2 The 4 Technology Solutions
REMEDIATION OF METALS-CONTAMINATED GROUNDWATERPART III: ELECTROCHEMICAL PROCESSES
An On-Line Version of a Column First Published in the:
National Environmental Journal Mar./Apr. 1993 Vol. 4 No. 2 Pages 22-23
by: David B. Vance
The remediation of groundwater contaminated with metals is constrained by the fact that metals can only be mobilized or immobilized, not destroyed as with the biodegradation of hydrocarbons. However, the use of electrochemical technology is an approach that is uniquely applicable to metals. The purpose of this column is to briefly review the application of electrochemical techniques to the remediation of groundwater contaminated with metals. Introduction to metal chemistry
Terminology for the technology includes electrokinetic remediation, electroreclamation, electrokinetic soil processing, and electrochemical decontamination. For purposes of this review electrochemical remediation processes are broken into two primary areas: electromigration and electroosmosis. Electromigration is used in relatively porous formations that are capable of supporting advective groundwater flow. Electroosmosis is applied to saturated fine grained formations such as silts or clays.
Electrochemical reactions can be used with any groundwater contaminant that has a charge such as soluble metal ions, other inorganic ions, and organic acids or phenols. Electrochemical reactions are stimulated by placing two or more electrodes in the subsurface which are connected to a direct current power source. The electrode with a positive charge is termed the cathode and that with a negative charge the anode. Cations migrate towards the negative electrode and anions towards the positive electrode. Power supplies should be capable of delivering currents around one amp per square foot of cross sectional area between the electrodes, and voltage potentials of a 10 to 30 volts per foot between electrodes. Estimates for the total power consumption required to treat a cubic yard of saturated soil are in the range of 50 to 250 Kilowatt Hours.
Electromigration is the process of mobilizing metal (or other) ions through an aquifer matrix without advective fluid flow. Mass transport is strictly by ion migration driven by an induced electrical field. The rate of migration for any given ion is dependent upon factors that include: the concentration of total ions (TDS) in the groundwater and the concentration of individual ions of concern, the charge of those ions, the electrical potential between the electrodes, the mobility of the individual ionic species, and the porosity and tortuosity of the pore spaces in the aquifer. Any groundwater will have multiple soluble ions that make up the TDS. The fraction of the total current carried by a particular ionic species in the TDS is represented by the transference number for that ion. The numerical value of the transference number is dependent upon the electromagnetic gradient (the greater the gradient the higher the mass transport rate) and concentration of the individual ionic species. The dynamics of this system is such that the application of electromigration to metal contaminated groundwater is best suited to conditions in which the metal ion makes up a major component of the TDS. Recovery efficiencies are very low if the metal ion is present at trace levels.
One peripheral, but inevitable, process that occurs, and has direct impact on the physical/chemical reactions induced by electromigration, is the electrolytic decomposition of water.
Water at the cathode undergoes the following reaction:
2H2O + 2e- H2 gas) + 2OH-
Water is oxidized at the anode by the following reaction:
2H2O - 4e- O2(gas) + 4H+
Hydrogen ions have an effective ionic mobility 1.8 times greater than that of the hydroxyl ion. Hydrogen ions lower the pH and their preferential migration through a formation will contribute towards the desorption of metal species from soil particles.
The term electroosmosis refers to a technology that has been used for decades by geotechnical engineers to remove water from clays, silts and fine sands. (The effect was first described in 1809). It has been used by the construction industry to stabilize embankments, dewater foundation soils, and increase the loading capacity of pilings. Electroosmosis is primarily an electrically induced hydraulic flow rather than an ionic flow. Water flows in the same direction as electrons in the electrical field loop, from anode to cathode.
The mechanism inducing the advective flow can be described as follows:
In fine grained soils pore space can be visualized as a capillary tube in which the tube walls (clay soil particles) are negatively charged. Water in such a tube exists as a double layer, with an adsorbed layer tightly held directly adjacent to the pore space wall. The water in the center of the pore space and the core of the tube is less strongly held by these attractive forces. The application of an electrical gradient across the tube will cause cations in the tightly adsorbed layer adjacent to the walls of the pore space to migrate towards the cathode, taking the free water in the core of the tube (and between the adsorbed layers) along with it. In contrast, under hydraulic pumping conditions, only the free water in the core of the pore space will flow. Contaminant mass transport is induced through hydraulic flushing, in addition to some electromigration of soluble metal ions.
The finer the grain of the pore space, the greater the significance of the electroosmotic effect. In fine grained soils electroosmotic induced mass transport can be two to three orders of magnitude higher than that capable of being induced hydraulically.
Potential problems in the field application of electrochemical technology include:
- High current densities and subsequent reactions at the anode can cause low pH conditions resulting in corrosion and dissolution of the electrode element.
- Precipitation of metal hydroxides or other TDS components near the cathode can cause pore space blockage.
- If contaminant and general TDS ion concentrations are not favorable, electrical costs can be unreasonably high because all ions (even undesirable ones) participate in the reactions.
- The number and spacing of electrodes can be prohibitive.
- Subsidence or settlement is a potential under some conditions.
Some of these issues can be addressed through process enhancements such as using additives to depolarize the cathode and anode.
Reactions induced in groundwater under an electric field are complex and will change with time. Low pH, high pH, ion mobilization, precipitation, dissolution and desorption are all reactions that can take place within different locations in the subsurface electrical cell at different times during the operation of the system. The electrochemistry in such a system evolves with time, successful utilization of this technology requires operation of the system to exploit those changes.
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