Effects of interaction of organic and inorganic P with ferrihydrite and kaolinite-iron oxide systems on iron release

The solubility of iron oxides in soils is governed by crystal size, crystal order, isomorphous substitutions, and associations with other minerals. Their dissolution occurs by protonation, reduction, or complexation with ligands such as phosphate and inositol phosphates. In this work, the interaction of inositol hexaphosphate (IHP) and phosphate (Pi) with ferrihydrite (Fh) or ferrihydrite-kaolinite systems (Fh-KGa2) was studied to assess the effects on iron oxide dissolution. Adsorption of IHP and Pi on a two-line Fh and Fh-KGa2 was performed in the range 0 to 0.004 mol P L-1, and the release of P and Fe from samples of Fh or Fh-KGa2 saturated with IHP or Pi was evaluated at different pHs. The amount of P adsorbed on Fh increased, reaching a plateau at 2.12 mumol m(-2) for IHP and 4.57 mumol m(-2) for Pi. Sorption on KGa2 was lower. It increased to 2.24 mumol m(-2) for IHP and to 2.96 mumol m(-2) for Pi on Fh-KGa2. On the basis of the IHP/Pi ratios, it was hypothesised that IHP interacted with Fh through two of its six phosphate groups, whereas it interacted with Fh-KGa2 through one P group. Phosphate desorption from these complexes occurred only at pH greater than or equal to 4.5 and was higher for Pi than for IHP and from the Fh-KGa2 system than from Fh. The desorption of IHP was followed by a consistent Fe release, higher at basic pHs. By contrast, Pi adsorption inhibited dissolution of both minerals, although the anion was desorbed in higher amounts compared with the P organic form.