Sorption and cosorption of 1,2,4-trichlorobenzene and tannic acid by organo-clays

Abstract

Organo-clay sorbents were prepared by treating montmorillonite with four different quaternary ammonium compounds. The sorbents were then evaluated for the ability to remove a lower molecular weight hydrophobic organic, 1,2,4-trichlorobenzene (TCB), from water in the presence of larger, more hydrophilic organics such as tannic acid. Removal of each sorbate species was characterized separately, then the organo-clays' removal of TCB was assessed in the presence of the tannic acid. The quaternary ammonium compounds used were tetramethylammonium chloride (TMA), hexadecyltrimethylammonium bromide (HDTMA), hexadecylbenzyldimethylammonium chloride (HDBDMA), and dioctodecyldimethylammonium bromide (DDDMA). The adsorption isotherms of tannic acid onto the organo-clays were all nonlinear; the TMA-modified clay led to unfavorable curvature, while the long-chain surfactants all produced favorable curvature. The ability of the organo-clays to remove tannic acid increased in the order TMA<HDBDMA, inconsistent with the organic carbon content of the clays. Adsorption, rather than partitioning, was the uptake mechanism. In contrast, sorption of TCB onto the organo-clays was by a partitioning mechanism, indicated by linear isotherms, uptake proportional to the percentage of the CEC exchanged by HDTMA, and a low heat of sorption. With two distinctive uptake mechanisms, the capacity of the modified clays for TCB was not affected by the presence of tannic acid, even with applied tannic acid concentrations of 50 mg/L. X-ray diffraction indicated that the tannic acid was excluded from internal sites, due to its size and polarity. Organo-clays thus may be usable to retain smaller, hydrophobic organic compounds such as TCB with no loss of capacity in the presence of tannic acid or similar natural organic matter present in surface and groundwaters. This was confirmed for TCB sorption in the presence of a humic acid sample.