Phosphate Adsorption and Desorption by Goethites Differing in Crystal Morphology
1990
Torrent, J. | Barrón, V. | Schwertmann, U.
Goethite is an important component influencing phosphate adsorption in many soils and sediments. Little is known, however, about the relationships between crystalline properties and phosphate adsorption and desorption by this mineral. In this study we investigated the phosphate adsorption and desorption properties of 31 synthetic goethites differing widely in specific surface area, crystal morphology, and AIOOH mole fraction. The amount of phosphate adsorbed per unit surface area at pH 6 and an equilibrium concentration of 6 mg P L⁻¹ was similar for all goethites (x̄ = 2.51, SD = 0.17 µmol P m⁻²), suggesting that the common crystal faces have similar P-sorbing capacities. This condition is compatible with the existence of only (110) faces; in fact, according to transmission electron microscope (tem) observations, both natural and synthetic goethites seem to be bound by only (110) faces. The amount of phosphate nonextractable by 0.1 M KOH, and the amount not extracted by dissolving one layer of one-unit-cell thickness with an acid (HCl-H₂SO₄) treatment, are related to crystal morphology: samples consisting of thin, multidomainic laths retain more P than those having thick, monodomainic crystals. It is hypothesized that the presence of V-shaped interdomainic grooves and/or the slit-shaped macropores of multidomainic laths accounts for stronger P bonding and/or low accessibility of the hydroxyl or hydronium ions.
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