The Effect of Iron on Some Physico-Chemical Properties of Bentonite Suspensions

Summary and ConclusionsElectrodialyzed bentonite suspensions were treated with FeCl₃ in quantities from 12.5 to 250 per cent of the adsorption capacity. They were washed until no Cl⁻ appeared in the washing and several physico-chemical properties of the Fe clays were studied. Some of the more important data are summarized below. The Fe was about all adsorbed from additions of FeCl₃ up to 2.5 symmetry concentration, in the presence of HCl, thus indicating that Fe⁺⁺⁺ is much better adsorbed than H⁺ at the existing pH values. After washing the pH of all suspensions was approximately 8.90. The amount of Cl⁻ not washed out was assumed to be a measure of the anion exchange capacity. An increase in Fe adsorption was associated with an increase in Cl⁻ adsorption, but not as a linear function. This was explained as a division of the Fe valences between the micelle and the Cl⁻, shifting in favor of the Cl⁻ as Fe⁺⁺⁺ formed the inner layer. This increase in the anion exchange capacity is important from a soil fertility standpoint in southern soils where large amounts of nitrates, phosphates, and other anionic fertilizer materials are used. It probably explains the observation that soils high in Fe fix large amounts of phosphate. Swelling of the dried colloid and hydration of the suspended colloid were more nearly proportional to the Cl/Fe ratio than to the total Fe. Low Cl/Fe ratios were associated with low hydration and small amounts of swelling. Thus, the proportion of Fe to anion and, perhaps the kind of anion, are important influences on the swelling and hydration of bentonite. The low swelling and hydration caused by the Fe is in agreement with the observations and experimental data on lateritic soils. Low hydration should give greater permeability and better aeration than is found in hydrated soils even if the total pore volume is the same. This is borne out by data and observations on the non-hydrated Cecil and hydrated Iredell soils. The increased aeration and permeability caused by the Fe are important factors in both soil fertility and soil conservation. Observations of the suspensions indicated that the red and yellow color was not directly proportional to the amount of Fe present and this was verified by color determinations. The hydration and swelling and the Cl⁻ adsorption data indicate that Fe is probably a very important factor influencing certain physico-chemical properties of lateritic soils.