Surface chemistry of ferrihydrite: Part 1. EXAFS studies of the geometry of coprecipitated and adsorbed arsenate

Reference LPI-ARTICLE-1999-017View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

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Metal Oxide Surfaces and Their Interactions with Aqueous Solutions and Microbial Organisms.

Publisher Summary This chapter reviews the work done with soils and with hydrous oxides of iron and aluminum, with particular reference to mechanisms and structural aspects of anion adsorption. Anion adsorption on clay minerals and calcium carbonate is also discussed. The determination of adsorption isotherms is one of the most useful experimental procedures in the study of the interaction of anions with hydrous oxides or soils. Anion adsorption involves an electrostatic interaction and some chemical interaction between the surface and the ion. The Langmuir model is incomplete in describing this adsorption because it takes no account of charge. The adsorption of anions is dependent on pH, with maximum adsorption usually occurring for fully dissociated ions at low pH where the surface becomes positively charged, owing to protonation of surface M·OH groups. Maximum adsorption of anions of incompletely dissociated acids occurs at a pH value close to the pK of the acid. Anion adsorption reactions in soil are complicated by competition for adsorption sites from anions, including carboxylates, and also by the presence of cations, such as calcium and aluminum.

Anion Adsorption by Soils and Soil Materials

http://www1.udel.edu/soilchem/Elzinga07JCIS.pdf

Phosphate adsorption onto hematite: An in situ ATR-FTIR investigation of the effects of pH and loading level on the mode of phosphate surface complexation

Adsorption isotherms were obtained for fulvic and humic acids on synthetic gibbsite and goethite and on a natural imogolite. The results were interpreted with the help of spectra of the adsorption complexes, and measurements of hydroxyl displaced. The mechanism of adsorption involved ligand exchange with the surface hydroxyl groups and hydrogen bonding. The hydroxyl groups displaced included singly coordinated species on the (100) face of goethite and on the edge faces of gibbsite. The doubly coordinated hydroxyl groups on the (001) face of gibbsite were not replaced, and this face adsorbed only un-ionized fulvic acid probably by a hydrogen bonding mechanism. Imogolite adsorbed most fulvic acid because of its high porosity. Only isolated places on its surface reacted with fulvic acid to form carboxylate groups.

Adsorption on hydrous oxides.III.Fulvic acid and humic acid on goethite,gibbsite and imogolite

The infrared absorption spectrum of phosphate adsorbed on α-FeOOD and α-FeOOH establishes the presence of binuclear (FeO)2PO. OH groups, bridging adjacent Fe3+ ions on (100) faces of the crystals. Examination of D2O-exchanged surfaces shows the predicted three species of OD on the goethite surface, two on fluoridated goethite, and four (including POD) on phosphated goethite. The spectrum of adsorbed pyridine indicates that both goethite and phosphated goethite have Lewis acid sites on edge faces only; neither exhibit Bronsted acidity. These observations confirm a proposed model of the (100) face.

James D. Russell

Papers

Confirmation of the surface structures of goethite (α-FeOOH) and phosphated goethite by infrared spectroscopy