Selective sorption of divalent cations using a high capacity sorbent

This manuscript describes the application of a novel sorbent, sodium titanate nanotube (STN) on partitioning of various divalent cations. Seven divalent cations, from alkaline earth, transition and post-transition groups, were used to determine the capacity and selectivity of STN. At pH 3 ± 0.02 and 0.1 M ionic strength, STN displayed high capacity for Pb and Cd (1.27 and 0.39 mmol/g, correspondingly). The affinity of divalent cations was in the order Pb ≫  Cd > Cu > Zn > Ca > Sr > Ni. For six of the tested cations, their sorption capacity can be linearly correlated to its hydrolysis constant and electronegativity. STN has unusually low affinity for Ni and correlations of sorption capacity of Ni falls outside the 95% confidence intervals. Furthermore, it exhibited sorption behavior similar to alkaline earth cations, significant uptakes occurred only when pH > point of zero charge. In competitive sorption tests, STN preferentially sorb Cd over other metals (Zn, Ni, Ca and Sr) which coexist in industrial wastewater. As such STN is a potential novel sorbent useful for partitioning Cd from other metals in industrial wastewater.