NEOCHIM: an electrochemical method for environmental application

Abstract

Ion migration and electroosmosis are the principal processes underlying electrokinetic remediation of hazardous wastes from soils. These processes are a response of charged species to an applied electrical current and they are accompanied by electrolysis of water at the electrodes through which the current is applied. Electrolysis results in the formation of OH − at the cathode and H + at the anode. The current drives the OH − and H + thus formed from the electrodes, through the soil and to the electrode of opposite charge. Introduction of OH − and H + into the soil being treated modifies soil chemistry and can interfere with either the collection or immobilization of hazardous waste ions. The introduction of either OH − or H + to the soil can be problematic to electrokinetic remediation but the problem caused by OH − has been the focus of most researchers. The problem has been addressed by flushing the OH − from the soil near the cathode or treating the soil with buffers. These treatments would apply as well to soils affected by H +. With the NEOCHIM technology, developed by the U.S. Geological Survey (USGS) for use as a sampling technique in exploration for buried ore deposits, OH − and H + are retained in the inner compartment of two-compartment electrodes and are thus prevented from reaching the soil. This enables the extraction of cations and anions, including anionic forms of toxic metals such as HAsO 2− 4. One of the principal attributes of NEOCHIM is the large volume of soil from which ions can be extracted. It is mathematically demonstrable that NEOCHIM extraction volumes can be orders of magnitude greater than volumes typically sampled in more conventional geochemical exploration methods or for environmental sampling. The technology may also be used to introduce selected ions into the soil that affect the solubility of ceratin ions present in the soil. Although field tests for mineral exploration have shown NEOCHIM extraction efficiencies of about 25–35%, laboratory experiments suggest that significantly higher efficiencies are possible. The attributes of NEOCHIM combined with relatively low cost of electrical power, indicate that the technology may be useful for remediation and monitoring of hazardous waste sites. Of particular importance is that NEOCHIM extractions affect only dissolved and electrically charged species, hence those prone to move in groundwater.