The constraints on oxidation of structural Fe(II) by aqueous Fe3+ and the pathways of electron transfer in clay minerals

Abstract

Iron-bearing clay minerals act as crucial redox-active metastable phases in the fate of redox-sensitive constituents in aqueous environments. This study focuses on the constraints and pathways of electron transfer between structural Fe(II) (Fe(II)_str) in clay mineral and aqueous Fe³⁺ (Fe(III)_aq). Various batch experiments were conducted using partially reduced clay minerals to investigate the impacts of clay mineral type, reduction extent, initial Fe(III)_aq concentration, and solution pH on the electron transfer process. The results demonstrate that higher reduction extents of clay minerals, higher initial Fe(III)_aq concentrations, and lower pH conditions enhance the electron transfer. Notably, the existence of Fe(II)_str is a prerequisite for its electron-donating to the Fe(III)_aq, but the occurrence also depends on the availability of electron “bridges” such as Fe(II)-O-Fe(III) entities in the clay mineral lattice or the ability of Fe(III)_aq to enter the interlayer space via ion-exchange. The pathway of Fe(II)_str- Fe(III)_aq electron transfer varies among different clay minerals: Fe(III)_aq acquires electrons from Fe(II)_str primarily through edge sites in nontronite, while through the basal plane in montmorillonite. In contrast, for the structurally stable illite without swelling properties, Fe(III)_aq has difficulty in acquiring electrons from Fe(II)_str. This study provides new insights into the geochemical cycling of iron in natural environments and its impact on redox-sensitive elements.