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

Abstract

A model to compensate the 2:1 layer having excess negative charge owing to the reduction of Fe 3+ to Fe 2+ by sodium dithionite buffered with citrate-bicarbonate in nontronite, beidellite, and montmorillonite is proposed. This model is based on reassessing published experimental data for Fe-containing smectites and on a recently published structural model for reduced Garfield nontronite. In the reduced state, Fe 2+ cations remain six-fold coordinated, and increases of negative charge in the 2:1 layer are compensated by the sorption of Na + and H + from solution. Some of the incorporated protons react with structural OH groups to cause dehydroxylation. Also, some protons bond with undersaturated oxygen atoms of the octahedral sheet. The amount of Na + (p) and H + (n~) cations incorporated in the structure as a function of the amount of Fe reduction can be described quantitatively by two equations: p = m/(1 + K0mrel) and rt i = KornmrJ(1 + K0rnrel); with K 0 = CEC (9.32 -1.06mto t + 0.02mt2ot), where into t is the total Fe content in the smectite, m is the Fe 2+ content, mr,~ is the reduction level (m/mtot) , CEC is the cation-exchange capacity, and K 0 is a constant specific to the smectite. The model can predict, from the chemical composition of a smectite, the modifications of its properties as a function of reduction level. Based on this model, the structural mechanism of Fe 3 § reduction in montmorillonite differs from that determined in nontronite and beidellite owing to differences in the distribution of cations over trans-and cis-octahedral sites.

The effect of iron on montmorillonite stability. (I) Background and thermodynamic considerations

“…Furthermore, aqueous suspensions of ferruginous smectite have an increased viscosity subsequent to Fe reduction due to increased interparticle attraction. Several mechanisms by which reduction of Fe(III) to Fe(II) may occur (involving interactions between OH (structural) , H 2 O (l) , and H + ) have been suggested (Stucki and Roth, 1977;Lear and Stucki, 1989;Drits and Manceau, 2000;Manceau et al, 2000b).…”

Evaluation of redox-active iron sites in smectites using middle and near infrared spectroscopy

“…Various spectroscopic studies have elaborated on the consequences of Fe reduction and re-oxidation for the structural environment of Fe within the octahedral sheet of iron-rich clay minerals and proposed reaction mechanisms (Lear and Stucki, 1987;Komadel et al, 1990;Hunter and Bertsch, 1994;Hunter et al, 1999;Drits and Manceau, 2000;Manceau et al, 2000b;Komadel et al, 2006). On the basis of insights gained from infrared (IR), UV-visible, and X-ray absorption spectroscopy, it was proposed that different Fe(II)/Fe(III) clusters form in the octahedral sheet upon changes of Fe redox state.…”

Method development for evaluating the redox state of Callovo-Oxfordian clayrock and synthetic montmorillonite for nuclear waste management

“…A reduction of 8% and 10% of Fe(III) occurs during the experiment, which is confirmed by the R factor which decreases after the reaction (0.32 and 0.28 respectively). Sodium dithionite is often assumed to be a powerful reactant to reduce ferric ions compared to H 2 (g) (Drits and Manceau, 2000;Komadel et al, 1990;Komadel et al,2006). However regarding the hydrogen reduction test, hydrogen gas reduces the claystone as much as sodium dithionite does in our experimental conditions.…”

“…However, H 2 (g) is not easy to handle and remains a less powerful reactant compared to sodium dithionite (Na 2 S 2 O 4 ) (Mayhew, 1978;Schwarzenbach et al, 2003). The latter has been extensively used to investigate the reduction of, e.g., clay structural Fe(III) (Drits and Manceau, 2000;Komadel et al, 2006;Lear and Stucki, 1985 Iron may be present as a structural specie in octahedral (Oh) or tetrahedral (Td) coordination, e.g. in smectite and illite (Reinholdt et al, 2001) (predominantly Fe III Oh), magnetite (Fe II Oh and Fe III Oh and Td), Fe(OH) 2 or pyrite FeS 2 (respectively Fe II Oh and Fe II Td), or as sorbed species, on the edge or in the interlayer space of clay minerals (mostly on Fe II and Fe III ) (Géhin et al, 2007;Silvester et al, 2005).…”