Polyhydroxoaluminum smectites

One way to prepare pillared smectites (and probably the simplest method for scaling up to industrial production) is introducing polyhydroxoaluminum cations by cation exchange. These cations are formed when NaOH is added to aluminum salt solutions in amounts NaOH/Al3+ < 3 (typical 1—2.5) [100—105], Usually, the polyhydroxoaluminum smectites are heated to 300—400 °C to dehydrate them into polyoxoaluminum smectites. As adsorbents they may also be used in the uncalcinated form.

Zielke and Pinnavaia [106] compared the adsorption of several chlorinated phenols by pillared montmorillonite and Laponite (Laponites are synthetic hectorite-like materials). Pentachlorophenol, which is the strongest Brensted acid, is best adsorbed by the polyoxoaluminum derivative and less by the polyhydroxo form. The adsorption capacity decreases strongly with increasing solution pH (from pH = 4.7 to 7.4), showing that this pollutant is adsorbed in undissociated form. The effectivity of the smectites (at pH = 4.7) is: polyoxoaluminum laponite > polyhydroxoaluminum laponite > polyoxoaluminum montmorillonite > polyoxochromium montmorillonite. Sodium laponite and montmorillonite show no tendency to adsorb the pollutant from aqueous solution.

The chief difference between laponite and montmorillonite is the high degree of disorder and delamina-tion of laponite, which also persists in the pillared forms. The enhanced adsorption in comparison with montmorillonite results from the increased accessible surface area and may also be related to the more hydrophobic character of the bare silicate surface [8].

Nolan et al. [107] reported the adsorption of chloro dibenzo-p-dioxins on polyhydroxoaluminum montmorillonite. High distribution coefficients (KD(m\lg) = amount X adsorbed per g solid (mg/g)/amount X in solution (mg/ml)) are obtained (octachloro dibenzodioxin on montmorillonite 2800 ml/g, on polyhydroxoaluminum montmorillonite 94000 ml/g). The distribution coefficient decreases sharply when the interlamellar polyhydroxoaluminum complexes are dehydrated to polyoxoaluminum pillars (A"d = 1100 and 1800 ml/g for polyhydroxoaluminum montmorillonite heated to 170° and 550 °C).

The adsorption properties of pillared clays can be improved by pre-adsorbing surfactants. Polyhydroxoaluminum montmorillonite covered with hexadecyl pyridinium ions is a better adsorbent for pollutants in industrial waste waters than hexadecyl pyridinium (HDPy) montmorillonite [108]. For benzopyrene KD is increased from 48000 ml/g (HDPy montmorillonite) to 95 800 ml/g (HDPy polyhydroxoaluminum montmorillonite) and for pentachlorophenol from 16 000 ml/g to 156 000 ml/g. No significant differences are noted in the sorption of 3,5-dichlorophenol on both adsorbents. The reduced adsorption of this pollutant may be related to its higher aqueous solubility.

The increased adsorption of certain pollutants on surfactant modified pillared montmorillonites results from the orientation of the surfactant cations different from that in the non-pillared smectites. As the polyhydroxoaluminium complexes are positively charged, the cations are assumed to be oriented with their positive head groups pointing away from the pillars (Srinivasan and Fogler, 1990). This makes the surface hydrophilic, whereas the space between the alkyl chains retains its hydrophobic character. It is also assumed that an electrostatic shielding against flocculation further increases the effective interface area.

Another way to modify pillared smectites is proposed by Michot and Pinnavaia [109], Sodium montmoril-lonite is reacted with a solution containing the [A11304(0H)24+x(H2012J(7"x)+ cation [110] and a technical alkyl pentaethylene oxide (Tergitol 15s-5). The reaction product is a pillared smectite loaded with the nonionic surfactant. The surfactant occupied micropores between the pillars are the adsorption sites for 3-chlorophenol, 3,5-di-chlorophenol, 3,4,5-trichlorophenol and pentachlorophenol. The uptake of the pollutants increases with the number of chlorine atoms. The toxicant loaded clay can be recycled by calcination at 500 °C and re-adsorption of the surfactant.

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