A new two-dimensional experimental apparatus for electrochemical remediation processes☆

Yingying Gu *,Rongbing Fu ,Hongjiang Li,Hui An

1 Department of Environmental&Safety Engineering,China University of Petroleum(East China),Qingdao 266580,China

2 Shanghai Academy of Environmental Sciences,Shanghai 200233,China

3 Qingdao Water Group Co.Ltd.,Qingdao 266002,China

Keywords:Electrochemical extraction 2D experimental apparatus Non-uniform electrical field Gas generation rate

ABSTRACT Electrochemical extraction of contaminants from soils is a promising soil decontamination technology.Various experiments have been conducted to study electrochemical reactions and geochemical processes in the electrochemical extraction using different experimental apparatuses.This paper presents the development of a new closed two-dimensional(2D)apparatus that can better simulate the field application of the technology and accurately monitor the most important electrochemical parameters to understand the process.The innovative features of the new apparatus include the outer and inner electrodes designed to apply a non-uniform electrical field across the specimen as in the field electrochemical remediation process,the probes installed to measure the 2D distribution of electrical voltage,and the gas and fluid volume measurement devices used to accurately monitor the gas generation and electroosmotic flow rates at both electrodes as a function of time.The components of this new apparatus and the features of each component are described.The operating procedure and some typical results from three experiments with the apparatus are demonstrated.The results show that the variation of the gas generation rate is in good agreement with the electric current.Their relation provides a valid evaluation for electrochemical behavior of the system and Faraday's laws of electrolysis.The 2D pro files of cadmium concentration and voltage distribution at the end of the experiment reveal the great effects of a non-uniform electrical field on the contaminant mobilization.

1.Introduction

Electrochemical remediation is a promising in situ decontamination technology for contaminated soils with low permeability.Various bench-scale experiments in the laboratory have been carried out to study the electrochemical reactions and geochemical processes in the electrochemical extraction process such as electroosmosis,electromigration,electrolysis of electrode fluids,pH gradient,complexation/chelation,oxidation–reduction reaction,electroosmotic flow,and electrophoretic mobility of soil particles,which prominently affect the efficiency of process[1,2].Many theoretical formulations and numerical models for the electrochemical remediation process need experimental evaluation[3,4].Therefore,it is essential to develop an experimental apparatus that simulates field extraction conditions and is capable of measuring various parameters as a function of time and space in the electrochemical extraction process.

The experimental apparatuses in use for bench-scale or pilot-scale electrochemical decontamination vary in materials,size,shape,and configuration for different functions and purposes[5].Some of them are open systems,i.e.,soil specimen is in direct contact with air[6–8].One or more pairs of electrodes have been used to generate a nonuniform electrical field to study the effects of electrode configuration on contaminant removal[9,10].Although the open systems simulate the field conditions better,electrochemical reactions are not well controlled and some parameters cannot be accurately measured due to the exchange with surroundings such as evaporation[5].Closed apparatuses avoid uncontrolled exchange of materials with the atmosphere.The most widely employed apparatus is a one-dimensional(1D)system with plate or mesh electrodes covering the cross section of the specimen to simulate 1D transport of contaminants[11–13].However,the 1D apparatus cannot replicate the non-uniform electrical field in field applications.Few 2D closed apparatuses have been employed for electrochemical extraction experiments.

In this study,a 2D closed electrochemical experimental apparatus is developed by modifying the most widely used 1D experimental apparatus for saturated or unsaturated soils.The innovative features of the new apparatus include:(1)the rectangular specimen cell between the top and base plates,which makes it much easier to sample at different locations for chemical analyses to establish 2D pro files of various soil parameters after experiments;(2)two sets of power electrodes applying a non-uniform electric field that simulates the field remediation process;(3)a series of voltage measurement probes uniformly installed in the specimen to monitor the 2D voltage distribution;and(4)the gas and fluid volume measurement devices to determine the gas generation rates and electroosmotic flow rates at both electrodes as a function of treatment time during experiments.The design and features of different components of the new apparatus are introduced in detail.The experimental procedures and some typical results of three bench-scale electrochemical experiments obtained using the apparatus are presented.

2.Design of the New Apparatus

The new apparatus consists of an electrochemical extraction apparatus for bench-scale electrochemical extraction experiments and a soil consolidation apparatus for preparing contaminated soil specimens.

2.1.Electrochemical extraction apparatus

The electrochemical extraction apparatus is an assembly consisting of a specimen cell,gas and fluid volume measurement devices,an electrical circuit,a dc power supply,and a data logger as shown in Fig.1.Detailed discussion on each individual component is given as follows.

Fig.1.The electrochemical extraction apparatus.(a)Schematic;(b)photo.

2.1.1.Specimen cell

The specimen cell is composted of a rectangular box,a top plate,a base plate,two sets of electrodes,and 71 voltage measurement probes uniformly distributed on the top plate.The internal dimensions of the rectangular specimen cell are 154 mm wide,254 mm long,and 90 mm deep.The schematic of the specimen cell is presented in Fig.2.

Fig.2.Schematic of the specimen cell.(a)Cross-section;(b)plan.

2.1.1.1.Top and base plates.Both top and base plates are equipped with three tubing nuts at each end for the installation of the outer and inner electrodes.Seventy one electrode nuts are equipped uniformly on the top plate of the specimen cell for installation of voltage measurement probes.The tailpiece adaptors through the edge of the top and base plates as shown in Fig.2(a)are used to sample the purging solution and to circulate the purging solution.

With a demountable base plate of the specimen cell,it is very simple to take samples at different locations after experiments for chemical analyses to investigate the remediation efficiency of contaminants and to better understand the electrochemical reactions and geochemical processes.

2.1.1.2.Electrodes.Two sets of electrodes are equipped in the specimen cell to apply a non-uniform electrical field for different performance.The outer electrodes are six hollow porous stainless steel tubes in direct contact with the soil specimen for the purging solution to flow through the electrodes freely.The porous part of the outer electrode is wrapped with filter papers to prevent intrusion of specimen through the pores.The inner diameter of the outer electrodes is 16 mm,providing adequate space for bubbles generated during the electrochemical process to release to the gas volume measurement devices.

Six 2-mm diameter stainless steel rods are installed through the electrode nuts as the inner electrodes through the outer electrodes concentrically.This configuration facilitates the study on the effects of purging solution chemistry on the electrochemical extraction process.Moreover,it facilitates the measurement of gas volume in the electrochemical extraction experiments.The outer electrodes or inner tube electrodes simulate the field application of the electrochemical remediation technology[14].

2.1.1.3.Voltage measurement probes.As contaminant migration mechanisms such as electromigration and electroosmosis are largely driven by the electrical gradient,accurate measurement of the voltage distribution as a function of time is essential to better understand the remediation process.Besides,energy consumption in the electrolytes and in the specimen cannot be determined separately without voltage measurement.

Many of the existing apparatuses are not equipped with voltage measurement probes[15–17].Some apparatus is capable of measuring the 1D voltage pro file along the length of the specimen[18,19].In this apparatus,71 stainless steel rods of 2-mm diameter are installed to monitor the 2D voltage distribution in the specimen as a function of time in the electrochemical extraction process.

2.1.2.Gas and fluid volume measurement devices

During the electrochemical process,gases are generated at the electrodes as a result of electrolysis of water.Simultaneously,protons and hydroxyl ions are generated at the electrodes.The electrolytic decomposition of water at the anode and cathode is described as follows.

As a result of electrolysis,a sharp pH gradient in the soil is generated,which has the most profound impact on the electrochemical remediation process such as electroosmotic flow,sorption/desorption of contaminants,complexation and precipitation of chemical species,greatly affecting the electrochemical remediation efficiency[20].The measurement of gas generation rates in the process provides an efficient approach to experimentally quantify the amount of hydrogen and hydroxyl ions generated at the electrodes[5].Comparison of the gas generation rates and the electric current passing through the system provides a valid evaluation on the electrochemical behavior of the system[21].

The gases produced at the electrodes may decrease the electroosmotic flow rate due to the progressive desaturation of soil[22].The outflow rate clearly depends on the amount of gas produced,revealed by experimental data and theoretical model.Other electrolysis products,such as chlorine gas or nitrogen gas,may depend on the species present in the electrolytes and their redox potentials[23,24],affecting the chemical composition of electrolytes and remediation efficiency[25,26].

Few existing apparatuses are equipped with gas volume measurement devices[27–29].In our new apparatus,gas and fluid volume measurement devices are designed to monitor gas generation rates and electroosmotic flow rates at both electrodes during experiments.Six 50-ml burettes are connected to the six outer power electrodes upside-down for gas volume measurement.When the volumes of gases generated through electrolysis of the purging solution are to be measured,the valves of burettes are turned off to collect the gases produced in the experiment.Otherwise,these valves are open to the atmosphere.The purging solution in the burettes is thus filled at atmospheric pressure.The solution in the burettes is kept at the same level as those in the polyethylene tubes.

The fluid volume measurement devices are polyethylene tubes connected to the specimen cell through the tailpiece adaptors installed at the edge of the base plate.A cross pipe is used to merge the three outlet ports into a single tube.The upper end of the tube is fixed through a T-shaped three-way pipe to keep the fluid level constant during the electrochemical extraction process as shown in Fig.1.Another burette full of electrolyte is used to supplement the electrolyte drop and to accurately measure the volume of electroosmotic flow into the specimen.

The electroosmotic flow rate and gas production rate in the experiment can be easily determined.When the valves of burettes are closed,electroosmotic flow rate and gas volume produced can be determined simultaneously,calculated by

where Δt is the time interval for measurement(s),Qeoais the electroosmotic flow rate at the anode in time interval Δt(m3·s?1)and Qeocis that at the cathode(m3·s?1),Vais the volume of effluent fluid collected from the tubing at the anode(m3)and Vcis that at the cathode(m3),and Vdais the total volume decrease in burettes at the anode(m3)and Vdcis that at the cathode(m3).A positive value of the electroosmotic flow rate indicates that the fluid flows from the anode towards the cathode.

where npis the amount of gas produced in time interval Δt(mol),Ptiis the final gas pressure in burette i at the end of time interval Δt(Pa),Vtiis the final volume of gas in burette i at the end of time interval Δt(m3),P0is the atmospheric pressure(1.01325×105Pa),V0iis the initial volume of air in burette i at the beginning of time interval Δt(m3),R is the gas constant(8.314 J·mol?1·K?1),and T is the absolute temperature(K).

The final gas pressure in burette i at the end of time interval can be calculated by

where ρ is the density of the purging solution,which is assumed to be 1000 kg·m?3,g is the acceleration due to gravity(9.81 m·s?2),and r is the internal radius of burettes(m).

The determination of electroosmotic flow rate is much easier when the gases produced in the experiment are not measured.The electroosmotic flow rates at the anode and cathode in time interval Δt are determined by

where Vais the volume of fluid added to the tubing at the anode to keep the fluid level at the initial position(m3)and Vcis the volume of effluent from the tubing at the cathode(m3).

2.1.3.Electrical circuit,power supply,and data logger

The electrical circuit is designed to conduct independent experiments simultaneously.A constant dc electrical potential is maintained across the specimen and the electric current passing through each specimen as a function of treatment time is measured independently.

An adjustable resistor of electrical resistance ranging from0 to 250Ω is installed in series with each specimen.The resistance is adjusted and fixed at an appropriate value so that the electrical potential difference across the resistor is kept at approximately 1–2 V during the electrochemical extraction experiments.

An IPS-3303 electric power supply provides dc electrical potential across the contaminated specimen in the electrochemical extraction experiments.It can be connected to the outer electrodes or inner electrodes,depending on the purpose of electrochemical experiments.The power supply can be operated in either constant current or constant voltage mode.

A TDS-303 data logger(Tokyo Sokki Kenkyujo Co.,Ltd)is used to monitor the voltage differences across the resistors and the voltage distribution at different locations in the specimen in the experiments.

2.2.Consolidation apparatus

The consolidation apparatus is designed to prepare a specimen of pre-determined porosity,initial moisture content,contaminant concentration,and soil pH.The consolidation apparatus is composed of the rectangular specimen cell,the top plate in the electrochemical extraction apparatus,a porous top plate,and a top collar.The schematic of the consolidation apparatus is presented in Fig.3.The top plate is used as the base plate for drainage of the slurry specimen in the consolidation process.The specimen cell is shared with the electrochemical extraction apparatus to eliminate the specimen transfer and to minimize any disturbance to the soil specimen.Another six stainless steel rods of the same diameter of the outer power electrodes are connected to the top of the outer power electrodes through the six ports to maintain the verticality of the power electrodes during consolidation and to guide the top plate.The porous top plate is wrapped with a filter paper and placed on top of the slurry.During consolidation,the fluid drains simultaneously from the top and the bottom of the apparatus.

Fig.3.Cross-section of the consolidation apparatus.

3.Materials and Experimental Procedures

Three bench-scale electrochemical extraction tests were conducted to evaluate the performance of the new apparatus.Two tests were conducted using the inner electrodes(tests EC-InV&EC-InI)to study the gas generation rates and electroosmotic flow rates as a function of treatment time in the remediation process.Another test was conducted using the outer electrodes(test EC-OutV)to evaluate the effects of a non-uniform electrical field on contaminant mobilization.The operational parameters of the three experiments are tab

ulated in Table 1.

3.1.Soil

The soil used in this study was collected in Nanhui County,Shanghai,China.It is an inorganic clay with low to medium plasticity with high acid/base buffer capacity.The organic content and the electrical conductivity of the soil are 0.18%and 0.339 dS·cm?1,respectively.More detailed descriptions of the clay soil are given by Gu and Yeung[30,31].

3.2.Experimental procedures

The electrochemical extraction experiments are conducted in three steps:(1)specimen preparation;(2)electrochemical extraction experiments;and(3)chemical analyses of soil samples.

3.2.1.Specimen preparation

In this experiment,a slurry specimen was prepared by adding approximately 5000 ml of Cd(NO3)2solution to 6.3 kg of air-dried soil.The slurry was manually mixed uniformly and allowed to equilibrate for 48 h before loaded into the consolidation apparatus.The slurry specimen was then consolidated using an unconfined compression apparatus at a very slow downward vertical displacement rate of 0.004 mm·min?1.

After consolidation,the excess consolidated soil extruding out from the specimen cell was trimmed with a wire saw and sliced uniformly into 9 sections as shown in Fig.4.Soil Sections 1–5 were analyzed to evaluate the uniformity of specimen and to establish its initial conditions.Each of Sections 1–5 was divided into five sub-samples for measurements of water content,soil pH,electrical conductivity,zeta potential of soil particle surfaces,and cadmium concentration.

Fig.4.Division of excess consolidated soil for analyses.

Water content was determined by dividing the mass of pore fluid by the mass of dry solid multiplied by 100%.Soil pH was measured using an Orion pH meter(dry soil to 0.01 mol·L?1CaCl2solution ratio of 1 g:5 ml).The measurement of electrical conductivity of soil pore fluid followed the method proposed by Rhoades[32].The zeta potential of each soil suspension sample was measured using a Delsa 440×micro electrophoresis instrument equipped with a microprocessor.The detailed measurement of the zeta potential of soil particle surfaces was described by Gu et al.[33].Cadmium concentration in soil samples was extracted using 5 mol·L?1HNO3solution and analyzed using a Perkin Elmer A Analyst 300 flame atomic absorption spectrometer.

Table 1 Operational parameters of electrochemical extraction experiments

3.2.2.Electrochemical extraction experiments

After the specimen was prepared,the electrochemical extraction apparatus was assembled with the gas and fluid volume measurement devices,dc power supply,electrical circuit,and data logger connected to the 71 voltage measurement probes as shown in Fig.5.Then electrolytes were added into the fluid and gas volume measurement devices.The electrochemical extraction apparatus was started by applying a constant dc electric potential or constant electric current through the inner or outer electrodes.In the electrochemical extraction experiments,electrical voltage,electric current,and gas and fluid volumes were monitored simultaneously as a function of time.

Fig.5.Sampling locations after the electrochemical extraction experiment.

3.2.3.Chemical analyses of soil samples

After the electrochemical extraction experiments,the dc power supply and fluid and gas volume measurement devices were removed.Soil samples at different positions of the specimen were taken with a rotary sampler with an inner diameter of18 mm and stored in zip lock bags for chemical analyses such as cadmium concentration.Sampling locations are depicted in Fig.5.

4.Results and Discussion

Some typical results of the three electrochemical experiments with the new apparatus are presented.It should be noted that they are by no means exhaustive in illustrating the applications of the apparatus.

4.1.Specimen consolidation

The soil parameters after specimen consolidation in test EC-InV are presented in Fig.6.The water content of the soil is in a narrow range of 31%–34%.For the soil specimen fully saturated after consolidation,the porosity of soil is approximately 0.45–0.47.Soil pH is around 7.7.A uniform electrical conductivity of soil pore fluid of approximately 0.32 dS·cm?1is measured in the specimen.The zeta potential of soil particle surfaces is within a range from?38.7 to?42.1 mV,which is in agreement with those determined at pH 7 in our previous batch experiments[33].Cadmium is in the range of 203–213 mg·kg?1.These results indicate that the soil specimen is fully saturated and uniform.

4.2.Electrochemical extraction experiment

4.2.1.Experiment using the inner electrodes

Two electrochemical extraction experiments were conducted with a constant electrical potential of 20.2 V(test EC-InV)or constant electric currents of 5,10,20,30,&40 mA(test EC-InI)applied across the specimen by connecting the inner electrodes.The inner electrodes were used as the power electrodes to better collect the gases into the gas measurement devices to assess the gas and fluid volume measurement performance of the new apparatus.

4.2.1.1.Gas generation rate measurement.The relationship between the gas generation rates and electric current passing through the system in test EC-InV is presented in Fig.7.The gas generation rate at the cathode decreased from 1.99 × 10?5mol·min?1to 1.64 × 10?5mol·min?1,while the electric current through the system decreased from 66.2 mA to 55.3 mA,which may be attributed to the migration of free ions in the soil specimen into electrolytes,decreasing the electrical conductivity of the specimen.The variation of gas generation rate with treatment time is in good agreement with that of electric current passing through the system.Similar result was also reported by Yeung et al.using a 1D apparatus[34].According to the chemical reactions of water electrolysis described in Eq.(2),0.5 mol of H2is generated at the cathode for 1 mol of electrical charge through the system.Comparison of the hydrogen generation rates and the electrical current passing through the system provides a valid evaluation of the electrochemical behavior of the system[21].

The relationship between the electric current passing through the system and gas generation rate at the cathode in tests EC-InV&ECInI is presented in Fig.8.According to Faraday's laws of electrolysis,the relationship is

where RH2is the generation rate of H2gas(mol·s?1),I is the electric current through the system(A),and F is the Faraday constant(96485 C·mol?1).The slope of the regression line is 3.045 × 10?7and 3.167 × 10?7mol·min?1·mA?1for tests EC-InV and EC-InI,respectively.The slopes differ from the theoretical value(3.109 × 10?7mol·min?1·mA?1)by only approximately 2.1%and 1.9%in tests EC-InV&EC-InI,respectively.Thus the experimental data are in good agreement with the theoretical values calculated by Eq.(9),indicating that the soil–chemical– fluid system is an active electrochemical system.The assumption of 100%Faraday efficiency for water electrolysis is still valid with all the impurities present in tap water.Similar results were obtained by Yeung[35]using a 1D apparatus,in which plate electrodes were used to apply a uniform electrical field during the electrochemical process.

4.2.1.2.Electroosmotic flow rate measurement.For tests EC-InV&EC-InI,electroosmotic flow rates through the specimen are simultaneously determined according to Eqs.(3)and(4).The electroosmotic flow rates at the anode and cathode decreased from approximately 5.7 ml·h?1to 2.0 ml·h?1as shown in Fig.9.A forward electroosmotic flow from the anode towards the cathode was maintained throughout the treatment duration.

According to the Helmholtz–Smoluchowski equation,the direction of electroosmotic flow is determined by the polarity of the zeta potential of soil particle surface and the magnitude of electroosmotic flow rate is proportional to the zeta potential.The forward electroosmotic flow throughout the experiment is due to the negative zeta potential of the soil in the wide pH range of 2–11[33].The decrease of electroosmotic flow rates during the experiment may be attributed to the increase of the zeta potential(becoming less negative)of the soil due to the progress of the acid front from the anode to the cathode[36].

Most of the existing closed experimental apparatuses are capable of measuring the ef fluent volume of electroosmotic flow as a function of treatment time[37,38].In this study,the electroosmotic flow rates at both electrodes can be accurately determined.It can be observed from Fig.9 that the out flow rate at the cathode is slightly higher than the inflow rate at the anode,especially at the beginning of the experiment.As the specimen is saturated,the difference between the in flow and out flow of electrolyte changes water content of the soil at different locations of the specimen(results not shown here).Few previous researches compared the difference of electroosmotic flow rates at the anode and cathode and reported this phenomenon,which needs further investigation.

Fig.6.Soil parameters after soil consolidation in test EC-InV.

4.2.2.Experiments using the outer electrodes

The third electrochemical extraction experiment with a constant electric potential of 19.8 V(test EC-OutV)applied across the specimen through four of the six outer electrodes in rows 2 and 6(as shown in Fig.5)was conducted for 130.5 h to evaluate the effects of a nonuniform electrical field on contaminant remobilization.0.1 mol·L?1EDTA was used as the electrolyte to enhance cadmium remobilization in soil.

4.2.2.1.Cadmium concentration.2D and 1D distributions of cadmium in the specimen before and after test EC-OutVare shown in Fig.10.The initial concentration of cadmium in the soil specimen was approximately 294 mg·kg?1.After approximately 5.4 days of treatment,cadmium spiked into the soil was significantly remobilized towards the anode due to the enhancement of EDTA.Final cadmium concentrations in columns 3–5 decreased after the experiment while those in the vicinity of the anode(columns 1 and 2)increased.Similar trends of heavy metal distribution with the enhancement of EDTA have been reported for 1D electrochemical extraction experiments[39,40].Fig.10 also shows non-uniform distributions of cadmium concentration in the same column due to the non-uniform electrical field across the specimen through the electrodes in rows 2 and 6 using the new 2D apparatus.Final cadmium concentrations in rows 2 and 6 at 2 cm from the anode(column 1)are nearly twice that of the initial concentration spiked into the soil while those in other rows barely change.Lopez-Vizcaino et al.[41]also observed non-uniform distributions of contaminant with rod electrodes installed in field or pilot studies.However,the ratio of electrode distance to the diameter of electrode in their study is much smaller than that in this study,providing a relatively uniform electrical field across the contaminated soil.By connecting different electrodes in the new apparatus,the effects of electrode configuration on contaminant mobilization can be further investigated.

Fig.7.Variations of gas generation rate and electrical current with treatment time during test EC-InV.

Fig.8.Gas generation rates at the cathode versus electrical currents in tests EC-InV&EC-InI.

Fig.9.Variation of electroosmotic flow rates with treatment time during test EC-InV.

Fig.10.Cd distributions in the specimen before and after test EC-OutV.(a)2D distribution;(b)1D distribution.

4.2.2.2.2D voltage distribution.The 71 stainless steel probes uniformly installed in the apparatus were used to monitor the 2D voltage distribution.On the one hand,the voltage distribution in the specimen helps explain the contaminant migration in the electrochemical remediation process.On the other hand,it represents the variation of electrical conductivity of soil that is primarily dependent on the water content of soil and ionic concentration in the soil pore fluid(results not shown here).In test EC-OutV,a non-uniform voltage distribution in the specimen was developed at the end of the experiment as shown in Fig.11.The electrical potentials increased from 0 V at the cathode to 19.8 V at the anode at the power electrodes in rows 2 and 6 while they increased from 1.4–2.4 V at the cathode to 17.4–17.8 V at the anode in other rows.The voltage differences among rows 2 and 6 and other rows at the anode resulted in a much higher final concentration of cadmium in rows 2 and 6 than that in other rows near the anode as presented in Fig.10.This is because negatively charged Cd–EDTA complexes migrate towards rows 2 and 6 from other rows under the localized electrical gradients near the anode region.On the other hand,the electrical potentials at the power electrodes(rows 2 and 6)were 1.4–2.4 V lower than those in other rows near the cathode,leading to the migration of negatively charged EDTA and Cd–EDTA complexes from rows 2 and 6 towards other rows.It well explains the fact that the final cadmium concentrations in rows 2 and 6 are slightly lower than those in other rows in column 5 as shown in Fig.10.

Fig.11.Voltage distribution at the end of test EC-OutV.

5.Conclusions

This paper demonstrates the design and innovative features of a new 2D electrochemical extraction experimental apparatus and some typical results of three experiments obtained using the apparatus.The following conclusions can be drawn.

1.A uniform soil specimen with a predetermined void ratio can be prepared by the consolidation apparatus.

2.The volume of gases generated and electroosmotic flow rates at both electrodes can be accurately measured as a function of time when the inner electrodes are connected as the power electrodes.Comparison of the gas generation rates and the electric current passing through the system provides a valid evaluation on the electrochemical behavior of the system.

3.The 2D profiles of physicochemical parameters such as voltage distribution and contaminant concentration during or after the experiment can be determined to better understand the effects of the non-uniform electrical field on the geochemical process.

In summary,the new 2D apparatus provides a useful tool for researchers to better understand the contaminant transport processes,electrochemical reactions,and physicochemical soil–contaminant interactions in the electrochemical extraction process.The experimental data can be used to evaluate theoretical formulations and numerical models for the process.