Adsorption of polycations on clays: an in situ study using 133Cs solution-phase NMR

Abstract

133 Cs solution-phase NMR has been evaluated as an in situ probe to study the adsorption of polycations, and other ions, onto clays in aqueous suspensions containing 2.5 mass% low-iron Texas bentonite. In particular, the effectiveness of the polycations FL15, FL16, F17, of general formula [(Me2NCH2CHOHCH2)n]n+, and Magnafloc 1697, [(CH2[graphic omitted]CH2)n]n+, at displacing Cs+ ions from the Texas bentonite is compared to Na+, K+, Me4N+(TMA+) and paraquat2+. This information has been correlated with that obtained from particle-size and electrophoretic measurements in aqueous solution, together with that obtained from adsorption isotherms, variable temperature X-ray diffraction (XRD) and thermogravimetry (TG) studies using dry powdered samples. FL15, FL17 and 1697 all exhibited high affinity adsorption isotherms on all the cation-exchanged forms of WL, whereas the adsorption of TMA+ ions, which represent the cationic portion of the polymers, was of lower affinity. The maximum amount of polymer adsorbed, Qmax, depended on the resident exchange cation, varying as Na+ > K+ > Cs+, and the molecular mass of the polymer. Qmax on the Na-clay approached twice the amount of polycation required to fulfil the cation exchange capacity (CEC) of the Texas bentonite. XRD profiles confirmed that the polycations resided between the clay lamellae except at low loadings on the Cs+-clay. In the absence of competing (poly)cations the 133Cs NMR exhibited a broad (1500 Hz), weak signal. As (poly)cation was added the signal narrowed, eventually to 15 Hz, and grew considerably in intensity. As the adsorption isotherms suggested, the polycations were extremely effective at displacing the resident Cs+ ions, closely followed by paraquat2+. TMA+ ions were considerably less effective and Na+ and K+ ions were ineffective. Magnafloc 1697 was not as effective at displacing Cs+ ions as the FL series of polymers, and this was reflected in the zeta potential (ζ) and particle-size measurements. This data suggested that the FL polycations were able to penetrate the interlayer more effectively than the bulkier 1697, which was only adsorbed up to 68% of the CEC.