ABSTRACT
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References ........................................................................................................................... 58
Electron transfer (ET) reactions are important in environmental chemistry as toxicity, bio-
availability, and fate are often dependent on the electronic state of pollutants. Two examples
to illustrate this are: (a) the transformation of aqueous hexavalent chromium (CrVI) to
trivalent chromium (CrIII) and (b) the dehalogenation of carbon tetrachloride (CCl4). Cr VI
is far more toxic than CrIII and is mobile in aerobic environments [1]. The transformation of
the hexavalent species to CrIII, however, leads to the formation of a CrIII-hydroxide under
most conditions, which effectively immobilizes the chromium and limits its bioavailability.
The reduction of chromium in soils is facilitated, and in some cases catalyzed, on mineral
surfaces [2]. CCl4 is a common pollutant in groundwater as a result of its use as an organic
solvent [1]. Through successive ET steps, CCl4 can be transformed to dissolved carbon
dioxide and chloride. ET reactions are often slow in homogeneous solutions, but can be
promoted by photochemical processes, enzymatic action, or the presence of mineral surfaces.
In this chapter, the role of mineral surfaces in promoting ET reactions is examined. Only
non-photochemical processes are considered in this chapter, and photochemical reactions are
examined elsewhere in this book. Following a brief review of the key concepts in the
mechanism of ET reactions, several types of ET reactions involving mineral surfaces will be
presented. Wherever possible, examples involving various types of pollutants are used to
illustrate the concepts presented in this chapter.
