Modified Clays for the Adsorption of Environmental Toxicants: Binding of Chlorophenols to Pillared, Delaminated, and Hydroxy-Interlayered Smectites

Abstract

Abstract--Due to their unique polarity, pore-size distribution, and high surface areas, pillared and delaminated clays are potentially useful materials for the adsorption of environmenlal toxicants. To determine their properties for adsorption of chlorinated phenols, alumina-pillared montmorillonite (APM), chromia-pillared montmorillonite (CPM), and alumina-delaminated Laponite (ADL) were reacted with aqueous pentachlorophenol (PCP) solutions in batch equilibrium experiments. An hydroxy-A1 Laponite (HAL) in which the Na + exchange ions were replaced by ions of the type AlI304(OH)(24+x)(H20)(I2 .)(7-• was included in the study. With ADL as the adsorbent, the extent of PCP adsorption increased with decreasing pH, and then became constant at pH _< pKa. Thus, the neutral phenol was preferred over the phenolate form. Binding of neutral PCP at pH 4.7 to all adsorbents never reached saturation values, and the loadings achieved were limited by the water solubility of the adsorbate. Among the pillared and delaminated clays investigated, ADL exhibited the largest capacity for physical adsorption of PCP at pH 4.7. Differences in the PCP binding capacities for APM, CPM, and ADL suggested that adsorption was dependent on the pore structure and surface composition of the modified clay adsorbent, not on surface area alone. HAL exhibited quantitative uptake of PCP at the 8 #mole/g level, indicating that a chemisorption mechanism may operate for PCP binding to this adsorbent. Adsorption of 3-chlorophenol, 3,5-dichlorophenol, and 3,4,5-trichlorophenol by ADL at pH 7.4 increased as the degree of hydrophobicity and chlorination of the phenol increased; hence, the binding capacity was not limited by the molecular size of the adsorbate. In contrast to the adsorption properties observed for pillared, delaminated, and hydroxy-interlayered clays, Na+-montmoriUonite and Na+-Laponite did not adsorb PCP from aqueous solution.

Adsorption of Chlorinated Phenols from Aqueous Solution By Surfactant-Modified Pillared Clays

“…At pH 10.5, neither of these two classes of sites is preferred by the phenolate anion. It is interesting to note that in the case of the adsorption of PCP on alumina pillared montmorillonire at acidic pH (Zielke and Pinnavaia, 1988), the adsorbed quantities were much smaller than those obtained for our surfactant-modified pillared clay. Clearly, chemical modification by eoadsorption of nonionic surfactant increases the binding energy of the surfaces, thus dramatically improving PCP adsorption.…”

“…In order to obtain a better understanding of the PCP adsorption mechanisms, it was necessary to replot the isotherms relative to the degree of undersaturation, as shown in Figure 5B. In a paper dealing with the adsorption of chlorinated phenols on pillared and delaminated clays, Zielke and Pinnavaia (1988) showed that pentachlorophenol adsorption was very much influenced by the equilibrium pH, with almost no measurable uptake for equilibrium pH > 7. In contrast, pentachlorophenol adsorption on organo-clays is also pH independent (Boyd et al, 1988).…”

“…We have proposed that microporous metal-oxide pillared clays and mesoporous delaminated smectite clays might be suitable as recyclable adsorbents for the adsorption of organic pollutants (Zielke and Pinnavaia, 1988). In pillared clays, the gallery surfaces of faceface aggregated layers are made accessible by the intercalation of metal-oxide aggregates formed by the dehydration and dehydroxylation of polycations such as the oligomeric Al1304(OH)24(H20)7a + cation.…”

Sorption of Organic Compounds by Al and Zr-Hydroxy-Intercalated and Pillared Bentonite

“…3-CP (pK1 = 9.02) was sorbed as a neutral molecule. According to Zielke and Pinnavaia (1989), neutral chlorophenol molecules are preferred by Al-pillared montmorillonite over the phenolate forms.…”

“…In this context, the possibility of sorbents in the purification of contaminated water, e.g., for in situ leachate treatment in waste-disposal containment systems, should be investigated. For some compounds, such as chlorophenols, it was shown that hydroxy-intercalated and pillared bentonites may be better sorbents than untreated bentonites owing to their high surface acidity and surface area (Zielke and Pinnavaia, 1989).…”