Na-kaolinite

Figure 6. Adsorption of calcium and its influence on adsorption of PAM and HP AM on Na-kaolinite.

Figure 11. Adsorption of HP AM on Na-kaolinite versus ionicity in the presence of calcium.

Materials. Na-Kaolinite A homoionic sample of kaolinite was prepared from a well-crystallized sample purchased from Source Clays, University of Missouri, using a standardized technique (14) which involved repeated washing with distilled water and by treatment with NaCl solutions to remove exchangeable ions such as Ca, and freeze-drying of the final product. Nitrogen specific surface area of this kaolinite was estimated to be 9.4nr/g and X-ray analysis showed the characteristic pattern of kaolinite. 

Figure 1. Adsorption and flocculation obtained with Na-kaolinite at pH 4.5 as a function of dosage of polyacrylamide (PAM 0.4-0).

Figure 7. Diagram illustrating the effect of polymer charge density on various flocculation responses of Na-kaolinite at pH 4.5.

Figure 9. Flocculation of Na-kaolinite at pH 4.5 as measured by the settling rate and the supernatant clarity as a function of particle surface coverage by 33% hydrolyzed polyacrylamide (HPAM 0.4-33).

Flocculation using polyacrylamide and hydrolyzed polyacrylamides was investigated by studying a number of flocculation responses of Na-kaolinite under controlled chemical and hydrodynamic conditions. ... 

Flocculation was correlated with both adsorption density and estimated surface coverage for the nonionic and 33% hydrolyzed polyacrylamides. Maximum settling rate was obtained with the nonionic flocculent at 0.1 and with the hydrolyzed sample at 0.2 surface coverage. Supernatant clarity showed a maximum at a surface coverage of Na-kaolinite by the hydrolyzed polyacrylamide of 0.1. At higher surface coverages (such as 0.5) considered in the past to be optimum for flocculation, complete dispersion was obtained with both the nonionic and the anionic polymer. 

A qualitatively similar relationship was observed for the hydrolysis of parathion on Ca- and Al-kaolinite (Figure 4). In order to explain the lower hydrolysis rate of parathion at Al-kaolinite than at Na-kaolinite surfaces, it was suggested (70) that steric hindrance may force the parathion molecule into a position or conformation less favorable to hydrolysis. On the other hand, the hydrolysis rate for methyl parathion on the Al-clay was higher than the one on the Na-clay (71), probably due to the smaller size of the methoxy group as compared to that of the ethoxy group of parathion... 

Parathion hydrolysis on clay surfaces also is affected by environmental factors, such as temperature and water content. A rise in temperature generally enhances parathion hydrolysis on kaolinite, but the effect is greater in Na+-kaolinite than Ca +-kaolinite. These differences are due to the different hydrolysis pathways in the presence of Na - and Ca +- saturating cations. In the limit of sorbed water, the addition... 

Figure 1. Adsorption excess isotherms on (o) Na-kaolinite, ( ) with methanol extracted Na-kaolinite in methanol)l)- -benzene(2) mixtures.

The basal planes and edges in clay minerals have different acid-base properties. PZCs of the edge (at pH 8) and of the Al layer (at pH 6) of kaolinite are reported in [761], The lEP of the edge surface of Na-kaolinite at pH 7.3 was obtained [762] as the intersection point of Bingham yield stress (pH) curves at various ionic strengths. 

Treated with Na2CO3 at 80°C for 2 d, and then with 1 M NaCI to obtain Na-kaolinite. 

Wang, J. et al.. Effects of chain length and structure of cationic surfactants on the adsorption onto Na-kaolinite, J. Colloid Interf. Sci., 213, 596, 1999. 

The cationic component of the clay has a paramount effect on the EOD. For example, for Na-kaolinite, EOD takes place at low (1-2 V) voltages, whereas for Ca-kaolinite, detectable dewatCT-ing occurs only above 10 V for Al-kaolinite, 25 V was needed just to initiate the dewatering, with appreciable continuing dewatering being observed only when 150-250 V were applied. 

This is attributed to the failure of the large molecules to enter the pores of the solid. More complex isotherm shapes are encountered as in the case of the adsorption of alkyl surfactants on silica and alumina. For example, the adsorption isotherm of sodium dode-cylsulfonate on alumina consists of four regions depending on the dominant adsorption mechanism. Adsorption of polymeric reagents on minerals typically results in a pseudo-Langmuirian type isotherm as shown in Fig. 4.7 for the adsorption of polyacrylamide on Na-kaolinite (Hollander et al., 1981). 

Fig. 4.7. Adsorption isotherm of polyacrylamide on Na-kaolinite as a function of residual polymer concentration and the corresponding Langmuirian plot.

Hollander, A.F., Somasundaran, R, Gryte, C.C., 1981. Adsorption of polyacrylamide and sulfonated polyacrylamide on Na-kaolinite. In Tewari, RH. (Ed.), Adsorption from Aqueous Solutions. Plenum Press. 

Figure 6. Isotherms for adsorption of several cationic pesticides on (top) Na-montmorillonite, (center) Na-kaolinite, (bottom) soil organic matter (63)...

It is apparent that at low moisture content (<10% for the Na-saturated clay mineral and <5% for the Ca-, or Mg-saturated clay mineral), where water is not available for hydrolysis, hydrolysis does not occur. This low moisture content corresponds with the saturation of the cation s first hydration shell. As the moisture content is increased to the upper limit of bound water (50% moisture content), a significant enhancement of the hydrolysis of the epoxide is observed. When the moisture content exceeds the upper limit of bound water (>50%), the rate constant for the hydrolysis of the epoxide was reduced by a factor of 4. It was concluded that water in excess of sorbed water diminishes the catalytic activity of clay surfaces by reducing the concentration gradient across the double layer, effectively raising the surface pH closer to that of the bulk water. In similar studies with MTC, the addition of water to oven-dried Na-montmorillonite and Na-kaolinite retarded the hydrolysis rate of the carbamate. This observation is consistent with the fact that MTC exhibits only neutral base-catalyzed hydrolysis. 

Pure quartzitic sand or a Na-kaolinite-sand mixture containing 2% W/W of clay were packed in glass columns giving porous media with pore volumes varying between 70 and 90 cm and lengths between 25 and 30 cm. The characteristics of these porous media are given in Table II. The BET specific areas of pure sand and Na-kaolinite are respectively 0.1 and 20 m /g. 

Fig. 3. Effect of precipitation on the shape of adsorption isotherm of sodium dodecylhenzenesulfonate on Na-kaolinite.

Figure 8. Microstructure of artificial Na+ kaolinite sediment obtained (a) in acid medium (pH = 3.5), (b) in alkaline medium (pH = 12), (c) scheme of electrostatic interaction between two particles in the acid medium.

Anionic Surfactants onto Kaolinite and lUite. In the investigation of the adsorption of sodium dodecylbenzenesulfonate (SDBS) and sodium dodecyl sulfate (SDS) onto asphalt covered kaolinite and illite surfaces, Siffert et al. [5S] observed Langmuir type I isotherms for SDS adsorption onto Na kaolinite and Na illite while the SDBS exhibited a maximum in adsorption with a decrease beginning near the CMC. Adsorption maxima were observed near the CMC for both surfactants in the Ca kaolinite and Ca illite systems. The adsorption behavior was explained as precipitation of the calcium salt of the surfactants (an idea supported by other studies), and the interaction of the aromatic ring in SDBS with the asphalt. This interaction favors desorption of the asphalt rather than adsorption of the SDBS. The amount of asphalt desorbed by SDBS was twice that desorbed by SDS. Other explanations for adsorption maxima include mixed micelle formation [55] and electrostatic repulsion of micelles from the bdayer covered surface [59]. 

---