Electrokinetic Treatment Research Articles

Electrokinetic treatment of soft soils: experimental study and numerical models

Soft soils have a high compressibility, and low shear strength, and constructions on such soils often require the use of ground-improvement techniques. This paper compares the use of an electrokinetic (EK) treatment of soft soils using the ionic solutions calcium chloride and sodium carbonate. The effects of the ionic-solution type, the EK-treatment duration, the cation exchange capacity (CEC), the specific surface area (Sa), the pH, the electrical conductivity (σ), and the ionic strength (Is) were considered in this study. Examining the parameters and evaluating their effects on soil behavior are difficult and complex. The design of experiments (DOE) software program was used to evaluate the effects of the parameters and determine the significant input factors for the EK treatment on soft soils. The analysis and optimization of the data produced the threshold values using the design-expert® software. In this study, the EK-treated soil with CEC = 4.9 meq 100/g, Sa = 4.5 m2/g , pH = 9.5, σ = 6.0 S/m, Is = 1.55·10-4 mol/L, and electrolyte-type setup of CaCl2-Na2CO3 gave better soil strengthening. The gain in strength is attributed to the flocculation and aggregation of the EK-treated soil particles. The analysis of the data by DOE indicated that it could be used to assess the significant effects of the input factors on the unconfined compressive strength, qu of the EK-treated soft soils.

Monitoring of electrokinetic treatment of landfill leachate-contaminated clay in Portugal

This paper describes how electrokinetic treatment (EKT) was used to decontaminate compacted kaolin artificially contaminated with leachate obtained from a landfill waste site near Lisbon, Portugal....

Ex situ remediation of sediment from Serbia using a combination of electrokinetic and stabilization/solidification with accelerated carbonation treatments.

The application of three simple and cost-effective technologies for ex situ remediation of the sediment of Begej River in Serbia is presented in this paper. In the first step, conventional electrokinetic treatment (EK) was carried out to reduce the amount of contaminated sediment and enhance the accumulation of metals. Subsequently, stabilization/solidification (S/S) treatment was applied to the remaining portion of polluted sediment to immobilize the accumulated metals. At the same time, the influence of accelerated carbonation on the effectiveness of the treatment was evaluated. The immobilizing agents used in this study included bio ash produced by combustion of wheat and soy straw mixture and bio ash derived from molasses incineration. After the treatments, the risk assessment was performed by using the sequential extraction procedure (SEP) and TCLP and DIN 3841-4 S4 leaching tests. The results obtained after the EK treatment revealed a reduction in the amount of polluted sediment to a half. Leaching tests and SEP performed on S/S mixtures after a 28-day maturation period indicated that accelerated carbonation decreased the mobility of critical metals, especially in wheat and soy straw mixtures. Moreover, based on the leaching tests, all prepared mixtures were categorized as non-hazardous and safe for disposal according to the relevant Serbian regulations. The newly developed method that combines EK and S/S treatments with the addition of accelerated carbonation produced reduced volumes of stabilized sediment which is safe for disposal.

Electro-bioremediation of a mixture of structurally different contaminants of emerging concern: Uncovering electrokinetic contribution

This study analyses the electrokinetic (EK) contribution to the removal from a clay soil of a mixture of 10 different contaminants of emerging concern (CECs; 17β-estradiol, E2; sulfamethoxazole, SMX; bisphenol A, BPA; ibuprofen, IBU; 17α-ethinylestradiol, EE2; oxybenzone, OXY; diclofenac, DCF; triclosan, TCS; caffeine, CAF; carbamazepine, CBZ). After 4 days, the CECs natural attenuation was between 0% (CBZ) and 90% (E2) yet increasing with the application of EK (20 mA, 12 h ON/OFF) to 14% (CBZ) and 100% (E2). When EK was applied, the CECs more recalcitrant to biodegradation (i.e. ≤ 13% biotic decay) mostly underwent electro-chemical induced degradation (OXY, DCF, TCS, CAF, CBZ). Daily irrigation enhanced the rates of the electro-oxidation -osmosis and -migration, increasing the CECs decay. After 8 days of EK treatment, the CECs decay increased, surpassing the decay lag phase of some compounds (OXY, TCS, and CBZ). Yet after 16 days, most CECs showed similar removals with and without EK, with EK only acting positively on SMX, OXY, TCS and CBZ (ca. +10%). Our results support that EK application can improve the removal of CECs from soil, however, under the conditions tested, 16-day treatment lead to pH alterations that decreased the bioremediation efficiency and inhibited electro-degradation near the cathode.

Relative relevance of factors affecting the electrokinetic dewatering of soft clayey soils

Electrokinetic (EK) treatment is becoming increasingly important in the geotechnical field, especially for treating fine-grained soils. Above all, dredged sediments (slurries) need to be treated in order to be reutilised. Applying an electric potential difference within the soil, three phenomena take place: electrophoresis, electro-osmosis and electromigration. Negative particles move towards the positive electrode while water and cations move towards the negative electrode. Although the literature is full of cases describing the parameters that affect the EK process, there is a need to understand its limits and fields of application. This paper summarises the results collected from several case studies to understand the influence of some parameters on the efficiency of EK treatment, specifically in terms of dewatering rate. Soil conditions (soil type, pH, pore fluid salinity and water content) and set-up design parameters (electrode materials, configurations and operational mode) are analysed. Finally, parameter ranges for assessing the acceptability of electro-osmotic treatment are proposed. The results show that there are optimum values for the previously mentioned parameters that improve the dewatering and thus enhance the behaviour of the treated soils. EK treatment can be effective when applied on dredged sediments with low salinity.

Electrokinetic Propagation of Acid and Base Fronts in Clayey Soil: An Experimental and Numerical Study

This paper presents an experimental and numerical investigation of the electrokinetic propagation of acid or base fronts through clayey soil. The experimental study included measuring the electric current, the mass of water extracted, and the advance of the acid and base fronts in synthetic sediment composed of 25% kaolinite and 75% silt. The results show that the flux rate remains constant over time and that the medium acidifies with time. The numerical model is based on the transport equations and, by considering adsorption and auto-ionization of water, it describes the temporal evolution of pH in the specimen. The experimental evaluation of the adsorption isotherms allows us to calculate the retardation factor, which is 69 and 19.5 for the hydronium and hydroxyl ions, respectively. Given these retardation factors, the results of the numerical model are consistent with experimental results and show how the acid and base fronts evolve in a saturated porous medium. Numerical model results show that electromigration is the major contributor to the migration of ions in the porous medium during electrokinetic treatment.

Electrokinetic and chemical treatment of oil sands mature fine tailings: dewatering and strengthening

Oil sands tailings have become a major challenge in Canada due to various geotechnical and environmental concerns. Mature fine tailings (MFT) typically contain 70% water and 30% silt- and clay-sized solids and have low hydraulic conductivities, which lead to difficulties in consolidation by nature. Electrokinetics (EK) has been proved to be effective at dewatering and strengthening oil sands tailings, but is only limited to the anode area. Further treatment is needed to improve the tailings around the cathode. Chemical stabilisation is a mature technique for ground improvement. However, there is limited information for the combined effects of EK and chemical treatment on high water content geomaterials such as MFT, slurry, sludge and so forth. This research study was carried out to evaluate the effects of combined EK and chemical treatment of MFT. Quicklime and Portland cement were selected as the chemical additives. The changes in water content, undrained shear strength, plasticity, pore-water pH and electrical conductivity and particle zeta potential after treatment were studied. It was found that the chemical treatment reduced the material property difference between the anode and cathode, whereas it reduced EK-induced water flow. Combined EK and chemical treatment of MFT may be beneficial at a low chemical dosage.

Effect of the pore fluid salinities on the behaviour of an electrokinetic treated soft clayey soil

Dredging activities of harbours and rivers are becoming very important in many countries all over the world and, as a consequence, the disposal of dredged sediments is a critical concern from an environmental point of view. In order to facilitate the disposal or the reuse of large volume of dredged soils, usually under-consolidated and with a high water content, an electrokinetic treatment can be adopted with the goal to dewater and strengthen the sediments.This paper presents the results of some electrokinetic tests performed on reconstituted clayey specimens at different pore fluid salinities (0.2 < sc < 30 g/l) treated with electrokinetic (EK) technique. The results indicate that the presence of small quantities of salts in the pore fluid enhances the electro-osmotic consolidation. On the contrary, for high salt concentrations of the pore fluid the electro-osmotic dewatering is significantly reduced. The mechanical behaviour of treated specimens has been investigated at the micro (SEM) and macro scale (triaxial and oedometer tests). The experimental results highlighted the relevant and expected contribution of the pore fluid characteristic on the effectiveness of the treatment as ground improvement technique.

Stabilization of Landslides Sliding Layer Using Electrokinetic Phenomena and Vacuum Treatment

The article presents the efficiency of application of cohesive soil dewatering for increasing its resistance to shearing, which influences the mass stability of flysch rock. Studies of the typical soil constituting the contact layer initiating the sliding of existing Carpathian flysch landslides were conducted. This aspect was examined because the water content of this soil decides its ability to form a sliding surface of the landslide block soil. The soil was subjected to changes in water content by dewatering with different methods. The influence of dewatering by self-acting gravitational outflow was examined and was additionally aided by two selected methods: electrokinetic phenomena and vacuum treatment. The model study conducted demonstrates the influence of the abovementioned dewatering methods on increasing the strength parameter of the soil at the contact layer in which sliding surfaces can be created. The paper also demonstrates the degree to which the application of the vacuum and electrokinetic treatment caused by DC current voltage influences the draining, decrease of plasticity, and increase of soil shear stress resistance. The application conditions and increase in effectiveness due to the application of the studied methods were determined. The proposed methods allowed for the strengthening of slopes for two exemplary landslides which formed in the area of occurrence of the Carpathian flysch.

The removal efficiency of lead from contaminated soil: modeling of cations and anions migration during the electrokinetic treatment

The effect of the presence of minerals in natural soil polluted with lead (II) was investigated to verify the efficiency of the electrokinetic remediation method. Natural soil “Sebkha of Oran” containing high calcite minerals and characterized by high salinity was used in experimental studies. This study investigates the effects of alkaline soil pH conditions on the transport and removal of lead by the electrokinetic treatment. XRD analyses were performed on the soil sample before and after electrokinetic treatment to determine any changes in mineral phases. Mathematical models using experimental data are developed to describe the mobility and diffusion coefficient of lead ions through the soil. Mathematical models were generated based on the physicochemical parameters characterizing the movement of cations and anions.

Development of laboratory-scale sequential electrokinetic and biological treatment of chronically hydrocarbon-impacted soils

This study focused on the remediation of a chronically diesel-polluted soil by combining an electrokinetic treatment with a variety of bioremediation approaches. Priority within the sequential treatment was given to electrokinetic remediation (EKR) since the application of natural attenuation (NA), biostimulation and site-specific bio-augmentation resulted in very low degradation performance for total petroleum hydrocarbons (TPH) and polycyclic hydrocarbons (PAH). The application of 20-day EKR (1.0 V cm−1 with polarity reversal) led to 47.2 % and 46.2 % removal of TPH and PAH, respectively, and exerted a negative impact on bacterial abundance, as determined indirectly by quantitative PCR of 16S rRNA genes and community function, as investigated by community-level physiological profiling. These adverse effects were transient and, after a 50-day NA treatment applied downstream from EKR, bacterial abundance was an order of magnitude higher than that found in the initial soil and TPH and PAH removals were significantly higher than those attained by EKR (64.1 % and 56.3 %, respectively). The combination of EKR with site-specific bioaugmentation led to the greatest TPH and PAH degradation (76.0 % and 78.6 %, respectively). The results indicate that bioremediation can be successfully applied downstream from EKR and that the adverse effects exerted by this physico-chemical approach on soil microbiota are reversible.

Effect of ionic solutions on the performance of electrokinetic treatment of soft soils

This study evaluates the performance of electrokinetic (EK) treatment to improve the engineering properties of soft soils. Laboratory tests were conducted using ionic solutions of calcium chloride, CaCl2 and sodium carbonate, Na2CO3. Atterberg limits, unconfined compression, electrochemical, thermal, and microscopic tests were performed in terms of treatment duration, anode-to-cathode distances, and different combinations of ionic solutions as calcium chloride-distilled water (CaCl2-DW), sodium carbonate-distilled water (Na2CO3-DW) and calcium chloride-sodium carbonate (CaCl2-Na2CO3). EK treatment caused a reduction in the specific surface area, Sa, cation exchange capacity, CEC, and plasticity resulting in 26%, 14%, 19% reduction using CaCl2-DW, Na2CO3-DW, and CaCl2-Na2CO3 ionic solutions, respectively. The formation of the new compounds, calcium aluminosilicate hydrates, CASH resulted in the strength gain of the EK treated soils and increased the strength from 21 kPa to 90 kPa, 63 kPa, and 92 kPa with the ionic solutions of CaCl2-DW, Na2CO3-DW, and CaCl2-Na2CO3, respectively.

Biosurfactants enhanced electrokinetic treatment of Cr from chromite ore processing residue based on chemical fractions

This study investigated the electrokinetic (EK) treatment of Cr from chromite ore processing residue (COPR) using rhamnolipid as the electrolyte. Chemical fractions were analyzed to better understand the transformation and migration of Cr from COPR with different electrolytes and time. The results showed the removal efficiency of Cr(Ⅵ) was enhanced to 90.04% with a concentration of rhamnolipid at 15 g/L, and the total Cr increased to 19.65% at 20 g/L; the above values are higher than 65.74% and 9.52% when using deionized water, respectively. After the EK treatment, the concentrations of Cr in the form of exchangeable (EXC), carbonates (CAR), Fe–Mn oxides (FM) and residual (RES) decreased while the organic matter (OC) form increased. Rhamnolipid greatly promoted a transformation between the chemical fractions and the migration of Cr. The transformation was most effective on the third day, and the removal efficiency gradually increased until reaching the best result on the fifth day; however, a further extension of time was detrimental to the transformation and migration of Cr. After the controlled experiment, the environmental toxicity of the remaining Cr decreased as the Cr in the high-risk fraction was effectively removed.

Electrokinetic treatment of desiccated expansive clay

Structures built on shallow foundations in expansive clay, where vegetation and weather conditions can significantly reduce the moisture content, must overcome the risk associated with excessive desiccation conditions, such as shrinkage and settlement. For many existing structures, removal of trees may not necessarily be an effective solution, as it can cause swelling and heave in the long term. This study explores a system that could be operated during prolonged drought periods to rehydrate and neutralise the negative pore pressures of expansive clay, avoiding the development of excessive desiccation. Electrokinetics is a proven method that has increasingly been adopted for dewatering and consolidation of fine-grained soils. The hypothesis presented is to reverse the electrokinetics process by extracting water from a saturated stratum below the groundwater level and force it into the ‘early-stage’ desiccated area, accelerating rehydration and reducing suction. The theory was examined by a laboratory model using moderately expansive clay. After draining the soil for 23 days to reach a moderate desiccation state, water was allowed into the lower part of the soil while applying the EK treatment using nine electrodes, placed in a radial pattern and inserted 110 mm into the soil with approximately 30% of their lengths below the water level. As a result of the electro-osmotic process, the average moisture content increased in the model by more than two-fold within 8 h. The suction measurement (taken in the middle of the desiccated area) showed an initial slow response followed by fast and consistent reduction rate, where suction dropped by 93% within 5 h and ultimately down to 1 kPa at the end of the treatment. This paper offers some insight into the factors controlling the proposed system and provides a good basis for future research to manage the risks associated with desiccation of expansive soils.

Heavy metal removal effectiveness, flow direction and speciation variations in the sludge during the biosurfactant-enhanced electrokinetic remediation

Heavy metal high content is strictly limited the land application of the sludge, which is considered as the best way for resource utilization. Hence, it is essential to explore a more effective way for reducing toxic metal. In this work, the combined biosurfactant and electrokinetic technique was used to promote toxic metals removal. Electroosmotic flow (EOF), sludge water content, electrolyte and sludge electrical conductivity variations, and removal efficiency, distribution, speciation and binding intensity of heavy metal variations were investigated during the electrokinetic process. The results demonstrated that the combined biosurfactant with electrokinetic technique could enhance toxic metals removal from sludge. In the enhanced electrokinetic treatment, the combined biosurfactant with electrokinetic technique can enhance cumulative electroosmotic flow, and the electrolyte electrical conductivity of anode and cathode, and the EK3 obtained a higher electrical conductivity (EC) than other EK treatments. Heavy metals obtained the highest removal efficiency during the EK3 treatment. Heavy metals distribution demonstrated that 58.63%, 71.34%, 63.04%, 52.95%, 61.74%, 56.24% of copper, zinc, chromium, lead, nickel and manganese migrated into the electrode solution, respectively, and 38.02%, 26.20%, 32.54%, 42.69%, 36.34%, 42.35% of copper, zinc, chromium, lead, nickel and manganese were still remained in the sludge. Heavy metal speciation indicated that the labile speciation percentage decreased. Whereas, the stable speciation percentage increased after the electrokinetic treatments. Heavy metal stability demonstrated that binding intensity increased, due to heavy metal have a higher stable speciation after the electrokinetic treatments.

Elec-accumulating behaviors of manganese in the electrokinetics-processed electrolytic manganese residue with carbon dioxide and oxalic acid

An electrokinetic treatment for the accumulation of manganese in electrolytic manganese residue (EMR) was investigated. Electrokinetic experiments were conducted in a self-made electrokinetic device with CO2 bubbled into the cathode region as well as deionized water (EKCa), Na2SO4 (EKCb), Na2C2O4 (EKCc) and H2C2O4 (EKCd) pretreating the EMR, respectively. The potential difference, the pH, the accumulated content of manganese, the manganese fractionation and the energy consumption were investigated during the electrokinetic experiments. The results revealed that the accumulated content of manganese at the cathode region during the electrokinetic experiments with CO2 and different pretreating reagents was higher than that during the direct electrokinetic experiment. The manganese content of the cathode region increased gradually with an increasing electrokinetic time during EKCa, EKCb, EKCc and EKCd. The total manganese content and the combinative exchangeable and carbonate content at the cathode region had the maximum 7.5% and 6.3%, respectively, during EKCd, and the energy consumption per gram accumulated manganese of EKCd was lowest. These patterns were because CO2 reacted with OH − ions generated at the cathode region to produce CO32− ions and reduced the resistance, and CaMn2O4 from the EMR was reduced to Mn2+ ions by H2C2O4 in acidic regions.

Combining nanoscale zero-valent iron with electrokinetic treatment for remediation of chlorinated ethenes and promoting biodegradation: A long-term field study

Nanoscale zero-valent iron (nZVI) is recognized as a powerful tool for the remediation of groundwater contaminated by chlorinated ethenes (CEs). This long-term field study explored nZVI-driven degradation of CEs supported by electrokinetic (EK) treatment, which positively affects nZVI longevity and migration, and its impact on indigenous bacteria. In particular, the impact of combined nZVI-EK treatment on organohalide-respiring bacteria, ethenotrophs and methanotrophs (all capable of CE degradation) was assessed using molecular genetic markers detecting Dehalococcoides spp., Desulfitobacterium spp., the reductive dehalogenase genes vcrA and bvcA and ethenotroph and methanotroph functional genes. The remediation treatment resulted in a rapid decrease of the major pollutant cis-1,2-dichloroethene (cDCE) by 75% in the affected area, followed by an increase in CE degradation products methane, ethane and ethene. The newly established geochemical conditions in the treated aquifer not only promoted growth of organohalide-respiring bacteria but also allowed for the concurrent presence of vinyl chloride- and cDCE-oxidizing methanotrophs and (especially) ethenotrophs, which proliferated preferentially in the vicinity of an anode where low levels of oxygen were produced. The nZVI treatment resulted in a temporary negative impact on indigenous bacteria in the application well close to the cathode; but even there, the microbiome was restored within 15 days. The nZVI-EK treatment proved highly effective in reducing CE contamination and creating a suitable environment for subsequent biodegradation by changing groundwater conditions, promoting transport of nutrients and improving CE availability to soil and groundwater bacteria.

Analysis and Optimization of Mn Removal from Contaminated Solid Matrixes by Electrokinetic Remediation.

Electrokinetic remediation is a useful technique for the removal of ionic contaminants in soils, sediments, sludges, and other solid porous matrixes. The efficiency of metal removal and the electricity consumption in the electrokinetic treatment of soils largely depend on electric and physicochemical conditions. This study analyzes the electrokinetic treatment of Mn contaminated kaolinite clay specimen and the influence of voltage, current intensity, moisture content, pH, and facilitating agents on metal removal and energy consumption. The objective of this study is to identify the influence of the typical variables used in electrokinetic remediation. The results showed that the operation at constant voltage or constant current intensity were equivalent in terms of metal removal and energy consumption, as long as the electric field intensity was kept low to minimize the consumption in parallel electrochemical reactions, especially the electrolysis of water. The moisture content had a significant influence on the Mn removal. Moisture content higher that 50 percent resulted in very effective Mn removal as compared with kaolinite specimens with lower moisture. The control of pH in the electrolyte solutions and the addition of facilitating agents (organic acids) enhanced the removal of Mn but increased the electric energy cost. Overall, the best conditions for Mn removal involved low to moderate electric potential difference (10 to 30 V), the use of citric acid as the facilitating agent, and the pH control in the cathode at a slightly acid pH. The electrokinetic treatment of a sludge from a water treatment plant contaminated with Mn was effective when pH control on the cathode was used. Mn and various metals (66% of Mn, 30% of Cu, 56% of Zn, 21% Sr, and 21% of Fe) were removed with moderate electricity and acid consumption.

Enhanced electrokinetic removal of problematic salts in arid and semi-arid areas

Recently, soil salinization in arid regions has caused considerable damage to ecosystems, agricultural productivity, and civil engineering infrastructure and facilities. This threat has become one of the most hazardous environmental issues and is the main reason for land degradation, desertification, and reduced soil load-bearing capacities. Conventional techniques of mitigating soil salinization such as soil irrigation, soil substitution, planting salt-accumulating plants, and soil amendment are expensive, time-consuming, and ineffective. Thus, the remediation of salt-affected soils represents a new challenge in the mitigation of soil salinization in arid and semi-arid lands. In this work, the feasibility of an electrokinetic treatment that removes salts from saline soil was evaluated. Three experimental electrokinetic tests were conducted with a laboratory-made electrokinetic apparatus using deionized (DI) water as the working solution. The potential gradient was set to either 1.5, 2.0, or 2.4 V/cm. This range of values was chosen to ensure low energy consumption and to suppress heating, which has adverse effects on the treatment. An enhanced electrokinetic approach (EEA) was developed by investigating the effects of diiferent electrical applied voltages (EAVs) on the removal of salts. The results revealed that the salt concentration gradually decreased and the salt removal rate increased as the EAV was increased. The highest salt removal rate occurred with an EAV of 2.4 V/cm, which yielded removal efficiencies of 89% and 88% for calcium (Ca2+) and sulfate (SO42−) ions, respectively. This research shows that the proposed electrokinetic technique has the potential to highly efficiently remediate and reclaim problematic salt-affected soils in arid and semi-arid areas.

Time-lapse monitoring of an electrokinetic soil remediation process through frequency-domain electrical measurements

The electrokinetic (EK) method is an emerging technique for soil remediation, even though a monitoring system of the contaminant removal through geophysical methods has not been developed yet. In this paper, frequency-domain time-lapse measurements are used on heavy-metal contaminated sediments for monitoring an EK remediation process in a small-scale measuring cell. Our goal is to monitor the development of the electrokinetic process within the sediment and to evaluate the total time needed for the treatment. In fact, frequency-domain electrical monitoring provides complex resistivity spectra at different time steps that can be correlated to changes in the physical properties of the sediments. We perform laboratory spectral induced polarization (SIP) measurements on different samples before, during and after the EK treatment, using different electrolyte solutions (acids and tap water), commonly employed in EK remediation. Direct-current measurements (resistivity and chargeability) were also acquired on one sample for testing the reliability of the system by a comparison with a widespread commercial instrumentation for field measurements.Results indicate that resistivity is a diagnostic parameter as long as it is linked to changes in water saturation, pH and ionic concentration and not to the percentage of metal extraction. The resistivity exhibited well-defined signatures as a function of time that changes depending on the conditioning agent and the grain size distribution. These peculiarities were used to understand the physical processes occurring within the cell and consequently to assess the effectiveness of the electrokinetic treatment.Conversely, the polarization effect was negligible using acids as conditioning agents at the electrolyte chambers. Therefore, the SIP method is not effective under these conditions, being the polarization effect significant only when tap water was used at both ends of the measuring cell. In this case, we were able to correlate changes in water saturation with the time-shift observed on relaxation time distributions (RTDs) after inversion of SIP data and to observe, using normalized chargeability, that polarization is stronger at high pH values.On these basis, resistivity is suitable to monitor the development of the remediation, to optimise the energy levels required for treatment and to assess the end time of the EK process (time when metal mobilization ends). In fact, the end time of treatment can be associated with the time at which resistivity becomes stable. This time is highly dependent on the particular working conditions and sediment grain size as demonstrated by our experiments.