Influence of Magnetic Field on Calcium Carbonate Precipitation: A Critical Review

Abstract

This review reports a critical study on the effect of magnetic fields on the precipitation process of calcium carbonate scale from hard water. Indeed, the harmful consequences of the water scaling phenomenon urged researchers to find effective solutions. One of the interesting antiscaling processes is the magnetic treatment of water, which triggers a reduction in the precipitation of calcium carbonate on the walls when in contact with hard water. In the present review, we discuss selected examples related to this process in a combined analysis of the latest advances and the mechanism of action of the magnetic field. Despite the diversity of studies investigating this phenomenon, the effectiveness of this treatment remains a controversial issue, and it is not possible to obtain a clear explanation of the phenomenon. This review proposes, finally, interesting hypotheses which can effectively explain the effect of magnetic treatment on the behavior of hard waters and the precipitation of calcium carbonate, which include magnetohydrodynamics and the hydration effect.

1. Introduction

Water is essential for every living organism, not only for drinking but also for most human activities. Despite the water supplies that cover the needs of the world’s population, the quality of water is still considered a hot topic and at the heart of research concerns. Unfortunately, water is not distributed evenly over the earth’s surface. While some countries have enormous renewable resources each year, others do not have enough and are facing enormous supply difficulties. The problem is acute in the countries with arid and semi-arid climatic weather. In these countries, in addition to water scarcity, there is the problem of poor quality. Indeed, waters are characterized by strong mineralization due to the high concentration of ions such as calcium, magnesium, carbonates, and sulfates. Some factors such as temperature and/or concentrations lead to the precipitation of poorly soluble salts commonly known as scale, mainly formed of calcium carbonate, on the walls of pipes, heat exchangers, etc. These deposits can be troublesome for several reasons and the consequences are multiple: hydraulic, technologic, energetic and economic. The most commonly encountered scale is that based on calcium carbonate, at least for sanitary, potable, irrigation and process water networks. Different sources that are likely to produce this type of deposit in domestic systems were selected and are presented by Boluda-Botella [1] in Figure 1.

Calcium carbonate scaling is a major hard-water problem in many parts of the world. In North America and particularly in the Midwest and Southwest United States, hard water is common. Some regions of Europe, such as the United Kingdom, Germany and France, also have high levels of water hardness. In Asia, particularly India and China, water hardness levels can vary considerably depending on local water sources. And finally, some regions of sub-Saharan Africa, especially those dependent on groundwater, experience hard-water problems.

Calcium carbonate deposition increases operating and maintenance costs, as it lowers the flow and heat transfer capabilities of hot water systems and increases the energy consumption of pumps in drinking water systems. In the last decade, there have been considerable advances in the development of chemicals to control scale deposits. However, the process is still expensive and harmful for both environment and health. In addition, CaCO₃ scaling mechanisms involve various physicochemical reactions related to the chemical composition of water, the nature of the walls involved, and the local variations in the composition appearing at the interface. Whenever water temperature or speed gradients are established, the phenomenon becomes more complex, and therefore, it is difficult to determine the exact amount of chemicals needed.

To avoid all the problems related to chemical treatments, various physical processes of antiscale treatment, such as electrical and magnetic processes, were proposed and employed. The latter method, which is the main objective of this review, has been studied by several researchers. Herein, we will present the main results, which will allow us to have clear ideas of the usefulness of this technique.

Several publications highlighted magnetic field application as a physical antiscaling method in the last decade. Among them, there are a large number of reviews [2,3,4,5]. A distinctive feature of this review is that general analysis of the state of the art is combined with detailed consideration of the most important studies on the theories of precipitation of calcium carbonate. This review aims to understand the mechanism of action of the magnetic field as well as the limitation of such a physical technique dedicated to water treatment for the prevention of scaling.

2. Magnetic Antiscaling Treatments

Whether for industrial or domestic use, there are many water treatment systems. These antiscaling procedures can be classified into two main classes:

-

Chemical processes such as the addition of acid [6], lime decarbonation and the addition of crystallizing inhibitors [7,8];

-

Physical processes based on the use of one or more physical techniques (electric field, magnetic field, ultrasound, etc.) [9,10,11,12,13,14] such as electrochemical softening and treatments by applying a magnetic field. The magnetic field is a fundamental physical measure that is crucial for the manipulation of various systems [15,16] found in research laboratories (nuclear magnetic resonance, mass spectroscopy, etc.) and hospitals (magnetic resonance imaging).

2.1. Magnetic Device Configuration

Various devices based on magnetic water treatment, domestic or industrial, are marketed throughout the world [17,18,19,20,21,22,23,24]. Some manufacturers use electromagnets, while others use permanent magnets that can be simple or arranged with different orientations of the magnetic field. Salman et al. [20] classified commercial magnetic-processing devices using permanent magnets into four categories (Figure 2 and Figure 3) depending on the orientation of the magnetic field, either perpendicular or parallel to the flow, in the intrusive and non-intrusive direction. In classes I and IV, the applied field is parallel to the flow direction, while in classes II and III, the field is perpendicular to the flow. Most of the scientists [20,21,23,25] used a device constructed of alternating pairs of permanent magnets, while Gryta [26] used a magnetic field originating from two S–S-type permanent magnets.

For the environmental impact of magnetic treatment, MWT has environmental benefits, including reducing the need for chemical inhibitors and improving water quality. However, it is essential to consider the environmental impact associated with the manufacturing and disposal of the equipment used. A thorough assessment will identify the best practices to minimize environmental impact while maximizing the benefits of magnetic treatment.

2.2. Permanent Magnetic Field

Using permanent magnetic devices, various studies have been performed in recent decades on drinking water [27,28,29,30], irrigation water [31,32,33,34,35,36,37,38,39,40,41,42,43,44], wastewater [44,45], seawater [46,47], synthetic (artificial) hard water [22,48,49,50,51,52,53,54], etc. Different parameters (morphology, hardness, density, etc.) were tested to determine the necessary conditions for the magnetic field influencing the behavior of water.

2.3. Alternating Magnetic Fields

Using rotating devices through which pulsed and alternating fields can be applied to calcocarbonic water, Oshitani et al. [55] observed the following:

-

The processing time required to reach the maximum effect in alternating or pulsed mode is much lower than in static mode.

-

The magnetic field frequency has a remarkable effect on the size and morphology of the formed calcium carbonate crystals.

-

Compared to static magnetic fields, the pulsed and alternating magnetic fields induce the formation of more stable crystals.

To assess the effectiveness of magnetic processing compared to traditional chemical processing methods, a cost–benefit analysis shows that long-term benefits, such as reduced operating costs and sustainability, could make magnetic processing more attractive despite potentially higher initial costs.

2.4. Effect of Dissolved Gases on Magnetic Treatment

Vallée et al. [56,57] evaluated the effect of alternating electromagnetic fields, with frequencies ranging from 5 to 400 kHz, on pure water using the elastic light scattering technique. Herein, the exposure of the pure water to the magnetic field reduces the maximum intensity of light scattering by 20 to 30%. This decrease is due to the reduction in the number of diffusers that are supposed to be microbubbles. The dynamic diffusion of the light shows that the diameter of the bubbles affected by the exposure to the electromagnetic fields is 300 nm. The double ionic layer at the gas/liquid interface stabilizes these bubbles in the liquid. Under the action of the field, this double ionic layer is disturbed, causing the nano-bubbles’ destabilization. Thus, the field would contribute to their disappearance. Moreover, the effect of a pulsed low-frequency electromagnetic field on the photoluminescence of water has been studied. The authors observed a decrease of 70% in the relative intensity of the photoluminescence of the emission band at 425 nm, which is attributed to the degassing of water. The magnetic field indirectly reduces the luminescence by destabilizing the bubbles due to the perturbation of their double ionic layer.

Similarly, Otsuka et al. [58] and Ozuki et al. [59] analyzed the Raman water bands by irradiating UV light at 400 nm using a 298 K spectrophotometer fluorescence. They did not notice any change in the water properties (no change in the Raman spectrum) even after the treatment of perfectly pure water (prepared by vacuum distillation of ultra-pure water) by a magnetic field of 6 T. However, when the same magnetic treatment is performed after the pure water has been exposed to oxygen, certain water properties such as the hydrogen bond vibration modes and the electrolytic potential change. The effect of magnetic treatment on water was quantitatively evaluated by measuring the contact angle of distilled water on a platinum plate. The measurements showed that the contact angle of the vacuum-distilled water was unchanged, which proves, according to the authors, that in this case the water was not “magnetized”. After stopping the magnetic treatment, the lowest contact angle is maintained for 20 min for the magnetic treatment in a closed system and 60 min for the magnetic treatment in air.

Crystallization tests of a mixture of CaCl₂ solution and Na₂CO₃ in both oxygenated and non-oxygenated medium were used as controls for the magnetic or non-magnetic effect on water. When vacuum-distilled water was employed, the magnetic treatment in a closed system had no effect on the formation of the residue under its calcite variety. However, using water, after exposure to oxygen or air, the X-ray powder diffraction (XRD) and Scanning Electron Microscope (SEM) analyses showed that CaCO₃ crystallizes under its aragonite variety. After the effect of the magnetic field has disappeared, the contact angle resumes its initial value, and CaCO₃ again precipitates as calcite.

2.5. Magnetic Treatment of Natural Waters

Crolet and Ledion [60] tested the effect of a magnetic field on the water of the Paris network. This water is slightly calcified and therefore presents a risk of scaling. For magnetic treatment, these authors used a commercial apparatus, for which they made several modifications. Various variants have been studied: the commercial apparatus, apparatus without a magnetic field, a magnetic device with a solid metal body, a magnetic device with a plastic body, etc. The tests carried out showed that only a certain number influenced the scaling character of the water. However, this effect appears to vary considerably depending on the employment conditions.

Moreover, only the magnetic device with a plastic body results in appreciable beneficial scaling effects. The substantial increase in the magnetic field does not seem to have a decisive effect on the effectiveness of the treatment. The authors conclude that the only possible interpretation seems to be a change in the load of certain colloids already in the water. The modification of their zeta potential could explain a subsequent modification of the germination phenomena on the walls located downstream of the treatment zone. Moreover, they suggest that these magnetic devices could not generate a new population of calcium carbonate colloids.

Lipus and Dobersek [61] conducted experiments with tap water containing concentrations of Mg²⁺ and Fe²⁺ ions sufficient to inhibit the precipitation of calcite, a variety that generally precipitates in tap water. The efficiency of the magnetic treatment was evaluated by comparing the quantities precipitated in boilers and pipes for three weeks in the presence and absence of magnetic treatment. The results show that the magnetic treatment can reduce the thickness of the scale deposited on the hot surfaces and protect the pipe from complete scaling. The aragonite variety of CaCO₃ constitutes all the tartars.

2.6. Effect of Magnetic Field on Seawater

Few studies have examined the effect of magnetic treatment on seawater. Gabrielli et al. [62] studied the efficiency of a permanent magnetic treatment with an intensity of 0.16 T on the precipitation of CaCO₃ in synthetic seawater. The electrochemical accelerated scaling method coupled with a quartz microbalance is used to evaluate the effect of the treatment. It was shown that the effect of the magnetic field is more critical for seawater than for fresh water. Indeed, the magnetic treatment of seawater causes a marked increase in the germination time of calcium carbonate compared to untreated water. The author attributed this effect to the very high conductivity of seawater.

Al-Qahtani [46] examined the effect of a 0.7 T magnetic field on dissolved ions in seawater, which were desalted by the reverse osmosis technique. Pipes and tanks are made of polyvinyl chloride (PVC). After treatment, Al-Qahtani noticed that the pH and conductivity of seawater had increased. In addition, he found that the salt concentrations of the treated solutions were consistently lower than those of the untreated seawater. This is why the separation of salts by reverse osmosis is improved when seawater solutions are magnetically treated. Therefore, it has been assumed that the magnetic device can cause physical and/or chemical changes in the nature of ions, which have a low possibility of passing through the membrane. Trueba [40] electromagnetically treated seawater used as a refrigerant fluid in a heat exchanger–condenser. He showed that this treatment accelerated the ionic nucleation of calcium and precipitated it as CaCO₃ in the bulk of the water, and this induced a decrease in the biofilm adhered to the internal surface of the tubes.

2.7. Effect of Flow Rate on the Efficiency of Magnetic Treatment

The flow of water through the treatment device may be one of the essential parameters that influence the efficiency of the magnetic field, which is why this parameter has been the subject of numerous studies.

Martemianov and Sviridov [63] studied the effect of a magnetic field on the hydrodynamic behavior of solutions near the wall during flow. To carry out this study, they used two experimental methods. The first is based on the direct mass transfer measurement using the radioactive ⁵⁹Fe^- isotope. Contamination with a known amount of radioactive material is carried out on the inner surface of a tube. By measuring the radiation rate of the fluid discharged, it is possible to estimate the rate of removal of the particles from the surface. The most important conclusion of this study is that a non-stationary magnetic field is more efficient than a stationary field. The second method is an electrochemical characterization of the flow based on measurements of the diffusion limiting current of a fast electrochemical reaction on a mini electrode placed in the flow. The experiments were carried out in steel tubes. The authors show that the magnetic effect is localized in the hydrodynamic layer.

2.8. Effect of Walls

Few studies have focused on the effectiveness of magnetic treatment as a function of the nature of the wall through which the magnetic field is applied [64,65,66,67,68,69,70,71].

Amiri and Dadkhah [63] studied the reduction in the surface tension of water under the influence of a magnetic field. Their measurements show that the surface tension of tap water and pure water ceases to decrease after several treatment cycles and then remains constant. However, a circulation without a magnetic field can also cause the same effect. Therefore, the main factor influencing the reduction in the interfacial tension would be related to the solid and dissolved impurities that can be introduced through the plastic pipe (Tygon) by erosion and dissolution mechanisms. The same interpretations have been proposed by Alimi et al. [54] who have tested pipes made of Tygon, Teflon (PTFE), PVC, copper, and stainless steel, and found that the Tygon pipe had the highest amounts of calcium carbonate precipitation while the PTFE pipe had the lowest.

Lipus and Dobersek [61] studied the effectiveness of magnetic tap water treatment on the formation of tartar in two types of pipes by evaluating the quantities adhered after three weeks. The tartar formed on a heated spiral copper pipe was 2.5 times less thick under the effect of magnetic treatment of water than in its absence. In a zinc-coated steel pipe, a fragile, brittle layer was formed in contrast to the case where no treatment was applied for which a hard inner lining was formed.

Busch et al. [66,67] circulated water in two types of pipes (PVC and metal) with and without magnetic treatment. The device consisted of a coil with two magnetized rods inserted into the pipe, symmetrically oriented along the axis. Two field strengths were chosen: 0.125 T and 0.15 T. The field was perpendicular to the fluid flow. The pH values were monitored within the solution and on the pipe walls. Experiments have shown that, without magnetic treatment, the surfaces are more alkaline than in the solution. When magnetic treatment is used, the surface of a metal pipe becomes more acidic than within the solution, while it becomes more alkaline near a PVC wall.

The most exciting results on the effect of the walls in the presence of a magnetic field were the subject of the work of Gabrielli et al. [62]. They examined the influence of four different materials (PVC I, PVC II, stainless steel and copper) through which a magnetic field with a density of 0.16 T was applied to a pure calcocarbon solution. It has been shown that the efficiency is optimal for a given flow rate of the water, which makes it possible to attribute the effect of the treatment to a magnetohydrodynamic process. However, the effectiveness of the magnetic treatment depends on the nature of the material used in the process of piping. Indeed, the results show that the effect is more pronounced for conductive materials than for insulation. However, this effect also exists near PVC II walls (PVC with additives). This indicates that electrokinetic phenomena could be involved, alone or in addition to magnetohydrodynamic processes.

A study of the heterogeneous precipitation of calcium carbonate on different wall types (aluminum, glass, stainless steel, copper) was also carried out [68]. It was noticed that in the absence of a magnetic device, the quantity deposited on the surface decreased as the temperature increased. When the walls on which the precipitation takes place are placed between two commercial permanent magnets of the south–south pole and of intensity 0.1 T for 2 h, the amount deposited decreases, and this decrease depends on the nature of the wall on which the precipitation has place. The most important effect was observed on the glass, at 60 °C, where the decrease in deposited precipitate reached 50%. However, no deposition was detected on aluminum at 80 °C.

Colic and Morse [69,70] also followed the precipitation of calcium carbonate on the metal/solution scaling interfaces. Zinc, used in industrial applications, was chosen as the test metal and the scaling solutions were saturated solutions of calcium carbonate. The measured weight change from the zinc sample (area 5 × 1 cm²) placed in the test water bath maintained at a constant temperature with a water flow of 0.5 μm⁻¹ showed that the treatment with (2 × 10⁴ V) decreased the deposition rate of calcium carbonate relative to the untreated solution, while the lower amplitude (6 × 10³ V) treatment increased the amount deposited.

For their part, Zienicke and Krasnov [71] observed that the velocity profile within the solution is influenced by the Lorentz force, which generates the formation of a thin boundary layer. The thickness reduction of this boundary layer under the influence of the magnetic field accentuates the effect of the nature of the wall and, consequently, the increase in the mass transfer from the wall to the solution.

In work published by Bayoumi et al. [72], the experimental data reveal an increase in the TDS of treated tap water with the intensification of the magnetic field intensity. This observation gives rise to the idea that the magnetic water treatment procedure could potentially facilitate the TDS suspended in the tap water, potentially leading to an increase in total dissolved salt levels. As the magnetic field intensity increases, there is a possibility of inducing changes in the interactions between suspended salts and water molecules, leading to the release of previously insoluble constituents into the aqueous solution.

In addition, the influence of the nature of the piping used during the treatment was investigated. Iron and PVC pipes consistently show a reduction in TDS levels as flow rates increase, suggesting that magnetic water treatment effectively changes water properties regardless of pipe material. Although TDS levels decrease in iron and PVC pipes, the processes that cause this phenomenon can vary between the two materials. The significant decrease in TDS levels in iron pipes is due to the impact of magnetic water treatment, enhanced by interactions between water and ferrous materials. These interactions are expected to significantly change the behavior of dissolved ions and enhance the effect of magnetic field strength on TDS concentrations. The inert nature of PVC pipes limits the possibility of substantial changes in water chemistry. However, the decline in TDS levels as flow rates increase indicates that magnetic water treatment can affect water quality in PVC pipe systems, although not as significantly as in iron pipes.

Although there are many observations on this work, the treatment time does not exceed fractions of a second, and there are no detailed data about the composition of the treated water or whether the water was filtered before treatment or not. There is also no information about the method of measuring dissolved salts or the accuracy of the device’s measurement, because sometimes the small observed differences can fall within the range of measurement errors.

2.9. Persistence of the Magnetic Field

The memory effect or “persistence” of magnetic treatment, often discussed in the context of precipitation and other chemical processes, refers to the ability of a solution to “remember” the effects of a magnetic field even after the latter has been removed.

The memory effect of magnetic treatment may play a crucial role in process efficiency, but it requires extensive investigation to determine its nature and durability. The need for repeated treatment will depend on the results obtained, the stability of the memory effect and the specific requirements of each application. It is believed that exposure to a magnetic field can change the structure of water molecules and dissolved ions. This could influence the dynamics of interactions between particles, even after the field is removed.

Some studies have shown significant memory effects, while others have failed to confirm these observations. This may be due to differences in experimental conditions, water composition, or other variables. The length of time this memory effect persists can vary. Studies indicate that the effect may wane rapidly over time, while others suggest that it may persist over prolonged periods.

The duration of the magnetic field effect persistence was estimated to be 120 h for Highashitani et al. [73] and Baker and Judd [74]; Coey and Cass [75] estimated it to be 200 h. Concerning Lv et an. [76], the magnetic memory time is affected by the intensity of the magnetic field employed in the treatment, while the treatment time and the hardness of the solution are secondary factors. On the contrary, Fathi [22] observed that the memory effect is influenced by all the treatment parameters (concentration, treatment time, piping materials, etc.).

We note an agreement between the various researchers concerning the existence of this time of persistence. Disagreement over duration can be attributed to experimental conditions. Additional studies will help clarify these questions and optimize treatment protocols.

3. Magnetic Treatment and Crystallization of Calcium Carbonate

The effect of the magnetic field on the precipitation of calcium carbonate is influenced by several parameters: pH, degree of supersaturation, ionic strength of the treated water, flow rate, treatment time or density of the field used. In this section, an analysis of the main studies that have been carried out is conducted, considering all these factors.

3.1. Crystal Growth

Different theories have been put forward to explain the phenomenon of crystal growth. These theories suggest that this growth can occur by one of the following processes:

-

Direct incorporation of the ions into the crystal formed;

-

Germination on the surface of the crystallite, also known as secondary germination;

-

Agglomeration of particles.

Researchers believe that these three processes can occur simultaneously at different points in the crystallization unit [77,78,79].

3.2. CaCO₃ Precipitation Models

The formation of a nucleus or embryo is achieved according to the following steps [77]:

-

Formation of the ion pairs by electrostatic interactions between the cations and the anions dissolved in the solution;

-

Aggregation of these ion pairs to form pregermination entities which are in dynamic equilibrium with the solution;

-

Growth of aggregate size to critical size. These aggregates are the result of the germination of the solid-state particles.

Most of the calculation models of the calcocarbonic system, including germination, form an integral part of the various anhydrous forms of CaCO₃ but never those of the hydrated forms. However, a gradual transformation of the initially precipitated phase from a metastable state to a more stable final crystal phase is generally necessary during precipitation. The metastable phase can be formed from an amorphous precipitate or a set of hydrated species that may contain contamination. This follows the rule introduced by Ostwald [80], which states that a metastable phase must necessarily pass through transient phases accompanied by progressive losses of free energy to transform into a more thermodynamically stable phase.

The formation in the supersaturated solution of precipitation precursors breaks the metastable equilibrium. The precipitation model is based on calculating the probability of the appearance of a critical germ, defined as a nucleus whose size is sufficient for growth, not dissolution. Thus, he assumed that the critical seed would be obtained from Ca²⁺ ions and associated with micelles whose complexity would increase with supersaturation. The disadvantage of this precipitation model is that it does not consider the hydrated, intermediate forms of the anhydrous varieties.

According to Ledion et al. [81], the scaling process follows three stages: germination, dehydration and growth. The process begins with the agglomeration of hydrated Ca²⁺ ions and CO₃²⁻; then, these ion pairs are grouped together to give a seed with an electrical charge that can grow and tends to dehydrate more and more to give a crystal that will have its own growth.

Gal et al. [82] propose a model in which the Ca²⁺ and CO₃²⁻ ions are surrounded by molecules of solvent to form a CaCO₃ precursor. Initially, these micelles would be highly hydrated and very disordered in images of amorphous CaCO₃. Consequently, the micelles would lose part of their hydration and would fit in a structure of order close to that of a less hydrated form of CaCO₃ solid. If the rate of CaCO₃ supersaturation of the solution is high, the precursors do not have the necessary time for their evolution, and the breakdown of the metastable state is linked to the precipitation of the amorphous carbonate. However, if the rate of supersaturation is low, the limit of the metastable domain coincides with a lower degree of supersaturation and the degree of dehydration progress achieved for a more ionic product. As soon as the solubility limit of one of the crystalline varieties is reached, the metastability state is broken by the precipitation of this form. This condition of exceeding the solubility product of the hydrated forms to trigger precipitation has been proven experimentally [83,84,85].

3.3. Effect of Magnetic Treatment on Precipitation Kinetics

One of the important parameters or physical properties characterizing the surface of a solid in contact with a liquid is the zeta potential, which contributes to repulsive or attractive interactions [86]. Its value and sign result from the electrical properties of the solid interfacial layer/electrolyte. Although calcium carbonate is not an oxide or a hydroxide, to some extent, its external charge depends indirectly on the pH of the solution due to the hydrolysis reactions taking place on its surface. Therefore, the zeta potential may also change as a function of the pH of the solution.

Applying the magnetic field induces a change in pH and influences the values of the zeta potential. This effect depends on the molar ratio. If this ratio is greater than 1, the zeta potential is positive and increases in value. Otherwise, the potential is negative. In the case of equimolarity, the initial positive value changes sign but has very little influence on the precipitation of calcium carbonate [87,88,89,90]. According to the author, all these effects have been explained by the fact that the magnetic field changes the state of hydration of the ions present (CO₃²⁻, HCO₃⁻, Ca²⁺ and H⁺). This change can affect the germination and velocity of CaCO₃ crystal growth.

This effect contradicts what Knez and Pohar [90] assert. These researchers showed that the magnetic field has no significant influence on the zeta potential of the calcium carbonate precipitate and that the effect observed on the crystal structure cannot be attributed to a magnetohydrodynamic effect. The results suggest that the magnetic field influences the phase equilibrium between the polymorphs by modifying the CO₂/water interface by ion hydration before forming stable nuclei in the solution. This has been proved by Highashitani et al. [72] and Barrett and Parsons [91] who studied the effect of the magnetic field applied to equimolar solutions (8 × 10⁻³ mol.L⁻¹) of CaCl₂ and Na₂CO₃ before mixing on the resulting properties of the precipitated calcium carbonate. The static magnetic field was applied at different flux densities (up to 0.6 T) for processing times up to 30 min. They found that the properties of the precipitated CaCO₃ are influenced only when the magnetic field is applied to the solution of Na₂CO₃. The minimum magnetic flux intensity necessary for the effect was evaluated at 0.3 T and the minimum processing time at 10 min. On the other hand, exposure to the magnetic field reduces the coagulation rate of colloids by more than 10% compared to the case in the absence of the magnetic field, which blocks the formation of CaCO₃ crystals, although the magnetic field density is not too high. Contrary to this observation, Du et al. [92] concluded through experiments that the application of an electromagnetic field even at low intensities (0.03 mT) favored the precipitation of CaCO₃ in bulk solutions.

However, regarding CaCO₃ precipitation kinetics, the available literature often has contradictory results. Some authors [72,93] show that the magnetic field slows germination and accelerates the rate of crystal growth, while others [94,95] argue that it accelerates germination (reduces germination time) and increases the rate of crystal growth.

Barrett and Parsons [91] used spectrophotometry to study the effect of the magnetic field on the formation of tartar from solutions of CaCl₂/Na₂CO₃ and CaSO₄/Na₂SO₄, and the results of this study agree with earlier studies [93]. Application of the magnetic field delays germination and accelerates crystal growth. In addition, they show the existence of the memory effect, the time that a magnetically treated solution retains the effect of this treatment.

The spectroscopic technique is also applied in the study by Kney and Parsons [90]. The results contradict those found by the same authors [96,97], since they observe that the magnetic field influences the treated solutions only when they contain solid particles of CaCO₃. Moreover, they observed that the reproducible results are recorded only in a pH range between 10.46 and 10.96. Furthermore, Parsons and his team show that the precipitation rate of calcium carbonate from a solution circulating orthogonally to a magnetic field with a density of 0.7 T is reduced to 80% and the size of the crystals formed decreases. This effect depends very much on the chemistry of the solution (composition and pH).

Tai et al. [98,99] studied the effect of a permanent magnetic field on the growth rate of calcite crystals for different working conditions, such as supersaturation, pH, and ionic strength. They have shown that this velocity decreases in the presence of the magnetic field. It decreases further when the intensity of the field increases.

3.4. Effect of Magnetic Treatment on the Crystallinity of CaCO₃

Abundant studies on the effect of the magnetic field on the nature of the precipitated CaCO₃ phase are found in the literature. Although there are contradictions, most researchers agree that water exposure to a magnetic field results in the precipitation of CaCO₃ under its aragonite variety.

Highashitani et al. [72] and Barrett and Parsons [89] argue that magnetic treatment favors the formation of aragonite at the expense of other anhydrous varieties of CaCO₃.

Similarly, Kobe et al. [100] show that the proportions of deposited calcite/aragonite/vaterite depend on the intensity of the magnetic field and the speed of water circulation through the system. The percentage of aragonite relative to calcite increases with the intensity of the field. This effect is less important on the rate of vaterite, which remains virtually unchanged under all conditions. This agrees with the work of Coey and Cass [75]. They show that passing calcocarbon water (containing 120 mg.L⁻¹ of initially dissolved CaCO₃) through a static magnetic field (0.1 T) increases the aragonite/calcite ratio.

This property of promoting the precipitation of aragonite in the presence of a static magnetic field was also observed by Chibowski et al. [87]. They exploited these data to explain the non-incrustation of the tartar formed on the walls of different materials exposed to the magnetic field. Indeed, unlike calcite, aragonite is known for its non-adherent nature. Similarly, Cefalas et al. [101] observed that the magnetic treatment favors the adhesion of aragonite and vaterite on a surface made of stainless steel 304 and inhibits the precipitation of the calcite variety. This effect is accentuated by the increase in applied field intensity.

Although most studies agree on the inhibitory effect of calcite formation in favor of aragonite when water is magnetically treated, some studies have contradictory results. Alimi et al. [22] observed that without any treatment, the crystals exhibit the characteristic morphology of the vaterite form. After MF treatment, the formation of the calcite form is promoted (Figure 4). Boluda-Botella et al. [1] observed that magnetic treatment limits the recrystallization of aragonite to calcite and decreases the formation of calcium carbonate on the wall of pipes. The work of Beruto and Giordani [102] showed that water treatment in a magnetic field of intensity 0.2 T with a flow rate of 0.7 m.s⁻¹ increases the calcite/aragonite ratio 3 times.

The study by Saban et al. [103] indicates that the size of the calcium carbonate particles formed decreases when the water is exposed to a magnetic field of density 0.75 T for 15 min. This has been linked to the increase in critical germs formed. In addition, the growth of these crystals takes place in the calcite structure. The same observation was shown by Mascolo [104]; by applying magnetic treatment to hard water at room temperature and 80 °C, the author observed a decrease in the mass of surface deposits, an increase in the size of calcite crystals on the surfaces, and an increase in the mass of CaCO₃ formed by homogeneous precipitation, as shown in Figure 5.

Contrary to the observations obtained by the majority of researchers in this field, Amer et al. [105] showed that the presence of a magnetic treatment reduces the deposition rate of CaCO₃, modifies the growth of calcite, and increases the formation of vaterite phase, which is the metastable phase, compared to the other anhydrous phases of calcium carbonate (calcite and aragonite). The mechanism by which MF affects the formation and growth of a particular phase was attributed to the Lorentz force hypothesis, which causes distortion of the ionic double layers of colloids and affects nucleation.

The contradictions concerning the observation effect of the magnetic field on the crystallinity of CaCO₃ may be due to the diversity of the magnetic devices used, the compositions of the solutions treated, the time of treatment, etc. Additionally, some studies applied magnetic treatment in static conditions while others used various dynamic conditions.

3.5. Influence of Foreign Ions

Natural waters often contain several ions foreign to the calcocarbonic system (Mg²⁺, Na⁺, SO₄²⁻, Cl⁻, etc.). Several studies show that these ions influence the process of crystallization of calcium carbonate. In most cases, they have an inhibitory effect on the germination and/or growth of the crystals. These charged particles could have other effects under the action of a magnetic field, which can “activate” them [53].

The effect of a static magnetic field on the precipitation of calcium carbonate was investigated in the presence of Mg²⁺ and Fe²⁺ ions [85,106]. The study consists of circulating solutions of CaCl₂ and Na₂SO₄ (8 × 10⁻³ mol.L⁻¹) in a plastic tube through a permanent magnet device (0.5 T) for 30 min. The results show that the magnetic field affects the zeta potential, pH, and rate of precipitation. These changes also depend on the foreign ions present in the solution. The presence of Fe²⁺ maintains the positive zeta potential for 2 h, whereas the other ions make it negative; the effect of Mg²⁺ on the zeta potential is less clear. The effects of Fe²⁺ probably result from their specific adsorption on the surface of CaCO₃.

Holysz et al. [107] showed that a static magnetic field (0.015 T) can cause changes in the conductivity of solutions containing dissolved salts such as NaCl, KCl, Na₃PO₄ or CaCl₂ and influence the amount of water which evaporates from the solutions. They suggest that magnetic treatment changes the structure of the water of hydration around the ions and therefore depends on the nature of the ions present.

On the other hand, Herzog et al. [108] observed that the effect of the magnetic field on the precipitation of calcium carbonate is due to the addition of ferric hydroxide particles in water. These crystals will support heterogeneous CaCO₃ germination. Without the magnetic field, none of the iron hydroxide minerals used (goethite, hematite, etc.) were particularly effective in inducing the germination of CaCO₃ from a supersaturated solution. In the presence of the magnetic field, the Fe²⁺ ion acts as a growth inhibitor of calcite. They suggest that the growth sites are blocked by the precipitation of FeCO₃ on the surface of the calcite crystals, as suggested by the following theoretical and experimental studies. The same effect is observed with Fe³⁺ ions but to a lesser extent.

It should also be pointed out that, according to Szkatula et al. [109], the success of using a magnetic field in industry for water treatment is due to silica as an impurity.

4. Mechanisms of the Magnetic Field Action

Several mechanisms have been proposed to explain the effect of the magnetic field on aqueous solutions. However, careful review of the literature indicates that several fundamentally different approaches predominate:

-

The magnetohydrodynamic phenomenon [21,110,111,112,113,114];

-

The hydration effect [24,86,94];

-

The gas/liquid interface effect [68,69].

4.1. The Magnetohydrodynamic Phenomenon (MHD)

The magnetohydrodynamic phenomenon results from the interaction of the velocity fields of the fluid with the electromagnetic fields. The force exerted on a charged particle (electron, ion) moving in an electromagnetic field is Lorentz’s force:

$$\overrightarrow{F}=q\left(\overrightarrow{E}+\overrightarrow{v}\times \overrightarrow{B}\right)$$

(1)

where

$\overrightarrow{E}$: sum of electric and electrostatic fields (V.m⁻¹);

$\overrightarrow{B}$: magnetic field induction (T);

$\overrightarrow{v}$: velocity of the particle (m.s⁻¹);

q: charge of the particle (C).

The Lorentz force causes an additional movement of charged particles, such as ions, in a direction orthogonal to υ and B. This magnetohydrodynamic effect affects not only the mass transfer but also the kinetics of the reactions and the amount deposited [115]. The MHD force can be amplified by increasing the conductivity of the medium.

To analyze this phenomenon, Lipus et al. [116] considered a basic cell (Figure 6) with the following working parameters:

$\overrightarrow{v}$: velocity of the water through the magnetic device;

$\overrightarrow{B}$: intensity of the applied magnetic field component;

τ: time of passage of the solution in the magnetic device.

In most magnetic treatment apparatuses, it has been shown that the Bτυ quantity is the most important parameter controlling the efficiency of the magnetic field.

The probability of ion collision within the solution depends on their thermal motion and drive forces, the average diffusion length, and convection-induced motions. The Lorentz force could influence this probability of collision by modifying the trajectories. For a thermal motion of ions with a velocity υ(t), the mean Lorentz force could be expressed simply by Equation (2) [115].

$$\begin{array}{ll}\mathrm{F}& ={\displaystyle \frac{1}{\mathrm{t}}}{\displaystyle \int \mathrm{F}\left(\mathrm{t}\right)\mathrm{dt}=\frac{1}{\mathrm{t}}{\displaystyle \int \mathrm{q}}}\left[\mathsf{\upsilon }\left(\mathrm{t}\right)+\mathsf{\upsilon }\right]\mathrm{dt}\times \mathrm{B}\\ & ={\displaystyle \frac{1}{\mathrm{t}}}{\displaystyle \int \mathsf{\upsilon }\left(\mathrm{t}\right)\mathrm{dt}\times \mathrm{B}.\mathrm{q}+\frac{1}{\mathrm{t}}{\displaystyle \int \mathrm{dt}.\mathrm{q}\mathsf{\upsilon }\times \mathrm{B}}}\end{array}$$

(2)

Hartmann and Lazarus [110,111] proved that if the magnetic field is strong enough, the velocity profile of a flowing fluid will be flattened in the center of the pipe and will develop in the vicinity of the pipe wall. They have also observed that even applying a relatively weak magnetic field orthogonal to the direction of a weakly turbulent flow reduces the pressure gradient required for a given flow. The primary effect is attributed to a reduction in the “electrical backwash” of the fluid. The magnetic field can also affect the velocity of the fluid flow, increasing or decreasing the state of rotation of the fluid according to experimental conditions.

In a cylindrical pipe, under laminar flow and in the absence of a magnetic field, the radial velocity profile is parabolic and is given by the following equation:

$$\mathsf{\upsilon }\left(\mathrm{r}\right)={\mathsf{\upsilon }}_0\left(1-{\displaystyle \frac{{\mathrm{r}}^2}{{\mathrm{a}}^2}}\right)$$

(3)

where

υ(r): The fluid velocity at each point at a distance r from the center of the pipe;

υ₀: Speed at the center of the pipe;

a: Radius of pipe.

The velocity is then maximum in the center of the tube and zero against the wall.

According to Busch et al. [112], the net effect of applying a magnetic field is the flattening of the flow velocity profile, which may ultimately result in the modification of the boundary layer (Figure 7). Thus, a greater gradient of velocity in the vicinity of the walls is established. This has the effect of increasing the shear and friction in the vicinity of the walls and promoting the probabilities of collision between the particles. This observation facilitated the interpretation of the results of the effect of the application of a magnetic field, which increases the rate of precipitation of the solutions treated. According to Busch, the field-induced ionic movements are equivalent to the circulation of a closed-circuit current.

Similarly, Martemianov and Sviridov [63] confirm, by an electrochemical method, that the profile of the flow of a conductive solution in a pipe is disturbed when a magnetic field is applied. At a constant flow velocity, the flow is accelerated along the walls and thus reduced to the center. This contributes to reducing the thickness of the boundary layer.

Alimi et al. [22,52] proposed the existence of a magnetohydrodynamic effect on electrically charged species present in water. This facilitates the formation of the ionic pairs, and this leads to a lowering of the supersaturation threshold, which must be crossed to allow the formation of the first nuclei of a solid phase. This effect would mainly benefit homogeneous germination, especially for low-hardness water. Similarly, this maturation could be favored by the hydrodynamic effects occurring in the vicinity of the walls under the effect of the partial velocity gradient. This could explain the influence of water circulation outside the magnetic field on the germination time, as demonstrated in their work.

4.2. Hydration Effect

The effects of a magnetic field were also explained by the change in the hydration of the solid hydrophobic surfaces, the gas/liquid interface, and the ions. These effects may explain the effects observed under static or stable treatment conditions (i.e., without the flow of the liquid phase treated through the magnetic device).

According to Lundage Madsen [94], the magnetic effect on the precipitation of calcium carbonate is related to the rate of transfer of protons in the process of crystallization:

$${\mathrm{Ca}}^{2+} + {\mathrm{HCO}}_3^{-} \to {\mathrm{CaCO}}_{3\left(\mathrm{S}\right)} + {\mathrm{H}}^{+}$$

(4)

Chibowski et al. [86] assume that the magnetic field can cause changes in the hydration layer of ions CO₃²⁻, HCO₃⁻, Ca²⁺ and H⁺, an effect that can be maintained long after treatment. This effect influences the germination and rate of crystal growth of the formed calcium carbonate.

According to Higashitani [117], the modification of the specific hydration of CO₃^2- ions could occur in the equilibrium between the various anhydrous phases during precipitation. Alimi et al. [22] assume that the magnetic field is likely to change the orientation of the spin of the proton and disturb the phenomena of dehydration.

Zheng et al. [118] concludes that Lorentz forces have a great effect on ions in water, by accelerating their migration and improving their accessibility to water clusters. Different ions exert a force on the dipoles of water molecules, which may promote or suppress hydrogen bond cleavage. As the number of hydrogen bond breaks increases, this results in an increase in wettability properties.

4.3. Effect on Gas/Liquid Interface

According to Lundage Madsen [94,95], the magnetic field is likely to change the solubility of CO₂ in water,

$${\mathrm{CO}}_2 + {\mathrm{H}}_2\mathrm{O} \iff {\mathrm{H}}_2{\mathrm{CO}}_3$$

(5)

and, consequently, that of CaCO₃.

Colic and Morse [68,69] have shown with molecular techniques that electromagnetic treatment changes the structure of water; this change is kept for hours. However, they realized that the degassing of water after treatment entirely affected the effects of the treatment. When working with radiofrequency and microwave fields, they also observed that the preliminary degassing of the water prevents the effect of the electromagnetic field on the behavior of suspensions and solutions. The gas/liquid interface is the first to be influenced by the action of the electromagnetic field and retains a memory effect.

Colic and Morse [68,69] postulate that the gas/liquid interface is disturbed by the microbubbles produced under the effect of an electromagnetic field. The only thing to notice is the detection of either an oscillation or a linear change in the solubility and diffusivity of the gases present, such as CO₂ or argon. The disturbance of the gas/liquid interfaces, generated by the electromagnetic field, can result in oscillating pH and change in the amounts of dissolved CO₂ and the formation of reactive oxygen or hydrogen species. These changes are mainly observed in the treatment with a high amplitude of radiofrequency.

4.4. Interaction Between Mechanisms

The interactions of magnetohydrodynamic effects and hydration effects create complex dynamics that can greatly improve the efficiency and control of the scaling/precipitation process. Indeed, increased mixing due to MHD effects can lead to higher local ion concentrations, while altered hydration can facilitate their reactivity. This combination can accelerate the nucleation process, potentially leading to rapid precipitation.

On the other hand, once nucleation occurs, the effects of MHD and hydration can influence the growth rates of the precipitates. The magnetic field can direct the movement of precipitated particles, affecting their aggregation and growth. Simultaneously, changes in hydration can impact how ions attach to growing precipitate surfaces.

4.5. Mechanisms of Action of the Magnetic Field Action on Calcium Carbonate Precipitation

Figure 8 shows a schematic representing the effect of magnetic water treatment on the precipitation of calcium carbonate as well as on the circulation of water. The image shows several phenomena occurring at the same time:

• The magnetohydrodynamic effect is the change in water flow from laminar to turbulent under magnetic field, where the boundary layer existing under laminar flow decreases or disappears after the flow change. This creates turbulence in the circulation of water, and the nature of the pipe materials greatly influences the precipitation of calcium carbonate.
• Hydration effect: The process by which a positive or negative ion attracts water molecules towards its immediate vicinity is called hydration. In this process, oxygen with the negatively charged side of the dipolar water molecule attracts and is attracted to the calcium ion in solution. Because of this ionic dipole force, water molecules cluster around calcium ions. Similarly, hydrogen, with the negative ends of water molecules, is attracted to HCO₃^- ions. Under the influence of the magnetic field, the polar water molecule changes its direction depending on the applied magnetic field, and, therefore, the ions present in the water will be released, which favors the probability of the formation of a stable seed of calcium carbonate. Thus, nucleation will predominate, and a precipitate containing many small particles will result.

Table 1. Treatment conditions used in some magnetic effect studies.

References	Water Hardness	MF Intensity (T)	Flow Rate	Time of Treatment	Important Results
Boluda-Botella et al. [1]	500 mg/L	0.012 T	800 L/h	-	Magnetic water treatment limits the recristallization of aragonite to calcite and decreases the formation of calcium carbonate on the wall of pipes.
Wang et al. [13]	Tap water 423 mg/L	100–400 mT	0.8 m/s	5 min	Increase in evaporation amount, decrease in specific heat and boiling point after magnetization; the changes depend on the magnetization effect.
F. Alimi et al. [22,51,52,53,54]	Synthetic water 30–50 °F	0.16 T	0.54, 0.74 and 0.94 L·min−1	5, 15 and 30 min	- Increase in the rate of homogeneous and heterogenous precipitation of CaCO3. - Acceleration of germination. - MF favors the formation of aragonite over calcite. Piping materials influence the effect of magnetic field on the precipitation of calcium carbonate.
Jiang et al. [48]	900 mg/L	0.5 T	0.17 m/s	54 h	- Magnetic field can promote the increase in the number of hydrogen bonds, which can inhibit the formation of CaCO3 formation. - The ratio of calcite, aragonite and vaterite will be changed at different magnetic field intensities, and the aragonite ratio will reach the peak at the optimum conditions.
Rouina et al. [50]	5.5 mmol/L	25 A and 50 Hz	4.3 m·min−1	420 min	- The salt rejection and permeate flow rate increased in reverse osmoses process. - CaCO3 precipitations formed were powdery.
Lv et al. [76]	Synthetic water 2–8 mmol/L	100–400 Gauss	0.2 m/s	12–48 h	Hydrogen bonds in aqueous solutions are distorted and even broken, and chunks of aggregated water molecules are split into smaller water molecules or monomers, resulting in increased activity of water molecules and increased salt solubility.
Bayoumi et al. [71]	Tap water 260 ppm	Coil turn to generate MF	1–6 mL/s	Single passage (less than second)	- Increase in the TDS of treated tap water with the intensification of the magnetic field intensity. - Magnetic water treatment can affect water quality in PVC pipe systems, but not as significantly as in iron pipes
Knez et al. [90]	100, 120°F	0.71 and 1.12 T	0.1 and 3 L·min−1	5.2–8.4 min	- Magnetic treatment promoted the precipitation of aragonite. The magnetic field does not significantly affect the zeta potential of the calcium carbonate formed at any stage of processing.
Wang and liang [119]	1000 mg/L	10–25 mT	0.4 m/s	The experiment was ended when fouling resistance became stable	- The experimental results indicate that the anti-fouling effect was related to the magnetic induction intensity at a specified electromagnetic frequency. - The conductivity increased by 84.87 µs/cm on average compared with that in the no-treatment group. - The average diameter of CaCO3 particles decreased.
Latifa et al. [120]	25°F	0.70 T	Static for 18 h	15 min, 30 min and 2 h	- Memory effect can be maintained for at least 24 h after treatment. - The precipitation of CaCO3 was delayed for 86 min after 18 h of magnetic treatment. - Aragonite was produced rather than vaterite and calcite.
Naderi et al. [121]	100–1000 mg/L	0.2 T and 0.5 T	50 L/h	10, 20 and 30 min	Increase in turbidity due to the occurrence of nucleation and crystallization in the homogenous phase.
F. Alimi [122]	Synthetic Seawater	0.16 T	0.94 L·min−1	30 min	Magnetic treatment enhanced the precipitation of CaCO3 in seawater
Liu et al. [123]	1000 ppm of CaCO3	65~70 kHz, 70~75 kHz, 75~80 kHz, and 80~85 kHz	0.6 m·s−1	4 h	The frequency of the applied electromagnetic fields stimulate a homogeneous crystallization of calcium carbonate in the solution.

brings together the water treatment conditions employed for various studies and the principal effect of magnetic treatment on the precipitation of calcium carbonate. In most cases described in the literature, the water studied is always very hard and supersaturated. It is then possible that the field already acts on germs or colloidal particles present in the solution.

The observed inconsistencies in the effects of magnetic treatment on precipitation, particularly from calcium carbonate, can be attributed to several factors:

• The chemical composition of water.
• The physical properties of treated water: In fact, the physical properties of water can influence the movement of particles and precipitation. Temperature variations, for example, modify these properties. The concentration of oxygen and other dissolved gases can also influence the chemistry of precipitation.
• Magnetic field intensity and configuration: The direction and configuration of the magnetic field (linear, uniform, or pulsed) can affect the way suspended particles interact, thereby changing the effectiveness of the treatment. Also, the intensity must be optimized for each system.
• Exposure time: Insufficient exposure time may fail to observe significant effects, while excessive time may result in undesirable effects or unexpected variations.
• Experimental methodology: Variations in experimental protocols, sample handling or instruments used may introduce inconsistencies. Additionally, the precision of measuring instruments and analysis methods may affect results, leading to inconsistencies in reported data.

We can thus say that the mechanisms proposed to explain the action of the magnetic field are only valid for well-defined treatment conditions.

5. Conclusions and Perspective

This review focused on the effect of magnetic fields on hard water. It made it possible to analyze in detail the degree of advancement of the scientific community in the field of magnetic antiscaling treatment, even if these studies always present great ambiguity in terms of determining the mechanism of action of the magnetic field.

Most studies agree on the inhibitory effect of calcite formation in favor of aragonite when water is magnetically treated and then has an inhibiting effect on the scaling phenomenon; some studies have contradictory results. Herein, we proposed a possible mode of action of the magnetic field. Indeed, MWT acts on seeds or colloidal particles in hard-water solutions. The diversity of the operating conditions may explain the differences between the results obtained and the interpretations proposed.

Therefore, the efficiency of the magnetic treatment depends on the conditions imposed on the fluid, the water composition and the construction of the treatment piping. Therefore, the proposed mechanisms, the magnetohydrodynamic effect and the hydration effect, are very specific and valid only for well-defined processing conditions.

Several directions for future research on magnetic water treatment can be considered:

• Test the effectiveness of magnetic treatment on wastewater from various industries (textiles, food processing, chemicals) to evaluate its decontamination potential.
• Develop mathematical models to simulate the interaction of magnetic fields with suspended particles, to identify optimal configurations for different types of treatments.
• Design and test prototypes of modular equipment that allow easy adjustment of magnetic field intensity and configuration according to specific treatment needs.
• Research the use of advanced magnetic materials (nanomaterials, composites) to improve the efficiency of treatment devices and reduce the manufacturing cost.
• Evaluate whether repeated treatments or combinations with other methods can improve or prolong the memory effect.

These research directions could contribute to a better understanding and optimization of magnetic water treatment, increasing its efficiency and practical application. In addition, they could help integrate this technology into a broader approach to sustainable water resource management.