Environ. Sci. Technol. 2005, 39, 9197-9204
Effect of Precipitation on Low Frequency Electrical Properties of Zerovalent Iron Columns Y U X I N W U , * ,† L E E D . S L A T E R , † A N D NIC KORTE‡ Department of Earth and Environmental Sciences, Rutgers University, Newark, New Jersey 07102, and 1946 Clover Court, Grand Junction, Colorado 81506
We conducted column studies to investigate the application of a noninvasive electrical method to monitor precipitation in Fe0 columns using (a) Na2SO4 (0.01 M, dissolved oxygen (DO) ) 8.8 ppm), and (b) Na2CO3 (0.01 M, DO ) 2.3 ppm) solutions. An increase in complex conductivity terms (maximum 40% in sulfate column and 23% in carbonate column) occurred over 25 days. Scanning electron microscopy (SEM) identified mineral surface alteration, with greater changes in the high DO sulfate column relative to the low DO carbonate column. X-ray diffractometry (XRD) identified reduced amounts of hematite/maghemite in both columns, precipitation of goethite/akaganeite in the sulfate column, and precipitation of siderite in the carbonate column. Nitrogen adsorption measurements showed increases in specific surface area of iron minerals (27.5% for sulfate column and 8.2% for carbonate column). As variations in electrolytic conductivity and porosity were minimal, electrical changes are attributed to (1) higher complex interfacial conductivity due to increased surface area and mineralogical alteration and (2) increased electronic conduction due to enhanced electron transfer across the iron-fluid interface. Our results show that electrical measurements are a proxy indicator of Fe0 surface alteration.
Introduction The Fe0 permeable reactive barrier (PRB) is an in situ technology for the remediation of chlorinated hydrocarbon, heavy metal, or radionuclide contaminated groundwater (15). Corrosion of the Fe0 surface and subsequent precipitation of iron minerals causes performance reduction (6-9) in both synthetic and in situ PRBs (6, 7, 9-12). A critical issue is how to monitor performance reduction caused by oxide formation and the decreased permeability associated with pore clogging. Electrical methods have been applied to study iron corrosion processes. Tafel scan and electrochemical impedance spectroscopy (EIS) analysis of iron has identified geochemical controls on the Fe0 corrosion process (13-15). Electrical measurements have also been applied at the fieldscale to image the in situ installation of Fe0 barriers (16). In this study, we utilize an electrical method to measure the electrical response of iron columns during corrosion and precipitation. The measurement is analogous to an upscaled EIS measurement and is transferable to field-scale monitoring of PRBs. * Corresponding author e-mail:
[email protected]; phone: (973) 596-5863. † Rutgers University. ‡ Consultant, Grand Junction, CO. 10.1021/es051052x CCC: $30.25 Published on Web 10/28/2005
2005 American Chemical Society
Electrical measurements on soils and minerals can be represented in terms of a measured frequency (ω) dependent complex conductivity σ*(ω)
σ*(ω) ) σ/(ω) + iσ//(ω)
(1)
where σ′ is the real part of σ*
(ω), being the conduction (energy loss) component, and σ′′ is the imaginary part of σ*(ω), being the polarization (energy storage) component (i ) x-1). The physicochemical controls on electrical properties of soils depend on frequency, being the subject of an extensive volume of literature (a review can be found in ref 17). At low frequencies (
