Adsorption behaviors of nano-sized ETS-10 and Al-substituted-ETAS-10 in removing heavy metal ions, Pb2+ and Cd2+

A new class of nano-sized large pored titanium silicate variety, ETS-10 and ETAS-10 with different Al2O3/TiO2 ratio were successfully synthesized and applied to the removal of heavy metals, in particular, Pb2+ and Cd2+. The study of equilibrium and adsorption isotherm at 298 K, 313 K and 328 K was conducted using batch mode. The uptake rates for both Pb2+ and Cd2+ were extremely rapid and fitted to the pseudo-second order. The sorption capacities of Pb2+ on ETS-10, ETAS-10(A) (Al2O3/TiO2 ratio of 0.1) and ETAS-10(B) (Al2O3/TiO2 ratio of 0.2) predicted by the pseudo-second order equation were to be 1.66 mmol/g, 1.68 mmol/g and 1.60 mmol/g while 1.25 mmol/g, 1.26 mmol/g and 1.12 mmol/g for Cd2+, respectively. Adsorption isotherms for both Pb2+ and Cd2+ were well fitted to Langmuir–Freundlich isotherm. The monolayer capacities predicted by Langmuir–Freundlich equation are quite well consistent with those by the pseudo-second order equation and those are, on the average for different temperature, 1.78 mmol/g on ETS-10, 1.80 mmol/g on ETAS-10(A) and 1.71 mmol/g on ETAS-10(B) for Pb2+ and 1.44 mmol/g, 1.33 mmol/g and 1.19 mmol/g for Cd2+, respectively. Finally, it is interesting to notice that as Al content increases in the framework of ETS-10, octahedrally coordinated Ti content decreases, indicating the partial incorporation of Al into octahedrally coordinated Ti as well as isomorphous substitution for tetrahedrally coordinated Si. It suggests that partial substitution of Al for octahedrally coordinated Ti rather leads to the reduction in sorption capacities due to the decrease in the number of exchangeable site in ETS framework.