Introduction

Soil is an essential resource for human survival and growth1. However, with the rapid expansion of industrial activities, various sources of contamination have emerged, leading to a significant rise in heavy metal pollution in soils on a global scale2,3,4,5.Industrial activities produce large quantities of heavy metal-contaminated sludge each year, leading to significant pollution of soil, water, and air, which accelerates environmental degradation6,7,8. According to the data from the 2014 National Soil Pollution Status Survey Bulletin, 16.1% of soil samples in China exceeded pollution thresholds, with Pb2+ levels surpassing the standard in 1.5% of the samples9,10,11. Heavy metal contamination is characterized by persistence, toxicity, bioaccumulation, and non-degradability12,13accumulating through food chains to cause long-term ecological damage, disrupt microbial communities, and threaten human health (cardiovascular diseases, neurological disorders)14,15,16. Research on soil remediation technologies for heavy metal contamination has been widely explored both domestically and internationally, focusing primarily on solidification/stabilization, soil leaching, and electrochemical remediation methods17,18,19. Ex-situ leaching techniques are significant for remediating heavy metal-contaminated soils and encompass physical classification as well as chemical leaching/extraction processes. Initially, high-pressure water spraying is employed to separate fine-grained soil (≤ 75 μm silt/clay) from coarse-grained soil. Fine-grained soil, with its extensive surface area and high capacity to adsorb heavy metals, is the focus of subsequent treatment. A suitable leaching agent is then chosen for chemical extraction to facilitate the transfer of heavy metals from the solid phase to the liquid phase, thereby altering their chemical forms20,21,22.

Ex situ leaching techniques, while effective for coarse soils, face challenges in fine soils due to complex interactions between leaching agents, soil type, and metal speciation23,24. Repeated leaching cycles prolong remediation, increase costs, and demand high operational precision25,26. The leaching-electrochemical combined remediation technology, a recent innovation, combines the benefits of soil leaching and electrochemical remediation techniques to enhance the removal of heavy metal pollutants from soil, improve remediation effectiveness, and restore soil environmental quality27,28. This technology has emerged as a prominent research focus in the field of soil remediation, demonstrating its potential for effectively removing heavy metals from contaminated soils. In the electrochemical remediation process, a direct current power supply is applied to both sides of the contaminated soil, establishing a closed circuit. Upon energization, water molecules in the electrode zones undergo electrolysis, producing OH− at the cathode and H+ at the anode. This generates an electrical potential gradient that drives the movement of charged particles. Electroosmosis promotes the movement of soil moisture by ions and electrons29. Electromigration assists in desorbing adsorbed pollutant ions into the liquid phase, guiding them toward the electrode regions30,31. Electrophoresis enables free colloids to transport pollutant ions, and the combined effects of these processes concentrate the pollutants in the electrode area, facilitating their discharge and achieving effective soil remediation. Numerous studies have explored the combined application of leaching and horizontal electrochemical remediation under constant voltage conditions for pollutant removal. For instance, Ng et al.32. examined the electrochemical enhancement effects of various leaching solutions, including sodium nitrate, nitric acid, citric acid, and EDTA, on Pb2+-contaminated soil. The results indicated that the combined remediation method achieved Pb2+ removal rates that were 2.52–9.08% higher in leaching and 4.98–20.45% higher in electrochemical remediation compared to traditional processes. Krishna et al.33. investigated the effects of leaching and electrochemical treatment technologies on the remediation of polycyclic aromatic hydrocarbons and heavy metal contamination. In treatments lasting up to 120 h, the removal rates of the heavy metals Zn2+, Pb2+, and Cu2+ were about 60%, 80%, and 30%, respectively. Wang N et al.34. studied the use of citric acid as a leaching agent for heavy metal removal, finding that 0.2 mol/L citric acid removed 61% of As3+, 46% of Cd2+, and 65% of Pb2+, which was more effective than the traditional stirring and leaching method.

This study focused on the ex-situ soil leaching process and examined the effectiveness of applying an electric field during the filter press dewatering stage following chemical leaching for deep remediation. The objective was to assess the feasibility and efficiency of electrochemical treatment for the removal of pollutants from the filter cake. Additionally, it aimed to evaluate the influence of various process parameters and contribute to the optimization of the remediation process. The study specifically targeted the filter cake that failed to meet regulatory standards after leaching. An innovative constant current vertical system was employed, with the goal of achieving Pb2+ levels within the soil pollution control standard within the specified time frame. This research explored the impact of parameters in the leaching-electrochemical combined remediation process on the deep removal of target pollutants from the soil, providing scientific insights for the remediation of heavy metal-contaminated sites and advancing efforts to enhance soil environmental quality.

Materials and methods

Experimental materials

The original soil samples(Commercial soil: It contains less organic matter compared to traditional soil) were obtained from the surface layer (0–20 cm) of fine-grained soil (0.075 mm > d > 0.005 mm) in Hebei Province. Prior to their utilization, the soil was naturally air-dried, ground, and sieved through a 0.075 mm pore size sieve to remove coarser particles35. The soil particle size distribution was determined according to the Standard for Geotechnical Specimen Methods (GB/T 50123–2019). Based on the “Particle Group Classification” in the Standard for Engineering Classification of Soil (GB/T 50145–2007), the soil samples were classified as fine-grained soils with coarse particles36. The Pb2+ concentration in the original soil was quantified using Inductively Coupled Plasma Mass Spectrometry (ICP-MS), resulting in a value of 0.84 mg·kg−1. A summary of the physico-chemical properties of the soil is provided in Table 1.

Table 1 The basic physical and chemical property parameters of the soil utilized in the experiment.
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Preparation of soil contaminated with Pb2+

In this study, soil contaminated with Pb2+ at a concentration of 1000 mg/kg was prepared to assess the influence of parameters in the leaching-electrochemical combined remediation process on Pb2+ removal. Spiked soil was chosen for its controllable composition, facilitating accurate evaluation of electrochemical effects while minimizing complex interferences, thus allowing a focused investigation on the research target. A stock solution of 800 mg/L lead nitrate was prepared by dissolving 0.80 g of lead nitrate in deionized water. The molar mass of lead nitrate (Pb(NO₃)₂) is 331.2 g/mol, calculated from the atomic masses of lead (Pb: 207.2 g/mol), nitrogen (N: 14 g/mol × 2), and oxygen (O: 16 g/mol × 6). Lead comprises approximately 62.56% of this mass (207.2/331.2). Consequently, in an 800 mg/L stock solution of lead nitrate, the concentration of lead ions (Pb²⁺) is approximately 500.5 mg/L. Following the results of the soil liquid limit test (Table 1), a slurry was prepared by mixing 150 g of raw soil (sieved through a 0.075 mm sieve) with 300 mL of the lead nitrate stock solution, maintaining a solid-liquid mass ratio of 1:2. Each gram of soil was exposed to 1.001 mg of lead ions (Pb²⁺) (500.5 mg/L × 2 mL).The Pb2+-spiked slurry was mixed for 3 h using an electric stirrer (SR-JB-90, 180 rpm). To ensure sample stability, the stirred slurry was sealed and allowed to age at room temperature for 15 days37.

Following the aging period, 20 mL of the supernatant was centrifuged at 3000 rpm for 10 min and then filtered through a 0.45 μm membrane filter. The Pb²⁺ concentration in the resulting supernatant was measured using an atomic absorption spectrophotometer (TAS-990). The soil demonstrated complete adsorption of lead ions, confirmed by the measured Pb²⁺ concentration in the post-centrifugation supernatant being below the detection limit (< 10⁻⁶ mg/L), indicating negligible loss to the liquid phase. The lead concentration in the prepared contaminated soil, determined via acid digestion followed by Flame Atomic Absorption Spectrometry (HJ 491-2019), was 998.28 mg/kg. This value represents a relative error of only - 0.17% compared to the target concentration (1000 mg/kg), verifying the accuracy of the preparation method and demonstrating quantitative adsorption of lead ions (derived from the lead nitrate) by the soil. Consequently, the lead concentration in the contaminated soil was confirmed to be approximately 1000 mg/kg (calculated as 1.001 mg Pb per kg of soil).

Selection of leaching agent

The prepared soil, containing a Pb2+ contamination level of 1000 mg/kg, was used to evaluate the effectiveness of different leaching agents. Citric acid solution, HCl solution, and FeCl3 solution were selected as leaching agents, with their respective concentrations set at 0.01 mol/L, 0.02 mol/L, and 0.05 mol/L. The solid-to-liquid ratio was maintained at 1:2. The optimal leaching agent was determined based on key indicators, including the Pb2+ removal rate, pH, and conductivity, as presented in Table 2.

Table 2 Comparison of leaching effect indicators for Pb-Contaminated soil using leaching agents at different concentrations.
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As presented in Table 2, under identical concentration conditions and a constant solid-to-liquid ratio, citric acid exhibited a measurable Pb2+ removal efficiency, with its effectiveness increasing progressively as its concentration was elevated. Although HCl demonstrated a higher Pb2+ removal efficiency, it posed a significant drawback by altering the physical and chemical properties of the soil38. In contrast, the Pb2+ removal efficiency of FeCl3 was inferior to that of citric acid. Regarding its impact on soil pH, citric acid induced relatively minor changes, whereas HCl caused severe acidification. FeCl3 also contributed to a decrease in soil pH, but its effect was less pronounced compared to HCl. In terms of conductivity, citric acid led to a relatively small increase, whereas HCl and FeCl3 significantly elevated soil conductivity. By comparing these key indicators, it was evident that citric acid, as a leaching agent, effectively removed Pb2+ while preserving the inherent physical and chemical properties of the soil to the greatest extent. This characteristic was found to be beneficial for the subsequent ecological restoration of the soil. As a tricarboxylic organic acid, citric acid demonstrated multiple advantages in the remediation of heavy metal-contaminated soil. Primarily, it exhibited strong complexation capabilities, enabling the formation of stable complexes with various heavy metal ions. This property facilitated the desorption of heavy metals from soil particle surfaces and enhanced their migration from the solid phase to the liquid phase, thereby improving overall removal efficiency39,40,41. Consequently, citric acid was selected as the leaching agent for the leaching-electrochemical combined remediation process in this study.

Reaction mechanism of leaching-electrochemical combined process

Figure 1 depicts the reaction mechanism diagram of the leaching-electrochemical combined process, which primarily occurred in two stages: the leaching stage and the electrochemical stage. During the leaching stage, citric acid partially ionized in the solution, releasing hydrogen ions H+ (Reaction 1). The Pb2+ adsorbed on the surface of soil particles underwent an ion exchange reaction with these hydrogen ions, leading to the desorption of Pb2+ into the liquid phase (Reaction 2). Additionally, the carboxyl and hydroxyl functional groups present in the citric acid molecule facilitated complexation with Pb2+, forming a stable Pb-citrate complex (Reaction 3). Significantly enhances the mobility of Pb²⁺ in the liquid phase:

$$\:{C}_{6}{H}_{8}{O}_{7}\rightleftharpoons\:{H}^{+}+{C}_{6}{H}_{7}{O}_{7}^{-}$$
(1)
$$\:Soil-{Pb}^{2+}+2{H}^{+}\rightleftharpoons\:Soil-2H+{Pb}^{2+}$$
(2)
$$\:{Pb}^{2+}+{C}_{6}{H}_{7}{O}_{7}^{-}\rightleftharpoons\:{\left[Pb{C}_{6}{H}_{7}{O}_{7}\right]}^{+\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:}$$
(3)

In the electrochemical stage, water electrolysis took place at the anode, producing H+ and O2(Reaction 4 and 5). The generated H+ migrated towards the cathode, and during its movement, it underwent ion exchange with Pb2+ adsorbed on soil particle surfaces, promoting the release of Pb2+ into the liquid phase. At the cathode, water underwent a reduction reaction, resulting in the formation of hydroxide ions (OH−) and hydrogen gas (H2). The Pb2+ in the liquid phase reacted with OH−, leading to the precipitation of Pb(OH)2 (Reaction 6). Under the influence of the applied electric field, both free Pb2+ and Pb2+ complexed with citric acid migrated towards the cathode. The reaction equation in the electrochemical stage is as follows:

$$\:2{H}_{2}O\to\:4{H}^{+}+{O}_{2}\uparrow\:+4{e}^{-}$$
(4)
$$\:2{H}_{2}O+2{e}^{-}\to\:2{OH}^{-}{H}_{2}\uparrow\:$$
(5)
$$\:{Pb}^{2+}+2{OH}^{-}\to\:Pb{\left(OH\right)}_{2}\downarrow\:$$
(6)

Two key transport mechanisms facilitated Pb2+ movement in this process: electroosmosis and electromigration. Electroosmosis enabled water molecules to carry Pb2+ towards the cathode, while electromigration directly drove Pb2+ movement under the influence of the electric field. Throughout the process, Pb2+ continuously transitioned between different forms, including the free ionic state in the liquid phase, the complexed state with citric acid, and the precipitated Pb(OH)2 solid state. Ultimately, these transformations contributed to the effective removal of Pb2+ from the contaminated soil.

Fig. 1
figure 1

Reaction Mechanism of Leaching Combined with Electrochemical.

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Experimental setup

This research employs a self-developed vertical electrochemical cell. This unique apparatus simultaneously facilitates electroosmotic dewatering and mechanical filtration, effectively simulating practical engineering filtration operations and enhancing the study’s practical relevance42,43. The experimental apparatus was custom-designed based on the core principles of leaching-electrochemical remediation, enabling the simultaneous integration of electro-osmotic dewatering and mechanical filtration under applied pressure, as illustrated in Fig. 2. The primary components of the experimental setup, including the piston and the filter chamber cartridge, were fabricated from nylon. The filter chamber cartridge had an internal diameter of 80 mm, while the piston’s outer diameter was slightly smaller to ensure a proper fit within the cartridge. A rubber O-ring was incorporated into the piston to seal the interface between the piston and the filter chamber cartridge, preventing leakage.

To exert pressure on the slurry within the filter chamber, an air compressor supplied controlled pressure to a cylinder, driving the piston plate downward. The applied pressure was regulated through an adjustment valve. To mitigate undesirable electrochemical reactions commonly associated with metal electrodes (e.g., copper), disc-shaped graphite electrodes (80 mm in diameter and 4 mm in thickness) were employed. These graphite electrodes contained multiple perforations (2 mm in diameter, with a density of 4 holes·cm−2) to facilitate efficient drainage. The electrode affixed to the piston functioned as the anode, while the electrode mounted on the base acted as the cathode.

The assembly sequence of the graphite electrode, Plexiglas sheet, and filter cloth on the base was depicted on the right side of Fig. 2, with corresponding structural components positioned at the bottom of the piston. The electrode affixed to the piston functioned as the anode, whereas the electrode mounted on the base acted as the cathode. A direct current (DC) power supply (160 V, 2 A, continuously adjustable) was utilized to generate an electric field across the filter cake, applying voltage between the anode and cathode to enhance electrochemical remediation.

Fig. 2
figure 2

Schematic representation of the experimental setup for filtration-electrochemical remediation of contaminated soil.

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Results and discussion

Influence of leaching process parameters on Pb2+ removal effect

The liquid-to-solid ratio of the drenching solution was utilized as a variable to examine its influence on the efficiency of Pb2+ removal from the soil. The findings, as illustrated in Fig. 3(a), demonstrated the impact of varying liquid-to-solid ratios on the removal efficiency of Pb2+. When the ratio was increased from 2 to 10, a statistically significant enhancement (P < 0.05) in the Pb2+ removal rate was observed, rising from 9.57 to 17.21%. Consequently, the Pb2+ concentration in the soil decreased from 902.74 mg/kg to 826.48 mg/kg. However, further increasing the liquid-to-solid ratio from 10 to 25 did not result in a significant change in Pb2+ removal efficiency. Based on these observations, a liquid-to-solid ratio of 10 was selected for subsequent single-factor screening experiments.

The influence of different citric acid concentrations on Pb2+ removal efficiency was presented in Fig. 3(b). The removal rate of Pb2+ progressively increased as the citric acid concentration was elevated. As the concentration increased from 0.01 mol/L to 0.05 mol/L, the Pb2+ removal rate increased from 8.76 to 12.54%. A significant increase (P < 0.05) in the removal rate from 12.54 to 17.21% was observed as the citric acid concentration was further raised from 0.05 mol/L to 0.10 mol/L. Increasing the concentration beyond 0.10 mol/L resulted in Pb2+ removal rates of 18.93% at 0.15 mol/L and 20.71% at 0.20 mol/L, corresponding to increases of 1.72% and 3.50%, respectively, compared to the removal rate at 0.10 mol/L (17.21%). The experimental results indicated that the increase in citric acid concentration from 0.10 mol/L to 0.20 mol/L resulted in a statistically significant (P < 0.05) improvement in Pb2+ removal efficiency. However, considering the leaching performance, economic feasibility, and environmental implications, a 0.10 mol/L citric acid solution was determined to be optimal for Pb2+ removal.

The effect of leaching time on Pb2+ removal efficiency was depicted in Fig. 3(c). The removal rate exhibited a general increasing trend with prolonged leaching duration. Specifically, within the initial 0.5 to 3 h period, the Pb2+ removal rate increased from 9.54 to 17.21%, leading to a reduction in Pb2+ concentration from 903.04 mg/kg to 826.48 mg/kg. However, during 3 to 6 h interval, the Pb2+ removal rate increased at a slower rate, approaching saturation, with values rising from 17.21 to 18.07%, corresponding to a marginal 1.05-fold increase. The residual Pb2+ concentration in the soil remained within the range of 817.89 to 826.48 mg/kg. The experimental findings demonstrated a significant increase (P < 0.05) in the Pb2+ removal rate during the initial 3 h of the leaching process. Considering both leaching efficiency and duration, a leaching time of 3 h was determined to be optimal for subsequent experiments.

The effect of pH on Pb2+ removal efficiency in soil was illustrated in Fig. 3(d). As the pH of the leaching environment increased, the Pb2+ removal rate exhibited a general decline. Specifically, within the pH range of 2 to 5, a significant reduction (P < 0.05) in Pb2+ removal efficiency was observed, decreasing from 22.31 to 3.24%. In contrast, within the pH range of 5 to 8, the decrease in Pb2+ removal efficiency became more gradual, with an average removal rate of 1.86%. Additionally, no statistically significant difference was detected in the Pb2+ removal rate between the pH ranges of 5 to 6 and 6 to 8. Based on these results, a pH of 3 was selected for subsequent experimental investigations.

Fig. 3
figure 3

Influence of leaching process parameters on Pb2+ removal effect. (a) Liquid-Solid ratio; (b) Effect of Leaching agent concentration; (c) Leaching time; (d) Leaching pH.

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Determination of the threshold point for electric filed application time

In the absence of an applied electric field, the filtration behavior of mud samples subjected to varying mechanical pressures within the filter chamber cylinder was illustrated in Fig. 4. As shown in Fig. 4(a), at an applied pressure of 0.31 MPa, the experimental results indicated a progressive increase in drainage volume during the initial 80 min. Throughout this period, the filtration coefficient exhibited an initial rise, followed by a decline, before reaching a stable state, characterizing the filtration phase. Beyond 80 min, the cumulative drainage rate decreased and stabilized, accompanied by a marked reduction in the filtration coefficient, indicating the transition to the pressure-tight phase. Therefore, 80 min at 0.31 MPa was established as the critical time threshold distinguishing the filtration phase from the pressure-tight phase. Similarly, as depicted in Figs. 4(b) and (c), the transition from the filtration phase to the compacting phase occurred at 60 min for 0.44 MPa and 42 min for 0.57 MPa, respectively.

Fig. 4
figure 4

The difference of filtration coefficient and drainage volume different pressures over time (a) 0.31 MPa; (b) 0.44 MPa; (c) 0.57 MPa.

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According to Ruth’s filtration theory44,45the ratio of filtrate volume (∆V) to the square root of the time interval (Δ√T) remains relatively stable over time. Since the filtration area was constant, this ratio could also be represented by the change in cake thickness (ΔL), implying that the ratio of ΔL to Δ√T remained unchanged throughout the filtration process (as described in Eq. (1))46. As filtration progressed, the process transitioned into the compacting stage once the filter chamber was completely occupied by a solid phase. In this stage, the ratio (∆V/Δ√T) began to decrease with time. If the penetration of fine particles into the filtration medium was minimal, the filtrate volume (∆V) could be directly correlated with the change in cake thickness (-ΔL). Based on Eq. (1), the variation in filter cake thickness (-ΔL) remained consistent in relation to Δ√T retention. Therefore, the application of the electric field was set at 20%, 40%, and 60% before reaching the respective time threshold points at different pressures. These corresponded to cake thickness values of 40 ± 0.92 mm, 35 ± 0.35 mm, and 28 ± 0.58 mm. The variations in time and cake thickness under different pressures were summarized in Table 3. The transition point between the filtration and compacting stages was determined by calculating the filtration coefficient using Eq. (1).

$$\:-\frac{\varDelta\:L}{\varDelta\:T}=i\sqrt{K}=const$$
(1)

\(\:i\sqrt{K}\)——Indicates the filtration factor;

L——Indicates the drop height of the specimen in the filter chamber, mm;

T——Experimental time; min.

Table 3 Changes of time and cake thickness under different pressures.
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Experimental design for leaching combined electrochemical restoration

Figure 4 illustrated the variation in Pb2+ content in the near-cathode and anode zones under optimal leaching conditions, including a solid-liquid ratio of 10, citric acid concentration of 0.1 mol/L, a leaching time of 3 h, and pH = 3.

A total of 150.00 g of Pb2+-contaminated soil was weighed and treated with a 0.10 mol/L citric acid leaching solution. The leaching process involved adding an equal volume of the citric acid solution at a liquid-to-solid ratio of 10, with the mixture stirred at 180 rpm for 3 h using an electric stirrer. After leaching, the drenched soil samples were transferred to the experimental setup for the combined leaching-electrochemical remediation process aimed at electrokinetic restoration. As presented in Table 4, the pressures applied in the electrochemical setup were P1, P2, and P3, set at 0.31 MPa, 0.44 MPa, and 0.57 MPa, respectively. These pressure values were selected based on preliminary experiments exploring combinations of pressure (P), temperature (T), and current (I). Specifically, P2 (0.44 MPa) was identified as the core pressure value, as it lies at the critical threshold for maintaining soil sample integrity without causing damage. The application times of the electric field were established as 20%, 40%, and 60% of the time before the threshold point for each pressure, labeled as T20, T40, and T60, respectively. Furthermore, based on the preliminary findings, a current density of 3 mA/cm² was established as the dominant parameter for the main study. This current level was chosen to mitigate interference from thermal effects associated with higher currents, thereby ensuring the repeatability of the electric field effects and the reliability of the data. The strength of the electric field was a critical parameter for electroosmotic and electrokinetic remediation. At a constant current, it was noted that exceeding a current density of 5 mA/cm2 resulted in a sharp decrease in electroosmotic efficiency due to the ohmic heating effect. To minimize this effect, three current densities, each below 5 mA/cm2 (specifically 2 mA/cm2, 3 mA/cm2, and 4 mA/cm2), were utilized in the study.

Table 4 Design of single factor experiment scheme of leaching combined with electrochemistry.
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Electrochemical response during remediation

Changes in voltage gradient

Fig. 5
figure 5

Influence of electric field application time on voltage gradient evolution.

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The change in voltage gradient, with the same pressure, electric field application time, and different times of electric field application, was shown in Fig. 5. The voltage gradient increased to its peak within 20 min of the start of energization, followed by a sharp decrease, and then gradually leveled off after 100 min of experimentation. The peak value of the voltage gradient gradually increased from 7.65 V/cm (P2T0I3) to 10.67 V/cm (P2T60I3) as the time of electric field application was extended. The rate of change in the cumulative drainage volume of the filtrate at the cathode and anode poles within the first 60 min of the experiment gradually diminished, while the liquid-solid ratio inside the filter chamber increased with filtration. Mechanical pressure readily removed free water from the sludge, and the application of the electric field led to electroosmosis, which removed the associated pore and weakly bound water, resulting in a higher degree of compaction of the sludge particles and increased resistance within the filtration chamber. According to Ohm’s law, a constant current with increased resistance led to an increase in voltage. The experimental results also showed that the application of the electric field advanced slightly decreased the initial value of the voltage gradient, from 6.94 V/cm (P2T0I3) to 6.91 V/cm (P2T20I3), 6.85 V/cm (P2T40I3), and 6.62 V/cm (P2T60I3). However, there was almost no effect on the final value of the voltage gradient, which fluctuated between 6.70 V/cm and 6.93 V/cm.

Changes in filtrate pH

Fig. 6
figure 6

Effect of applied electric field on pH of anode and cathode filtrat.

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Soil pH is a critical indicator reflecting the mobility of heavy metal ions. Figure 6 presents the pH evolution of the filtrate discharged from the anode and cathode during the P₂T₄₀I₃ experiment. The pH changes can be divided into two distinct phases: before and after electric field application.Pre-application phase: The initial pH of the leachate discharged from both electrodes fluctuated between 4.01 and 4.47, showing no significant difference between anode and cathode. Both remained stable during this period.Post-application phase (48–180 min): Upon electric field application, a clear divergence occurred. The pH of the cathode filtrate increased from 4.01 to 8.12, while the anode filtrate pH decreased from 4.05 to 2.37.Without the applied electric field, the system essentially operated under pure pressure filtration. The initial filtrate pH from both electrodes exceeded 3, attributed to the consumption of H⁺ ions from citric acid during the prior leaching stage, which desorbed Pb²⁺ from the solid phase into the liquid phase.The application of the electric field altered the system pH over time due to electromigration, electrophoresis, and electroosmosis. These findings are consistent with the observed pH changes in the anode and cathode filtrates. During electrochemical remediation, water hydrolysis reactions generate substantial H⁺ and OH⁻ ions. This leads to H⁺ accumulation in the anode region and OH⁻ accumulation in the cathode region. The direct current applied via the electrodes consequently induces an acid front at the anode and a base front at the cathode.

Factors influencing the experimental process of leaching-electrochemical combined restoration

Effect of filtration pressure on Pb2+ removal effect

According to the parameters specified in Table 4, the time threshold points for the filtration and compaction stages varied with different pressures. The influence of different pressures (P1T40I3, P2T40I3, P3T40I3) on the combined leaching-electrochemical restoration process was investigated. The impact of these pressures on Pb2+ removal during the combined leaching-electrochemical remediation was presented in Fig. 7(a). The removal rate of Pb2+ in the near-cathode zone showed minimal variation across the different pressures, with no significant differences observed. In contrast, the Pb2+ concentration in the near-anode zone decreased progressively as the pressure increased, resulting in an enhanced removal rate. At 0.44 MPa (P2T40I3), the Pb2+ removal rate in the near-anode zone increased from 36.43% (P1T40I3) to 37.75%, reflecting a 1.32% improvement. However, at 0.57 MPa (P3T40I3), the removal rate of Pb2+ in the near-anode zone rose from 36.43% (P1T40I3) to 38.31%, which was a statistically significant increase (P < 0.05). In comparison, the Pb2+ removal rate in the near-anode zone at 0.57 MPa was 0.56% higher than at 0.44 MPa, though this difference was not significant. The total Pb2+ removal in the near-cathode and near-anode zones was 64.57%, 65.68%, and 66.37% for P1T40I3, P2T40I3, and P3T40I3, respectively. The combined leaching and electrochemical remediation process facilitates the transfer of pollutants from the solid phase to the liquid phase47. The primary goal of the end-press process in leaching is to separate the solid and liquid phases, thereby reducing the water content in the soil and subsequently decreasing the Pb2+ content in the solid phase.

Effect of electric field application time on Pb2+ removal effect

The effect of electric field application time on Pb2+ removal efficiency was illustrated in Fig. 7(b). The experiments were conducted with a current density of 3 mA/cm2, and the electric field was applied at 20%, 40%, and 60% of the time threshold to assess its influence on Pb2+ removal from soil48. Using P2T0I3 as a baseline, the results revealed that earlier application of the electric field led to an increase in Pb2+ removal in the near-anodic zone. Specifically, the Pb2+ removal rates increased by 4.66%, 0.59%, and 1.86% for P2T20I3, P2T40I3, and P2T60I3, respectively. Notably, the Pb2+ removal rate in the near-anodic region for P2T20I3 was significantly higher (P < 0.05) than for P2T0I3. When comparing P2T20I3 with P2T40I3 and P2T60I3, the Pb2+ removal rates in the near-anodic zone decreased by approximately 4.07% and 2.80%, respectively. In contrast, the Pb2+ removal rate in the near-cathode zone showed little variation when the electric field was applied earlier, fluctuating between 25.14% and 27.93%. The highest Pb2+ removal rates were achieved by applying the electric field 20% before the time threshold (P2T20I3), with removal rates of 25.14% and 41.82% observed for the near-cathode and near-anode zones, respectively.

Effect of electric field strength on the removal effect of Pb2+

Applying a direct current electric field across both sides of the filter press not only enhanced the dewatering process but also contributed to the removal of residual target pollutants in the filter cake49. The strength of the electric field is a critical factor in electroosmosis and electrokinetic restoration. However, when the current density exceeds 5 mA/cm2 under constant current conditions, the ohmic heating effect significantly reduces electroosmosis efficiency50. In this study, the pressure was maintained at 0.44 MPa, and the electric field was applied 40% of the way before the time threshold (i.e., for 36 min) to investigate the impact of electric field strength (P2T40I2, P2T40I3, and P2T40I4) on Pb2+ removal. Current densities of 2, 3, and 4 mA/cm2 were used, with the electric field applied 40% before the time threshold, to assess the effect of varying current densities on Pb2+ removal in both the near-cathode and near-anode zones, as shown in Fig. 7(c). The results revealed that higher current densities led to greater pollutant removal when the electric field was applied for the same duration under constant current conditions. Among the different current densities tested, the highest current density (4 mA/cm2) achieved the greatest Pb2+ removal, with removal rates of 29.09% in the near-cathode zone and 41.10% in the near-anode zone.

Fig. 7
figure 7

Influencing factors during the experimental process of combined leaching electrochemical remediation (a) filtration pressure; (b) electric field application time; (c) electric strength.

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Remediation efficiency and pb²⁺ speciation analysis

Removal of Pb2+ using combined remediation processes

The findings of the experiment on the combined remediation of Pb2+-contaminated soil through leaching and electrochemical methods were summarized in Table 5. This integrated approach exhibited a superior Pb2+ removal rate compared to the conventional leaching method under all treatment conditions. Specifically, the removal rates of Pb2+ in the regions near the cathode and anode increased by approximately 9.28% and 21.20%, respectively. Moreover, the water content of the filter cake after remediation was considered an indicator of Pb2+ removal efficiency in the near-anodic zone, where a lower water content corresponded to a higher Pb2+ removal rate. the water content of the filter cake under varying pressures was 20.03% (P1T40I3), 20.32% (P2T40I3), and 19.80% (P3T40I3). The experimental findings indicated that higher applied pressures during the leaching-electrochemical remediation process resulted in lower water content in the cake and a concomitant reduction in Pb2+ content in the near-anode zone. Consequently, Pb2+ content in the near-anode zone steadily decreased with increasing pressure, leading to a gradual increase in the removal rate.

Under the P2T20I3 experimental condition, the lowest water content of 16.32% in the filter cake was recorded, resulting in the highest Pb2+ removal rate of 41.82% in the near-anode region. In contrast, Ng et al. (2015) documented a Pb2+ removal rate of 84.14% following 24 h of leaching and electrochemical coupling remediation at 7.58 V and a citric acid concentration of 0.057 mol/L. Although the present study attained a maximum Pb2+ removal rate of 41.82%, its primary focus was to enhance the efficiency of heavy metal removal within a significantly shorter duration (< 180 min). Despite not reaching the removal rate of Ng et al.‘s study, the Pb2+ removal achieved in this study within a brief period was still significant.

Table 5 Effect of combined leaching and electrochemical repair process on the removal rate of Pb2+.
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Morphological analysis of Pb2+ after remediation

The morphological analysis results of Pb2+ in soil samples, following leaching and experimental remediation with P2T40I2, P2T40I3, and P2T40I4, were presented in Fig. 8. Using the BCR sequential extraction method, the heavy metals were classified into four distinct morphologies: F1 exchangeable (EXC), F2 reducible (RED), F3 oxidizable (OXI), and F4 residual (RES). The results indicated that the Pb2+ forms removed from the regions near the cathode and anode were primarily in the reducible and acid-soluble states, with a smaller proportion found in the oxidizable state. When compared to the soil after leaching, the electric field application led to a reduction in the amount of Pb2+ in the reducible state in both the near-cathode and anode zones, from 59.72% (P2T0I0) to 39.53%, 26.76% (P2T40I2), 35.43%, 24.94% (P2T40I3), and 28.02%, 21.10% (P2T40I4), respectively. The proportion of Pb2+ in the reducible state in both the cathode and anode zones exhibited a consistent decrease with increasing current density. Regarding the acid-soluble state Pb2+, the concentrations at the cathode and anode were reduced from 15.36% (P2T0I0) to 8.81%, 5.63% (P2T40I2), 9.82%, 5.89% (P2T40I3), and 8.49%, 6.22% (P2T40I4), respectively. Additionally, the oxidizable state Pb2+ at both the cathode and anode was reduced by an average of 4.61% and 4.65% at various current densities.

Fig. 8
figure 8

Fractional changes in Pb2+ before and after remediation: (a) cathode; (b) anode.

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Citric acid considerably improved the effectiveness of electrochemical leaching by utilizing its acidic nature and ability to form metal complexes51. The acidic properties of citric acid facilitated the release of Pb2+ ions, which then moved toward the cathode due to the applied electric field. At the same time, citric acid formed negatively charged complexes with Pb2+, which were directed to the anode by the electric field. Moreover, the electric field promoted the dissolution and migration of the metal through mechanisms like electrophoresis. The combined actions of these processes resulted in enhanced remediation outcomes. In studies examining the combined remediation process, Pb2+ removal rates in the contaminated soil reached approximately 26.49% near the cathode and 38.41% near the anode. These rates met the target screening value of 800 mg·kg as defined by the “Soil Pollution Risk Control Standard for Soil Environmental Quality Construction Land” (GB 36600–2018). Following treatment, there was a substantial reduction in the acid-soluble and reducible fractions of heavy metals, while the residual state proportion increased. This indicated a successful treatment, lowering the mobility and bioavailability of heavy metals, and thereby reducing their environmental impact.

Analysis of the significance of parameters in the combined restoration process

The influence of pressure, electric field application time, and current density on Pb2+ removal efficiency was evaluated through a three-factor, three-level orthogonal experiment. In this analysis, a higher value of Ki and ki (i = 1, 2, 3) signified a more effective Pb2+ removal, while a larger R value indicated a greater contribution of the respective factor to the Pb2+ removal rate. The results of the experiments are summarized in Table 6.

Table 6 Analysis of the results from the orthogonal experiments on leaching electrochemical combined remediation.
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By evaluating the magnitude of the polar deviation (R) for each factor, the factors influencing the Pb2+ removal rate were prioritized in the order of C (current density) > A (pressure) > B (electric field application time). The optimal levels for each factor in the single-factor experiments were A3 (0.57 MPa), B1 (20%), and C2 (3 mA/cm2), which aligned with the findings from the single-factor experiments on the leaching electrochemical combined remediation. The best combination of factors was A3B1C2, meaning the optimal process parameters for the leaching electrochemical combined remediation were: a pressure of 0.57 MPa, an electric field application time of 20% before the threshold filtration time, and a current density of 3 mA/cm2. Under these conditions, the Pb2+ removal rate from the soil reached 70.1%.

Conclusion

In the investigation of the leaching process, the optimal conditions for the removal of Pb2+ from soil were determined to be a liquid-solid ratio of 10, a citric acid concentration of 0.1 mol/L, a leaching duration of 3 h, and a pH of 3 for the leaching environment.

During the filtration and pressure-tightening phases, threshold pressure values were set at 0.44 MPa for 60 min and 0.57 MPa for 42 min. Based on the filtration coefficient, the electric field was applied at 20%, 40%, and 60% before reaching these threshold points, corresponding to positions of 40 ± 0.92 mm, 35 ± 0.35 mm, and 28 ± 0.58 mm. The results demonstrated that, under constant repair time and current density, the Pb2+removal rate in the near-cathode region remained largely unaffected by changes in pressure. However, the removal rate in the near-anode region exhibited a significant increase as pressure increased. Under constant current, pressure, and repair time, an increase in current density further enhanced the Pb2+ removal rate, with the P2T20I3 experiment achieving removal rates of 29.09% near the cathode and 41.10% near the anode. Compared to non-energized experiments, the Pb²⁺ fractions removed from both the cathode and anode zones were predominantly in the exchangeable (F1) and reducible (F2) states, with a smaller proportion in the oxidizable (F3) state, indicating that the mobility and bioavailability of Pb²⁺ were significantly reduced, thus posing relatively lower environmental hazard risks.

Orthogonal experimental analysis of the combined leaching-electrochemical remediation process revealed that current density exerted the greatest influence on Pb2+ removal efficiency, followed by pressure and electric field application time. The optimal process parameters were identified as a pressure of 0.57 MPa, an electric field application at 20% prior to reaching the filtration time threshold point (36 min), and a current density of 3 mA/cm2, which resulted in an impressive Pb2+ removal rate of 70.1%. These findings emphasized the remarkable efficiency of the combined leaching-electrochemical remediation process in significantly enhancing Pb2+ removal from contaminated soil.

This integrated technology offers rapid, operationally simple remediation for heavily contaminated sites, achieving regulatory compliance within hours. Future work should explore field-scale applications and multi-contaminant (e.g., Cd, Zn and Cr) scenarios.