP under the same test condition. Therefore, it can be inferred that SiO2–NH2/PASP has a much stronger chelating ability with Ca2+ than PASP.

Characterization of CaSO4 and CaCO3 scales

The crystal morphology exploration tests were repeated three times. The morphology of CaSO4 scales formed under different concentrations of SiO2–NH2/PASP and PASP are shown in Fig. S5. The calcium sulfate formed in the presence of SiO2–NH2/PASP is of disordered, irregular, and patchy morphology; and that formed after the addition of PASP has a bar-shape morphology as well as increased particle size and length. Figure 5 shows the SEM images of calcium sulfate crystals in the absence and presence of SiO2–NH2/PASP scale inhibitor. The CaSO4 crystals formed in the absence of antiscalants have a regular rod-shaped as well as smooth surface and compact crystal structure (Fig. 5a, a’). When 2 mg/L SiO2–NH2/PASP is added, CaSO4 crystal with irregular shape and relatively rough surface is formed (Fig. 5d, d’); and CaSO4 macrocrystals in good dispersion state are formed after the addition of 4 mg/L SiO2–NH2/PASP (Fig. 5e, e’)4,33. As the concentration of PASP increases from 2 to 4 mg/L, strip-like CaSO4 crystal with increased size is formed (Fig. 5b, c, b’, c’). This demonstrates that the introduction of antiscalants, especially SiO2–NH2/PASP, leads to significant changes in the morphology as well as average length and diameter of CaSO4 crystals, which could be because the carboxylic group not only can chelate with Ca2+ to block the active growth point of CaSO4 crystal but also can dwell on CaSO4 microcrystal surfaces via adsorption to disperse the microcrystal via charge repulsion10,34. In other words, the inhibition effect of SiO2–NH2/PASP against the CaSO4 scale is dominated by the chelating ability of SiO2–NH2/PASP and its adsorption on CaSO4 microcrystal surfaces.

Fig. 5: SEM images of CaSO4 scale at different magnifications.
figure 5

a, a’ Without antiscalant, b, b’ with 2 mg/L PASP, c, c’ with 4 mg/L PASP, d, d’ with 2 mg/L SiO2–NH2/PASP, e, e’ with 4 mg/L SiO2–NH2/PASP.

The SEM photographs of CaCO3 crystals formed under different static scale inhibition test conditions are shown in Fig. 6. The CaCO3 crystal obtained without scale inhibitor has a regular shape and smooth surface. With the increase of PASP concentration, the morphology of calcium carbonate becomes disordered and its particle size increases. With 10 mg/L SiO2–NH2/PASP, irregular CaCO3 crystal forms, and its sharp edges and acute corners disappear. When the concentration of SiO2–NH2/PASP increases to 30 mg/L, the cubic shape of the calcium carbonate scale is destroyed, and its surface is relatively rough; and its shape and size are increased. This could be attributed to two antiscaling mechanisms. On the one hand, the antiscalants can prevent more scale-forming positive ions in the solution from being precipitated through complexation action. On the other hand, the antiscaling chemicals can interact with mineral nuclei to disrupt the crystallization process and keep the crystal particles dispersed in aqueous suspension, thereby inhibiting their sedimentation or adhesion35,36. Therefore, it can be inferred that multiple inhibition effects, including lattice distortion, chelation, and dispersion, work together to significantly inhibit the formation of the CaCO3 scale.

Fig. 6: SEM images of CaCO3 scale at different magnifications.
figure 6

a, a’ Without antiscalant, b, b’ with 10 mg/L PASP, c, c’ with 30 mg/L PASP, d, d’ with 10 mg/L SiO2-NH2/PASP, e, e’ with 30 mg/L SiO2–NH2/PASP.

The crystal structures of CaSO4 and CaCO3 scales are further analyzed by X-ray diffraction (XRD), and corresponding XRD patterns are shown in Fig. 7. In the absence of the scale inhibitors, the major XRD peaks at 2θ = 11.53°, 20.69°, 23.31°, and 29.02° (Fig. 7a) are assigned to gypsum (CaSO4·2H2O). After the addition of SiO2–NH2/PASP or PASP, the resultant CaSO4 does not undergo crystal transition, but its crystallization behavior and particle size change to some extent. This further confirms that SiO2–NH2/PASP functions to inhibit the formation of CaSO4 scale mainly through chelation and adsorption, which is consistent with relevant SEM results. As to the aqueous solution of CaCO3, calcite and aragonite are the primary crystals formed in the absence of the scale inhibitors (Fig. 7b)37. After the addition of SiO2–NH2/PASP, the XRD signals of the (104) and (110) planes of calcite disappear, and the strong diffraction peak of vaterite emerges at 2θ = 29.21o. Calcite is the most thermodynamically stable, whereas vaterite is the least stable38,39. This indicates that SiO2–NH2/PASP can inhibit the growth of calcium carbonate crystals and completely block the conversion path from amorphous calcium carbonate to calcite as compared with PASP. In other words, the SiO2–NH2/PASP scale inhibitor functions to inhibit the formation of CaCO3 scale through chelating reaction in association with lattice distortion effect.

Fig. 7: XRD patterns of an aqueous solution of scale.
figure 7

a CaSO4 scale, b CaCO3 scale.

Scale inhibition mechanism of SiO2–NH2/PASP against CaSO4 and CaCO3

To further explore the adsorption characteristics of SiO2–NH2/PASP on scale surface, we conducted X-ray photoelectron spectroscopy (XPS) analyses of the CaSO4 and CaCO3 scales formed under static scale inhibition tests. The high-resolution Ca 2p-XPS spectra of CaSO4 and CaCO3 scales formed without the scale inhibitors and in the presence of PASP and SiO2–NH2/PASP are given in Figs. 8 and 9. In the absence of the scale inhibitors, the CaSO4 scale shows the distinctive peaks of Ca 2p1/2 and Ca 2p3/2 at 351.42 and 347.84 eV, respectively (Fig. 8). After the addition of 4 mg/L PASP, the Ca 2p1/2 and Ca 2p3/2 peaks shift to 351.53 and 347.93 eV, which corresponds to a change in the chemical surroundings of Ca2+ ion upon the introduction of PASP. The Ca 2p1/2 and Ca 2p3/2 peaks further shift to 351.58 and 347.99 eV after the addition of 4 mg/L SiO2–NH2/PASP, which gives further evidence to the uptake of scale inhibitors on the surface of calcium sulfate scales. Similarly, the introduction of PASP and SiO2–NH2/PASP leads to shifting in the Ca 2p peaks of the CaCO3 scale by 0.17 and 0.42 eV, respectively (Fig. 9). These findings imply that the SiO2–NH2/PASP scale inhibitor can be adsorbed on the surface of CaSO4 and CaCO3 scales to affect the growth, morphology, and size of scales, thereby effectively retarding their formation in aqueous solutions.

Fig. 8: Ca 2p XPS spectra of CaSO4 scales.
figure 8

a Without scale inhibitor, b with 4 mg/L PASP, c with 4 mg/L SiO2–NH2/PASP.

Fig. 9: Ca2p XPS spectra of CaCO3 scales.
figure 9

a Without scale inhibitor, b with 30 mg/L PASP, c with 30 mg/L SiO2–NH2/PASP.

The inhibition mechanism of SiO2–NH2/PASP against CaSO4 and CaCO3 scales is shown in Fig. 10. In the absence of the scale inhibitors, calcium sulfate and calcium carbonate form stable scales on the pipe surface, as shown in Fig. 10a, b. When SiO2–NH2/PASP is dissolved in water, the negatively charged carboxylic group of SiO2–NH2/PASP coordinates with positively charged Ca2+ ions to prevent them from forming calcium carbonate and sulfate. Meanwhile, SiO2–NH2/PASP can be adsorbed on the surface of newly generated scale crystal to increase the inter-particle repulsive force and prevent intercrystalline bonding, thereby accommodating good dispersion of the scales and retarding their deposition on the inner walls of the pipes (Fig. 10c, d). As a result, SiO2–NH2/PASP can effectively inhibit the formation of CaSO4 and CaCO3 scales in aqueous solutions.

Fig. 10: Schematic illustration of inhibition mechanism of SiO2–NH2/PASP against CaSO4 and CaCO3 scales.
figure 10

a The inside of the pipe without heating and adding SiO2–NH2/PASP, b the inside of the pipe after heating, c the inside of the pipe after adding SiO2–NH2/PASP, d the inside of the pipe after heating and adding SiO2–NH2/PASP.

In summary, SiO2–NH2/PASP, an organic-inorganic hybrid as a potential green scale inhibitor for industrial circulating water, is synthesized via a facile liquid-phase polymerization route. Static scale inhibition tests demonstrate that SiO2–NH2/PASP exhibits much better inhibition performance against CaSO4 and CaCO3 scales than PASP, and has good environmental adaptability. This is because, as evidenced by SEM, XRD, and XPS analyses, the carboxylic group of SiO2–NH2/PASP can chelate with Ca2+ to block the active growth points of the inorganic scales while its adsorption on scale crystal surface also contributes to retarding the formation of the scales. Furthermore, SiO2–NH2/PASP can completely change the conversion path from amorphous calcium carbonate to calcite, which could also account for its desired inhibition ability against the CaCO3 scale. The as-prepared SiO2–NH2/PASP scale inhibitor with good environmental acceptance could be promising in preventing the formation of CaSO4 and CaCO3 scales on the inner walls of the pipes for industrial circulating water.

Methods

Materials

Reactive nanosilica (SiO2–NH2) surface modified with amino (adsorption aperture: 19.2 nm) was provided by Henan Wangwu Nanotechnology Company Limited (Jiyuan, China). Industrial grade polysuccinimide (PSI, Mw = 7000) was purchased from Wuhan Yuancheng Gongyi Technology Company Limited (Wuhan, China). Anhydrous calcium chloride, disodium edetate, anhydrous sodium sulfate, borax, and potassium hydroxide were purchased from Tianjin Komi Chemical Reagent Company Limited (Tianjin, China). Anhydrous sodium hydrogencarbonate was commercially obtained from Tianjin Deen Chemical Reagent Company Limited (Tianjin, China). Anhydrous ethanol was bought from Anhui Ante Food Company Limited (Suzhou, China). Deionized water (DI) was used as the solvent and for rinsing as well.

Preparation of SiO2–NH2/PASP

A certain amount of SiO2–NH2 was dispersed in the mixed solution of ethanol and DI. The as-obtained dispersion (0.013 g) was placed into a round bottom flask, followed by the addition of 0.984 g of PSI under stirring at 60 °C for 1 h. The resultant mixed dispersion was allowed to react for 24 h thereat while its pH was adjusted to be 8–9. Upon completion of the reaction, the product was precipitated with absolute ethanol for 12 h and dried at 60 °C for 24 h to afford SiO2–NH2/PASP solid product. PASP was prepared in the same manner while no SiO2–NH2 was used19.

Characterization of SiO2-NH2/PASP

The morphology of SiO2–NH2 nanohybrid was observed with a transmission electron microscope (TEM; JEM-2010, JEOL, Japan). The structure of SiO2–NH2/PASP was characterized by Fourier transform infrared spectroscopy (VERTEX 70 FTIR spectrometer, Bruker Optics, Germany), 1H nuclear magnetic resonance spectroscopy (1H NMR; AVANCE 400 MHz NMR spectrometer, Bruker Optics, Germany), and scanning electron microscopy inconnection with energy dispersive spectrometry (SEM-EDS; JSM-7610F, Hitachi, Japan).

The carboxyl content of PASP and SiO2–NH2/PASP was determined by conductance titration. First, excess HCl solution was added to acidize PASP and SiO2–NH2/PASP, then titrated with NaOH standard solution, and the volume and conductivity values of the consumed NaOH standard solution were recorded40. The conductivity curve is shown in Fig. S6. The carboxyl group content (W) can be calculated by Eq. (2)40

$$W = \frac{{M \times (V_2 - V_1) \times 45}}{{1000m}} \times 100\;\%$$
(2)

M is the concentration of the NaOH solution, mol/L; V1 is the volume of the NaOH standard solution consumed by excess hydrochloric acid, mL; V2 is the volume of the NaOH standard solution consumed by excess hydrochloric acid and the carboxyl group contained in the polymer, mL; V2V1 is the volume of the standard solution of NaOH consumed by the carboxyl group in the polymer, mL; W is the mass of the sample, g40.

Static scale inhibition tests

The antiscalant performance of SiO2–NH2/PASP against calcium sulfate was tested by the static scale inhibition method. Briefly, proper amounts of CaCl2 (6800 mg/L Ca2+) and Na2SO4 (7100 mg/L of SO42−) were mixed at 70 °C for 6 h; and the filtrate was titrated with ethylene diamine tetraacetic acid (EDTA) standard solution to determine the concentration of calcium ion. The scale inhibition efficiency (E, %) against CaSO4 is calculated as

$$E = \frac{{X_2 - X_1}}{{X_0 - X_1}} \times 100\;\%$$
(3)

where X0 (mg/L) is the initial concentration of Ca2+ in the water samples before heating, X2 and X1 (mg/mL) are the concentration of Ca2+ after reaction in the presence and absence of scale inhibitor, respectively. The static inhibition tests were repeated three times, and the average of the repeat tests was cited to minimize data scattering.

An aqueous solution containing 240 mg/L Ca2+ and 732 mg/L HCO3 was prepared and heated with a water bath at 80 °C for 10 h. At the end of heating, the solution was cooled to room temperature; and the Ca2+ concentration in the supernatant was determined by EDTA titration. The scale inhibition efficiency (η, %) against CaCO3 is calculated as

$$\eta = \frac{{C_2 - C_1}}{{C_0 - C_1}} \times 100\;\%$$
(4)

where C0 (mg/mL) is the concentration of Ca2+ in the solution before the static inhibition test, C1 and C2 (mg/mL) are the concentration of Ca2+ in the absence and presence of scale inhibtor, respectively. All the static inhibition tests were conducted in triplicate to ensure reproducibility.

Surface analysis of scale crystal

The surface morphology of the as-formed scale crystal was observed with a scanning electron microscope (JSM-7610F, JOEL, Japan); and its crystal configuration was identified by XRD (D8 Advance, Bruker Optics, Germany). Moreover, the interaction between the scale inhibitor and scale crystals was investigated by XPS (ESCALAB 250Xi, Thermo, USA; monochromated Al Kα X-ray source,  = 1486.6 eV).