Selective electrodialysis process for the recovery of potassium from multicomponent solution systems

Potassium fertilizer is a strategic resource indispensable for food grain production, while nearly half of China's potassium fertilizer is imported due to the shortage of potassium resources (Chen et al. 2020). How to effectively recover and utilize the potassium resources and hence accelerate the development of agriculture in China is an important issue (Li 2013). It was reported that most potassium resources are extracted from soluble potash ores and Salt Lake brines (Ciceri et al. 2015; Hongwen et al. 2015). Recently, crop straw has been regarded as a promising raw material to recover potassium since potassium fertilizer applied to crops can also be a raw material for obtaining potassium resources (Hou et al. 2021; Ru Fang et al. 2022; Wang et al. 2023), which provides benefits in terms of potassium life cycle and sustainable development (Yakovleva et al. 2021; Song et al. 2022). In addition, wine lees and food waste can also be critical raw materials of potassium resources extraction (Madejón et al. 2001), whereas the obtained potassium resource usually contain other coexisting ions, typically Mg²⁺.

Separation of monovalent K⁺ and multivalent Mg²⁺ is crucial to recover potassium resources with high purity. Commonly, mono- and multivalent ions can be separated by adsorption (Guo et al. 2020; Zhang et al. 2021a), membrane technology (Li et al. 2022; Liu et al. 2023a) and precipitation (Jumaeri et al. 2021; Liu et al. 2023b). However, adsorption is limited by the adsorption capacity. Traditional membrane technology typically uses nanofiltration membranes, which requires severe operating conditions but possesses somewhat lower selectivity. Post-treatment of the precipitation method will be relatively troublesome. To overcome the limitations facing the aforementioned methods, electrodialysis technology is developed as a new type of electrically driven membrane separation technology to separate ions. It can deal with solutions in large quantities without investing large amounts of chemical reagents. It is, therefore, used in several industries for ion separation (Abou-Shady 2017; Sedighi et al. 2023). Selective electrodialysis (SED) is a unique technique that can separate mono- and multivalent ions. The monovalent ion exchange membrane used in SED enables the smooth passage of monovalent ions but blocks the migration of multivalent ions. Anna Siekierka and Fatma Yalcinkaya (Siekierka & Yalcinkaya 2022) developed a monovalent ion exchange membrane with high cobalt ion selectivity. They ultimately enabled cobalt the removal of cobalt from multicomponent solutions up to 91% with a separation factor of 5.6 for lithium and cobalt. Zhang et al. (Zhang et al. 2021b) developed a selective electrodialysis process in the feed-discharge mode that allows continuous lithium extraction from brines with low Li⁺ concentrations to obtain high-purity lithium carbonate after SED. The use of SED is becoming more widespread as the need to extract high-value products from wastewater increases. Many studies have been reported on the extraction of lithium from lithium-magnesium coexistence systems, however, limited studies have been performed on K⁺/Mg²⁺ separation. Considering the separation of K⁺/Mg²⁺ is demanded in a wide range of application scenarios (Hancer & Miller 2000; Duan et al. 2006; Widyatmoko & Burgman 2006), we developed an SED process to efficiently recover K⁺ from K₂SO₄/Mg₂SO₄ solution. The effects of current density, potassium/magnesium ion concentrations and their ratios, as well as temperature, on the recovery rate of K⁺ and separation selectivity of K⁺/Mg²⁺ were systematically investigated, with the influencing mechanisms analyzed by examining the migration pathways of target (K⁺) and impurity (Mg²⁺) ions.

The reagents used in the experiments were analytically pure, and the water used was deionized water produced in the laboratory. A five-compartment polytetrafluoroethylene (PTFE) electrodialysis test tank was used for the experiment. The selective cation exchange membrane was purchased from Astom of Japan, and the anion exchange membrane (AEM) was purchased from YDS Environmental Equipment Co., Ltd (Langfang, China).

Both the concentration and auxiliary chambers were configured with 0.1 L K₂SO₄ solution, with an initial concentration being 0.1 mol/L. 0.1 L of solution containing both K₂SO₄ and MgSO₄ with a certain concentration were filled in the desalination chamber. 0.2 L of K₂SO₄ solution with an initial concentration of 0.5 mol/L served as the electrode solution. The solution from each chamber was pumped into the membrane stack to circulate the solution and to make the solution more uniform in each chamber. The experimental process was carried out for 120 min, during which the solution samples in the concentration and desalination chambers were taken out every 20 min. Each group of experiments was performed three times, and the average value of the concentration of K⁺ and Mg²⁺ was taken.

To investigate the effects of current density, potassium ion concentration, magnesium ion concentration, and temperature on ion migration, experiments were conducted under different conditions in this study, and the parameters of the experimental conditions are shown in Table 1.

The concentration of K⁺ was determined by sodium tetraphenylborate-quaternary ammonium titration and atomic absorption spectrophotometer (TAS-990F). The concentration analysis of Mg²⁺ was performed by an automatic potentiometric titration analyzer (Titration Excellence T7, Mettle Toledo, Switzerland) and atomic absorption spectrophotometer. The analytical methods and standards for each ion concentration are shown in Table 2.

The conditions affecting the competitive migration of coexisting ions are complex, and the following experiments were conducted to investigate the effects of various factors on the separation performance of K⁺/Mg²⁺.

Current density determines the migration ability of positively and negatively charged particles (Ozkul et al. 2023). This section, hence, examines the effect of current density (100, 150, 200, and 250 A/m²) on electrodialysis, and all the studied current densities are within the range of the limiting current density (Min et al. 2021).

As shown in Figure 2(c), the initial membrane stack resistance is the highest since at this time, the solution in each chamber is heterogeneous and the ion concentrations in the concentration chamber are low. K⁺ gradually migrates to the concentration chamber as the experiment proceeds, and the solutions in each chamber tend to be homogeneous in their own uniform. This leads to a better conductivity and decreased resistance of the solutions. In addition, the migration amount of K⁺ at low current density is less than that at high current density. Therefore, in the later stage of the SED, the ion concentration in the desalination chamber increases with the decreasing current density as evidenced by Figure 2(b), leading to the lowest resistance at 100 A/m² (Szczygiełda & Prochaska 2017)^. The variation of with recovery rate of K⁺ at different current densities is shown in Figure 2(d). The overall upward trend of with the increase of rate is due to the relatively high selectivity of the membrane. It is known from Figure 2(a) that the higher the current density is, the more K⁺ will migrate across the membrane. At the same time, the impurity ion Mg²⁺ will also migrate faster with the increased electric field driving force. The selectivity is highest at a current density of 150 A/m², followed by current densities of 200 and 250 A/m². Although low current density can retard the leak of Mg²⁺, the mobility of K⁺ decreases at low current densities (Jaroszek et al. 2016). Therefore, as the current density decreases, becomes smaller.

It can be found that low current density leads to a decrease in ion migration rate and efficiency, while high current density can accelerate ion migration but lead to a relatively high leak of Mg²⁺. At 200 A/m², more potassium ions were migrated, and the leakage of Mg²⁺ was relatively small. This leaded to an effective improvement in selectivity while ensuring efficiency, therefore, a current density of 200 A/m² was chosen for the subsequent experiments.

It is well documented that initial ion concentration significantly impacts ion migration (Zhu et al. 2020). In this section, the competitive migration mechanism of ions was explored by analyzing the variation of concentration of K⁺, the variation of a flux of Mg²⁺, and the variation of with recovery rate at different Mg²⁺ concentrations.

Figure 4(c) shows the effects of Mg²⁺ concentration on the resistance. It is regarded that a single valence selective ion exchange membrane has a high migration resistance to Mg²⁺. Therefore, Mg²⁺ will accumulate on the membrane surface, forming a stronger positive layer and a greater repulsive force against K ⁺. At the same time, the amount of K⁺ that can migrate from the desalination compartment to concentration chamber decreases, thus the membrane stack resistance will have a slight upward trend after the initial decrease, as shown in Figure 4(c).

The and recovery rate of K⁺ were plotted in Figure 4(d). When K⁺/Mg²⁺ = 0.4, Mg²⁺ with high concentration results in great migration competition with K⁺ and lower recovery rates. When K⁺/Mg²⁺ = 0.7 and 1.0, the final values of and recovery rate are similar but higher than that at K⁺/Mg²⁺ = 0.4. When K⁺/Mg²⁺ = 1.3 and 1.6, the concentrations of Mg²⁺ are similar and relatively low, therefore, both the recovery rate and are improved.

In summary, changes in Mg²⁺ concentration can affect the migration of both ions. The greater the concentration of impurity ions is, the greater the effects on the migration of the target ions are. Therefore, the concentration of Mg²⁺ was chosen as 0.5 mol/L for the following experimental investigation.

Competitive migration between ions is influenced by the initial concentration of the two competing ions (Ding et al. 2021). In this section, the migration patterns of ions under different concentrations of K⁺ (0.20, 0.35, 0.50, 0.65, and 0.80 mol/L) were investigated while the Mg²⁺ concentration was kept at 0.50 mol/L.

As shown in Figure 6(c), the membrane stack resistance decreases as the experiment proceeded. Higher K⁺/Mg²⁺ ratio leads to higher conductivity and lower membrane stack resistance. It may be because the high concentration of K⁺ increases the concentration gradient on both sides of the ion exchange membrane and enhances the migration tendency of ions. Therefore, ions are more likely to pass through the ion exchange membrane, leading to a decrease in membrane resistance. At the same time, the migration of the impurity ion Mg²⁺ is inhibited as the concentration of the target ion K⁺ increases, which reduces the contribution of Mg²⁺ to the resistance of the ion exchange membrane, resulting in a decrease in resistance. Figure 6(d) shows that both and recovery rate gradually decreases with the increase of K⁺/Mg²⁺ ratio. This is because the increase of the initial K⁺ concentration cannot result in equivalent increase in K⁺ flux due to the constant membrane separation ability and electric field driving force.

Whereas when the initial concentrations of K⁺ are 0.65 and 0.80 mol/L, the K⁺ fluxes are almost the same. This is probably because the processing capacity of the ion exchange membrane is limited, and the solution viscosity increases with the increase of ion concentration (Weisbrod et al. 2016), which is unfavorable for the migration of target ions. The flux of Mg²⁺ leaked into the concentration chamber shows a continuous decreasing trend with the increase of the initial concentration of K⁺. The increase in the migration of the target ion K⁺ decreases the leakage of the impurity ion Mg²⁺, which is due to the competitive migration of ions and the ion exchange capacity is fixed within a certain range.

In summary, a high initial concentration of K⁺ can induce a larger K⁺ flux while suppress the leakage of Mg²⁺ toward concentration chamber. Therefore, we chose to conduct further experimental investigations with the initial concentration of K⁺ of being kept at 0.65 mol/L.

As can be seen from Figure 8(a), the increase in temperature has a promoting effect on the migration of K⁺ to the concentration chamber, and the concentration of K⁺ in the concentration chamber shows an increasing trend with the increase of temperature, which is consistent with the variation trend shown in Figure 8(b). This is mainly because the viscosity of the solution decreases with the increase in temperature. Also, the increased molecular kinetic energy at high temperature accelerates the ion migration rate. Figure 8(c) shows that the resistance decreases as the temperature increases. This can be explained by the effects of temperature on diffusion, the hydration layer, and the pore size of the membrane. High temperature increases the diffusion coefficient and accelerates the ion migration rate, thus resulting in a faster decrease in resistance. In addition, ions migrate across the membrane in a hydrated form (Chen et al. 2013). The increase in temperature is more likely to break the hydration layer, making it easier for ions to migrate. Also, high temperature can increase the pore size of the membrane, which can improve the mass transfer rate. Although the increase of temperature accelerated the migration of the target ion K⁺, it also aggravated the leakage of Mg²⁺. As shown in Figure 8(d), when the temperature is low (20 and 25 °C) the leakage amount of Mg²⁺ is small, resulting in high selectivity. At 30 and 35 °C, although the migration amount of K⁺ is higher when compared with that at low temperatures, the facilitation effects of high temperature on the migration of Mg²⁺ is higher than that on K⁺ migration (Benneker et al. 2018), which decreased the selectivity.

In order to effectively achieve the selective separation of mono- and multivalent ions, in this study, the effects of current density, target ion concentration, impurity ion concentration, and temperature on the competitive migration of K⁺ and Mg²⁺ were investigated. It was found that both the migration fluxes of K⁺ and Mg²⁺ increased with the increasing current density. A noticeable competitive migration between K⁺ and Mg²⁺ was demonstrated: an increase of the initial concentration of K⁺ favors the migration of itself while inhibiting the leakage of Mg²⁺. While the increase of initial concentration of Mg²⁺ also causes an inhibitory effect on the migration of K⁺. There is an optimal temperature at which the highest can be obtained. The final results show that the current density has the greatest effect on the ion flux, and the target ion concentration has the greatest effect on the selectivity of mono- and multivalent ions. However, a combination of and ion fluxes, the final choice of current density was 200 A/m² and the initial concentration of target ion K⁺ was 0.65 mol/L and the initial concentration of Mg²⁺ was 0.5 mol/L. This work provides a good guide for the recovery of K⁺ by SED from complex systems and the competitive migration of mono- and multivalent ions.

This work is funded by Tianjin Science and Technology Project (20JCZDJC00450), Natural Science Foundation of Hebei Province (B2020202029), Central Guidance on Local Science and Technology Development Fund of Hebei Province (226Z3102G), Fundamental Research Funds of Hebei University of Technology (JBKYTD2001).

The authors declared that there is no conflict of interest.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.