Preparation method of low-calcium and magnesium and high-strontium water and application thereof

Abstract

The application discloses a preparation method of low-calcium-magnesium strontium-rich water and application thereof. The method comprises the following steps: filtering strontium-containing raw water through quartz sand and activated carbon in sequence, and performing primary nanofiltration separation through a polyamide composite nanofiltration membrane; and then performing secondary nanofiltration separation through a sulfonated polyether sulfone composite nanofiltration membrane. According to the strontium content in the water produced by the secondary nanofiltration, the water can be mineralized through a selective strontium-rich mineralization filter element made of strontium spar, celestite and palygorskite which are sintered and subjected to carbonation treatment. Through the cascade synergistic effect of the two-stage nanofiltration, the application realizes efficient removal of calcium and magnesium ions, and obtains strontium-rich water with a strontium content of 2.0-8.5 mg / L and a significantly reduced calcium, magnesium and chlorine ion content, which can be applied in the fields of medicine, beverage, drinking water or health food.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, specifically to a method for preparing low-calcium, magnesium-rich strontium water and its application, particularly a method for selectively increasing the strontium ion content in water while reducing the content of hardness ions such as calcium and magnesium. Background Technology

[0002] Strontium is an essential trace element for the human body, playing a vital physiological role. Studies have shown that strontium, along with calcium, can activate osteoblast activity, increase bone density, and promote fracture healing, potentially shortening the healing period by 30%. Therefore, strontium-rich natural mineral water, drinking water, and functional beverages are highly sought after in the global market. However, the vast majority of water sources in nature, including high-quality natural mineral water, typically have low natural strontium ion content, often ranging from 0.1 mg / L to 2.0 mg / L. Simultaneously, these water sources generally contain high concentrations of other cations, particularly calcium, magnesium, and potassium ions, often tens or even hundreds of times higher than strontium ions. For example, in typical mineral water, calcium ion concentrations can reach 50-150 mg / L, and magnesium ion concentrations can reach 20-50 mg / L.

[0003] The concentration of strontium in my country's public drinking water ranges from 0.005 to 3.11 mg / L, with an average of only 0.360 mg / L. Meanwhile, tap water contains relatively high levels of calcium and magnesium ions. Long-term consumption of water with high calcium and magnesium content may increase the risk of kidney stones, and a small number of people may experience gastrointestinal discomfort. Furthermore, the scale formed by high calcium and magnesium ions can severely damage kettles, water heaters, and other appliances, increasing energy consumption and causing blockages or overheating malfunctions. In addition, hard water can also impart a bitter taste. Moreover, these calcium and magnesium ions are chemically similar to strontium ions. Using ordinary reverse osmosis (RO) technology, while efficiently removing calcium and magnesium ions, it also removes strontium ions and other potentially beneficial minerals almost proportionally, posing a technical challenge to the selective enrichment of strontium ions. Existing technologies for increasing strontium content in water mainly include the following:

[0004] Direct addition of strontium salts: This method increases the strontium content in water by adding soluble strontium salts such as strontium chloride and strontium nitrate. Strontium chloride is readily soluble in water; its hexahydrate has a solubility as high as 106.2 g / 100 ml of water at 25°C. However, this method also increases the chloride ion content in the water, which does not meet the requirements for healthy drinking water.

[0005] Ion exchange method: This method utilizes ion exchange resins to enrich strontium ions. Studies have shown that the affinity order for cations in montmorillonite and soil clay is K < Mg < Ca < Sr < Ba, while in biotite it is Mg < K < Ca < Sr < Ba. However, traditional ion exchange techniques, while enriching strontium, also increase the content of calcium, magnesium, and potassium ions, making selective separation impossible.

[0006] Natural mineral filter media method: This method utilizes strontium-containing minerals such as celestite for mineralization. For example, Gree Electric's patented strontium mineralized filter element uses an inner layer of silicate ore powder and an outer layer of strontium ore powder, with a mass ratio of strontium ore powder to silicate ore powder of 1~6:1. However, this method makes it difficult to control the strontium release rate and cannot reduce the content of other ions.

[0007] Therefore, developing a technology that can effectively increase the strontium content in water while selectively reducing the relative contents of calcium, magnesium, potassium, and chloride ions is of great significance for improving drinking water quality and promoting public health. Summary of the Invention

[0008] The purpose of this invention is to provide a method for preparing low-calcium, magnesium, and strontium-rich water and its application. This method can significantly reduce the relative contents of calcium, magnesium, potassium, and chloride ions while increasing the strontium ion content in water, thereby obtaining strontium-rich drinking water that meets health requirements.

[0009] This invention provides a method for preparing low-calcium, magnesium-rich, strontium-rich water, comprising the following steps:

[0010] Step S1: The strontium-containing raw water is passed through a quartz sand filter and an activated carbon filter in sequence to remove suspended solids, colloids and residual chlorine in the water to obtain pretreated effluent;

[0011] Step S2: The pretreated effluent obtained in step S1 is subjected to primary nanofiltration separation through a first nanofiltration membrane module to obtain primary nanofiltration permeate; the membrane material of the first nanofiltration membrane module is a polyamide composite nanofiltration membrane with a pore size of 0.5-2 nm.

[0012] Step S3: The primary nanofiltration permeate obtained in step S2 is subjected to secondary nanofiltration fine separation through a second nanofiltration membrane module to obtain secondary nanofiltration permeate; the membrane material of the second nanofiltration membrane module is a sulfonated polyethersulfone composite nanofiltration membrane with a sulfonation degree of 20% to 40%.

[0013] Preferably, the operating pressure of step S2 is 0.4-0.8 MPa and the operating temperature is 15-35°C; the operating pressure of step S3 is 0.3-0.6 MPa and the operating temperature is 15-35°C.

[0014] Preferably, the strontium-containing raw water in step S1 is at least one of high-strontium carbonate fresh water and high-calcium magnesium groundwater.

[0015] Preferably, in step S2, the membrane material of the first nanofiltration membrane assembly is preferably a polyamide composite nanofiltration membrane NF270 type nanofiltration membrane.

[0016] Preferably, the operating pressure of the first-stage nanofiltration in step S2 is 0.5 to 0.7 MPa, and the operating temperature is 30 to 35°C.

[0017] Preferably, the membrane material of the second nanofiltration membrane assembly in step S3 is NTR-7450 sulfonated polyethersulfone nanofiltration membrane.

[0018] Preferably, the operating pressure of the secondary nanofiltration in step S3 is 0.4 to 0.5 MPa, and the operating temperature is 30 to 35°C.

[0019] In a preferred embodiment, based on the strontium content in the strontium-containing raw water and the strontium concentration in the secondary nanofiltration permeate, such as when the strontium content in the secondary nanofiltration permeate is less than 1.0 mg / L, the method further includes step S4, selective strontium enrichment mineralization: the secondary nanofiltration permeate obtained in step S3 is passed through a selective strontium enrichment mineralization filter element for a contact time of 30–60 min to obtain the final permeate.

[0020] Preferably, the selective strontium-enriched mineralized filter element is obtained by mixing 10-25 parts of strontium ore, 5-15 parts of celestite, 80-100 parts of attapulgite and 5-20 parts of binder, pressing them into shape, and then sintering them at 500-750℃ for 1-3 hours.

[0021] Preferably, the selective strontium-enriched mineralization filter element is further subjected to treatment in a CO2 atmosphere for 1 to 3 hours after sintering, at a pressure of 0.1 to 0.3 MPa and a temperature of 20 to 30°C.

[0022] Preferably, the average particle size of the strontium ore, celestite, and attapulgite is 100-250 mesh, and the binder is at least one of polyolefin, polyamide, epoxy resin, and acrylic resin.

[0023] Preferably, the present invention also provides a strontium-rich water with low calcium and magnesium content, wherein the strontium-rich water with low calcium and magnesium content is prepared by any of the above-described water treatment methods.

[0024] Preferably, the strontium content in the low-calcium, magnesium-rich strontium water of the present invention is 2.0–8.5 mg / L, the calcium ion content is 5.0–15.0 mg / L, the magnesium ion content is 0.5–5.0 mg / L, and the chloride ion content is 0.5–2.5 mg / L.

[0025] Preferably, the present invention also provides the application of low-calcium, magnesium-rich strontium water in pharmaceuticals, beverages, drinking water or health foods.

[0026] This invention utilizes the cascaded synergistic effect of two-stage nanofiltration membranes. The first-stage nanofiltration membrane employs a sieving mechanism to filter and retain Ca2+ with larger hydration radii through pore size separation. 2+ and Mg 2+ Sr with a smaller hydration radius 2+ Then it can preferentially pass through the filter membrane. The secondary nanofiltration membrane utilizes the strong Dominis repulsion effect generated by the high density of sulfonic acid groups on the membrane surface to remove residual Ca. 2+ and Mg 2+ Perform deep retention, while Sr 2+ Due to its low charge density, it can still maintain high transmittance. The two-stage membrane series design ensures both the stepwise deep removal of calcium and magnesium hardness ions and avoids excessive loss of strontium through the complementarity of membrane materials.

[0027] Based on the strontium concentration in the secondary nanofiltration permeate, if the strontium content is below 1.0 mg / L, a selective strontium-enriched mineralization filter cartridge is used to adjust the strontium concentration. This cartridge uses strontium ore as the primary strontium source, supplemented by celestite to provide additional strontium sources and sulfate ions. Attapulgite acts as a buffer carrier, providing a porous framework and supplementing trace elements. The main mineral component of strontium ore is SrCO3; high-temperature calcination partially decomposes and activates its crystal lattice, improving the solubility of strontium ions. Celestite's main component is SrSO4, and its solubility characteristics are improved after calcination. Attapulgite, as a naturally porous mineral, has a low strontium content but excellent adsorption and ion exchange capabilities. It can slowly release trace beneficial elements into the water while simultaneously adsorbing some heavy metal ions that may dissolve from the ore, thus playing a role in water purification and safe buffering.

[0028] This invention utilizes the complementary mechanism of nanofiltration deionization and selective mineralization. First, it removes most of the Ca2+ from the raw water through two-stage nanofiltration. 2+ Mg 2+ and Cl - The process involves removing strontium to obtain a base water body with low mineralization. Then, selective strontium enrichment mineralization is carried out in this low-strontium water body to ensure that the strontium ions released by the mineralization material are not disturbed by high concentration ions. This allows the target strontium concentration to be achieved with a lower amount of mineralization material, while avoiding the introduction of excessive calcium and magnesium ions.

[0029] The advantages or beneficial effects of the method for preparing low-calcium, magnesium-rich, and strontium-rich water provided by this invention and its application include at least the following:

[0030] This invention achieves highly efficient and selective separation of calcium and magnesium ions with high rejection rate and strontium permeability through the cascade synergy of a polyamide composite membrane and a sulfonated polyethersulfone membrane, under an operating pressure of only 0.3–0.8 MPa. The resulting strontium-rich water has a strontium content of 2.0–8.5 mg / L, and the contents of calcium, magnesium, and chloride ions are significantly reduced, which can meet the needs of various high-end applications such as drinking water, medicine, and health food. Detailed Implementation

[0031] To more clearly illustrate the purpose, technical solution, and advantages of this invention, the technical solution of this invention will be described in detail below through specific embodiments. It should be noted that these embodiments are only for illustrating this invention and not for limiting its scope of protection; the actual scope of protection of this invention should be determined by the claims.

[0032] Unless otherwise specified, the materials and reagents used in the following examples and comparative examples are commercially available. Unless otherwise specified, the amount of each component in the following examples is 1 g per part by weight.

[0033] I. Raw materials used in the following embodiments

[0034] 1. First nanofiltration membrane module: NF270-4040 type, polyamide composite nanofiltration membrane, purchased from Dow Chemical Company, USA.

[0035] 2. Second nanofiltration membrane module: NTR-7450 sulfonated polyethersulfone nanofiltration membrane with a sulfonation degree of 22%, purchased from Nitto Denko Corporation, Japan.

[0036] 3. Reverse osmosis membrane module (for comparative example): BW30-4040 type, polyamide composite reverse osmosis membrane, purchased from Dow Chemical Company, USA.

[0037] 4. Strontium ore: Natural strontium ore with SrCO3 content ≥85%, crushed and sieved to 100-150 mesh for use, commercially available.

[0038] 5. Celestite: Natural celestite ore with SrSO4 content ≥80%, crushed and sieved to 100-150 mesh for use, commercially available.

[0039] 6. Attapulgite clay: Attapulgite clay, purity ≥90%, average particle size 150 mesh, specific surface area ≥200 m² / g, commercially available.

[0040] 7. Adhesives: Polyethylene adhesive, powder, melt index 8-12 g / 10min, purchased from Dow Chemical Company, USA; epoxy resin adhesive, E-44 type, purchased from Nantong Xingchen Synthetic Materials Co., Ltd.

[0041] 8. ICP-OES: Inductively Coupled Plasma Optical Emission Spectrometer, Agilent 5110, purchased from Agilent Technologies (China) Co., Ltd.

[0042] 9. Source of raw water:

[0043] Raw water A is high-strontium carbonate freshwater, and its main water quality parameters are: Sr 2+ Content 7.55 mg / L, Ca 2+ Content 75.54 mg / L, Mg 2+ Content 9.19 mg / L, Cl - Content 6.23 mg / L, pH value 7.24.

[0044] Raw water B is low-strontium, high-calcium, high-magnesium groundwater. Its main water quality parameters are: Sr 2+ Content 2.28 mg / L, Ca 2+ Content 142.31 mg / L, Mg 2+ Content 38.41 mg / L, Cl - Content 10.54 mg / L, pH value 7.52.

[0045] II. Experimental examples and comparative examples for preparing low-calcium, magnesium-rich strontium water.

[0046] Example 1

[0047] Step S1: Raw water A passes through a quartz sand filter and an activated carbon filter sequentially at a flow rate of 1.5 L / min to obtain pretreated effluent.

[0048] Step S2: The pretreated effluent enters the first nanofiltration membrane module, with an operating pressure of 0.7 MPa and a temperature of 32°C, and the first-stage nanofiltration permeate is collected.

[0049] Step S3: The primary nanofiltration permeate enters the secondary nanofiltration membrane module at an operating pressure of 0.5 MPa and a temperature of 30°C to obtain secondary nanofiltration permeate.

[0050] Strontium concentration detection: A sample of the secondary nanofiltration permeate was taken and analyzed using ICP-OES. The final permeate water quality was: Sr 2+ 3.82 mg / L, Ca 2+ 7.46 mg / L, Mg 2+ 1.08 mg / L, Cl - 1.32 mg / L. The resulting Sr in the water 2+ The concentration was 3.82 mg / L, higher than 1.0 mg / L, and Ca... 2+ Mg 2+ Cl -The requirements are met, so step S4 is not performed, and the secondary nanofiltration permeate is used directly as the final permeate.

[0051] Example 2

[0052] Steps S1-S3: Using raw water B, the operation is the same as in Example 1, except that the operating pressure of the first-stage nanofiltration is adjusted to 0.6 MPa and the operating pressure of the second-stage nanofiltration is adjusted to 0.4 MPa, with the temperature at 28℃ for both. A sample of the second-stage nanofiltration permeate is taken for testing, and Sr... 2+ The concentration is 0.82 mg / L, which is lower than 1.0 mg / L, therefore step S4 is performed.

[0053] The specific preparation method of the selective strontium-enriched mineralized filter element in step S4 is as follows: 20 parts of strontium ore with an average particle size of 150 mesh, 10 parts of celestite with an average particle size of 150 mesh, and 90 parts of attapulgite with an average particle size of 150 mesh are mixed evenly with 15 parts of polyethylene binder and pressed into a cylindrical filter element blank with an outer diameter of 60 mm, an inner diameter of 28 mm, and a length of 250 mm. The blank is placed in a muffle furnace and heated to 650℃ at a rate of 5℃ / min, and sintered at that temperature for 2 h. Then, it is cooled to room temperature with the furnace. Subsequently, the filter element is placed in a sealed container, CO2 gas is introduced, and carbonation treatment is carried out at a pressure of 0.2 MPa and a temperature of 25℃ for 2.5 h. After removal, it is air-dried to obtain the selective strontium-enriched mineralized filter element.

[0054] The prepared filter element was loaded into a filter column, and secondary nanofiltration permeate was introduced. The contact time was controlled at 50 minutes, and the collected effluent was the final permeate. ICP-OES analysis showed that the final permeate quality was: Sr 2+ 1.85 mg / L, Ca 2+ 9.53 mg / L, Mg 2+ 1.61 mg / L, Cl - 1.36 mg / L.

[0055] Example 3

[0056] Similar to Example 2, the only difference is that in step S4, when preparing the mineralized filter element, the sintered filter element is used directly without CO2 carbonation treatment.

[0057] The specific preparation method of the selective strontium-enriched mineralized filter element in step S4 of Example 3 is as follows: 20 parts of strontium ore with an average particle size of 150 mesh, 10 parts of celestite with an average particle size of 150 mesh, and 90 parts of attapulgite with an average particle size of 150 mesh are mixed evenly with 15 parts of polyethylene binder and pressed into a cylindrical filter element blank with an outer diameter of 60 mm, an inner diameter of 28 mm, and a length of 250 mm; the blank is placed in a muffle furnace and heated to 650°C at 5°C / min, held at that temperature for 2 h, and then cooled to room temperature with the furnace.

[0058] The prepared filter element was loaded into a filter column, and secondary nanofiltration permeate was introduced. The contact time was controlled at 40 minutes, and the collected effluent was the final permeate. ICP-OES analysis showed that the final permeate quality was: Sr 2+ 1.55 mg / L, Ca 2+ 10.64 mg / L, Mg 2+ 1.76 mg / L, Cl - 1.53 mg / L.

[0059] Example 4

[0060] Step S1: Raw water B is used and passed sequentially through a quartz sand filter and an activated carbon filter at a flow rate of 2.0 L / min to obtain pretreated effluent.

[0061] Step S2: The pretreated effluent is fed into the first nanofiltration membrane module at an operating pressure of 0.8 MPa and a temperature of 35°C. The recovery rate is controlled at 80%, and the first-stage nanofiltration permeate is collected.

[0062] Step S3: The primary nanofiltration permeate is fed into the second nanofiltration membrane module at an operating pressure of 0.3 MPa and a temperature of 35°C, with the recovery rate controlled at 85%, to obtain the secondary nanofiltration permeate.

[0063] A sample of the permeate from the secondary nanofiltration stage was taken and analyzed using ICP-OES. Sr 2+ The concentration is 0.78 mg / L, which is lower than 1.0 mg / L, therefore step S4 is performed.

[0064] The specific preparation method of the selective strontium-enriched mineralization filter element in step S4 is as follows: 25 parts of strontium ore with an average particle size of 150 mesh, 15 parts of celestite with an average particle size of 150 mesh, and 80 parts of attapulgite with an average particle size of 100 mesh are mixed evenly with 18 parts of epoxy resin binder and pressed into a cylindrical filter element blank with an outer diameter of 60 mm, an inner diameter of 28 mm, and a length of 250 mm. The blank is placed in a muffle furnace and heated to 550℃ at a rate of 5℃ / min, and sintered at that temperature for 2.5 h, then cooled to room temperature with the furnace. After sintering, the filter element is soaked in pure water for 16 h for pre-dissolution, and the leachate is drained. Subsequently, the filter element is placed in a sealed container, CO2 gas is introduced, and carbonation treatment is carried out at a pressure of 0.3 MPa and a temperature of 25℃ for 2 h. After removal, it is air-dried to obtain the selective strontium-enriched mineralization filter element.

[0065] The prepared filter cartridge was loaded into a filter column, and secondary nanofiltration permeate was introduced, with a contact time controlled at 45 minutes. The collected effluent was the final permeate. ICP-OES analysis showed the final permeate quality to be: Sr 2+ 2.08 mg / L, Ca 2+ 11.25 mg / L, Mg 2+ 1.92 mg / L, Cl- 1.45 mg / L.

[0066] Example 5

[0067] Step S1: Raw water A passes through a quartz sand filter and an activated carbon filter sequentially at a flow rate of 1.6 L / min to obtain pretreated effluent.

[0068] Step S2: The pretreated effluent enters the first nanofiltration membrane module, with an operating pressure of 0.5 MPa and a temperature of 30°C, and the first-stage nanofiltration permeate is collected.

[0069] Step S3: The primary nanofiltration permeate enters the secondary nanofiltration membrane module at an operating pressure of 0.4 MPa and a temperature of 30°C to obtain secondary nanofiltration permeate.

[0070] Strontium concentration detection: A sample of the secondary nanofiltration permeate was taken and analyzed using ICP-OES. The final permeate water quality was: Sr 2+ 3.15 mg / L, Ca 2+ 8.82 mg / L, Mg 2+ 1.35 mg / L, Cl - 1.48 mg / L. The resulting Sr in the water 2+ The concentration was 3.15 mg / L, higher than 1.0 mg / L, and Ca... 2+ Mg 2+ Cl - The requirements are met, so step S4 is not performed, and the secondary nanofiltration permeate is used directly as the final permeate.

[0071] Comparative Example 1

[0072] Step S1: Raw water A passes through a quartz sand filter and an activated carbon filter sequentially at a flow rate of 1.5 L / min to obtain pretreated effluent.

[0073] Step S2: The pretreated effluent is fed into the first nanofiltration membrane module (NF270-4040 type) at an operating pressure of 0.7 MPa and a temperature of 32°C to collect the first-stage nanofiltration permeate.

[0074] According to ICP-OES testing, the final product water quality is: Sr 2+ 5.21 mg / L, Ca 2+ 28.35 mg / L, Mg 2+ 4.82 mg / L, Cl - 3.56 mg / L.

[0075] Comparative Example 2

[0076] Step S1: Raw water A passes through a quartz sand filter and an activated carbon filter sequentially at a flow rate of 1.5 L / min to obtain pretreated effluent.

[0077] Step S3: The pretreated effluent is directly fed into the NTR-7450 sulfonated polyethersulfone nanofiltration membrane of the second nanofiltration membrane module at an operating pressure of 0.5 MPa and a temperature of 30°C to obtain secondary nanofiltration permeate. The resulting permeate is the final permeate, without going through steps S2 and S4.

[0078] After testing, the final product water quality was: Sr 2+ 3.05 mg / L, Ca 2+ 15.28 mg / L, Mg 2+ 3.62 mg / L, Cl - 2.18 mg / L.

[0079] Comparative Example 3

[0080] Raw water A is processed through a reverse osmosis membrane at an operating pressure of 1.5 MPa and a temperature of 25°C, and the effluent is collected.

[0081] After testing, the final product water quality was: Sr 2+ 0.15 mg / L, Ca 2+ 1.52 mg / L, Mg 2+ 0.25 mg / L, Cl - 0.32 mg / L.

[0082] The results are analyzed as follows:

[0083] 1. Examples 1 and 5 employ a two-stage nanofiltration system, combining a first-stage polyamide nanofiltration membrane (NF270) with a second-stage sulfonated polyethersulfone nanofiltration membrane (NTR-7450), achieving excellent performance with high calcium and magnesium rejection rates and high strontium ion permeability. In contrast, Comparative Example 1 and Comparative Example 2, using only a single-stage polyamide nanofiltration membrane and a single-stage sulfonated polyethersulfone nanofiltration membrane respectively, showed significantly lower calcium and magnesium ion rejection rates compared to the examples. The core reason lies in the complementary and synergistic effect of the two nanofiltration stages: the first-stage polyamide membrane primarily uses pore size sieving, and its pore size of 0.5–2 nm can effectively retain calcium ions with larger hydration radii. 2+ and Mg 2+ While allowing for smaller hydration radii, Sr 2+ The first stage allows for the initial removal of calcium and magnesium ions, creating a low-ionic-strength environment for the second stage. The second-stage sulfonated membrane primarily utilizes dominance repulsion; its surface high-density sulfonic acid groups carry a strong negative charge, generating a strong electrostatic repulsion against residual divalent cations, achieving deep hardness removal. In Comparative Example 2, which lacks the first stage, a high concentration of Ca... 2+ Mg 2+The charge sites on the sulfonated membrane surface are rapidly occupied and saturated, shielding the Donnan effect and causing a significant decrease in retention capacity. Without the second stage (Comparative Example 1), pore size sieving alone cannot reduce the hardness to an extremely low level. Therefore, the sequential tandem connection and complementary mechanism of the two-stage membranes are key to achieving efficient removal of calcium and magnesium strontium.

[0084] 2. Examples 1 and 5 maintained high strontium permeability while removing calcium and magnesium, and were superior to the reverse osmosis of Comparative Example 3. The fundamental reason lies in the different separation mechanisms of the two membranes: the reverse osmosis membrane has an extremely small pore size (<0.5 nm), and based on the dissolution-diffusion mechanism, it can remove all Sr... 2+ Ca 2+ Mg 2+ Cl - Since ions are almost indiscriminately retained, they cannot be selectively separated by differences in radius or charge between ions. Therefore, the removal of calcium and magnesium ions inevitably leads to the removal of strontium ions. In contrast, nanofiltration membranes have nanoscale pores (0.5~2nm) and surface charges, enabling them to utilize both sieving and Donald's effects to separate strontium ions, which have small hydration radii and relatively low charge densities. 2+ Preferentially permeable, Ca with large hydration radius and high charge density 2+ Mg 2+ It is effectively trapped. In addition, the operating pressure of reverse osmosis (1.5 MPa) is much higher than that of nanofiltration membrane (0.3~0.8 MPa), resulting in a significant increase in energy consumption.

[0085] 3. A comparison between Example 2 and Example 3 shows that the strontium concentration in the water produced by the mineralized filter cartridge after carbonation treatment in Example 2 is 1.85 mg / L, which is higher than the 1.55 mg / L in the untreated filter cartridge of Example 3. Furthermore, the Ca concentration in Example 2 is higher than that in Example 3. 2+ Mg 2+ Cl - The concentrations of impurity ions were all lower than in Example 3. A possible reason is that when in contact with pure water, the uncarbonated strontium ions react rapidly, causing the Sr ions in the liquid layer near the filter element surface to rise. 2+ Instantaneous supersaturation of concentration can easily promote Sr 2+ With CO3 in the solution 2- or SO4 2- Recombining actually blocked the internal strontium dissolution channels, causing a sharp decrease in the subsequent dissolution rate and limiting the total dissolution amount. In Example 2, the sintered filter element was subjected to carbonation treatment under a CO2 atmosphere, generating a dense, low-solubility carbonate layer. This carbonate buffer layer, on the one hand, controls the release rate of the internal strontium source through its own slow dissolution, allowing strontium ions to dissolve stably and resulting in a higher cumulative dissolution amount; on the other hand, this covering layer also effectively suppresses internal impurity ions, such as Ca... 2+ Mg 2+The rapid initial dissolution of strontium reduces the content of impurity ions while increasing the target strontium concentration.

[0086] It should be clarified that the above embodiments are merely illustrative of specific implementations of the present invention and do not constitute a limitation on the scope of protection of the present invention. Based on the technical content disclosed in this invention, those skilled in the art can make various modifications, adjustments, or equivalent substitutions within its basic principles and design concepts. These modifications and improvements need not be listed exhaustively, but should all be considered to fall within the scope of protection of this invention.

Claims

1. A method for preparing low-calcium, magnesium-rich, strontium-rich water, characterized in that, Includes the following steps: Step S1: The strontium-containing raw water is passed through a quartz sand filter and an activated carbon filter in sequence to remove suspended solids, colloids and residual chlorine in the water to obtain pretreated effluent; Step S2: The pretreated effluent obtained in step S1 is subjected to primary nanofiltration separation through a first nanofiltration membrane module to obtain primary nanofiltration permeate; the membrane material of the first nanofiltration membrane module is a polyamide composite nanofiltration membrane with a pore size of 0.5-2 nm. Step S3: The primary nanofiltration permeate obtained in step S2 is subjected to secondary nanofiltration fine separation through a second nanofiltration membrane module to obtain secondary nanofiltration permeate; the membrane material of the second nanofiltration membrane module is a sulfonated polyethersulfone composite nanofiltration membrane with a sulfonation degree of 20% to 40%.

2. The method according to claim 1, characterized in that, The strontium-containing raw water in step S1 is at least one of high-strontium carbonate type water and high-calcium magnesium type groundwater.

3. The method according to claim 1, characterized in that, The operating pressure of step S2 is 0.4–0.8 MPa, and the operating temperature is 15–35°C; the operating pressure of step S3 is 0.3–0.6 MPa, and the operating temperature is 15–35°C.

4. The method according to any one of claims 1-3, characterized in that, In step S2, the membrane material of the first nanofiltration membrane assembly is a polyamide composite nanofiltration membrane NF270 type nanofiltration membrane; in step S3, the membrane material of the second nanofiltration membrane assembly is an NTR-7450 type sulfonated polyethersulfone nanofiltration membrane.

5. The method according to any one of claims 1-3, characterized in that, The method further includes step S4: passing the secondary nanofiltration permeate obtained in step S3 through a selective strontium-rich mineralization filter cartridge for a contact time of 30 to 60 minutes to obtain the final permeate.

6. The method according to claim 5, characterized in that, The selective strontium-enriched mineralized filter element comprises the following raw materials in parts by weight: 10-25 parts strontium ore, 5-15 parts celestite, 80-100 parts attapulgite, and 5-20 parts binder; the preparation method of the filter element comprises: mixing the raw materials, pressing them into shape, and then sintering them at 500-750℃ for 1-3 hours to obtain a sintered filter element.

7. The method according to claim 6, characterized in that, The method for preparing the selective strontium-rich mineralized filter element further includes, after the sintering step, placing the sintered filter element in a CO2 atmosphere and treating it at a pressure of 0.1–0.3 MPa and a temperature of 20–30 °C for 1–3 h.

8. The method according to claim 6, characterized in that, The average particle size of the strontium ore, celestite, and attapulgite is 100-250 mesh, and the binder is at least one of polyolefin, polyamide, epoxy resin, and acrylic resin.

9. A strontium-rich water with low calcium and magnesium content, characterized in that, The strontium-rich water with low calcium and magnesium content has a strontium content of 2.0–8.5 mg / L, a calcium ion content of 5.0–15.0 mg / L, a magnesium ion content of 0.5–5.0 mg / L, and a chloride ion content of 0.5–2.5 mg / L. It is prepared by the preparation method described in any one of claims 1–8.

10. The use of the low-calcium, magnesium-rich strontium water of claim 9 in the preparation of pharmaceuticals, beverages, drinking water or health foods.