Formation of colloidal particles of hydrous iron oxide by forced hydrolysis

The mechanism of formation and growth of hydrous iron oxide (FeOOH) during the initial stages of forced hydrolyses of ferric chloride aqueous solution was studied by small angle X-ray scattering (SAXS). The effect of the hydr olysis temperature (60 degrees C, 70 degrees C and 80 degrees C) and of the addition of urea on the formation of colloidal particles under isothermal conditions were investigated. Based on the experimental scattering function s in the Guinier range, we suggest the presence of elongated colloidal part icles. The particle diameter and length, and their variation with time, wer e determined by fitting the form factor of prolate ellipsoids to the experi mental scattering functions. We have assumed that our solutions are in a di lute state and that all colloidal particles are approximately of the same s ize. The colloidal particles have geometrical shapes similar to those of th e subcrystals that build up the superstructure of beta-FeOOH crystals, indi cating that the formation of this hydrous iron oxide is governed by an aggr egation process. Otherwise, the addition of urea hinders the growth and yie lds smaller particles, with a reduction in size greater than 50%. (C) 2000 Elsevier Science B.V. All rights reserved.