Restrictive Entry of Aqueous Molybdate Species into Surfactant Modified MontmorilloniteA Breakthrough Curve Study

Molybdenum sorption behavior and the nature of breakthrough curves have been studied in a fixed-bed column containing montmorillonite clay modified by treatment with cationic surfactant hexadecyltrimethylammonium (HDTMA) bromide. Different amounts of the surfactant loaded into the interlayer region of the montmorillonite to increase the interlayer spacing is found to enhance molybdate sorption. The sorption was found to be at a maximum in acidic medium (pH <0) and was almost constant between pH 3 and 4. Beyond pH 4, adsorption decreased rapidly and was almost two to four times lower than that in the acid medium. It is known from the equilibria of aquo molybdate species that polymolybdate species Mo 8 O 26 4 - and Mo 7 O 24 6 - are prevalent in the highly acidic medium; the species H 2 MoO 4 and HMoO 4 - are stable between pH 0 and 4, and MoO 4 2 - is predominantly present above pH 5. The variation in adsorption of Mo is interpreted on the basis of the type of species prevalent at different pHs. The sample loaded at 0.5 CEC of surfactant with an interlayer spacing of 5.2 Å was found to adsorb negligible amounts of molydate, whereas that loaded with surfactant corresponding to 1.5 times its CEC is able to admit HMoO 4 - and MoO 4 2 - species into the interlayer of clay sheets but not the larger polymolybdate ones. The samples loaded with surfactant corresponding to 2.0 times its CEC swells enough (32 Å) to admit polymolybdate species Mo 7 O 24 6 - (23.8 Å) and Mo 8 O 26 4 - (27.2 Å). Shapes of breakthrough curves obtained at different surfactant loadings clearly support the restricted entry of the molybdate species. The study has demonstrated, for the first time, the possibility of sieving of the anionic species in the aqueous medium by carefully manipulating the gallery height of a layered material. Sorption energy calculated from the DKR equation shows that the interaction between the surfactant and molybdate follows an ion-exchange mechanism. Washing with dilute alkaline solution releases the molybdates quantitatively and regenerates the column material. The regenerated column material shows no change in the breakthrough curves when repeatedly used for molybdate adsorption.