. 2000. Kinetics of phosphate adsorption on iron oxides formed under the influence of citrate. Can. J. Soil Sci. 80: [445][446][447][448][449][450][451][452][453][454]. The influence of organic acids on the formation of Fe oxyhydroxides and oxides has been intensively studied. However, scant attention has been paid to the subsequent effect on surface chemistry of the Fe oxides formed. The kinetics and mechanisms of phosphate adsorption by the Fe oxides formed in the presence of citrate ligands at initial citrate/Fe(II) molar ratios (MR) of 0, 0.001, 0.01, and 0.1 were investigated using the conventional batch method. The adsorption studies were conducted at the initial phosphate concentration 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-exponential factor of phosphate adsorption by the Fe oxides formed greatly varied with their structural and surface properties. These properties, which included crystal structure, specific surface area, surface porosity, surface geometry, and point of zero salt effect (PZSE), differed significantly with the initial citrate/Fe(II) MR at which Fe oxides were formed. The results of this study have cast the light on the role of organic acids such as citric acid in influencing the surface chemistry of naturally occurring Fe oxides through fundamental structural perturbation and the impact on the dynamics of phosphate in terrestrial and aquatic environments.Key words: Kinetics, activation energy, pre-exponential factor, phosphate, iron oxides, citric acid, structural perturbation Liu, C. et Huang, P. M. 2000. Cinétique de l'adsorption des phosphates sur les oxydes de fer formés sous l'influence des citrates. Can. J. Soil Sci. 80: 445-454. Bien que l'influence des acides organiques sur la formation des oxyhydroxydes et des oxydes de fer ait fait l'objet d'études intensives, on s'est peu intéressé à l'effet des oxydes ainsi formés sur la chimie du sol de surface. Nous étudions selon la méthode classique par fractionnement la cinétique et les mécanismes de l'adsorption des phosphates sur les oxydes de Fe formés en présence de ligands de citrate, pour des rapports molaires (RM) citrates/Fe (II) de 0, 001, 0,01 et 0,1. Les études d'adsorption étaient réalisées à la concentration initiale de phosphate de 0,5 mM au pH 4,0 durant des périodes allant de 2 min à 56 h, à 278, 288, 298 et 313 K. Les résultats obtenus montrent que l'adsorption du phosphate suivait une cinétique multiple du second degré comportant deux taux distincts, dans chaque système de réaction. La quantité, le coefficient de taux, l'énergie d'activation et le facteur pré-exponentiel d'adsorption des phosphates sur les oxydes de Fe formés variaient grandement selon les propriétés structurales et les propriétés de surface des oxydes : structure cristalline, surface spécifique, porosité, ...

Taranakite is an important reaction product of monoammonium phosphate fertilizer with soils. Its formation affects the transformation of nutrients in soils. The effect of different molar ratios of Fe(II)/Al and Fe(III)/Al on the formation of taranakite at pH 4.0 was investigated in this study. The results show that Fe(II) ion significantly perturbed the formation of taranakite at the Fe/Al molar ratio of 1.2 When the Fe/Al molar ratio was increased to 2.5, the formation of taranakite was completely inhibited by Fe(II), whereas, under the same condition, some crystalline taranakite was still observed in the Fe(III) system. Although Fe(III) had less effect on the crystallization of taranakite than Fe(II) at lower Fe/Al molar ratios, it also completely inhibited the formation of taranakite at the molar ratio of Fe(III)/Al ≥ 5. The solid products formed in the Fe(III) or Fe(II) system contained a substantial amount of Fe(III) and a much higher proportion of phosphate than that was required for the formation of NH4–taranakite. As indicated by the solution phase analysis at the end of the experiment, more Fe ions were present in the solution in the Fe(II) system, compared with the Fe(III) system, to perturb the nucleation and crystallization of taranakite. Since iron is a very common element in soil, taranakite formation may be perturbed in soils with high Fe content, especially under reduced and acidic conditions.