| Abstract
| - Mineral-bound Fe(II) species represent important naturalreductants of pollutants in the anaerobic subsurface. At clayminerals, three types of Fe(II) species in fundamentallydifferent chemical environments may be present simultaneously, i.e., structural Fe(II), Fe(II) complexed by surfacehydroxyl groups, and Fe(II) bound by ion exchange. Weinvestigated the accessibility and reactivity of these threetypes of Fe(II) species in suspensions of two differentclay minerals containing either ferrous iron-bearing nontroniteor iron-free hectorite. Nitroaromatic compounds (NACs)exhibiting different sorption behavior on clays were usedto probe the reactivity of the various types of reduced ironspecies. The clay treatment allowed for a preparation ofnontronite and hectorite surfaces with Fe(II) adsorbed bysurface hydroxyl groups at the edge surfaces. Furthermore,hectorite suspensions with additional Fe(II) bound tothe ion exchange sites at the basal siloxane surfaces wereset up. We found that both structural Fe(II) and Fe(II)complexed by surface hydroxyl groups of nontronite reducedthe NACs to anilines. An electron balance revealed thatmore than 10% of the total iron in nontronite was reactiveFe(II). Fe(II) bound by ion exchange did not contribute tothe observed reduction of NACs. Reversible adsorption of theNACs at the basal siloxane surface of the clays stronglyretarded NAC reduction, even in the presence of highconcentrations of Fe(II) bound by ion exchange to the basalsiloxane surfaces. Our work shows that in natural systemsa fraction of the total Fe(II) present on clays maycontribute to the pool of highly reactive Fe(II) species inthe subsurface. Furthermore, this work may help to distinguishbetween Fe(II) species of different reactivity regardingpollutant reduction. Although structural iron in clays representsonly a small fraction of the total iron pool in soils andaquifers, reactive Fe(II) species originating from the reductionof structural Fe(III) in clays may contribute significantlyto the biogeochemical cycling of electrons in the subsurfacesince it is not subject to depletion by reductive dissolution.
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