Fe2+-Fe3+ Transformations in Clay and Resin Ion-Exchange Systems

Abstract

Abstract----The pH, Eh, electrical conductivity (EC), and the amounts and valency of replaceable iron were measored periodically on Fe ~+-and Fe3+-saturated montmorillonite and cation-exchange resin at three temperatures. Differences in the pattern of change of pH, Eh, and EC with time appear to be related more to the histories and modes of preparation of the systems than to intrinsic differences in the hydrolysis of the iron in them. Electron transfer reactions involving crystal components of the clay can cause oxidation of adsorbed Fe 2 § ions; the activation energy (Ea) for oxidation on the clay's surface was 6 kcal/mole, less than a third of the activation energy reported for Fe z+ oxidation in solution. In the Fe2+-resin, where Ea = 10.7 kcal/mole, perturbed surface-water molecules may act as electron acceptors enhancing Fe 2+ oxidation.Polymerization and precipitation of the adsorbed iron is affected by the necessity to maintain electroneutrality, the ability of the iron-hydroxy ions and small polymers to move about in the voids of the ion exchanger, and the steric hindrance posed by the matrix of the ion exchanger to the formation of large polymers. In resin, little or no iron precipitates, probably due both to steric hindrance and the inability of the resin to release ionic components to maintain electroneutrality. In clays, steric hindrance is small, and AI and Mg are released from the crystal to maintain electroneutrality, thus the precipitation of iron is abundant and is controlled by the rate of release of AI and Mg from the crystal.

Brønsted Acidification Observed during Hydrothermal Treatment of a Calcium Montmorillonite

“…While this is in accord with previous findings, the most unexpected result of the study was the extremely fast exchange and thus hydrolysis reaction that led very quickly to low pH values. This is in contrast to results obtained previously on other acidic clays (Eberl, 1978;Gerstl and Banin, 1980) that showed similar but much slower pH changes.…”

Characterization of Adsorbed Iron in Montmorillonite by Mössbauer Spectroscopy

“…If Fe3+(a) cations were in sites similar to FeZ+(a), namely as partially solvated free ions, a break should have been detected in the In A(T) vs. Fe 3+ plot at T = 210~ the temperature at which part of the interlayer water started to melt. This observation may be explained as follows: Fe3+(a) tends to hydrolyze and polymerize on the clay's surface (e.g., Gerstel and Banin, 1980) forming hydroxyferric aggregates. The ferric ion in this state of polymerization is insensitive to water solvation.…”

Oxytetracycline sorption onto Iraqi montmorillonite

“…The analysis was done in duplicates. Preparation of Fe-modified clay (Gerstl and Banin 1980) In a plastic centrifuge tube, 4.0 g of clay was shaken in the presence of 20 mL of 1 M FeCl 3 solution for 30 min. The tube contents were centrifuged at a speed of 3,000 rpm for 10 min.…”