Charge properties of mixtures of minerals with variable and constant surface charge

Abstract

Surface charge of mixtures of lepidocrocite and illite determined by adsorption of potential-determining and electrolyte ions is studied as a function of pH and NaCl concentration. Deviations between results obtained by both methods were a function of the illite content of the mixtures, and can be mainly attributed to coatings of hydroxyaluminium complexes on the constant charge surfaces of the clay mineral, which can react with H + ions or release exchange sites by precipitation of well-formed hydroxides. These coatings of aluminium complexes can cause H + consumptions similar to those due to lepidocrocite.Some ideas are put forward for modelling charge properties of mixed systems.

Adsorption of phosphate by two iron oxides in relation to their porosity

“…4), of 2.2 pg ml-' at pH 4.12 (7% of P adsorbed), 3 pg ml-' at pH 6.83 (14% of P adsorbed) and 3.2 pg ml-at pH 9.66 (33% of P adsorbed). After 1 d, there was a marked decrease in the solution concentration, similar to the apparent readsorption found by Madrid & Posner (1979) for goethite and by Mums & Fox (1976) for some soils. The amount readsorbed corresponded to about 3% of the P adsorbed in each case.…”

“…This explanation is confirmed by a readsorption which is sometimes found when suspensions of phosphated iron oxide or soils are diluted with phosphatefree solutions (Munns & Fox, 1976;Madrid & Posner, 1979). This heterogeneity has been related to the porosity of the solid surfaces (Vaidyanathan & Talibudeen, 1968;Cabrera et al, 1981); but little evidence has been forthcoming to confirm this relationship.…”

“…The lepidocrocite was described in a previous paper (Madrid et al, 1984). It has a specific surface area (BET) of 116 mz g-'.…”

“…Many studies of desorption of phosphate from metal oxides and soils have reported a lack of reversibility often explained by the existence of a slow component of the sorption reaction (Hingston et al, 1974;Munns & Fox, 1976;Madrid & Posner, 1979), attributed to heterogeneity of the surface. This explanation is confirmed by a readsorption which is sometimes found when suspensions of phosphated iron oxide or soils are diluted with phosphatefree solutions (Munns & Fox, 1976;Madrid & Posner, 1979).…”

Role of organic matter and sesquioxides on variable charge of three soils from Galicia, Spain

“…Bonding of acetate to positive-charged Al or Fe oxyhydroxides may release negative-charged groups on clay surfaces (Madrid et al, 1984) or on organic matter surfaces that were previously bound to those sesquioxides, thus increasing the CEC. On the other hand, specific adsorption of negative-charged molecules in the coordination layer (inner Helmholz plane), completing the coordination of Fe or Al ions, can also add negative charges to the solid phase (Täte and Theng, 1980).…”