NEOCHIM: an electrochemical method for environmental application
Introduction
Ion migration and electroosmosis are the two principal processes in electrokinetic remediation. Ion migration is the movement of ionic species present in the pore fluid, or soil moisture, that is induced by an applied electric current. The positive and negatively charged ions move to the electrode of opposite charge. Electroosmosis results from differences in the distribution of electrical charge on the soil capillary, or pore wall, and the adjacent pore water. The pore wall generally carries a negative charge which attracts the positive pole of water molecules present in the pore space. Upon application of an electric current, the aligned positive poles of water molecules are drawn toward the cathode, dragging along substances dissolved or suspended in the water. Pamukcu and Wittle (1992)note that electroosmosis produces a rapid flow of water in low-permeability soils (that) contributes significantly to the decontamination process in clay soils. In electrokinetic remediation, ion migration and electroosmosis are accompanied by electrolysis of water, which forms hydrogen gas and hydroxyl ions at the cathode and oxygen gas and hydrogen ions at the anode. The hydroxyl and hydrogen ions are driven from the electrode by the current into the soil being treated and to the electrode of opposite charge. The processes of ion migration and electroosmosis, along with a third process, electrophoresis, the migration of charged colloids or micelles, and the attendant process of electrolysis are illustrated in Fig. 1.
The formation, transport, and effects of OH− and H+ have been universally recognized by researchers in electrokinetic remediation (Acar, 1990Acar, 1992; Hamed et al., 1991; Acar et al., 1992; Pamukcu and Wittle, 1992; Wittle and Pamukcu, 1993; Shapiro and Probstein, 1993; Probstein and Hicks, 1993; Runnels and Wahli, 1993; Eykholt and Daniel, 1994; Patterson and Runnels, 1996). Although Hamed et al. (1991)note that the formation and transport of H+ from the anode may benefit the removal of Pb from the soil, the effects of both OH− and H+ on remediation can be problematic for the following reasons: (1) OH− may react with cations of interest, immobilizing them by precipitation and preventing their collection. The effect of OH− on soil pH and the immobilization of Zn is illustrated in Fig. 2a, from Probstein and Hicks (1993). (2) Hydrogen ions may react with soil constituents, mobilizing cations not previously mobile. (3) Electroosmosis decreases with increasing ion concentration in pore water, therefore, introduction of OH− and H+ ions may reduce the transport of fluid, and attendant ions. (4) The transport of OH− and H+ consumes a disproportionate amount of the electric current intended for the transport of hazardous ions because of their greater ionic mobilities (Table 1). Of the adverse effects of OH− and H+ on remediation, the immobilization of toxic metal ions by precipitation in soils alkalinized by OH− is considered the most debilitating.
Various means have been proposed for preventing the harmful effects imparted by OH−. Popular among these is the removal of OH− by flushing the cathode. Probstein and Hicks (1993)remove OH− ions by flushing the area around the cathode with water (Fig. 3a). The design of the field installation is based on successful results obtained in laboratory experiments (Fig. 2b) using an apparatus similar to the one shown in Fig. 3b. Probstein and Hicks (1993)note that purging the area around the anode (Fig. 3) with suitable non-toxic fluids improves the electroosmotic remediation of the soil. Such fluids might include buffering compounds to control pH and solutions that enhance desorption of metals or increase metal solubility. The remediation of soils using electroosmosis with purging fluids, as reported in Probstein and Hicks (1993)has been patented (Probstein et al., 1991).
The NEOCHIM technology obviates the OH− and H+ problems with a two-compartment electrode (Fig. 4). A salt bridge prevents the ions, produced by electrolysis in the inner compartments of the cathode and anode, from reaching the outer compartments, which contain a conducting fluid (electrolyte) that receives ions transported from the soil. Electrical contact of the electrode with the soil is made through a semi-permeable parchment membrane at the base of the outer compartment. The membrane allows the passage of ions from the conducting fluid into the soil and from the soil into the fluid, while retaining the fluid in the compartment. A second membrane at the base of the inner compartment retains the salt bridge in the compartment. The NEOCHIM technology was developed by the USGS following the lead of Russian scientists conducting research on CHIM, a method of electrogeochemical sampling for use in exploration of buried mineral deposits. The design, development and testing of the NEOCHIM electrode is detailed in Leinz et al. (1998).
The development of NEOCHIM for mineral exploration showed that the technology possessed certain attributes: (1) Both cations and anions can be extracted from the soil directly into fluids in the outer sampling compartments of separate cathode and anode electrodes. The fluids can be readily removed for processing or disposal. (2) Sampling volumes were calculated to be many orders of magnitude larger than volumes typically sampled in more conventional geochemical sampling methods. (3) The method can be used to alter the solubility of certain ions in the soil through the introduction of selected counterions. (4) Experimental results suggest extraction efficiencies, or the ratio of the number of equivalents of ions collected to the equivalents of charge delivered to the electrodes, of 50% or more. With these attributes it is concluded that NEOCHIM may be useful for monitoring and remediating hazardous waste sites.
Section snippets
Transport of cations and anions into separate electrodes
As shown in Fig. 2a, without flushing the area around the cathode, the electrokinetic transport of Zn is impeded by the high pH front that migrates from the cathode causing the Zn to precipitate. Because OH− is retained in the inner compartment of the cathode during a NEOCHIM extraction, there is no migrating pH front and transition metal ions such as Cu2+, Pb2+, and Zn2+ are transported directly into the cathode as shown in tests conducted by Leinz and Hoover (1994)over metal-enriched soils.
An experimental evaluation of NEOCHIM
The NEOCHIM method has been extensively tested for mineral exploration in the field (Leinz et al., 1998). A more recent experimental evaluation of NEOCHIM was conducted in the laboratory, under more controlled conditions, to further evaluate the attributes of the method. In one experiment Li isotopes were used in the extraction of Li+ through a column of soil. The extraction of Zn2+ and K+ through a column of soil was evaluated in a second experiment.
Energy costs of remediation with NEOCHIM
The average efficiency of NEOCHIM extractions conducted in Nevada in 1995 and 1996 was nearly 37% for the metals analyzed (Leinz et al., 1998). Of the metals collected in the cathodes Ca predominated, presumably as Ca2+. The amount of charge delivered to the cathodes was approximately 1250 A h, corresponding to 46 equivalents of charge (26.8 A h/equivalent). At an efficiency of 37%, this corresponds to the collection of 343 g of Ca as Ca2+. In the Nevada tests, arrays of 20–25 cathodes were
Conclusion
NEOCHIM, an electrogeochemical sampling method developed by the USGS for use in mineral exploration is based on the electrokinetic migration of ions residing in soil moisture into the outer compartment of dual-compartment electrodes. The design of the electrode, which incorporates a salt bridge between the compartments, prevents migration into the soil of OH− and H+ formed by electrolysis in the inner compartments of the cathode and anode, respectively. This eliminates the problems caused by OH−
Acknowledgements
We are grateful to the USGS Mineral Resources Program for supporting our research and acknowledge David J. Grimes, Robert J. Horton, and Dr. Misac N. Nabighian for their critical reviews of the manuscript.
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