Smectites exchanged to various degrees with mono- or biprotonated 1,4-diazobicyclo(2,2,2)-octane (Dabco) have a porous structure in which the organic molecule acts as a “pillar.” Less Dabco2+ than Dabco1+ was necessary both to initiate and to complete expansion, whereas for both Dabco1+ and Dabco2+ the threshold for expansion increased with the charge density of the clay. Interstratification of 9.7-Å and 14.5-Å (Dabco1+) or 14.1-Å (Dabco2+) layers was observed before full expansion occurred.

Nitrogen sorption isotherms on partially exchanged Dabco1+- and Dabco2+-clays are of the BET II and Langmuir type respectively, whereas n-butane adsorbed according to BET type II for both cationic forms. The specific surface area was governed by the number of Dabcoz+ pillars, irrespective of the inorganic cation, and increased with Dabcoz+ content even beyond the composition where full expansion was reached and notwithstanding the increasing number of silicate layers in the c direction of the crystal units. This relationship is explained by an enhanced random interleaving of clay platelets in the stacking units containing Dabcoz+.

By ion exchanging expandable clay minerals with large, cationic oxyaluminum polymers, “pillars” were introduced that permanently prop open the clay layers. On the basis of thermal, infrared spectroscopic, adsorption, and X-ray powder diffraction (XRD) analysis, the interlayering of commercial sodium bentonite with aluminum chlorohydroxide, [Al13O4(OH)24(H2O)12]+7, polymers appears to have produced an expanded clay with a surface area of 200–300 m2/g. The pillared product contained both Brönsted and Lewis acid sites. XRD and differential scanning calorimetry measurements indicated that the micropore structure of this interlayered clay is stable to 540°C. Between 540° and 760°C, the pillared clay collapsed with a corresponding decrease in surface area (to 55 m2/g) and catalytic cracking activity for a Kuwait gas oil having a 260°-426°C boiling range.

In environments contaminated with Cr, the interlayers of expandable layer silicates may serve as sinks for this potentially toxic element. As a means of determining the potential for smectites to serve as sinks for Cr, the precipitation products of Al and Cr in the interlayers of a montmorillonite were examined. Five montmorillonite (SWy-1) clay suspensions were treated with preweighed amounts of AlCl3 and CrCl3 to give five Al/(Al + Cr) molar ratios (1.0, 0.67, 0.5, 0.33, 0) with a total trivalent cation (M3+) concentration of 600 cmol(+)/kg clay. The clay-cation suspensions were titrated with 0.1 N NaOH to give a NaOH/M3+ molar ratio of 2.5. Analysis of the solid-phase reaction products showed that the cation exchange capacity and specific surface of all clays were reduced. Chromium reduced the exchangeability of the interlayers while Al increased the thermal stability. X-ray diffraction analysis revealed that all Al-containing interlayer materials formed similar gibbsitelike polymers. Data from infrared spectroscopy indicated that both Al and Cr were present within the same polymer. Differential thermal analysis and thermogravimetric tracings showed that the rapid collapse of the interlayer in the Cr end-member upon heating was due to a low-temperature loss of hydroxyls. It was not possible to identify all interlayer structures in the Cr end-member. Data from X-ray photoelectron spectroscopy showed all Cr to be Cr(III). Displacement of the interlayer material became more difficult as Cr content increased. The least exchangeable interlayers, therefore, may be found in environments containing the most Cr.

In soil environments, the surfaces of clay minerals are often coated with hydrolytic products of Al. However, limited information is available on the effect of hydroxyaluminum coatings on the interlayering of enzymes for montmorillonite. The objective of this study was to compare the adsorption of tyrosinase onto montmorillonite as influenced by levels of hydroxyaluminum coatings. Tyrosinase is one of the strongest catalysts in the transformation of phenolic compounds. Adsorption of tyrosinase onto Ca-montmorillonite (Ca-Mte) and different hydroxyaluminum-montmorillomte complexes (Al(OH)x-Mte), containing 1.0, 2.5 and 5.0 mmol coated Al/g clay, was studied both in the absence and in the presence of a phosphate buffer at pH 6.5 and 25°C. Except for Ca-Mte in the absence of phosphate where the adsorption isotherm was of C type (linear), the adsorption isotherms were of L type (Langmuir). More tyrosinase molecules were adsorbed onto Ca-Mte than onto the Al(OH)x-Mte complexes, both in the absence and in the presence of phosphate. This indicated the easy accessibility of the enzyme to the uncoated Ca-Mte surfaces. The presence of phosphate did not significantly affect the amount of tyrosinase adsorbed onto Ca-Mte, but substantially reduced the adsorption of tyrosinase onto Al(OH)x-Mte complexes. The higher the level of hydroxyaluminum coatings, the lower the amount of tyrosinase was adsorbed. Because of their affinity to the aluminous surfaces, phosphate ions evidently competed strongly with tyrosinase for Al(OH)x-Mte complexes adsorption sites. The intercalation of tyrosinase by Ca-Mte was indicated by the increased d-spacing of the complex as the amount of the enzyme adsorbed increased. The infrared spectra of tyrosinase-Ca-Mte complex showed that the amide II band of tyrosinase at 1540 cm-1 was practically unaffected by adsorption. The amide I band at 1654 cm-1 was shifted toward a higher frequency, indicating a slight perturbation in the protein conformation. This perturbation became more noticeable in the presence of Al(OH)x-Mte complexes. The data indicated that hydroxyaluminum coatings play an important role in retarding the adsorption of tyrosinase by montmorillonite, and phosphate effectively competes with tyrosinase for the adsorption sites on Al(OH)x-Mte complexes.

There is a clear gap in the understanding of the desorption patterns of metals sorbed on soils and clays, despite their importance in the mobility, transport and fate of metals in natural environments. In this study, we investigated the desorption behavior of Cd, Zn and Pb sorbed on montmorillonite (Mt) and on hydroxyaluminum (HyA)- and hydroxyaluminosilicate (HAS)-Mt complexes. At pH 6.5, 2.5 g L–1 of HyA-Mt and HAS-Mt sorbed almost all of the 10–6 M Cd, Zn or Pb, while Mt under the same condition sorbed ~48, 49 and 55% of the added Cd, Zn and Pb, respectively. Based on pH50 values, the selectivity of metal sorption on Mt was Pb > Zn > Cd, and on the complexes, it was Pb ≫ Zn = Cd. In general, larger fractions of sorbed metals were remobilized from Mt than from the complexes. Again, in comparison with Pb, larger fractions of sorbed Cd and Zn were remobilized from different clays. Reducing the pHs of the equilibrium sorption systems from a fixed point (6.5) to different points (6.0, 5.5, 5.0, 4.5, and 4.0) and from different points (6.5, 6.0, 5.5, 5.0, and 4.5) to a fixed point (4.0) both yielded hysteretic metal desorption patterns. The fractions of Cd and Zn desorbed through Na and Cu exchange from the clays, especially from the complexes, were very different, indicating the existence of cation exchangeable metal sorption sites of weak and strong affinities on the complexes. Based on the EDTA-extractable fractions of Cd and Zn from HAS–Mt and HyA-Mt, it appeared that HyA–metal bonds are stronger than the HAS–metal bonds. Compared with other agents, acetic acid remobilized the highest fractions of all metals irrespective of the type of clays, with a concomitant release of Al or Al + Si. The Pb-HyA/HAS-Mt bonds were, however, still much too strong to be broken substantially by this mechanism. The results accomplished in this study suggest further attention to the fundamental understanding of the mobility, fate, bioavailability and toxicity of the concerned metals in soils and related environments.