Effect of Phosphate on the Formation of Nanophase Lepidocrocite from Fe(II) Sulfate

Jesús Cumplido, Vidal Barrón and José Torrent
Departamento de Ciencias y Recursos Agrícolas y Forestales, Universidad de Córdoba, Apdo. 3048, 14080 Córdoba, Spain
E-mail of corresponding author: crltocaj@uco.es

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 (α-FeOOH) or lepidocrocite (γ-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.

Key Words: Goethite • Lepidocrocite • Nanophase • Phosphate • Rietveld Analysis

Clays and Clay Minerals; October 2000 v. 48; no. 5; p. 503-510; DOI: 10.1346/CCMN.2000.0480502
© 2000, The Clay Minerals Society
Clay Minerals Society (www.clays.org)