3.1 Sustained-release behavior and release kinetics
In order to study the slow-release properties of the composites, static release experiments were carried out and the results are shown in Fig. 1. The mass loss data of static dissolution experiments of PLA, PLA/EDTMPA0.09, PLA/EDTMPA0.16, and PLA/EDTMPA0.23 in water and simulated solution (simulated solution configured according to GB/T16632-2019) are shown in Fig. 1 (a1) and Fig. 1 (b1), respectively. The percentage of mass loss of scale inhibitors in the composites in water and simulated solution calculated from the mass loss are shown in Fig. 1(a2) and Fig. 1(b2), respectively. From Fig. 1, it can be seen that the proportion of mass loss of slow-release scale-inhibiting materials in both aqueous and simulated solutions gradually increases over 70 days, while the mass of PLA pure material remains almost unchanged, so the mass loss of the composites was the release of the scale inhibitor, indicating that the scale inhibitor was slowly released during this process. Moreover, the rate of mass loss of the composites in the simulated solution is faster than in water. The reason could be that in the simulated solution containing the calcium ions, the scale inhibitor is released into the solution and then chelates with calcium ions. This leads to a decrease in the concentration of scale inhibitor in the simulated solution, which increases the concentration difference between the inside and outside of the composite and increases the diffusion drive of the scale inhibitor. Therefore, the diffusion rate of the scale inhibitor in the simulated solution is greater than that in the ultrapure water.
The influence of diffusion driving force on the diffusion rate of scale inhibitors is not only reflected in different external environments but also in the slow-release process. From Fig. 1, we can see that in the initial stage, the diffusion rate of the scale inhibitor shows a tendency to be faster and then slower, both in the ultrapure water and in the simulated solution. This can also be explained from the perspective of diffusion driving force. At the beginning of the experiment, the concentration of scale inhibitor in the external solution is low, and the diffusion driving force of scale inhibitor is larger, resulting in a larger diffusion rate. With the extension of time, the concentration of scale inhibitor in the external solution gradually increases, the diffusion driving force of scale inhibitor gradually decreases, and the diffusion rate decreases.
Through the analysis of the release behavior of the slow-release scale-inhibiting materials, we obtain the process of the slow-release scale-inhibiting materials to achieve the scale inhibition effect. Firstly, the scale inhibitor has to diffuse and release from the composite materials, and then chelate with the calcium ions in the solution, so as to reduce the generation of precipitation and achieve the scale inhibition effect. The release of the scale inhibitor and its anti-scaling mechanism is shown in Fig. 2.
In order to further explore the release behavior of the scale inhibitor in the slow-release scale-inhibiting materials and to predict the possible release time of the scale inhibitor, a kinetic fit of the static release experimental curves was carried out, as shown in Fig. 3. The model contains simulation curves of PLA/EDTMPA0.23 samples in water and simulated solution. The specific values are shown in Table 1, the formula for this model is as follows:
Where
Q is the cumulative release ratio of the scale inhibitor,
t is the dissolution time of the material.
The goodness of fit of the first order-type kinetic fitting curves in both water and simulated liquids is greater than 0.99. According to the model prediction, after 120 days, the proportion of scale inhibitor lost from the samples would be approximately 50% in the water and 80% in the simulated liquids, indicating that the scale inhibitor could be released continuously for more than 120 days.
Table 1
Fitting curve parameter table
parameter | a | b | R2 |
PLA/EDTMPA0.23 (water) | 52.5137 | 0.0344 | 0.9992 |
PLA/EDTMPA0.23 (simulated liquids) | 82.2388 | 0.0296 | 0.9986 |
To further investigate the slow-release properties of the composites, a dynamic release experiment was conducted on PLA and composites. In the dynamic release experiment, it can be seen from Fig. 3 that the mass of PLA remains almost unchanged, while the proportion of mass loss of PLA/EDTMPA0.09, PLA/EDTMPA0.16, and PLA/EDTMPA0.23 gradually increases, and the trend is basically consistent with the static release experiment so that the composites can still continuously release scale inhibitor during the dynamic release process. Then comparing the dynamic release experiment (Fig. 4) with the static release experiment (Fig. 1), there is no significant difference in the amount of scale inhibitor released when the slow-release scale-inhibiting material is dissolved in water for the same time, which may be attributed to the fact that the scale inhibitor per unit of the solution was small, resulting in a smaller amount of scale inhibitor that can be released so that the updating of solution does not have a large effect on the release of the scale inhibitor.
3.2 Anti-scaling performance
In order to test whether the scale inhibitors released by slow-release scale-inhibiting materials have the effect of scale inhibition, scale inhibition performance tests were conducted according to GB/T 16632 − 2019. The experimental solution was derived from the solution containing scale inhibitor obtained from the dynamic release experiments, in order to evaluate the long-term scale inhibition performance of the composites. The scale inhibition rate obtained through the scale inhibition experiment is shown in Fig. 5(a), which shows that the scale inhibitor released through the slow-release scale-inhibiting materials still has the scale inhibition effect, indicating that the slow-release scale-inhibiting materials have the long-term scale inhibition ability.
According to GB/T 16632 − 2019, the water treatment agent sample solution (each 1 ml contains 0.5 mg of water treatment agent) was prepared, and the amount of water treatment agent added was controlled by adding different amounts of sample solution, and the result of scale inhibition rates for different scale inhibitor additions are shown in Fig. 5(b), which shows that when the amount of scale inhibitor added is between 5 and 25mg, the scale inhibition rate is between 40% and 60%. The mass loss of the composites measured in the dynamic dissolution experiments in Fig. 5(c) shows that most of the mass loss of the samples is around 5–10 mg, which corresponds to a scale inhibition rate of 40–60% in Fig. 5(b). This data suggests that the lower scale inhibition efficiency measured for the solutions in the dynamic experiments mostly near 60% is not due to the impaired scale inhibition ability of the scale inhibitor during the slow-release process, but rather is related to the fact that the scale inhibitor content in the solution is not very high. Therefore, the scale inhibitor is still able to resist scale during sustained release, and the prepared slow-release scale-inhibiting materials have the ability of slow-release and long-term scale inhibition.
3.3 Crystallographic analysis of scale samples
To further confirm the anti-scaling properties of EDTMPA, the structure of calcium carbonate powders with and without added scale inhibitors collected in the anti-scaling test were analyzed by XRD (Fig. 6). The diffraction peaks at 23.0° (012), 29.4° (104), 31.4° (006), 36.0° (110), 39.4° (113), 43.1° (202), 47.5° (018), 48.5° (116) and 57.4° (122) are the characteristic peaks of calcite in Fig. 6(a1), indicating that in the absence of EDTMPA, the crystal structure of calcium carbonate is predominantly calcite[4, 32]. Analyzing the scale produced after the addition of scale inhibitor, the characteristic diffraction peaks can be seen in Fig. 6(b2) of the XRD pattern appearing at 21.0° (004), 24.9° (110), 27.0° (112), 32.8° (114), 43.8° (300), 49.1° (304), 50.1° (118), and 55.8° (224), which corresponds to the unstable crystal morphology of calcium carbonate - vaterite[33]. Meanwhile, the diffraction peak (104) corresponding to calcite is significantly weakened. This indicates that in the presence of scale inhibitors, the normal growth of calcium carbonate is disrupted, less calcite is produced, and more formation of unstable crystal forms that can be easily removed.
The morphology of scale samples with and without added scale inhibitors was analyzed by scanning electron microscopy (SEM) observation (Fig. 6). In Fig. 6(a2), it can be seen that the surface of the calcium carbonate generated without the addition of scale inhibitors is smooth, with a regular angular shape and densely arranged crystal cells, for the calcite structure. In the morphology diagram of the scale sample with added scale inhibitor (Fig. 6(b2)), it can be seen that the morphology of calcium carbonate has changed significantly, the crystal edges have disappeared, the structure is no longer regular and looser, which indicates that the addition of scale inhibitor has distorted the calcium carbonate crystals generated subsequently.
The results of XRD and SEM indicate that the presence of scale inhibitors causes lattice distortion of the formed scale, inhibits the normal ordered growth of crystals, and transforms stable calcite into unstable crystals[34], resulting in a loose structure of the formed scale and making it easier to remove. Therefore, the scale inhibitors released through slow-release scale-inhibiting materials still have scale inhibition ability.
3.4 Morphology of Samples
In order to observe the release behavior of slow-release scale-inhibiting materials, the PLA pure material and composites were subjected to scanning electron microscopy (Fig. 7). As can be seen in Fig. 7, comparing the images before the dissolution of PLA pure material in Fig. 7(a1) and composite material in Fig. 7(b1), 7(c1), 7(d1), proves that scale inhibitor particles have been successfully doped into the composite material; comparing the composite material images before dissolution in Fig. 7(b1), 7(c1), 7(d1) and after 7 days in Fig. 7(b2), 7(c2), 7(d2) and 14 days in Fig. 7(b3), 7(c3), 7(d3) of dissolution, it can be seen that there are holes in the material after 7 days and 14 days of dissolution, indicating that the scale inhibitor in the composite material has successfully entered into the solution.