Kinetics of phosphate adsorption on iron oxides formed under the influenceof citrate

The influence of organic acids on the formation of Fe oxyhydroxides and oxi des has been intensively studied. However, scant attention has been paid to the subsequent effect on surface chemistry of the Fe oxides formed. The ki netics and mechanisms of phosphate adsorption by the Fe oxides formed in th e presence of citrate ligands at initial citrate/Fe(II) molar ratios (MR) o f 0, 0.001, 0.01, and 0.1 were investigated using the conventional batch me thod. The adsorption studies were conducted at the initial phosphate concen tration of 0.5 mM and pH 4.0 during the reaction period from 2 min to 56 h at 278, 288, 298, and 313 K. The results show that the phosphate adsorption followed multiple second-order kinetics and had two distinct rates in each reaction system. The amount, rate coefficient, activation energy and pre-e xponential factor of phosphate adsorption by the Fe oxides formed greatly v aried with their structural and surface properties. These properties, which included crystal structure, specific surface area, surface porosity; surfa ce geometry, and point of zero salt effect (PZSE), differed significantly w ith the initial citrate/Fe(II) MR at which Fe oxides were formed. The resul ts of this study have cast the light on the role of organic acids such as c itric acid in influencing the surface chemistry of naturally occurring Fe o xides through fundamental structural perturbation and the impact on the dyn amics of phosphate in terrestrial and aquatic environments.