Interaction of mugineic acid with iron (hydr)oxides: Sulfate and phosphate influences

Interactions of 100 mu mol L-1 mugineic acid (MA) with synthetically produced Fe (hydr)oxides (goethite, hematite, lepidocrocite, and ferrihydrite) as influenced by 100 mmol L-1 SO42- and phosphate were studied at equilibrium in the pH range from 3 to 11, In the presence of SO42-, the concentrations of total MA [i.e., free MA + Fe(III)-MA complexes] and Fe(III) dissolved by MA in the filtrates were higher than those in the presence of 100 mmol L-1 Cl- at pH < 7. Sulfate increased the formation of Fe(III)-MA complexes indirectly by reducing the adsorption of MA and Fe(III)-MA by the Fe (hydr)oxides. The adsorption of MA by the Fe (hydr)oxides and consequently the MA-promoted dissolution of Fe(III) from ferrihydrite and lepidocrocite were greatly inhibited by the adsorption of phosphate. The slight dissolution of Fe(III) between pH 5 and 9 is partly attributed to the formation of soluble Fe(III)-phosphate complexes. Although MA does not react directly with Fe (hydr)oxides in the presence of excess phosphate, MA mould promote the dissolution reaction of Fe(III) by phosphate, The sequence of adsorption affinity for the Fe (hydr)oxides was found to be: phosphate >> MA > Fe(III)-MA, SO42-> Cl-. Our results suggest that SO42- and especially phosphate applied as fertilizers may inhibit the MA-promoted dissolution of Fe(III) from Fe (hydr)oxides in soils mainly by displacement of MA from the surface of Fe (hydr)oxides.