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Article

Elucidating the Memory Effects of Magnetic Water Treatment via Precipitated Phase Changes of Calcium Carbonate

by
Aly Ahmed Mohamed Sayed
*,
Soumya Basu
,
Takaya Ogawa
,
Keito Inagawa
and
Hideyuki Okumura
*
Graduate School of Energy Science, Kyoto University, Kyoto 606-8501, Japan
*
Authors to whom correspondence should be addressed.
Eng 2025, 6(2), 26; https://doi.org/10.3390/eng6020026
Submission received: 16 December 2024 / Revised: 23 January 2025 / Accepted: 28 January 2025 / Published: 1 February 2025
(This article belongs to the Section Chemical, Civil and Environmental Engineering)
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Versions Notes

Abstract

:
Research on the effects of magnetic fields on water and aqueous solutions has produced various findings, such as the suppression of scale formation in pipes and boilers, inhibition of metal corrosion, enhancement of concrete strength, and changes in properties like viscosity and electrical conductivity. However, the challenges in quantifying these effects, the issues with reproducibility affected by trace elements in the water used in the experiments, and the involvement of complex parameters and mechanisms have led to ongoing debates, with some questioning the very existence of magnetic field effects. The “memory effect”, where the impact of magnetic exposure persists for a certain period, further complicates explanations of these phenomena. To fully elucidate and enable practical applications of these effects, further research is essential. In this study, we aimed to investigate the magnetic field effects on water, including memory effects, where the quantification and elucidation potentially lead to various applications, including environmentally friendly solutions on scale suppression and life science issues. The results revealed that the vaterite phase precipitation ratio significantly increased in magnetically treated water, reaching up to 51%, from 26% without the treatment, which is high reproducibility; furthermore, a reduction in mean particle size was observed when using magnetically treated water, suggesting that it may help prevent scaling. Furthermore, when solutions of calcium carbonate, calcium chloride, and sodium bicarbonate were individually subjected to magnetic treatment, the most notable increase in the vaterite phase precipitation ratio was observed when calcium chloride and sodium bicarbonate solutions were magnetically treated separately and then reacted to precipitate calcium carbonate.

1. Introduction

In recent years, climate change has been observed worldwide, bringing about a wide array of challenges. First, rising temperatures are causing polar ice to melt, leading to sea level rise and increasing the risk of coastal flooding. Additionally, the increase in extreme weather events, such as droughts, heavy rainfall, and storms, has resulted in frequent natural disasters that severely impact agriculture and food supply [1]. These climate-related natural disasters have caused significant economic losses, amounting to approximately USD 131.7 billion in 2018 alone [1]. Furthermore, ecosystems are changing, leading to habitat loss and the extinction of various plant and animal species. Climate change also poses risks to human health by raising the incidence of heatstroke and infectious diseases, and it is associated with water shortages, malnutrition, and social unrest due to competition for resources. For these reasons, climate change is regarded as one of the greatest challenges of the 21st century, spurring various efforts to prevent further deterioration [1].
The primary cause of climate change is the increase in greenhouse gas emissions from human activities. In particular, the use of fossil fuels like oil and coal, which surged after the Industrial Revolution, along with deforestation, industrial processes, and agricultural practices, has increased the levels of carbon dioxide (CO2) and methane (CH4) in the atmosphere, raising the concentration of greenhouse gases. These gases accumulate in the atmosphere, trapping heat and contributing to the global warming phenomenon. Currently, efforts to combat climate change are shifting toward clean energy; however, fossil fuels remain the main energy source.
In recent years, increasing the efficiency of fossil fuel use and, thereby, output has become one of the key discourses of combating climate change. However, the very nature of fossil fuels is that they are available from the earth, where a variety of contaminants are found, leading to multiple issues in efficient extraction. One such example is the issue of scale formation, which reduces energy efficiency. This is basically the precipitation of mineral components such as calcium [2,3,4], magnesium [2], and silicon [3] present in water. Because scale has significantly lower thermal conductivity [5] compared to metals, its accumulation on equipment such as heat exchangers and boilers impedes heat transfer, thereby reducing thermal efficiency. Consequently, scale formation is a significant issue in many industrial operations, particularly in sectors such as the oil and gas industry [6,7], water treatment [8,9], and cooling systems [10,11]. Traditional methods for preventing scale formation use chemical inhibitors [12]. These inhibitors work by either preventing the formation of scale deposits or dispersing scale particles to prevent their adhesion to surfaces. Common inhibitors include polyphosphates, anionic polymers, and cationic polymers [13]. Additionally, when scale has already precipitated and requires removal, chemical treatments are often used [14], employing various acids such as hydrochloric acid, phosphoric acid, and acidic surfactant mixtures. However, both scale prevention and removal methods are costly and can harm the environment due to the use of strong chemicals [15]. Some of these agents are also corrosive to piping, potentially causing embrittlement. In recent years, more environmentally friendly technologies have been developed to tackle this issue. Recent advancements offer promising alternatives to chemical treatments in the form of various anti-scaling systems: (1) acoustic anti-scaling [16,17], (2) electrochemical systems [18,19], (3) liquid-infused porous surfaces [20,21], (4) ceramic balls [22], and (5) magnetic water treatment [15,23,24]. The following sections provide an overview of each technology and its specific characteristics.
Acoustic Anti-Scaling Technology: High-frequency vibrations in ultrasonic waves detach mineral particles from surfaces, thus inhibiting the formation of solid scale layers. Frequencies typically range between 20 kHz and 100 kHz, depending on the application and the required level of scale prevention. This method has proven effective in several industrial systems, including desalination plants and cooling towers, particularly for soft scales like calcium carbonate. However, it is less effective for harder scales, such as barium sulfate. The effectiveness of this method is also influenced by factors such as water chemistry, flow rate, and temperature, highlighting the need for further research to enhance its applicability across diverse conditions [16].
Electrochemical Scale Control: These cause scale-forming minerals to precipitate in a controlled environment, such as a seeded crystallization vessel, rather than on critical surfaces. This approach significantly reduces the required electrode area while maintaining high energy efficiency. Experiments to date show a reduction in the specific cathode area by more than a factor of 10 without increasing energy demand, suggesting that electrochemical scale control could be widely applicable in scale prevention [18].
Liquid-Infused Porous Surfaces (LIPS): These infuse low-surface-energy lubricants into microporous coatings, creating liquid-repellent, slippery surfaces. Experimental studies have shown that these surfaces can significantly reduce calcium carbonate deposition, with some coatings decreasing scale formation by up to 18 times compared to untreated surfaces. This approach underscores the potential of surface engineering as a sustainable solution for scale prevention [20].
Ceramic Balls: This impacts the nucleation and growth of scale-forming minerals. According to mathematical models, ceramic balls can reduce the surface tension energy of calcite particles, thus inhibiting their growth. Experimental studies have shown the effectiveness of ceramic balls in preventing scale formation, making them suitable for both industrial and household applications [22].
Magnetic Water Treatment (MWT): This is a non-chemical approach that utilizes permanent magnets to alter the properties of water and reduce the potential for scale formation. The efficiency of this method depends on factors such as treatment duration, flow velocity, and pipe material composition [23,25]. For example, in terms of processing time, the duration for which water is exposed to the magnetic field can have a significant impact. Longer exposure times may enhance the interaction between the magnetic field and water, thereby improving the efficiency of the treatment. In terms of flow rate, a lower flow rate increases the residence time of water within the magnetic field, thereby enhancing the treatment effect. Conversely, a higher flow rate shortens the interaction time, potentially reducing the efficiency of the treatment. The material of the pipe significantly affects the crystallization process of calcium carbonate. Magnetic treatment increases the total amount of precipitate and promotes the formation of precipitation in the bulk solution rather than on the pipe walls. This effect is more pronounced with non-conductive materials, and surface roughness also plays an important role [25]. Experimental models have proposed methods to predict the effectiveness of magnetic treatment under various conditions. Although MWT is gaining attention for its cost-effectiveness and simplicity, the precise mechanisms underlying its effects remain under investigation, necessitating further research [15]. Models regarding changes in water structure suggest that magnetic treatment alters the structure of water, affecting hydrogen bonding and cluster formation. Non-classical nucleation theories, such as the formation of Dynamically Ordered Liquid-Like Oxyanion Polymers (DOLLOP), have been proposed to explain these effects. It is also suggested that the gradient of the magnetic field is more critical than its strength [26].
The focus of this paper is MWT, as it stands out for its cost-efficiency and ease of application. Moreover, since it is a non-chemical application, this can be adapted for a wide range of power plants and processes, making the utility almost universal from a non-reactive viewpoint. Existing studies indicate that magnetic treatment significantly affects calcium carbonate precipitation and scaling, with reports showing reduced scale deposits on surfaces, a marked decrease in surface deposits, and an increase in bulk precipitate mass [27].
Additionally, magnetic fields are reported to influence the crystal morphology and particle properties of calcium carbonate. Magnetic treatment has been shown to produce smaller, irregularly shaped crystals, increase the zeta potential of precipitated particles, and alter particle size distribution [28,29]. Furthermore, magnetic treatment can shift the precipitation phase of calcium carbonate from the stable calcite phase to the metastable aragonite phase, which is easier to remove from surfaces [30]. While various studies have reported these effects, the efficacy of magnetic treatment remains a topic of debate [15].
Beyond scale control, magnetic treatment has garnered attention for its reported impacts on metal corrosion inhibition [31], concrete strength enhancement [32,33], changes in solution properties such as viscosity [34,35] and electrical conductivity [36,37], effects on plant growth [38,39], and even influences on cancer cells [26,40]. However, due to inconsistencies in results, the lack of a universal mechanism, and numerous influencing factors, further research is necessary to enable reliable applications of this technology.

Hypothesized Mechanisms of Magnetic Water Treatment

  • Hydrogen Bonding and Water Structure: Magnetic fields can affect hydrogen bonding in water, altering the arrangement within water clusters and impacting its physical properties [41].
  • Crystal Morphology and Nucleation: Magnetic treatment may modify nucleation processes and crystal structures, resulting in hydrophilic crystals that are less likely to adhere to surfaces [42].
  • Magnetohydrodynamic Effects: Magnetic fields may influence fluid dynamics, affecting the aggregation or dispersion of colloidal particles [43].
  • Zeta Potential Changes: Alternating magnetic fields can alter the zeta potential of colloidal particles, reducing the likelihood of scale formation [44].
The key influencing factors for MWT are flow velocity and treatment duration, where water flow velocity and exposure time to the magnetic field influence the anti-scaling effect. Optimizing these parameters can enhance treatment efficiency [23]. Secondly, higher temperatures and stronger magnetic fields generally improve the effectiveness of magnetic treatment in preventing scaling [45,46]. In this paper, we vary the duration of the magnetic treatment and the field strength to give key insights into reducing the scaling effects of the mineral forms of calcium carbonate (CaCO3). The first issue that we try to tackle is the various discrepancies in reported results for MWT, which may stem from the use of non-standardized methods, variations in water composition, or differences in operational processes [47]. Many studies lack clear standard operating procedures, with key parameters such as exposure duration and magnetic field characteristics being partially reported at best [15]. Therefore, quantitative analysis and standardized metrics are essential for elucidating the mechanisms of magnetic water treatment and establishing it as a viable technological solution.
The second key goal of this study is to elucidate the “memory effect” in magnetically treated water through quantitative evaluation and parameterization of the magnetic field’s effects on water and calcium carbonate. A phenomenon known as the “memory effect” has also been reported, wherein magnetically treated water retains certain properties of the magnetic treatment for an extended period despite being a non-magnetic substance. For instance, Otsuka et al. prepared magnetically treated water by reciprocating water in a static magnetic field and reported that water with dissolved oxygen exhibited a reduction in contact angle, with this effect lasting up to 20 min [36]. Various studies have reported similar memory effects, with durations ranging from 20 min [36] to 24 h [48] and even up to 200 h [49]. To achieve this objective, we conducted a comparative analysis of calcium carbonate precipitation—a common scale component—in magnetically treated and untreated water samples. Precipitation phases and particle size characteristics were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), and dynamic light scattering (DLS). The novelty of our results lies in not only standardizing the process of MWT but also experimentally and quantitatively reporting the phases of the CaCO3-associated minerals at each temperature and exposure duration to magnetic fields. These results are crucial for developing an inexpensive and accessible method for preventing scale formations in existing and new power plants.

2. Materials and Methods

2.1. Magnetic Treatment of Water and Solution

Figure 1 illustrates an overview of the apparatus used for magnetic water treatment. In this study, ultrapure water (18.2 MΩ at the time of collection, ORGANO PX-0060 Puric-µ system ultrapure water unit with direct connection to tap water, from ORGANO CORPORATION, Tokyo, Japan) was used after exposure to the atmosphere for over 24 h. Approximately 1200 mL of ultrapure water was prepared and circulated through a 6 mm inner-diameter silicone tube at a flow rate of 3 m/s using a pump (M100-60CF, from Marintec Co., Ltd., Fukuoka, Japan). The tube was fixed inside a permanent neodymium magnet with a magnetic flux density of 700 mT to apply magnetic treatment. The magnetic flux density was confirmed by a Tesla meter and remained unchanged during the experiment. The length of the magnetic field exposure was 6.5 cm. The container holding the water was maintained at a constant temperature (within ±0.5 °C) using a thermostatic bath. The temperature was kept constant (±0.5 °C) using a heater (thermal robo tr-1a from Azwan Corporation, Osaka, Japan), which automatically heated to the target temperature, and a cooler (Neocool Dip BE200 from Yamato Scientific Co., Ltd., Tokyo, Japan) simultaneously. At the same time, monitoring was performed visually with a thermometer. The magnetic treatment duration was set to 3 h, and the treatment temperatures were set at 5 °C, 15 °C, 25 °C, 35 °C, and 45 °C.
Additionally, for comparative testing, water was circulated through the system under identical conditions without applying a magnetic field. This water was otherwise treated in the same way and circulated using the pump but without magnetic exposure. It was also confirmed by a Tesla meter that there was no influence of external magnetic fields, and the pumps were separated by about 1 m to ensure that vibrations had as little effect as possible. For simplicity, this paper refers to the former as magnetically treated water (MTW) and the latter as normal water (NW). The dissolved oxygen (DO) and pH of the water (25 °C) before the start of the experiment were measured to be 7.8 mg/L and 6.0, respectively.
Additionally, solutions of calcium chloride (0.1 mol/L, purity ≥ 95.0%, from NACALAI TESQUE Inc., Kyoto, Japan), sodium bicarbonate (0.2 mol/L, purity 99.5–100.3%, from NACALAI TESQUE Inc., Kyoto, Japan) and calcium carbonate (0.1 mol/L, prepared by reacting calcium chloride and sodium bicarbonate) were prepared. The water used to prepare these solutions was the same ultrapure water (18.2 MΩ at the time of collection) as that used for NW and MTW, and it was exposed to the atmosphere for over 24 h prior to use. The prepared solutions were circulated through a silicone tube (inner diameter 6 mm) at a flow rate of 3 m/s using a pump. The tube was fixed inside a permanent magnet with a magnetic flux density of 700 mT to apply magnetic treatment. The length of the magnetic field exposure was set to 6.5 cm. The container holding the solutions was maintained at a constant temperature of 25 °C (within ±0.5 °C) using a thermostatic bath.
For comparative testing, the solutions were also circulated through the system under identical conditions without applying a magnetic field. In this case, the solutions were circulated using the pump, but no magnetic treatment was applied.

2.2. Calcium Carbonate Precipitation

For each of the MTW and NW samples prepared at 5, 15, 25, 35, and 45 °C in Section 2.1, we prepared 200 mL. To each sample, we added 0.02 mol of calcium chloride (purity ≥ 95.0%, from NACALAI TESQUE Inc., Kyoto, Japan) and 0.04 mol of sodium bicarbonate (purity 99.5–100.3%, from NACALAI TESQUE Inc., Kyoto, Japan). These compounds were dissolved by stirring for 1 min, allowing calcium chloride to react with sodium bicarbonate as follows:
CaCl2 + 2NaHCO3 → CO2 + CaCO3 + H2O + 2NaCl
The reaction was allowed to precipitate calcium carbonate for 4 min while maintaining the temperature. After 4 min, the prepared solution was filtered over a period of 5 min to recover the precipitated calcium carbonate. The wet calcium carbonate was then air-dried for 5 min before being used for measurement. The preparation conditions for each sample are summarized in Table 1.
Similarly, samples were prepared by precipitating calcium carbonate under the following conditions: calcium chloride powder was added to the treated sodium bicarbonate solution to produce a final calcium carbonate concentration of 0.1 mol/L; sodium bicarbonate powder was added to the treated calcium chloride solution to achieve the same concentration; and calcium chloride solution was added to the treated sodium bicarbonate solution for calcium carbonate precipitation. For each case, the solutions were stirred for 1 min immediately after the addition of the powder or solution to facilitate the reaction between calcium chloride and sodium bicarbonate. The reaction was carried out for 4 min while maintaining a constant temperature to allow calcium carbonate to precipitate. After 4 min, the resulting solution was filtered over a period of 5 min to recover the precipitated calcium carbonate. The wet calcium carbonate was air-dried for 5 min and subsequently used for measurements. Magnetically treated calcium carbonate solutions were handled similarly; after magnetic treatment, the solutions were maintained at a constant temperature for 5 min to allow precipitation. The solutions were then filtered for 5 min to recover the calcium carbonate, which was air-dried for 5 min before use in measurements. The preparation conditions for each sample are summarized in Table 2.

2.3. Verification of Memory Effects

Water was magnetically treated under the same conditions as in Section 2.2. Three types of water were prepared: magnetically treated water immediately after preparation and magnetically treated water that had been kept at a constant temperature for 10, 20, 30, and 40 min. 200 mL of each type of water was prepared, and calcium carbonate was precipitated using the same procedure as in Section 2.2.

2.4. Characterization Instruments

The microstructure of the samples was observed by a SU6600 (from Hitachi High-Technologies Corporation, Tokyo, Japan) scanning electron microscope (SEM) using secondary electron (SE).
Phase identification was determined through X-ray diffraction using the Mini Flex 600C (from Rigaku Holdings Corporation, Tokyo, Japan) with Cu-Kα radiation, Ni filter, tube voltage of 40 kV, tube current of 15 mA, step of 0.010°, and scan range from 3 to 90°.
Particle size measurements were conducted using an ELZ-2000ZS (from Otsuka Electronics Co., Ltd., Osaka, Japan). Samples 1 to 10 were dissolved in ultrapure water at a concentration of 1% wt/v, then stirred using an ultrasonic cleaner before being injected into a glass cell for measurement. Since calcium carbonate precipitates quickly, we measured it with an accumulation count of 25.

3. Results

3.1. SEM Particle Characterization

The morphology of samples 1 to 10 are evaluated through SEM observation at a magnification of 1200×, and the results are shown in Figure 2.
At 5 °C under NW conditions (Figure 2a) and MTW conditions (Figure 2b), no significant changes were observed in the precipitated calcium carbonate. Rhombohedral crystals, a calcite phase, were observed.
At 15 °C under NW conditions (Figure 2c) and MTW conditions (Figure 2d), in addition to the rhombohedral calcite phase crystals observed in Figure 2a,b, spherical crystals of the vaterite phase appeared. Furthermore, a comparison of Figure 2c,d shows that a higher amount of spherical crystals precipitated under MTW conditions (Figure 2d) than under NW conditions.
At 25 °C under NW conditions (Figure 2e) and MTW conditions (Figure 2f), there was a significant increase in the precipitation of spherical crystals of the vaterite phase compared to the results at 15 °C. Moreover, when comparing NW conditions (Figure 2e) with MTW conditions (Figure 2f), the size of spherical particles precipitated under MTW conditions was generally larger than those under NW conditions. Additionally, under MTW conditions (Figure 2f), smaller particles were more frequently observed than under NW conditions (Figure 2e).
At 35 °C under NW conditions (Figure 2g) and MTW conditions (Figure 2h), both conditions resulted in the precipitation of rhombohedral crystals of the calcite phase and spherical crystals of the vaterite phase.
At 45 °C under NW conditions (Figure 2i) and MTW conditions (Figure 2j), similar to the results at 35 °C, both rhombohedral calcite phase crystals and spherical vaterite phase crystals were observed. Focusing on the spherical crystals presumed to be of the vaterite phase observed in samples 1 to 10, many of these were not perfect spheres but exhibited partially defective shapes or hollow structures, as shown in Figure 3. The reason why spherical crystals presumed to be vaterite phase were not observed at 5 °C is thought to be because the vaterite phase is reported to form at around room temperature [50,51], and the temperature at which vaterite phase formation becomes active had not been reached. This explanation aligns with the appearance of spherical crystals at temperatures of 15 °C and above.
Additionally, the absence of needle-shaped crystals presumed to be of the aragonite phase under all conditions can be explained by the lack of the requisite formation conditions, which are reported to occur at 50 °C [50,51].

3.2. XRD Characterization

Figure 4 shows the XRD patterns of calcium carbonate powders obtained from samples 1–10. Peaks visible in the patterns were marked to identify different crystal phases: squares (□) indicate the calcite phase, circles (○) the vaterite phase, and crosses (×) the aragonite phase.
The XRD patterns were analyzed using Rietveld refinement (via Profex) to quantify the relative proportions of calcite, vaterite, and aragonite phases. Each sample was measured five times, and the average proportions with standard errors are summarized in Table 3.
From Figure 4a,b, it is evident that peaks corresponding to the calcite phase dominate under all conditions, with no visually discernible vaterite phase peaks. This result is reflected in Table 3, where calcite constitutes over 97% of the precipitated calcium carbonate in all samples, while vaterite and aragonite phases were nearly absent.
In contrast, Figure 4c,d shows that while calcite remained the primary phase, small peaks corresponding to the vaterite phase appeared under MTW conditions (Figure 4d, 15 °C). Quantitative results in Table 3 confirm this observation: the vaterite proportion increased from approximately 4% under NW conditions to about 9.5% under MTW conditions, representing a twofold increase.
In Figure 4e,f, peaks for the vaterite phase, previously indistinct, became more pronounced. Moreover, these peaks were more significant under MTW conditions (Figure 4f) than NW conditions (Figure 4e). Table 3 corroborates this, showing that vaterite increased from 26% under NW conditions to 51% under MTW conditions, while calcite proportions decreased from over 90%.
At 35 °C and 45 °C (Figure 4g–j), peaks for both calcite and vaterite phases were observed in both conditions. Under MTW conditions, vaterite peaks were slightly more pronounced than under NW conditions. Table 3 indicates that at 35 °C, the vaterite proportion increased from about 29% (NW) to 35% (MTW), and at 45 °C, from about 23% (NW) to 41% (MTW), showing an 18% increase under MTW conditions. These findings are further plotted in Figure 5, which illustrates the vaterite proportions in samples 1–10.
These results indicate that magnetic treatment (MTW) had minimal impact on calcium carbonate crystal formation at 5 °C or lower. However, from 15 °C to 45 °C, MTW conditions led to a decrease in calcite proportions and an increase in vaterite proportions. At 25 °C, magnetic treatment increased vaterite formation by approximately 25%, and at 15 °C and 45 °C, it nearly doubled vaterite proportions. Additionally, aragonite formation remained below 1% under all conditions, consistent with prior studies [50,51].
The XRD results align with the SEM observations in Figure 3. It is generally known that amorphous calcium carbonate (ACC) precipitates first, transitioning over time into calcite or vaterite phases [52]. As vaterite is thermodynamically unstable, it tends to transition to calcite at low temperatures and to aragonite at higher temperatures [52].
In this study, magnetic treatment promoted vaterite stabilization at temperatures above 15 °C, suggesting that MTW inhibited the transition from vaterite to calcite. This aligns with previous research indicating that magnetic treatment delays phase transitions of calcium carbonate crystals [53].
Figure 5 shows that under NW conditions, vaterite proportions remained stable from 25 °C to 45 °C. However, under MTW conditions, vaterite proportions peaked at 25 °C and then decreased at 35 °C and 45 °C. This decline may be attributed to the dissolution or transition of vaterite. As vaterite is thermodynamically unstable and has higher solubility compared to calcite and aragonite, it is prone to dissolution in aqueous solutions [54]. This suggests that the dissolution of vaterite and its transfer to calcite is accelerated in high-temperature water, resulting in decreased vaterite presence. This is a second key significant result that points towards MTW segregating the scale crystals at room temperatures, which can be achieved under any normal condition.

3.3. DLS Results and Characterization

Figure 6 shows the particle size distribution of calcium carbonate powder obtained from samples 1 to 10, as measured by Dynamic Light Scattering (DLS). The peak particle sizes, distribution widths, and average particle diameters obtained from these measurements are summarized in Table 4.
In Figure 6a, under the 5 °C conditions, when comparing NW and MTW, the minimum value of the particle size distribution for MTW is slightly smaller than that for NW. However, no significant differences were observed in the peak particle size or distribution width. On the other hand, in Figure 6b–e, under conditions of 15 °C and higher, it was observed that the particle size distribution width for MTW became narrower, and the maximum particle size was significantly smaller for MTW compared to NW. These results suggest that calcium carbonate particles precipitated under MTW conditions are more concentrated around the peak particle size compared to those under NW conditions.
Furthermore, in terms of the average particle diameter, calcium carbonate precipitated under MTW conditions showed smaller values than those obtained under NW conditions for all temperatures above 5 °C. Based on these results, the average particle sizes of calcium carbonate precipitated under NW and MTW conditions are plotted in Figure 7. Figure 7 shows that the average particle diameter tends to increase with temperature for both NW and MTW, suggesting that as the temperature increases, crystal growth is more favored over nucleation. This trend is consistent with reports on the increase in particle size with temperature for aragonite [55], and this study indicates a similar temperature–particle size relationship for calcite and vaterite.
The reason for the smaller average particle size in MTW compared to NW at temperatures of 15 °C and above is likely due to the increased precipitation of vaterite, which forms smaller crystals compared to calcite, as observed in SEM images. Moreover, since the horizontal axis in the DLS results is on a logarithmic scale, at temperatures of 15 °C and above, the minimum values for both NW and MTW are similar, but the maximum value for NW is larger than for MTW.
Previous studies have reported a delay in particle growth [56], and similar results were found in this study. This delay can likely be attributed to the magnetic treatment, which may slow down the crystallization transition. During the formation of calcium carbonate crystals, amorphous calcium carbonate (ACC) initially forms, followed by the precipitation of vaterite and calcite. The magnetic treatment likely delayed nucleation, leading to suppressed calcite growth within the precipitation time, resulting in smaller crystals. Therefore, calcium carbonate precipitated using magnetic-treated water exhibited smaller average particle sizes.
At temperatures above 15 °C, the particle size distribution range of NW was wider than that of MTW, and both smaller and larger particles appeared compared to MTW. The reason for this is that when calcium carbonate precipitates in NW, both calcite and vaterite are precipitated in the initial stage of the precipitation process, but as time passes, the thermodynamically unstable vaterite dissolves and the vaterite particles become smaller. The dissolved vaterite is considered to grow into larger calcite crystals when it transitions to stable calcite, which has existed since the early stage of precipitation [57]. Therefore, vaterite in NW is considered to be relatively small, while calcite is considered to be relatively large. However, since the particle distribution width of MTW is smaller, the calcite and vaterite particles that appear are relatively the same size as those of NW. In fact, the SEM image in Figure 2 shows that this is the case for samples at 15 °C or higher. Therefore, it is thought that calcium carbonate vaterite deposited by MTW does not re-dissolve or re-precipitate like vaterite deposited by NW. This suggests that MTW inhibits or delays the re-precipitation of the vaterite phase of calcium carbonate. From these findings, we can conclude that magnetic treatment can suppress the growth of scale-forming vaterite crystals without interfering with any other chemical reaction.

3.4. XRD Characterization for the Different Solution Samples

In Figure 8A,B, the XRD patterns of calcium carbonate powders prepared by processing the solutions of samples A–H are shown. In each XRD pattern, markers were placed to identify crystals corresponding to visually visible peaks. Specifically, squares (□) indicate peaks in the calcite phase, circles (○) in the vaterite phase, and butts (×) in the aragonite phase. These XRD patterns were analyzed by Rietveld analysis (using Profex), and the results are summarized in Table 5, where the presence ratios of calcite, vaterite, and aragonite phases are calculated (Mean and standard error of three times).
In the results of samples A (Figure 8A) and B (Figure 8B), where sodium bicarbonate solution was circulated, calcite was prominently observed in both cases. On the other hand, vaterite was observed in small amounts, and no aragonite was detected. According to Table 5, calcite was the main phase, and the amount of vaterite increased from approximately 12.2% in the non-magnetic-treated NW to approximately 18.2% in the magnetic-treated MTW.
For samples C (Figure 8C) and D (Figure 8D), where calcium chloride solution was circulated, calcite was the dominant phase, and no patterns of vaterite or aragonite were observed. In Table 5, calcite accounted for nearly the entire content, with vaterite only slightly increasing from about 6.1% in the NW to about 7.9% in the MTW.
Next, for samples E (Figure 8E) and F (Figure 8F), where sodium chloride-containing calcium carbonate solution was circulated, calcite remained the predominant phase, and vaterite and aragonite were scarcely detected. In Table 5, the calcite content was approximately 97.9% for NW and 97% for MTW, with other phases being minimal.
Finally, when comparing samples G (Figure 8G) and H (Figure 8H), where sodium bicarbonate and calcium chloride solutions were circulated, calcite was observed as the major peak, with no aragonite detected. However, vaterite, which was almost absent in sample G, appeared significantly in sample H. According to Table 5, sample G contained about 93.9% calcite, while sample H had approximately 58.2% calcite and 41.3% vaterite. This is the key novelty in terms of results since the previous literature did not compare or report the percentages of the phases of crystals for different solutions. These results have the potential to open pathways for several different applications of MTW, using various solutions for different purposes.

3.5. Memory Effect

The vaterite phase precipitation fractions in the calcium carbonate powder obtained in Experiment 2.3 are shown in Figure 9. The proportion of vaterite phase precipitated in calcium carbonate that was left to precipitate immediately after magnetic treatment without leaving the water for 10 min showed a similar trend, and the proportion of vaterite phase precipitated was still around 50% after 20 min and leaving the water for 20 min. However, when the magnetically treated water was left for 30 min before being used for calcium carbonate precipitation, the proportion of vaterite phase precipitation decreased to about 30%, which was the same as the amount of vaterite phase precipitation when calcium carbonate was precipitated in water without magnetic treatment, and the results were similar for 40 min. These results indicate that the magnetically treated water in this experiment remembers the state immediately after magnetic treatment for about 20 min after the magnetic field is applied and returns to a state similar to normal water after about 30 min.

4. Discussion

Experimental results so far show that the proportion of vaterite phase precipitation increased by about 25% in the case of MTW with calcium carbonate precipitation by magnetic treatment of ultrapure water at 25 °C. However, when various aqueous solutions were magnetically treated, the increase in vaterite phase precipitation was only as small as MTW, less than 10%, except in the case of sodium bicarbonate and calcium chloride solutions. Comparing the results, it became clear that only when sodium bicarbonate and calcium chloride solutions were simultaneously treated with a magnetic field did the precipitation of the vaterite phase significantly increase. In other solutions, the effect of magnetic treatment was limited; for example, in the sodium bicarbonate solution, the increase in vaterite phase was only from about 10% to 16%, while in the calcium chloride and calcium carbonate solutions, little to no change was observed. In the case of the calcium carbonate solution, the solution used in this experiment contained 0.1 g of calcium carbonate per liter, which significantly exceeds its solubility (0.0014 g/L at 25 °C), suggesting that most of the nuclei had already formed before the treatment.
On the other hand, when pure water was treated magnetically, the vaterite phase was significantly precipitated, suggesting that the water itself was influenced by the magnetic field, changing its properties and potentially affecting nucleation. Furthermore, when sodium bicarbonate and calcium chloride solutions were simultaneously magnetically treated, it was confirmed that both substances were fully dissolved in the solution due to their high solubility, and no nucleation occurred. This again suggests that the water’s characteristics were altered by the magnetic field.
Existing studies report that magnetic field application affects the structure of adsorbed water on solid surfaces and the structure of ion-water complexes, which can influence crystal nucleation and the size of small particles [58]. Therefore, in this study, it was expected that when only one of the solutions was magnetically treated, the magnetic field would affect the water in the solution, leading to an increase in the precipitation ratio of the vaterite phase in both cases. However, when only one of the solutions was actually magnetically treated, an increase in the vaterite phase was observed only in the sodium bicarbonate solution. In this case, the vaterite phase increased 1.5 times, from about 12.2% to 18.2%. In contrast, no significant change was observed in the calcium chloride solution, suggesting that the magnetic field has a specific effect on the sodium bicarbonate solution.
According to existing research, the crystallization of calcium carbonate is a very complex process that occurs in several stages [59]. It has been suggested that in aqueous solutions, Ca2+ and CO32− ions form hydrated shells and weakly bond to each other, forming ion pairs. During this process, the water shell weakens, and disordered hydrated micelles are formed, which eventually leads to the precipitation of solid calcium carbonate. Furthermore, existing research suggests that magnetic field treatment specifically affects the hydration of CO32− ions and may directly influence the polymorphic equilibrium during the precipitation process [60,61]. It is suggested that the reason for the increased precipitation of the vaterite phase when aqueous sodium bicarbonate solutions were treated with magnetic fields in this study is due to the change in the hydration state of the CO32− ions caused by the magnetic field.
According to a report by Otsuka et al., when pure water containing dissolved oxygen was magnetically treated, changes in its properties and memory effects were observed, whereas no such changes were seen in deoxygenated water [36]. It is explained that the dissolved oxygen is affected by the magnetic field, forming a metastable structure of hydrated oxygen, which may influence the crystallization process. It has also been reported that the effect of magnetic treatment disappears when ethanol is added to magnetically treated water due to the strong interaction between ethanol and water [36]. In our study, we applied a magnetic field to water and various solutions for a fixed duration, followed by the precipitation of calcium carbonate in a non-magnetic environment. The experimental results suggest that magnetic treatment alters the properties of water and solutions, which in turn affects the precipitation of calcium carbonate. Furthermore, since these changes persisted even in the absence of the magnetic field, a memory effect can be confirmed quantitatively.
The reason for the lack of a noticeable magnetic treatment effect in the calcium chloride solution may be that calcium chloride readily forms hydrates, which could have inhibited the formation of oxygen-induced hydrates. To elaborate, calcium chloride dissolves in water as Ca2+ ions, which are structure-forming ions with an enthalpy of hydration of −1616 kJ/mol, making it easy to form strong hydration. Sodium bicarbonate, however, dissolves in water as HCO3− ions. This ion has a hydration enthalpy of −383 kJ/mol, making it weakly hydrated compared to the Ca2+ ion. At this time, the calcium chloride solution is thought to inhibit the hydration of oxygen before it is formed by the magnetic treatment due to the strong hydration of the Ca2+ ions. Therefore, the total amount of oxygen hydration, which is believed to affect the precipitation of calcium carbonate, is lower than in the sodium bicarbonate solution, and the magnetic field effect is less likely to appear.
Furthermore, existing studies have shown that carbon dioxide promotes the formation of clathrate structures in water [62]. The results of this study also confirmed that when CO32− ions are present in the solution, the magnetic field effect is more pronounced. It should be noted that the pure water used in this study was exposed to air for 24 h before the experiments to ensure sufficient dissolution of oxygen and carbon dioxide. Therefore, it is believed that dissolved oxygen triggered the memory effect, with dissolved carbon dioxide further enhancing this effect.

5. Conclusions

Magnetic field treatment affected calcium carbonate crystal formation above 15 °C, reducing the proportion of the calcite phase and increasing the precipitation of the thermodynamically unstable vaterite phase by a factor of 1.2 to 2 (depending on treatment temperature). The vaterite phase reached its peak at 25 °C. This was presumably due to the accelerated re-dissolution of the vaterite phase and its transition to the calcite phase.
In addition, under magnetic field-treated water, the particle size distribution of the crystals narrowed, and the average particle size decreased at all temperatures above 5 °C. This is thought to be due to the fact that the vaterite phase did not re-dissolve as it should have, suggesting that magnetically treated water inhibits the re-dissolution and transition of unstable vaterite.
A ‘memory effect’ was identified whereby magnetic field treatment altered the properties of the water and solution, with lasting effects on crystal nucleation and growth. In particular, dissolved oxygen and carbon dioxide were suggested to contribute to this effect, with increased formation of vaterite phases in magnetic field-treated water. In addition, this study has succeeded in indexing the memory effect of water and stabilizing the precipitation of vaterite, which is an unstable phase, at around 25 °C. Quantitative measurements of the vaterite and calcite phases at various temperatures assure standardization at 25–45 degrees Celsius, where the vaterite phase, which is less likely to adhere to the pipe wall, appears more efficiently. In addition to the quantitative measurements, the standardization is extended to multiple calcium salt solutions, with Calcium chloride and Calcium bicarbonate mixture providing the most optimum outcome.
Based on these results, this study quantitatively evaluated and parameterized the effects of magnetic fields on water and calcium carbonate. Magnetic field treatment shows the potential to control the crystallization process of calcium carbonate and has potential applications in well water and water pipe scale control and water flow system design, especially when operated at around 25 °C. In order to investigate the complex effects of magnetic water treatment in real situations, the following priorities for future research directions were established:
i.
In existing power plants and new technology power plants (such as ultrasupercritical coal, water gas shift reactors, etc.), as to how the process can be optimized, including life-cycle assessments [63];
ii.
Novel technologies such as Ocean thermal and sea-water air-conditioning systems, which rely heavily on mineralized sea water and are prone to scale formation, should be investigated;
iii.
Hydraulic systems that suffer from scaling effects should be optimized in future research to test the efficacy of magnetic fluid treatment.
From an academic front, the effects of the duration dependency of magnetism exposure should be quantified. In addition, the reasons why vaterite phases are stabilized by magnetically treated water should be further investigated to determine how nucleation and hydration are affected by magnetic treatment. This simple technology can prove to be a major eco-friendly way of achieving waste treatment and could increase the efficiency and lifetime of thermal power generation, playing a major role in decoupling.

Author Contributions

Conceptualization, A.A.M.S., K.I. and H.O.; methodology, A.A.M.S., K.I. and H.O.; software, A.A.M.S.; validation, A.A.M.S., K.I., T.O. and H.O.; formal analysis, A.A.M.S.; investigation, A.A.M.S. and K.I.; resources, A.A.M.S.; data curation, A.A.M.S.; writing—original draft preparation, A.A.M.S. and S.B.; writing—review and editing, T.O., S.B. and H.O.; visualization, A.A.M.S.; supervision, H.O.; project administration, S.B. and H.O.; funding acquisition, T.O. and H.O. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not Applicable.

Informed Consent Statement

Not Applicable.

Data Availability Statement

Data will be made available upon request to the first author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Magnetic Processing Equipment Overview. (The arrows show the direction of fluid flow).
Figure 1. Magnetic Processing Equipment Overview. (The arrows show the direction of fluid flow).
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Figure 2. SEM images of calcium carbonate precipitated under NW and MTW conditions at (a) NW at 5 °C, (b) MTW at 5 °C, (c) NW at 15 °C, (d) MTW at 15 °C, (e) NW at 25 °C, (f) MTW at 25 °C, (g) NW at 35 °C, (h) MTW at 35 °C, (i) NW at 45 °C, and (j) MTW at 45 °C.
Figure 2. SEM images of calcium carbonate precipitated under NW and MTW conditions at (a) NW at 5 °C, (b) MTW at 5 °C, (c) NW at 15 °C, (d) MTW at 15 °C, (e) NW at 25 °C, (f) MTW at 25 °C, (g) NW at 35 °C, (h) MTW at 35 °C, (i) NW at 45 °C, and (j) MTW at 45 °C.
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Figure 3. Hollow calcium carbonate spherical crystals.
Figure 3. Hollow calcium carbonate spherical crystals.
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Figure 4. XRD patterns of calcium carbonate precipitated under NW and MTW conditions at 5 °C to 45 °C.
Figure 4. XRD patterns of calcium carbonate precipitated under NW and MTW conditions at 5 °C to 45 °C.
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Figure 5. Precipitation ratio of vaterite phase under NW and MTW conditions.
Figure 5. Precipitation ratio of vaterite phase under NW and MTW conditions.
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Figure 6. DLS results of calcium carbonate precipitated under NW and MTW conditions at 5 °C to 45 °C.
Figure 6. DLS results of calcium carbonate precipitated under NW and MTW conditions at 5 °C to 45 °C.
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Figure 7. Plot of the average particle diameter obtained from Table 4.
Figure 7. Plot of the average particle diameter obtained from Table 4.
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Figure 8. XRD patterns of calcium carbonate precipitated from various solutions (samples A to H of Table 2) using NW and MTW.
Figure 8. XRD patterns of calcium carbonate precipitated from various solutions (samples A to H of Table 2) using NW and MTW.
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Figure 9. Percentage of vaterite phase precipitation in calcium carbonate precipitated by MTW left for a certain time after magnetic treatment.
Figure 9. Percentage of vaterite phase precipitation in calcium carbonate precipitated by MTW left for a certain time after magnetic treatment.
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Table 1. Sample preparation conditions (NW and MTW).
Table 1. Sample preparation conditions (NW and MTW).
Sample No.Type of WaterWater Temperature [℃]
Sample 1NW5
Sample 2MTW5
Sample 3NW15
Sample 4MTW15
Sample 5NW25
Sample 6MTW25
Sample 7NW35
Sample 8MTW35
Sample 9NW45
Sample 10MTW45
Table 2. Sample preparation conditions (solutions).
Table 2. Sample preparation conditions (solutions).
Sample NameType of WaterCirculating Solution
Sample ANWNaHCO3
Sample BMTWNaHCO3
Sample CNWCaCl2
Sample DMTWCaCl2
Sample ENWCaCO3(+NaCl)
Sample FMTWCaCO3(+NaCl)
Sample GNWNaHCO3 and CaCl2
Sample HMTWNaHCO3 and CaCl2
Table 3. Rietveld analysis results of the XRD patterns obtained from Figure 4.
Table 3. Rietveld analysis results of the XRD patterns obtained from Figure 4.
NWMTW
CalciteVateriteAragoniteCalciteVateriteAragonite
5 °Caverage (%)97.11.30.497.91.11
standard error1.20.90.40.70.60.1
15 °Caverage (%)94.84.00.489.79.50.9
standard error3.33.40.30.90.80.2
25 °Caverage (%)73.226.00.848.150.90.9
standard error1.51.60.22.22.20.2
35 °Caverage (%)7029.40.664.435.00.6
standard error3.33.40.13.33.30.1
45 °Caverage (%)77.622.90.458.940.60.5
standard error3.53.90.24.74.60.2
Table 4. Summary of the DLS Results obtained from Figure 6.
Table 4. Summary of the DLS Results obtained from Figure 6.
Peak Center (µm)Range (µm)Average (µm)
5 °CNW2.081.202.08
MTW2.051.122.01
15 °CNW2.542.382.63
MTW2.401.262.42
25 °CNW2.722.812.75
MTW2.601.602.66
35 °CNW2.813.832.97
MTW2.802.392.83
45 °CNW2.883.893.11
MTW2.782.172.83
Table 5. Rietveld analysis results of the XRD patterns obtained from Figure 8.
Table 5. Rietveld analysis results of the XRD patterns obtained from Figure 8.
NWMTW
CalciteVateriteAragoniteCalciteVateriteAragonite
Pure Wateraverage (%)73.226.00.848.150.90.9
standard error1.51.60.22.22.20.2
NaHCO3average (%)87.012.20.880.518.21.3
standard error3.52.10.32.82.00.3
CaCl2average (%)92.26.10.791.27.90.9
standard error2.01.50.22.12.40.4
CaCO3
(+NaCl)		average (%)97.91.70.497.02.10.9
standard error2.12.50.33.33.30.5
NaHCO3 and CaCl2average (%)93.96.50.658.241.30.5
standard error2.22.70.43.53.20.2
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MDPI and ACS Style

Sayed, A.A.M.; Basu, S.; Ogawa, T.; Inagawa, K.; Okumura, H. Elucidating the Memory Effects of Magnetic Water Treatment via Precipitated Phase Changes of Calcium Carbonate. Eng 2025, 6, 26. https://doi.org/10.3390/eng6020026

AMA Style

Sayed AAM, Basu S, Ogawa T, Inagawa K, Okumura H. Elucidating the Memory Effects of Magnetic Water Treatment via Precipitated Phase Changes of Calcium Carbonate. Eng. 2025; 6(2):26. https://doi.org/10.3390/eng6020026

Chicago/Turabian Style

Sayed, Aly Ahmed Mohamed, Soumya Basu, Takaya Ogawa, Keito Inagawa, and Hideyuki Okumura. 2025. "Elucidating the Memory Effects of Magnetic Water Treatment via Precipitated Phase Changes of Calcium Carbonate" Eng 6, no. 2: 26. https://doi.org/10.3390/eng6020026

APA Style

Sayed, A. A. M., Basu, S., Ogawa, T., Inagawa, K., & Okumura, H. (2025). Elucidating the Memory Effects of Magnetic Water Treatment via Precipitated Phase Changes of Calcium Carbonate. Eng, 6(2), 26. https://doi.org/10.3390/eng6020026

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