Role of Structural Hydrogen in the Reduction and Reoxidation of Iron in Nontronite

The effects of reduction and reoxidation of octahedral Fe 3+ on the exchange of structural hydrogen in nontronite were determined using tritium ( 3 H) as a label element. The uptake of H from the surrounding solution of nontronite suspensions increased as the reduction of structural Fe 3+ increased. Similarly, the loss of H from the structure increased as the reduction increased. The results are generally consistent with a reduction mechanism involving the loss of structural OH, leaving the affected Fe sites with less than six-fold coordination. The attenuation of increased negative charge on the clay layer, however, was less than predicted by such a mechanism. During the reoxidation of reduced nontronite in suspension, about one-third of the H remaining as part of the structure following reduction was lost, whereas twice that amount of H was incorporated into the structure from the surrounding solution. A reoxidation mechanism is proposed whereby H 2 O from the surrounding solution is incorporated into the mineral structure followed by the elimination of a hydrogen ion, returning the Fe to six-fold coordination. This mechanism implies the reversibility of Fe reduction in nontronite.