EFFECT OF PHOSPHATE ON THE FORMATION OF NANOPHASE LEPIDOCROCITE FROM Fe(II) SULFATE

Abstract—The effect of phosphate on the formation of Fe oxides from Fe(II) salts is important because phosphate is a ubiquitous anion in natural environments. For this reason, the products formed by oxidation of phosphate-containing Fe(II)SO4 solutions neutralized with bicarbonate were characterized. The rate of oxidation of Fe(II) increased with increasing P/Fe atomic ratio to 0.2 in the initial solution. Goethite (a-FeOOH) or lepidocrocite (g-FeOOH) or both were produced and identified by powder X-ray diffraction (XRD). The ratio between lepidocrocite and goethite increased with increasing P/Fe. In the 5–8.5 pH range, the formation of goethite predominated at P/Fe, 0.005, but only lepidocrocite was detected by XRD for P/Fe. 0.02. Thus, phosphate favors lepidocrocite formation because lepidocrocite has (1) a layered structure (like its precursor green rust), and (2) a structure less dense than that of goethite, thereby requiring less complete removal of the green-rust interlayer phosphate to form. The lepidocrocite crystals were platy, with prominent {010} faces and the thickness of the plates decreased with increasing P/Fe from.25 nm for P/Fe, 0.005 to,5 nm for P/Fe. 0.1. The solubility of lepidocrocite in acid oxalate was nearly complete for P/Fe. 0.03. The lepidocrocite contained occluded phosphate, i.e., phosphate that could not be desorbed by alkali treatment. The decrease in the b unit-cell length with increasing P/ Fe suggests that lepidocrocite may contain structural P