Lithium, as a critically important strategic reserve metal, is experiencing increasingly high demand in today's rapidly advancing technological era, particularly in the fields of batteries and energy storage^[69]. In China, lithium reserves in brine resources account for 71.9% of the total lithium resources^[70]; therefore, extracting lithium from brine not only holds significant strategic resource value but also immense economic potential. However, the process of lithium extraction from brine is far from straightforward. Many salt lake brines in China have a high magnesium-to-lithium ratio, posing severe challenges to traditional lithium extraction methods^[71]. Due to the extremely similar physicochemical properties of magnesium and lithium, it is difficult to effectively distinguish between the two during separation, greatly increasing the difficulty and complexity of the process^[72]. Electrodialysis technology utilizes selectively permeable membranes, and under the influence of an electric field, it can promote the preferential migration of lithium ions while blocking magnesium ions on the other side of the membrane, thereby significantly improving the recovery rate and purity of lithium and effectively overcoming the shortcomings of traditional methods.

The advantages of electrodialysis technology in lithium extraction from brine are multifaceted. In addition to significantly enhancing lithium recovery rate and purity, it can also markedly reduce energy consumption^[73]. Compared with traditional evaporation-concentration methods, electrodialysis does not rely on large amounts of chemical reagents or high-temperature operations; instead, it can efficiently extract and concentrate lithium at a lower energy consumption level. Moreover, electrodialysis is easy to operate and highly automated, making it ideally suited for large-scale industrial production and providing strong technical support for the industrialization of lithium extraction from brine.

The primary methods of electrodialysis for lithium extraction from brine involve brine concentration and selective separation of lithium ions. Electrodialysis can serve as the core process for lithium extraction (separation of lithium ions) or be used as a front-end or back-end process (concentration of lithium-containing solutions). Zhou et al.^[74]focused on the challenge of concentrating lithium salts from primary resources, introducing electrodialysis (ED) technology to concentrate lithium sulfate solutions. Figure 3illustrates a schematic diagram of electrodialysis concentration. The study examined the effects of factors such as ion-exchange membrane type, applied voltage, and operation mode. In terms of operation mode, "high volume ratio concentration" offers advantages in energy consumption, while "multi-stage concentration" can achieve higher concentrations; for example, when using a two-stage electrodialysis with a volume ratio of 1∶3, a Li₂SO₄solution with an initial mass fraction of 6% can be concentrated to 17.43%. In terms of energy consumption and cost, the minimum energy consumption can reach 30.9 kW·h/m³. Compared to the traditional evaporation-concentration method, which has an energy cost of 16 $/t, electrodialysis has an energy cost of 3.1 $/t. Thus, ED is a competitive method compared to traditional evaporation, both in terms of energy consumption and operating time per unit volume.

Jiang et al^[61]were the first to apply bipolar membrane electrodialysis (BMED) technology to the development and utilization of brine lithium resources. They initially used conventional electrodialysis (ED) to concentrate the brine, followed by precipitation of Li₂CO₃using Na₂CO₃. Subsequently, the low-concentration Li₂CO₃was converted into LiOH using BMED. This process is illustrated in Figure 4, yielding products with 95% purity for both Li₂CO₃and LiOH, while reducing production costs by 82.5% compared to previous methods. Zhao et al^[62]combined BMED with NF, RO, and ED to produce LiOH from brines in salt lakes with Mg²⁺/Li⁺ratios greater than 30. The two-stage NF process effectively reduced the Mg²⁺/Li⁺mass ratio to below 0.5, simultaneously recovering over 92% of lithium. Through the RO-ED process, the lithium concentration in the nanofiltration (NF) permeate exceeded 14 g/L, and the lithium-rich solution was then fed into the bipolar membrane electrodialysis (BMED) process to produce a LiOH solution. Finally, an evaporation and crystallization process yielded LiOH·H₂O with a purity of 99.6%. Thus, the integrated membrane process demonstrated effective separation of magnesium and lithium, as well as continuous preparation of lithium hydroxide with the desired purity from salt lake brines with high Mg²⁺/Li⁺ratios.

Zhang et al.^[75]concentrated brine from the Dongtaijiner Salt Lake in Qinghai, with an Mg²⁺/Li⁺ ratio of 60, using continuous selective electrodialysis. The Li⁺ concentration was enriched from 0.26 g/L to 0.55 g/L, reducing the Mg²⁺/Li⁺ ratio to 5.2, with a lithium recovery rate of 68.6%. Subsequently, lithium precipitation was performed on the concentrated brine, yielding lithium carbonate with a purity exceeding 99.6%. Nie et al.^[76]used selective electrodialysis to separate magnesium and lithium from the old brine of Dongtaijina, which had an initial Mg²⁺/Li⁺ ratio of 20.7, reducing the ratio to 2.07 while achieving a Li⁺ recovery rate of up to 90.5% and an energy consumption of 4.5 kW·h/kg Li⁺.

In the critical field of lithium extraction from salt lake brines, electrodialysis technology plays an irreplaceable role. Particularly when confronted with the challenging issue of the commonly high magnesium-to-lithium ratio in salt lake brines, this technology, with its unique separation principles and advantages, is transitioning from the research exploration phase to practical applications in industrial production. From the perspective of R&D progress, electrodialysis technology has already achieved remarkable and significant results at both laboratory and pilot scales. In regions rich in salt lake resources, such as China and South America, numerous enterprises and specialized research institutions have actively engaged in trials and promotional efforts aimed at applying electrodialysis technology to the industrial-scale extraction of lithium^[77]. Currently, electrodialysis technology is predominantly employed in combined process systems integrated with traditional lithium extraction methods. By leveraging its outstanding magnesium-lithium separation efficiency, it significantly enhances the processing efficiency of subsequent lithium extraction procedures, effectively optimizing the entire lithium production chain. Although electrodialysis technology has demonstrated exceptional performance in laboratory and pilot-scale studies, it still faces a series of pressing challenges that need to be addressed in large-scale industrial production scenarios. For instance, the relatively high initial equipment procurement costs undoubtedly raise the financial threshold for enterprises seeking large-scale implementation; the durability of membrane materials remains insufficient^[78], affecting the long-term stable operation of the equipment, and its selectivity also requires further optimization and improvement to achieve more precise and efficient ion separation.

Overall, electrodialysis, as an efficient separation technology, demonstrates significant advantages in treating salt lake brines with high magnesium-to-lithium ratios. With continuous technological innovation and improvement, coupled with gradually decreasing costs, electrodialysis is expected to gain wider application in the coming years and is highly likely to become one of the mainstream technologies in the field of lithium extraction from salt lake brines.

Boron, as an important non-metallic mineral, is an indispensable chemical raw material with a wide range of applications spanning glass manufacturing, agriculture, the chemical industry, and more. It also plays a crucial "nutrient" role in future industrial development^[79]. Notably, boron reserves in brine resources are quite substantial, accounting for 33% of the country's total reserves^[80]. Extracting boron from salt lake brines has thus become an important production method. Due to boric acid's low ionization constant and the fact that most brines have a weakly acidic, neutral, or slightly alkaline pH environment^[81-82], boron predominantly exists in brines as boric acid molecules. These electrically neutral boric acid molecules cannot migrate under the influence of an electric field, and their relatively large molecular size also makes migration via diffusion difficult. Therefore, it is typically necessary to create an alkaline environment to convert boron into the borate ion form, facilitating further extraction. Electrodialysis technology offers a new and effective approach for boron extraction, utilizing selectively permeable membranes to achieve efficient separation of borate ions under electric field drive. Particularly in complex salt mixtures, electrodialysis can precisely separate borate ions from other ions in brines, significantly enhancing boron recovery rates and purity, and demonstrating unique advantages over traditional methods.

In recent years, numerous studies have explored the use of BMED for boron recovery from aqueous solutions. Bunani et al.^[83]investigated the effectiveness of bipolar membrane electrodialysis (BMED) in separating and recovering lithium and boron from aqueous solutions. They examined the impact of process conditions such as applied potential, initial sample volume, and pH on BMED performance, finding that lithium and boron recovery rates increased with higher applied voltage but decreased with larger initial sample volumes. Under optimal conditions of 15 V and an initial sample volume of 0.5 L, lithium separation and recovery rates were 99.6% and 88.3%, respectively, while boron rates were 72.3% and 70.8%, respectively. Additionally, increasing pH had a more pronounced effect on promoting boron separation and recovery. This study provided an effective method for simultaneously separating and recovering lithium and boron from aqueous solutions, offering valuable reference for resource recovery and utilization; however, it did not address the treatment of actual complex water samples. Noguchi et al.^[84]proposed multi-step bipolar membrane electrodialysis (BPED) for concentrating boron in brine. The study utilized a device composed of bipolar membranes and anion exchange membranes, conducting experiments in a semi-batch operation mode. When the concentration ratio of concentrated solution to dilute solution was 4∶1 and the initial boron concentration was 100 mg/L, after 60 minutes of the first BPED step, the boron concentration in the concentrated solution increased from 100 mg/L to 373 mg/L, while the boron concentration in the dilute solution decreased to 9.8 mg/L. After four steps of operation, the boron concentration in the concentrated solution approached the solubility of boric acid (approximately 10 g/L), achieving efficient boron concentration and recovery. İpekçi et al.^[68]studied the application of bipolar membrane electrodialysis (BMED) for simultaneously separating and recovering lithium and boron from aqueous solutions. In laboratory-scale experiments, they investigated the effects of acid and base solutions' types and concentrations in the acid and alkali chambers, as well as the applied voltage, on process efficiency. The results indicated that when using 0.05 mol/L HCl in the acid chamber and 0.05 mol/L NaOH in the alkali chamber, simultaneous separation and recovery of boron and lithium could be achieved. At a voltage of 30 V, boron and lithium separation rates were 86.9% and 94.7%, respectively, with recovery rates of 50% and 62%, respectively, at a power consumption of 0.79 kW·h/kg. Hung et al.^[65]explored the influence of various process parameters on boron removal and recovery efficiency in boron-containing wastewater treatment through batch and continuous BMED modes. Their research showed that higher initial boron concentrations reduced the boron treatment rate, and high initial pH values slowed down the boron removal rate, whereas increasing current density improved these conditions. When the solution pH was controlled between 9.5 and 10.5, the current density was 6.36 A/m²,and the maximum voltage was 12 V, boron removal and recovery efficiencies reached 98.6% and 86.5%, respectively. This study provided data support and theoretical basis for boron-containing wastewater treatment, but further feasibility studies, such as large-scale or field-scale feasibility assessments, are still needed before large-scale applications can be implemented.

Electrodialysis technology has unique applications in the extraction of boron from salt lake brines. Its advantages are significant, with high separation efficiency enabling precise separation of boron from other ions such as sodium, potassium, and magnesium in the brine, greatly enhancing the purity of boron extraction. Compared to traditional methods, it reduces the use of chemical reagents and minimizes the risk of introducing impurities. Additionally, the technology operates at room temperature, consumes low energy, causes minimal equipment corrosion, and offers high safety and stability, while also facilitating automated continuous production, aligning well with large-scale industrial production needs. However, electrodialysis technology also has limitations. The first is the generally high cost associated with electrodialysis. Furthermore, the complex composition of salt lake brines leads to competitive ion migration, interfering with boron separation, and requires stringent brine pretreatment, including meticulous impurity removal and pH adjustment; otherwise, the ion-exchange membranes may easily be damaged, complicating the process^[85]. Overall, electrodialysis technology boasts prominent advantages, but its challenges cannot be overlooked.

Potassium plays a vital role in agricultural production and is often referred to as the "food for food" ^[86]. As a major agricultural country, China has vast areas of crop cultivation, resulting in an extremely strong demand for potassium fertilizers. In terms of resource distribution, China's potassium resources are primarily found in salt lake brines, existing in the form of liquid potash deposits ^[87]. In current research and practice on potassium extraction from brine, achieving efficient separation and purification of Mg and K is a critical step for obtaining high-quality potassium fertilizers and improving the utilization rate of brine potash resources, which holds significant importance for ensuring the sustainable development of China's agriculture ^[88].

Zhang et al^[64]studied the effect of electrodialysis on the separation of magnesium and potassium under different Mg²⁺/K⁺conditions. The results indicated that the concentration of Mg²⁺is a key factor influencing the selectivity of ion-exchange membranes; an increase in Mg²⁺concentration reduces the selectivity coefficient. After 2.5 hours of electrodialysis, the Mg²⁺/K⁺ratio decreased from 10.31 to 1.15, with a potassium recovery rate of 89.06% and an energy consumption of 0.089 kW·h/mol K. Meanwhile, almost all sulfates were removed from the solution, contributing to improved purity of the KCl product. Guo et al^[89]focused on recovering potassium ions from complex coexisting ion systems using selective electrodialysis technology. They investigated the effects of current density, K⁺/Mg²⁺concentration, and temperature on the K⁺/Mg²⁺separation process, aiming to explore the competitive migration mechanisms of monovalent and polyvalent ions. The study found that ion concentration plays a crucial role in the selectivity of potassium-magnesium ion separation, with higher concentrations reducing selectivity, and potassium ion concentration having a more significant impact. Higher current densities can increase ion flux but have little effect on separation selectivity. Although increasing temperature enhances ion flux, it reduces the proportion of potassium ions in the solution. This research provides important insights for the high-value recovery of potassium resources and the study of ion competitive migration. Zhang et al^[63]used monovalent-selective electrodialysis (MSED) to study the recovery of potassium ions from complex coexisting ion systems. The results showed that MSED can effectively separate Mg²⁺from K⁺. Increasing Na⁺concentration increases the competitive migration of Na⁺against K⁺, significantly reducing the recovery efficiency of K⁺. As the concentration of introduced Mg²⁺increases, the hindrance to K⁺recovery gradually intensifies, and this can be mitigated by gradually reducing the voltage during MSED. Preliminary application to actual brine solutions achieved a K⁺recovery rate of 74.83%, reducing the Mg²⁺/K⁺ratio from 7.72 to 0.92, with an energy consumption of 0.116 kW·h/mol K⁺.

From the perspective of advantages, it boasts high separation efficiency, enabling rapid separation of potassium ions from other ions such as sodium and magnesium in brine, thereby accelerating potassium extraction while also reducing energy consumption. Moreover, considering the significant differences in composition among various salt lake brines, researchers have already made certain progress in developing customized electrodialysis processes. However, to achieve large-scale industrial application of this technology, substantial breakthroughs are still needed in several key areas.

Mg²⁺, Ca²⁺, and other ions in brine have received relatively little research attention regarding extraction using electrodialysis. The main issue is that precipitates formed by Mg²⁺and Ca²⁺can clog membrane pores, leading to a decline in membrane flux and selectivity. Chen et al.^[66]proposed the use of bipolar membrane electrodialysis combined with crystallization chamber technology to capture CO₂from seawater and extract magnesium resources for the preparation of MgCO₃·3H₂O. By measuring the metastable zone and exploring various influencing factors, the study showed that increasing current density and temperature is beneficial for magnesium extraction, while enhancing flow rate can promote its absorption. Under optimal conditions, the magnesium extraction rate reached 56.71%, with an energy consumption of 1.67 kW·h/kg MgCO₃·3H₂O, providing theoretical support for related industrial applications. Trihasti Kartika et al.^[67]studied the use of cylindrical membrane electrodialysis technology for mineral recovery from brine, achieving a high desalination rate. The results indicated that at a flow rate of 30 L/h with recirculation, 10 A, and 40 ℃, the ion product recovery rates for Ca²⁺and SO₄ ^2-were 30.17% and 27.62%, respectively.

In the study of calcium and magnesium extraction from salt lake brine using electrodialysis, the main challenges are the high concentrations of calcium and magnesium ions and the difficulty in separating them from other ions^[90]. Research has primarily focused on developing membrane materials with high selectivity and durability to improve the separation efficiency of calcium and magnesium ions^[91]. Additionally, optimizing process flows, reducing energy consumption, and integrating electrodialysis with other separation techniques, such as precipitation and reverse osmosis, have also become key research areas. Although laboratory studies have made certain progress, further refinement and improvement are still needed for industrial-scale applications.