Redox transformation of structural iron in nontronite induced by quinones under anoxic conditions

In natural anoxic subsurface environments, the geochemical cycles of iron are largely associated with the migration and transformation of organic matter. Intensive attention has been paid to the redox interaction of organic matter with aqueous Fe and iron (hydr)oxides. Whereas, the abiotic redox cycling of structural Fe in clay minerals induced by quinones has not been well understood. In this study, we selected nontronite (NAu-2) as a model Fe-bearing phyllosilicate clay mineral and 1,4-hydroquinone (H₂Q)/1,4-benquinone (BQ) as a model quinone couple. Our results show that the structural Fe(III) in NAu-2, with tetrahedral Fe(III) priority, can oxidize H₂Q into BQ, and octahedral Fe(II) in NAu-2 can reduce BQ to H₂Q, with semiquinone radicals (SQ^-) as intermediate. The extent of the redox reactions depends on the reduction potential difference between NAu-2 and H₂Q/BQ. However, a fraction of Fe(II)-Fe(III)-OH and Fe(II)-Fe(II)-OH groups in the octahedral sheet are difficult to be oxidized by BQ, because the reduction potential gradient decreases to a low level as the reaction proceeds. And the structure of NAu-2 can only partially restored upon re-oxidation with tetrahedral Fe(III) irreversibility. Output of this study replenishes the understanding regarding redox cycling of structural Fe in clay minerals induced by quinones.