Due to their ubiquitous occurrence in nature and their engineering applications, smectite clay minerals have been extensively researched.  Nevertheless, there are limited studies which have attempted to examine the redox properties, and the secondary minerals formed, when these clay minerals react with aqueous Fe(II) - conditions expected in redox dynamic subsurface environments or clay barriers.  In this study, we observed that green rust forms on both montmorillonite and nontronite when reacted with 1 mM Fe(II) at pH 7.8.  The green rust precipitated within 5 minutes and remained stable for at least 18 days. Over this time period the reduction potential of all three suspensions increased by 50 to 150 mV and equilibrated at approximately -100 mV vs SHE.  The reduction potential of green rust aged for 18 days at this pH was much lower (~270 mV vs SHE ) indicating that green rust alone was not controlling the reduction potential of these green rust/smectite systems.  On the contrary, our results demonstrate that Fe(III) solubility was enhanced, most likely through partial dissolution of the clay minerals initiated by electron transfer from Fe(II) to structural Fe(III). The reduction kinetics of a model organic contaminant, 4-chloronitrobenzene (4-CINB), by Fe(II) became increasingly slower over time and reduction was completely inhibited after 18 days. It was concluded that the rise in reduction potential and increasing silicate sorption to Fe redox active sites resulted in the drastic loss in reductive reactivity towards 4-ClNB.  This study highlights the important roles of secondary mineral formation, electron transfer and primary mineral dissolution to temporal changes in the reactivity of the Fe(II)-Fe(III) redox couple and are likely to be relevant in many natural and engineered environments where smectite clay minerals are present.<br/>

Green rust formation induced by reaction between aqueous Fe(II) and smectite clay minerals

254th National Meeting (2017)

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