Chemical Reductants for ISCR: The Potential for Improvement

The abiotic reduction reactions that form the basis of in situ chemical reduction (ISCR) have been studied extensively, but there has been little effort to develop a unified understanding of what factors control ISCR performance. Some of the key factors are summarized here, with the ultimate goal of building a basis for rational design of better ISCR reductants. INTRODUCTION The fact that groundwater and sediment contaminants can be reduced by pathways that are abiotic (i.e., do not directly involve microorganisms) has been well documented in the literature for more than 30 years. Some of the early work on this has been reviewed (Tsukano 1986; Tratnyek et al. 1989; Wolfe et al. 1992), and since then there have been many academic studies of organic contaminant degradation reactions using model systems designed to represent the natural reductants that are most likely to be responsible for abiotic reduction reactions in soils, sediments, and groundwaters. These putative abiotic reductants are of three types: minerals that derive their reducing capacity from Fe [e.g., magnetite (Lee et al. 2002; Gorski et al. 2010), green rust (Erbs et al. 1999; O'Loughlin et al. 2004), goethite (Amonette et al. 2000; Elsner et al. 2004), and clays (Cervini-Silva et al. 2002; Elsner et al. 2004)] or S [e.g., mackinawite (Butler et al. 1999) and pyrite (Kriegman-King et al. 1994; Lee et al. 2002)] and redox-active moieties associated with natural organic matter [e.g., quinones (Tratnyek et al. 1989; Schwarzenbach et al. 1990; Uchimiya et al. 2009)]. Despite this background, most practitioners—and some researchers—have assumed that in situ abiotic reduction of contaminants (e.g., as a component of natural attenuation) in the environment is insignificant. Recently, however, several studies have characterized sites where a significant portion of contaminant degradation appears to be due to direct reaction with reducing mineral phases (Ferrey et al. 2004; Darlington et al. 2008), and these studies have renewed interest in the role that these phases might play in natural attenuation and the prospects for manipulating such systems to generate more or better reductants in situ (Becvar et al. 2008). An example of such a manipulation is the technology known is “in situ redox manipulation” [ISRM (Fruchter et al. 2000; Szecsody et al. 2004)], where dithionite (a soluble chemical reductant) is injected to reduce native ferric iron to produce adsorbed and structural ferrous iron, which is a fairly effective reducing agent for easily reduced contaminants (like chromate, carbon tetrachloride, and munitions compounds). Coincident with the above developments, there has been rapid development of remediation technologies that involve emplacement of zero-valent iron (ZVI) to serve as the chemical reductant of contaminants (Tratnyek et al. 2003). This field has become fairly mature in recent years, and there is now considerable competition among vendors of ZVI for remediation applications. This has led to efforts to engineer better forms of Proceedings of the 7th International Conference on Remediation of Chlorinated and Recalcitrant Compounds 24-27 May 2010, Monterey, CA. (Submitted 21 April 2010)