ON THE SORPTION OF ANIONS BY FERRIC-OXIDE GEL .3. RATE-CONTROLLING MECHANISM AND EFFECT OF ORGANIC-SOLVENTS IN CHLORIDE-ION EXCHANGE

A ferric oxide gel (Fe-I), prepared by adding ammonia to ferric chlori de and washing with dilute ammonia and water, and the gel (Fe-II), obt ained by further washing of Fe-I with more concentrated ammonia (which removed more of the Cl- ions from the gel) and water were studied. It was found that the apparent capacity for Cl- ion adsorption by ligand exchange with surface OH groups is much higher for Fe-II than for Fe- I. The Fe-II displayed a complex variation of this capacity with alcoh ol addition, whereas Fe-I displayed a continuous increase in sorption with alcohol concentration. The latter increase is in the order propan -1-ol > ethanol > methanol, and may be attributed to a stronger intera ction of Cl- ions with the surface. For Fe-II, the complex behaviour m ay in part be connected with the development of micropores. The latter effect changed the rate-controlling mechanism from chemical for Fe-I to particle diffusion for Fe-II, and may be accompanied by an increase in the surface area, leading to an increase in the capacity. For the Cl- ion exchange, the values of the rate constant for Fe-I and the int ernal diffusion coefficient ($) over bar D-i for Fe-II decreased progr essively with alcohol addition. This pointed to an imbibition of alcoh ols by both gels. The order of the ($) over bar D-i values for the var ious anions is not changed by the addition of alcohol, which probably shows that ($) over bar D-i is governed by the complexing ability of t he anions with the surface.