INTERACTIONS OF METAL HYDROUS OXIDES WITH CHELATING-AGENTS .1. BETA-FEOOH-EDTA

The adsorption of ethylenediaminetetraacetic acid (EDTA) on a well-defined hydrosol consisting of rod-like β-FeOOH particles was studied as a function of pH and temperature. It was found that at 22°C the uptake of EDTA decreases with increasing pH (from pH 2.5 to 9) and it becomes negligible at pH > ∼ 9. At higher temperatures (50 and 90°C) an analogous trend is observed, although the adsorbed amount is smaller at the same pH. The data can be fitted to a Langmuir-type adsorption isotherm, from which the standard adsorption free energy was calculated to decrease from -9.5 to -8.2 kca/mole when the pH rises from 2.8 to 8.2 (at 22°C). Dissolution of ferric ions from the sol particles was determined in the same systems. In the absence of EDTA very small amounts of released Fe3+ ions could be detected over the pH range of 2-13 during 48 hr of equilibration. When EDTA was added little leaching of ferric ions took place up to pH values of ∼7.5 and above 12. In the range of pH 7.5-12 considerable dissolution of Fe3+ was found and the amount of released ferric species increased with increasing concentration of added EDTA. Electrokinetic data showed that the sol had an isoelectric point at pH ∼6.4; however, in the presence of EDTA this was shifted to considerably lower pH values. A mechanism which explains these effects is offered. © 1979.