Reduction of Structural Iron in Ferruginous Smectite by Free Radicals

Abstract

Abstract--The oxidation state of structural iron greatly influences the physical-chemical properties of clay minerals, a phenomenon that may have significant implications for pollutant fate in the environment, for agricultural productivity, and for industrial uses of clays. Knowledge of redox mechanisms is fundamental to understanding the underlying basis for iron's effects on clays. Past studies revealed that the extent of Fe reduction varied depending on the reducing agent used, but this variation may not have been a simple function of the reduction potential of the reducing agent. The objective of this study was to identify the relationship between the Fe reduction mechanism and free radical activity in the reducing agent. Several reducing agents and their mixtures with the Na-saturated, 0.5 to 2 #m size fraction of ferruginous smectite (SWa-1) were analyzed by electron spin resonance (ESR) spectroscopy to determine the presence of unpaired electrons or free radicals. Only NazS204 exhibited paramagnetic free-radical behavior with a signal at about g = 2.011, which was attributed to the sulphoxylate (SO2-.) free radical. The free radical was labile in aqueous solution, and the ability of NazSzO4 solution to reduce structural Fe in the smectite decreased with age of the solution and paralleled the disappearance of the free radical signal in the ESR spectrum. The paramagnetic species was preserved and enhanced ifNa2SzO4 was added to the clay suspension, indicating that either the clay surface stabilized the SO2 9 radical or the additional unpaired electrons were produced in the clay structure.

A Model for the Mechanism of Fe3+ to Fe2+ Reduction in Dioctahedral Smectites

“…The model of Gan et al (1992) Electron spin resonance (ESR) spectroscopy was used to determine the reductant species responsible for the transfer of electrons during reduction. In solution, sodium dithionite (Na28204) dissociates to Na § and (8204) 2 ions.…”

“…The disulfite anion also dissociates in water, partly according to 2($zO4) 2 + H20 --~ (8203) 2 + 2(HSO3)---* ($203) 2-+ 2(803) 2 -{-2H +, and partly into the paramagnetic free radical species, SO2 -~ which contains unpaired electrons as indicated here by "o" (Dunitz, 1956;Lynn et al, 1964;Rinker et al, 1958). Gan et al (1992) showed that in Na28204 solutions, these free radicals are responsible for the reduction of structural Fe 3+. They observed a correlation between the concentration of SO2-" in solution and the reducing ability of the reagent.…”

Nitrate removal from water using UV-M/S2O4 2− advanced reduction process

“…Dithionite is a high production volume chemical, primarily because of its industrial use in bleaching wood pulp (Dence 1996;Ellis 1996;Lindholm 1999;Pemberton, et al 1995) and its use in dyeing textiles, which helps to maintain its relatively low price of $0.76/lb (The Innovation Group 2008). In addition to its industrial uses, dithionite is widely used by soil scientists to reductively extract iron from soils (Gan et al 1992). The ability to reduce iron has also been applied to developing in situ aquifer remediation technology where iron in aquifer solids is reduced by dithionite and the reduced iron subsequently reduces oxidized contaminants (Szecsody et al 2004;Ludwig et al 2007).…”

Treatability Study of In Situ Technologies for Remediation of Hexavalent Chromium in Groundwater at the Puchack Well Field Superfund Site, New Jersey

“…Hydrogen sulfide is highly reactive, causing the reductive dissolution of some amorphous and poorly crystalline Fe(III) oxides (e.g., ferrihydrite) and the subsequent precipitation of the Fe(II) as amorphous FeS (Matsunaga et al 1993). The reduction of structural Fe(III) in layer silicates without dissolution may also occur (Gan et al 1992). These reduced phases store reductive capacity in sediments and subsequently can reduce soluble contaminants such as Cr(VI) to form insoluble precipitates (Amonette et al 1994;Fredrickson and Gorby 1996;Deng and Stone 1996).…”