Properties and Quantitative Estimation of Poorly Crystalline Components in Sesquioxidic Soil Clays

Abstract

Abstract--Sesquioxidic soil clays from Oxisols in South Africa, Australia and Brazil, and two clays from Andosols in Japan and New Zealand, were investigated by selective dissolution techniques. Acid ammonium oxalate (pH 3) was found to be superior to currently popular alkaline reagents for extracting amorphous aluminosilicates and alumina from these clays. Boiling 0.5 N NaOH dissolved large amounts of finely-divided kaolinite and halloysite, while hot 5~ Na2CO 3 reaction was too slow (partial dissolution of synthetic amorphous aluminosilicates with one extraction) and insufficiently selective (gibbsite and kaolin of poor crystallinity dissolve to a variable extent). On the other hand, synthetic gels (molar SiO2/A120 ~ ranging from 0.91 to 2.55) dissolved completely after 2 hr shaking in the dark with 0.2 M acid ammonium oxalate (0.2 ml/mg). Specificity of oxalate for natural allophane was indicated by removal of similar quantities of silica and alumina using different clay:solution .ratios. Oxalate extraction data indicated that allophane is absent in Oxisol clays. Allophane was determined quantitatively in volcanic-ash soil clays by allocating hydroxyl water content to oxalate-soluble silica plus alumina on the basis of an ignition weight los~chemical composition function for synthetic amorphous aluminosilicates. Parameters of chemical reactivity and distribution of electric charges following various chemical pretreatments of allophane were found to correspond closely to those predicted on the basis of synthetic gel behaviour. Results for Oxisol clays suggested that the role of amorphous (oxalate-soluble) alumina in governing physicochemical properties is generally less than that of the poorly-crystalline, Al-substituted iron oxide component which is removed by deferration with citrate~tithionite-bicarbonate reagent.

Transformation of 2-line ferrihydrite to 6-line ferrihydrite under oxic and anoxic conditions

“…A subsample of the Ni ferrihydrite coprecipitate consisting of ~0.01 mol/L Fe 3+ was treated with 100 mL of acidified 0.2-mol/L ammonium oxalate (pH 3.0 with HCl) solution for either 2 or 24 h at room temperature in light-excluded containers (Fey and LeRoux 1977). Following the desired extraction period, the suspension was centrifuged at 5000 rcf for 10 min.…”

Dissimilatory bacterial reduction of Al-substituted goethite in subsurface sediments

“…Hydroxylamine hydrochloride (Chao and Zhou, 1983) was used for the extraction of amorphous iron oxyhydroxides and reducible manganese oxides. Ammonium oxalate (Schwertmann, 1964;Fey and LeRoux, 1977) was used for the removal of amorphous aluminosilicates as well as poorly crystalline forms of iron oxides. Dithionite-citrate-bicarbonate (DCB) (Mehra and Jackson, 1960) was used for the complete extraction of reducible Fe(III) oxides.…”

Structure of Some Allophanes from New Zealand

“…The treatment dissolved allophane (Fey and Le Roux, 1977;Higashi and Ikeda, 1974), but left a trace of residue which was probably mainly glass. The suspensions (50 mg in 20 ml) were also treated with dithionite (0.2 g), sodium citrate (0.6 g), and sodium bicarbonate (2 ml, 1 M) at 90~ for 15 min.…”