The Effects of Iron Oxidation State on Surface Chemistry and Structure of Iron-Bearing Smectites: Cation Fixation and Selectivity, Structural Iron Environment, and Dissolution

Abstract

Smectites are widespread as a major constituent of soils and sediments and include a substantial amount of structural Fe. The reduction of structural Fe(III) invokes partial dehydroxylation, protonation, and site translocation of Fe(II) to alter the crystal structure. The structural changes, therefore, influence their surface properties such as layer charge, cation-exchange capacity (CEC), and cation fixation and selectivity. The objective of this study was to investigate the effects of the oxidation state of structural Fe on cation fixation and selectivity and smectite structure. Experimental results revealed that reductive K fixation in smectite suspension occurs under the condition that K was present in the interlayer prior to or during Fe reduction. It indicates that the full or partial layer collapse by reduction causes K fixation. The fixed K decreased with the decrease of layer charge and the presence of TMPA (trimethylphenylammonium) cations in smectite layers. TMPA cations inhibit the expanded layers from collapsing upon Fe reduction, The selectivity of the oxidized surface of SWa-1 was much greater for TMPA than for K whereas in the reduced, higher-charged state, the selectivity was reversed and K dominated the exchange complex over TMPA. Mossbauer analysis of redox-treated Garfield showed that the reduction and subsequent reoxidation of structural Fe significantly changes local environment of Fe in the nontronite. The irreversibility became more pronounced as Fe reduction levels increased. FTIR analysis of redox-modified smectite structure revealed that the overall effects of bacterial Fe reduction on smectite structure were similar compared to chemical Fe reduction, when the smectite was reduced to similar levels as by either by Shewanella oneidensis or by pH-buffered dithionite. The dissolution of smectites by bacterial reduction without organic acids was nonstoichiometric and insignificant to moderate levels. In general, the changes in smectite structure during redox reactions indicates that redox processes in smectites are irreversible.