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Article

Effect of Pulsed Electric Field on the Cations Removal from Salt-Affected Soils to Optimize Energy Use Efficiency in Reclamation

by
Ahmed Abou-Shady
1,2
1
Soil Physics and Chemistry Department, Water Resources and Desert Soils Division, Desert Research Center, El-Matariya, Cairo 4540031, Egypt
2
Laboratory of Water & Soil Chemistry, Water Resources and Desert Soils Division, Desert Research Center, El-Matariya, Cairo 4540031, Egypt
Resources 2025, 14(1), 16; https://doi.org/10.3390/resources14010016
Submission received: 25 November 2024 / Revised: 10 January 2025 / Accepted: 16 January 2025 / Published: 20 January 2025
(This article belongs to the Special Issue Assessment and Optimization of Energy Efficiency)
Browse Figures
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Abstract

:
In arid and semi-arid zones, reclaiming/restoring salt-affected soil is considered a significant challenge because of the limited amount of water available for soil washing. The reclaimed salt-affected soil is regarded as a valuable resource for increasing the production of food and feed. In the current study, soil electrokinetics (SEK) under pulsed-mode electric field operation was used to evaluate and optimize energy use efficiency for reclaiming salt-affected soils, which is one of the electro-agric technology branches that was suggested in 2021 to address the water crisis in arid and semi-arid regions. Under a fixed applied voltage of 5 V, or 1 V/cm, the calcareous, highly salinized soil under investigation was reclaimed. A 25% reduction in applied voltages with time OFF set at 15, 30, 60, and 120 min and a 50% reduction with time OFF set at 15, 30, 60, and 120 min were the two pulsed electric field techniques that were examined. The findings demonstrated that the removal of Na+ surpasses half (50%) in the majority of pulsed-mode studies. By decreasing the removed K+, which is crucial for plant growth, the pulsed modes of electric fields 25 and 50% showed an economic advantage over the control experiment, which operated with a continuous electric field. Throughout the control experiment, very little Ca2+ was removed. However, the amount of Ca2+ removed rose when the electric field’s pulsed mode was applied, and the removal percentages were higher for the pulsed 50% strategy than the pulsed 25% strategy. In nearly every segment of every experiment (control, pulsed 25%, and pulsed 50%), the pH levels exceeded the initial value of 8.05. The pulsed 25% strategy of the OFF time showed an improvement in current passing at the longest interval of 120 min; the pulsed 50% strategy of the OFF time showed an improvement in current passing at the shorter and longer intervals of 15, 60, and 120 min; however, the interval of 30 min had a negative effect. The cumulative EO flow at the time OFF interval of 60 min was improved by the pulsed 25% strategy throughout the first seven days of operation, and by the end of the trial, the control experiment exhibited high values. The highest values, however, were displayed by the pulsed 50% field at the time OFF interval of 60 min. The anolyte pH decreased for the majority of the time OFF intervals over the first seven days of the trial for both the 25% and 50% pulsed strategies. Lastly, in order to minimize the overall energy consumption, it is strongly advised that the pulsed mode of the electric field be used while reclaiming salt-affected soil.

1. Introduction

1.1. Calcareous Soil

The Food and Agriculture Organization states that the calcium carbonate (CaCO3) content of calcareous soils is frequently greater than 15% in various forms. About 30% of the world’s land area, including cultivated areas, is covered by calcareous soils. They are extensively found in arid and semi-arid areas with abundant mineral resources. These soils include CaCO3 that either precipitated in place or was inherited from parent material (primary carbonates). After Ca oxalate-containing plant tissues are released (usually by litterfall) and oxidized and degraded by oxalotrophic bacteria and fungi, CaCO3 can also occur [1,2,3,4,5,6]. In the process of forming aggregates, CaCO3 is an essential cementing material, especially in calcareous soils with low levels of clay and organic matter. The primary element governing the environmental functional characteristics of calcareous soils is CaCO3. CaCO3 affects the drainage characteristics, soil porosity, and the distribution of soil particle and pore sizes. The pH of calcareous soil is nearly 8, primarily due to buffering effects from interactions with CaCO3 and atmospheric CO2. Whenever CaCO3 dissolves, calcium (Ca2+) and carbonate ( C O 3 2 − ) ions are released [1,2,3,4,5,6].

1.2. The Function of Ca2+, K+, and Na+ in Soil and Plants

Plant growth and development depend on the nutrient Ca2+. Plant leaves require this element; a lack of Ca2+ causes pale leaves, and a severe Ca2+ deficiency causes the plant to dry up, usually starting at the leaf tips and ending with the loss of the leaf buds [7]. After nitrogen and potassium, calcium is the third most prevalent of the basic plant nutrients. Ca2+ (1–3 mM) is necessary for plants to grow and develop normally. All phases of development are impacted by the biological processes that Ca2+ contributes to, including signaling, plant metabolism, and cell proliferation [8]. K+ is an essential macronutrient whose availability in the soil affects both crop productivity and plant growth and development [9]. When plants do not get enough K+ from supplements, they develop poorly, grow slowly, produce fewer seeds, have lower yields, and are more vulnerable to pests and illnesses. The synthesis of starch, ATP production, water and nutrient movement, legume-based nitrogen fixing, sugar translocation, protein synthesis, and enzymes are all aided by soil K+ [10]. High Na+ soil concentrations increase diffusive and biochemical barriers to photosynthetic CO2 uptake, alter the production of starch and soluble carbohydrates, reduce transpiration rates, and diminish the amount of pigment present in leaves. Furthermore, an excess of Na+ reduces the uptake of nutrients by the roots by altering the trans-membrane transport of ions, which causes plant cells to lose their turgor and further damages their membranes due to the production of reactive oxygen species [11]. Na+ can cause macropore blockage and the dispersion of soil clay particles. When paired with Na+-induced clay dispersion, Mg2+ can generate soil disaggregation, which may lead to soil cracking [12].

1.3. An Overview of Soil Electrokinetics

According to the information mentioned by Hansen et al. [13], L. Casagrande conducted the first field experiments of the electroosmosis (EO) concept using actual soil in the 1930s and 1940s, despite the fact that F. F. Reuss had discovered in 1809 that an electric direct current (DC) field might be used to move water through a porous material some 210 years earlier. Attempts have been undertaken over the last 60 to 70 years to exploit the electrokinetic (EK) phenomenon—which happens when electric DC fields are applied—to solve a variety of emergent and practical issues. Early in the 1990s, pioneering scientists introduced standards and concepts which helped SEK, a physiochemical approach, acquire prominence [13,14]. SEK has been used in a variety of scenarios to accomplish specific goals, including overcoming problems in agriculture [15], consolidation [16], soil nutrient availability [17], reclaiming salt-affected soils [18], sedimentation [19], the remediation of polluted soil (both organic and inorganic) [20,21,22], phosphorus management in soils and sewage sludge [23], and dewatering [24].
Combining SEK with other chemical, biological, and physical techniques can increase its efficiency [14,25,26]. SEK experiments have encouraged phyto/bioremediation and caused the movement of charged and uncharged elements, mainly through electromigration and EO in the target soil, by applying an electric field in two forms, alternating current and DC, either together or independently. Since SEK cannot distinguish between specific soil components, the introduction of electric fields causes bulk motions [24,27,28,29,30]. Elemental separation, stability, and synergistic separation with SEK were all improved by scientists with the addition of other compounds [31,32,33,34,35]. A recent analysis of pulsed electric fields showed the benefits of this strategy for SEK research [22], including a number of benefits such as (a) lowering the overall costs of SEK application by reducing energy consumption, (b) obtaining sufficient remediation, (c) encouraging residual fractions to change into weakly bound fractions, which makes it easier to move between places when an electric field is present, (d) improving charged species movement and/or desorption, (e) enabling the transfer of contaminants from the solid to the liquid (interstitial fluid) phases, (f) preventing voltage loss if a cation exchange membrane is used, (g) permitting pollutant dispersion through the soil’s pores while the applied voltage is OFF, (h) reducing concentration polarization, (i) producing a large electroosmotic flow, (j) avoiding corrosion on the electrode, etc. The pulse electric field could be used during inorganic pollutant remediation [36,37,38,39,40,41,42], simultaneous remediation of organic and inorganic pollutants [43,44,45,46,47,48,49], organic pollutant remediation [50,51,52,53,54,55], salt removal [56,57,58,59,60], and anion removal [61,62] and could be integrated with reverse polarity [63,64,65,66], phytoremediation and bioremediation [67,68,69], and dewatering and consolidation [70,71,72,73,74,75,76,77,78,79]. The use of electric fields is also expanded to include desalination and wastewater treatment, which is thought to be beneficial for enhancing SEK performance when it comes to treating contaminants that contain an anolyte or catholyte. For instance, the electric field could be used to recover metals through electrodeposition or electrolysis [80], treat wastewater that contains both organic and inorganic pollutants [81,82], and increase the effectiveness of wastewater purification by combining it with other techniques like Fenton oxidation and coagulation [83], oxidizing organic materials in wastewater [84], electrodeionization [85], reusing or recycling water for irrigation [86], and electrocoagulation [87].
In this study, we examine how Na+, K+, and Ca2+ behave during SEK operation under different pulsed electric field circumstances. Two pulsed electric field approaches were investigated: a 25% decrease in applied voltages with time OFF set at 15, 30, 60, and 120 min and a 50% reduction with time OFF set at 15, 30, 60, and 120 min. Our latest assessment, which focused on the analysis of pulsed electric fields over a 31-year period (1993–2023) and was published in 2024, indicates that the idea of applying a different mode of pulsed electric field to the reclamation of highly salty calcareous soil was not investigated [22]. Also, other publications released in 2024 revealed a similar tendency [88,89,90]. Nine electrokinetic experiments were conducted to accomplish this task: four experiments examined a 25% reduction in the amount of energy consumed, four experiments examined a 50% reduction in the amount of energy consumed, and one experiment was devoted to a control (i.e., performed with continuous application of applied voltages). In order to replicate the conditions of dry and semi-arid regions, where water resources for land reclamation are limited, electrokinetic studies were also conducted under the depletion of water in the anode reservoirs (anolyte). Soil pH fluctuation upon experiment termination, electrolyte pH, current passage, and EO flow rate were also examined. Using an electric field to recover soils and move beyond barriers caused by high salt concentrations [91,92,93,94] could be a benefit of this work’s results.

2. Materials and Methods

Ras Sidr Research Station in South Sinai, Egypt, provided the soil sample form the top layer (0–20 cm). In September 2024, the soil samples were collected. The soil under study was calcareous and highly salinized. The texture of the soil was sandy loam [95,96]. The soil’s electrical conductivity was 15.3 dS/cm. While the pH was 8.05, the total CaCO3 content was 50.21%. The Na+, K+, and Ca2+ total contents were 2027.05, 1236.95, and 77,486.12 mg/kg, respectively. To simulate the conditions of arid and semi-arid countries with severely limited water resources, the SEK unit was operated in the anode reservoir under unstable/hydrostatic water head. The SEK unit may suffer from water depletion if it is used extensively in the arid and semi-arid regions. After two, four, and seven days, the anode reservoir was refilled with tap water (Ec = 358 µS cm−1, pH = 7.2) [24]. There were about fifty milliliters of tap water in the anode reservoir. The current investigations were conducted without the installation of separate electrolyte reservoirs, as was the case with the prior SEK remediation, in order to replicate the field application. In order to separate the soil, anolyte, and catholyte, the electrodes were placed directly inside the SEK unit (see Figure 1). As seen in Figure 1, a 24 h plug-in timer was utilized to regulate the transition between the applied voltages’ ON and OFF modes. The electrodes were made to be extremely resistant to corrosion, particularly for the anode. They were 20 cm long and 1.5 cm in diameter, made of titanium. The iron electrode was not included in this study since it might have an impact on the pH drop near the anode [97]. Because wax was used to keep the electrode in place and prevent leaks, the electrodes’ effective length inside the SEK box was about 16 cm. Tissue paper was positioned underneath the anode to keep soil particles out of the anode and cathode reservoirs. The internal distance between the anode and cathode was five centimeters. To ensure that the gases generated by water electrolysis would escape outside the laboratory, the SEK box, shown in Figure 1, was made of clear plastic that was 2 mm thick and had a plastic cover attached to the nozzle. The SEK box’s top surface measured 12.5–18.5 cm, while its bottom layer measured 11.5–17.5 cm.
The SEK box measured 8.5 cm in height. The soil in the SEK box was 1.5 cm high. A 10 mL plastic syringe was used to collect the anolyte solution in order to measure the pH fluctuation in the anolyte. In contrast, the pH of the catholyte was measured in the exit EO flow or, in the absence of an outlet EO flow from the catholyte compartment, using a plastic syringe. Throughout the ten days of the experiment, the applied voltage was kept constant at 5 V, or 1 V/cm. The current study’s experimental design is detailed in Table 1. Following the completion of the experiments, four 1.25 cm thick slices of the treated soil were taken from the anode side and dried for 24 h at 100 °C in an oven. The soil was then grounded and placed in a plastic bag for further examination. To provide steady DC of 1 V/cm, the Chinese-made RXN-305D and RXN-3010 power supplies (0–30 V and 30–60 V, respectively) were used. In order to measure the current flowing through the SEK unit precisely, an avometer (UT61E, manufactured in Beijing, China) was attached to the electric circle. It was discovered that the current passing values that showed up on the power supply were not accurate in the mA range. An EC meter (JENWAY, MODEL: 4510) and pH meter (JENWAY, MODEL: 3510) were used to measure the electrical conductivity and pH, respectively. Ca2+, K+, and Na+ were detected using a Flamephotometer (JENWAY, MODEL PFP7, Made in Chelmsford, UK), and the total content of CaCO3 was determined using the Volumetric Calcimeter method [98]. Following the SEK tests in different conditions, the fluctuation in Na+, K+, and Ca2+ was analyzed using the concentration ratio (CR) equation as follows:
C R = T h e   c o n c e n t r a t i o n   o f   i o n   a f t e r   t r e a t m e n t T h e   c o n c e n t r a t i o n   o f   i o n   b e f o r e   t r e a t m e n t

3. Results

3.1. Effects of Different Pulsed Electric Field Types on the pH Distribution of Soil

The soil under investigation had an alkaline pH of 8.05. Figure 2A,B depict the pH variation in the investigated soil following the termination of the SEK experiments conducted under the pulsed 25% and 50% strategies. Except for section A of the 30 min OFF period of the pulsed 50% strategy, the pH of the investigated soil in every segment was higher than the initial values of 8.05 (the initial soil pH) in both the pulsed 25% and 50% strategies. While higher pH values in the pulsed strategy of the 50% time OFF interval were found in section C near the cathode, the higher pH rate in the pulsed 25% instance was located in the middle of the SEK unit (i.e., sections B and C). Section D, which is directly behind the cathode, had lower pH values than section C, which came after it, in both of the 25% and 50% reduction energy methods of the pulsed mode. The pH values with the 15 min OFF break were almost the same when comparing the 25% pulsed-mode approach to the control trial, whereas the pH values with the other intervals of OFF time were lower for sections A and B. It was observed that, with the exception of the 60 min time OFF interval, all of the time OFF intervals in sections C and D had higher pH values when compared to the control experiment. For every period of OFF time, sections A and B displayed pH values in the 50% pulsed method that were lower than those of the control experiment; in contrast, sections C and D displayed the reverse trend, with higher pH values. During the 25% pulsed OFF intervals, the pH levels next to the anode were in ascending order: 60 min OFF time < 120 min OFF time < 30 min OFF time < control experiment ≤ 15 min OFF time. However, adjacent to the cathode, the pH levels during the 25% pulsed OFF intervals were in descending order: 30 min OFF time ≥ 120 min OFF time > 15 min OFF time > control experiment ≤ 60 min OFF time. During the 50% pulsed OFF intervals, the pH levels next to the anode were in ascending order: 30 min OFF time < 15 min OFF time < 120 min OFF time < 60 min OFF time < control experiment. The pH levels adjacent to the anode were in ascending order throughout the 50% pulsed OFF intervals: 30 min OFF time < 15 min OFF time < 120 min OFF time < 60 min OFF time < control experiment. On the other hand, during the 50% pulsed OFF intervals, the pH levels next to the cathode were in decreasing order: 120 min OFF time > 60 min OFF time > 30 min OFF time > 15 min OFF time > control experiment.

3.2. Impact of Various Pulsed Electric Field Modes on the Na+, K+, and Ca2+ Distribution

In Figure 3, the response of Na+ to various pulsed electric field techniques with 25% and 50% OFF times at varied intervals is illustrated. Comparing the pulsed 25% and 50% strategies to the control tests, which involved a continuous electric field except for the pulsed 50% time OFF strategy for 15 min intervals in sections B and C, revealed a greater removal of Na+. With the exception of the pulsed 25% time OFF strategy for 60 min intervals, when the removal of Na+ close to cathode was higher than that of other sections (i.e., A, B, and C), the removal of Na+ close to anode was higher than that close to cathode in the pulsed 25% strategy. With the exception of the 15 min OFF period, during which the removal of Na+ was stable in sections B, C, and D, the same tendency was also noted with the pulsed 50% time OFF interval. Since the presence of Na+ is thought to be the primary problem, the majority of pulsed-mode experiments show that the removal of Na+ exceeds half (50%) and emphasize the employment of the pulse mode technique to lower the energy consumption in reclaiming salt-affected soil.
K+’s reaction to different pulsed electric field approaches with 25% and 50% OFF times at different intervals is shown in Figure 4. The elimination of K+ from section A near the anode was greater in the pulsed 25% of time OFF interval than in the control experiment, which operated with a constant electric field; however, the opposite tendency was noted near the cathode. The removed K+ for the pulsed 25% time OFF interval in section D next to the cathode was arranged in the following descending order: interval 30 min > control experiment = interval 15 min > interval 120 min > interval 60 min. With the exception of the pulsed 50% 15 min time OFF interval, where the removed K+ was equal to that obtained in the control experiment, the removal of K+ near the anode (in section A) was greater in the pulsed 50% time OFF interval than in the control experiment, which was run with a continuous electric field application. In all of the pulsed 50% time OFF intervals, the removal of K+ was lower than the control experiment. Compared to the control trial, the pulsed modes of electric fields 25 and 50% demonstrated an economic advantage by reducing the removed K+, which is important for plant growth.
Figure 5 illustrates how Ca2+ responds to various pulsed electric field techniques with 25% and 50% OFF times at various intervals. The amount of Ca2+ removed from all sections of the control experiment, which was run with an electric field applied continuously, was negligible. In contrast, the application of the electric field’s pulsed mode increased the amount of Ca2+ removed, and the removal percentages were higher in the case of the pulsed 50% strategy than the pulsed 25% strategy. When the time was set to OFF in the following order, the application of pulsed 25% time OFF intervals demonstrated higher removal percentages: interval 120 min > interval 60 min > interval 30 min > control experiment > interval 15 min. With the exception of the 120 min OFF interval, during which the removed Ca2+ decreased in sections C and D, the pulsed 50% time OFF intervals showed removed Ca2+ between 50 and 60% for all time OFF intervals.

3.3. The Effects of Different Pulsed Electric Field Modes on the pH of Anolyte and Catholyte

Figure 6 and Figure 7 show how the catholyte and anolyte solutions’ pH changes in response to various pulsed electric field types. After 24 h, the catholyte pH was the same as in the control experiment for the smaller and medium intervals (15–60 min OFF) in the case of a pulsed electric field with 25% energy decrease (pulsed 25%); however, the catholyte pH rose when the time OFF interval was extended to 120 min (Figure 6). In comparison to the control experiment, the catholyte pH generally increased in the case of the pulsed 25% strategy with intervals of 30, 60, and 120 min (OFF time) after two days had elapsed until the experiment’s completion (after ten days). Pulsing 25% at intervals of 120 min (OFF time) caused the catholyte pH to drop sharply as the trial duration increased. This trend was also seen with pulsed 25% at intervals of 30 min (OFF time) and the control experiment. After passing four days of operation, the pulsed 25% strategy with intervals of 15 and 60 min (OFF time) tended to gradually increase. The pulsed 25% strategy with intervals of 15 and 60 min (OFF time) tended to progressively rise after four days of operation. While the catholyte pH was higher, the 30 and 60 min time OFF intervals revealed a similar pattern to the control experiment in the case of a pulsed electric field with 50% energy decrease (pulsed 50%). The cathode reservoir’s dryness and leaking made it challenging to measure the catholyte pH for the pulsed 50% strategy at 15 and 120 min intervals (OFF time), respectively. Figure 7 illustrates the effects of varying pulsed electric fields on the anolyte solution’s pH for the 25% and 50% strategies. The anolyte pH was lower in the pulsed 25% case at all intervals (OFF time) than in the control experiment, and it tended to decrease as the experimental period was extended. When 25% was pulsed at 60 min intervals, the anolyte pH decreased slightly (not as much as it did during other intervals), which led to an increase in anolyte pH in comparison to the control trial. Within the first five days of starting the studies, the pH for each of the several pulses 25% was as follows: control experiment > interval 60 min > interval 15 min > interval 30 min; nonetheless, an osculated behavior was observed at interval 120 min. The pulsed 50% approach showed the same behavior as the pulsed 25% strategy, with the exception of the pulsed 50% strategy with a 15 min OFF period, where the anolyte pH was higher than the control experiment 24 h after the trial started. Leakage made it challenging to assess the changes in the anolyte’s pH after the experiment’s 50% pulse strategy with a 15 min break.

3.4. Effects of Different Electric Field Pulse Types on Current Passing

Figure 8 illustrates the change in current passing throughout the examination of several pulsed techniques (25% and 50% OFF duration with varying intervals). When trials began with a 25% pulse, the current was increased and dropped significantly after a day, which was consistent with the control experiment’s behavior. A modest decrease in the current passing was then seen as the experimental duration was extended. The pulsed 25% time OFF intervals of 15 and 30 min had the same current passing during the 10-day experiment as the control study. Comparing the pulsed 25% interval period to the control experiment, increasing it to 60 min reduced current passing, while increasing it to 120 min increased current passing. It was noticed that after 2 days of initiating experiments with the pulsed 25% strategy, the current passing was slightly improved compared with the first day. The current passing in all interval levels improved when the pulsed 50% time OFF interval was applied, with the exception of the 30 min time OFF intervals, which were the same as the control experiment for the first four days before declining. After 24 h of the experiment, the anolyte pH reduction for the pulsed 50% strategy was somewhat higher than that for the pulsed 25% strategy.

3.5. Effects on the Cumulative Electroosmosis Flow Rate of Different Pulsed Electric Field Modes

Figure 9 shows the cumulative EO flow rate of several pulsed electric field modes. In comparison to the control trial, the cumulative EO flow rate decreased for the pulsed 25% time OFF interval of 15 min. This behavior was also seen with the greatest values in the time OFF interval of 120 min. During the first seven days, the pulsed 25% time OFF period of 30 min behaved exactly like the control experiment; however, after that, the EO outlet flow dropped. During the first seven days, the pulsed 25% time OFF period of 60 min had the maximum EO flow; however, the EO flow rate decreased thereafter. When comparing the pulsed 25% EO flow rate to the control experiment, the cumulative EO flow rate fell into the following order: control experiment > 60 min interval > 15 min interval > 30 min interval > 120 min interval. Due to leaks and a decrease in catholyte reservoir solution or dryness, the cumulative EO rate was not evaluated for the 15 and 120 min time OFF intervals for the pulsed 50% time OFF period approach. The cumulative EO flow rate of pulsed 50% time OFF at 30 and 60 min intervals was higher than the control experiment over the first four days of the experiment. For the pulsed 50% time OFF interval of 60 min, this behavior continued until the experiment’s termination, but not for the time OFF interval of 30 min. In the 60 min pulsed 50% time OFF strategy, the maximum cumulative EO flow rate was recorded.

4. Discussion

According to Abou-Shady and El-Araby [22], the use of pulsed electric fields has shown a number of benefits, including (a) lowering the overall costs of SEK application by reducing energy consumption, (b) obtaining sufficient remediation, (c) encouraging residual fractions to change into weakly bound fractions, which makes it easier to move between places when an electric field is present, (d) improving charged species movement and/or desorption, (e) enabling the transfer of contaminants from the solid to the liquid (interstitial fluid) phases, (f) preventing voltage loss if a cation exchange membrane is used, (g) permitting pollutant dispersion through the soil’s pores while the applied voltage is OFF, (h) reducing concentration polarization, (i) producing a large electroosmotic flow, (j) avoiding corrosion of electrode, etc. In addition to lengthening the time needed for remediation, the use of pulsed electric fields may also result in the diffusion of contaminants via soil pores, which are barriers for SEK [22].
Water electrolysis produces alkaline media at the cathode and acid media next to the anode after the electric field is connected between the electrodes. The alkaline media in the cathode zone will migrate from the cathode side to the anode, whereas the acid media on the anode side will act similarly toward the cathode [22,29]. The high concentration of CaCO3 (50.21%) in the soil under study in this work will interact with the H+ generated near the anode and ultimately prevent a significant change in the pH of the soil next to the anode. However, there will be a more noticeable pH shift close to the cathode, which could account for the higher soil pH values in sections A, B, C, and D as compared to the initial values of 8.05. In contrast to sections B and C, section D’s pH drop was more obvious, highlighting the function of electroosmosis flow from the anode to the cathode. In the majority of pulsed-mode investigations, the removal of Na+ exceeds half (50%) of the total. The pulsed modes of the 25 and 50% electric fields demonstrated an economic advantage over the control experiment, which operated with a continuous electric field, by lowering the removed K+, which is essential for plant growth. Very little Ca2+ was removed from any of the sections during the control experiment, which was carried out with a constant electric field applied. However, using the electric field’s pulsed mode increased the amount of Ca2+ removed, and the removal percentages were higher for the pulsed 50% strategy than the pulsed 25% strategy. Monovalent ions like Na+ and K+ were removed at quicker rates than divalent ions like Ca2+. This could be because monovalent ions have lower adsorption forces than divalent ions.
The electrolyte reservoirs and treated soil are separated by electrodes in the current design, allowing the electrolytes to pass through tissue paper that is placed beneath the electrodes. Furthermore, the existing design’s frequent refilling of the anode reservoirs may have reduced the moisture content between the anode and the treated soil. Similarly to the conditions of a portion of our study, a recent study by [61] investigated the effects of pulse-enhanced SEK remediation on fluorine-contaminated soil using a pulsed 50% strategy at equal time ON intervals (30, 60, 90, and 120 min) and time OFF intervals (30, 60, 90, and 120 min). The results showed a similar tendency to SEK studies, where the current flowing increased significantly during the first few hours of operation before progressively declining. When the interval time was shortened (e.g., 30 min), the highest current passing values were achieved [61]. The larger current passing values observed in the system with the shorter pulsed electric field interval can be explained by the pulse current system’s capacity to effectively control concentration polarization and lower ion exchange membrane resistances [61]. The majority of pulsed 25 and 50 time OFF applications in our study resulted in an improvement in current passage; the 50% pulsed method outperformed the 25% pulsed strategy.
Zhou et al. [61] state that the EO flow is precisely proportional to the electric potential gradient if both porosity and the coefficient of electroosmotic permeability remain constant. The 30 min interval pulsed period displayed the highest electrical potential gradient and the largest EO when compared to other experimental runs. The control experiment’s EO, which operated with a constant applied voltage, showed the largest cumulative EO flow in our analysis when compared to the 25% pulsed time OFF interval approach. This figure was nearly identical to the one that was achieved by applying the 25% pulsed 60 min OFF interval method. However, when compared to the control experiment, the 60 min OFF break during the pulsed 50% method should provide optimal values. Because our research was conducted in an unstable anode reservoir, these results might not be consistent with those of other studies.
Recent studies have documented the beneficial effects of pulsed electric fields during soil electrokinetics. A recent study by Diba et al. on the improved remediation of clay soils contaminated with Cu utilizing SEK that included intermittent power and purging solutions showed that it is still difficult to fully prevent the negative consequences of focusing bands, even with the increased rate of Cu removal from the application of chemical agents [88]. To overcome this obstacle, a large number of enhancers may be required in order to completely eradicate the environmental hazards associated with heavy metals. By using the pulsed mode of electric fields, which provides a great opportunity for desorption/dissolution reactions to predominate over precipitation mechanisms during the electrokinetic process that will transfer Cu ions in the form of more solubilized species, this restriction can be overcome [88]. Zhou et al. pointed out that various periodic frequencies and power-off intervals all increased the removal rates of PAHs, especially those with large ring numbers, as compared to the continuous current application [89]. According to the Wang et al.’s study, increasing the voltage gradient and power-on duration helps to increase the transport of rare earth elements and increase their recovery rate [90].

5. Perspectives for the Future of Using Soil Electrokinetics to Reclaim Soil Damaged by Salt (Salt-Affected Soil)

The SEK design used in this experiment was quite simple; the electrodes were placed directly into the salty–calcareous soil, separating it from the anolyte and catholyte reservoirs. Additionally, tissue paper was placed underneath the electrodes to keep soil particles out of the electrolyte reservoirs and only permit the flow of EO or electrolyte solution to pass through. In the 33 years after the 1993 introduction of the fundamentals and principles of SEK, a number of SEK processes have been introduced [14,27,28,29]. The current SEK design utilized in this work is a horizontal design; however, we have not examined vertical or mixed (horizontal and vertical) designs, which will be our next task. Several developments in SEK designs occurred between 1993 and 2000: SEK integrated with permeable reactive barrier [99], PCPSS [100], different electrode configurations [101], scaling up SEK [44,102], SEK using a proton pump approach [103], SEK with a salt bridge system [104], SEK rotational operation [105], SEK-assisted phytoremediation [106,107], combined SEK with solar cells [108], vacuum electrokinetics [109], different electrode positions [110], novel electrode modules [111], integrated SEK and anaerobic digestates [112], etc. However, SEK use was significant in 2021–2023 and studies included different designs like using perforated pipes [113], an upward gradient EKG [114], wet sponges [115], cartridges with a cation exchanger [116], magnetic induction-assisted EK [117], auxiliary electrodes [118], dielectric barrier discharge plasma [119], flexible ion adsorption electrodes [120], etc. All of these advances could be applied to improve the electrokinetic management of salt-affected soil, as per the study’s main objective. Additionally, even though the current work used a pulsed electric field and constant applied voltage, the SEK for slat removal did not demonstrate the impacts of the stepped-up voltage mode at the start of the SEK test, which uses continuous voltage climbs to prevent high temperatures. The application of lateral anodes [24], the polarity reversal technique [121,122] and nozzles [28]; conditioning the electrolyte solution [123]; applying a continuously reoriented electric field [30,124]; and avoiding the emergence of cracks are additional enhancing strategies that may be essential for improving the performances of SEK for salt removal but were not thoroughly investigated.

6. Conclusions

In order to assess and maximize energy use efficiency for salt-affected soil reclamation, the SEK approach is employed. Restoring or recovering salt-affected soil is considered a significant problem in dry and semi-arid locations because of the lack of water for soil washing. Reclaimed soil that has been impacted by salt is thought to be a great resource for increasing food and feed production. In our study, the applied voltage was set at 5 V, or 1 V/cm. Two pulsed electric field approaches were investigated: a 25% decrease in applied voltages with time OFF set at 15, 30, 60, and 120 min and a 50% reduction with time OFF set at 15, 30, 60, and 120 min. Calcareous, highly salinized soil was the subject of the investigation. The results collected are summarized as follows:
  • At the longest interval of 120 min, the pulsed 25% time OFF strategy demonstrated an improvement in the current passing; at the shorter and longer intervals of 15, 60, and 120 min, the pulsed 50% time OFF strategy demonstrated an improvement in the current passing; however, a negative impact was noted with the interval of 30 min.
  • In the majority of pulsed-mode investigations, the removal of Na+ exceeded half (50%) of the total.
  • The 25 and 50% pulsed modes of the electric field demonstrated an economic advantage over the control experiment, which operated with a continuous electric field, by lowering the removed K+, which is essential for plant growth.
  • Very little Ca2+ was removed from any of the sections during the control experiment, which was carried out with a constant electric field applied. However, using the electric field’s pulsed mode increased the amount of Ca2+ removed, and the removal percentages were higher for the pulsed 50% strategy than the pulsed 25% strategy.
  • The pulsed 25% showed an improvement in the cumulative EO flow at the time OFF interval of 60 min during the first seven days of operation, and by the end of the trial, the control experiment shown high values; however, the pulsed 50% time OFF interval of 60 min displayed the highest values.
  • In contrast to the control experiment, the majority of the time OFF intervals for the pulsed 25% strategy showed high catholyte pH values, with the exception of the time OFF interval of 15 min. In contrast, the time OFF intervals for the pulsed 50% strategy showed higher pH values for the time OFF intervals of 30 and 60 min.
  • For either pulsed strategy of 25% or 50%, the anolyte pH dropped during the majority of time OFF intervals over the first seven days of the experiment.
Finally, it is highly recommended that the pulsed mode of the electric field be utilized while recovering salt-affected soil in order to reduce the overall energy usage.

Funding

This research was funded by the Desert Research Center, Egypt.

Data Availability Statement

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest

The author declares no conflicts of interest.

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Figure 1. An illustration of the SEK experiment which includes electrodes, a nozzle, and bottle for collecting the EO flow, as well as a cover (A). A plug−in timer that enables precise control over switching between ON and OFF time (B). The timer’s usage instructions to explain how it works (C), and a pre−operational photo of SEK experiment units showing soil loading (D).
Figure 1. An illustration of the SEK experiment which includes electrodes, a nozzle, and bottle for collecting the EO flow, as well as a cover (A). A plug−in timer that enables precise control over switching between ON and OFF time (B). The timer’s usage instructions to explain how it works (C), and a pre−operational photo of SEK experiment units showing soil loading (D).
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Figure 2. The change in soil pH levels following the termination of the SEK experiments: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment. The anode and cathode distance was fixed at 5 cm, and sections A, B, C, and D were taken from the anode side to the cathode at a distance of 1.25 cm apiece.
Figure 2. The change in soil pH levels following the termination of the SEK experiments: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment. The anode and cathode distance was fixed at 5 cm, and sections A, B, C, and D were taken from the anode side to the cathode at a distance of 1.25 cm apiece.
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Figure 3. The variation in Na+ concentration ratios after terminating the soil electrokinetic experiments: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment. The anode and cathode distance was fixed at 5 cm, and sections A, B, C, and D were taken from the anode side to the cathode at a distance of 1.25 cm apiece.
Figure 3. The variation in Na+ concentration ratios after terminating the soil electrokinetic experiments: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment. The anode and cathode distance was fixed at 5 cm, and sections A, B, C, and D were taken from the anode side to the cathode at a distance of 1.25 cm apiece.
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Figure 4. The variation in K+ concentration ratios after terminating the soil electrokinetic experiments: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment. The anode and cathode distance was fixed at 5 cm, and sections A, B, C, and D were taken from the anode side to the cathode at a distance of 1.25 cm apiece.
Figure 4. The variation in K+ concentration ratios after terminating the soil electrokinetic experiments: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment. The anode and cathode distance was fixed at 5 cm, and sections A, B, C, and D were taken from the anode side to the cathode at a distance of 1.25 cm apiece.
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Figure 5. The variation in Ca2+ concentration ratios after terminating the soil electrokinetic experiments: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment. The anode and cathode distance was fixed at 5 cm, and sections A, B, C, and D were taken from the anode side to the cathode at a distance of 1.25 cm apiece.
Figure 5. The variation in Ca2+ concentration ratios after terminating the soil electrokinetic experiments: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment. The anode and cathode distance was fixed at 5 cm, and sections A, B, C, and D were taken from the anode side to the cathode at a distance of 1.25 cm apiece.
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Figure 6. The pH variations in the catholyte solution at various experimental times during SEK treatment: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment.
Figure 6. The pH variations in the catholyte solution at various experimental times during SEK treatment: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment.
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Figure 7. The pH variations in the anolyte solution at various experimental times during SEK treatment: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment.
Figure 7. The pH variations in the anolyte solution at various experimental times during SEK treatment: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment.
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Figure 8. The fluctuation in current passing during the operation of the SEK experiments: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment.
Figure 8. The fluctuation in current passing during the operation of the SEK experiments: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment.
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Figure 9. The variation in the cumulative electroosmosis flow (mL) across SEK tests: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment.
Figure 9. The variation in the cumulative electroosmosis flow (mL) across SEK tests: (A) the 25% pulsed time OFF interval strategy in comparison to the control experiment and (B) the 50% pulsed time OFF interval strategy in comparison to the control experiment.
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Table 1. An experimental configuration using the pulsed mode of electric field with a 25% and 50% energy consumption reduction target.
Table 1. An experimental configuration using the pulsed mode of electric field with a 25% and 50% energy consumption reduction target.
Trials No.Strategy for Reducing Energy UseThe Period of Applied Voltage Connection (Time ON)The Period of Applied Voltage Disconnection (Time OFF)Applied Voltage
1Control experiment24 hZero1 V/cm
225%45 min15 min1 V/cm
325%90 min30 min1 V/cm
425%180 min60 min1 V/cm
525%360 min120 min1 V/cm
650%15 min15 min1 V/cm
750%30 min30 min1 V/cm
850%60 min60 min1 V/cm
950%120 min120 min1 V/cm
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Abou-Shady, A. Effect of Pulsed Electric Field on the Cations Removal from Salt-Affected Soils to Optimize Energy Use Efficiency in Reclamation. Resources 2025, 14, 16. https://doi.org/10.3390/resources14010016

AMA Style

Abou-Shady A. Effect of Pulsed Electric Field on the Cations Removal from Salt-Affected Soils to Optimize Energy Use Efficiency in Reclamation. Resources. 2025; 14(1):16. https://doi.org/10.3390/resources14010016

Chicago/Turabian Style

Abou-Shady, Ahmed. 2025. "Effect of Pulsed Electric Field on the Cations Removal from Salt-Affected Soils to Optimize Energy Use Efficiency in Reclamation" Resources 14, no. 1: 16. https://doi.org/10.3390/resources14010016

APA Style

Abou-Shady, A. (2025). Effect of Pulsed Electric Field on the Cations Removal from Salt-Affected Soils to Optimize Energy Use Efficiency in Reclamation. Resources, 14(1), 16. https://doi.org/10.3390/resources14010016

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