Showing papers in "Clay science in 2001"

Effect of molybdate adsorption on some surface properties of nano-ball allophane.

Abstract: Cation exchange capacity (CEC) of nano-ball allophane measured at same equilibrium pH increased with molybdate adsorption at initial molybdate concentrations of 0.1 and 1.6mM. The increase in CEC was attributed either to deprotonation of silanol group near the adsorption site and/or negative charge carried by the molybdate. Great differences in the amounts of molybdate adsorption between the two initial molybdate concentrations (0.1mM and 1.6mM) was not reflected in the change in CEC. This may be due to polymerization of molybdate at higher concentrations (more than 0.2mM), therefore, increase in CEC was ascribed to adsorption of monomeric molybdate species. Increase in CEC was greater for allophane samples with lower Si/Al ratios than for allophane sample with higher Si/Al ratio, in agreement with the trend of adsorption. Ab inito molecular orbital calculations indicated that Bonsted acidity of silanol group of allophane near adsorption site increased with the molybdate adsorption. The increase in acidity together with free Mo-OH or Mo-Ogroups of molybdate adsorbed in monodentate form contributes the increase in CEC with the molybdate adsorption.

Zeta potentials of clay minerals estimated by an electrokinetic sonic amplitude method and relation to their dispersibility

Abstract: Zeta () potential of clay minerals, which affects the dispersion and the flocculation in their water suspension, is generally determined by an electrophoresis method. This method is, however, required the diluted suspension system which is thoroughly dispersed, and needs considerably long times for the measurement. An electrokinetic sonic amplitude (ESA) method for the (-potentials was applied to four kinds of Nat-saturated clay minerals, kaolinite, halloysite, montmorillonite and allophane. The pH-dependence of (-potential was discussed comparing with the changes of their mean particle or aggregate sizes determined by a laser scattering method. The (-potential of kaolinite, halloysite, montmorillonite and allophane was 22, -15, -25 and -50 mV in the clay samples saturated with Na+ at about pH 7.0, respectively. These approximated to the values reported in the previous studies. The (-potential of clay minerals estimated with the ESA method was clearly related with the dispersibility of their suspension at various pHs. The ESA method was effective for the estimation of (-potential of clay minerals, in conclusion.

Artificial weathering of granite under earth surface conditions - the influence of acid rain on rocks and minerals (Part 3).

Abstract: In order to predict the effect of acid precipitation on building stone, artificial chemical weathering of polished plates of Kitagi Granite, Okayama, southwestern Japan, was conducted. An improved Soxhlet extraction apparatus was used with distilled water and HNO3, H2SO4 and HC1 solutions of pH 3 at 55•Ž for different periods of time up to 824 days. The granite was composed mainly of quartz, plagioclase (Ab86An12Or2), alkali feldspar (0r92Ab8) and biotite. The leached sample solutions were analyzed for major and minor elements using ICP, ICP-MS and ionchromatography. Morphological and chemical changes of each mineral surface were studied by SEM, EPMA and microscopic techniques. Alteration products were collected from the surface of each mineral on the polished plates by hand picking and examined using an XRD. Molar ratios of each element in the leached solutions to those in the fresh granite varied depending on the experimental period. The ratios of elements such as Mn, Fe, Mg, Sc and Zn, which reflect the dissolution of biotite, were relatively high. Similarly, the ratios of Ca, Na, Gd, Sm, Nd, La and Sr, which reflect the dissolution of plagioclase, were relatively high. It seems, therefore, that plagioclase and biotite are easily weathered. It was also estimated from the ratios that the reactivity order of the acid solutions for plagioclase is H2SO4, HC1 and HNO3, and for biotite is HNO3, H2SO4, and HCl. It is evident that biotite changed into mica-clay minerals and/or interstratified minerals of mica and vermiculite by exposure to the various acid solutions used in the experiment. Smectite appeared to be formed from plagioclase during the artificial chemical weathering. However, altered products from quartz and alkali feldspar, were not detected.