IRON OXIDATION-STATE EFFECTS ON CATION FIXATION IN SMECTITES

Cation fixation is of great importance to agricultural productivity and to the chemical and physical properties of soils. Reduction of Fe(III) to Fe(II) in clay minerals is known to enhance the potential for K+ fixation on drying, but further studies are needed to determine whether K+ fixation also increased when reduction occurs without drying, and to measure the potential for fixation of other cations. The objective of this study was to address these questions. The effect of Fe oxidation state on cation fixation was determined by chemically reducing aqueous suspensions of a ferruginuous smectite and a mont-morillonite with Na2S2O4, then saturating the exchange complex with either K+, Ca2+, Cu2+, or Zn2+ using inert-atmosphere techniques. The amount of fixed cation was calculated as the difference between the total and the exchangeable (relative to Mg2+) amounts. Both smectites displayed substantial capacity to fix interlayer cations (without drying) as the level of Fe(II) increased in their structures, depending on the cation and the Fe(II) content. For a given Fe(II) content, the amount of cation fixation increased in the order Cu2+ < Zn2+ < Ca2+ < K+, which follows precisely the inverse of the hydration energies (DELTA-H(hyd)) of the cations and suggests that reduction of octahedral Fe encourages dehydration of the interlayer region. In both clays, the amount of exchangeable K+ was largely independent of Fe(II) content, but virtually all additional negative charge due to Fe reduction was reflected in the nonexchangeable K+ fraction.