Formation of complexes between bentonite and different cationic polyelectrolytes and their use as sorbents for non-ionic and anionic pollutants

Four different cationic polymers that varied in chemical type, cation density and molecular weight were reacted with a Wyoming bentonite (<2 mu m fraction) over a wide range of polymer/clay ratios. Each of the polycations [a polydiallyldimethylammonium chloride (poly-DADMAC), a polyamine, a polystyrene and a polyacrylamide] is a quaternary ammonium derivative of a polymer. The polyacrylamide actually comprises a copolymer of polyacrylamide. Carbon content, basal spacing by X-ray diffraction (XRD), elemental composition by X-ray fluorescence and zeta potential were measured for each of the polymer-clay complexes. The various polymer-clay complexes were studied for their capacities to remove toluene from water, and for the stabilities of sorbed toluene. All of the polycations reacted to form interlayer complexes with the clay, although the clay association with a low-charge density polyacrylamide was weak. Except for the polyacrylamide, which displaced little Na+ and no Ca2+, polycations displaced most exchangeable Na+ from the clay at low loadings (< 0.25 g polymer g(-1) clay), but only displaced most of the exchangeable Ca2+ at quite high loadings (up to 2 g polymer g(-1) clay). Clay-polymer reactions were largely completed at quite low polymer/clay ratios, although the polydiallyldimethylammonium chloride (poly-DADMAC) reacted, at a reduced efficiency, at loadings up to 4 g polymer g(-1) clay. Only the poly-DADMAC showed distinct high-spacing XRD peaks for multilayer intercalation of polymer by the clay. Clay-polymer complexes formed with the polystyrene polycation were the most effective for removing toluene from solution. Clay-polystyrene complexes had a similar uptake of toluene and their association with toluene is of similar stability as complexes of the clay with a double-chain alkyl ammonium cation. Where it occurred, uptake of toluene by clay-polymer complexes reached a maximum at quite low loadings of polymer ( similar to 0.5 g g(-1) clay). Uptake was maintained at or near the maximum level when polymer loadings increased to quite high values. This suggests direct interaction of the toluene with the polycation on the clay rather than its uptake into spaces between polymer pillars. Complexes involving the polyamine, poly-DADMAC and polystyrene, but not the polyacrylamide, all developed a positive zeta potential at high loadings of polymer. Except for the polyacrylamide complexes, zeta potentials steadily became less negative as polymer content in complexes increased from zero, indicating some polycations occupying external surfaces as well as interlayers. However, charge reversals only took place at high loadings of polymers, when the amounts of polycations sorbed were close to that required to satisfy the clay cation exchange capacity. (C) 2002 Elsevier Science B.V. All rights reserved.