Sorption of tannic acid, phenol, and 2,4,5-trichlorophenol on organoclays

Sorption of the organic pollutants phenol, 2,4,5-trichlorophenol, and tannic acid (Fluka) onto montmorillonite-based sorbents was studied. The sorbents were homoionic Na- or Ca-montmorillonite, montmorillonite completely exchanged with the cationic organic surfactant dimethyldistearylammonium chloride (or DDA-M), and DDA-M partially re-exchanged with Ca2+ (Ca-DDA-M). The influence of pH on sorption of phenol and trichlorophenol was also investigated. The sorption capacities for phenol, trichlorophenol, and tannic acid were significantly enhanced by surfactant treatment of the homoionic clay. Uptake onto the organoclays increased in the order phenol < tannic acid < trichlorophenol, which corresponds to the order of increasing hydrophobicity. The significant role played by hydrophobicity was also observed in pH effects which generally favored sorption when the pH was low enough to assure that the uncharged (protonated) species was predominant. In the case of trichlorophenol, sorption could be shown to be quantitatively attributable to the neutral species. The results of X-ray diffraction measurements showed increased d001 spacings when the DDA was intercalated into the interlamellar spaces, increasing the available sorbent porosity. This, and the linkage of uptake to solute hydrophobicity, indicates that uptake is by a partitioning mechanism, though non-linear isotherms may be caused by site limitations. Surprisingly, if a portion of the intercalated DDA is removed by exchange with Ca2+, the d001 spacing decreases but the sorption capacity of the organoclays is improved. One possible mechanism is improved elementary sheets cohesion due to calcium ions, imparting molecular sieve characteristics to the solid. The extent and possible selectivity of sorption using these organoclays suggests that they may provide an alternative to activated carbon sorbents in water treatment.