Controllable dual electrochemical reactive barriers for the remediation of TCE co-contaminated groundwaterXuhui Mao 1, Songhu Yuan 1, Noushin Fallahpour 1, Joniqua Howard 2, Ingrid Padilla 2, Akram N. Alshawabkeh 1

The project described was supported by Award Number P42ES017198 from the National Institute Of Environmental Health Sciences. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute Of Environmental Health Sciences or the National Institutes of Health.

Acknowledgements

Validity on mixed contaminationIntroduction

Summary• A novel electrochemical barrier consisting of iron anode and porous cathode is proposed for the remediation of synthetic groundwater with mixed contamination.

•Cast iron anode produces ferrous species, which not only creates an electrolyte-based “barrier” that enhances the reduction of TCE on cathode, but also abates other contaminants including dichromate, selenate and arsenite.

•  The electrical currents passing the two anodes can be adjusted and different redox conditions of electrolytes can be controlled.

•The overall system, comprising the electrode-based and electrolyte-based barriers, can be engineered as a versatile and integrated remedial method for a relatively wide spectrum of contaminants and their mixtures.

Future work-- The long-term validity of the system in this study is going to be tested.-- The column experiment using limestone column, mimicing the Karstic aquifer, was scheduled for in the near future.-- Researches regarding the Physiochemical and biological changes that the iron electrolysis may induce are underway.

Summary and Future work

Experimental method and materials

Results and discussion

Schematic of the experimental setup

Fig. 1 Schema of the electrochemical induced dual reactive barriers for the in situ remediation of the mixed contamination in groundwater

Fig. 4 ORP and pH changes under different electrode configurations and different current intensities. (a) MMO anode (at Anode-1 position of Column A) and copper foam cathode; (b) Cast iron anode (at Andode-1 position of Column A) and copper foam cathode; (c) cast iron anode (at Anode-1 position), MMO anode (at Anode-2 position) and copper foam cathode, with different current distributions on the two anodes. Electrolytes were made by dissolving salts in tap water (containing around 20 mg L-1 chloride ions). Flow rate of solution was 2 mL/min for all the experiments.

Fig. 5 (a) Decay of the aqueous TCE concentration after the electrodes (samples collected from Port-S4 of Column B), under different electrode configurations and arrangements. (b) The ORP values of the electrolytes before cathode (water samples from the Port-S3 of Column B). 90 mA current and 4 mL/min liquid flow rate were applied to all the experiments. For the two anodes experiment, the current distribution was 45 mA (cast iron) to 45 mA (MMO). In the figure legend, the A-1 and A-2 refer to the Anode-1 and Anode-2 position, respectively. The letter C means the cathode. The symbol “||”denotes the electrolytes between the electrodes.

Evaluation indicators

When the reaction reaches a steady-state condition, The removal efficiency of aqueous TCE, and the treating capacity of TCE were proposed for the evaluation of these different electrolysis procedures, where Cin and Cen are the TCE concentration (mg/L) in the influent and in the effluent, respectively; Q is the flow rate of electrolyte solution (L/h).

Fig. 3 Schematic of (a) column A, (b) column B and (c) electric connections of three electrodes. The dimensions are millimeter. Four stainless steel bolts were used to connect each electrode inside the column, and the bolts were sealed with some modified Swaglok nuts and speta. All sampling ports were sealed with Swaglok nuts and septa, which allow multiple punctures. The void volumes of Column A and column B are 1.2L and 0.8 L, respectively. All samples were collected from the central of column using syringes.

RetreatFebruary 23-26, 2012Dorado, Puerto Rico

The electrolysis consisting of inert anode and copper cathode will increase the ORP value of groundwater because of the presence of electrochemically induced oxidative substances, such as active oxygen and chlorine. When the iron anode is used in the electrolytic reactor, the pH reaches alkaline condition and ORP goes to reducing values. The redox conditions of electrolytes can be exquisitely regulated by varying the current on the two anodic reactions. Unlike the e-barrier based on inert electrodes,the electrolysis using anode not only shifts the redox condition in the immediate vicinity of electrode, but also provides reducing substances like ferrous species to electrolytes, which may also contribute to the cleanup of contaminants.

Effect of electrode arrangement on TCE removalpH and redox regulation using iron electrolysis

Fig. 2 Cleanup processes of mixed contamination using dual electrochemical reactive barriers

Effluent

pH prober

ORP prober

Port-S5

20.83

Port-S3

Port-S2

Cathode

Anode-1

Influent

Anode-2

Port-S1

Port-S4

Column I.D = 6.35

44.83

4.45

3.81

11.94

1.78

3.81

3.56

Port-S3

Port-S2

Cathode

Anode-1

Influent

Effluent

Anode-2

Port-S1

Port-S4

3.81

3.56

23.5

4.45

3.81

11.94

1.78

Column I.D = 6.35

Adjustable resistance

Anode-2

Cathode

Anode-1 A

(a) (b)

(c)

RE= (Cin-Cen)/Cin × 100% (Removal efficiency of TCE)

TC= Q ×(Cin-Cen) (Treating capacity )

View of Column A designThe arrangement of three electrodes (iron anode, copper foam, MMO anode, from the bottom)

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ORP, MMO(30 mA): Iron (60 mA)

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No.3, MMO(A-1) || Cu foam (C) No.4, Cu foam(C) || MMO(A-2) No.5, Iron (A-1) || Cu foam (C) No.7, Iron (A-1) || Cu foam (C) || MMO (A-2) No.6, Cu foam (C) || MMO(A-2), deoxygenated solution

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No.1, Iron (A-1) || Iron (C) No.2, MMO(C) || MMO(A-2) No.3, MMO(A-1) || Cu foam (C) No.4, Cu foam(C) || MMO(A-2)

No.5, Iron (A-1) || Cu foam(C) No.6, Cu foam(C) || MMO(A-2),

deoxygenated solution No.7, Iron(A-1) || Cu foam(C) || MMO(A-2)

(a)

(a)

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mg/

h)

Foam cathode thickness (cm)

4 mL/min 2 mL/min

Effect of operation variables on TCE removal

Fig. 6 (a) TCE removal efficiency at different electrolyte flow rates, as a function of the thickness of foam electrode. 90 mA current and influent containing 20 mg/L TCE were applied for the experiments. Samples were collected from Port-S4 of Column B when steady-state concentration reached. Iron (A-1)||Cu foam(C)||MMO(A-2) electrode configuration and equal current distribution on two anodes (45 mA for each). (b) Contour plot of the TCE removal efficiency (at steady-state condition, at Port-S4) as a function of applied current and influent TCE concentration. 2 mL/min electrolyte flow rate, 1.27 cm thick copper foam electrode and equal current distribution on the two anodes (iron anode and MMO electrode) were applied for the experiments.

(b)(a)

Fig. 7 (a) Normalized Cr(VI) concentrations at Port-S4 of Column B. Two anodes (iron anode and MMO electrode) configuration and equal current distribution. The cathode was Cu plate if without elsewise specification. The electrolytes contained dichromate and 5.0 mg/L TCE. (b) Normalized concentration profiles of Cr(VI), selenate, nitrate and TCE. Column A was used for this experiment. 60 mA current and Iron (A-1)||Cu foam(C)||MMO(A-2) electrode configuration. The influent included 0.5 mg/L Cr2O7

2-, 2.2 mg/L SeO42-, 20 mg/L NO3

- and 5.4 mg/L TCE. 2 mL/min electrolyte flow rate for all the experiments in this figure.

Rational electrodes arrangement, longer residence time of electrolytes and higher surface area of foam electrode improve the reductive transformation of TCE. More than 82.2% TCE removal efficiency is achieved at lower influent concentration (< 7.5 mg/L) and higher current (>45 mA).

Observations and implicants

The cleanup of Cr(VI), selenate ions and As(III) primarily relates to the reducing electrolyte containing ferrous species, although further studies are still needed for details of the mechanism of cleanup.

Observations and implicants

Two kinds of cleanup mechanisms proceed in the system. TCE and nitrate are basically removed through the electrode-based electrochemical processes, therefore their removal efficiencies are subjected to the factors that are related to the mass transfer effect, such as the hydraulic residence time of the synthetic groundwater and the surface area of electrode.

The column experiments show that removal efficiencies on these contaminants are more than 80%.

Different from the conventional electrochemical processes only using inert anodes like MMO and carbon materials, the iron anode generates ferrous species, which can serve either as electron donors for the reduction of contaminants or as adsorbents for the immobilization of contaminants. Therefore, an electrolyte-based “barrier” consisting of ferrous species was built for the cleanup of some groundwater contaminants. Unlike the zero valent iron permeable reactive barriers, the reactivity of the dual electrochemical barriers here can be easily controlled by varying the applied current in term of the type and level of the contaminants. Besides, introducing iron anode and MMO anode simutaneously (or individually) provides a facile way for the in situ regulation of the redox condition of groundwater. The redox conditions can be varied based on the properties of target contaminants. Suitable redox condition, in combination with the electrochemically generated gases (O2, H2), may stimulate the anerobic or aerobic activities of the bacteria in the groundwater, facilitating the degradation of contaminants.

Implications for groundwater remediation

1. Civil and Environmental Engineering Department, Northeastern University, Boston, MA, 02115, USA2. Department of Civil Engineering and Surveying, University of Puerto Rico, Mayaguez, Puerto Rico, 00681

Contaminated soil and groundwater are major problems in the US and in Puerto Rico. Presence of over150 contaminated sites, including 15 Superfund sites, results in high potential for contamination exposure and risks to public health. Trichloroethylene (TCE), often used as a metal degreaser, is one of the predominant contaminants that may contribute to serious health problems.

The Puerto Rico Testsite for Exploring Contamination Threats (PROTECT) research Center that studies the effects of environmental contamination on human health and aims to reduce the impacts on public health. As part of the Center, Project 5 will develop a green remedial technology to treat the TCE in groundwater. In this study we propose an electrochemically induced dual barriers for transformation of TCE co-contaminated groundwater. An example of a potential in-situ scheme of the proposed technology is shown below.