Showing papers on "Goethite published in 1973"

Adsorption of the herbicide 2,4‐d on goethite

Abstract: Summary Adsorption, of the herbicide 2,4-Dichlorophenoxyacetic acid (2,4-D) on an iron oxide, goethite, was studied in aqueous suspensions as a function of solution pH, ionic strength of the medium, and initial 2,4-D concentration. The 2,4-D anion was reversibly adsorbed on positively charged goethite surfaces, maximum adsorption being observed near the pKa of 2,4-D (2.73) and at low ionic strength. Within certain levels of adsorption (5–22 mg 2,4-D adsorbed/g goethite) the complex became hydrophobic and floated to the liquid surface. This flotation effect disappeared on further adsorption. It is suggested that adsorbed 2,4-D anions are orientated with their hydro-phobic aromatic ends directed towards the solution, the carboxyl groups being weakly bound to positive sites on the oxide surface. At high levels of adsorption, some of the anions are orientated in the opposite direction by π–π interaction, with the first adsorbed layer and the surface reverts to its hydrophilic nature.

Iron-bearing Oolites and the Present Conditions of Iron Sedimentation in Lake Chad (Africa)

Abstract: The oolites are found at a depth of 1 to 3 meters of water, in a zone of about 2700 sq. km, off the Chari delta. The grain size is around 0. 250 mm. Montmorillonite nuclei are surrounded by goethite and silica, the iron content attains a maximum value of 49% Fe2O3. Pollen analyses show a lacustrine formation. The present chemical conditions lead to a scheme of iron behaviour in which colloidal or adsorbed iron from the solid load of the incoming rivers (94000 tons reactive iron in 1970–71) is separated from kaolinite, which becomes unstable in the lake, and forms a coprecipitate with silica.

Rust in the Apollo 16 rocks

Abstract: Apollo 16 samples of all four rock types and from all stations contain evidence for hydration and oxidation - i.e., the presence of hydrated iron oxide, probably goethite. Rock 66095 contains native FeNi grains with a characteristic intergrowth of schreibersite and, to lesser extents, of cohenite. Troilite also contains sphalerite. The goethite contains 1.5-4.6 wt.% chlorine and occurs mainly on the edges of FeNi metal, causing a rust color in the cracks and space around the native metal grains, which also contain abundant chlorine. This observation suggests the presence of lawrencite (FeCl2), a phase that deliquesces and oxidizes very rapidly upon exposure to water or to a moist atmosphere.

Method of digesting bauxite via the Bayer process with the addition of reducing agents

Abstract: The settling rate of caustic alkali-digested goethite-containing bauxite or laterite, or the like, is increased by carrying out the digestion in the presence of a reducing agent capable of reducing the trivalent iron component of the goethite to a lower valence so as to convert the goethite to hematite, magnetite, metallic iron, or combinations thereof, which enhance the settling rate of the digestion mass.

The dissolution of goethite in aqueous solutions of sulphur dioxide

Abstract: The leaching of goethite in perchloric acid ("direct leaching") and i n acidified sulphur dioxide solutions ("reductive leaching or dissolution") has been investigated. The effects of temperature, hydrogen ion concentration, sulphur dioxide partial pressure, s t i r r i n g speed and sample weight on the reductive leach rate have been studied. The enhancement of the reductive leach rate as a consequence of adding cupric ions was also investigated. A leaching mechanism was proposed for the direct and uncatalyzed reductive dissolution. Protonation of the oxide surface sites, described by a Langmuir Adsorption Isotherm, resulted in the formation of ferric-hydroxy species. These either desorbed (the single solution step i n direct leaching) or underwent the adsorption of a sulphur dioxide molecule possibly followed by another hydrogen ion. The overall reductive leach rate was a function of the desorption of the intermediate and the f i n a l surface species. Cupric ions did not appear to directly catalyze a step in the postulated reductive dissolution mechanism. Rather, the essential feature of their catalytic role was thought to be an electron exchange reaction between Cu^ and Fe^* on the oxide surface to form cupric and ferrous ions i n solution. An anionic cuprous species capable of adsorbing on the oxide surface was formed by cupric ion reduction with aqueous sulphur dioxide and subsequent complexing with bisulphite and/pr sulphite ions. A comparison of the reductive leach rates of magnetite and hematite with goethite was made both with and without the presence,of cupric ions.

Natural Rust on Stone

Abstract: The iron content of the earth’s crust averages 5%. At the earth’s surface iron is tied up as green or black ferrous-ferric iron in the ferromagnesian silicates, as the black ferrous-ferric oxide magnetite, as the yellowish ferrous sulphides, pyrite and marcasite; as the grey to dark-brown ferrous carbonate siderite, and as the red or black ferric oxide hematite. The last is not only a common pigment in rock but can also accumulate as our most important iron ore. In humid atmospheres the brown to ochre-brown ferric hydroxide, goethite, is the most important mineral of the common “rust”, alpha-FeOOH. Goethite is usually accompanied by amorphous (not yet crystallized) ferric hydroxide of the same color. Natural rust is summarized as the “mineral” limonite which is not a mineral in the true sense. All the minerals mentioned here tend to adjust to the humid or semi-humid atmospheric surface conditions as they weather to ferric hydroxide or limonite. Metallic iron also changes to rust, mostly amorphous ferric hydroxide with some magnetic brown maghemite, gamma-Fe2O3. Crystallization or aging of the non-crystalline ferric hydroxide leads to the formation of submicroscopic goethite. In some rare instances deep orange gamma-FeOOH, lepidocrocite, can form.