Surface reactions of goethite with phosphate

Abstract

Reactions carried out between synthetic goethite (α-FeOOH) and aqueous phosphoric acid (1 mmol dm^–3) at pH 3 at 25 °C for 18 days yield mixtures of goethite and tinticite [Fe₆(PO₄)₄(OH)₆·7H₂O], as has been shown by scanning transmission electron microscopy (STEM) and selected-area electron diffraction. X-Ray photoelectron spectroscopy (X.p.s.) indicated the presence of small, variable quantities of phosphate (ca. 2%). The presence of phosphate was indicated by X.p.s. only after mild grinding of the product, indicating that sorption of phosphate at the goethite surface is relatively unimportant. (X.p.s. is particularly sensitive to the first few atomic layers in a solid.) The results are important in resolving the difficult question of phosphate availability in soils, particularly in the presence of iron oxides.