Abstract: The morphology of the precipitates of ferric hydrous oxides formed by forced hydrolysis of acidic ferric salts solutions at elevated temperature is strongly affected by the addition of EDTA to such system before aging. At lower content of EDTA, the sizes of the particles formed increase with EDTA until certain value which corresponds to point B in the typical Langmuir-type adsorption isotherm of EDTA on growing nuclei or tiny particles. The opposite phenomena have been observed at higher EDTA concentrations. These are due to the formation of ferric-DETA complex, which restricts the hydrolysis of free ferric ions and therefore the growth of crystals. In addition, any factor, e.g. anions presented in the aging solution, which will affect the adsorption of EDTA on growing nuclei or the complexing of EDTA with ferric ions might influence the extent of the effect of EDTA.

Key words: Adsorption, Complex, Coagulation, Critical supersaturation, Hydrolysis